Soil Stabilization Using Precipitated Calcium Carbonate (PCC) Derived from Sugar Beet Waste

Abstract

The objective of this research is to examine the use of precipitated calcium carbonate (PCC), obtained during the production of sugar from sugar beets, and to stabilize subgrades beneath highway pavements or to stabilize foundations built on loess (windblown silt). The research also aims to permanently capture the carbon from PCC in soil. The experimental process involved the collection of representative loess samples, the addition of variable percentages of PCC, and conducting laboratory experiments on compacted PCC soil mixes to evaluate the effect of PCC on subgrades beneath pavement and foundations beneath buildings. Samples of PCC were obtained from the Amalgamated Sugar Corporation, located 187 km away from Pocatello. In addition, soil was collected from local sources in which saturation collapse and damage have occurred in the past. Unconfined compressive strength tests, which index subgrade bearing failures, were performed on both untreated and PCC-treated soils to evaluate the effect of PCC in stabilizing pavement subgrades and foundations as well as sequestering carbon. The experimental test results revealed a significant average increase of 10% to 28% in the strength of loess samples stabilized with 5% PCC compared to the native soil. The chemical composition and microstructure of PCC were further analyzed through energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) tests. EDX analysis unveiled a carbon content of 9% by weight in PCC, which could contribute to the carbon footprint when it breaks down. Additionally, SEM images displayed an unsymmetrical and sub-rounded microstructure of PCC particles. Based on these findings, the study suggests that utilizing PCC could improve the resistance of loess to saturation collapse and potentially reduce carbon emissions associated with cement or lime production while offering an opportunity to use PCC in soil application.

1. Introduction

1.1. Soil Modification/Stabilization

Two general methods to improve soil performance are soil modification and soil stabilization, which are carried out using calcium-based materials such as Portland cement, quicklime, and hydrated lime [1]. The initial step for soil stabilization is soil modification, which means altering the physical properties of the soil (for instance, reduction in water content) with or without using additives (such as lime) to improve the engineering properties of fill and create a stable working surface during construction [2]. On the other hand, soil stabilization is achieved when soil containing proper quantities of binders (cement/lime) has cured long enough to allow the pozzolanic reactions to bind the soil particles [1,3].

In soil stabilization, Portland cement is used for granular materials such as sand and fine materials such as silt. Certain types of cement have the potential to increase the strength of soil–cement mix in significant amounts, and in some instances produce a fivefold increase in compressive strength. Types I and III Portland cement are typically used in geotechnical engineering applications [4]. The advantages of using cement are to improve compressibility, durability, intact strength, stiffness, elastic modulus, resistance to freeze and thaw, fracture, fatigue, and moisture degradation, as well as to reduce swelling, permeability, plasticity, and settlement of structures [5,6,7,8]. In addition, cement helps to accelerate construction schedules by strengthening weak soils (such as loess or windblown silt) and reducing soil excavation depths down to a stronger stratum. The use of the appropriate type of cement is based on soil cohesion and the environmental conditions, including the amount of water, impurities, specific surface area, and organic matter present in the soil [4]. The presence of organic matter, sulfates, clay minerals, carbonates, and silica–alumina, as well as the pH of the soil, the natural drainage, and weathering conditions of the soil, have a significant effect on soil stabilization [9,10]. To reduce the use of cement, lime is commonly used in soil stabilization applications.

Lime is used in soil modification to decrease the plasticity and swelling behavior of clay soils [11]. Calcium cations from lime are exchanged with sodium cations of clay to stabilize the space between the silica tetrahedra. Moreover, the calcium ions in the lime react with aluminates and silicates in clays and form a calcium silicate or calcium silicate hydrate (CSH), which is very important in soil–lime bonding [3,12]. A lime content of 4% to 6% is typically used for civil works projects [13]. However, large amounts of lime are needed for most adverse conditions. When lime is properly blended to a suitable depth, it can increase the soil resilient modulus of fine-grained soils by a factor of 10 or more and increase the shear strength by a factor of 20 or more [14]. Furthermore, lime reduces the plasticity index and thus the plastic behavior of clay soil [15,16]. However, lime modification or stabilization is affected by soil and lime type, amounts of lime, moist-curing period, and use of fly ash, Portland cement, sodium metasilicate, sodium sulfate, and sodium hydroxide [15]. The impacts of using lime for soil stabilization include a reduction in the pH, nutrient level, microbial activity, and health and safety risks for workers and communities [16]. In addition, lime stabilization should be performed at a temperature above 40 degrees Fahrenheit because lime requires sufficient heat for proper interaction with the soil [3].

From an environmental standpoint, the production of Portland cement and lime generates significant quantities of carbon dioxide. The production of Portland cement alone contributes approximately 7% to global carbon emissions [17]. Both cement and lime manufacturing processes involve heating limestone (CaCO₃) at high temperatures, which releases carbon dioxide (CO₂). There is an emission of about 0.9 kg of carbon dioxide (CO₂) for every kilogram of cement production [18]. Moreover, production of one ton of lime produces about 0.75 tons of CO₂ [19,20]. Quicklime is dusty and is transported as gravel with a grain size equal to or greater than a number 4 sieve. To help mitigate the dust problem, quicklime is mixed with water to make a slurry [16]. Because of carbon emissions from Portland cement and lime, the engineering community is studying new methods and products to stabilize soil. If successful, precipitated calcium carbonate (PCC) could be used instead of Portland cement and lime, which will result in a lower carbon footprint. In the carbon footprint picture, it must be understood that the present production of sugar from beets generates about 0.68 to 0.72 kg of CO₂ emissions per kilogram of sugar beet [21]. These amounts of CO₂ are produced even though PCCs are not utilized for soil stabilization. However, the use of PCC reduces sugar beet waste that is being landfilled and reduces CO₂ emissions by limiting the use of Portland cement and lime.

1.2. Precipitated Calcium Carbonate (PCC)

In the world, about 32% of sugar is produced from sugar beet [22,23,24]. The major contributors to sugar production from sugar beets are the United States (US), China, Ukraine, Egypt, and Turkey. In the US, an average individual consumes 126.4 g of sugar daily, which proves that a large quantity of sugar is consumed and substantial amounts of waste products from the sugar beet industry are generated [24]. Various studies mention that nanoplatelets of PCC can improve the mechanical properties of concrete [25]. Nanoplatelets are microscopic nanoparticles that have angular and irregular shapes. It is believed that the presence of nanoparticles in PCC also helps to stabilize soil.

There has been no research found to date (2023) on utilizing PCC in soil stabilization. Information on the possible use of PCC as a cementing agent in soil is taken from the concrete industry. Gharieb M. and Rashad A.M. performed studies related to the use of PCC, also known as carbonation lime residue (CLR), in concrete by adding 5% to 25% PCC by weight of Portland cement [23]. The sugar beet waste utilized in a study by Gharieb and Rashad was collected from the Saudi Sugar Company (Jeddah, Saudi Arabia). As per their study, the use of PCC in concrete results in lower density but higher porosity. Gharieb and Rashad also revealed relationships between PCC content and unconfined compressive strength/water absorption [23]. As the amount of PCC increased from 5% to 25%, the compressive strength decreased but the water demand increased to create a workable mix. The highest concrete strengths at PCC contents of 5% by weight cement were essentially the same as in the baseline mix.

The research on concrete conducted by our team found that Portland cement can be replaced by PCC up to 30% by weight while still maintaining the compressive strength of concrete above 27.6 MPa (4000 psi) [26,27]. However, there is a decreasing trend in compressive strength as the amount of PCC content increases, as shown in Figure 1. It was challenging for the ISU team to ascertain whether PCC had a binding effect on concrete. Thus, this study serves as supporting research for the concrete research mentioned in [26,27].

1.3. Problem Statements

• Portland cement and lime can be used to stabilize loess, however, both cement and lime contribute to carbon emissions and are costly. Thus, alternative materials that can replace Portland cement and lime need to be discovered. PCC is one such material that helps to utilize the waste product as well as help to replace portions of Portland cement or lime.
• Loess beneath the pavement and foundation in Southeast Idaho is susceptible to collapse during saturation. Loess has poor bearing capacity because of a lack of cohesion and does not have the strength to support foundations. Thus, these types of soil need to be stabilized before construction.

1.4. Research Gaps and Motivation

There are a limited number of products used in the soil modification and stabilization process. Portland cement and lime (calcium oxide or hydroxide) are used in many applications. Fly ash is one of the supplementary cementitious materials (SCMs) added to reduce lime/cement usage. However, fly ash does not provide the level of bond strength compared to Portland cement and lime. There is a very limited amount of research using locally available waste materials such as PCC. The beneficial effect of using PCC is not only cost but also sequestering carbon, which does not occur in the cement industry. PCC is available locally (within 120 miles of Pocatello, ID 83209, USA) at no cost other than transportation to the job site.

This research marks progress towards the goal of attaining sustainability within geotechnical engineering by aiming to support the reduction of carbon emissions to net zero. Net zero refers to reducing carbon emissions as much as possible to create a balance between emissions and the removal of carbon dioxide [27,28]. Approximately 70 countries have set a common goal of achieving net zero by 2050 [29]. The preliminary target for achieving net zero planned was set by the Paris Agreement by reducing 45% of carbon emissions by the year 2030. The major players in reducing carbon emissions include the European Union and the United States. Over 1000 cities/educational institutions and over 400 financial institutions are also closely working to support the goal of net zero [29]. Net zero carbon emission can be facilitated if PCC can be substituted for Portland cement and/or lime in soil stabilization applications.

1.5. Research Objectives and Scope

The objectives of this research are summarized in the following points.

• To determine an alternative material, which can replace Portland cement or lime.
• To reduce the carbon dioxide emissions from cement and lime by substituting them with waste products generated from the sugar beet industry.
• To determine the optimum amounts of PCC required to stabilize weak soil found in subgrades beneath the pavement and below the foundation of the building.

The research on PCC stabilization of loess subgrades by the Idaho State University (ISU) team was conducted at the engineering laboratory at ISU. The transportation and geotechnical engineering group at ISU have studied this problem for more than 20 years and has instituted research programs in soil stabilization, including the use of PCC, to improve pavement life and reduce transportation costs in the state. The initial phase of this study involved identifying the chemical elements present in PCC. The second phase involved evaluating the unconfined compressive strength on compacted soil samples mixed with different percentages of PCC to evaluate its effectiveness in stabilizing subgrades. If left untreated and exposed to water, compacted windblown silt subgrades experience strength loss, resulting in significant pavement damage, which ultimately reduces the life span of the pavement. In this project, samples were moist-cured for 1 h, 7 days, and 28 days and tested to evaluate whether any strength was gained as a function of increasing time.

2. Materials and Methods

2.1. Soil, PCC, Portland Cement, and Lime Collection

Loess (see Figure 2a) samples were collected from the basement of Colonial Hall at ISU in Pocatello, ID, United States. The Colonial Hall building was built in 1910 and has undergone settlement (seismically induced saturation collapse) of more than 7 in during the past 110 years [30]. The soil collected for this study allows us to properly investigate the binding effect of PCC in the weak soil, which is susceptible to erosion. Loess samples were collected, and samples were taken to the geotechnical laboratory for testing. Unwanted particles like stones and plants from loess were removed with the help of a number 40 sieve. The reason behind removing stone and plant matters was to determine whether PCC could provide proper binding to loess or not. The bucket containing loess was sealed with a lid throughout the investigation period to prevent moisture loss from the sample. Fine-grained soils such as those used in this study are classified based on Atterberg limits tests, which yield moisture contents in the clays and silts that behave as liquids (liquid limit) and semi-solid materials (plastic limit). The soil used in this project has a liquid limit of 20.9%, a plastic limit of 21.61%, and is non-plastic. The liquid and plastic limits were determined by following the test procedures given in ASTM D4318 [31,32]. The soil is classified as an elastic silt (ML) in the Unified Soil Classification System [33,34]. In the AASHTO classification system, the loess is classified as A-4 (0).

PCC used in this study was collected from the Amalgamated Sugar Corporation in Twin Falls, ID 83211, USA. PCC samples (see Figure 2b) were taken from stockpiles and ranged from nanosized to 1.5 cm. The large PCC particles were ground to provide the proper binding between soil samples and the PCC. The Amalgamated Sugar Corporation’s sugar factory is the nearest and is only 187 km from ISU. Like the loess sample collection, PCC was collected in buckets with the help of a shovel and the bucket was sealed with a lid to prevent moisture loess from PCC.

Various proportions of PCC or Portland cement/lime (2.5% to 50% by dry weight of loess) and about 14% to 16% of water by dry weight of loess were added to prepare the soil cylinder samples. The native loess used in this study effervesces in 10 normal hydrochloric acids because of the presence of calcium carbonate, which is also a cementing agent in soil. The baseline mix using Portland cement (see Figure 2c) was produced by the Ash Grove Cement Company, and type S hydrated lime (see Figure 2d) was produced by Chem-star (Chemical Lime Company, USA). Both the Portland cement and type S hydrated lime were purchased from the local Home Depot.

2.2. Sieve Analysis of Project Materials: Loess and PCC

The sieve analysis of the loess was performed using ASTM D422 [35,36] (see

Table 1. Sieve analysis [27].

Sieve Size	Sieve Size (mm)	Per Cent Passing
		Loess	PCC
3/8 in.	9.5	100	95.26
No. 4	4.76	98.55	90.03
No. 10	2	95	82.94
No. 40	0.42	90.44	67.69
No. 100	0.149	84.62	47.45
No. 200	0.074	78.22	33.93

). The sieve analysis of loess showed passing proportions of 100%, 98.43%, 94.93%, 90.40%, 84.49%, and 78.37% for 3/8 in, number 40, number 4, number 10, number 100, and number 200 sieves, respectively. This indicates that loess samples are mostly finer than the number 40 sieve. On the other hand, the sieve analysis of oven-dried PCC had passing proportions of 95.26%, 90.03%, 82.94%, 67.69%, 47.45%, and 33.93% for 3/8 in, number 40, number 4, number 10, number 100, and number 200 sieves, respectively. Sieve analysis of loess and PCC showed that PCC is coarser than loess samples utilized in this study.

2.3. Sample Preparation

The loess was prepared by first sieving the silt through a number 40 sieve to remove the unwanted materials and gravel-sized particles. PCC proportions of 2.5%, 5%, 7.5%, 25% and 50% of 491 g of dry weight of loess were mixed with the soil. Samples were prepared by adding 14% to 16% water (by dry weight of loess) to meet the required pre-compaction water content. The soil mix was thoroughly blended to obtain a uniform moisture content and then compacted using a Harvard miniature compaction device (see Figure 3b). The soils were compacted in the Harvard device using the following procedures.

• Lubricate mold (see Figure 3b) with form oil to facilitate sample extraction.
• Add soil mix to the mold in one-third height increments.
• Compact 25 times for each lift using the standard Harvard miniature tamper (see Figure 3c) to ensure proper compaction.
• Remove mold from the clamp, spacer disk and collar, then insert the sample into the ejector.
• Push the sample out of the mold with an ejector (see Figure 3d). Press the piston down firmly to push the soil sample out of the mold (see Figure 3e).

The effects of PCC on stabilizing loess subgrades were evaluated by performing unconfined compressive strength tests on samples containing varying amounts of PCC including 2.5% to 50% by dry weight of loess. The baseline soil mix was prepared by mixing soil with Portland cement/lime (5 and 25% by dry weight of loess) with local collapsible soils at the same water content of 14% to 16% by dry weight of loess. The natural water content (weight water divided by weight solids) in the loess was typically 2.7% (2% to 3%), while the water content in the PCC samples was about 27%. Free water was present on the bucket lid. After preparing the cylindrical soil samples with typical dimensions (

Table 2. Dimensions for compacted loess and PCC/Portland cement samples [27].

Diameter (cm)	Height (cm)	Area (cm2)	Area (m2)	Volume (m3)
3.175	7.3025	7.916	7.92 × 10−4	5.78 × 10−5

), unconfined compression strength tests were carried out to determine the optimum amount of PCC required for soil stabilization.

2.4. Unconfined Compressive Strength Tests

Unconfined compressive strength tests are used as an indicator of the load-bearing capacity of soil. This is also known as uniaxial compressive strength, and refers to the ability of a sample to tolerate axial forces [37]. In this study, unconfined compressive strength tests were conducted on compacted cylindrical samples consisting of loess and loess mixed with PCC/Portland cement/type S hydrate lime following the guidelines from ASTM D 2166-06 [38]. Once the soil cylinder sample was prepared, the weight of the samples was recorded to determine the unit weight of soil samples and correlate the unit weight with compressive strength. The prepared soil samples were then placed in a compression testing machine—Durham Geo Slope E-40520—as illustrated in Figure 4. Compression tests were conducted at approximately 1 h, 7 days, and 28 days after mixing. These tests aimed to find the optimum PCC content in stabilizing loess, to permanently trap the carbon from PCC into loess, and to reduce the use of Portland cement, as well as reduce the use of type S hydrated lime. Companion tests were performed on soil mixed with Portland cement/type S hydrated lime for comparison purposes with the PCC test results. The maximum load and deflection at failure were recorded and the compressive strength of samples was determined using Equation (1):

$$f_c^{\prime }=\frac{P_{max}}{A_c}$$

(1)

where $f_c^{\prime }$ = unconfined compressive strength (psi); P_max = maximum applied load until failure (N); and A_c = corrected cross-sectional area of soil cylinder for radial distortion (mm²).

3. Results

3.1. Composition of PCC

X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) analyses conducted on the precipitated calcium carbonate (PCC) samples provided valuable insights into their composition and microstructure. The combination of XRD, EDX, and SEM analyses offers a comprehensive understanding of PCC, allowing researchers and practitioners to make informed decisions regarding its use in various applications, including those related to carbon sequestration. XRD is a technique that reveals the crystallographic structure of a material. In the case of PCC, the XRD analysis showed a higher concentration of calcium carbonate, specifically calcite (CaCO₃), and quartz (SiO₂). This information is crucial for understanding the fundamental mineralogical composition of PCC. It is important to note that XRD provides information about crystal structures, but not specific element concentrations.

Energy-dispersive X-ray spectroscopy (EDX) analysis delved deeper into the elemental composition of PCC. The EDX spectrum displayed X-ray intensity at different energy levels, with higher peaks indicating more intense X-ray emissions and consequently higher concentrations of the corresponding elements. The analysis revealed that PCC contains significant amounts of calcium (45.9% by total weight of PCC), oxygen (39.4%), and carbon (9%). This detailed elemental information is crucial for understanding the chemical makeup of PCC and its potential applications.

SEM analysis provided a high-resolution visual representation of PCC’s surface (see Figure 5). The SEM image displays irregular and partially rounded microstructures of PCC particles, offering insights into its microscopic physical features. The significance of these findings lies in the comprehensive characterization of PCC, which is essential for various applications. For instance, the high concentration of calcite identified through XRD confirms PCC’s calcium carbonate content. The elemental composition data from EDX is valuable for applications such as soil–cement use, where the carbon content (9%) can contribute to carbon sequestration. Moreover, SEM images aid in understanding the physical morphology of PCC particles, which is crucial in applications in which particle shape influences performance.

3.2. Unconfined Compressive Strength Test Results

The compressive strength tests were conducted on compacted soil samples including native loess, loess with variable percentages of PCC, loess with different amounts of Portland cement, and loess with various amounts of type S hydrated lime. All samples were prepared at moisture contents varying from 14% to 16% by total solid weight. The samples were moist-cured above a water bath before undergoing the compression tests. Moist-curing is defined as samples being placed in a water bath above the water enclosed in a container to prevent moisture loss and to hydrate the samples. This type of curing method is different from typical hydration methods in which samples are placed in the water.

3.2.1. Unconfined Compressive Strength Test Results for Loess Mixed with PCC

Unconfined compressive strength tests were performed on compacted samples of loess to determine the effect of adding PCC on the strength of the loess. The samples were moist-cured to normalize the testing procedures used in the mix designs. Unconfined compressive strength tests were conducted at 1 h, 7 days, and 28 days moist-curing time. The overall average compressive strength of the compacted loess itself in the one-hour moist-cured test was 0.144 MPa. There was no evidence of crack formation in the samples after preparation or before performing the tests. In the next series of tests, various proportions of PCC—2.5%, 5%, 7.5%, 25% and 50%—by dry weight of loess were added to the loess. The resulting mixes were compacted and moist-cured for 1 h, 7 and 28 days. The testing sequence in each set began with sample 3 (the final prepared sample) and concluded with sample 1 (the initial prepared sample).

Table 3. Unconfined compressive strength for one-hour moist-curing time [27].

Sample Number	Cement Type	Cement (%)	Maximum Load before Failure (N)	Average Unconfined Compressive Strength (MPa)	Average Effective Unit Weight (N/mm3)	Water Content at the End of Test (%)
1	None	0	120.1	0.0058	3.62 × 10−5	15.3
2			129.0			14.4
3			129.0			14.5
1	PCC	2.5	111.2	0.0058	3.60 × 10−5	15.9
2			111.2			15.8
3			137.9			17.8
1		5	129.0	0.0056	3.46 × 10−5	13.4
2			142.3			12.9
3			137.9			13.3
1		25	120.1	0.0053	3.30 × 10−5	12.5
2			142.3			12.1
3			137.9			12.4
1		50	142.3	0.0048	3.01 × 10−5	15.4
2			169.0			15.3
3			142.3			12.4

displays the unconfined compressive strength test results of compacted loess with and without PCC for a one-hour moist-curing time.

Table 3 shows that the addition of 2.5% PCC (by dry weight of loess) does not increase the one-hour strength. The average compressive strength of 2.5% PCC blended with loess was 0.139 MPa, whereas the average strength of compacted loess itself was 0.144 MPa in the one-hour test. The lower strength may be related to a higher water content (+2% to +4%) in the PCC–loess sample. When the PCC content was increased from 2.5% to 5%, the early unconfined compressive strength test results showed a slight increase (approximately 0.0144 MPa) in one-hour strength. However, when the PCC content was increased from 5% to 25%, the early unconfined compressive strength did not have a significant gain in one-hour strength. The reason might be associated with shorter moist-curing time, which was not sufficient for pozzolanic reactions to occur or to bind the soil particles. The study was continued by adding 50% PCC to the same field sample. The strength of compacted loess with 50% PCC was higher than the strength of compacted loess with 2.5% to 25% PCC. However, the practical application of adding 50% PCC to a fill to increase the short-term (one-hour) strength is questionable and impractical. The next phase of the study was conducted for 7-day strength using the same procedures as in the first set of tests. Results are displayed in

Table 4. Unconfined compressive strength for 7-day moist-curing time [27].

Sample Number	Cement Type	Cement (%)	Maximum Load before Failure (N)	Average Unconfined Compressive Strength (MPa)	Average Effective Unit Weight (N/mm3)	Water Content at the End of Test (%)
1	None	0	120.1	0.0058	3.60 × 10−5	13.3
2			142.3			13.3
3			106.8			13.1
1	PCC	5	169.0	0.0058	3.59 × 10−5	13.5
2			160.1			14.1
3			164.6			13.3
1		25	155.7	0.0055	3.44 × 10−5	15.4
2			146.8			15.3
3			169.0			15.0
1		50	204.6	0.0048	3.01 × 10−5	15.4
2			226.9			15.3
3			191.3			15.0

.

Based on Table 4, the test results show an increase in the 7-day compressive strength of loess containing 5% PCC (by dry weight of loess). At 25% PCC, the strength was higher than the loess itself, but was essentially the same as with 5% PCC. Like the one-hour tests, there was a significant increase in strength at 50% PCC. It is interesting to note the corresponding decrease in dry unit weight, which is typically inconsistent with unit weight/strength correlations. This appears to mean that the void ratio is higher, but the silt particles are cemented by the PCC and thus are responsible for the increased strength.

To further evaluate the effect of PCC on loess, the samples were moist-cured for 28 days following the strategy used in concrete, because concrete gains more than 95% of its strength when hydrated for 28 days.

Table 5. Unconfined compressive strength for 28-day moist-curing time [27].

Sample Number	Cement Type	Cement (%)	Maximum Load before Failure (N)	Average Unconfined Compressive Strength (MPa)	Average Effective Unit Weight (N/mm3)	Water Content at the End of Test (%)
1	None	0	97.9	0.13	1.86 × 10−5	15.3
2			93.4			15.5
3			124.5			14.6
1	PCC	5	120.1	0.16	1.83 × 10−5	13.7
2			155.7			14.1
3			129.0			13.6
1		25	160.1	0.16	1.63 × 10−5	12.6
2			146.8			12.3
3			115.7			13.6
1		50	191.3	0.20	1.60 × 10−5	19.2
2			182.4			17.7
3			173.5			18.1

presents the unconfined compressive strength, unit weight and water content of PCC–loess samples at 28 days. The tests were performed late in the investigation on samples that may be significantly different from the ones used in the 1 h and 7-day tests.

Based on the test results, the 28-day compressive strength increased with PCC like the results with lesser curing times. The difference in strength between the 7- and 28-day tests appears to be related to the lower water content in the 28-day samples. The lower water content may have a significant impact on the pozzolanic effect of the PCC. Further study on this subject is recommended. To provide easy visualization for comparison purposes, the unconfined compressive tests at the 1 h, 7-day, and 28-day tests are plotted in the graph in Figure 6. The test results clearly show the expected strength gain with increasing time. Based on the minimal standard deviation error observed for compressive strength test results for the soil mixes, ranging from 10⁻³ to 10⁻⁴ MPa, standard deviations are not shown in the graph.

Based on Figure 6, the unconfined compressive strength of loess containing 50% PCC is higher than soil containing 2.5% to 25% PCC by dry weight of loess. However, the loess containing 5% PCC shows a gain of strength for 1 h, 7 and 28 days compared to loess samples without PCC. Although PCC provides strength, it is not sufficient for soil stabilization. Moreover, the addition of PCC does not show any strength gain with increasing moist-curing time. Thus, this study further focused on attempting to increase the pozzolanic activity of PCC by drying and grinding.

3.2.2. Use of Dry Ground PCC

Fine powder has more surface area and is usually more reactive than coarser material. To increase the pozzolanic activity of PCC, the PCC was dried and sieved through a number 200 sieve. The proportions of dry PCC utilized for the analysis were 5%, 25%, and 50% by dry weight of loess. Like normal PCC, the dry ground PCC mixed with loess was tested after moist-curing for 1 h, 7 days, and 28 days. Figure 7 displays the one-hour unconfined compressive strength test results for the dry PCC–loess mix. The highest unconfined compressive strength test results for loess were found with the addition of 25% dry ground PCC. The highest strength might be associated with the lower water content.

The seven-day unconfined compressive strength of soil mixed with dry ground PCC was found to be greater at 25% content than at 5% (Figure 7). The loess sample gained strength by the addition of 25% dry ground PCC at 7 days when compared to 1 h moist-curing time. An increase in the strength of loess of about 0.091 MPa was observed with 25% dry ground PCC. However, the strength of PCC with the addition of 5% dry PCC fluctuated: strength decreased at 7 days and increased at 28 days.

To further evaluate the change in strength with finer dry ground PCC powder, compressive strength tests were carried out after curing samples for 28 days, and recorded data are presented in Figure 8. The unconfined compressive strength of loess with 25% dry ground PCC decreased, while loess with 5% dry ground PCC increased at 28 days when compared to 7-day test results. Unlike the significant increase in compressive strength at 7 days, the unconfined compressive strength decreased at 28 days compared to 7 days, but slightly increased at 28 days compared to 1 h. The slight increase in compressive strength signifies that the compressive strength of the soil–PCC mix decreases over time after 7 days. There might be other reasons behind the decrease in strength. Further tests on soil–PCC mixes at 28 days are highly recommended to better understand and evaluate the long-term behavior. However, the compressive strength of loess with 5% dry PCC increases at 1 h, decreases or returns to original strength at 7 days, and again increases at 28 days. The reason behind this might be that for each test time, different soil–PCC mixes were prepared. This also explains that unlike Portland cement or lime, the compressive strength of the soil–PCC sample does not increase significantly over time.

The loess sample containing 25% dry ground PCC by dry weight of loess showed an increase in strength (see Figure 7) compared to normal PCC. This proves that dry ground PCC is more reactive than wet PCC. However, the loess sample containing 5% dry ground PCC did not show a significant increase in strength when compared to normal PCC. Thus, further investigation is needed to discover a technique to increase the pozzolanic activity of PCC.

3.2.3. Modes of Failure

Two modes of failure were observed in all the samples (containing PCC, lime, and Portland cement) at the end of the unconfined compression strength tests. Vertical cracks (extension failure) and diagonal cracks (shear failure) along with combinations of both failure modes were observed in the samples (see Figure 8).

3.2.4. Use of Type S Lime

Lime modification and stabilizations are in practical use for many geotechnical applications. Companion strength tests on loess with and without type S hydrated lime were also performed and are presented in Figure 9. Like the PCC–loess tests, cylindrical samples were prepared for 1 h, 7 days and 28 days. In addition, 5% type S hydrated lime by dry weight was added to the Colonial Hall loess. For comparison, the strength of the compacted native loess was in the range of 0.13 to 0.15 MPa. The tests indicate a one-hour strength increase of roughly 0.0144 MPa, which can be sufficient to increase the bearing of subgrades during construction. Tests at 7 days and 28 days show a significant fivefold and eightfold increase, respectively, during the curing periods.

In the next tests, the type S hydrated lime content was increased from 5% to 25% by dry weight of loess. Test results are provided in Figure 9. The one-hour (short-term) strength increased significantly (1.7 times) with a 20% increase in lime. This result is important in accelerating the placement of fill during construction. However, at 7 and 28 days, the strength of the lime-cemented loess was significantly lower than at the 5% concentration, but also much higher than the loess itself. Tests conducted at Idaho State University demonstrate the beneficial use of 5% type S lime by dry weight of soil in long-term soil stabilization of loess. Proportions above 5% may benefit the placement and stability of loess in fill, but not abide by environmental restrictions. In addition, 5% type S hydrated lime increases the compressive strength more than with the addition of 25% type S hydrated lime to the loess (refer to Figure 9). This is one of the justifications for the usage of type S hydrated lime from 2% to 5% by dry weight of soil.

3.2.5. Portland Cement-Stabilized Loess

Companion tests were performed on soil–Portland cement mixes in which 5% and then 25% cement (by dry weight of loess) were added to the Colonial Hall Loess. Local type I/II Portland cement was used in the experiments. Like loess with PCC and loess with lime, cylindrical samples were moist-cured for 1 h, 7-day and 28-day periods. Uniaxial compression test results are provided in Figure 10.

Based on Figure 10, at 5% type I/II Portland cement by dry weight of loess, the average one-hour unconfined compressive strength of loess–Portland cement samples (0.28 MPa) is much higher than the loess itself and the lime-cemented loess. The average unconfined compressive strength underwent a significant (6-fold) increase at 7 days and a further increase (11-fold) at 28 days. However, the strengths of the Portland cement–loess were much less than the strength of lime–loess at 7 and 28 days. Additional companion tests were performed on loess samples combined with 25% type I/II Portland cement by dry weight of loess to evaluate the differences in gain of strength with the addition of PCC, type S hydrated lime, and Portland cement. The one-hour strength for loess with 25% and 5% Portland cement are essentially the same; however, the long-term strength of the samples with the 25% cement concentration was much higher than those with only 5% Portland cement. The strength of loess with 5% cement increased by approximately 0.13, 1.53, and 3.06 MPa at 1 h, 7 days, and 28 days, respectively. However, the short-term strength (1 h) of loess is higher with 25% cement than with 5% Portland cement. The unconfined compressive strength of loess with 25% type I/II Portland cement is 7.31 and 5.81 MPa, much greater than the strength of loess with 5% type I/II Portland cement at both 7 and 28 days. It is worth noting that the strength of loess containing 25% cement is 1.50 MPa higher at 7 days than at 28 days. However, the compressive strength of loess with 5% Portland cement is higher at 28 days than at 7 days and 1 h. This proves that loess gains strength over time with the addition of 5% Portland cement by dry weight of loess.

4. Discussion

After construction, subgrades built on compacted loess became soft and provided inadequate bearing beneath pavements. This project intended to investigate various alternatives for subgrade stabilization by adding chemicals to the soil before compaction. In the case of precipitated calcium carbonate (PCC), there is an additional benefit of sequestering carbon formed during the manufacture of sugar from sugar beets. The test results show an increase in the compressive strength of loess subgrades with the addition of PCC before compaction. The optimum content of PCC recommended from this study is 5% by weight of dry loess. Within 1 h, the compressive strength, which controls bearing capacity, increased by roughly 10% when PCC was blended with the loess. At 7 days, the soil strength increased 30% above the untreated loess with the addition of 5% PCC. The highest strengths compared with baseline values were achieved at 50% PCC after a 28-day moist-curing time; however, the use of 50% PCC exceeds the practical limit in subgrade construction. Even though the strength values are low for cemented soils (0.14 to 0.19 MPa), the soil cement benefits the project by increasing the resistance to softening and loss of subgrade strength ultimately facilitating construction by lowering the water content.

Subgrade stability in areas with loess deposits can and has been achieved by adding lime and Portland cement to fill materials or cut surfaces. In practice, stabilization is typically accomplished using lime in concentrations up to 5% by weight. According to test results, type S hydrated lime has little to no effect on increasing the short-term (one-hour) strength of compacted loess. However, there was a significant increase in strength at 7 and 28 days. At 25% lime content, the short-term strength almost doubled, which is important in stabilizing wet subgrades, but again is not utilized in practice. Further, the 28-day strength of mixes with 25% lime was lower than samples prepared with 5%. This condition is typical of early high-strength soil cement, in which strength gain is rapid at the start of hydration, but increases at a slower rate than in soils with lower lime content.

Stabilization for fine-grained soils can also be achieved by adding Portland cement before compaction. In practice, concentrations of 5% (by dry weight of soil) are generally used to stabilize subgrades. In this research, the unconfined compressive strength of loess–type I/II Portland cement increased both the short-term (1 h) and long-term (28 days or more) strength of compacted loess. Moreover, the short-term strength increased by 90% and the long-term strength increased by more than 20 times the strength of compacted loess itself. With 25% by weight Portland cement, the short-term compressive strength increased by 70% (which is incrementally lower than the 5% Portland cement mix); however, the long-term strength was 37 times greater than the strength of loess by itself. Again, in soil stabilization practice, the proportion of Portland cement added to stabilize soil is typically 5% or less by weight. In some applications, where Portland cement is needed to reduce the water content of the soil, the proportion has been from 5% to 15% depending on the project types, durations, and situations.

Comparisons of compressive strength values for untreated loess, PCC-cemented loess, lime-cemented loess, and Portland cemented loess at 5% concentration are given in

Table 6. Summary of one-hour compressive strength of loess and 5% PCC–Portland cement of dry weight of loess [27].

Moist Cured Time	Average Unconfined Compressive Strength (MPa)
	Loess	PCC–Loess	Type S Hydrated Lime–Loess	Portland Cement–Loess
1 Hour	0.14	0.16	0.20	0.28
7 Days	0.15	0.19	0.95	1.67
28 Days	0.13	1.57	1.57	3.17

Lower 28-day strength of % PCC explained by the difference in material sources from those in the 1-h and 7-day tests.

. The test results show that there was an increase in the strength of the loess (0.014 to 0.05 MPa) with the addition of 5% PCC. Much higher strengths were achieved with lime and Portland cement, but lacked the beneficial environmental effect of sequestering carbon in the manufacture of sugar.

According to this study, the addition of PCC to loess shows an increase in strength. However, the strength gained by loess by incorporating 2.5% to 25% PCC might not be sufficient for soil stabilization in practice. This is the first attempt to use PCC, a waste product from sugar beets, in the soil to stabilize the weak soil, and further investigation on this subject matter is highly recommended. The following section details the limitations of this study, and it is recommended to analyze those limitations further.

5. Limitations of This Study

This study has various limitations as it was an initial study, and it lacks various means and methods. Some of the known limitations are as follows.

• Variation in water content: PCC samples have a higher water content than loess samples, making it difficult to evenly distribute water in the samples. Uniform water content and unit weight for various samples are not possible to achieve.
• Sample size: PCC samples were coarser than loess, Portland cement, and lime, resulting in less effective outcomes due to less surface area. Drying and grinding processes with an oven and hammer require more time and they are not cost-effective.
• Unknown levels of calcium carbonate: The amount of calcium carbonate contained by various types of PCC is unknown. Samples were collected from different stockpiles and buckets with potential variations in the amounts of calcium carbonate.
• Impurity levels: This study lacks a method to determine the amounts of impurity found in PCC, leaving the presence of various amounts of unwanted particles in various samples.

6. Conclusions

One of the goals of this study was to address the susceptibility of windblown silt (loess) in the near-surface soils in Southeast Idaho to saturation collapse and settlement in compacted loess because of its high void ratio. This study also aimed to utilize waste products from sugar beet production, specifically precipitated calcium carbonate (PCC), to improve the performance of subgrades beneath pavement and foundation. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) provided valuable insight into the chemical and physical properties of PCC. XRD analysis proved the presence of a higher concentration of calcium carbonate/calcite in PCC. Furthermore, EDX analysis provided further details on the elemental composition by weight, showing that PCC contains calcium, oxygen, and carbon of 45.9%, 39.4%, and 9%, respectively, by weight. The scanning electron microscopy (SEM) analysis revealed partially rounded and unsymmetrical microscopic shapes of PCC particles.

This study not only provides information on the utilization of a waste product generated from local sugar production but also shows its potential for sequestering carbon emissions associated with sugar production from beets. Various proportions of PCC (ranging from 2.5% to 50%) were blended with loess to evaluate the optimum concentration. Based on the test results, the addition of 5% PCC by dry weight of loess resulted in a notable increase in unconfined compressive strength, promising for stabilizing collapsible windblown silt. Test results indicated a 10% (short-term) to 28% (long-term) increase in the unconfined compressive strength. When 5% PCC by dry weight of loess is mixed with loess, the resulting strength is 0.01 MPa to 0.05 MPa higher than uncompacted untreated loess. The optimum content to stabilize windblown silt is 5% PCC (by dry weight of soil). The addition of PCC can be used to stabilize collapsible loess subgrades in Southeast Idaho and at the same time sequester carbon for the benefit of the planet. The strength in loess samples with 2.5% and 7.5% PCC had minimal impact on one-hour strength, while the addition of 50% PCC showed a significant increase in strength by 0.09 MPa at 1 h, 0.17 MPa at 7 days, and 0.07 MPa at 28 days. However, using higher percentages of PCC by dry weight of loess is impractical in practice. The use of alternative approaches like drying and grinding PCC and sieving through a number 200 sieve showed enhanced strength at 25%, but is not effective for the cost.

Companion tests were conducted to compare the use of PCC in loess stabilization with lime and Portland cement-stabilized subgrades. A substantial increase in strength was observed at 5% cement concentration, with continued gain at 25%. This shows that the strength of loess with binders should rise with the increase in moist-cured/hydration time. The tests clearly show much higher strengths with the addition of lime and Portland cement, which are used in soil stabilization. However, the increased strength comes at an environmental cost: increased global temperatures and an enhancement of carbon dioxide emissions. The main expense with the use of PCC is the transportation costs. Sources of PCC are within 187 km of Pocatello. However, if the PCC has limitations on stabilization (such as in wet soils), Portland cement and type S lime can be added to the PCC to further strengthen the subgrade. Preliminary tests show that the addition of 2.5% PCC, lime, Portland cement and fly ash yields a soil-mix strength of roughly 10 times that of the PCC alone.

Overall, the results show that incorporating PCC in pavement subgrades or building foundations can provide several benefits, including improving unconfined compressive strength and thus the bearing strength. These studies can provide further information on the potential use of PCC in highways and building applications. A few recommendations are listed below to increase the pozzolanic activity of PCC samples.

7. Recommendations

This is the preliminary study on the use of PCC from sugar beets to stabilize weak soil. There are a few recommendations below for possible uses of PCC in soil.

• The chemical analysis of PCC should be properly studied to identify the differences and similarities between PCC and Portland cement/lime. Amounts of impurities and organic matter in PCC should be identified, if available. In addition, the possible effect of those impurities on soil stabilization should be studied.
• PCC should be combined with other commonly used binders like lime or Portland cement or additives such as fly ash, slag cement, quicklime, silica fume, and other available supplementary cementitious materials (SCMs) to identify if portions of cement or lime can be substituted by PCC and if it can increase the strength of weak soil.
• The lower strength provided by PCC might be due to chemically stable calcium carbonate. The means and method should be developed to extract calcium or calcium oxide from PCC and should be tested to determine if it can provide a binding effect on soil.
• PCC should be added to clay to determine its effect on clay soil.
• It is crucial to assess the water absorption capacity of PCC to ascertain the optimal water quantities required for achieving its maximum binding effectiveness.
• PCC should be used to replace limestone in cement and lime production to find the differences between limestone or calcium carbonate and PCC from sugar beets.
• The California bearing ratio (CBR) test should be conducted to determine the possibility of using PCC to prevent/reduce settlement and bearing capacity of weak soil.