Building a Sustainable Future from Theory to Practice: A Comprehensive PRISMA-Guided Assessment of Compressed Stabilized Earth Blocks (CSEB) for Construction Applications

Abstract

Compressed stabilized earth blocks (CSEBs) offer a cheaper and environmentally sustainable alternative to traditional building materials for construction. In addition to addressing waste disposal difficulties, the inclusion of waste additives may improve the characteristics of compressed earth blocks (CEBs). This article attempts to outline the findings of researchers who have utilized the various manufacturing processes and investigated the influence of binders and fibers on the properties of CEBs. A systematic search of Web of Science and Scopus electronic databases for works on soil blocks published between 2012 and 2022 yielded 445 articles, while reports, case studies, conference papers, and non-English articles were omitted. Keywords such as “Soil blocks”, “Earth bricks”, and others were used to identify eligible studies. This study has been segmented into five sections, including a descriptive examination of articles and authors who have investigated soil blocks, a comparative analysis based on their manufacturing processes, and physical, mechanical, and durability aspects of the CSEBs, which were analyzed to determine the impact of additives. The PRISMA 2020 standards were followed in the evaluation of each record, which resulted in the identification of 61 articles that were pertinent to the study’s objective. The comparative analysis of the articles reveals that the binders were more significant in improving the compressive strength, cyclic wetting-drying and erosion (durability) aspects of the soil blocks, while fibers were effective in enhancing their flexural and thermal performance. The literature review indicates that if the minimum permissible limits are met, waste materials have the potential to partially replace the soil. In addition, this study suggests establishing standardized manufacturing norms and testing protocols to ascertain the quality and safety of CSEBs used in construction. However, this study is constrained by the limited databases used, governed by keywords, electronic resources and timeframe that could be used as research avenues in the future.

1. Introduction

Materials such as soil, wood, and stone are traditional building materials that have been utilized for construction by mankind for millions of years. As the more popular and abundantly available natural building material, soil is typically used to construct residential structures, with mud being used explicitly in hot, arid, and temperate climatic regions such as India and North Africa [1]. The filtering process of articles based on the PRISMA, 2020 guidelines is illustrated in Figure 1, depicting various stages. The geographic distribution of earthen construction around the world is depicted in Figure 2. The United Nations Centre for Human Settlements (UNCHS) data suggest that as much as 40% of the world’s population still resides in these earthen structures [2]. This is credited to the construction practice associated with these materials as being simple, requiring minimal technology and tools, not requiring skilled labor, and requiring low energy consumption, thus proving to be cost-effective simultaneously [1,3]. However, due to technological advancement and economic development, the practice of construction with earth has fallen by the wayside over the last few centuries, resulting in the extensive utilization of contemporary building materials such as concrete, aluminum, glass, steel and others. In addition to being expensive and resource-intensive, these modern building materials also leave a massive carbon footprint [4]. Long abandoned in favor of materials such as concrete and steel, earthen construction is gaining traction in the current context of sustainable development [3,5]. Using locally accessible natural materials for building construction is an effective and economical alternative that can attenuate emissions and subsequently reduce the carbon footprint of structures [1,6,7].

For centuries, compacted earth has been used worldwide conventionally for the construction of dwellings, and it relies on practices, namely “Adobes”, “Rammed earth”, “Cob”, and “Wattle-and-daub” type of constructions. “Adobes” are bricks or blocks made from a mixture of earth and water that may or may not include other stabilizing or reinforcing agents such as lime, cow dung, rice husk straw, or wheat straw that are allowed to dry in the sun. “Rammed earth”, on the other hand, is a technique for the construction of walls by compacting moist earth in layers within a wooden formwork, either by hand or by the use of a mechanical rammer [8,9]. “Cob” is another ancient building method used since prehistoric times. Cob structures are formed in layers, and each layer is first kneaded into a solid, dough-like consistency. The layer is then allowed to cure before successive layers are stacked one over another. In addition, unlike “Rammed earth” construction, this method does not use formwork, making it the simplest to implement. The cob is placed on the ground, pressed with the hands or feet, and molded into the desired shape. Sand, straw, clay-rich soil, and water are the constituents of cob [10,11,12]. Lastly, construction with wattle-and-daub has a long history that spans the Americas, Europe, Africa, the Middle East, and all of Asia. This construction method was popular among the ancient populations of Mesopotamia, Rome, and several other cultures and is reported to be the first construction technique, prior to adobe and rammed earth, utilizing tree branches for the supporting structure, with mud poured into a nest of intertwined branches [12,13]. Due to its speedier construction, many nomadic tribes employed wattle and daub to build temporary homes. This method involves weaving together thin branches (wattle) as a base for a daub, or mud plaster, which is applied to a lattice of sticks, bamboo or boards (wattle) [11,12,14].

CSEBs are regarded as an evolution from adobes and rammed earth since the manufacturing process remains identical [15]. Locally available soil is utilized as the parent material, while stabilizing agents such as lime and cement are introduced into it to enhance the mechanical and durability performance of the blocks. The mix is then compacted using suitable machinery to manufacture these blocks. However, the compaction method has significantly improved with a breakthrough in the 1950s by G. Ramires, who successfully developed one of the first manual presses, CINVA-RAM, which pressed the moist soil into a block form [9]. The present state, however, takes a different approach with the evolution of mechanical presses with hydraulic compaction capabilities for a more uniform and homogenous brick quality.

Nevertheless, the earthen bricks prepared by the methods mentioned earlier have still not been accepted as a mainstream building material and are considered non-standard due to concerns of quality control and the unindustrialized manufacturing process. Heterogeneousness and inconsistent soil properties are the major reasons for labeling the bricks as unsuitable for modern construction, possessing lower mechanical strength and durability properties. As a result, earthen structures are challenging to design and have received little acceptance as building materials by designers and governments worldwide. This is easily seen because only a few countries have specific guidelines in terms of standard codes for carrying out construction using earth bricks, including ASTM E2392/E2392M, NMAC (USA), NZS 4297, 4298, 4299 (New Zealand), SCEB-1382 (Sri Lanka), and IS-13827 (India). There is also a stigma associated with using earthen material for construction, which is considered substandard and is frequently associated with poverty. There is also a psychological effect on the dwellers’ minds, as earthen houses are labeled as unsafe and prone to accelerated deterioration.

However, the statements mentioned above may not be entirely accurate. Over the last few years, there has been a shift in the perspective with more focus on developing sustainably. A rise in construction activities utilizing earthen materials has been attributed to the potential benefits that are now slowly being recognized globally [15]. Better acoustics, noise isolation, better thermal performance, and the ability to control moisture while promoting a comfortable and healthy interior environment are critical points leading to this shifting viewpoint [1,3,16]. In general, CO₂ emissions due to earthen structures are also relatively lower than the existing building materials [6,17]. Additionally, these materials can be easily disposed of and do not require further processing.

Earthen blocks, on the other hand, possess inherent limits such as poor compressive strength, volume changes, shrinkage cracking, strength loss due to saturation, erosion owing to direct or in-direct rainfall, and durability concerns [4,5,9,18]. Earth blocks manufactured in conjunction with stabilizing agents were introduced to fix these flaws. Traditional stabilizers such as cement [3,5,17,19,20] and lime [19,21,22,23] have been used extensively in this domain. “Compressed Stabilized Earth Blocks” (CSEBs) are essentially manufactured by mixing a stabilizer in a definite proportion to the soil and then pressing either manually or mechanically. The type of stabilizer, the number of stabilizers used, and the amount of water used are all parameters that typically influence the effectiveness of a stabilizer. These are also critical factors influencing the mechanical response of CSEBs. These unfired stabilized earth blocks have enormous potential and are widely used in the construction industry, primarily in masonry blocks, interlocking pavement tiles, and other structures.

The existing literature focuses primarily on improving the mechanical and durability properties of CSEBs. While CSEBs are generally easy to construct, more research is needed to understand the best techniques for using CSEBs in different types of building projects. This could include research on the use of CSEBs in multi-story buildings as well as on the best techniques for incorporating windows, doors, and other features into CSEB walls. The mix design of CSEBs can have a significant impact on their strength and other properties. Further research is needed to determine the optimal mix design for different types of soils and stabilizers, as well as to identify any potential drawbacks or limitations. Currently, there is no universal standard for CSEBs. This can make it difficult to compare different products and ensure consistent quality. Establishing a set of standards for CSEBs could help improve their reliability and make them more widely accepted.

Compressed stabilized earth blocks (CSEBs) are gaining appeal as a green alternative to traditional concrete, but there is a paucity of research summarizing the current state-of-the-art methods in this area. This review covers that knowledge gap by giving a thorough overview of what is currently known about CSEB manufacturing, properties, and its mechanical and durability performance. In addition, this review has revealed that there are important gaps in the current testing and production standards. This would aid in identifying areas that require additional research, and it might also act as a reference for individuals interested in employing CSEBs in their own construction projects. Overall, this review paper offers a thorough and current overview of the state-of-the-art CSEBs, constituting a fresh contribution to the area.

This study therefore attempts to present a comprehensive analysis of the diverse types of binders and fibers employed by multiple research scholars over the last decade to analyze the physical, mechanical, and durability aspects of CSEB. Furthermore, this study will attempt to analyze and comment on the feasibility, applicability, and usefulness of such waste in improving the properties mentioned earlier in the CEB to create an environmentally friendly, lightweight, and economically viable block compared to its traditional counterparts.

This study’s structure includes an introduction, a brief history of CSEBs, and the methodology employed in this review. In addition, a section is dedicated to discussing the authors and publication patterns of those who have researched CSEBs from a variety of perspectives. The subsequent section describes the types and methodologies utilized by researchers in the production of blocks. This is followed by an examination of the physical, mechanical, and durability aspects of CSEBs, and the paper concludes with a discussion of the findings and recommendations for future research.

2. Methodology of Review

This study aims to provide a complete overview of various materials used in the construction industry over the past decade by multiple research scholars, as well as to identify the major obstacles and opportunities that could be investigated using CSEBs to increase their viability and acceptance. This was achieved through a thorough examination of the relevant literature. This systematic literature assessment follows the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) approach to ensure that the study can be reproduced [24]. It is a widely used methodology for conducting literature reviews and gives scholars a transparent, unambiguous picture, making it reproducible. The systematic review was carried out in stages and started with identification, then moved on to screening articles based on a predetermined list of guidelines, validating the eligibility of the chosen articles based on their content, and it concluded with establishing inclusion criteria for the articles to be reviewed. The methodology followed is depicted in Figure 1.

Figure 1. Different stages of filtering of articles according to PRISMA, 2020 guidelines.

Figure 1. Different stages of filtering of articles according to PRISMA, 2020 guidelines.

2.1. Identification of Articles

Article identification is critical and necessary because it provides reproducibility and transparency to the review paper. The Web of Science electronic resource was examined for articles regarding soil blocks published in the last ten years, or between years 2012 and 2022. The following keywords were used to identify as many eligible studies as possible: “Soil blocks”, “Earth bricks”, “Unburnt soil bricks”, “Compressed soil blocks”, and “Compressed stabilized earth blocks”, with the advanced search option used to broaden the scope of article identification. The reference lists of the eligible articles found through the electronic search were also manually searched. A total of 445 articles were identified through this process.

2.2. Screening and Selection Criteria

The screening process was used to select pertinent articles after identification. This procedure was followed as per the selection and rejection criteria. The first stage was to remove duplicate articles, which left 312 articles for further analysis. This was followed by screening of articles based on their title and keywords, through which 114 articles were omitted. Furthermore, 52 more records were removed due to their irrelevance to the research work’s topic. The abstracts of the remaining 76 articles were read, of which 15 were rejected, leaving 61 articles based on the selection and rejection criteria.

2.3. Eligibility Evaluation of Full-Text Articles

Full-text articles that passed the initial screening were then read to determine which ones would be included. A content-based evaluation was performed on the remaining 61 full-text articles. The articles were carefully read, and the articles that discussed masonry walls using soil blocks, utility of mortar in earthen construction, testing methods and standards and other similar research articles were discarded. A detailed inclusion–exclusion criteria is explained in the following sub-section.

2.4. Inclusion Criteria

To address our specific research theme, we included all papers that described research, examining the mechanical, physical, or durability characteristics of soil blocks. Only articles published in journals were used for this study since the journal articles were peer-reviewed with in-depth analysis of the subject of interest, implying that they had quality and structured content. While book chapters and case studies are good sources for references, they are often limited to a particular case or topic in their entirety. As a result, this review eventually included 61 papers. The different metrics that were established to approve and reject potential research articles for this study are presented in

Table 1. Inclusion and exclusion criteria for the identification of pertinent articles.

Metric	Inclusion Criteria	Exclusion Criteria
Article category	Journal	Non-journal, review, conference, case studies, book chapter and comments
Article title	Associated with soil blocks or contains keywords in the title	Unassociated with soil blocks or lacks the keywords
Abstract	The abstract discusses soil bricks or block testing, or as the keywords indicate	Tests on soil blocks or bricks are not discussed in the abstract or in the keywords
Communication medium	English	Non-English (French, German, etc.)
Full-text	Available	Unavailable
Content	Emphasis on physical, mechanical and durability characteristics of earthen block	Emphasis on analysis of masonry wall made using earthen blocks, testing methodology for earthen block, or on design of block manufacturing equipment

.

2.5. Analysis of the Selected Articles

The 61 papers were analyzed, and their data were tabulated using Microsoft Word and Excel programs. The data included a variety of details, such as the affiliations and countries of origin of the various authors, journals and publishers chosen by various researchers, and the various types of fibers and binders used in conjunction with their relative proportions. Along with depicting the type of block and the machines used in block manufacturing, the initial, final, and increment or decrement in the values of the CSEBs’ physical, mechanical, and durability properties had been studied. In case information was unclear or unavailable, the corresponding value has been left blank.

Figure 2. Geographic distribution of earthen construction around the world [25].

Figure 2. Geographic distribution of earthen construction around the world [25].

2.6. Authors and Publication Trends

This segment discusses the dynamics of the publication of the selected articles in terms of year, journal name, and publisher chosen by the researchers. Authors’ affiliations, as well as their country of origin, are discussed in this section.

2.6.1. Year of Publication

The yearly publication pattern of the articles that were chosen for review is covered in this section. The articles chosen were explicitly published between 2012 and 2022, out of which 61 articles pertinent to this review were chosen. This time frame was implemented in order to examine the recent innovations and advancements in the context of CSEB manufacturing and the utilization of binders and fibers alternative to their traditional equivalents. It is clear that in the early years, there were very few articles published on this subject. The year 2013 had only one article, while the years 2014 to 2019 showed an increasing trend in the number of articles published, with the year 2020 having the most, with 16 articles published. Years 2021 (n = 10) and 2022 (n = 5) display a reasonable number of published articles. Figure 3 depicts the year-wise publication trend of the articles selected for review.

2.6.2. Journal and Publisher

This segment focuses on classifying the pertinent articles (n = 61) based on the journals and publishers that were chosen by the researchers for their research article. A variety of journals and publishers were found during this analysis, which displays the extent and the reach of their efforts. During this analysis, a total of 34 journals and 18 publishers were identified. Construction and Building Materials, which published 29.5% of the selected articles and was followed by the Journal of Building Engineering and Materials Today: Proceedings, were the two most popular journals for publishing articles about soil blocks.

From the analysis, it is evident that Elsevier was the most preferred publisher, with 49% of the total articles published by Elsevier alone, followed by MDPI, Springer, and others. Figure 4 and Figure 5 show the publication trend in terms of publishers and journals preferred by researchers.

2.6.3. Author’s Affiliation

It was discovered during the analysis of the relevant articles that authors from various countries were actively researching the field of soil blocks. The analysis was restricted to full-text articles in English. It was discovered that 76 institutions or universities worked on the topic of soil blocks, with Bangladesh University of Engineering and Technology and North Carolina State University appearing in 5 articles, followed by the University of Education Winneba and National Institute of Technology Agartala appearing in 4 and 3 articles, respectively. According to the data obtained from the n = 61 articles, 80 countries conducted studies on CSEBs. The majority of the authors who published such articles came from India (12.5%), followed by the US (11.2%) and Algeria (6.2%).

Table 2. Number of authors and their affiliated institute and country for the (n = 61) articles reviewed.

Reference(s)	Author(s) Affiliation	Country	Number of Authors
[26]	University of Sharjah	United Arab Emirates	4
[27]	University of Lome	Togo	3
[28]	Universidad Politécnica de Madrid	Spain	3
	University of Valladolid
[29]	University Lille	France	4
	University of Houston	USA	1
[30]	Universidade Federal da Paraíba	Brazil	2
[22]	Universidade de Lisboa	Portugal	3
[31]	Universidade de Brasília	Brazil	2
	Universidade de Aveiro	Portugal	3
[32]	National Institute of Technology Karnataka	India	2
[33]	University of Education Winneba	Ghana	2
[34]	University of Education Winneba	Ghana	1
[35]	University of Education Winneba	Ghana	1
[7]	University of Education Winneba	Ghana	1
[36]	University of Florida	USA	2
[37]	Pennsylvania State University	USA	2
[19]	Giza High Institute of Engineering and Technology	Jordan	4
	Yarmouk University	Egypt	1
[38]	Bangladesh University of Engineering and Technology	Bangladesh	2
	North Carolina State University	USA	1
[4]	Military Institute of Science and Technology	Bangladesh	3
	Bangladesh University of Engineering and Technology
	Pabna University of Science and Technology
	North Carolina State University	USA	1
[39]	Bangladesh University of Engineering and Technology	Bangladesh	4
	North Carolina State University	USA	1
[18]	University of Sharjah	United Arab Emirates	2
[40]	University of Seville	Spain	2
	University of Strathclyde	UK	1
[5]	The University of Ngaoundere	Cameroon	2
	National School of Mineral Industry ENIM	Morocco	1
[41]	University of Bechar	Algeria	4
[42]	Bangladesh University of Engineering and Technology	Bangladesh	3
	North Carolina State University	USA	1
[6]	Bangladesh University of Engineering and Technology	Bangladesh	4
	North Carolina State University	USA	1
[43]	National Institute of Technology Calicut	India	2
[44]	National Institute of Technology Agartala	India	2
[3]	Felix Houphouet Boigny	Côte d’Ivoire	5
	University Peleforo Gon Coulibaly University
[17]	University of M’sila	Algeria	3
[1]	Université d’Abomey-Calavi	France	5
	University of Nantes
[45]	University of Sciences and Technology of Oran Mohamed Boudiaf	Algeria	1
	IMT Nord Europe	France	2
[46]	University of Liege	Belgium	3
[47]	De La Salle University	Philippines	2
[16]	De La Salle University	Philippines	5
[48]	University of Yaoundé	Cameroon	4
[49]	Université Polytechnique de Bobo-Dioulasso	Burkina Faso	2
	Université de Ouagadougou
	Université de Toulouse	France	2
	Université de Lyon
	Coventry University	UK	1
[50]	University of Yaounde	Cameroon	6
	University of Douala
[51]	University of Alabama	USA	2
[52]	Universidad Aut’onoma de Chile	Chile	2
	Universidad de La Frontera
	Universidad Internacional de La Rioja	Spain	2
	Universidad Cat´olica Del Maule
[53]	BMS College of Engineering	India	4
	Indian Institute of Science
[54]	Institut International d’Ingénierie de l’Eau et de l’Environnement (2iE)	Burkina Faso	2
	Université de Liège	Belgium	2
[55]	Institut International d’Ingénierie de l’Eau et del’Environnement (Institut 2iE)	Burkina Faso	2
	Université de Liège	Belgium	1
[56]	Institut International d’Ingénierie de l’Eau et de l’Environnement (2iE)	Burkina Faso	3
	Université de Lyon	France	2
	Université de Toulouse
[57]	University of Jaffna	Sri Lanka	4
[58]	Curtin University	Australia	4
[59]	National Institute of Technology Agartala	India	2
[60]	National Institute of Technology Agartala	India	4
[9]	University of Trás-os-Montes e Alto Douro	Portugal	6
	University of Minho
[61]	Chandigarh University	India	2
[8]	University of Minho	Portugal	6
	University of Trás os Montes e Alto Douro Vila Real
[62]	Chandigarh University	India	4
	Guru Ghasidas University
[63]	Federal University of Rio Grande do Norte (UFRN)	Brazil	7
	Federal Institute of Education, Science and Technology of Paraíba (IFPB)
[64]	BMS College of Engineering	India	2
[21]	University of Biskra	Algeria	2
[15]	University of Biskra	Algeria	4
	University of Ouargla
	University of Djelfa
[65]	University of Minho	Portugal	6
	Federal Center for Technological Education “Celso Suckow da Fonseca” (CEFET/RJ)	Brazil	1
[66]	Indian Institute of Technology Guwahati	India	2
[67]	University of Moratuwa	Sri Lanka	2
	University of Ruhuna
[23]	Instituto Tecnologico de Pachuca	Mexico	5
	Universidad Autonoma de Tamaulipas
	Universidad Nacional Autonoma de México
[68]	Universidad Industrial de Santander	Colombia	4
[69]	University of Kelaniya	Sri Lanka	2
[70]	Quaid-e-Awam University of Engineering	Pakistan	3

lists the number of researchers along with their corresponding institutional and national affiliations.

2.6.4. Geographic Location of Experimentation

A geographical distribution analysis was undertaken on the relevant articles (n = 61) in order to understand the location of the experimentation or to determine the location of the soil that was experimented on.

Figure 6 and Figure 7 depict the country- and continent-wide dispersion of the authors’ research work, respectively. The majority of the articles published came from India, with 10 (12.5%) articles published; followed by the United States, with 9 (11.2%) articles; and Algeria, France, Bangladesh, and Portugal, each with 5 (6.2%) articles published.

While leading numbers do not depict a country’s prominence in a specific research field, they provide an overview of the current research trend. It was found that 26 countries were actively involved in soil block research. It was also discovered that as many as seven researchers from various institutions/universities in various countries collaborated for research purposes. A continent-wise distribution of the relevant articles published shows that Asia had the highest number of publications (31%), followed by Africa (25%), Europe (18%), North America (10%), South America (6%), and Oceania (1%). In contrast, no publications from Antarctica were identified.

Figure 7. Continent-wise distribution of the (n = 61) articles reviewed.

Figure 7. Continent-wise distribution of the (n = 61) articles reviewed.

3. Earthen Blocks: Types and Manufacturing Methods

This section examines the different aspects involved in the manufacturing of CSEB. According to the articles under analysis, solid or hollow blocks made up the majority of the manufactured specimens, although a few cylindrical samples were also observed. A few articles also mentioned producing blocks as cuboidal solid blocks or full-length beams, which were then divided into smaller blocks to conduct tests in accordance with set testing criteria. Because there were no set standards, the manufactured samples’ dimensions varied greatly. However, multiple researchers reportedly used a dimension of 100 mm × 100 mm × 100 mm. Standardization of block manufacturing becomes necessary due to the numerous physical, structural, and durability tests performed on the produced samples.

The production of these blocks depends heavily on machinery, and as technology has advanced, the block-making process has significantly improved. Old block-making machines such as the CINVA-RAM, which employs a hand-operated toggle lever and piston system to manually compress the moist soil in the mold, are still in use. However, newer upgrades such as the BREPAC and Astram, developed in the UK and India, can exert compacting forces between 16MN/m² and 5MN/m² [71]. These block-making machines can use both static and hydraulic compaction. The Auram Earth Block Press 3000, Test Mark CM-500, Tinius Olsen H50KS, and NANNETI compacting machine were also used by researchers in the analyzed articles. Some researchers also used the standard proctor method for producing compressed blocks, which involved dynamic compaction with a rammer to provide the necessary compaction effort to compress the soil blocks. Earth block manufacturing is carried out by placing the soil mixture into the mold and then in the press chamber [72,73,74]. Force is then applied either manually or mechanically to the soil mix, which compresses it, thereby reducing voids and increasing density [75,76]. The specification of the equipment and manufacturing processes used by the authors are listed in

Table 3. Manufacturing techniques adopted by authors.

Reference(s)	Block Type	Mold Dimension (mm3)	Equipment Type	Methodology	Pressure/Compaction Energy
[1]	Solid	40 × 40 × 160	Hydraulic press	Uniaxial compaction	5 MPa
[3]	Solid	40 × 40 × 160	Hydraulic press	Static compaction	40 MPa
[4]	Solid	19 × 9 × 9	Tamping rod	Dynamic compaction	-
[6]	Solid	240 × 115 × 90	Auram earth block press 3000.	Static compression	-
[7]	Solid	290 × 140 × 100	BREPAC	-	10 MPa
[8]	Hollow	280 × 140 × 110	Terstaram press	Static compression	>2 MPa
[15]	Solid	100 × 100 × 200	Hydraulic press	Static compaction	10 MPa
[17]	Solid	70 × 70 × 280	Hydraulic press	Static compression	6 MPa
[18]	Solid	300 × 140 × 100	Unconfined compression machine	Static compression	2 MPa
[19]	Hollow	30 × 15 × 8	Compaction machine	Manual pressing	0.1471 MPa
[22]	Solid	295 × 140 × 90	Terstaram press	Static compression	3.6 MPa
[23]	Solid	140 × 80 × 105	CINVA-RAM press	Manual pressing	6.58–8.72 MPa
[27]	Solid	100 × 100 × 100	Terstaram press	Hydraulic press	3000 KN
[28]	Solid	294 × 141 × 97	CINVA-RAM	-	4.4 MPa
[32]	Solid	305 × 143 × 105	Block making machine	Single acting rammer	2–3 MPa
[33]	Solid	290 × 140 × 100	BREPAC	Manual pressing	5 MPa
[34]	Cylindrical	40 × 125	Tinius Olsen H50KS	Manual pressing	Compacting rate (mm/min)—0, 5, 10, 15
[35]	Solid	280 × 140 × 100	BREPAC	Manual pressing	5 MPa
[37]	Beam	413 × 102 × 102	Test mark CM-500	Static compression	2224 KN
[38]	Solid, Cylindrical	254 × 127 × 76, 100 × 200, 38 × 76	Tamping rod	Manual compaction	-
[43]	Solid	305 × 143 × 100	Mardini press	Manual pressing	2.5 MPa
[47]	Solid	295 × 140 × 100	Hand-pressed machine	Static compression-	-
[48]	Solid	215 × 105 × 55	CINVA-RAM	Static compression	60 kN
[50]	Solid	200 × 90 × 40	NANNETI compacting machine	Static compaction	15 MPa
[51]	Solid	120 × 120 × 90	CEB’s manual machine	Manual pressing	-
[53]	Solid	305 × 145 × 100	ASTRAM	Toggle level mechanism	-
[55]	Solid	295 × 140 × 95	Terstaram press	-	3.5 MPa
[56]	Solid	140 × 140 × 95	TERSTARAM manual press	Static compaction	3.5 MPa
[59]	Solid	100 × 100 × 100	Standard proctor	IS 2720 Part VII	-
[60]	Solid	100 × 100 × 100	Tamping rod	Standard proctor	-
[61]	Solid	190 × 90 × 40	Tamping rod	Standard proctor	-
[64]	Solid	230 × 110 × 75	Mardini press	Toggle level mechanism	-
[65]	Solid	300 × 150 × 70	Mechanical tapping machine	-	-
[66]	Solid	100 × 100 × 100	Steel rammer with mould	Dynamic compaction	-
[68]	Solid	320 × 80 × 150	CINVA-RAM	-	-
[70]	Beam	1980 × 165 × 400	Mechanized system	Static compression	2–6 MPa

.

4. Binders and Fibers Utilized in CSEB Manufacturing

Numerous studies have utilized various binders and fibers as additives to enhance the mechanical and durability performance of CEBs. Research scholars have observed that these additions considerably enhance the soil’s properties, bringing the properties of CSEBs up to par with their conventional counterparts. This section focuses on the binders and fibers used by authors in the soil mix, including the variety and frequency of these additives.

4.1. Frequency of Use of Binders and Fibers

The (n = 61) articles under this review’s analysis provided a glimpse into the range of binders and fibers that research scholars are experimenting with to enhance the mechanical, physical, and durability qualities of CEBs. A total of 45 binders were identified during this review, wherein some of them have been grouped on the basis of common parent materials such as hydraulic lime, hydrated lime, clay and clay pozzolana; ground blasted furnace slag and granulated blast furnace slag are among the others. Due to its binding properties that form a gel-like structure that binds the soil particles in the soil-cement mixture, cement has been used as a traditional stabilizer for improving soil properties.

Cement was incorporated into the soil mix by 34 articles, demonstrating its superiority over other materials. Ten researchers used lime in this review of n = 61 articles. Lime is a less expensive alternative to cement-admixed soil and has been widely used in the last decade [77]. It has binding properties similar to cement. Other binders used included ground granulated furnace slag, sugarcane bagasse ash, Rice Husk Ash (RHA), saw dust ash and others widely utilized to stabilize soils [78]. However, in the last decade, there has been a shift toward non-traditional binders such as calcium carbide residue, cow dung, and aloe vera mucilage. Because of their binding properties, resins such as bael, wood apple, jack resin, and others have also been undertaken by a few researchers. In these review articles, researchers also experimented with materials such as brick waste, cassava peels, and shea butter waste.

Researchers usually add fibers to improve the flexural and tensile mechanical properties of CEBs. However, many researchers have found that adding these fibers decreases the physical properties of the CSEBs. A total of 19 different fibers were found while conducting the review analysis of the chosen articles, including both natural and synthetic fibers. Some fibers had been grouped together since they belong to the same parent material, such as coconut fiber, coconut husk and polypropylene as a synthetic fiber have been grouped into a single fiber type for ease of understanding and simplicity of measurement in this study. Date palm fiber was preferred the most, with an occurrence of five, followed by synthetic and coconut fibers, with a frequency of five, four and three, respectively. Some researchers also employed wood chips and rubber powder as fiber reinforcement. Other fibers incorporated include Kraftterra fiber, paddy straw, pig hair, and sugarcane bagasse.

While there is a clear demarcation in the utilization of cement as a binder, the same is not reciprocated in the context of fibers. This is due to the clear distinction in the improvement of properties such as strength and durability upon the inclusion of cement as a binder across varied soil compositions [15,17,20], whereas the inclusion of binders may affect certain properties positively [1,41] or can have adverse impacts as well [17,51].

Table 4. Different binders and fibers identified in the relevant articles.

Reference(s)	Binder	Fiber
[1]	-	Kenaf
[3]	Cement, Shea Butter Waste	-
[4]	Cement, Saw Dust Ash	-
[5]	Cement	-
[6]	Cement, Sand	-
[7]	-	Coconut Husk, Sugarcane Bagasse, Oil Palm Fruit Fiber
[8]	Alkaline-Activated Fly Ash	-
[9]	Cement, Fly Ash, Glass Powder, Recycled Alkaline Cleaning Solution	-
[15]	Cement	Date Palm
[16]	Powdered Green Mussel Shells	Pig Hair
[17]	Cement, Brick Waste	Sisal
[18]	Cement	White Synthetic Fiber (WF), Black Synthetic Fiber (BF), Date Palm
[19]	Cement, Lime, Sodium silicate	-
[21]	Lime	Date Palm
[22]	Cement, Lime	-
[23]	Lime, Aloe Vera Mucilage	Coconut
[26]	Cement, Clay Modifier (Synthetic Termite Saliva)	Date Palm, Wood Chips
[27]	Cement	-
[28]	Hydraulic Lime	-
[29]	Fly Ash, Ground Blasted Furnace Slag	-
[30]	Cement	-
[31]	Cement	Kraftterra
[32]	Cement, Granulated Blast furnace Slag (GBFS)	-
[33]	Clay Pozzolana	-
[34]	-	-
[35]	Pidiproof LW	-
[36]	Cement	Polypropylene
[37]	Cement	Polypropylene
[38]	Cement, Fly Ash	-
[39]	Cement, Fly Ash	-
[40]	Alginate, Lignin	Wool
[41]	Cement, Lime	Date Palm
[42]	Cement, Fly Ash	-
[43]	Cement, Natural Rubber Latex	-
[44]	Crushed Brick Waste	-
[45]	Clay, Blast Furnace Slag, Glass Powder Activated with NaOH	-
[46]	Limestone, Sandstone, Porphyry	-
[47]	Cement	Abaca and Maguey
[48]	Cement	Steel
[49]	Cow Dung Powder	-
[50]	Phosphate	-
[51]	Cement	Banana
[52]	Bottom Biomass Ash	-
[53]	Cement, Lime	-
[54]	Calcium Carbide Residue, RHA	-
[55]	Cement, Calcium Carbide Residue, RHA	-
[56]	Metakaolin, Sodium Hydroxide solution	-
[57]	RHA, Eggshell Powder, Caustic Soda	-
[58]	Cement, Microbially Induced calcium Carbonate Precipitation (MICP)	Crumb Rubber
[59]	Cement	Coconut Coir
[60]	Cement	Coconut
[61]	Marble Dust, Sugarcane Bagasse Ash	Paddy Straw
[62]	Cement, Sugarcane Bagasse Ash	Wheat Straw
[63]	Cement, Cassava Wastewater	-
[64]	Cement, Lime, Terrazyme	-
[65]	Hydraulic Lime, Hydrated Lime	-
[66]	Cement	-
[67]	Dawal Kurudu, Pines Gum, Sugarcane Bagasse, Bael Resin, Jack Resin, Agarwood Resin, Wood Apple Resin	-
[68]	Coal Ash, Cassava Peels	-
[69]	RHA, Resin Adhesive	-
[70]	-	Jute

lists the various types of binders and fibers uncovered throughout the literature, whereas

Table 5. Frequency of utilization of binders and fibers as reported in the relevant articles.

Type of Binder	No of Articles	Type of Binder	No of Articles	Type of Fiber	No of Articles
Cement	34	Pines Gum	1	Date Palm	5
Lime	10	Porphyry	1	Synthetic	4
Fly Ash	6	Sandstone	1	Coconut	3
RHA	4	Resin Adhesive	1	Crumb Rubber	1
Sugarcane Bagasse Ash	3	Sand	1	Oil Palm Fruit	1
GBFS	3	Metakaolin	1	Kenaf	1
Calcium Carbide Residue	2	Saw Dust Ash	1	Maguey	1
Brick Waste	2	Shea Butter Waste	1	Abaca	1
Clay	2	Sodium Silicate	1	Pig Hair	1
Natural Rubber Latex	1	Terrazyme	1	Jute	1
Bael Resin	1	Wood Apple Resin	1	Wool	1
Cassava Peels	1	Alginate	1	Banana	1
Cassava Wastewater	1	Bottom Biomass Ash	1	Steel	1
Caustic Soda	1	Agarwood Resin	1	Wheat Straw	1
Phosphate	1	Coal Ash	1	Sugarcane Bagasse	1
Eggshell Powder	1	Cow Dung Powder	1	Sisal	1
Glass Powder	1	Dawal Kurudu	1	Kraftterra	1
Jack Resin	1	Limestone	1	Paddy Straw	1
Lignin	1	Pidiproof LW	1	Wood Chips	1
Microbially Induced Calcium Carbonate Precipitation (MICP)	1	Recycled Alkaline Cleaning Solution	1
Clay Modifier (Synthetic Termite Saliva)	1	Sodium Hydroxide Solution	1
Powdered Green Mussel Shells	1	Marble Dust	1
Aloe Vera Mucilage	1

depicts the frequency with which binders and fibers were utilized in the relevant publications.

4.2. Types of Tests Conducted

The different features of the CSEBs studied during the various tests conducted by the researchers are analyzed and presented in this segment. In addition to the physical and mechanical aspects of the CSEB, this segment also assesses their microstructural and durability characteristics. Based on the data collected from the analyzed research papers (n = 61), it was found that a total of 246 separate experiments were conducted. The authors discovered that the a major portion (94 times) of these tests were based on the mechanical properties of the CSEB, including tests such as compressive strength, flexural strength, splitting tensile strength, and the triaxial test. This was followed by tests on the physical properties of the CSEB (86 times), which included testing the bulk density, porosity, thermal conductivity, water absorption, and linear shrinkage characteristics. Durability and microstructural analysis tests were conducted less frequently—26 and 30 times, respectively. Durability tests on earth blocks included efflorescence, erosion, cyclic wetting drying, abrasion resistance, and acid resistance [79]. Fourier-Transform Infrared Spectroscopy, Energy Dispersive Spectroscopy, X-ray Diffraction, and Scanning Electron Microscopy were among the microstructural tests [80]. Other tests performed on the soil blocks included ultrasonic testing, dynamic young modulus, life cycle analysis, electrical resistivity, and cost analysis [81,82]. It is obvious from the analysis that the compressive strength of the CSEB was the chief focus of the majority of the researchers, as 53 of the (n = 61) selected articles incorporated this test, followed by the water absorption test, which was performed 32 times. Other mechanical tests, such as flexural and splitting tensile strength, were investigated infrequently, with 27 and 11 instances, respectively. Necessary tests for identifying brick quality, such as efflorescence and linear shrinkage, were conducted in only three and four articles, respectively. Furthermore, the manufactured specimen’s cost-effectiveness was only briefly analyzed, despite being an essential part in determining its advantage over its traditional counterparts. The outcomes in Table 6 pertain to the bulk density of CSEB findings conducted by researchers as documented in scholarly publications. Table 7 and Table 8 list all the tests identified during the literature study.

5. Physical Characteristics of CSEBs

This segment critically examines the numerous physical characteristics that were the focus of investigation by researchers in the publications selected for this review. The following discussion covers characteristics such as bulk density, porosity, water absorption, linear shrinkage, and thermal conductivity of CSEBs.

5.1. Bulk Density

Bulk density refers to the ratio of a soil block’s mass to its volume. Several authors have used a wide range of binders and fibers, both alone and in combination, to investigate the bulk density of a CSEB. In the literature studied, twelve binders and one fiber (out of forty-five and nineteen binders and fibers, respectively) were used for the bulk density testing of soil blocks. Table 6 summarizes the bulk density of the reviewed literature. From the relevant literature studied, it was observed that traditional binders such as cement and fly ash, when used with the soil matrix to stabilize soil blocks, displayed enhancement in the values of bulk density [39,57,60]. The use of other binders, such as bottom biomass ash, resin adhesive, eggshell powder with caustic soda, dawal kurudu, pines gum, and sugarcane Bagasse, exhibited a decrement in the bulk density property with the rise in the concentration of the additions [52,67]. Reference [21] suggested that the reduction in density was caused by a combination of factors, including a less homogeneous soil matrix and an increase in voids in the mix. Reference [67] utilized natural polymers (Dawal Kurudu, Pines Gum, Sugarcane Bagasse) to stabilize CEBs and found that the density improved significantly in tandem with the rise in the proportion of gravel in the soil mix. Reference [28] employed two types of hydraulic lime to improve the characteristics of a CEB and discovered that as lime content increased, bulk density declined, owing to the partial replacement of soil by lime, along with the flocculation in the clayey soil elements due to Van der Waal’s force of attraction. Similar assessments were made by [52]. The authors also mention that the density slowly began to increase over the lime fixation point. Reference [69] also came to similar conclusions. They found that when clay soil was partially replaced with RHA and resin adhesive as an additive to the soil matrix, the density of the CSEB decreased by 1.43%. In the reviewed literature, fiber utilization was limited to a single publication, in which cement and coconut fiber were utilized to stabilize the CEB. The reduction in CSEB density was attributed to the resistance offered by the fibers during compacting [60]. Nonetheless, the maximum value of the CSEB’s bulk density exhibited a negligible reduction from the control or the initial values of bulk density. This has been mainly attributed to the partial substitution of soil by these binders, which have relatively lower density than soil, in comparison to other binders such as RHA, bottom biomass ash and calcium carbide residue (CCR) [28,52,55,57]. Therefore, the results were not consistent across all binders.

Table 6. Bulk density of CSEB undertaken by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Initially Stabilized/Control Sample Bulk Density (kg/m3)	Post-Stabilization Maximum Bulk Density (kg/m3)	Increment (+)/Decrement (−)(%)
[28]	Hydraulic Lime	3–12	UNE EN 772-13	1785	1675	−6.12
[39]	Cement	4–10	Modified Proctor	1684	2016	19.71
	Fly Ash	0–30
[52]	Bottom Biomass Ash	0–20	ASTM D7263	1790	1740	−2.79
[55]	8% Cement + CCR	0–25	-	1800	1721	−4.39
	8% Cement + CCR and RHA	20:0–12:8		1522	1578	3.67
[57]	Cement	10–20	ASTM C140	1706	1864	9.26
	Eggshell Powder: RHA	10–40: 90–60			1634	−4.22
[60]	Cement	0–10	Standard Proctor	1730	1930	10.36
	Coconut Fiber	0–1		1720	1700	−1.16
[67]	Dawal Kurudu	5–15	-	1854	1841	−0.71
	Pines Gum		-	2052	2005	−2.29
	Sugarcane Bagasse		-	1819	1813	−0.33
[69]	Cement: Clay + RHA + Resin Adhesive	1:15 + 0–20 + 20 mL/kg cement	-	1812	1786	−1.43

Table 6. Bulk density of CSEB undertaken by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Initially Stabilized/Control Sample Bulk Density (kg/m3)	Post-Stabilization Maximum Bulk Density (kg/m3)	Increment (+)/Decrement (−)(%)
[28]	Hydraulic Lime	3–12	UNE EN 772-13	1785	1675	−6.12
[39]	Cement	4–10	Modified Proctor	1684	2016	19.71
	Fly Ash	0–30
[52]	Bottom Biomass Ash	0–20	ASTM D7263	1790	1740	−2.79
[55]	8% Cement + CCR	0–25	-	1800	1721	−4.39
	8% Cement + CCR and RHA	20:0–12:8		1522	1578	3.67
[57]	Cement	10–20	ASTM C140	1706	1864	9.26
	Eggshell Powder: RHA	10–40: 90–60			1634	−4.22
[60]	Cement	0–10	Standard Proctor	1730	1930	10.36
	Coconut Fiber	0–1		1720	1700	−1.16
[67]	Dawal Kurudu	5–15	-	1854	1841	−0.71
	Pines Gum		-	2052	2005	−2.29
	Sugarcane Bagasse		-	1819	1813	−0.33
[69]	Cement: Clay + RHA + Resin Adhesive	1:15 + 0–20 + 20 mL/kg cement	-	1812	1786	−1.43

Table 7. Physical and mechanical tests undertaken by researchers as reported in relevant publications.

Reference(s)	Physical Properties					Mechanical Properties
	Bulk Density	Porosity	Thermal Conductivity	Water Absorption	Linear Shrinkage	Compressive Strength	Flexural Strength	Splitting Tensile Strength	Triaxial Test
[1]		●	●			●	●
[3]		●		●		●	●
[4]	●	●		●		●	●
[5]						●
[6]	●			●		●
[8]						●	●
[9]				●		●
[15]				●		●		●
[16]						●	●
[17]	●					●		●
[18]						●
[19]	●			●		●	●
[21]	●		●			●	●
[22]	●	●	●	●		●	●	●
[23]		●	●	●	●	●			●
[26]	●			●		●
[27]						●
[28]	●					●	●
[29]						●
[31]			●
[32]				●		●
[33]	●			●		●		●
[34]	●					●		●
[35]	●			●		●		●
[36]						●	●
[37]							●
[38]	●	●				●	●	●	●
[39]	●	●		●		●			●
[40]						●	●
[41]	●	●		●		●	●
[42]	●	●		●		●
[43]	●		●	●		●	●
[44]	●			●		●	●
[45]	●			●		●
[46]						●
[47]						●	●
[48]						●	●	●
[49]				●	●	●	●
[50]	●			●		●
[51]				●		●	●
[52]	●	●	●			●	●
[53]				●
[54]						●
[55]				●			●
[56]	●	●	●			●	●
[57]	●					●	●
[58]			●	●		●
[59]	●			●		●		●
[60]	●					●		●
[61]	●
[62]				●		●
[63]	●			●		●
[64]				●			●
[65]			●	●		●
[66]	●					●
[67]	●			●	●	●		●
[68]				●		●	●
[69]	●			●		●
[70]					●	●	●
Count	29	11	10	32	4	53	27	11	3

Table 7. Physical and mechanical tests undertaken by researchers as reported in relevant publications.

Reference(s)	Physical Properties					Mechanical Properties
	Bulk Density	Porosity	Thermal Conductivity	Water Absorption	Linear Shrinkage	Compressive Strength	Flexural Strength	Splitting Tensile Strength	Triaxial Test
[1]		●	●			●	●
[3]		●		●		●	●
[4]	●	●		●		●	●
[5]						●
[6]	●			●		●
[8]						●	●
[9]				●		●
[15]				●		●		●
[16]						●	●
[17]	●					●		●
[18]						●
[19]	●			●		●	●
[21]	●		●			●	●
[22]	●	●	●	●		●	●	●
[23]		●	●	●	●	●			●
[26]	●			●		●
[27]						●
[28]	●					●	●
[29]						●
[31]			●
[32]				●		●
[33]	●			●		●		●
[34]	●					●		●
[35]	●			●		●		●
[36]						●	●
[37]							●
[38]	●	●				●	●	●	●
[39]	●	●		●		●			●
[40]						●	●
[41]	●	●		●		●	●
[42]	●	●		●		●
[43]	●		●	●		●	●
[44]	●			●		●	●
[45]	●			●		●
[46]						●
[47]						●	●
[48]						●	●	●
[49]				●	●	●	●
[50]	●			●		●
[51]				●		●	●
[52]	●	●	●			●	●
[53]				●
[54]						●
[55]				●			●
[56]	●	●	●			●	●
[57]	●					●	●
[58]			●	●		●
[59]	●			●		●		●
[60]	●					●		●
[61]	●
[62]				●		●
[63]	●			●		●
[64]				●			●
[65]			●	●		●
[66]	●					●
[67]	●			●	●	●		●
[68]				●		●	●
[69]	●			●		●
[70]					●	●	●
Count	29	11	10	32	4	53	27	11	3

Table 8. Durability tests and microstructural analysis undertaken by researchers as reported in relevant publications.

Reference(s)	Durability Properties					Microstructural Evaluation
	Efflorescence	Erosion Test	Cyclic Wetting Drying Test	Abrasion Resistance Test	Acid Resistance	Scanning Electron MICROSCOPY	X-ray Diffraction	Fourier Transform Infra-Red Spectroscopy	Energy Dispersive Spectroscopy
[1]						●
[3]						●			●
[4]	●
[6]						●
[7]		●
[9]		●				●		●
[16]		●
[19]	●	●
[22]		●
[23]				●
[27]		●	●
[29]						●		●
[30]					●	●	●
[33]		●
[34]		●
[38]	●		●
[39]			●			●
[42]						●
[43]			●			●
[44]			●			●	●
[45]						●
[46]				●
[50]						●	●	●
[52]		●				●	●		●
[54]							●
[55]						●
[58]		●
[59]						●	●
[63]			●			●	●
[64]			●			●
[65]		●
[67]						●
[69]		●			●
Count	3	12	7	2	2	18	7	3	2

Table 8. Durability tests and microstructural analysis undertaken by researchers as reported in relevant publications.

Reference(s)	Durability Properties					Microstructural Evaluation
	Efflorescence	Erosion Test	Cyclic Wetting Drying Test	Abrasion Resistance Test	Acid Resistance	Scanning Electron MICROSCOPY	X-ray Diffraction	Fourier Transform Infra-Red Spectroscopy	Energy Dispersive Spectroscopy
[1]						●
[3]						●			●
[4]	●
[6]						●
[7]		●
[9]		●				●		●
[16]		●
[19]	●	●
[22]		●
[23]				●
[27]		●	●
[29]						●		●
[30]					●	●	●
[33]		●
[34]		●
[38]	●		●
[39]			●			●
[42]						●
[43]			●			●
[44]			●			●	●
[45]						●
[46]				●
[50]						●	●	●
[52]		●				●	●		●
[54]							●
[55]						●
[58]		●
[59]						●	●
[63]			●			●	●
[64]			●			●
[65]		●
[67]						●
[69]		●			●
Count	3	12	7	2	2	18	7	3	2

5.2. Water Absorption

The absorption value indicates the total capacity of the unit to absorb moisture. The water absorption test is an essential durability indicator, as samples that absorb a significant amount of moisture tend to degrade rapidly [83,84]. The standard procedure for determining water absorption is almost consistent across all standard codes. The dry samples are weighed first before being submerged in water for 24–48 h at room temperature. After the sample is retrieved from the water, the mass of the wet specimens is determined and weighed against the mass of the dry specimens. ASTM C830, IS 3495 Part 2, and LNEC E394, among others, have been frequently used to carry out testing based on the location of the study. The standard codes recommend a maximum water absorption value of 20%, as found in the reviewed articles. However, not all standards recommend the same limits for water absorption values.

Table 9. Water absorption of CSEB undertaken by the researchers reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Initially Stabilized/Control Sample Water Absorption (%)	Post-Stabilization Minimum Water Absorption (%)	Increment (+)/Decrement (−) (%)
[4]	Cement	4–10	ASTM C830	11	6.89	−37.36
	Saw Dust Ash	0–10
[6]	Cement	4–9	ASTM C830	18.5	7.9	−57.29
	Sand	20–70
[9]	Precursor (Fly Ash + Glass Powder)	50–50	UNE 41410	16.49	15.92	−3.45
	Activator (Alkaline Activated Cement)	50–50
	Precursor/Activator	0.5–0.75
[22]	Cement	8	LNEC E394	Disintegrated	13.60%	NA
	Cement + Lime	4 + 4
[23]	10% Lime + 1 % Aloe Vera Mucilage + Coconut Fiber	0–0.5	NMX-C-37-ONNCE-205	33.1	36.75	11.027
[32]	20% GBFS + Cement	2–8	-	Disintegrated	10.9	NA
[33]	Clay Pozzolana	0–30	BS EN 772-11	16	4.6	−71.25
[39]	Cement	4–10	ASTM C830	17.92	7.94	−55.69
	Fly Ash	0–30
[41]	Cement	0–10	BS 3921	21.39	11.88	−44.46
	Lime	0–10
	Date Palm Fiber	0–0.5
[42]	Cement	5–10	ASTM C830	18.7	10.2	−45.45
	Fly Ash	5–25
[43]	Cement	8	IS 3495 Part 2	19.2	18	−6.25
	Natural Rubber Latex	1–5
[44]	Crushed Brick Waste	0–24	IS 3495 part II	8.41	9.06	7.72
[45]	10% Clay + 10% Blast Furnace Slag + 30 gm Glass Powder In 100 mL NaOH Solution	2–8	-	21.27	18.25	−14.19
	10% Clay + 10% Blast Furnace Slag + 40 gm Glass Powder In 100 mL NaOH Solution	2–8		21.42	19.35	−9.66
[49]	Cow-Dung Powder	0–3	NF P13-305	Disintegrated	5.64	NA
[50]	Phosphate	0–20	ASTM C642-06	9.83	9.36	−4.78
[51]	Cement	7	-	7.4	10.6	43.24
	Banana Fiber	0–0.35
[53]	Cement	4–8	IS: 1725	14	7	−50
	Lime	0–4
[58]	6% Cement + MICP	0–25 mL sprayed	HB 195	Disintegrated	5.33	NA
	Rubber Crumb	5–20
[59]	Cement	10	HB 195	Disintegrated	10.35	NA
	Coconut Coir	0–5
[62]	5% Cement + Sugarcane Bagasse Ash	4–10	-	21.9	11.8	−46.11
	Wheat Straws	4–10
[63]	Cement	6,12	NBR 8492	15.42	11.88	−22.95
	Cassava Wastewater	10.01
[64]	Cement	2–8	IS 3495 Part 2	12.5	9.5	−24
	Lime	2–6
	Terrazyme	0.05 mL/kg
[65]	Hydraulic Lime	6	LNEC E394	8.7	7.7	−11.49
	Hydrated Lime	1
[67]	Dawal Kurudu	5–15	BSEN-772-1	15.32	9.5	−37.98
	Pines Gum
	Sugarcane Bagasse
[68]	Coal Ash	0–10	ASTM C67-11	28.42	26.65	−6.22
	Cassava Peels	0–5
[69]	Cement: Clay + RHA + Resin Adhesive	1:15 + (0–20) + (20 mL/kg of cement)	SLS 1382	17.2	15.6	−9.30

concludes the effects of the various fibers and binders studied in this study.

Most researchers found that incorporating binders, fibers, or both produced satisfactory results. Cement stabilization effectively reduced water absorption in the soil blocks, as found in a study by [58], which is congruous with the findings of multiple authors [41,63,68]. Additionally, many unstabilized samples disintegrated during testing, which is consistent with earlier findings and has been attributed to the unstabilized block’s weak cohesion [22,32,49,58,59]. During the review of the pertinent articles, researchers discovered that they had used binders, including fly ash, glass powder, lime, coal ash, and cow dung powder, among others. It was observed that the use of all of these binders positively impacted the water absorption characteristics of the CSEBs and effectively subdued the block’s maximum water absorption capacity [85,86]. The results were more pronounced when the cement and fly ash were used as binders to stabilize the earth blocks. Reference [42] incorporated cement and fly ash into the soil mix and were able to reduce the water absorption from 17.92% to 7.94%, a reduction of 55.69%. The authors also observed that as cement content increased, water absorption capacity decreased for any amount of FA [87]. At the same time, water absorption decreased in all cases as FA content increased for specific cement content. Reference [33] observed a gradual decrease in water absorption as the block specimen’s pozzolana content rose. The water absorption values varied from a maximum of 16% to a minimum of 4.6%, a decrease of approximately 75% compared to the control sample. Blocks that had been stabilized with pozzolana were less porous and thus absorbed less water than control samples, owing to their fineness and ability to fill voids. Similar findings have also been reported by multiple researchers [6,39,42].

When soil blocks were stabilized with fibers, mixed results were observed. According to [51], employing banana fibers to reinforce soil blocks caused an increment in water absorption values from 7.4% to 10.6% when compared to control soil blocks, negatively affecting the block’s durability. Similar findings were made by [23], who discovered that the incorporation of coconut fiber increased the porosity of the CEBs, increasing their water absorption capacity, with no detrimental effects on the mechanical properties. The high capillarity, porous structure, and hydrophilic nature of the fibers are responsible for their increased water absorption, as determined by the authors [23,41,51]. Similarly, it has been reported that the rise in the proportion of admixed fibers (cassava peels, date palm fiber and others) in the CEBs was accompanied by increased water absorption values in the CSEBs [41,62,63,68]. Thus, when using fibers to stabilize soil blocks, two main parameters—fiber concentration and fiber length—need to be adjusted [23].

5.3. Thermal Conductivity

Urbanization and population growth increase energy demand, making it important to consider a material’s thermal conductivity, which is its capacity to transfer heat through conduction. A large amount of energy is consumed worldwide to maintain pleasant indoor temperatures. Therefore, building materials must be assessed for their thermal conductivity in order to regulate and adjust energy consumption sustainably. In tropical regions such as India, it is advised that the thermal conductivity of masonry construction be kept to a minimum in order to maximize energy savings and eventual economic benefits. During the analysis of the relevant literature, we found that different instruments were used to measure thermal conductivity. These instruments included a commercial CT meter, a TPS1500, a KD2pro thermal characteristics meter with an SH-1 sensor, and others. Each of these devices uses a different set of approaches to measure the thermal conductivity, with the TPS 1500 using a hot disk transient plane source in accordance with ISO 22007-2, and the CT-meter using a transient hot wire following ISO 8894-1:1987 standards. The thermal evaluation of the soil blocks is depicted in

Table 10. Thermal conductivity of CSEB undertaken by the researchers reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Equipment	Method	Testing Standard	Initially Stabilized/Control Sample Thermal Conductivity (W/m K)	Post-Stabilization Minimum Thermal Conductivity (W/m K)	Increment (+)/Decrement (−) (%)
[1]	1.2% Kenaf Fibers	Fiber Length 10–40 Mm	-	Hot Film Method	-	2	0.95	−52.5
[21]	10% Lime +Untreated Date Palm Fiber	0–0.2	CT-Meter Device	Transient Hot Wire Method	ISO 8894-1	0.824	0.761	−7.6456
	10% Lime +Alkali Treated Date Palm Fiber					0.837	0.805	−3.8232
[22]	Cement,	8	ISOMET 2114	Dynamic Transient Pulse Method	-	0.58	0.65	12.069
	Cement + Lime	4 + 4				0.58	0.61	5.17241
[23]	10% Lime + 1% Aloe Vera Mucilage + Coconut Fiber	0–0.5	KD2pro (Decagon Devices)	Linear Source of Transient Heat	ASTM D5334-14	0.41775	0.35975	−13.884
[43]	Cement	8	TPS 500 S	Hot Disk Method	-	1	0.96	−4
	Natural Rubber Latex	1–5
[52]	Bottom Biomass Ash	0–20	TPS1500	Hot Disk Transient Plane Source Method	ISO 22007-2	0.86	0.78	−9.3023
[56]	Metakaolin and Sodium Hydroxide Solution	5–20	Despro Therm	Asymmetric Hot Plane Method	-	0.6	0.71	18.3333

by various equipment, their working methods, and the applicable standards. Only seven of the sixty-one pertinent articles evaluated the thermal conductivity of the soil blocks. The researchers utilize binders such as lime, cement bottom biomass ash, and aloe vera mucilage, among others. At the same time, only coconut, kenaf, and date palm fibers—alkali-treated and untreated—were utilized.

According to [22], the composition (additives + soil) of CEB had a minimal impact on its thermal conductance. Control/unreinforced/unstabilized samples outperformed stabilized blocks when lime and cement were utilized as binders to stabilize the soil block, independent of the moisture content used for manufacturing the CEB. The authors add that the thermal conductance of a CEB is significantly influenced by its density and moisture content, which is congruous with the assessments of multiple authors [21,52,56]. In contrast, most of the studies incorporating binders and fibers into the soil matrix showed positive results. The thermal conductivity of the CEB declined with increasing fiber content, according to [21], who employed cement and date palm fiber treated with and without alkali solution. this is attributed to the thermal conductance of the fiber being lower than that of the soil matrix. Fibers in the matrix generate voids, increasing the soil block’s porosity and lowering its density simultaneously [52,56]. The minor decrease in thermal conductivity suggested that, when compared to other parameters such as compaction pressure and porosity, introducing fibers in the soil mix had a minimal effect on the CEBs’ thermal conductivity. The thermal conductivity of an earth block reinforced with date palm fiber turned out to be lower than that of the unstabilized sample; however, the authors concluded that the fiber alkali treatment had a detrimental impact. Similar research was conducted by [1], in which the author used Kenaf fibers of varying lengths to stabilize soil blocks and discovered that the thermal conductivity decreases as fiber length increases. Similar reductions in thermal conductivity following the incorporation of fiber into the soil mix have been discovered in the relevant literature, as reported here. It can thus be stated that the inclusion of cement and lime as a binder individually in the CSEBs negatively impact their thermal conductively, i.e., they increase the thermal conductance of the blocks, but this is also associated with an increase in the mechanical properties of the blocks. Authors utilizing fibers in conjunction with these traditional binders have increased the mechanical and decreased the thermal conductivity of the soil blocks [21]. A similar observation was made for other binders as well (bottom bio ash, natural rubber latex, aloe-vera mucilage) [23,43,52].

6. Mechanical Properties of CSEBs

In this section, the compressive strength, flexural strength, and splitting tensile strength tests for CSEBs have been analyzed to assess their mechanical characteristics. The various binders and fibers used, the testing standards followed, and the increase or decrease in the results from the initial or un-stabilized mix are discussed in this section.

6.1. Compressive Strength

Compressive strength is an essential metric for any masonry unit since it is a gauge of a unit’s strength and it indicates density and porosity [15,28,39,52]. A review of the pertinent articles revealed that this test had been performed most frequently (53 out of 61 articles). Multiple standard codes were used, including ASTM D5102, XP P13-901, IS 4332 Part 5, and others, depending on the location of the study.

Table 11. Compressive strength of CSEB investigated by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/ Fiber Proportion (%)	Testing Standard	Initially Stabilized/Control Sample Strength (MPa)	Post-Stabilization Maximum Strength (MPa)	Increment (+)/Decrement (−) (%)
[1]	1.2% Kenaf Fibers	Fiber length 10–40 mm	EN 196-1	4.23	6.27	48.22
[3]	5% Cement + Shea Butter Waste	0–10	EN 196-1	2.64	3.16	19.69
[4]	Cement	4–10	ASTM D5102	0.758	2.001	163.98
	Saw Dust Ash	0–10
[6]	Cement	4–9	ASTM D5102	1.37	5.62	310.21
	Sand	20–70
[8]	Alkaline Activated Fly Ash	10,15	EN 772-1	0.34	3.08	805.88
[15]	Cement	5–8	XP P13-901	7	12.75	82.14
	Date Palm Fibers	0.05–0.20
[16]	Powdered Green Mussels Shells	0–10	NMAC	2.36	4.16	76.21
	Pig Hair Fiber	0–1
[17]	7% Cement + 20% Brick Waste + Sisal Fibers	0–0.5	XP P13-901	6.98	8.1	16.04
[21]	10% Lime +Untreated Date Palm Fiber	0–0.2	XP P13-901	7.9	10.2	29.11
	10% Lime +Alkali Treated Date Palm Fiber			7.9	10.6	34.17
[28]	Hydraulic Lime	3–12	EN 1015-11	3.36	4.56	35.71
[29]	Fly Ash	10–40	NF EN 196-1	3.52	8.94	153.97
	10% GBFS + Fly Ash	9–36		5.57	16.53	196.76
[33]	Clay Pozzolana	0–30	BS EN 772-1	1.73	3.75	116.76
[35]	Pidiproof LW+	0.5–1.5	BS EN 772-1	0.99	2.8	182.82
[38]	Cement	0–10	ASTM D5102	0.56	7.1	1167.85
	Fly Ash	0–30
[39]	Cement	4–10	ASTM D 5102	2.25	6	166.66
	Fly Ash	0–30
[41]	Cement	0–10	NF EN 196-1	0.78	1.93	147.43
	Lime	0–10
	Date Palm Fiber	0–0.5
[42]	Cement	5–10	ASTM D5102	2.82	8.1	187.23
	Fly Ash	5–25
[43]	Cement	8	IS 3495 Part1	3.48	4.37	25.57
	Natural Rubber Latex	1–5
[45]	10% Clay + 10% Blast Furnace Slag + 30 gm Glass Powder in 100ml NaOH Solution	2–8	-	3.59	5.95	65.73
	10% Clay + 10% Blast Furnace Slag + 40 gm Glass Powder in 100ml NaOH Solution	2–8		3.44	6.63	92.73
[48]	Cement	6	ASTM D 2166-00e1	0.89	11.6	1203.37
	Steel Fibers	1–2.7
[49]	Cow-Dung Powder	0–3	NF P14-306	2.11	2.81	33.17
[51]	Cement	7	ASTM C67-07	3.39	6.3	85.84
	Banana Fiber	0–0.35
[52]	Bottom Biomass Ash	0–20	ASTM C67	3.42	3.9	14.03
[54]	Calcium Carbide Residue (CCR) and RHA	9:1–6:4	XP P13-901	1.9	6.6	247.36
[55]	8% Cement + CCR	0–25	XP P13-901	1.1	4.6	318.18
	8% Cement + CCR -RHA	20:0–12:8		1.1	7	536.36
[56]	Metakaolin and Sodium Hydroxide Solution	5–20	XP P13-901	1.36	8.9	554.41
[57]	RHA, Eggshell Powder, Caustic Soda	10–20	ASTM C109	0.48	1.85	285.41
	Eggshell Powder: RHA	10–40:90–60
[59]	Cement	10	IS 4332 Part 5	3.82	7.63	99.73
	Coconut Coir	0–5
[63]	Cement	6,12	NBR 8492	1.93	8.89	360.62
	Cassava Wastewater	10.01
[65]	Hydraulic Lime + Hydrated Lime	6 + 1	NP EN 772-1	11	9.7	−11.81
[66]	Cement	0–10	IS 4332 Part 5	1.1	9.73	784.54
[67]	Dawal Kurudu	5–15	BS EN 772-1	0.28	0.41	46.42
	Pines Gum			2.22	2.93	31.98
	Sugarcane Bagasse			0.46	0.5	8.69
[68]	Coal Ash	0–10	ASTM D 2166-00e1	2.03	3.37	66.00
	Cassava Peels	0–5
[69]	Cement: Clay + RHA + Resin Adhesive	1:15 + 0–20 + 20 mL/kg cement	SLS 1382	3.25	4	23.07

displays the various binders and fibers utilized individually and in combination and the increase or decrease in compressive strength values.

Nearly all researchers have reported growth in the compressive strength values of the CEBs, which was discovered during a review of the pertinent articles. The authors of [4] observed a 164% increase in the values of compressive strength when soil was admixed with cement and saw dust ash. The authors of [8] utilized alkaline-activated fly ash as a binder and found a substantial increase in the compressive strength values. Similar results were reported by [38,42,54,57]. Reference [29] used fly ash with ground blasted furnace slag to stabilize soil blocks and found that the 28-day compressive strength was significantly greater than that of the preceding studies. The CSEB displayed a compressive strength of 16.53 MPa. This is linked to the formation of hydrated calcium alumina silicate coexisting with geopolymer gels to form a vast, three-dimensional array of aluminosilicate [29].

As reported by multiple researchers, the inclusion of fibers slightly increased the compressive strength [15,17,41,51]. According to the findings of researchers, this is because the fibers enhance friction in the soil matrix by bearing a portion of the applied load. As a result, this raises the contact forces amongst soil particles, thereby improving the compressive strength of the soil. However, the inclusion of fibers beyond a specific limit or the optimum value has detrimental effects on compressive strength [15,17,51].

6.2. Flexural Strength

The ability of a material to withstand stress when bent without permanently deforming is referred to as its flexural strength. During an assessment of the relevant literature, it was discovered that 27 researchers had conducted flexural analyses on soil blocks. Most of these publications had fibers present in the soil matrix, whereas a handful performed flexural testing on soil blocks in the absence of fibers. The flexural strength test of the reinforced soil blocks was performed using a variety of testing standards depending on the accessibility of the testing equipment and the location of the study. The literature study revealed that the incorporation of binders and fibers positively impacted the flexural characteristics of the CEBs.

All of the articles reviewed reported growth in the soil block’s flexural strength. The authors of [57] used RHA and egg shell powder mixed with 10% cement to stabilize the soil blocks. The soil blocks showed an increase in flexural strength with an increasing binder proportion up to a certain percentage, and then, a decline in the flexural strength was detected. The drop in tensile strength noticed due to the replacement of RHA by eggshell powder in the CSEB could be related to the eggshell powder’s poorer binding capacity compared to RHA, which can be explained as a consequence of insufficient adhesion between the soil and the RHA + eggshell powder mix [57]. Similar results of the decline in flexural strength after a certain percentage of binder, fiber or both has been reported by multiple authors [16,21,36,40,41]. However, some authors have also reported increasing flexural strength with an increase in the binder or fiber content [49,70].

The incorporation of fibers in the soil blocks drastically improved the deformation characteristics as well as the load–deflection characteristics of the unreinforced specimens [88]. Authors have reported instant brittle failure of the unreinforced soil blocks when tested for flexural strength [37,48,50,51]. However, the fiber-reinforced blocks exhibited ductile behavior post cracking [36,37,48]. This ductile behavior is credited to the fibers that bridge the crack and hold cracked regions together [36,37,48,51,70]. The literature studied for this review revealed a total of fifteen binders and nine fibers used to carry out the flexural strength test, as shown in

Table 12. Flexural strength of CSEB investigated by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Initially Stabilized/Control Sample Strength (MPa)	Post-Stabilization Maximum Strength (MPa)	Increment (+)/Decrement (−) (%)
[3]	5% Cement + Shea Butter Waste	0–10	EN 196-1	1.83	2.18	19.13
[8]	Alkaline Activated Fly Ash	10,15	EN 772-6	1.8	2.3	27.78
[16]	Powdered Green Mussels Shells	0–10	NMAC	0.106	1.053	893.39
	Pig Hair Fiber	0–1
[21]	Lime	8–12	XP P 13-901	1.1	1.25	13.64
	Date Palm Fiber	0–0.2
[22]	Cement,	8	EN 772-6	0.25	1.19	376
	Cement + Lime	4 + 4
[28]	Hydraulic Lime	3–12	EN 1015-11	0.46	0.74	60.87
[36]	8% Cement + Polypropylene Fiber	0.2–1	-	0.79	1.02	29.11
[37]	8% Cement + Polypropylene Fiber	0.2–1	ASTM 106	0.51	0.87	70.58
[38]	Cement	0–10	-	0.18	1.37	661.11
	Fly Ash	0–30
[40]	Alginate, Lignin	19.5, 0.5	EN 1015-11	1.12	1.45	29.46
	Wool Fiber	0.25
[41]	Cement	0–10	NF EN 196-1	0.21	0.77	266.67
	Lime	0–10
	Date Palm Fiber	0–0.5
[43]	Cement	8	ASTM C293/C293M	1.26	1.91	51.59
	Natural Rubber Latex	1–5
[44]	Crushed Brick Waste	0–24	HB 195	2.19	2.65	21.00
[47]	Cement	5–12	-	0.424	0.495	16.74
	Abaca And Maguey Fiber	0.25
[48]	Cement	6	ASTM D 1635-00	0.51	2.6	409.80
	Steel Fibers	1–2.7
[49]	Cow-Dung Powder	0–3	NF P14-306.	0.11	0.54	390.91
[51]	7% Cement + Banana Fiber	0.175–0.35	ASTM C67-07	0.62	1	61.29
[52]	Bottom Biomass Ash	0–20	ASTM C348-19	1.27	2.91	129.13
[56]	Metakaolin and Sodium Hydroxide Solution	5–20	-	0.43	2.2	411.62
[57]	Cement	10	ASTM C348	0.03	0.21	600.00
	Eggshell Powder: RHA	10–40:90–60
[64]	Cement	2–8	ASTM C 67-02C	0.116	0.216	86.21
	Lime	2–6
	Terrazyme	0.05 mL/kg
[68]	Coal Ash	0–10	ASTM D 1635-00	0.68	1.09	60.29
	Cassava Peels	0–5
[70]	Jute Fiber	0.5–2	ASTM C293	0.69	0.74	7.25

.

6.3. Splitting Tensile Strength Test

The tensile strength of a material is gauged as its ability to withstand a certain amount of tension before breaking or fracturing. Multiple studies have reflected upon the brittle nature of the earth blocks. Therefore, the authors have incorporated fibers to reinforce the soil blocks to possess sufficient tensile strength to resist fracture. The split tensile testing of the soil blocks was conducted according to several testing standards. Due to the lack of standards that focus precisely on the tensile strength testing of soil blocks, most standards employed were those for concrete testing specimens. RILEM TC 164-EBM and the Centre for Development of Enterprise (C.D.E) were the only standard codes that focused on assessing the split tensile strength of the compressed earth blocks.

Table 13. Splitting tensile strength of CSEB undertaken by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Content (%)	Testing Standard	Initially stabilized/Control Sample Strength (MPa)	Post-Stabilization Maximum Strength (MPa)	Increment (+)/Decrement (−) (%)
[15]	Cement	5–8	C.D.E-Compressed earth blocks: Testing procedures	0.85	1.5	76.47
	Date Palm Fibers	0.05–0.20
[17]	7% Cement + 20% Brick Waste + Sisal Fibers	0–0.5	RILEM TC 164-EBM	0.6	1.3	116.67
[22]	Cement	8	EN 12390-6	0.08	0.61	662.50
	Cement + Lime	4 + 4		0.08	0.21	162.50
[33]	Clay Pozzolana	0–30	BS EN 1338	0.53	0.87	64.15
[35]	Pidiproof LW+	0.5–1.5	EN 12390-6	0.27	0.54	100.00
[38]	Cement	0–10	ASTM C 496	0.11	0.78	609.09
	Fly Ash	0–30
[48]	Cement	6	C.D.E Compressed earth blocks: Testing procedures	0.26	1	284.62
	Steel Fibers	1–2.7
[59]	Cement	10	IS 5816	0.14	1.2	757.14
	Coconut Coir	0–5
[60]	Cement	2.5–10	IS 5816	0.096	0.17	77.08
	Coconut Fiber	0.2–1

illustrates the findings of various authors on splitting tensile strength values.

Cement stabilization reportedly improved the tensile strength characteristics of the earthen blocks [15,38,48,59]. Reference [38] observed that the strength improved up to a specific optimal value when fly ash content was increased with a constant cement proportion. Any subsequent growth in fly ash content resulted in a decline in splitting tensile strength. The authors of [60] reported similar findings, wherein the authors observed that the tensile strength initially decreased because of a low percentage of fibers, but subsequently increased up to the addition of an optimal fiber dosage, which begins to decline gradually as the dosage exceeds the optimum dosage. The authors argued that the phenomenon is due to the formation of fiber bulk in the soil matrix, which decreases the strength.

In contrast, some authors have claimed that increasing the binder or fiber content increases the splitting tensile strength value. The authors of [59] found that increasing the dosage and length of the coconut fibers in the soil blocks increased the strength value. The authors of [35] made similar remarks when the soil blocks were stabilized by Pidiproof LW+ binder. The reinforced blocks demonstrated excellent performance in terms of their splitting tensile strength and displayed ductile behavior along with better post-failure performance when compared to unstabilized soil blocks. This has been attributed to the bridging action across the cracks and the good adhesion between the soil and the fibers. In the reviewed articles, a total of five fibers and six binders were employed when conducting the splitting tensile strength testing of the soil blocks. This implies that additional fibers uncovered throughout the review process could potentially be utilized to enhance the tensile strength of the CEBs, indicating that there is room for their applicability. This is crucial given that using fiber as reinforcement in rammed earth walls provides safety during earthquakes and the unexpected failure of structures.

7. Durability Properties of CSEBs

This section describes the resilience characteristics of the CSEBs that several researchers have analyzed through testing. The relevant literature revealed tests such as efflorescence, cycle wetting and drying, abrasion resistance, acid resistance, and erosion. Most of the articles concentrated on assessing the erodibility of soil blocks due to their use as masonry units and their vulnerability to deterioration from atmospheric preconditioning owing to the weathering and pitting of the block surface. Of the 61 publications examined, 12 performed erosion tests, making it the most frequently performed test for analyzing a soil block’s durability. This is followed by seven articles that tested for cyclic wetting and drying of the soil blocks, which emphasizes loss in strength or mass to realistically evaluate a soil blocks’ durability. Only three articles tested for efflorescence, and just two articles tested for abrasion and acid resistance, as shown in Table 8.

7.1. Erosion Test

Durability is mainly associated with challenges encountered due to the presence of water, among other environmental or anthropogenic events. Moisture penetration and subsequent surface erosion, characterized as penetration depth and pitting depth, are the two significant factors that have been identified to impact durability negatively and reduce the construction life [22,52,58,65,69]. Even though there is no global agreement on evaluating resistance to erosion, researchers have concluded that this phenomenon is induced by direct or indirect (wind-driven) precipitation.

Table 14. Erosion characteristics of CSEB undertaken by researchers as reported in relevant publications.

Reference (s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Testing Methodology	Penetration Depth (mm)		Pitting Depth (mm)		Erosion Rate (mm/min)
					Initial Value	Final Value	Initial Value	Final Value	Initial Value	Final Value
[7]	Coconut Husk Fiber	1	The Australian Earth Building Handbook. HB 195	Spray Jet	-	-	Fully eroded	39.6	-	0.66
	Sugarcane Bagasse Fiber							43.3		0.72
	Oil Palm Fruit Fiber							41.2		0.69
[9]	Precursor (Fly Ash + Glass Powder)- Activator (Alkaline Activated Cement)	50–50	UNE 41410	Drip	-	8	-	0	-	-
	Precursor/Activator	0.5–0.75
[19]	Cement	8	Earth-wall construction. National Building Technology Centre, Australia	Drip	Not assessed	0	Not assessed	1	-	-
	Lime	8				0		15
	8% Cement + Sodium silicate	2				10		25
[22]	Cement,	8	NZS 4298	Spray jet	Failed	40	Failed	0	23	Nil
	Cement + Lime	4 + 4		Drip	57.7	0	4.3	0	-	-
[27]	Cement	3–12	-	Spray jet	Failed	No trace	-	-	-	-
[33]	Clay Pozzolana	0–30	NZS 4298	Spray jet	-	-	-	-	2	1.5
[34]	-	-	NZS 4298	Drip	-	-	6.2	8.2	-	-
[52]	Bottom Biomass Ash	0–20	UNE 41410	Drip	-	-	Not assessed	<10	-	-
[58]	6% Cement + MICP	0–25 mL spray application	The Australian Earth Building Handbook. HB 195	Spray jet	-	-	-	-	18.8	7.1
	Rubber Crumb	5–20
[65]	Hydraulic Lime + Hydrated Lime	6 + 1	NP EN 12504-4	Spray Jet	-	-	-	-	Not assessed	0–0.33 mm/h
[69]	Cement: Clay	1:15	-	Spray jet	-	-	2.14	3.42	0.037	0.058
	RHA	0–20
	Resin Adhesive	20 mL/kg cement

tabulates the various testing standards adopted by the researchers. The researchers devised and employed two distinct techniques to simulate the impact of rain on a block’s surface to assess its resistance to erosion. Water is applied to the block surface by spraying it through a pressured jet nozzle or by dripping it from a fixed height with a constant head. The stabilized and reinforced blocks were found to perform better in the test than the unstabilized blocks observed from the analysis of the relevant articles.

Reference [58] reported that cement-stabilized rammed earth blocks displayed minimally detectable erosion compared to unstabilized rammed earth blocks, which eroded fast with complete block penetration. Unstabilized sample erosion was found to be significantly reduced by MICP surface treatment and the addition of crumb rubber to the mixture. Cement stabilization has displayed greater erosion resistance, which is congruous with the assessments of other researchers [19,22,27,58]. Densifying the blocks may affect their erosion rate, as [34] found that pitting depth increased with increasing compaction rates. Similar observations have been made by other authors as well [65]. Reference [7] examined the impact of fiber on the erosion characteristics of soil blocks using Coconut Husk Fiber, Sugarcane bagasse fiber, and oil palm fruit fiber. The author noted that the fibers reduced the erodibility of the blocks, although there were apparent traces of degradation. The exterior surface of the fiber-reinforced blocks deteriorated more swiftly than the inside due to the clustering of fibers within the interior versus the presence of fewer fibers on the surface.

7.2. Cyclic Wetting–Drying Test

The cyclic wetting–drying test for CSEBs is the second most recommended durability test performed by researchers, as revealed by an analysis of the pertinent literature. Only three testing standards were identified, which served as a framework for assessing the soil blocks’ wetting and drying cycle. The standards include the Brazilian standard code NBR 13554; the Indian standard IS 1725, and the American standard ASTM D559. The test can be performed in two ways: (1) by calculating mass loss % and (2) by determining loss in strength due to saturation or after completion of wetting–drying cycles. A review of the relevant literature found that the cyclic wetting–drying test on soil blocks was only conducted on soil blocks stabilized with binders. There were no publications found that also had fibers in the mixture. This provides an adequate opportunity for future research avenues to assess the influence of fibers alone and in conjunction with binders on the cyclic wetting–drying characteristics of CSEBs.

The analysis of pertinent articles revealed that soil blocks were imperiled to 5, 6, and 12 cycles of wetting and drying with a duration of 48 h between each cycle, as shown in

Table 15. Cyclic wetting–drying analysis of CSEB undertaken by researchers as reported in relevant publications.

Reference(s)	Binders/Fibers	Binder/Fiber Proportion (%)	Testing Standard	Cycles Performed	Maximum Strength/Loss in Mass (%) before Wetting-Drying Cycle		Maximum Strength/Loss in Mass (%) after Wetting-Drying Cycle
[27]	Cement	3–12	-	5 cycles, each cycle 48 h	3.83 MPa	7.963%	10.2 MPa	0.002%
[38]	Fly Ash	0–30	ASTM D559	12 cycles, each cycle 48 h	7.11 MPa	9.31%	6.26 MPa	3.76%
	Cement	0–10
[39]	Cement	4–10	ASTM D559	12 cycles, each cycle 48 h	8.69%		1.38%
	Fly Ash	0–30
[43]	Cement	8	IS 1725	12 cycles, each cycle 48 h	1.35%		0.96%
	Natural Rubber Latex	1–5
[44]	Crushed Brick Waste	0–24	IS 1725	12 cycles, each cycle 48 h	9.57 MPa	0.94%	14.05 MPa	4.72%
[63]	Cement	6,12	NBR 13554	6 cycles, each cycle 48 h	1.84%		0.3%
	Cassava Wastewater	10.01
[64]	Cement	2–8	IS 1725	12 cycles, each cycle 48 h	4.56 MPa	2.98%	6.78 MPa	2.46%
	Lime	2–6
	Terrazyme	0.05 mL/kg

. Reference [27] performed the test on the soil blocks with different dosages of cement and observed that the increase in the cement dosage in the soil block made it more resistant and, therefore, more durable. When the cement dosage is greater than 10%, the mass loss is negligible, at just 0.002%. These observations are in line with other authors’ findings [39,43,63,64]. Reference [38] reported retention in the dry strength (up to 75%) of the soil blocks after the wetting–drying cycles credited to the presence of fly ash and cement in the soil mix. Similarly, [27] noticed increased dry compressive strength. In order to enhance the performance characteristics of CEBs, [44] employed crushed brick waste as an additive. The authors discovered that the compressive strength of the stabilized soil blocks that underwent wetting–drying cycles was significantly higher than the initial values. By adding 24% crushed brick waste, a maximum strength of 14.05 MPa was attained. These increases came about as a result of the cement’s rapid hydration and the pozzolanic reaction of the brick debris. After submerging soil blocks in water and performing a subsequent brushing operation, it was found that mass loss increased and was attributable to an increase in pore-water pressure. The authors add that crushed brick waste has greater water absorption, resulting in significant swelling, shrinking, and consequent mass loss.

8. Conclusions

The use of soil in construction dates back to prehistoric times. Compressed earth bricks offer a cheaper and more environmentally friendly alternative to the ever-increasing carbon footprint of construction materials. This research aims to summarize the findings of previous studies of compressed earth blocks to provide a comprehensive analysis of their manufacturing, physical, mechanical, and durability characteristics. Incorporating binder, fibers, or both into the manufacturing of these compressed earth blocks has resulted in enhanced performance and characteristics on par with their traditional counterparts. While binders displayed considerable influence upon the compressive strength and durability of the CEBs, fibers enhanced their flexural and thermal performance. However, it was revealed from the review that there is a predominant utilization of cement as a binding and stabilizing agent for CSEB. Cement gives CSEB its high compressive strength and reduces water absorption, making the blocks resistant to water damage. Nevertheless, the manufacturing of cement has significant effects on the environment, air quality, water quality, and human health due to the release of greenhouse gases, air and water pollution, and soil deterioration. Furthermore, cement is a nonrenewable resource, and its overuse in CSEB manufacturing may contribute to resource depletion in the long run. Therefore, it becomes imperative to move towards alternative binders (industrial waste such as waste foundry sand and blast furnace slag, among others), which also possess pozzolanic properties and have reportedly enhanced the mechanical properties of the CSEBs. However, also worth mentioning are the possible environmental impacts and long-term durability aspects of these alternatives, which could be future research avenues. Natural fibers make up the majority of the fibers, and while they passively improve mechanical qualities, their susceptibility to deteriorate over time raises severe questions about their use. This issue could be remedied with the integration of synthetic fibers, necessitating additional investigation into their potential application. The detailed findings of this study are discussed in the following subsections.

• It was discovered that compressive strength, flexural strength, water absorption, bulk density, and SEM analysis had been routinely utilized to evaluate the performance characteristics of CSEBs. The findings of the majority of these tests indicate that the addition of fiber, binder, or both to soil block manufacturing improves its characteristics. These findings were further corroborated by the minimum recommended values specified by their respective standards, and most of the compressed stabilized earth blocks fulfilled the criterion.
• Thermal conductivity, splitting tensile strength, porosity, cycle wetting–drying, and erosion resistance tests were discovered to be reported half as frequently as compressive strength and other tests. Despite being crucial metrics for evaluating the functionality and durability indicators of earth blocks, they were reported in fewer articles. In addition, only one or two studies reported the efflorescence and linear shrinkage qualities of earth blocks, despite the fact that earth blocks are utilized as masonry units in construction practice. This might be identified as a gap in the research, and these tests could be conducted on earth blocks using binders, fibers, or both to better analyze the performance and durability of these stabilized earth blocks.
• It was perceived that the application of binders significantly increased the compressive strength of CEBs, with cement exhibiting the best performance of all the binders used by different researchers. Due to the densely packed structure owing to the presence of binders, the earth blocks were also resistant to water absorption and displayed increased erosion resistance.
• Soil blocks with added fibers demonstrated better flexural characteristics and thermal performance. The fibers could bridge the cracks, thus preventing the earth blocks’ brittle failure. The lower thermal conductivity of the fibers made them more resilient to thermal fluctuations than un-stabilized mixes.
• Apart from SEM, other microstructural analytical methods, e.g., X-ray diffraction, Fourier Transform Infra-Red Spectroscopy, and Energy dispersive spectroscopy, were rarely used. In addition, other evaluations, such as life cycle analysis and cost analysis, were presented in just one and three articles, respectively, which can provide insight into the practicability of using earth blocks stabilized by binders and fibers.
• It was identified that there are not many standard codes and guidelines present worldwide to establish manufacturing standards, and the testing standards used to carry out important mechanical and durability analyses of earth blocks were based on those of the fired clay bricks, which needs to be addressed at the earliest in order to present reliable results. These identified gaps in the studied literature can serve as a foundation for future research.