Geopolymers as Sustainable Material for Strengthening and Restoring Unreinforced Masonry Structures: A Review

Abstract

Unreinforced masonry (URM) structures are vulnerable to earthquakes; thus, materials and techniques for their strengthening and restoration should be developed. However, the materials used in some of the existing retrofitting technologies for URM and the waste produced at its end-of-life are unsustainable. The production of ordinary Portland cement (OPC) worldwide has enormously contributed to the global carbon footprint, resulting in persistent environmental problems. Replacing OPC with geopolymers, which are more sustainable and environmentally friendly, presents a potential solution to these problems. Geopolymers can replace the OPC component in engineering cementitious composites (ECC), recommended to strengthen and restore URM structures. In the present paper, the state-of-the-art knowledge development on applying geopolymers in URM structures is discussed. The discussion is focused on geopolymers and their components, material characterization, geopolymers as a strengthening and restoration material, and fiber-reinforced geopolymers and their application to URM structures. Based on this review, it was found that the mechanical properties of geopolymers are on par with that of OPC; however, there are few studies on the mentioned applications of geopolymers. The characterization of geopolymers’ mechanical and physical properties as a restoration material for URM structures is still limited. Therefore, other properties such as chemical interaction with the substrate, workability, thixotropic behavior, and aesthetic features of geopolymers need to be investigated for its wide application. The application method of geopolymer-based ECC as a strengthening material for a URM structure is by grouting injection. It is also worth recommending that other application techniques such as deep repointing, jacketing, and cement-plastering be explored.

1. Introduction

Concrete is ubiquitous in our growing urban landscape, and ordinary Portland cement (OPC), a vital component of concrete, has been shown to contribute significantly to the greenhouse gases in the atmosphere [1]. This problem can be circumvented by replacing OPC with sustainable and environmentally friendly alternatives that are on par in strengthening and restoring capabilities. Currently, geopolymers are extensively studied as a potential replacement for OPC. Saloni et al. [2] explained that geopolymers are sustainable materials because industrial by-products allow for lower energy consumption. Additionally, replacing OPC, produced using a massive amount of energy, could lower carbon footprint. Neupane et al. [3] reported that geopolymer concrete has higher indirect-tensile and flexural strengths than OPC concrete. Aside from this, its thermal stability and resistance to harsh environments make it a preferred material to restore historical structures [4]. Historical structures are commonly built using unreinforced masonry (URM), because OPC-based concrete was invented and used in the 19th century and established its market dominance in the early 20th century [5]. Masonry buildings are made of units—cast stone, clay masonry units, concrete masonry units, natural stone, terra cotta, adobe, pisé de terre, cobb—laid in a mortar [5]. These materials have low tensile strength and are intended to withstand compression loading only [6]. According to ICOMOS (2003), crushing, buckling, and brittle failure are among the most common causes of damage to masonry structures. The unreinforced masonry building’s low tensile capacity makes it vulnerable for earthquake-induced loads—in-plane lateral loads or out-of-plane or eccentric loads—which may cause diagonal cracks, sliding, or the rotation of a wall [6]. Recent studies recommend the use of engineered cementitious composites (ECC) such as fiber-reinforced and fabric-reinforced cementitious composites for strengthening of masonry walls against out-of-plane loading [7,8] and in-plane loading [7,9]. Geopolymers can replace the OPC of these ECC to make the material more sustainable and environmentally friendly; in addition, ECC can be improved, since geopolymers are reported to have a higher mechanical strength and durability than OPC-based ECC. Despite this, geopolymers have not replaced OPC widely in strengthening and restoring URMs. In terms of application as a restoration material, aspects such as its chemical interaction with the substrate, workability, thixotropic behavior, and aesthetic features are yet to be established [4]. On the other hand, geopolymer-based fiber-reinforced composites have only been studied as grouts; there are other applications of geopolymers as strengthening materials for URMs—deep repointing, jacketing, and cement-plastering—that are yet to be explored.

This paper is the first to present a review of the application of geopolymers as strengthening and restoration materials for URM structure. The discussion includes the method of data collection, geopolymers and their characterization techniques, their application as strengthening and restoration material for URM structures, and the application of fiber-reinforced geopolymer (FRG) in URM structures.

2. Collection and Sources of Literatures

Two databases were used in this review: the Scopus and ScienceDirect databases. To show the current state of geopolymer research, the number of documents related to geopolymers in the Scopus and ScienceDirect databases from 2015 to 2021 are determined. The methodology of choosing the papers used in this review article is shown in Figure 1.

2.1. From the ScienceDirect Database

According to Tober [10], the ScienceDirect database provides the scientific citation databases, Scopus and Scirus. Additionally, it can provide a broader range of hits with high precision compared to other databases [10]. Figure 2 shows the number of research papers about geopolymers in the ScienceDirect database. It can be observed that research on geopolymers remained dormant from 2000 to 2011; however, it started to increase gradually from 2012 to 2016 and rapidly from 2017 to 2019, reaching a peak in 2019–2020. The increasing trend in geopolymer research may indicate that researchers are becoming more inclined to develop more environmentally friendly materials specifically for use in the construction industry, which is known to contribute significantly to global greenhouse gas emissions [1].

Currently, around 7327 results are obtained from the database when the word “geopolymer” is used in the search. The number of articles is narrowed down by adding words to the search keyword depending on the subtopics of this article. The following are the search words used to obtain the research articles included in this paper: mechanism of geopolymerization (1324 results), geopolymer components (4667 results), geopolymer alkali activators (4092 results), geopolymer precursors (2760 results), geopolymer additives (2172 results), geopolymer characterization methods (4696 results), geopolymer as a strengthening material (1096 results), geopolymer as a restoration material (211 results), geopolymer reinforced with fiber (1568 results), geopolymer as a strengthening material for unreinforced masonry structures (32 results), and geopolymers used in the restoration of unreinforced masonry (4 results).

2.2. From the Scopus Database

Currently, around 8758 results are obtained from the database when the word “geopolymer” is used in the search. Figure 3 shows that research on geopolymers remained dormant from 2000 to 2011; however, it started to increase gradually from 2012 to 2016 and rapidly from 2017 to 2019, reaching a peak in 2019–2020. Figure 3 shows a similar trend as observed in the ScienceDirect database.

The number of articles is narrowed down by adding words to the search keyword depending on the subtopics of this article. The following are the search words used to obtain the research articles included in this paper: mechanism of geopolymerization (183 results), geopolymer components (492 results), geopolymer alkali activators (519 results), geopolymer precursors (678 results), geopolymer additives (355 results), geopolymer characterization methods (17 results), geopolymer as a strengthening material (7 results), geopolymer as a restoration material (5 results), geopolymer reinforced with fiber (755 results), geopolymer as a strengthening material for unreinforced masonry structures (2 results), and geopolymers used in the restoration of unreinforced masonry (no results).

3. Geopolymerization Process and Components of Geopolymers

The mechanical properties of geopolymers heavily rely on how the geopolymerization process takes place and what the products are. According to John et al. [11], the process of geopolymerization involves three steps in the following order: (1) dissolution, (2) orientation, and (3) polycondensation. In the dissolution step, alkaline activation is a vital process. It is the reaction of the alumina and silicates initially contained in the raw materials called precursors (e.g., fly ash, blast furnace slag, etc.) with an alkaline solution prepared through the mixing of alkali components (e.g., NaOH, Na₂SiO₃, etc.) that produces a polymer with Si–O–Al–O bonds [11]. Zhou et al. [12] showed a schematic diagram of the metakaolin particles reacting with the alkali activator. The role of Na⁺ is to neutralize the negative charge caused by the presence of monomers of silica and alumina [12]. As shown in Figure 4, the sides of the metakaolin particles erode first. It is followed by the attack of Na⁺ and OH^- ions at the center of the particles during a process called solvation.

Zailani et al. [13] emphasized that the alkaline activator should have a high concentration to accelerate the dissolution of Si⁴⁺ and Al³⁺ from precursor leading to the formation of sodium alumina–silicate hydrate (N-A-S-H) and calcium alumina–silicate hydrate (C-A-S-H) gels. Equation (1) represents the reaction of the silicates and aluminates present in fly ash, an example of a precursor, with the NaOH solution [13].

(SiO, Al O from fly ash) + 2NaOH + 5H₂O → Si(OH) +2Al(OH)₄⁻ + 2Na^+.

(1)

Geopolymers are also known to contain linked SiO₄ and AlO₄ with cations such as Na⁺ and K⁺ that are interspersed to neutralize the negative charge of the polymer [11]. Other products are also formed during geopolymerization. The presence of Fe₂O₃ in the precursor can lead to the formation of ferrosialate gel, which can also strengthen the resulting material [14]. Other products of geopolymerization are beidellite ((Na,Ca) Al₂Si₄O₁₀(OH)₂) and fayalite (Fe₂SiO₄), which are products of dissolution of other components of the precursor and are produced through the use of a sufficient concentration of alkali activator. Equations (2) and (3) show the reactions involved in the formation of beidellite and fayalite [13].

Na⁺ +Ca²⁺ +Si(OH)₄⁻ +2Al(OH)₄⁻ → (Na, Ca)Al₂Si₄O₁₀(OH)₂ (beidellite)

(2)

OFeOH·H₂O + Si(OH)₄ → Fe(OH)₃·SiO₂ (fayalite) + 2H₂O

(3)

In a high calcium-based system, Chindaprasirt et al. [15] enumerated the possible reactions occurring, which are as follows:

i.

Dissolution of SiO₂, Al₂O₃ and calcium sources (CaSO₄ and CaO).

SiO₂ + Al₂O₃ + OH⁻ → SiO₂ (OH)₂²⁻ or SiO(OH)₃⁻¹ + Al(OH)₄⁻

(4)

CaSO₄, CaO → Ca²⁺ +SO₄²⁻ + OH⁻

(5)

ii.

Precipitation reactions

Ca²⁺ + SiO₂ (OH)₂²⁻ or SiO(OH)₃⁻¹ + Al(OH)₄⁻ → CASH gel

(6)

Na⁺ + SiO₂ (OH)₂²⁻ or SiO(OH)₃⁻¹ + Al(OH)₄⁻ → NASH gel

(7)

Figure 5 shows the general components of geopolymers such as precursor, alkali activator, water, and additives.

In the following subsections, the components of geopolymers—alkali activators (Section 3.1), precursor (Section 3.2), and additives (Section 3.3)—and their properties are discussed.

3.1. Alkali Activators

Alkali activators are usually composed of NaOH, KOH, Na₂SiO₃, and K₂SiO₃. They dissolve Si and Al monomers from the precursors to create Si and Al cross-links [16]. The compressive strength increases as the hydroxide molarity increases (range of 10 to 16 M). However, increasing the molarity can only increase the compressive strength to a certain extent [17]. Rattanasak and Chindaprasirt [18] explained that a higher NaOH concentration could dissolve more Si and Al ions from the precursor, resulting in a greater degree of polymerization. In geopolymers with NaOH concentration higher than 16 M, the excess sodium ions may have caused premature gel precipitation in the matrix, as well as causing reduced workability, hindering the leaching of the Si and Al species. The use of KOH instead of NaOH may increase the extent of geopolymerization, since K⁺ may increase the rate of polycondensation compared to when Na⁺ is the ion involved. K⁺ allows the polycondensation rate to increase by binding more water molecules than Na⁺ [19]. The ratio of NaOH to Na₂SiO₃ of the alkali activator also affects the workability aside from the compressive strength. In the study by Leong [20], the highest workability of 250 mm is obtained for Na₂SiO₃/NaOH/KOH of 0.5, with KOH-based geopolymer having more excellent workability than NaOH-based geopolymer. However, in the study by Quiatchon et al. [21], the water-to-solids ratio affected the compressive strength more significantly than the ratio of NaOH to Na₂SiO₃.

3.2. Precursors

Precursors are raw materials that are the primary source of aluminum and silicon that participate in the geopolymerization reaction due to their high reactivity. These precursors are of a geological origin or derivative manufacturing processes, thus ensuring greater sustainability than conventional materials such as OPC. The precursors used most often in geopolymer mixes are fly ash, clay, metakaolin, and granulated blast furnace slag (GGBS) [22].

Moradi et al. [23] defined metakaolin as a cementitious material derived by heating kaolin at 500–600 °C. On the other hand, fly ash is a waste product of the coal-fired power generation industry that can harm the environment when not disposed of correctly [24]. Wang et al. [25] mentioned that fly ash is used in the construction industry as an admixture that strengthens and enhances the durability of concretes. Blast furnace slag is also a calcium-rich waste product of the iron industry. It is produced through a smelting process; limestone or dolomite is added to the heat carrier to remove the impurities of the iron ore, followed by their decomposition producing magnesium oxide and calcium oxide and melting with other ashes producing the aluminosilicate material, which is the blast furnace slag [26,27]. Other unpopular precursors used in the preparation of geopolymers are rice husk ash [28], red mud [29], nickel laterite mine waste [30], perlite, kaolinic clay [31], and red clay ceramic powders [22]. In some studies, the mentioned materials are mixed in different proportions to serve as the precursor for geopolymers.

One crucial factor that influences the suitability of precursors is the particle size, as the exposed surface area directs the kinetics of the geopolymerization reaction. This feature is also why GGBS had the most positive effect in promoting early-stage compressive strength, as its median diameter is the most uniform and the smallest of the frequently used precursors. Metakaolin and fly ash have similar characteristics of irregular PSD, with fly ash having a larger median diameter [32]. The increase in volcanic ash content in the study by Tigue et al. [33] caused a decrease in the unconfined compressive strength due to the irregular profile of volcanic ash particles and its relatively large particle size distribution. This increase also plays a role in the required quantity of activator solutions, as more are needed to activate fewer reactive precursors, further driving the entailed costs and environmental impacts.

The study by El Idrissi et al. [32] points out that when precursors are used singlehandedly, the low-calcium fly ash has the greatest ultimate strength of all the commonly used precursors. However, it is much slower to achieve this effect due to the longer dissolution rate of the fly ash particles, while having a lower specific surface compared to metakaolin. Nickel laterite mine waste was used to improve the geopolymer synthesis and required at least 600 °C thermal activations and milling for two hours at 443 rpm to optimize the cementitious activity during mix design [13].

Geopolymer blends, which are considered a mixed binder with fly ash, were facilitated in several studies to balance the elemental species that participate in the geopolymerization reaction and utilize more waste feedstock, hence lowering the global warming potential (GWP) of the concrete [34]. This is carried out while ensuring that the target application is met, such as mass and mechanical strength. These systems require optimization studies to check the limits of component additions in the mix designs. The development of paving blocks from the study by Kumar and Kumar [35] showed that compressive strength increases up to a specific limit in the addition of red mud. A dosage of 10 to 20% meets material specifications. The increase was observed due to the increased aluminosilicate dissolution caused by red mud but only to a certain extent. The study by Kalaw et al. [36] considered the ternary binder system of coal fly ash, coal bottom ash, and rice hull ash. This system was optimized to use the three feedstocks effectively in targeting the required mechanical strength and fire resistance properties. Lastly, a geopolymer made from the mixture of cement kiln dust and silica fume was assessed by Yaseri et al. [37]. Compressive strength results showed that the dual system reaches a maximum limit at 55% silica fume and 45% cement kiln dust and is caused by the increase in the available aluminosilicate components in the system. At higher silica fume content, however, the excess silicon species had no oligomeric reaction sites, causing the downtrend of the compressive strength [37].

Figure 6 shows a bar graph presenting the number of articles about different precursors used in preparing geopolymers found in the ScienceDirect and Scopus databases.

3.3. Additives

Aside from alkali activator solutions and precursors, additives such as superplasticizer, silica fume, quartz, and alumina improve geopolymers’ mechanical properties. Figure 7 shows a bar graph presenting the number of articles in the ScienceDirect and Scopus databases about different additives incorporated in geopolymers.

Superplasticizer is commonly added to address the flow and enhance the workability of the geopolymer. In the study by Moni et al. [38], the effect of the molarity of NaOH on the viscosity of the geopolymer paste is adjusted by adding a superplasticizer. The addition of a superplasticizer reduced the viscosity of the geopolymer pastes, but the excessive addition of the superplasticizer can affect the resulting mechanical performance of the geopolymer paste. Adding a superplasticizer beyond 5% resulted in reduced strength [15,39]. Another common additive is silica fume. Thokchom et al. [40] observed that the addition of silica fume up to 5% resulted in reducing the porosity and water absorption of fly ash geopolymer mortar. Silica fume also increased compressive strength, but excessive addition resulted in deterioration of strength [41]. They suggested an optimum silica fume content of 4% by weight of the total binder. Rashad et al. [42] investigated the effect of quartz powder in fly ash geopolymer pastes subjected to elevated temperatures and observed that the 28-day compressive strength of the 30% quartz powder mix increased by 3.42 times. Alumina, on the other hand, enhanced microstructure properties and helped in the early strength of the geopolymer [43].

4. Geopolymer Characterization Methods

The chemical properties of the starting materials such as alkali activators, precursors, and additives affect the mechanical properties of resulting geopolymers. Section 4 discusses different characterization methods—Fourier transform infrared radiation (FT-IR) spectroscopy (Section 4.1), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) (Section 4.2), the Brunauer–Emmett–Teller (BET) method (Section 4.3), X-ray fluorescence analysis (Section 4.4), X-ray diffraction analysis (Section 4.5), and nuclear magnetic resonance (NMR) spectroscopy (Section 4.6)—that can be used to determine the chemical properties of the starting materials and resulting geopolymers.

4.1. Fourier Transform Infrared Radiation (FT-IR) Spectroscopy

Yu et al. [44] describe FT-IR spectroscopy as a method that provides a bulk analysis of the molecular adsorption bands of the raw materials and the changes that have occurred during product formation. Pasupathy et al. [45] determined the FT-IR bands observed in fly-ash-based geopolymer (GPC) and ordinary Portland cement (OPC) concretes. GPC concrete has major bands, such as 785, 946, 1152, 1313, and 1635 cm⁻¹, and in OPC concrete, the major bands are 856, 965, 1062, 1116, 1289, 1363, 1511, and 2948 cm⁻¹. Additionally, the Si–O–Al/Si band in GPC is found at 946 cm⁻¹ and 965 cm⁻¹ in OPC. O-H bending vibrations are observed at 1313 and 1635 cm⁻¹ in GPC and 785 cm⁻¹ in OPC (Figure 8).

Chen et al. [46] reported the FT-IR bands observed in reactive ultrafine fly ash (RUFA)-based geopolymer consistent with the literature. The absorption bands at 1650 cm⁻¹ and 3480 cm⁻¹ are the bending vibrations of H-O-H and –OH, confirming the presence of water molecules in the sample, and 460 cm⁻¹ is the in-plane bending vibration of Si–O–Si, while the band at 728 cm⁻¹ indicates the presence of Si–O–Al bending vibration. At 1093 cm^–1 of fly ash, the band shifted after alkali activation to 1044 cm⁻¹, 1026 cm⁻¹, 1009 cm⁻¹, and 1002 cm⁻¹ [47]. This phenomenon is attributed to the increasing number of silicon sites with nonbridging oxygen or the increase in the number of substituents of Al in the geopolymer.

Figure 8. FT–IR peaks observed in fly-ash-based GPC and OPC samples [47,48].

Figure 8. FT–IR peaks observed in fly-ash-based GPC and OPC samples [47,48].

4.2. Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS)

Lee and van Deventer [48] and Sasui et al. [49] described SEM analysis as a method used to check the particle surface morphology of the raw material and formed material and verify the formation of the gel phase. On the other hand, EDS analysis provides an elemental analysis (area or point) of the developed material or confirms the presence of certain elements such as Si, Al, Mg, O, and Na [48,50].

Eliche-Quesada et al. [51] used SEM to observe the effects of the geopolymerization process on the microstructures of the geopolymers. In the control sample, the formation of hydrated sodium aluminosilicate gel (Na-A-S-H) is observed. At the same time, the addition of different types of clay to the geopolymer resulted in a C-A-S-H or Ca-Na-S-H type of gel, unreacted clay platelets, and unreacted bottom ash around the gels. In the study by Riahi et al. [52], the effect of the alkali activator and the extent of geopolymer adherence to the fiber were determined through SEM. It was found that the interfacial bonding between the geopolymer and steel fiber is weak because of the interfacial debonding observed in the SEM images (Figure 9).

SEM can also be used to study the morphology of the starting materials such as precursors [53,54]. Long et al. [55] studied the morphology of municipal solid waste incineration fly ash (MSWI-FA) and ferronickel slag (FNS), known geopolymer precursors, to determine whether these precursors can have an excellent interfacial bonding. MWSI-FA is registered as irregular and spherical particles, indicating a high surface area, while FNS is rough and porous, creating a good bonding interaction with the binder (Figure 10) [55].

4.3. Brunauer–Emmett–Teller (BET) Method

Brunaeur–Emmett–Teller or BET method is used to determine the surface area (m²g⁻¹) of materials to correlate the surface area to the mechanical and chemical properties of the materials [56]. Güngör and Özen [56] concluded that the higher the surface area, the greater the positive effect on the compressive strength of the geopolymer mortars because geopolymerization is more effective for smaller particles. Wang et al. [57] determined the surface area of the raw materials they used and the resulting geopolymers made of coal gangue and red mud. The researchers observed that there is enhanced adsorption for samples with a large surface area and that geopolymerization and the incorporation of red mud Na₂SiO₃ as an activator increase the surface area [11]. However, in the study by Freire et al. [58], the opposite was observed: BET surface area and adsorption capacity of the geopolymer have no relationship.

On the other hand, Saretta [59] determined that the BET surface area of 17.181 m²g⁻¹ for a ZnO/geopolymer nanocomposite photocatalyst is enough for the photocatalytic reactions to occur on the surface of the material. In the study by Liu et al. [60], BET results showed that the specific surface area ratio of rice-hush-ash-based geopolymer and metakaolin-based geopolymer increases with increased heat erosion temperature because the water bound in the geopolymer evaporates faster at a high temperature. The study by Nieves et al. [61] showed that the surface area determined through BET affects the workability of geopolymer paste and the reactivity of the starting materials; therefore, the surface area of the materials should be part of decision making when formulating geopolymers.

4.4. X-ray Fluorescence Analyis

Hui-Teng et al. [62] determined the chemical composition of fly ash (FA) using the XRF technique. They found that the main compounds present in FA are SiO₂ and Al₂O₃ with a total of 84.3% (w/w), while CaO is also current but only with a low amount of 3.89% (w/w). Zawrah et al. [63] determined the oxides present in the feeders’ and cyclones’ waste clays, and they found that these starting materials are mainly composed of silicon and aluminum oxides.

Jamil et al. [64] determined the chemical composition of kaolin and ground granulated slag (GGBS), as shown in

Table 1. Oxide composition of different raw materials.

Composition (%)	Source	SiO2	Al2O3	Na2O	MgO	MnO	P2O5	K2O	SO3	TiO2	ZrO2	CaO	Fe2O3
Coal Fly Ash (Class F)	[65]	56.5	22.5	0.48	3.16			1.49		0.86		10.2	5.78
Coal Fly Ash (Class C)	[49]	38.8	19.5	0.87				1.84				26.2	2.87
Coal Bottom Ash	[66]	39.4	34.3		1.57			1.19	0.90			6.3	15.0
Palm oil fuel Ash	[67]	46.2	10.4	7.28				-	0.67			7.4	5.47
Rice Husk Ash	[66]	78.0	0.54		0.50			1.94	0.25			1.14	0.16
Ceramic Waste Powder	[68]	72.8	12.2	13.5	1.00							0.01	0.56
Calcined Gangue	[58]	53.4	42.3	0.16	0.21		0.89	0.89	0.23	1.33		0.34	1.06
Kaolin	[54]	54.0	31.7			0.11		6.05		1.41	0.10		4.89
Metakaolin	[66]	50.6	45.7		0.23			0.11	0.14			0.2	0.31
GGBS	[55]	30.4	10.5		3.20	0.71				0.98	0.05	50.4	0.53

. Both precursors are found to contain mainly SiO₂ and Al₂O₃. K₂O present in kaolin contributes to its lower melting temperature, while MgO and CaO of GGBS reduce the sintering temperature and trigger the geopolymerization process.

The addition of calcium components in the geopolymerization reaction is primarily seen during the addition of GGBS. The reactive Ca in the GGBS source enhances the mechanical strength of the geopolymer concrete due to the formation of calcite and calcium silicate hydrate (C-S-H) gel in the structure, while decreasing the porosity and setting time [49]. The reactive Mg in high MgO sources such as calcined hydromagnesite caused an increase in the early-stage compressive strength for low calcination temperatures and late-stage compressive strength for high-temperature calcination [69].

XRF analysis is a technique used to determine the ratio of Si/Al and Na/Al, which affects the mechanical strength of the final product up to an optimum. Based on the data extracted by Sadat et al. [70], the source materials of pure fly ash as a precursor should have a Si/Al ratio of 1.95 and Na/Al of 1.0 to maximize mechanical strength. If added with other raw materials such as metakaolin, mine tailings, kaolinite, and albite, the only important factor would be a Si/Al ratio of 2.0 to 2.1. The Si/Al ratio variation can improve the applied technologies and product properties, as shown in Figure 11.

4.5. X-ray Diffraction Analysis

Huo et al. [71] explained that the XRD technique determines the phases of starting materials and resulting geopolymers and examines their microstructure. They observed that the diffraction peaks for calcium magnesium aluminosilicate present in ordinary Portland cement (OPC) are not present in geopolymer samples, indicating that the minerals in OPC have already reacted after alkali activation. Kuri et al. [50] reported that the minerals present in fly ash and ground ferronickel slag (GFNS) comprise mainly amorphous phases, 86.0% and 61.3%. In addition, the crystalline phases of fly ash are primarily quartz and mullite, while GFNS has forsterite and quartz.

In a study by Yu et al. [72], the effect of mechanical activation was seen in the diffraction peaks of copper mine tailings. Figure 12 shows the decline of crystalline silica peaks (label 1) in quartz (SiO₂) after three hours of mechanical activation through a laboratory ball mill. Nonetheless, the albite peaks (label 2) increased after activation but are confirmed in concurrent FT-IR analysis of the raw and pretreated samples to reduce the polymerization degree. Moreover, the introduction of alkaline roasting as an additional activation step after grinding further disintegrated silicon and alumina species in the quartz and albite peaks, as shown in Figure 13.

These resulted in increased cementitious activity during geopolymerization and highlighted the potentiality of copper mine tailings as a precursor. XRD is conducted to investigate the reactivity of participating materials such as rice husk ash [73] and to check if additional steps are necessary, such as milling and stirring [74].

The study by Sasui et al. [49] shows the effect of the silica source to determine the reactivity of the geopolymer precursor. Different combinations of a Class C fly ash and ground granulated blast furnace slag (GGBS) were mixed. XRD patterns show more amorphous peaks or less crystallinity as the amount of GGBS was increased. This result also corresponded with increasing compressive strength upon geopolymerization, as more reactive species from the GGBS source are observed.

4.6. Nuclear Magnetic Resonance (NMR) Spectroscopy

Zhou et al. [12] detected the NMR of the geopolymerization process through vacuum dehydration. The deconvolution peaks of the NMR spectra revealed a four-step geopolymer process of:

• Dissolution of raw materials into mass monomers.
• Initial polymerization.
• Polymer collapse and quick rearrangement are brought about by dehydration.
• Subsequent polymerization persists mostly on fragmented bonds of the structure.

The stages are seen on the evolution of the NMR peaks to show the individual resonances. The in-situ observation of the geopolymerization process has been a difficult task for researchers because of restrictions in methodologies. They studied the geopolymerization process by employing vacuum dehydration and ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy analysis. The vacuum dehydration process is executed using a circulating gas system activated by an oil pump and a nitrogen condenser pipe, wherein water is removed for around 40 min. This technique is used to arrest the geopolymerization states of the samples so the geopolymerization process can be studied.

Table 2. Chemical shifts of vacuum dehydrated geopolymers observed through ²⁷Al and ²⁹Si NMR.

Chemical Shift (ppm)	Interpretation	Source
50 and 70	aluminosilicates	[12]
0–20	six-coordinate aluminum resonating from [Al(H2O)6]3+
25	Al(V) peak
72–110	Si atoms with various linkages

shows the chemical shifts that the researchers and their interpretations observe.

Yuan et al. [75] also reported the NMR observation that as the atomic number of the alkali-activated ions increases, the degree of geopolymerization also increases.

4.7. Mercury Intrusion Porosimetry (MIP)

Mercury intrusion porosimetry is a means of obtaining the pore size and structure of the samples after precision cutting into a box and being dried under vacuum [75]. The pore size distribution is a manifestation of the specific volume of mercury, which was intruded in the sample through the sample’s capillaries and pores [76]. The sample is placed in a penetrometer where the vacuum is increased while the readings of the pore diameters are indicated through an analyzer [32]. Pore analysis is important in checking the relationships of mechanical strength and other properties such as thermal properties and composition of materials. In the study by Lin et al. [77], phosphate-activated metakaolin geopolymer (PMG) pore structure after a two-stage curing process, including a pre-curing at 40 °C for 24 h and secondary curing at 80 °C for another 24 h, is shown in Figure 14. The MIP result shows that the most probable aperture of PMG is about 100 nm, and the total porosity of PMG is about 13.6%. On the other hand, Hu et al. [78] concluded that the pore structure of micropore-foamed geopolymer has capillary pores of 15 μm–72 μm, which indicated that an increase in Ca(OH)₂ limits the foaming property. Hager et al. [79] observed that the total porosity values of the unheated unblended mortar and after exposure to 400 °C were comparable. However, in the mortar of fly-ash-based geopolymer with slag, the increase in porosity was significant at 200 °C.

5. Geopolymers as a Strengthening and Restoration Material

Figure 15 shows a substantial number of studies on geopolymers’ application as a strengthening and restoration material; however, there are very few studies on the application of geopolymers as a strengthening and restoration material, specifically for URM structures relative to the total number of research articles about geopolymers. This indicates that more studies about geopolymer application in URM structures can be further explored. Section 5.1 narrates the studies about geopolymers as a strengthening material in general and in URM structures. On the other hand, Section 5.2 enumerates the studies about geopolymers as a restoration material in general and in URM structures.

5.1. Geopolymers as a Strengthening Material

The strength capacity refers to the permissible strength or deformation that a component can withstand for a specific load demand [80]. While the structural application of geopolymers is still limited to local modification of structural members, the strengthening application of the matrices shows promising results. A summary of studies regarding the use of geopolymers as a strengthening material is shown in

Table 3. Summary of studies on geopolymers used as a strengthening material.

Authors	Technique	Unreinforced Geopolymer or with Fiber Reinforcement
I. Beams
[81]	Jacketing	Ultrafine blast furnace slag based
[82]	Jacketing	Steel-reinforced geopolymer matrix
[83]	Jacketing—analytical method	Steel-reinforced geopolymer matrix
[84]	Jacketing	Carbon-fiber-reinforced phosphate-based geopolymer (PBG)
[85]	Glass-fiber-reinforced geopolymer bars	Fly-ash-based geopolymer beams
[86]	Thin deflection hardening fiber-reinforced layers	Fly-ash-based geopolymer beams
II. Wall
[87]	Thickening	Polypropylene-reinforced fly-ash based
[88]	Thickening	Metakaolin-slag-sodium-silicate geopolymer (GP)
[89]	Grouting	Additive styrene butadiene (SB) latex geopolymer grout
III. Thin plates
[90]	Carbon textile-reinforced geopolymer composite	Fly-ash based
[91]	Textile-reinforced geopolymer composite	Hybrid PVA fiber and AR-glass textile-reinforced geopolymer composites
IV. Bond strength
[92]	Single-lap shear test	Steel-reinforced geopolymer matrix
[93]	Single-lap shear test	Fly-ash with slag geopolymer
[22]	Interfacial transition zone	Fly-ash based
[94]	Exposure to high temperature	Fly-ash based
[76]	Molar concentration	Steel-fiber-reinforced geopolymer
[95]	Slant shear test	Metakaolin geopolymer against cement mortar
V. Mechanical tests only
[96]	Compressive strength only	Activated quartz based
[38]	Effect of sodium chloride	Slag composite matrix

.

The study by Ding et al. [84] showed preliminary results of about a 100% increase in ultimate load capacity for reinforced concrete (RC) beams jacketed with carbon-fiber-reinforced PBG. While for the studies of Bencardino and Condello [92], RC beams reinforced with steel-fabric-reinforced geopolymer matrix at the bottom resulted in an increase of 16% of ultimate load capacity compared with unreinforced RC beams. The study by Maras et al. [87] investigated the use of geogrid geopolymer panels (GGP) applied to the surface of wall samples through vertical and diagonal loading systems. The compressive strength and ductility of the walls improved after using GGP; the strengthened samples’ shear strength also improved remarkably. It is also observed that the metakaolin-based geopolymer has a better bond strength at 3.5 MPa compared to geopolymers with silica fume, polymer concrete adhesive, styrene butadiene resin (SBR), and micro-steel-fiber-reinforced concrete with around 1 to 2.5 MPa only [87].

About 1096 results are obtained when the phrase used in the search is “geopolymers as strengthening material”. However, this is reduced to 32 results when the words used are “geopolymers as a strengthening material for unreinforced masonry structures”. Out of the 32 articles, only one research article substantially and directly discussed the application of geopolymers in strengthening URMs. Calabrese et al. [97] mentioned that URMs are only used for vertical load and that this makes URMs vulnerable to earthquakes that particularly target the structures horizontally. Thus, it is necessary to develop materials that can be used to strengthen URMs. In Tamburini et al.’s [88] study, geopolymer grout is made using metakaolin–slag–sodium silicate binder and aggregates of fine, coarse river sand and fibrous wollastonite filler. They confirmed that the grout made successfully adhered to two clay bricks, especially the soft mud clay bricks with mortar compressive strength of 45 MPa, low leaching, and low drying and thermal linkage [97].

The performance of geopolymer can also be affected by aggressive environments (AE) or conditions such as in coastal areas and sulphate attacks. Amran et al. [98] stated that compared to normal cement-based concrete, geopolymer matrices demonstrates better aggressive resistance against aggressive environment due to having less porous structures. The effect of AE to geopolymer is influenced by how fast the geopolymerization process occur. A study by Razak et al. [99] presented the performance of fly-ash-based geopolymer immersed in seawater up to 60 days. The tests include a compressive strength test, water absorption test, and density test before and after seawater exposure. It is observed in their study that the hydration reaction of alkali solution and fly ash is faster than the hydration process of the ordinary cement, leading to a higher density for the geopolymer sample.

The bond strength between the substrate concrete and the geopolymer matrix is also important if it is to be used as a strengthening or repair material [100]. Lashkari et al. [101] presented the investigation of geopolymer-based coating exposed to harsh environments along the Persian Gulf. It is reported that slag in geopolymer exposed to such environments has better ability to fill voids, which influences the lower effect of chloride ions. It is observed that a 30% slag with 7.5% silica fume demonstrates an optimal design for AE in terms of the resulting compressive strength, with a 66% increase in tensile bonding strength (pull-off test) compared to ordinary-Portland-cement-based samples.

5.2. Geopolymers as a Restoration Material

ICOMOS (2003) explained that restoration is carried out to preserve and reveal the aesthetic value of historic structures. According to Ricciotti et al. [4], characteristics such as high workability, chemical compatibility, and good thixotropic behavior are needed in restoration. They also highlighted that factors such as chemical interaction with the substrate, interface behavior, consistency, penetration capacity, workability, good mechanical properties, low creeping and shrinkage, chemical and thermal resistance, similar aesthetic features, and adaptation to the masonry movements might be used in choosing the restoration material.

Ricciotti et al. [4] also mentioned that adding pigment to the geopolymer slurry can be carried out to change its colors to match that color of the surface it will be applied on; therefore, it is also essential to consider the chemical compatibility of restoration materials with pigments that will be used. About 211 results are collected when the phrase used in the search is “geopolymers as a restoration material”. However, only four research papers appeared when the words used are “geopolymers used in the restoration of unreinforced masonry”. Out of these four papers, none directly discussed the application of geopolymers as a restoration material for URMs. Being one of the oldest structure types, most historical buildings are made of URMs; with this knowledge, the phrase used in the search was changed to “geopolymers used in the restoration of historical buildings”, with 53 results to obtain more information that can present the potential application of geopolymers in the restoration of URMs.

Allali et al. [102] mentioned that cement-based mortars produce salts that can damage the stone and cause structural problems due to their low flexibility. The preliminary study by Occhipinti et al. [103] pointed out the evidence of conservation problems in many historical buildings and archeological remains, thus needing modern techniques and materials to restore historic constructions where a crucial preservation criterion must be followed. Their study presented pumice-based geopolymer as a restoration material. They noted that to guarantee the chemico-physical and mechanical affinity in the field of restoration, there must be a selection of products with compatible characteristics with the original substrate. They further say that this choice to complement the original substrate provided a respectful authenticity to the restoration of the ancient material. Furthermore, it was also noted that the material showed higher deformability and lower final strength than most natural rocks. This is notable, as restoration materials must be less rigid and robust than the original materials to avoid excessive induced stresses on the building [104].

Moutinho et al. [104] studied the potential of metakaolin-based geopolymers in conserving tile facades by filling gaps/lacunae in glazed ceramic tiles, a characteristic element of Portuguese architectural heritage. The researchers found that the geopolymer samples have favorable characteristics, making them fitted in bonding ceramics fragments and filling gaps in old ceramic tiles such as resistance to changes in temperature and humidity and compact structure or fewer pores. However, they note that there is a need to study the physical properties further, focusing on optimizing adhesion between ceramic bodies and geopolymers. In deciding which material to use in restoration, the leachability property should also be considered. Kohout et al. [105] observed that the metakaolinite-rich materials exhibited a leachable characteristic independent of the kaolinitic claystone. There was no formation of dense non-filtrable gel, even after weeks, for the sample with an s/L ratio of 0.4, while Al and Si concentrations increased in just the first couple of minutes of leaching for the sample with an s/L ratio of 0.65.

6. Geopolymers Used as Grouting

Güllü et al. [106] had a study with the rheology (flow) and the strong performance of cold-bonded geopolymer used for grouting and deep mixing. Deep mixing is usually used as a method of strengthening undisturbed soil on surface grounds. It is carried out by injecting cementitious grouts that fill voids, making the soil behave similarly to a soft rock. The cold-bonded geopolymer used in the study is made of stabilizers such as limestone dust and bottom ash. The high unconfined compressive strength (UCS) obtained by the 10% bottom-ash-based geopolymer replacement to the cement-based grout is influenced by its rheology that has good workability resulting in fewer pores. Another study by Güllü et al. [107] assessed the rheology and strength of metakaolin-based and slag-based geopolymer for grouting. It is observed that the higher replacements of metakaolin to the mix did not have flow responses, making them inappropriate to be used as grout. It also followed the same amount replacement of 10% to provide higher UCS.

In another application of grout, the binding mortar between building blocks that are stacked to form the walls of a building must be considered when strengthening structures. Through the injection method, Murat Maras [89] presented a study by geopolymer grout applied to masonry walls. Two factors are to be considered when using grout injection: (1) viscosity—a thicker consistency is used to cover the larger cracks and protect the lower viscosity geopolymer injection on falling off the wall—and (2) mechanical properties, as the grout is used not only to fill in gaps but to strengthen the overall performance of the wall. Lower-viscosity grout is used, as it can quickly enter small cracks to fill in the wall gaps.

Various stabilizers are used for geopolymer grouts and deep mixing in a study by Canakci et al. [108]. The study compares the performances of fly ash (FA)-, slag (SL)-, glass powder (GP)-, marble powder (MP)-, bottom ash (BA)-, rice husk ash (RHA)-, silica fume (SF)-, and metakaolin (MK)-based geopolymer grout for ground enhancement. It is observed that the unconfined compressive strength (UCS) responses of FA-, GP-, MK- and BA-based geopolymer grouts are lower than of cement-based grout. Slag-based geopolymer grouts gave the most favorable UCS response, followed by grouts with silica fume addition. Increasing stabilizer proportions up to 25% increases UCS and curing time. In a separate study, Canakci et al. [109] also stated that as the amount of glass powder in the grout increases, the setting times of grout pastes also increase. Optimum inclusion of 3% glass powder has been observed to have better UCS compared to cement-based grouts.

7. Sustainability of Geopolymers in Unreinforced Masonry Structures (URMs)

Historical structures are commonly built using unreinforced masonry because Portland-cement-based concrete was only invented and used in the 19th century and established its market dominance in the early 20th century [5]. Masonry buildings are made of units (cast stone, clay masonry units, concrete masonry units, natural stone, terra cotta, adobe, pisé de terre, and cobb) laid in mortar [5]. These materials have very low tensile strength and are intended to withstand compression loading only [6]. According to ICOMOS (2003), crushing, buckling, and brittle failure are among the most common causes of damage for masonry structures. With the goal of preservation of the historic fabric of a masonry building, ASTM C1713-17 laid out the Specification for Mortars for the Repair of Historic Masonry. To prevent collapse due to compression and for early detection of damage, a properly designed repointing mortar is recommended [5]. Alkali-activated aluminosilicates can be used to replace the cementitious component for the repointing of masonry after a rigorous characterization of the material properties of the existing mortar. Optimizing the sustainable use of fly ash for alkali-activated aluminosilicates or geopolymers is one of the projects that the industry can carry out as one of the primary stakeholders [110]. ASTM C1713-17 emphasizes the importance of matching the material properties of the repair material to the existing mortar to not induce further damage to the structure. Water retention, air content, total porosity, water vapor permeability, minimum compressive strength, maximum compressive strength, flexural bond strength, and absorption rate are among the physical and mechanical properties recommended by the standard [5].

The unreinforced masonry building’s low tensile capacity makes it vulnerable to earthquake-induced loads (in-plane lateral loads or out-of-plane or eccentric loads), which may cause diagonal cracks, sliding, or the rotation of a wall [6]. Surface treatments, shotcrete, grout/epoxy injection, repointing, external reinforcement, glass FRPs, junction strengthening, confinement, and mesh reinforcements are among the existing retrofitting technologies for URM [111]. However, the materials used in some of these technologies and the waste produced at its end-of-life renders the solutions unsustainable.

Recent studies also recommend the use of fabric-reinforced cementitious matrix (FRCM) for strengthening of masonry walls against out-of-plane loading [7,111] and in-plane loading [7,94]. After an extensive analysis of the structural behavior of the wall and the mechanical properties of FRCM (flexural capacity, bond strength, and crushing capacity), FRCM composite has been shown as an effective retrofit measure by applying it on the wall surface [111]. Geopolymer can also replace the cementitious component in the FRCM composite, as reported by Longo et al. [112], and provide adequate thermal insulating capacity as well.

8. Fiber-Reinforced Geopolymers (FRG) Used as Unreinforced Masonry Structures (URMs)

Reinforcing with fibers helps in controlling the drying shrinkage of geopolymers [101]. Guo et al. [113] mentioned that the incorporation of fibers in geopolymers could also reduce cracks. Guo et al. [114] observed that the addition of fibers increases the specific surface area of the pores, while decreasing the average pore diameter, and the smaller the fiber, the higher the decrease in the surface area of the pores. The greater the pore complexity, the better the mechanical properties of the geopolymer [114].

The addition of fibers in a geopolymer matrix can reduce the propagation of microcracks. A study by Wang et al. [115] explains the mechanism between the fibers and the matrix and implies that the fibers act as a bridge to sustain and transfer energy absorbed by the matrix. The interfacial bond between the surface of the fiber and the matrix contributes mainly to the branching effect and crack deflection that will prevent crack localization along a path.

Fiber-reinforced geopolymers (FRG) can be used to strengthen and restore URMs; however, there are few studies on this subject matter. When the phrase “fiber-reinforced geopolymers” is used in the search, about 1571 search results can be seen. However, when the search phrase is made more specific and changed to “fiber-reinforced geopolymers used in unreinforced masonry structures”, only 44 results can be seen. Out of these 44 results, only one research paper, the study by Tamburini et al. [99], substantially discussed the use of FRG in URMs, as previously mentioned. Geopolymer grout is reinforced using fiber meshes and steel fabrics; the researchers observed that the reinforced geopolymer grouts have excellent potential to strengthen existing masonry structures [112]. In the following sections of this paper, different kinds of fibers are used in reinforcing geopolymers. Since only one research paper is collected about the specific use of FRG in URMs, the next subsections present the various fiber types—natural fibers, synthetic polymers, metallic fibers, and other inorganic and organic fibers—used in making FRG, which can give light to the potential use of FRG in URMs in future studies.

8.1. Natural Fibers

Natural fibers promote a more sustainable and environmentally friendly material production. Camargo et al. [116] presented recent developments on using plant-derived natural fibers as organic fiber reinforcement in cement and geopolymer composites. Fibers can be derived from the different parts of the plant, as shown in Figure 16. Silva et al. [17] presented jute fiber reinforcement in a geopolymer mortar with no variation in fiber length and variable fiber content (0.5%, 1.0%, 1.5%, and 2% of weight of the specimen). The results showed that an optimum jute fiber content of 1.5% attained good results for compressive and splitting tensile tests. The fiber content optimization, however, differs from one fiber to another. The study by Abbass et al. [117] showed that 0.2% by weight of coconut fibers increased the compressive strength of fly-ash- and slag-based geopolymer by 5.13% but declined as the fiber content increased.

Challenges can occur when using natural fibers on cementitious matrices. One example is natural fibers having smooth surface areas that lead to poor adhesion to the geopolymer. In a study by Huang et al. [118], weak bond strength is observed between the untreated rice straw fibers and the slag-based geopolymer matrix. The alkali-treated rice straw, treated mainly with NaOH for 4 h, helped increase the surface area for bonding resulting in around 4 MPa of additional strength in bending. To further increase the fiber’s surface that will be in contact with the geopolymer matrix, pre-treatment and treatment procedures can also be done. Pre-treatment includes water run-off for several days to remove dirt or starch and alkali treatment to induce rougher surfaces for better adhesion and bonding between the fiber and matrix [119]. The alkali treatment comprises soaking the fiber in NaOH solution to strip off unwanted waxes, hemicellulose, and lignin. Other treatments that can be carried out are chemical coupling, oxidation, plasma, ultrasound, and enzyme treatments [120]. Another challenge is the even distribution of fibers along the volume of the specimen.

In terms of acid resistance, Ribeiro et al. [121] investigated the resulting behavior of unreinforced geopolymer and bamboo-fiber-reinforced geopolymer (GPBF) immersed in sulfuric or hydrochloric acid solutions. It is observed that GPBF showed higher compressive strength degradation when exposed to sulfuric acid with a decrease of 2 MPa in strength after 28 days of exposure. However, the resulting strength still complies with strength requirements for wastewater tanks and sewage systems that usually have <15% sulfuric acid environments.

8.2. Synthetic Polymers

Noushini et al. [122] determined the effect of monofilament and fibrillated polypropylene fibers and monofilament structural polyolefin on the mechanical properties of the fly-ash-based geopolymer. The group observed a slight reduction in the compressive strength of all the fiber-reinforced composites by around 1–7% for polypropylene (PP) fiber-reinforced composites and a significant decrease in polyolefin-reinforced samples. The decreased mechanical strength is attributed to the voids introduced to the concrete upon the addition of the fiber. Zhong et al. [123] studied the effect of replacing polyvinyl alcohol (PVA) fibers with recycled tire polymer (RTP) fibers on the mechanical properties of the fly-ash- and slag-based strain hardening geopolymer composite. Their results indicate that the addition of RTP fibers lowered the compressive strength and the uniaxial tensile behavior. On the other hand, the addition of 1.5% and 2.0% PVA resulted in a 14.49% and 24.94% reduction in compressive strength because of the more entrapped air that reduced the compactness of the geopolymer when fibers are added.

Kheradmand et al. [124] reinforced fly-ash-based geopolymeric mortars with short hybrid polymeric fibers (SHPF) and observed increased flexural strength but reduced compressive strength and flexural stiffness. The addition of SHPF did not influence the 28-day compressive strength, but a minimal reduction is observed after 7 and 14 days of curing. This behavior called the crack-arresting capacity of fibers, highest at 0.8% SHPF, served as evidence that the increased porosity after 7 and 14 days of curing is later reduced when the binder hardened after 28 days of curing.

Geopolymers with high porosity can be reinforced by low viscosity polymeric resin. In the study by Fiset et al. [125], the effect of the incorporation of unsaturated orthophtalic polyester resin to highly porous metakaolin-based geopolymers is investigated. The group introduced open pores to the metakaolin-based geopolymers using hydrogen peroxide and saponified canola oil, followed by the polyester resin that polymerizes together with the geopolymeric framework. They observed that the mechanical properties of the reinforced geopolymers are significantly enhanced by 12–40× for compressive strength and 2× for ductility. For Shaikh [91], recycled polyethylene terephthalate (PET) fiber is used as the reinforcing material for cement–fly ash (CFA) geopolymer mortar. They observed that PET fiber-reinforced geopolymer has a higher compressive strength than cement and CFA composites. However, composites with polypropylene fiber exhibited higher compressive strength than composites reinforced with PET fiber. Still, a reduction in strength is observed when the amount of both fibers is increased from 1% to 1.5%.

Haddaji et al. [126] used a combination of polypropylene (PP) fiber and glass fiber as reinforcing materials to metakaolin-based geopolymer. They observed that the flexural strength of the composites increases with the increasing amount of the fibers, with 1% as the maximum amount. The resulting geopolymer reinforced with PP fiber also exhibited high ductility and good fracture mode. Catauro et al. [127] used polyethylene glycol (PEG) as reinforcement to metakaolin-based geopolymer. They found that 3% and 6% PEG resulted in an increase in flexural strength, which also increases with curing time. In the study by Chen et al. [128], polyethylene glycol (PEG), polyacrylamide (PAM), and sodium polyacrylate (PAAS) are used to reinforce the slag-based geopolymer. The group observed that the bending toughness coefficient of the reinforced samples is enhanced by around 50% by the incorporation of 0.6 wt.% of PAAS due to the polymers filling the large pores of the samples.

8.3. Metallic Fibers

The addition of metal fibers as reinforcement is a means to improve the material’s flexural strength and to overcome the inherent brittleness of geopolymer concrete [129]. A study by Bernal [130] showed that increasing the fiber loading has a decreasing trend with the mechanical properties of GP slag concrete, except for splitting tensile and flexural strengths. A similar study by Shadnia et al. [131] reveals that steel-fiber-reinforced geopolymer concrete (SFRGC) had superior durability properties than an equivalent grade of conventional concrete. The addition of fibers noticeably enhanced the durability characteristics of geopolymer concrete up to a certain threshold [132]. Similar results are seen upon increasing the length of the loaded fibers. Another study showed that the combination of silica fume and recycled steel fiber influenced the product’s impact resistance and mechanical properties [86]. Compared to polymeric fibers such as PP, steel fibers were seen to have better adhesion forces with the geopolymer paste.

Steel fibers tend to have a hydrophilic property due to their corrugated surface, resulting in better energy absorption and flexural strength without causing any adverse effects to the compressive strength [129]. Abu Obaida et al. [93] used a mix of steel fiber and plastic fibers from waste tires. The strength results for mix designs are acceptable but showed the highest strength when only pure steel fibers were used, possibly due to the contamination of the shredded car tires [133]. Ranjbar et al. [129] mentioned that plastic fibers tend to show more geopolymer shrinkage due to the hydrophobic characteristics of the PPF, mainly caused by its smooth surface. The weaker fiber–GP matrix resulted in lower mechanical performance.

8.4. Other Inorganic and Organic Fibers

In a study conducted by Masi et al. [133], they compared the mechanical and thermal properties of two commonly used fibers, which are PVA and basalt.

Table 4. Physical, mechanical, and thermal behaviors of PVA and basalt fibers.

Fiber	Physical Behavior	Mechanical Behavior	Thermal Behavior	Source
Polyvinyl alcohol (PVA)	1% (v/v) resulted in 40% loss in flowability of geopolymer	Has high ductile behavior and high flexural strength Direct influence on improving flexural strength of geopolymer Did not affect the compressive strength of geopolymer	Flexural strength did not change significantly in the range of 600 °C and 700 °C	[133]
Basalt	1% (v/v) resulted in 26% loss in flowability	Suffers from brittle failure mode even with high tensile and flexural strength Did not influence flexural strength of geopolymer Did not affect the compressive strength of geopolymer	Temperatures of 200 °C and 250 °C caused a decrease in values of flexural strength of 35% and 70%

shows the result of investigating the physical, mechanical, and thermal behaviors of PVA and basalt fibers by Masi et al. [133].

According to Qin et al. [134], geopolymers with high brittleness and poor toughness have limited use as a building material. Their study reinforced metakaolin-based geopolymer with chitosan to improve its mechanical toughness, especially its flexural strength. At 1.6 % chitosan, the flexural strength has reached its maximum value. They have concluded that the mechanical property of the reinforced geopolymer is greatly affected by the concentration of the alkali activator. In the study conducted by Sathanandam et al. [135], glass fibers are added to increase the split-tensile, flexural, and compressive strength of geopolymers; they concluded that too many glass fibers decreased the samples’ flexural strength, with 0.3% of glass fibers producing the best result. A mixture of metakaolin and slag is used to make the geopolymer in the study conducted by Sittinun et al. [136]. This geopolymer mix is combined with different kinds of organic polymers (sodium polyacrylate, polytetrafluoroethylene, and polyurethane) to make organic–inorganic hybrid composite films as flame-retardant coatings. The specimens are found to be thermally stable up to 200 °C, which shows that the fiber-reinforced metakaolin–slag mix has a flame-retardant property.

9. Conclusions

Unreinforced masonry structures (URMs) have a low tensile capacity, making them susceptible to destruction by earthquake loads; it is recommended to strengthen and restore them using engineering cementitious composites (ECC). ECC is usually made of ordinary Portland cement (OPC), in which the production contributes a high carbon footprint, therefore causing several environmental problems. Thus, the OPC component of ECC should be replaced with more sustainable and environmentally friendly material. This review showed the potential of geopolymers, a more environmentally friendly material, to replace OPC as the matrix part of ECC. As reported in this review, the strength and durability of geopolymers are on par with OPC; however, this review article also showed very few studies on the use of geopolymer-based ECC in the strengthening and restoration of URMs. This review recommends that more aspects of geopolymers—chemical interaction with the substrate, workability, thixotropic behavior, and aesthetic features—be studied further before using geopolymers as a restoration material for URMs. In addition, the only application of geopolymer-based ECC that has been studied is its application in grouting injections; other applications can be explored, such as in deep repointing, jacketing, and cement-plastering of URMs. Future studies can also consider the development of geopolymers that can match the properties of the substrate mortars to prevent further damage to the structures.