Laboratory Study on Improvement of Expansive Soil by Chemically Induced Calcium Carbonate Precipitation

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Article Laboratory Study on Improvement of Expansive Soil by Chemically Induced Calcium Carbonate Precipitation

Shaoyang Han 1,2, Baotian Wang 1, Marte Gutierrez 2,*, Yibo Shan 3 and Yijiang Zhang 3

1 Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, 1 Xikang Rd., Nanjing 210098, China; [email protected] (S.H.); [email protected] (B.W.) 2 Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, USA 3 Geotechnical Engineering Department, Nanjing Hydraulic Research Institute, 34 Hujuguan Rd., Nanjing 210024, China; [email protected] (Y.S.); [email protected] (Y.Z.) * Correspondence: [email protected]

Abstract: This paper proposes the use of calcium carbonate (CaCO3) precipitation induced by the addition of calcium chloride (CaCl2) and sodium carbonate (Na2CO3) solutions as a procedure to stabilize and improve expansive soil. A set of laboratory tests, including the free swell test, unloaded swelling ratio test, unconﬁned compression test, direct shear test, scanning electron microscopy (SEM) test, cyclic wetting–drying test and laboratory-scale precipitation model test, were performed

under various curing periods to evaluate the performance of the CaCO3 stabilization. It is concluded from the free swell tests and unloaded swelling ratio tests that the addition of CaCl2 and Na2CO3 can profoundly decrease soil expansion potential. The reduction in expansion parameters is primarily attributed to the strong short-term reactions between clay and stabilizers. In addition, the formed Citation: Han, S.; Wang, B.; cementation precipitation can decrease the water adsorption capacity of the clay surface and then Gutierrez, M.; Shan, Y.; Zhang, Y. consequently reduce the expansion potential. The results of unconﬁned compression tests and Laboratory Study on Improvement of direct shear strength tests indicated that the addition of CaCl and Na CO has a major effect on Expansive Soil by Chemically 2 2 3 Induced Calcium Carbonate geotechnical behavior of expansive soils. Based on the SEM analyses, new cementing crystalline Precipitation. Materials 2021, 14, 3372. phases formatted by sequentially mixing CaCl2 and Na2CO3 solutions into expansive soil were found https://doi.org/10.3390/ma14123372 to appear in the pore space, which results in a much denser microstructure. A laboratory-scale model test was conducted, and results demonstrate the effectiveness of the CaCO3 precipitation technique Academic Editors: Angelo in stabilizing the expansive soil procedure. The test results indicated that the concentration of CaCl2 Marcello Tarantino and higher than 22.0% and Na2CO3 higher than 21.2% are needed to satisfactorily stabilize expansive soil. Jacek Tejchman It is proposed to implement the precipitation technique in the ﬁeld by the sequential permeation of

CaCl2 and Na2CO3 solutions into soils in situ. Received: 28 April 2021 Accepted: 15 June 2021 Keywords: expansive soil; precipitation technique; calcium carbonate; soil improvement Published: 18 June 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- iations. Expansive soils are known to exhibit large volume changes due to moisture ﬂuctua- tions [1,2]. These soils expand due to an increase in water content and shrink due to drying. The swelling and shrinkage of expansive soils often induce large swelling pressures that can cause severe damage to shallow engineering structures such as runways, building foundations, roads and embankments [3–5]. In the US alone, it is estimated the expansive Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. soils cause amounting to about US$27 billion in pavement maintenance in 2013 [6]. One of This article is an open access article the most effective solutions to mitigate the negative impacts of expansive soils is soil stabi- distributed under the terms and lization by treating and improving expansive soils to reduce their swelling and shrinkage conditions of the Creative Commons potential. Attribution (CC BY) license (https:// Several methods have been introduced for the treatment of expansive soil [7–9]. Chem- creativecommons.org/licenses/by/ ical stabilization is one of the effective techniques to overcome the undesirable behavior of 4.0/). expansive soils. Among the chemical stabilization methods, traditional treatments such

Materials 2021, 14, 3372. https://doi.org/10.3390/ma14123372 https://www.mdpi.com/journal/materials Materials 2021, 14, 3372 2 of 23

as physical mixing of lime, cement, fibers and fly-ashes with soils have been commonly used for the soil treatment [10–15]. However, these traditional methods show some limitations in engineering applications. For example, the addition of lime or cement may have environmental impacts since the production of lime and cement is energy-intensive and emits substantial greenhouse gas into the atmosphere [16,17]. Moreover, the physical mixing of the stabilizers with soils is costly and resource-intensive, making it unsuitable for laboratory-scale applications. In addition, due to high plastic nature of expansive soils, working with and achieving appropriate pulverization in the field are difficult tasks [18]. In-situ stabilization methods using lime column, lime piles and lime slurry injection have been widely used in Japan, China, United States and Sweden. However, being limited by the often low permeability and stiff nature of expansive soil, these in situ methods are inef- fective or difficult to implement in some regions [19,20]. Accordingly, in situ stabilization of expansive soil without the need for extensive earthwork is of interest in civil engineering. Recently, calcium carbonate (CaCO3) precipitation has been extensively used to en- hance the engineering properties of various materials, such as soil, mortar, concrete and recycled aggregates [21–23]. Microbially induced calcite precipitation (MICP) produces carbonate ions from the hydrolysis of urea, which has been investigated by many researchers [24,25]. However, a high concentration of ammonium is generated and accumu- lated as the by-product during the hydrolysis of urea, which can be harmful to aquatic ecosystems and can pollute the atmosphere [26]. To overcome the limitations of MICP, directly incorporating precipitated calcium carbonate into clay has also been employed to improve the properties of clay [27]. In fact, unlike soft soils, expansive soils have a stiff to very stiff consistency, and consequently, directly physical mixing of calcium carbonate, and clay incur substantial cost in terms of construction equipment and labor. Additionally, numerous researchers have studied chemically induced calcium carbonate precipitation (CCP), which involves CO2 exposure or direct injection to hydrated mortar and cemented soils to accelerate the calcium carbonate precipitation on the surface of recycled aggregates and soils [28,29]. In the practical application of CCP, pipes are injected into the ground while controlling the pump pressure in order to supply CO2 gas and Ca(OH)2 solution in soils. However, injecting Ca(OH)2 (irrespective of quantities) is great of harm to plants and soils due to the highly alkaline feature of Ca(OH)2 with potential hazards and toxicity [30]. The aim of this paper is to present a new method of a mixing-induced precipitation reaction in expansive soils by the successive permeation of calcium chloride (CaCl2) and sodium carbonate (Na2CO3) solutions into expansive soil according to the following reaction: CaCl2 + Na2CO3 → CaCO3 ↓ +2NaCl (1)

The resulting calcium carbonate (CaCO3) precipitation is expected to improve the properties of expansive soil by flocculation–agglomeration, densification and cementation effects [16,31–33]. This process is simply based on the reaction between calcium ions 2+ 2− (Ca ) and carbonate ions (CO3 ). A calcite crystal is usually obtained by this reaction, which is the most stable phase of calcium carbonate form at room temperature and normal atmospheric conditions [34,35]. The proposed CaCO3 precipitation technique will be far superior to the traditional lime pile technique or the CCP method due to the very high diffusion rates of the solutions in soils. In practical applications, in situ stabilization of expansive soils can be conducted by the successive permeation of CaCl2 and Na2CO3 solutions through the pipes in expansive soils. Moreover, the two solutions can be spread on the surface of the ground and allowed to seep naturally into the soils to stabilize them in situ. Moreover, Na2CO3 is an important ingredient in many household products, especially in cleaning and disinfecting products. Only in very concentrated solution or in solid form is Na2CO3 potentially harmful [36]. In addition, CaCl2 is extensively introduced into naturally contained soluble carbonate formations for in situ restoration of contaminated soils and groundwater [37]. As the precipitate forms, substantial quantities of heavy metal ions coprecipitate with the calcium carbonate precipitate, and the coprecipitates are insoluble in formation fluids, including Materials 2021, 14, 3372 3 of 23

groundwater. Compared with the MICP method, the proposed technique is relatively not toxic to humans and vegetation and does not pollute the atmosphere. Therefore, this method can be utilized without posing safety and health problems to humans and relatively minimize the impact on the environment. A comparative study was conducted by sequential mixing of CaCl2 and Na2CO3 solutions with expansive soil in different concentrations. A series of macro- and microlevel tests, including a free swell test, unloaded swelling ratio test, unconﬁned compression test, direct shear strength test, scanning electron microscopy (SEM), cyclic wetting–drying test and laboratory-scale precipitation model test, were performed to evaluate the performance and efﬁciency of the proposed soil stabilization technique.

2. Materials and Methods 2.1. Test Materials Expansive soils from Gaochun District, Nanjing City, China, were used in this investigation. The soil was air-dried, and two separate test batches were obtained by passing soil samples through 0.5 and 2 mm sieves. The 0.5 mm sieved soil was used for the free swell test, and the 2 mm sieved soil was used for determining the standard Proctor compaction, unloaded swelling ratio test, direct shear test and unconﬁned compression test. The physical and engineering properties of the soil used in the study were determined according to Chinese Standard GB/T 50123-1999 [38] and are presented in Table1. The data for the untreated natural soils include soil index properties, Atterberg limits, shear strength parameters and unconﬁned compressive strength. Commercial-grade CaCl2 and Na2CO3 were used as stabilizers for the stabilization of the expansive soil.

Table 1. Engineering properties of the expansive soil used in the study.

Property Value Speciﬁc gravity 2.74 Liquid limit (%) 51.6 Plastic limit (%) 23.2 Plasticity index 28.4 Free swell ratio (%) 56.0 Maximum dry density (g/cm3) 1.55 Optimum moisture content a (%) 22.3 Cohesion a (kPa) 18.9 Friction angle a (◦) 11.9 Unloaded swelling ratio a (%) 9.3 Unconﬁned compressive strength a (UCS) (kPa) 137 a Sample compacted at optimum moisture content to maximum dry density.

The mineral composition of the expansive soils was characterized by the X-Ray Pow- der Diffraction (XRD) test. Figure1 and Table2 show the XRD pattern and quantitative percentage of the different minerals, respectively, which corresponds to the different crystalline phases, among which montmorillonite was the dominant mineral. The chemical components of the expansive soils were obtained by performing Scanning Electron Mi- croscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS, Nanjing Forestry University, Nanjing, China). The chemical elements of the expansive soil are shown in Table3. Ele- ments Si and Al were characterized as the dominant elements in the expansive soils. Materials 2021, 14, x FOR PEER REVIEW 4 of 23

Materials 2021, 14, 3372 4 of 23 Nanjing, China). The chemical elements of the expansive soil are shown in Table 3. Ele- ments Si and Al were characterized as the dominant elements in the expansive soils.

Figure 1. XRDFigure pattern 1. XRD of the pattern expansive of the soil. expansive soil.

Table 2.2. The results ofof XRDXRD teststests onon thethe expansiveexpansive soil.soil.

MontmorilloniteMontmorillonite QuartzQuartz VermiculiteVermiculite HydromicaHydromica KaoliniteKaoliniteFeldspar Feldspar CalciteCalcite (%)(%) (%) (%)(%) (%)(%) (%)(%) (%)(%) (%)(%) 2525 23 20 20 1515 1515 11 11

Table 3. Chemical elements of the expansive soil. Table 3. Chemical elements of the expansive soil. Si Al Fe K Mg C Si Al Fe K Mg C (%)(%) (%)(%) (%)(%) (%)(%) (%) (%) 41.02 14.26 6.62 3.33 1.08 1.37 41.02 14.26 6.62 3.33 1.08 1.37 2.2. Sample Preparation 2.2. Sample Preparation As stated, CaCO3 precipitation can be promoted by a double-exchange reaction be- As stated, CaCO precipitation can be promoted by a double-exchange reaction tween CaCl2 and Na2CO3 3 solutions in expansive soil. According to the reaction in Equation between CaCl and Na CO solutions in expansive soil. According to the reaction in (1), 1 mol of CaCl2 2 (110 g)2 reacts3 with 1 mol of Na2CO3 (106 g) to produce 1 mol of CaCO3 Equation (1), 1 mol of CaCl2 (110 g) reacts with 1 mol of Na2CO3 (106 g) to produce 1 mol (100 g); thus, the molar mass of CaCl2 and Na2CO3 combining to form CaCO3 is in the ratio of CaCO3 (100 g); thus, the molar mass of CaCl2 and Na2CO3 combining to form CaCO3 of 1.04 (110/106). Maintaining the same ratio, CaCl2 solution and Na2CO3 solution were is in the ratio of 1.04 (110/106). Maintaining the same ratio, CaCl2 solution and Na2CO3 mixed sequentially with the expansive soil to precipitate CaCO3 in the soil. solution were mixed sequentially with the expansive soil to precipitate CaCO in the soil. Prior to mixing, all the expansive soils were oven-dried and passed through3 a 2-mm Prior to mixing, all the expansive soils were oven-dried and passed through a 2-mm sieve. Then, the expansive soils were separately and thoroughly mixed with the two solu- sieve. Then, the expansive soils were separately and thoroughly mixed with the two tions in equal amounts to an optimum moisture content (OMC) of 22.3%. CaCl2 and solutions in equal amounts to an optimum moisture content (OMC) of 22.3%. CaCl2 and Na2CO3 solutions were prepared by dissolving solid CaCl2 and solid NaOH, respectively, Na2CO3 solutions were prepared by dissolving solid CaCl2 and solid NaOH, respectively, in water to each achieve 50% concentration. Subsequently, the prepared CaCl2 solution in water to each achieve 50% concentration. Subsequently, the prepared CaCl2 solution (or (or NaCO3 solution) was homogeneously mixed to the needed amount of air-dried soils NaCO solution) was homogeneously mixed to the needed amount of air-dried soils until until they3 achieved a uniform color. The samples were then placed in an airtight bag for they achieved a uniform color. The samples were then placed in an airtight bag for moisture moisture equilibration for 1 h. Subsequently, the prepared Na2CO3 solution (or CaCl2 so- equilibration for 1 h. Subsequently, the prepared Na2CO3 solution (or CaCl2 solution) was lution)mixed manuallywas mixed with manually the soil with samples the soil until samples the mixture until the achieved mixture aachieved homogeneous a homoge- and neousuniform and appearance uniform appearance then placed inthen an placed airtight in container an airtight for moisturecontainer equilibration for moisture for equili- 24 h. Thebration main for steps 24 h. in The the main test proceduresteps in the are test shown procedure in Figure are2 shown. in Figure 2.

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Figure 2. The test procedure: ( a)) initial initial soil blocks, ( b) soil after sieving, ( c) moisture moisture equilibration for 1 h and ( d) moisture equilibration for 24 h.

Thyagaraj et al. [39] [39] found that the sequence of mixing solutions can affect the utility of stabilization; therefore, thethe expansive soil samples in these tests were mixed with CaClCaCl22 and Na 2COCO33 inin two two different different sequences. The The first ﬁrst sequence is mixing CaClCaCl2 solution in expansive soil followed by Na2CO33,, which which is is called Series 1, whilewhile inin SeriesSeries 2,2, NaNa22COCO33 solutionsolution was mixed before CaCl 2 solution.solution. To reveal the effect ofof excessiveexcessive CaClCaCl22 on the soil properties, two additional mixing methods were designed and tested: (1) (1) Series 1A and Series 2A where the same reaction ratioratio is maintained while graduallygradually increasingincreasing thethe concentrationsconcentrations ofof CaClCaCl22 andand NaNa22COCO33 solutionssolutions to raise CaCO 3 precipitationprecipitation in the soil, and (2) Series 1B and Series 2B where the CaCO33 precipitationprecipitation is is kept kept constant constant while while mixing mixing excessive excessive CaCl CaCl22 inin the the expansive expansive soil. soil. Table4 4 gives gives thethe designationsdesignations andand detailsdetails of of the the mixing mixing method. method.

Table 4. Designations and details of mixing method.

PercentagePercentage of of Solution Concentration Concentration SeriesSeries Designation (%)(%) Natural SoilSoil -- SeriesSeries 1A-1 1A-1 11.0% 11.0% CaClCaCl210.6% 10.6% Na NaCO2CO3 SeriesSeries 1A 1A 2 2 3 SeriesSeries 1A-2 1A-2 22.0% 22.0% CaClCaCl2221.2% 21.2% Na Na2CO2CO3 3 Series 1: (complete reaction) Series 1: (complete reaction) Series 1A-3 27.5% CaCl2 26.5% Na2CO3 CaCl2 followed by Na2CO3 Series 1A-3 27.5% CaCl2 26.5% Na2CO3 CaCl2 followed by Na2CO3 Series 1B Series 1B-1 27.5% CaCl2 21.2% Na2CO3 Series 1B Series 1B-1 27.5% CaCl2 21.2% Na2CO3 (CaCl2 in excess) Series 1B-2 33.0% CaCl2 21.2% Na2CO3 (CaCl2 in excess) Series 1B-2 33.0% CaCl2 21.2% Na2CO3 Series 2A-1 10.6% Na2CO3 11.0% CaCl2 Series 2A Series 2A-1 10.6% Na2CO3 11.0% CaCl2 Series 2A-2 21.2% Na2CO3 22.0% CaCl2 Series 2: (completeSeries 2A reaction) SeriesSeries 2A-2 2A-3 21.2% 26.5% NaNa22COCO33 27.5%22.0% CaCl CaCl2 2 Na2CO3 followedSeries 2: by CaCl2 (complete reaction) Series 2B SeriesSeries 2A-3 2B-1 26.5% 21.2% NaNa22COCO33 27.5%27.5% CaCl CaCl2 2 Na2CO3 followed by CaCl2 Series(CaCl2 2Bin excess) SeriesSeries 2B-1 2B-2 21.2% 21.2% NaNa22COCO33 33.0%27.5% CaCl CaCl2 2 (CaCl2 in excess) Series 2B-2 21.2% Na2CO3 33.0% CaCl2 2.3. Testing Procedures 2.3. Testing Procedures It has been known that the stabilization of expansive soils can not only eliminate the swellingIt has potential been known of soils that but the also stabilization improve their of mechanicalexpansive soils properties can not that only are eliminate of interest the in swellingcivil engineering potential applications of soils but also [40,41 improve]. Therefore, their mechanical the free swell properties test and unloadedthat are of swelling interest inratio civil test engineering were carried applications out to evaluate [40,41]. theTheref effectivenessore, the free of swell the stabilizers test and unloaded in eliminating swell- ingswell ratio potential test were ability. carried In addition,out to evaluate the direct the effectiveness shear test and ofunconﬁned the stabilizers compression in eliminating test swellwere usedpotential to determine ability. In the addition, utility of the the direct stabilizers shear on test improving and unconfined the geotechnical compression behavior test wereof expansive used to determine soils. To further the utility investigate of the st theabilizers mechanisms on improving of the soil the stabilization geotechnical process, behav- iorthe of microstructure expansive soils. of theTo further natural investigate and treated the soil mechanisms samples was of monitoredthe soil stabilization using SEM. pro- In cess,addition, the microstructure to verify the effectiveness of the natural of and the treated CaCO3 soilprecipitation samples was technique monitored for using stabilizing SEM. Inexpansive addition, soil to verify in situ, the CaCl effectiveness2 and Na2CO of the3 solutions CaCO3 precipitation were sequentially technique permeated for stabilizing into the expansive soil in situ, CaCl2 and Na2CO3 solutions were sequentially permeated into the

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compacted expansive soil through a central vertical hole, then representative soil samples were collected after 28 days for evaluating the efficiency of in situ CaCO3 precipitation technique by performing the free swell tests, unloaded swelling tests, direct shear tests and unconfined compression tests. All the testing procedures used in this paper follow the Chinese Standard GB/T 50123-1999 [38]. The free swell ratio is a simple experimental index to identify the expansion potential of soils [42]. For performing free swell tests, 10 mL of representative oven-dried soil (passing a 0.5 mm sieve) was slowly poured into a 50 mL measuring jar filled with 45 mL distilled water and 5 mL sodium chloride. The soils were then allowed to reach a volume equilibrium state without any change in the volume of the solids in 24 h. Then the final volume of the soils was recorded. The free swell ratio is defined as the increase in the volume of the soil expressed as a percentage of the initial volume of the soil. Apart from the free swell ratio assessment, the swelling behavior of compacted expansive soil samples is usually identified as an important swelling characteristic that is required for evaluation. An unloaded swelling ratio test was conducted to investigate the compression behavior of the natural and treated soils. The wet homogenized natural and treated soils were compacted in oedometer rings of 60 mm diameter and 20 mm height to the maximum dry density of 1.55 g/cm3 using a static press with a piston and then cured with a controlled condition for 1, 7 and 28 days. After curing, the samples were placed between two porous stones with filter papers and set up in an oedometer assembly, and then a dial gauge was installed to measure vertical displacement. A seating load of 1 kPa was applied to make the sample contact with the upper and lower parts of the oedometer assembly, and then the sample was immersed with distilled water for swell potential determination. Dial gauge readings were taken every 2 h until the difference between the two readings is smaller than 0.01 mm. The unloaded swelling ratio is expressed as the ratio of an increased amount of the sample height to initial height after water immersion under no-load conditions. To investigate the effect of the stabilizers on the unconfined compressive strength of expansive soils, a series of unconfined compression tests were performed. The samples were compacted in molds with 39.1 mm diameter and 80 mm height to the maximum dry density of 1.55 g/cm3. To ensure uniform compaction, each sample was evenly divided into five layers of compaction. After each curing period (i.e., 1, 7 and 28 days), the samples were extruded from the molds, and the loading tests were performed. Shear strength parameters (i.e., cohesion and friction angle) are the principal engineering properties of soil that control the stability of a soil mass under load [43]. To ensure the stability of the constructed facilities on expansive soils, the shear strength parameters of the stabilized compacted soil are evaluated using the direct shear test. The samples for the direct shear test were prepared in a way similar to that used for unloaded swelling tests. Vertical stresses of 50, 100, 150 and 200 kPa were applied and were kept constant as the soil samples were sheared under constant horizontal shear displacement with a velocity of 0.8 mm/min until the horizontal deformation reached the set limit of 4 mm. To evaluate the micromechanisms of the soil stabilization process by induced CaCO3 precipitation, the soil samples were studied and characterized using scanning electron microscopy (SEM). Oven-dried soil samples from direct shear tests were used for SEM analyses. Images of the soil samples were magnified 600, 1200 and 5000 times, respectively. The significant damage caused by expansive clay to engineering structures is primarily due to cyclical swell-shrink behavior [44,45]. Therefore, a series of studies were performed to determine the influence of cyclic wetting and drying on the swelling and cracking behavior of natural and treated soil samples. The samples for cyclic wetting–drying tests were prepared similarly to the unloaded swelling test. Apart from determining the influence of cyclic wetting and drying on swelling and cracking behavior of natural and treated soil samples after curing for 7 days, the effects of cyclic wetting–drying on shear strength are also discussed. The samples were performed in conventional oedometer test assemblies with a free surface load. They were allowed to swell upon wetting, and the Materials 2021, 14, 3372 7 of 23

changes in the thickness of the samples were monitored by using dial gauges. The final thickness upon swell equilibrium was measured, and then the sample was gently removed and placed in a ventilated container, the samples were allowed to shrink, and the masses of the samples were continuously monitored by a balance until the initial mass was reached. The laboratory-scale model test procedure was conducted according to Thyagaraj et al. [46]. The portion of the natural expansive soil passing the 2-mm sieve was divided into 6 equal parts and thoroughly mixed with the desired amount of distilled water to reach the maximum water content of 22.3%. The 6 parts of moist soils were placed to equilibrate in airtight containers for a minimum period of 24 h. At the end of the equilibration period, the 6 parts of soils were thoroughly mixed prior to compaction. The laboratory-scale precipitation model test was conducted in a plexiglass cylindrical mold of 300 mm diameter and 200 mm height (Figure3). The expansive soil was statically compacted to a thickness of 150 mm (each layer was 30 mm) in a cylindrical test mold. The soil was compacted to a maximum dry density of 1.55 g/cm3 at its optimum water content of 22.3%. Each test required 20.09 kg of moist expansive soil (w = 22.3%) for compacting the soil to a dry density of 1.55 g/cm3. A central hole in 70 mm diameter was created in the compacted soil mass from the top to depth of 100 mm. The volume of this hole is 385 cm3, and the hole was filled with coarse sand. Then 2000 mL of 27.5% CaCl2 solution was permeated into the soil mass through this central hole in approximately 10 days. Subsequently, 2000 mL of 21.2% Na2CO3 solution was permeated into the soil mass through the central hole in approximately 10 days to facilitate CaCO3 precipitation, according to Equation (1). Then the soil mass was covered with a wet cloth and cured for a period of 28 days. After 28 curing days, the thin-wall sampling tubes with a diameter of 39.1 mm and 80 mm height was pushed into the soil mass with the depth of 120 mm at the radial distance of 0.8·D and 1.9·D (where D = diameter of central hole = 70 mm), from the central hole, respectively, which were used for determining the unconfined compressive strength. Representative soil samples were also collected at a radial distance of 0.8·D and 1.9·D for the determination of the free swell index. The oedometer rings with a diameter of 60 mm and length of 20 mm were pushed into soil mass with the depth of 50 mm at the radial distance of 1·D and 1.3·D from the central hole, respectively. The oedometer samples were used for evaluating the unloaded swelling ratio and shear strength. Sequentially, a permeate with CaCl2 and Na2CO3 solutions into the compacted expansive soil through the central hole in the mold. The introduction of the permeate into the sample increased the water content from 22.3% to around 25.0% and reduced the dry density from 1.55 g/cm3 to 1.51 g/cm3. Therefore, the unconfined compression test and oedometer test samples performed on natural soil under compacted conditions correspond to that of laboratory-scale model test samples. The unconfined compressive strength, free swell index, unloaded swelling ratio and shear strength of laboratory-scale model test stabilized samples were evaluated as per the previously described procedures. Materials 2021,, 14,, 3372x FOR PEER REVIEW 88 of of 23

Figure 3. The laboratory-scale model test mold ( D: diameter of central hole, 70 mm).

3. Results and Discussion 3.1. Free Swell Tests The deﬁnition of the free swell ratio δef (%) is given as The definition of the free swell ratio δef (%) is given as

Vwe𝑉−−𝑉V0 δe𝛿 f = = ××100% 100% (2) V0𝑉

where Vwewe isis the the final ﬁnal volume volume of of 10 10 mL mL oven oven dried dried soil, soil, and and V0 Vis0 theis the initial initial soil soil volume volume (10 mL).(10 mL). Table5 5 gives gives thethe resultsresults ofof thethe freefree swellswell ratioratio teststests withwith thethe differentdifferent concentrationsconcentrations of solutions in the two test sequences. The calculatedcalculated percentage of precipitation by dry weight of of soil soil is is also also presented presented in in Table Table 5.5 .According According to toEquation Equation (1), (1), the theamount amount of pre- of cipitationprecipitation in the in the soil soil can can be becalculated. calculated. For For example, example, ifif 11.2 11.2 mL mL of of 11% 11% CaCl CaCl2 reactsreacts with 11.2 mL of 10.6% Na2CO3,, the the amount amount of of precipitation precipitation is is calculated calculated as as follows: follows: 100 100 11.1511.15× ×11% 11%× × ==1.121.12 g g (3) 110110 The percentage of precipitation by the dry weight of soil is calculated as follows: 1.12 × 100% = 1.12% (4) 100

Table 5. The results of free swell tests.

Free Swell Ratio Calculated Percentage of Precipitation by Dry Weight of Soil Series Designation (%) (%) Natural Soil 56

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Table 5. The results of free swell tests.

Calculated Percentage of Precipitation by Dry Free Swell Ratio Series Designation Weight of Soil (%) (%) Natural Soil 56 Series 1A-1 45 1.12 Series 1A-2 38 2.23 Series 1A-3 30 2.79 Series 1B-1 27 2.23 Series 1B-2 24 2.23 Series 2A-1 48 1.12 Series 2A-2 40 2.23 Series 2A-3 35 2.79 Series 2B-1 32 2.23 Series 2B-2 30 2.23

The percentage of precipitation by the dry weight of soil is calculated as follows:

1.12 × 100% = 1.12% (4) 100

As can be seen in Table5, an increase in the percentage of CaCO 3 precipitation profoundly decreased the free swell ratio in both series. The signiﬁcant reduction of free swell ratio is mainly due to the fact that the divalent calcium ions (source from CaCl2) can easily substitute the monovalent ions (e.g., Na+) in the diffuse double layer (DDL) of the clay particles [47], thereby leading to a strong short-term reaction between clay and stabilizers, allowing for their ﬂocculation [48]. Therefore, at any given percentage of precipitation, the cases with large amounts of CaCl2 show better performance because more Ca2+ is provided. Taking the calculated percentage of precipitation of 2.23% in Series 1, for example, the free swell ratio is less than 27% for the case with excessive CaCl2, whereas the free swell ratio is 38% for the complete reaction case. Moreover, the addition of CaCl2 and Na2CO3 solutions may increase the salinity of pore liquid, which can decrease the repulsive pressure between clay surfaces, causing the soil skeleton to shrink [40] and therefore leading to a decrease in free swell ratio. According to Chinese Standard GB 50112-2013 [49], stabilizers can be considered effective by reducing the free swell ratio to less than 40%. Therefore, stabilizers with CaCl2 concentration higher than 27.5% and Na2CO3 concentration higher than 21.5% were selected for subsequent experiments. Furthermore, Series 1 shows higher utility on decreasing the value of free swell ratio than Series 2. Consequently, four cases, namely sequential mixing of 22.0% CaCl2 and 21.2% Na2CO3 solutions, 27.5% CaCl2 and 26.5% Na2CO3 solutions, 27.5% CaCl2 and 21.2% Na2CO3 solutions and 33.0% CaCl2 and 21.2% Na2CO3 solutions, were the preferred proposed mixtures for the subsequent experiments.

3.2. Unloaded Swelling Ratio Tests

The unloaded swelling ratio δe (%) at time t is measured using:

zt − z0 δe = × 100% (5) h0

where zt is the dial gauge reading at time t (mm), z0 is the initial dial gauge reading (mm), and h0 is the initial height before water immersion (20 mm). Table6 presents the results of the unloaded swelling ratio test with different concentrations of solutions. MaterialsMaterials2021 2021, 14, 14, 3372, x FOR PEER REVIEW 10 ofof 2323

TableTable 6. The 6. results The results of the of unloaded the unloaded swelling swelling ratio tests. ratio tests. Calculated Percentage of Precipitation Curing Periods Unloaded Swelling Ratio Calculated Series Designation Unloaded Swellingby Dry WeightPercentage of Soil of Curing Periods Series(d) Designation (%) Ratio Precipitation by Dry (d) (%) Natural Soil 9.3 (%) Weight of Soil (%) 1 1.5 2.23 Natural Soil 9.3 Series 1A-2 7 1.1 2.23 28 0.91 1.5 2.23 2.23 Series 1A-2 7 1.1 2.23 1 1.228 0.9 2.79 2.23 Series 1A-3 7 0.71 1.2 2.79 2.79 28Series 1A-3 0.57 0.7 2.79 2.79 1 1.028 0.5 2.23 2.79 Series 1B-1 7 0.81 1.0 2.23 2.23 28Series 1B-1 0.67 0.8 2.23 2.23 28 0.6 2.23 1 1.61 1.6 2.23 2.23 Series 1B-2 7Series 1B-2 1.37 1.3 2.23 2.23 28 1.128 1.1 2.23 2.23

FigureFigure4 4shows shows the the effecteffect ofof differentdifferent stabilizersstabilizers onon the unloaded swelling swelling ratio ratio for for 1, 1,7 7and and 28 28 days days of of curing curing time, time, respectively. respectively. Apparently, Apparently, the the unloaded unloaded swelling swelling ratio ratio of oftreated treated soil soil samples samples is all is allless less than than 2%, 2%,while while the unloaded the unloaded swelling swelling ratio ratioof natural of natural soil is soil9.3%. is 9.3%.The dramatic The dramatic reduction reduction in the unloaded in the unloaded swelling swelling ratio by ratio mixing by mixing CaCl2 and CaCl Na2 and2CO3 Nasolutions2CO3 solutions can be explained can be explained by the diminishment by the diminishment of repulsive of repulsive forces. The forces. clayThe particles clay particlesflocculate ﬂocculate by approaching by approaching each other each to other form to formmore morecompact compact particle particle accumulation accumulation and andaggregation aggregation [50,51], [50, 51and], and thus, thus, the theinteraction interaction between between the theclay clay surface surface and and water water is re- is reduced.duced. Besides, Besides, as as subsequently subsequently discussed, discussed, th thee decrease decrease in in the the unloaded unloaded swelling swelling ratio ratio is isalso also attributed attributed to to the the formation formation of ofnew new cementation cementation precipitation precipitation by by solutions solutions reacting reacting by bybinding binding the the clay clay particles particles together together and and filling ﬁlling the the soil soil voids, voids, causing causing the thedecrease decrease in hy- in hydration.dration. Moreover, Moreover, the the results results in in Figure Figure 4 show thatthat thethe slightslight decrease decrease in in the the unloaded unloaded swellingswelling ratioratio with increasing increasing curing curing period period is is may may be be due due to tothe the production production of new of new pre- precipitationscipitations and and the the growth growth of ofprecipitation precipitation crystal crystal [52]. [52 ].

FigureFigure 4.4. TheThe effecteffect ofof differentdifferent concentrationsconcentrations ofof solutionssolutions andandcuring curingperiods periodson on the the unloaded unloaded swellingswelling ratio. ratio.

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3.3. Unconﬁned Compression Tests Table7 gives the results of the unconﬁned compression tests with samples prepared by mixing expansive soil with CaCl2 and Na2CO3 solutions in different concentrations.

Table 7. The results of unconﬁned compression tests.

Calculated Percentage of Curing Periods UCS Series Designation Precipitation by Dry Weight of Soil (d) (kPa) (%) Natural Soil 137 1 180 2.23 Series 1A-2 7 237 2.23 28 268 2.23 1 270 2.79 Series 1A-3 7 325 2.79 28 370 2.79 1 272 2.23 Series 1B-1 7 389 2.23 28 400 2.23 1 196 2.23 Series 1B-2 7 240 2.23 28 248 2.23

Figure5 shows the variations of unconfined compressive strength (UCS) with different concentrations of solutions versus the curing periods of 1–28 days. The stress–strain behavior of all the samples at different curing periods are presented in Figures6 and7. It is observed that the treated clay samples show a strengthening trend with the increase in the calculated percentage of precipitations for complete reaction cases. In comparison, the natural soil exhibits UCS of 137 kPa. The significant improvement in the UCS of expansive soil is again primarily attributed to the strong short-term reaction between clay and stabilizers, allowing for flocculation. Besides, according to the reaction in Equation (1), sequential mixing of CaCl2 and Na2CO3 into the expansive soil can form CaCO3 precipitation, which will be absorbed on the surface of clay particles and form a package between clay particles, thus generating cementation or bond between clay particles. Moreover, the particle void filled with precipitated CaCO3 may increase the soil density in terms of increasing the UCS [28]. A similar stress–strain curve shape is observed in most stabilized cases, which reaches the peak values at lower strains, indicating a significant influence of the stabilization on the material stiffness and overall rigidity. It is interesting to note that the UCS appears to decrease when the concentration of CaCl2 is above 27.5% for a constant precipitation content for the curing time of 28 days. Taking the calculated percentage of precipitation of 2.23%, for example, the UCS for the case with 27.5% of CaCl2 and 21.2% of Na2CO3 after 7 days is 389 kPa, whereas the UCS for the case with 33.0% of CaCl2 and 21.2% of Na2CO3 after 7 days is 240 kPa. The reduction in UCS when the concentration of CaCl2 is beyond 27.5% may be due to the absorption of more moisture at a higher concentration of CaCl2 [53]. As discussed previously, the increase in UCS with the curing period is also contributed by the formation of new precipitations and the growth of precipitation crystals. Materials 2021, 14, 3372 12 of 23 Materials 2021, 14, x FOR PEER REVIEW 12 of 23 Materials 2021, 14, x FOR PEER REVIEW 12 of 23

Figure 5. Variations of UCS with different concentrations of solutions versus curing periods. FigureFigure 5.5. VariationsVariations ofof UCSUCS withwith differentdifferent concentrationsconcentrations of of solutions solutions versus versus curing curing periods. periods.

Figure 6. The stress–strain behavior of Series 1 samples with different curing periods. FigureFigure 6.6. TheThe stress–strainstress–strain behaviorbehavior ofof SeriesSeries 11 samplessamples withwith differentdifferent curingcuring periods.periods.

Figure 7. The stress–strain behavior of Series 2 samples with different curing periods. FigureFigure 7.7. TheThe stress–strainstress–strain behaviorbehavior ofof SeriesSeries 22 samplessamples withwith differentdifferent curingcuring periods.periods. 3.4.3.4. DirectDirect ShearShear TestsTests 3.4. Direct Shear Tests TableTable8 8shows shows the the results results of of the the direct direct shear shear tests tests under under different different CaCl CaCl2 2and and Na Na2CO2CO33 Table 8 shows the results of the direct shear tests under different CaCl2 and Na2CO3 concentrations.concentrations. FiguresFigures8 8 and and9 9show show the the shear shea stress-shearr stress-shear displacement displacement curves curves from from the the concentrations. Figures 8 and 9 show the shear stress-shear displacement curves from the directdirect shearshear teststests ofof thethe soilsoil samples samples treated treated with with 27.5% 27.5% CaCl CaCl2 2followed followed by by 26.5% 26.5% Na Na2CO2CO33 direct shear tests of the soil samples treated with 27.5% CaCl2 followed by 26.5% Na2CO3 solutionssolutions andand 27.5%27.5% CaClCaCl22 followedfollowed byby 21.2%21.2% NaNa22COCO33 solutions, respectively.respectively. ForFor reasonsreasons solutions and 27.5% CaCl2 followed by 21.2% Na2CO3 solutions, respectively. For reasons ofof simplicitysimplicity andand clarity,clarity, thethe graphsgraphs ofof thethe otherother casescases areare notnot presentedpresented herehere asas theythey showshow of simplicity and clarity, the graphs of the other cases are not presented here as they show thethe samesame trends.trends. ItIt cancan bebe seenseen thatthat allall curvescurves showshow similarsimilar shapes,shapes, andand thethe interfaceinterface shearshear the same trends. It can be seen that all curves show similar shapes, and the interface shear

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stress increases with the increase in shear displacement until it reaches the maximum shear strength, after which it decreases slightly or remains constant. Figure 10 shows the influence of concentrations of solutions versus curing periods on the cohesion of treated soil samples. As illustrated in Figure 10, the increase in CaCO3 precipitation results in a significant improvement in the cohesion of the treated samples. This is possibly due to more CaCO3 precipitations formed lead to particle–particle cementation and provide a significant improvement in soil properties. On the other hand, the cohesion increases with the concentration of CaCl2 until it reaches 27.5%; that is, the cohesion of soil samples will decrease with more addition of CaCl2. This phenomenon can be explained by the replacement of native monovalent exchangeable cations by divalent calcium ions, resulting in the loss of double-layer water, which leads to an increase in attraction between soil particles; thus, the effect of preventing the formation of shear slip surface is increased [54,55]. However, excessive CaCl2 is more likely to form a face-to-face aggregation or edge-to-edge flocculation, resulting in the gradual increase in the size of the void between particle clusters. Therefore, the cohesive force of clay decreases when the concentration of CaCl2 exceeds 27.5%, indicating that a high concentration of CaCl2 may have a negative effect on the cohesion of clay [56]. Figure 11 shows the variations in friction angle with concentrations of solutions versus curing periods. As can be seen, the friction angle slightly increases with the calculated percentage of precipitation and concentration of CaCl2. This is because the double-layer water is like lubrication between particles, and the strong short-term reaction may decrease the thickness of the diffused double layer, which weakens the lubricating effect and results in the increase in friction angle.

Table 8. The results of direct shear tests.

Curing Friction Calculated Percentage of Series Cohesion Periods Angle Precipitation by Dry Weight of Soil Designation (kPa) (d) (◦) (%) Natural Soil 18.9 11.9 1 20.2 12.1 2.23 Series 1A-2 7 26.8 13.7 2.23 28 30.6 13.9 2.23 1 30.5 13.8 2.79 Series 1A-3 7 32.0 14.5 2.79 28 34.6 14.3 2.79 1 35.5 13.3 2.23 Series 1B-1 7 37.4 14.7 2.23 28 42.2 14.9 2.23 1 22.3 12.8 2.23 Series 1B-2 7 32.0 15.1 2.23 28 32.8 15.3 2.23 Materials 2021, 14, 3372 14 of 23 Materials 2021, 14, x FOR PEER REVIEW 14 of 23 Materials 2021, 14, x FOR PEER REVIEW 14 of 23 Materials 2021, 14, x FOR PEER REVIEW 14 of 23

Figure 8. The stress–displacement curve of the soil samples treated with 27.5% CaCl2 followed by Figure 8. The stress–displacement curve of the soil samples treated with 27.5% CaCl2 followed by Figure 8. The stress–displacement curve of the soil samples treated with 27.5% CaCl2 followed by 26.5% Na2CO3 solutions. 26.5% Na CO solutions. 2 26.5%Figure Na 8.2 2TheCO3 stress–displacementsolutions. curve of the soil samples treated with 27.5% CaCl followed by 26.5% Na2CO3 solutions.

Figure 9. The stress–displacement curve of the soil samples treated with 27.5% CaCl2 followed by Figure 9. The stress–displacement curve of the soil samples treated with 27.5% CaCl2 followed by Figure21.2% Na 9. 2TheCO3 stress–displacement solutions. curve of the soil samples treated with 27.5% CaCl2 followed by 2 21.2%Figure Na 9. 2TheCO3 stress–displacementsolutions. curve of the soil samples treated with 27.5% CaCl followed by 21.2% Na2CO3 solutions. 21.2% Na2CO3 solutions.

Figure 10. Variations of cohesion with different concentrations of solutions versus curing periods. Figure 10. Variations of cohesion with different concentrations of solutions versus curing periods. FigureFigure 10.10. VariationsVariations ofof cohesioncohesion withwith differentdifferent concentrationsconcentrationsof ofsolutions solutionsversus versuscuring curing periods. periods.

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FigureFigure 11. 11.Variations Variations of of friction friction angles angles with with different different concentrations concentrations of solutions of solutions versus versus curing curing periods. periods. 3.5. Scanning Electron Microscopy (SEM) 3.5. ScanningFigure 12 a,c,eElectron show Microscopy the scanning (SEM) electron micrographs of natural clay samples, while FigureFigure 12b,d,f 12a,c,e present show the the samples scanning treated electron with micrographs 27.5% CaCl of2 naturaland 21.2% clay Nasamples,2CO3 solu-while tionsFigure after 12b,d,f 7 days present at 600, the 1200samples and treated 5000 times with 27.5% magnification, CaCl2 and respectively. 21.2% Na2CO As3 solutions can be seen,after the7 days natural at 600, clay 1200 sample and 5000 has atimes dispersed magnification, structure respectively. and obvious As large can pore be seen, spaces the (Figurenatural 12 claya,c,e sample). In contrast, has a thedispersed treated samplestructure exhibits and obvious more densification large pore spaces and pore-size (Figure refinement12a,c,e). In (contrast,Figure 12 theb,d ).treated The denser sample microstructure exhibits more maydensification be due toand the pore-size precipitations refine- formedment (Figure in the treated12b,d). samplesThe denser (Figure microstructure 12f), which distributemay be due spatially to the withinprecipitations the pore formed space andin the bind treated soil particlessamples together,(Figure 12f), forming which a dist moreribute integrated spatially composition. within the pore The space effective and cementationbind soil particles and densification together, forming of the soil a more provide integrated significant composition. improvement The of effective soil properties, cemen- whichtation isand in gooddensification agreement of withthe soil previous provide works. significant improvement of soil properties, which is in good agreement with previous works.

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Figure 12. SEM micrographs of: (a) natural sample at 600 times magnification; (b) treated sample with 27.5% CaCl2 and Figure 12. SEM micrographs of: (a) natural sample at 600 times magnification; (b) treated sample with 27.5% CaCl2 and 21.2% Na2CO3 at 600 times magnification after 7 days; (c) natural sample at 1200 times magnification; and (d) treated 21.2% Na2CO3 at 600 times magnification after 7 days; (c) natural sample at 1200 times magnification; and (d) treated sample sample with 27.5% CaCl2 and 21.2% Na2CO3 at 1200 magnification after 7 days; (e) natural sample at 5000 times magnifi- with 27.5% CaCl2 and 21.2% Na2CO3 at 1200 magnification after 7 days; (e) natural sample at 5000 times magnification; and cation; and (f) treated sample with 27.5% CaCl2 and 21.2% Na2CO3 at 5000 times magnification after 7 days. (f) treated sample with 27.5% CaCl2 and 21.2% Na2CO3 at 5000 times magnification after 7 days.

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3.6. Cyclic Wetting–Drying Tests

The wetting–drying process was repeated up to four cycles. The vertical strain δi (%) after each cycle is calculated by

hw − hs δi = × 100% (6) hs

where hw is the thickness of the samples at the end of swelling procedure at cycle i = 1, 2, 3, 4; hs is the thickness of the samples at the end of shrinking procedure at cycle (i − 1); when i = 1, hs is the initial thickness of the compacted samples. Table9 presents the results of direct shear tests in terms of cohesion and friction angle after different wetting–drying cycles for compacted expansive soil samples. To simplify the experiment process, the samples after 0, 2 and 4 wetting–drying cycles were used for the shear tests.

Table 9. Direct shear test results after wetting–drying cycles.

Cohesion Friction Angle Series Designation Numbers of Cycles (kPa) (◦) 0 18.9 11.9 Natural soil 2 12.3 10.0 4 7.9 9.9 0 26.8 13.7 Series 1A-2 2 22.6 11.1 4 19.4 10.2 0 32.0 14.5 Series 1A-3 2 28.7 12.7 4 24.5 12.3 0 37.4 14.7 Series 1B-1 2 30.6 13.5 4 25.9 13.0 0 32.0 15.1 Series 1B-2 2 25.7 13.4 4 22.1 13.1

Figure 13a presents the crack patterns of natural soil samples after each wetting– drying cycle. It can be noted that several microcracks appear on the surface of the natural soil sample after the ﬁrst cycle, then additional new microcracks were induced by the growing cracks with the increase in wetting–drying cycles. This continuous development and propagation of cracks may form a crack network and weaken the strength of the soil [57]. Figure 13b,c show the soil samples treated with 27.5% CaCl2 followed by 26.5% Na2CO3 solutions and 27.5% CaCl2 followed by 21.2% Na2CO3 solutions, respectively. For reasons of simplicity and clarity, the graphs of the other two cases are not presented here as they show the same results of no cracking. Notably, there are no distinct cracks on the surface of samples after the fourth cycle for both treated soil samples, showing good improvement of the stabilizers. Materials 2021, 14, 3372 18 of 23 Materials 2021, 14, x FOR PEER REVIEW 18 of 23

Figure 13. PhotographsPhotographs of of surfaces surfaces of of samples samples afte afterr wetting–drying wetting–drying (W–D) cycles for: ( aa)) natural natural sample; sample; ( (bb)) treated treated sample sample with 27.5% CaCl2 and 26.5% Na2CO3; and (c) treated sample with 27.5% CaCl2 and 21.2% Na2CO3. with 27.5% CaCl2 and 26.5% Na2CO3; and (c) treated sample with 27.5% CaCl2 and 21.2% Na2CO3.

FigureFigure 1414 presentspresents the the change change in in cohesion cohesion for for soil soil samples samples after after different different wetting– wetting– dryingdrying cycles. cycles. Cohesion Cohesion for for all all samples samples decreases decreases with with increasing increasing wetting–drying wetting–drying cycles. cycles. ThisThis can can be explained by by the successive deve developmentlopment of of cracks, cracks, as as shown shown in in Figure Figure 13,13, which destroys the integrity of soil samples and provides channels for the infiltration inﬁltration of water, resulting inin aa reductionreduction in in cohesion cohesion [58 [58].]. In In comparison, comparison, there there is stillis still a high a high value value for forthe the cohesion cohesion of of treated treated soil soil samples samples after after experiencing experiencing four four wetting–drying wetting–drying cycles.cycles. This observationobservation is is consistent consistent with with no no distinct cracks appearing on on the surface of the samples forfor both both treated treated soil soil samples samples (Figure (Figure 13b,c). 13b,c). In addition, In addition, the theresults results in Figure in Figure 15 show 15 show that wetting–dryingthat wetting–drying cycles cycles have havea minor a minor effect effecton the on friction the friction angle angleof soil of samples. soil samples. The mois- The turemoisture content content and clay and content clay content account account for the for change the change in friction in friction angle, angle,as shown as shown by He byet al.He [59]. et al. [59].

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Figure 14. Variations in cohesion versus wetting–drying cycles. FigureFigure 14.14. Variations inin cohesioncohesion versusversus wetting–dryingwetting–drying cycles.cycles.

Figure 15. Variations in friction angle versus wetting–drying cycles. FigureFigure 15.15. Variations inin frictionfriction angleangle versusversus wetting–dryingwetting–drying cycles.cycles. Figure 16 compares the vertical strain calculated from Equation (6) of each cycle cal- Figure 16 comparescompares the the vertical vertical strain strain calc calculatedulated from from Equation Equation (6) (6)of each of each cycle cycle calculated versus four wetting–drying cycles for natural and treated samples cured for 7 calculatedculated versus versus four four wetting–d wetting–dryingrying cycles cycles for for natural natural and and treated treated samples samples cured cured for for 7 days. Overall, there is a decrease in the vertical strain with the increase in wetting–drying 7days. days. Overall, Overall, there there is is a adecrease decrease in in the the vertical vertical strain strain with with the the increase in wetting–drying cycles for natural soil samples. To discuss in detail, an abrupt decrease on the second ver- cyclescycles for natural natural soil soil samples. samples. To To discuss discuss in indetail, detail, an anabrupt abrupt decrease decrease on the on second the second vertical strain of the natural sample occurs after the first cycle, and the decrease is getting less verticaltical strain strain of the of thenatural natural sample sample occurs occurs after after the thefirst first cycle, cycle, and and the thedecrease decrease is getting is getting less significant in the later three cycles. This is mainly due to the continuous rearrangement of lesssignificant significant in the in thelater later three three cycles. cycles. This This is mainly is mainly due due to the to the continuous continuous rearrangement rearrangement of soil particles and thus leading to the destruction of the internal clay structure [45]. In con- ofsoil soil particles particles and and thus thus leading leading to the to thedestruction destruction of the of theinternal internal clay claystructure structure [45].[ In45 ].con- In trast, both stabilized soil samples show a slight change of vertical strain with wetting– contrast,trast, both both stabilized stabilized soil soil samples samples show show a aslig slightht change change of of vertical vertical strain strain with wetting– drying cycles, which is attributable to effects of cementation and densification of CaCO3 dryingdrying cycles,cycles, whichwhich isis attributableattributable toto effectseffects ofof cementationcementation andand densificationdensification ofof CaCOCaCO33 precipitation, as discussed previously. precipitation,precipitation, asas discusseddiscussed previously.previously.

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FigureFigure 16. Variations inin vertical strain versus wetting–dryingwetting–drying cyclescycles forfor naturalnatural andand treatedtreated samples.samples.

3.7.3.7. Laboratory-Scale Precipitation ModelModel TestTest Table 10 10 compares compares the the results results from from Tests Tests 1 and1 and 2 samples 2 samples with with those those of the of naturalthe natural soil samples.soil samples. The The unconfined unconfined compressive compressive strength strength of Test of Test 1 samples 1 samples increased increased from from 137 kPa 137 tokPa 270 to kPa270 andkPa 256and kPa256 atkPa radial at radial distances distances of 0.8 of·D 0.8·andD and 1.9·D 1.9·, respectively.D, respectively. Similarly, Similarly, the unloadedthe unloaded swelling swelling ratio ratio of Test of Test 2 samples 2 samples decreased decreased significantly significantly from from 9.3% 9.3% to 0.7–1.0%. to 0.7– The1.0%. cohesion The cohesion of the of Test the 2 Test samples 2 samples increased incr fromeased 18.9% from to18.9% 28.2–32.4%. to 28.2–32.4%. The friction The friction angle alsoangle slightly also slightly increased increased from from 11.9 11.9 to 13.0 to 13.0 and and 13.3%, 13.3%, respectively. respectively. The The free free swell swell ratioratio drasticallydrastically reduced to to 38–39% 38–39% from from 56.0%. 56.0%. The The laboratory-scale laboratory-scale model model test test proves proves the the ef- effectiveness of the CaCO precipitation technique in the in situ stabilization of expansive fectiveness of the CaCO3 3precipitation technique in the in situ stabilization of expansive soil.soil. Table 10. Comparison of physicochemical and geotechnical behavior of tests 1 and 2 with natural samples. Table 10. Comparison of physicochemical and geotechnical behavior of tests 1 and 2 with natural samples. Unloaded Free Swell Radial UnconfinedUnconfined Compressive Cohesion FrictionFriction Angle Free Swell Sample SwellingUnloaded Ratio Swelling Cohesion ◦ Ratio Sample RadialDistance DistanceStrength Compressive (UCS) (kPa) (kPa) ( ) Angle Ratio (%)Ratio (%) (kPa) (%) Natural Soil -Strength 137 (UCS) (kPa) 9.3 18.9 11.9(°) 56.0(%) NaturalTest 1 Soil 0.8D - 270 137 - 9.3 - 18.9 - 11.9 38.0 56.0 TestTest 1 1 1.9D 0.8D 256 270 ------40.0 38.0 TestTest 2 1 1.0D 1.9D - 256 1.0 - 32.4 - 13.0 - 40.0- Test 2 1.3D - 0.7 28.2 13.3 - Test 2 1.0D - 1.0 32.4 13.0 - Test 2 1.3D - 0.7 28.2 13.3 - 4. Conclusions

4. ConclusionsChemically induced CaCO3 precipitation by sequentially introducing and mixing CaCl2 and Na2CO3 solutions into expansive soil was used as a stabilization and soil Chemically induced CaCO3 precipitation by sequentially introducing and mixing improvement procedure. Based on the results of free swell tests, unloaded swelling CaCl2 and Na2CO3 solutions into expansive soil was used as a stabilization and soil im- tests, geotechnical strength tests, SEM imaging and wetting–drying tests, the following provement procedure. Based on the results of free swell tests, unloaded swelling tests, conclusions can be drawn: geotechnical strength tests, SEM imaging and wetting–drying tests, the following conclu- •sionsCaCO can be3 precipitationdrawn: produced by CaCl2 and Na2CO3 solutions is observed to cause a signiﬁcant improvement in stabilizing expansive soils. Sequentially mixing CaCl2 • CaCO3 precipitation produced by CaCl2 and Na2CO3 solutions is observed to cause a followed by Na2CO3 solution shows a better performance than mixing Na2CO3 significant improvement in stabilizing expansive soils. Sequentially mixing CaCl2 fol- followed by CaCl2 solution on free swell ratio, decreasing free swell ratio from 32% lowed by Na2CO3 solution shows a better performance than mixing Na2CO3 followed for the latter case with 21.2% Na2CO3 and 27.5% CaCl2 to 27.0% for the former case by CaCl2 solution on free swell ratio, decreasing free swell ratio from 32% for the

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with 27.5% CaCl2 and 21.2% Na2CO3, both of which are almost half of the value of 56.0% for natural soil sample. • Unloaded swelling ratio is decreased from 9.3% for the natural soil sample to 0.6% for the treated sample with 27.5% CaCl2 and 21.2% Na2CO3 after curing periods of 28 days. • Evident increases in UCS and cohesion are noted with the addition of CaCl2 and Na2CO3 solutions. Furthermore, there is an extra minor increase in the strength of soil samples when the concentration of CaCl2 is slightly excessive. However, it is found that there may be a slight decrease in the UCS and cohesion of soil when the concentration of CaCl2 is higher than 27.5%. Meanwhile, the concentration of solutions has a minor influence on friction angle. • According to SEM analysis, the treated sample exhibits more densification and pore- size refinement, presenting denser microstructure than the natural sample. The proposed chemical improvement method can suppress crack formation effectively during the wetting–drying process, and thus keeping the strength parameters at a relatively high value. The results of the laboratory-scale precipitation model test show that the precipitation technique significantly reduces the swelling potential and increases the strength properties of the expansive soil. Therefore, the presented laboratory investigation clearly demonstrates the effectiveness of the CaCO3 precipitation technique in stabilizing expansive soil. The laboratory-scale model studies also aid in further investigation pilot-scale field trials for evaluating the practical engineering significance. In terms of field implementation of the stabilization procedure, it is also proposed to spray the two solutions on the surface of soils and allow for the solution to seep naturally into the ground to improve expansive soils in situ. This method will not require extensive earthwork and re-working of the soil and is deemed more cost-effective than existing methods. In further investigation, it is worthy to conduct tests with different concentration types so as to establish a model to predict the strength properties.

Author Contributions: Conceptualization, S.H. and M.G.; methodology, B.W.; writing—original draft preparation, S.H.; writing—review and editing, M.G.; supervision, B.W. and M.G.; data curation, Y.S. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX17_0469) and the Fundamental Research Funds for the Central Universities (No. 2017B647X14). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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