materials Article
Effects of Initial Water Content on Microstructure and Mechanical Properties of Lean Clay Soil Stabilized by Compound Calcium-Based Stabilizer Chenglong Yin, Wei Zhang , Xunli Jiang and Zhiyi Huang * College of Civil Engineering and Architecture, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China;
[email protected] (C.Y.);
[email protected] (W.Z.);
[email protected] (X.J.) * Correspondence:
[email protected]; Tel.: +86-0571-8820-8702 Received: 15 September 2018; Accepted: 8 October 2018; Published: 10 October 2018
Abstract: Initial water content significantly affects the efficiency of soil stabilization. In this study, the effects of initial water content on the compressibility, strength, microstructure, and composition of a lean clay soil stabilized by compound calcium-based stabilizer were investigated by static compaction test, unconfined compression test, optical microscope observations, environment scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The results indicate that as the initial water content increases in the range studied, both the compaction energy and the maximum compaction force decrease linearly and there are less soil aggregates or agglomerations, and a smaller proportion of large pores in the compacted mixture structure. In addition, for specimens cured with or without external water supply and under different compaction degrees, the variation law of the unconfined compressive strength with initial water content is different and the highest strength value is obtained at various initial water contents. With the increase of initial water content, the percentage of the oxygen element tends to increase in the reaction products of the calcium-based stabilizer, whereas the primary mineral composition of the soil-stabilizer mixture did not change notably. Keywords: initial water content; clay soil; calcium-based stabilizer; soil stabilization; compressibility; strength; microstructure and composition
1. Introduction From a historical perspective, clayey soil, derived from the rock weathering process of the geological cycle [1,2], already existed long before humankind. From the moment this material was used, its volume and strength changing property with seasonal moisture variation has frequently been detrimental to buildings and structures constructed on it, which threatens people’s safety and costs a large sum of money annually [3–7]. In order to control or constrain the deflections and movement of clayey soils and enhance its compressibility, strength, stiffness, the resistance to water and cracking, and other engineering properties, a variety of inorganic, organic, and biological materials such as lime [8–11], Portland cement [12–15], fly ash [16,17], granulated blast furnace slag [18–20], cement kiln dust [21,22], rice husk ash [23,24], polyacrylamide copolymers [25,26], bioenergy coproduct [27,28], and so on, in the form of powder or liquid, was mixed with this problematic material with or without extra water before compaction. This operation is usually referred as soil stabilization. Soil stabilization is a traditional but cost-effective technique in civil engineering, and finds its prevailing applications in pavement base, subbase, or embankment, canal or reservoir lining, shallow building foundations, and stabilized rammed earth constructions among others, especially for locations where relatively high-cost materials such as gravel and crushed stone are unavailable or have a long transportation distance while the budget of the construction project is probably limited [7,29,30].
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Lime, Portland cement, and fly ash are the most commonly used binding materials in soil stabilization and they all comprise of a calcium element, which can compose two main kinds of binding gels in soil stabilization: calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH), and because of which they are called calcium-based stabilizer. These calcium-based stabilizers are usually in the form of grinded powder. During soil stabilization practice, they are combined with soil and a certain amount of water to form the soil–water–binder reaction system. It is the chemical or physico-chemical reactions (or both) in the system that essentially transforms the properties of the soils. There are many factors affecting the efficiency of soil stabilization. Terashi [31] has categorized them into four groups: (a) characteristics of soils (soil structure, clay minerals, particle size distribution, plasticity index, cation exchange capacity, pH, contents of sulphate and organic matter, and so on); (b) characteristics of binders (binder types, dosage methodology, corresponding reaction processes such as dissolution, diffusion, and precipitation, products of chemical or physico-chemical reactions, and so on); (c) mixing and compaction procedures (pulverization, mixing uniformity, initial water content (IWC), dry density, and the like); and (d) curing procedures (temperature, air moisture, curing time, and the like). Among all the influential factors of soil stabilization, initial water content is of first-level importance. Firstly, the reaction process of the soil–water–stabilizer reaction system can be simplified as dissolution-precipitation [32–34], in which calcium-based binding materials and reactive minerals of soil first dissolve into water and then precipitate on the surfaces of soil particles to fill the soil pores (macropores and micropores [35]). Therefore, without water the reaction in the soil–water–stabilizer system will not occur. Secondly, water is a strong polar molecule and has a very powerful affinity to soil minerals. Once water goes into the soil structure, a diffused double-layer (DDL) microstructure [36,37] is formed around soil particles and influences the pore size distribution, matric suction, compressibility, and shear strength of the soils. Initial water content as the first-considered and quality-controlled factor in soil stabilization, has been studied by other researchers. Most of the research focuses on the effects of initial water content (IWC) on the strength property of the soil treated by stabilizers, but different studies report different results and conclusions. For example, Ramesh and Sivapullaiah [38] compared the development of strength of black cotton soil (a type of vertisol with high volume change capacity) stabilized by lime under different IWCs and found that the strength of lime-stabilized black cotton soil increases rapidly when the specimens are compacted at a water content slightly lower than the optimum moisture content (the water content corresponding to the maximum dry density on the curve of Proctor compaction test). However, Guo et al. [39] analyzed the influence of IWC on the strength of lime-modified expansive soil, and considered that the optimum IWC in the construction of lime-modified expansive soil should be about 3 percentage points higher than the optimum water content. Consoli et al. [40] studied the effects of IWC on the strength of a lime-treated sandy lean clay and drew the conclusion that at the same curing age, the strength was not affected by the IWC. Besides, Consoli et al. [41] also investigated the effects of IWC on the strength of cement-stabilized clayey sand and found that the unconfined compressive strength (UCS) first increased, then was followed by a decrease with the increase of IWC. He speculated that this was attributed to the different structure formed during compaction where IWC played a fundamental role. Arora and Aydilek [42] investigated the relation between UCS and IWC of a cement-treated sandy soil-fly ash mixture and concluded that after the same curing period, the increase of IWC generally resulted in the decrease of the UCS. In addition, they ascribed this to the cementitious reactions, in which higher water/cement ratio impaired the strength development. As a result, these contradictory results could not lead to a defined recognition on the determination of optimum IWC in soil stabilization and there was not enough information in the existing research to explain the reason why the IWC affects the strength evolution of the stabilized soils. Herein, the objective of this study is to investigate the effects of IWC on the mechanical properties (compressibility and strength), microstructure, and composition of a lean clay soil stabilized by a
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stabilized by a compound calcium-based stabilizer comprised of cement, lime, and fly ash, and explain how the microstructure and composition variation of the soil-stabilizer mixture with different compound comprised of cement, lime, aand fly of ash, andcompaction explain how the IWCs affectscalcium-based the mechanicalstabilizer properties. To achieve this objective, series static tests, microstructure and composition variation of the soil-stabilizer mixture with different IWCs affects unconfined compression tests, optical microscope observations, Environment Scanning Electron the mechanical properties. To achieve this objective, series of Microscope static compaction tests,with unconfined Microscope (ESEM) scanning, Environment Scanninga Electron combined Energy compression tests, optical microscope observations, Environment Scanning Electron Microscope Dispersive X-ray (ESEM-EDAX) analysis, and X-ray Diffraction (XRD) were conducted. Hopefully, (ESEM) scanning, Environmentcan Scanning Microscopeofcombined Energy Dispersive the results of this manuscript help inElectron the determination optimumwith initial water content ofX-ray soil (ESEM-EDAX) analysis, and X-ray Diffraction (XRD) were conducted. Hopefully, the results of this stabilization practice. manuscript can help in the determination of optimum initial water content of soil stabilization practice. 2. Materials and Methods 2. Materials and Methods 2.1. Material Properties 2.1. Material Properties 2.1.1. 2.1.1. Soil Soil The The soil soil used used in in the the tests tests was was aa natural natural yellowish-brown yellowish-brown muddy muddy soft soft soil soil taken taken (according (according to to ASTM D6282/D6282M-14 [43]) from the bottom of a 1.5 m deep borrow pit near Hangzhou, ASTM D6282/D6282M-14 [43]) from the bottom of a 1.5 m deep borrow pit near Hangzhou, China, China, which did not not contain contain large largeparticles particlesofofsand, sand,gravel, gravel,orororganic organic matter. After drying in the oven at which did matter. After drying in the oven at the ◦ the temperature of 105 °C for 3 days, the soil was pulverized to pass No. 4 (4.75 mm) sieve and temperature of 105 C for 3 days, the soil was pulverized to pass No. 4 (4.75 mm) sieve and deposited deposited a sealed plastic drum the tests to JTG [44].size Thedistribution particle size in a sealedin plastic drum for the tests for according to according JTG E51-2009 [44].E51-2009 The particle of distribution driedinsoil is shown in 1Figure Table 1 shows the physical of the XRD soil. the dried soilofisthe shown Figure 1. Table shows1.the physical properties of the properties soil. Combining Combining XRD with ESEM-EDAX analysis, can foundcontained that the soil clay with ESEM-EDAX analysis, it can be found thatitthe soilbemainly clay mainly mineralscontained of clinochlore, minerals of clinochlore, montmorillonite, and illite; and other minerals such as quartz and muscovite. montmorillonite, and illite; and other minerals such as quartz and muscovite. The principal mineral The principalof mineral of the the XRD soil ispattern marked on theinXRD pattern in content Figure 2.inThe composition the soilcomposition is marked on shown Figure 2. Theshown organic the organic content in the soil was low. According to the Unified Soil Classification System (ASTM soil was low. According to the Unified Soil Classification System (ASTM D2487-2017 [45]), the soil is D2487-2017 the soil classified as lean clay (CL) type. classified as[45]), lean clay (CL)istype.
Percentage finer by weight (%)
100 90 80 70 60 50 40 30 20 10 0 0.001
0.01
0.1
1
Particle size (mm)
Figure Figure 1. 1. Soil Soil particle particle size size distribution. distribution. Table 1. Physical properties of soil sample. Table 1. Physical properties of soil sample. Natural DryDry Dried Natural Dried Density Moisture Density Moisture (g/cm3 ) Content (%)
(g/cm3) 1.64
1.64
Content (%) 2.94 2.94
Liquid Plastic Specific Liquid Plastic Specific Gravity LimitLimit (%) Limit (%) Limit Gravity
2.69
2.69
(%) 37.8
37.8
(%) 19.3
19.3
Plasticity
Plasticity Index (%) Index (%) 18.5
18.5
Activity
Activity of Clay of Clay 2.02
2.02
pH
pH
6.55
6.55
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Intensity (a.u.) Intensity (a.u.)
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♦ Quartz ♣ ♦ Muscovite-2M1 Quartz ♥ ♣ Clinochlore-1M1b Muscovite-2M1 ♠ ♥Montmorillonite-15A Clinochlore-1M1b ∇♠Illite-2M1[NR] Montmorillonite-15A ∇ Illite-2M1[NR]
5000 5000
∇♦ ♠ ♦ ∇ ∇♥ ∇♠ ♥ ♦ ♣ ♥♦ ♦ ♣ ♦ ♠♣ ♣ ♣ ♦ ♥ ♦♦ ∇ ♣♣ ∇ ♠ ♣ ∇ ♦ ∇♦ ♦♦♦♦ ♣♦ ♦ ♦ ♥♠ ♣ ♥ ∇ ♦ ♦ ♣♣ ♣ ♦ ♣♣ ♠ ♣ ♦♦ ♦ 0 ♦ ∇ ♦ ♦♦♦♦ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 ♠∇ ♥♣ ♥ ♠∇ ♥♣ ♥
0
5 10 15 20 25 30 35 402θ(°) 45 50 55 60 65 70 75 80 85
2θ(°)
Figure mineral composition compositionofofsoil. soil. Figure2.2. Principal Principal mineral Figure 2. Principal mineral composition of soil.
2.1.2. Compound 2.1.2. CompoundCalcium-Based Calcium-BasedStabilizer Stabilizer 2.1.2. Compound Calcium-Based Stabilizer The usedininthe the study was composed of cement, lime,flyand Thecalcium-based calcium-based stabilizer stabilizer used study was composed of cement, lime, and ash.fly Theash. The cement was Type 325 Ordinary Portland cement, which was produced by local Qianchao Portland The calcium-based stabilizer used in the study was composed of cement, lime, and fly ash. The cement was Type 325 Ordinary Portland cement, which was produced by local Qianchao Portland Cement Company (Hangzhou, China). The lime used in the research was provided by Hangzhou cement was Type 325 Ordinary Portland cement, which was produced by local Qianchao Portland Cement Company (Hangzhou, China). The lime used in the research was provided by Hangzhou Cement Company(Hangzhou, (Hangzhou, China). The lime used the research was powder provided by Hangzhou Tuohai Corporation China), which waswas a finely ground limelime powder with with 85% total Tuohai Corporation (Hangzhou, China), which a in finely ground 85% content total Tuohai Corporation (Hangzhou, which aC finely ground limeobtained powder with Shaoxing 85% total of content CaO and MgO. TheMgO. fly ash was high calcium Class fly ash from Shaoxing Shangyu of CaO and The flyChina), ash was high was calcium Class Cobtained fly ash from content ofHangzhou-union CaOcogeneration and MgO. cogeneration The flyLtd. ash(Shaoxing, was calcium Class flyTable ash obtained from Shaoxing Shangyu Co., high Ltd. (Shaoxing, China). 2the presents thecomposition chemical Hangzhou-union Co., China). Table 2Cpresents chemical Shangyu cogeneration Co., Ltd. (Shaoxing, China). Tablecalcium-based 2 presents thestabilizer chemical of the three additives. The size distribution of the mixed of composition the three Hangzhou-union additives. The particle sizeparticle distribution of the mixed calcium-based stabilizer is shown composition of the three additives. The particle size distribution of the mixed calcium-based stabilizer shown in is Figure 3. in Figure 3. is shown in Figure 3. Table2.2.Chemical Chemicalcomposition composition of Table of the the calcium-based calcium-basedstabilizers. stabilizers. Table 2. Chemical composition of the calcium-based stabilizers. Chemical Compositions (Mass Fraction, %) Chemical Compositions (Mass Fraction, %) Materials Compositions (Mass Fraction, %) SiO2 AlChemical 2O3 Fe2O 3 CaO Na 2O K2O MgO Materials Materials SiO2 Al22 O3Al2O Fe32 O3Fe2OCaO NaNa MgO 2 O 2O K 22O SiO 3 CaO K O MgO Portland cement 18.04 8.79 4.96 54.14 0.12 0.32 3.56 Portland cement 18.04 18.04 0.32 3.56 Portland 54.14 0.120.12 0.32 3.56 Limecement - 8.79 8.79 - 4.96 4.96 - 54.1486.26 0.68 LimeFly - 11.61 -0.68 Lime - - 21.73 - - 1.75 - 86.2640.28 86.26 -0.95 0.68 ash 1.36 0.49 Fly ashFly ash 11.61 11.61 21.73 21.731.75 1.7540.2840.28 0.950.95 1.36 0.49 1.36 0.49
TiO2 TiO TiO - 22 -- - 1.66 1.66 1.66
SO3 SO3SO3 1.77 1.77 - 1.77 - 0.61 0.610.61
Volume percentage Volume percentage (%)(%)
5 5 4 4 3 3 2 2 1 1 0 0.1 0 0.1
1 10 1 Particle size 10 (μm)
100 100
Particle size (μm) Figure 3. Particle size distribution of compound calcium-based stabilizer. Figure3.3.Particle Particlesize sizedistribution distribution of compound compound calcium-based Figure calcium-basedstabilizer. stabilizer.
2.2. Experimental Program 2.2.2.2. Experimental Program Experimental Program The experimental program included two parts. First, the effects of IWC on the compaction The experimental included two First, the theeffects effects IWC onthe thecompaction compaction The experimental included two parts. parts. First, ofofIWC on properties and UCS program ofprogram the soil calcium-based stabilizer mixture were investigated by static properties andand UCSUCS of theofsoil stabilizer mixturemixture were investigated by static compaction properties thecalcium-based soil calcium-based stabilizer were investigated by static
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tests and unconfined compression tests. Then, the variation of microstructure and composition of compaction tests tests and and unconfined unconfined compression compression tests. Then, Then, the the variation variation of of microstructure microstructure and and compaction the mixture with different IWCs were studied bytests. optical microscope observation, ESEM scanning, composition of the mixture with different IWCs were studied by optical microscope observation, compositionanalysis, of the mixture with different IWCs were studied by optical microscope observation, ESEM-EDAX and XRD spectra analysis. ESEM scanning, scanning, ESEM-EDAX ESEM-EDAX analysis, analysis, and and XRD XRD spectra spectra analysis. analysis. ESEM 2.2.1. Mixture Design 2.2.1. Mixture Mixture Design Design 2.2.1. As is shown in Table 1, the soil is slightly acidic with a pH of 6.55. According to former As is is shown shown in in Table Table 1, 1, the the soil soil is is slightly slightly acidic acidic with with aa pH pH of of 6.55. 6.55. According According to former former studies studies studiesAs [46,47], cement cannot fully hydrate and harden to form CSH and CAHtogels until the pH [46,47], cement cannot fully hydrate and harden to form CSH and CAH gels until the pH arrives at aa [46,47], cannot fully hydrate and harden to form CSHofand until pH arrives at arrives atcement a certain value. Therefore, we conducted a series pHCAH testsgels using thethe method presented certain value. value. Therefore, Therefore, we we conducted conducted aa series series of of pH pH tests tests using using the the method method presented presented by by Eades Eades and and bycertain Eades and Grim [48] to determine reasonable initial lime and cement content. Figure 4 shows Grim [48] to determine reasonable initial lime and cement content. Figure 4 shows that lime at the Grim [48] to determine reasonable initial lime and cement content. Figure 4 shows that lime at the that lime of at1% thedry weight of 1% dry soil can of promote theup pH of soil slurry up tothe 12.3 and after that, weight soil can promote the pH soil slurry to 12.3 and after that, increase of lime weight of 1% dry soil can promote the pH of soil slurry up to 12.3 and after that, the increase of lime thecontent increase of lime content cannot change pH significantly, so the initial lime content was set to cannot change change pH pH significantly, significantly, so so the the initial initial lime lime content content was was set set to to 1% 1% weight weight of of dry dry soil. soil.1% content cannot weight soil. Figure indicates that in the cement content studied, cement content Figureof55dry indicates that in in 5the the cement content content range studied, whenrange cement contentwhen reaches 4%, the the pH Figure indicates that cement range studied, when cement content reaches 4%, pH reaches 4%, the pH value of diluted cement slurry is stable and the cement-soil slurry curve is not value of diluted cement slurry is stable and the cement-soil slurry curve is not far from that of cementvalue of diluted cement slurry is stable and the cement-soil slurry curve is not far from that of cementfarsoil from thatThus, of cement-soil Thus, 4% weight of dry soil was chosen to beThe thecontent initial cement slurry. 4% weight weightslurry. of dry dry soil soil was chosen to be be the initial initial cement content. of fly fly soil slurry. Thus, 4% of was chosen to the cement content. The content of content. The aims content of fly ash, which aims to to provide pozzolans, was set to Considering be 1% weight offly dry ash, which to provide pozzolans, was set be 1% weight of dry weight. that ash, which aims to provide pozzolans, was set to be 1% weight of dry weight. Considering that fly weight. Considering that fly ash can consume a certain amount of lime by pozzolanic reaction, the lime ash can consume a certain amount of lime by pozzolanic reaction, the lime content was accordingly ash can consume a certain amount of lime by pozzolanic reaction, the lime content was accordingly content wasto adjusted 2% weight of the drymixture soil. Consequently, the was designed adjusted toaccordingly 2% weight weight of of dry soil. soil.toConsequently, Consequently, the mixture was designed designed tomixture be cement:lime:fly cement:lime:fly ash:to adjusted 2% dry was to be ash: bedry cement:lime:fly ash: dry soil equal to 4:2:1:100. Afterwards, we conducted a series of unconfined dry soil equal to 4:2:1:100. Afterwards, we conducted a series of unconfined compressive tests with soil equal to 4:2:1:100. Afterwards, we conducted a series of unconfined compressive tests with four different different combinations of cement, cement, lime, fly fly ash, ash, and dry drylime, soil (4:1:1:100; (4:1:1:100; 4:2:1:100; 3:3:1:100; compressive tests with four different combinations of cement, fly ash, and dry soil3:3:1:100; (4:1:1:100; four combinations of lime, and soil 4:2:1:100; 2:4:1:100). The results show that the combination with cement: lime: fly ash: dry soil equal to 4:2:1:100 4:2:1:100; 3:3:1:100; 2:4:1:100). Thethe results show that combination cement: lime:tofly ash: dry 2:4:1:100). The results show that combination withthe cement: lime: flywith ash: dry soil equal 4:2:1:100 has the highest highest unconfined compressive strength, and was was thereby thereby adopted as thereby the final finaladopted mixtureas soil equal to 4:2:1:100 has the highest unconfined compressive strength, and was has the unconfined compressive strength, and adopted as the mixture proportion for the the tests. theproportion final mixture proportion for the tests. for tests. 13 13 12 12 11 11
pH pH
10 10 99 88 77 66 55 0 0
11
22
33
44
55
66
77
88
99
10 11 11 12 12 10
Lime content content (%) (%) Lime
Figure4.4. 4.pH pH variation variation with with lime lime content. Figure with lime content. content. Figure pH variation 13 13 12 12 11 11
pH pH
10 10
diluted cement cement slurry slurry diluted cement-soil slurry slurry cement-soil
99 88 77 66 0 0
11
22
33
44
55
66
77
88
99
10 11 11 12 12 10
Cement content content (%) (%) Cement
Figure5.5. 5.pH pH variation variation with cement content. Figure with cement cementcontent. content. Figure pH variation
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2.2.2. 2.2.2. Molding Points Points Design Design As As the the reference reference point, point, the the maximum maximum dry dry density density and and optimum optimum moisture moisture content content of of the the mixture mixture were and 15%, 15%, respectively, respectively, by modified Proctor compaction test (Figure 6). were tested tested to to be be 1.79 1.79g/cm g/cm33 and Specimens Specimens with with different different IWCs IWCs (11%, (11%, 13%, 13%,15%, 15%,17%, 17%,19%, 19%,21%) 21%)and andtwo twodry drydensities, densities,1.79 1.79g/cm g/cm33 3 3 (100% degree) and and 1.72 1.72g/cm g/cm(96% (96% compaction degree), were made for tests. the tests. (100% compaction degree) compaction degree), were made for the The The molding points A and B)shown are shown in Figure For each dry density and water content, molding points (line(line A and B) are in Figure 6. For6.each dry density and water content, there there 30 parallel specimens. are 30are parallel specimens. 1.85
Dry density (g/cm3)
1.80
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
A6
1.75 1.70 1.65 line A (100% compaction degree) line B (96% compaction degree) modified Proctor compaction curve
1.60 1.55
8
10
12
14
16
18
20
22
Water content (%) Figure 6. 6. Modified Modified Proctor Proctor compaction compaction curve curve and and the the molding molding points. points. Figure
2.2.3. 2.2.3. Specimen Specimen Molding Molding Procedures Procedures The The specimens specimens made made in in the the study study were were cylindrical, cylindrical, 50 50 mm mm high, high, and and 50 50 mm mm in in diameter. diameter. During During the molding process, each specimen was strictly made according to the following molding procedures: the molding process, each specimen was strictly made according to the following molding •procedures: First, the water contents of the oven-dried soil and the calcium-based stabilizer were measured
• • •
with of T0801-2009 standard soil [44].and the calcium-based stabilizer were measured First,the themethod water contents of the in oven-dried Then, the amount of dry soil and stabilizer needed for 6 specimens (controlled by the blender with the method of T0801-2009 in standard [44]. used) calculated, weighed, andstabilizer mixed byneeded the blender for about 5 (controlled min until the Then, was the amount of dry soil and for 6 specimens bydry the mixture blender was uniformly consistent. used) was calculated, weighed, and mixed by the blender for about 5 min until the dry mixture • Next, the calculated volume of water was added and mixed for another 5 min until a homogenous was uniformly consistent. soil-stabilizer mixture formed. that, the mixture carefully with a5sealing film toa • Next, the calculated volumeAfter of water was addedwas and mixedcovered for another min until prevent moisture loss. homogenous soil-stabilizer mixture formed. After that, the mixture was carefully covered with a sealing film to prevent Finally, the quantity of themoisture mixture loss. for one specimen was weighed and put in the mold that had • • Finally, the quantity of the mixture two cylindrical compaction blocks. for one specimen was weighed and put in the mold that had two cylindrical compaction blocks. 2.2.4. Static Compaction Test 2.2.4.The Static Compaction compaction testTest was performed on a 30 kN hydraulic pressing machine according to JTG E51-2009 [44]. After being filled up, the mold was moved on the pressing machinetotoJTG be The compaction test was performed on a 30 kNrapidly hydraulic pressing machine according statically The compaction process was displacement-controlled a displacement E51-2009 compacted. [44]. After being filled up, the mold was rapidly moved on thewith pressing machine torate be of 1 mm/min. During the compaction process, the compaction data of all specimens were recorded statically compacted. The compaction process was displacement-controlled with a displacement rate in by the sensors frequency of 50 Hz) installed thespecimens pressing machine. When of detail 1 mm/min. During the(sampling compaction process, the compaction data on of all were recorded in the specimens were compacted the designated dimension of 50 high and machine. 50 mm in When diameter, detail by the sensors (samplingtofrequency of 50 Hz) installed on mm the pressing the the load was keptcompacted stable for two minutes beforedimension unloading.ofThen, thehigh specimens wereindemolded specimens were to the designated 50 mm and 50 mm diameter,and the the height, andfor diameter of eachbefore specimen was measured and documented. loadmass, was kept stable two minutes unloading. Then, the specimens were demolded and the mass, height, and diameter of each specimen was measured and documented.
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2.2.5. Specimen Curing Process To make comparisons, after compaction half of the specimens were placed in plastic sealing bags to avoid external moisture intrusion and the other half were not treated. Then, all the specimens were transferred to the curing room with a temperature at 20 ± 1 ◦ C and humidity at 95 ± 5% in accordance with JTG E51-2009 [44]. Each of the 6 specimens (3 in bags and the other 3 without bags) were marked as a group. Thus, specimens with the same IWC and dry density were divided into five groups which were respectively cured to 1, 3, 7, 14, and 28 days. In order to only investigate the effects of IWC, all the specimens were kept as they were after different curing ages and they were not submerged into water to saturate. 2.2.6. Unconfined Compression Test The unconfined compression tests were conducted on the 30 kN hydraulic pressing machine at a displacement rate of 1 mm/min according to ASTM D2166 [49]. Before the unconfined compression test, the mass, height, and diameter of each specimen were measured again. 2.3. Microstructure and Composition Research After the compression test, all the tested specimens were submerged into anhydrous ethyl alcohol for 1 day and oven-dried at 70 ◦ C for another day to terminate the hydration process. Small parts from the unbroken district of the specimens were carefully taken out for microstructure and composition research. 2.3.1. Optical Microscope Observation To quickly and simply investigate the structure change of the dried soil, calcium-based stabilizer, and soil-stabilizer mixture with the variation of IWC in real time, the optical microscope observation was performed. The optical microscope used in the study is the Lecia DM750 (Lecia Microsystems Inc., Buffalo Grove, IL, America) equipped with the Leica ICC50 (Lecia Microsystems Inc., Buffalo Grove, IL, America) digital camera module, which can readily take photos and preserve the images. During observation, the soil, stabilizer, and the mixture were evenly placed on an object slide as a thin layer and there was not a coverslip on them. To protect the microscope, the object lens cannot be pushed too close and thus the magnification times was limited to 40. 2.3.2. ESEM Scanning and ESEM-EDAX Analysis The morphological structure of the specimens cured in plastic bags, in a much smaller scale than that of the optical microscope, was obtained by ESEM scanning. The ESEM used here was FEI Quanta 650 FEG (FEI., Hillsboro, OR, America) with an acceleration voltage range of 200 V–30 kV and maximum beam current of 200 nA. The ESEM was also equipped with an EDAX detector, by which the element composition of the designated points of the specimens was acquired. 2.3.3. XRD Analysis The mineral change of the specimens with different IWCs at different curing ages was analyzed by means of powder XRD. The XRD patterns were collected by PANalytical X’Pert PRO diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu-Kα radiation (λ = 1.5418 Å); exploration range 5◦ to 80◦ ·2θ; steps of 0.026◦ ·2θ; and goniometer speed of 0.001◦ ·2θ·s−1 . 3. Results 3.1. Static Compaction Test Results Although there are some differences among each curve even in the same IWC group, typical compaction curves of specimens with different IWCs are shown in Figure 7. The small horizontal
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Compaction Compactionforce force(kN) (kN)
arrows on the curves mark the points where the specimens are compacted to the designated dimension. dimension. The The curve curve for for IWC IWC at at 21% 21% does does not not have have an arrow arrow because because the the specimens specimens were were saturated saturated dimension. The curve for IWC at 21% does not have an arrow an because the specimens were saturated during during compaction and could not be fully compacted. Figure 8 shows that as the IWC increases from during compaction and could not be fully compacted. Figure 8 shows that as the IWC increases compaction and could not be fully compacted. Figure 8 shows that as the IWC increases fromfrom 11% 11% to to 19%, the the maximum maximum compaction compaction force force decreases decreases from from around around 17 to to 12 kN, kN, while while the the total total axial axial 11% to 19%,19%, the maximum compaction force decreases from around 1717 to 1212kN, while the total axial deformation increases increases from from about about 29 to to 34 34 mm. mm. The The slope of of each each curve, curve, which which reflects the the soil soil deformation deformation increases from about 29 29 to 34 mm. The slopeslope of each curve, which reflects reflects the soil stiffness stiffness under different stress states, also varies slightly with different IWCs. Figure 8 shows the stiffness under stress different stress varies slightly with different Figure 8 shows the under different states, alsostates, varies also slightly with different IWCs. FigureIWCs. 8 shows the secant slopes secant slopes slopes from from original original point to to points points of of half half maximum maximum compaction compaction force. It It indicates that that the secant from original point to pointspoint of half maximum compaction force. It indicatesforce. that theindicates mean stiffnessthe of mean stiffness of the soil decreases in the form of cubic function as IWC increases from 11% to 19%. mean stiffness of the soil decreases in the form of cubic function as IWC increases from 11% to 19%. the soil decreases in the form of cubic function as IWC increases from 11% to 19%. 25.0 25.0 22.5 22.5 20.0 20.0 17.5 17.5 15.0 15.0 12.5 12.5 10.0 10.0 7.5 7.5 5.0 5.0 2.5 2.5 0.0 0.0 0 0
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Figure 7. 7. Typical Typical compaction compaction curves. curves. Figure
Figure 8. 8. The The effect effect of of initial initial water water content content (IWC) (IWC) on on secant secant slope. slope. Figure 8. The effect of initial water content Figure (IWC) on secant slope.
To To better better understand understand the the effects effects of of IWCs IWCs on on the the compaction compaction properties properties of of the the mixture, mixture, the the To better understand the effects of IWCs on the compaction properties of the mixture, the maximum compaction force (Figure 9a) and compaction energyenergy (Figure(Figure 9b) of two groups of specimens maximum compaction force (Figure 9a) and compaction 9b) of two groups of maximum compaction force (Figure 9a) and compaction energy (Figure 9b) of two groups of at 100% compaction degree are further analyzed. The compaction energy is computed bycomputed the integral specimens at 100% compaction degree are further analyzed. The compaction energy is by specimens at 100% compaction degree are further analyzed. The compaction energy is computed by of compaction curve with the equation: thethe integral of the the compaction compaction curve with the the equation: equation: the integral of curve with n
Ec = ( Fi ++ =∑ 0.5 0.5( +Fi−1 )( )(xi −− −xi−1 ))) = 0.5( )( i =1
(1) (1) (1)
isthe thecompaction compaction energy; energy; Fi and andFi−1 are arethe thenumber numberiiiand andiii− compaction force, force, where the the Ec isis where compaction force, the compaction energy; and are the number and −− 111 compaction where the respectively; xi . and andxi−1 are arethe thenumber numberi iand i and andi i− i −−111displacement displacementrecorded recordedby bysensors. sensors. respectively; respectively; and are the number displacement recorded by sensors. It is observed that both the maximum compaction force and compaction energy decrease linearly linearly It is observed observed that that both both the the maximum maximum compaction compaction force force and and compaction compaction energy energy decrease decrease linearly It is with the the increase increase of of IWC. IWC. The The largest largest decline decline percentages percentages for for the the maximum maximum compaction compaction force force and and with with the increase of IWC. The largest decline percentages for the maximum compaction force and compaction energy are calculated to be 45.9% and 53.4%, respectively. compaction energy are calculated to be 45.9% and 53.4%, respectively. compaction energy are calculated to be 45.9% and 53.4%, respectively.
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(a) (b) (a) (b) Figure 9. Results of static compaction test: (a) maximum compaction force; (b) compaction energy. energy. Figure 9. Results of static compaction test: (a) maximum compaction force; (b) compaction energy.
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Unconfined Unconfinedcompressive compressivestrength strength(MPa) (MPa)
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3.2. 3.2. Unconfined Unconfined Compression Compression Test TestResults Results 3.2. Unconfined Compression Test Results Figure and IWC IWC at at different different curing curing ages ages (1day, (1day, 3days, 3days, Figure 10 10 presents presents the the relationship relationship between between UCS UCS and Figure 10 presents the relationship between UCS and (with IWC at different curing ages (1day,and 3days, 77 days, 14 days, and 28 days) under two curing conditions and without external water) days, 14 days, and 28 days) under two curing conditions (with and without external water) and two two 7degrees days, 14 days, and 28 days) under96%). two curing conditionsfor (with and without external water) and two degrees of of compaction compaction (100% (100% and and 96%). The The test test results results for specimens specimens cured cured without without external external water water degrees of compaction (100% and 96%). The test results for and specimens cured without external water supply degree) Figure 10b 10b (96% compaction degree). supply are are shown shown in in Figure Figure 10a 10a (100% (100% compaction compaction degree) and Figure (96% compaction degree). supply are shown in Figure 10a (100% compaction degree) and Figure 10b (96% compaction degree). Both Both of of the the two two figures figures show show the the same same variation variationtrend trendof ofUCS UCSregardless regardlessof ofthe thecompaction compactiondegree: degree: Both of the two figures show the same variation trend of UCS regardless of the compaction degree: 1. At early early curing curingage age(1 (1day dayand and33days) days)the theUCS UCSdecreases decreaseslinearly linearly IWC increases from 11% 1. At asas IWC increases from 11% to 1. At early curing age (1 day and 3 days) the UCS decreases linearly as IWC increases from 11% to to 19%; 19%; 19%; 2. After curing curing for 7 days and 14 days, the UCS value does not change 2. After change significantly significantly with with the the 2. After curing for 7 days and 14 days, the UCS value does not change significantly with the variation of IWC; variation of IWC; 3. Through a curing time of 28 days, a parabolic relationship between UCS and IWC can be 3. Through be 3. Through a curing time of 28 days, a parabolic relationship between UCS and IWC can be observed: the the UCS UCS first first increases increases and and then decreases with the increase of IWC, reaching peak observed: observed: the UCS first increases and then decreases with the increase of IWC, reaching peak value at 15% IWC, which is equal to the optimum optimum water content content derived derived from the the modified modified value at 15% IWC, which is equal to the optimum water content derived from the modified Proctor compaction test. Proctor compaction test. 2.6 2.6 2.4 2.4 2.2 2.2 2.0 2.0 1.8 1.8 1.6 1.6 1.4 1.4 1.2 1.2 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 10 10
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(a) (b) (a) (b) Figure 10. Effects of initial water content on unconfined compressive strength of specimens cured in Figure 10. Effects Effects of initial initial water water content content on on unconfined unconfined compressive compressive strength strength of specimens specimens cured in plastic bags: (a) dry density 1.79 g/cm333; (b) dry density 1.72 g/cm33.3 (a) dry density 1.79 g/cm g/cm; ;(b) (b)dry drydensity density1.72 1.72g/cm g/cm. . plastic bags: (a)
Additionally,ititcan can befound found thatininthe the first7 7days days the UCS develops fast and the increase rate Additionally, and the increase rate of Additionally, it canbe be foundthat that in thefirst first 7 daysthe theUCS UCSdevelops developsfast fast and the increase rate of UCS accelerates with higher IWC; but afterwards the strength development gets slow and the UCS accelerates withwith higher IWC;IWC; but afterwards the strength development gets slow and the and increase of UCS accelerates higher but afterwards the strength development gets slow the increase rate firstthen enlarges theninreduces in therange variation range of IWC, reachingatmaximum at 15% rate first enlarges reduces the variation of IWC, reaching maximum 15% IWC. increase rate first enlarges then reduces in the variation range of IWC, reaching maximum at 15% IWC. IWC.
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Unconfinedcompressive compressivestrength strength(MPa) (MPa) Unconfined
As Figure 11 11 presents thethe unconfined compression test test results for specimens cured As comparison, Figure presents unconfined compression results for Asa aacomparison, comparison, Figure 11 presents the unconfined compression test results for specimens specimens incured moist air. It can be clearly found that after a curing time of 14 and 28 days, for specimens with in moist air. It can be clearly found that after a curing time of 14 and 28 days, for specimens cured in moist air. It can be clearly found that after a curing time of 14 and 28 days, for specimens with 96% of the UCS is the IWC is 15%, 96% degree of compaction, the highest UCS value is obtained when when the IWC 15%, equals with 96% degree degree of compaction, compaction, the highest highest UCS value value is obtained obtained when the is IWC is which 15%, which which equals the optimum water content of modified Proctor compaction test; whereas under 100% degree the optimum water content of modified Proctor compaction test; whereas under 100% degree equals the optimum water content of modified Proctor compaction test; whereas under 100% degreeof of specimens compacted at IWCs (11% and 13%) tend have higher UCSs. compaction, the the specimens compacted at lower IWCs (11% and 13%) tend toto have higher UCSs. of compaction, compaction, the specimens compacted at lower lower IWCs (11% and 13%) tend to have higher UCSs. 7days 7days curing curing age, age, 96% 96% compaction compaction degree degree 14days 14days curing curing age, age, 96% 96% compaction compaction degree degree
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Initial Initial water water content content (%) (%) Figure 11. Effects of initial water content unconfined Figure compressivestrength strengthof ofspecimens specimenscured curedin Figure11. 11.Effects Effectsof ofinitial initialwater water content content on on unconfined unconfined compressive compressive strength of specimens cured inin moist air. moist moistair. air.
3.3. Optical Microscope Observations 3.3. 3.3. Optical Optical Microscope Microscope Observations Observations Figure 12 shows the comparison ofthe the structure of wet soil and soil. dry soil. When theovensoil is Figure Figure 12 12 shows shows the the comparison comparison of of the structure structure of of wet wet soil soil and and dry dry soil. When When the the soil soil is is ovenoven-dried to a water content about 3%, smaller particles and aggregates are generally attracted dried dried to to aa water water content content about about 3%, 3%, smaller smaller particles particles and and aggregates aggregates are are generally generally attracted attracted to to the the tosurfaces the surfaces of larger ones by attractive forces (mainly electrostatic forces according to former of larger ones by attractive forces (mainly electrostatic forces according to former research surfaces of larger ones by attractive forces (mainly electrostatic forces according to former research research [1,50]), forming aand spatial and porous However, structure. after However, after water intothe the [1,50]), aa spatial porous structure. water into the [1,50]), forming forming spatial and porous structure. However, after water intrusion intrusion intointrusion the dry dry soil, soil, the dry soil, the original structure immediately breaks down to a dispersed fabric with clearer particle original original structure structure immediately immediately breaks breaks down down to to aa dispersed dispersed fabric fabric with with clearer clearer particle particle boundaries boundaries boundaries and better transparency. When part of the wet soil is moved with the tip of and better transparency. When part of the wet soil is moved with the tip of toothpick, the adjacent and better transparency. When part of the wet soil is moved with the tip of toothpick, the toothpick, adjacent the adjacent soil particles do not move. However, when the dry soil is moved in the same way, soil particles do not move. However, when the dry soil is moved in the same way, the adjacent soil particles do not move. However, when the dry soil is moved in the same way, the adjacent soil soil the adjacent soil matric particles and aggregates in a certain distance move together. This optical matric matric particles particles and and aggregates aggregates in in aa certain certain distance distance move move together. together. This This optical optical microscope microscope observation indicates water decrease the force between soil and make the microscope indicates that water can decrease the attractive between soil particles observationobservation indicates that that water can can decrease the attractive attractive force between force soil particles particles and make the soil structure easier to change. and make the soil structure easier to change. soil structure easier to change.
Figure 12. Structure of wet and dry soil (right). Figure12. 12. Structure Structure of of wet wet soil soil (left) (left) Figure soil (left) and and dry dry soil soil(right). (right).
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Another Another interesting interesting phenomenon phenomenon from from Figure Figure 13 13 is is that that when when the the water water boundary boundary gets gets close close to to Another interesting phenomenon from Figure 13quickly is that when the water boundary gets close to the the dry soil, the whole aggregate will be pulled into the water, while when the water the dry soil, the whole aggregate will be pulled quickly into the water, while when the water dry soil, the whole aggregate the will be pulledinquickly into the water, while when the water boundary is boundary boundary is is approached approached by by the stabilizer stabilizer in the the same same way, way, the the stabilizer stabilizer particles particles have have difficulty difficulty in in approached by the stabilizer in the same way, the stabilizer particlesabsorbed have difficulty in being attracted being being attracted attracted into into the the water water and and some some of of them them are are even even just just absorbed on on the the water water surface. surface. into the water and some of them are even just absorbed on the water surface. According to the theory According According to to the the theory theory of of surface surface thermodynamics thermodynamics [51–53], [51–53], this this phenomenon phenomenon indicates indicates that that the the soil soil ofparticles surfacehave thermodynamics [51–53], this phenomenon indicates that the soil particles have higher higher surface energy than the stabilizer particles and can be more easily wetted particles have higher surface energy than the stabilizer particles and can be more easily wetted by by surface energy than the stabilizer particles and can be more easily wetted by water. water. water.
Figure 13. Figure surface of of water. water. Figure 13. 13. Stabilizer Stabilizer on on the the surface surface of water.
Figure and it seemsthat thatthe thereaction reactionproducts products Figure 14 exhibits some reaction products Figure14 14exhibits exhibitssome somereaction reaction products products of of the the stabilizer stabilizer and and ititseems seems that the reaction products tend the water, water, though though there there are are aaa few fewpieces piecesof ofradial radial tend be principally formed on the tendtoto tobe beprincipally principallyformed formed on on the the boundary boundary of of the the water, though there are few pieces of radial formations which look like ettringite located in the water area, shown 14a. formations which look like ettringite located in the in Figure 14a. formations which look like ettringite located in the water area, shown in Figure 14a.
(a) (a)
(b) (b)
Figure 14. Reaction products calcium-based stabilizer (the red arrows pointing); (a) reaction time Figure14. 14.Reaction Reactionproducts productsofof ofcalcium-based calcium-basedstabilizer stabilizer(the (thered redarrows arrowspointing); pointing);(a) (a)reaction reactiontime timeof Figure of 0.5 h; (b) reaction time of 2 h. of 0.5 h; (b) reaction time of 2 h. 0.5 h; (b) reaction time of 2 h.
3.4. 3.4. ESEM Scanning and ESEM-EDAX Analysis 3.4.ESEM ESEMScanning Scanningand andESEM-EDAX ESEM-EDAX Analysis Analysis The scanning (Figure (Figure15). 15).The Theresults results The microstructure of the specimens was Themicrostructure microstructureof ofthe thespecimens specimens was was investigated investigated by by ESEM ESEM scanning scanning (Figure 15). The results show that as the IWC increases, the number of large aggregates agglomerations reduces while show that as the IWC increases, the number of large and agglomerations reduces show that as the IWC increases, the number of large aggregates and agglomerations reduces while while the quantity of individual particles increases; and the number of large inter-aggregate pores the quantity of individual particles in turn the number of large inter-aggregate pores the quantity of individual particles increases; and in turn the number of large inter-aggregate pores decreases, leading to the soil structure becoming decreases, 15a–d). In Inaddition, addition,it decreases,leading leading to to the the soil soil structure structure becoming becoming fine fine and and dense dense (Figure (Figure 15a–d). 15a–d). In addition, ititis isis observed that in 1 dayand 3 day-specimens, the hydration products are too few to be easily found. observed that in 1 dayand 3 day-specimens, hydration products are too few to be easily found. observed that in 1 day- and 3 day-specimens, the hydration products are too few to be easily found. Only specimens curing for days will the hydration Only the calcium-based calcium-based compound compound Onlyinin inspecimens specimenscuring curing for for 777 days days will will the the hydration hydration products of of the calcium-based compound stabilizer start to notably come up (Figure 15e,f). stabilizer Furthermore, owing to to the the hydration hydrationproducts products stabilizerstart startto tonotably notably come come up up (Figure (Figure 15e,f). 15e,f). Furthermore, Furthermore, owing owing to the hydration products
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mainly generating in the pores of soil matric particles, specimens with higher IWCs which have a more mainly generating in the pores of soil matric particles, specimens with higher IWCs which have a dispersed pore structure seem to have a much more homogeneous structure (Figure 15c–f). more dispersed pore structure seem to have a much more homogeneous structure (Figure 15c–f).
(a) (w = 11%, curing age of 14 days)
(b) (w = 15%, curing age of 14 days)
(c) (w = 11%, curing age of 7 days)
(d) (w = 17%, curing age of 7 days)
(e) (w = 11%, curing age of 7 days)
(f) (w = 17%, curing age of 7 days)
Figure Figure 15. 15. Effects Effects of of initial initialwater watercontent contenton onmicrostructure. microstructure.
During the the ESEM scanning process, During process, we we also also used usedEDAX EDAXto toidentify identifythe theelement elementcomposition compositionofof the hydration hydration products in different the different areas areas of of the the specimens specimens(Figure (Figure16b,d). 16b,d).ItItwas wasfound foundthat thatthough though the hydration products (which are already known as CSH and CAH) are mainly composed of oxygen, silicon, aluminum, and calcium, the percentage of each element in different scanning spots varies a
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the hydration products (which are already known as CSH and CAH) are mainly composed of oxygen, Materials 2018, 11, x FOR PEER REVIEW 13 of 18 silicon, aluminum, and calcium, the percentage of each element in different scanning spots varies a lot even in the specimen. However, generally, it seemsitthat for the products ofproducts specimens lot even insame the same specimen. However, generally, seems thathydration for the hydration of with higher IWC, the percentage of oxygen element tends to be a little higher, probably because more specimens with higher IWC, the percentage of oxygen element tends to be a little higher, probably free water transforms intotransforms structural into water during the hydration because more free water structural water during process. the hydration process.
(a) (w = 13%, curing age of 28 days)
(b) (Element composition around red dot in (a))
(c) (w = 19%, curing age of 28 days)
(d) (Element composition around red dot in (c))
Figure 16. Effects content on on chemical chemical composition. composition. Figure 16. Effects of of initial initial water water content
3.5. XRD Results Results 3.5. XRD Figure 17 presents presents the the X-ray X-raydiffractograms diffractogramsofofpowder powdersamples samplestaken takenfrom from specimens cured Figure 17 specimens cured to to 1, 1, 14, and 28 days in plastic bags. It can be found that the diffractograms of samples with different 14, and 28 days in plastic bags. It can be found that the diffractograms of samples with different IWCs IWCs and various ages exhibit little difference withother, each especially other, especially the characteristic and various curingcuring ages exhibit little difference with each for thefor characteristic peaks peaks of primary minerals (principally and muscovite the which soil), which indicates that IWC has of primary minerals (principally quartzquartz and muscovite in thein soil), indicates that IWC has little little effect on the crystallized primary mineral composition of the soil-stabilizer mixture. However, effect on the crystallized primary mineral composition of the soil-stabilizer mixture. However, the the characteristic peaks of clay minerals seem change withouta aclear cleartrend, trend,which whichmight might be be ascribed ascribed characteristic peaks of clay minerals seem to to change without to sampling differences and inherent inhomogeneity of the natural soil. The expected dispersed to sampling differences and inherent inhomogeneity of the natural soil. The expected dispersed characteristic peaks of of poorly poorly crystallized crystallized hydration hydration products, products, CSH CSH and and CAH CAH [54], [54], are are not not notable, notable, characteristic peaks which which may may be be attributed attributed to to the the relatively relatively small small addition addition of of calcium-based calcium-based compound compound stabilizer stabilizer and and insufficient crystallization time. insufficient crystallization time.
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Figure Figure 17. 17. Effects Effects of of initial initial water water content content on on mineral mineral composition. composition.
4. 4. Discussion Discussion Combining Combining the the mechanical mechanical property property study study with with the the microstructure microstructure and and composition composition research, research, the effects of initial water content on the soil-stabilizer mixture studied can be revealed as follows. the effects of initial water content on the soil-stabilizer mixture studied can be revealed as follows. 4.1. The Effects of Initial Water Content on the Compaction Process 4.1. The Effects of Initial Water Content on the Compaction Process The results of optical microscope observation identify the foundational effect of the molding The results of optical microscope observation identify the foundational effect of the molding water: it breaks down the soil aggregates and agglomerations into individual particles and makes the water: it breaks down the soil aggregates and agglomerations into individual particles and makes the soil structure more easily able to be changed. This is also confirmed by other research [55–57] and can soil structure more easily able to be changed. This is also confirmed by other research [55–57] and be explained by the diffused double layer (DDL) theory. More water enlarges the thickness of the DDL can be explained by the diffused double layer (DDL) theory. More water enlarges the thickness of the of the soil particles and even free water will come up, lessening the shear resistance of the soil. As a DDL of the soil particles and even free water will come up, lessening the shear resistance of the soil. result, on the scale level of the whole mixture, the stiffness of the mixture declines (Figure 8) and if As a result, on the scale level of the whole mixture, the stiffness of the mixture declines (Figure 8) and there is no excess pore water pressure, it will need less force and energy to densify the mixture, which if there is no excess pore water pressure, it will need less force and energy to densify the mixture, is testified by the static compaction test (Figure 9). Meanwhile, more water in the mixture means more which is testified by the static compaction test (Figure 9). Meanwhile, more water in the mixture individual soil particles exist and thus during the densification process, small particles can more easily means more individual soil particles exist and thus during the densification process, small particles move to the pores between larger particles or aggregates to minify the pores, which was identified by can more easily move to the pores between larger particles or aggregates to minify the pores, which the ESEM scanning (Figure 15) and other relevant research [57–60]. Therefore, right after compaction, was identified by the ESEM scanning (Figure 15) and other relevant research [57–60]. Therefore, right the specimens with different IWCs have different original structures: specimens with higher initial after compaction, the specimens with different IWCs have different original structures: specimens water content have less aggregates and agglomerations, and a smaller proportion of large pores. with higher initial water content have less aggregates and agglomerations, and a smaller proportion of large pores.of Initial Water Content on Strength Development 4.2. The Effects During the process, the water reactsDevelopment with the calcium-based stabilizer to produce the 4.2. The Effects of curing Initial Water Content on Strength bonding gels. Specimens with different IWCs have different reaction rates (Figures 10 and 11). During the curing process, the water reacts to with thedifferent calcium-based to produce the The produced gels change the mixture structure yield a UCSstabilizer at different curing ages bonding gels. Specimens with different IWCs have different reaction rates (Figures 10 and 11). The for different specimens. The UCS is just the reflection of the structural properties of the mixture at a produced gels change the mixture structure to yield different a UCS at different curing ages for certain time. different specimens. The UCS is just the reflection of the structural properties of the mixture at a certain time.
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For the specimens cured without external water, in the earlier days (1 day and 3 days) of the curing process, the stabilizer has not fully reacted and the mixture structure mainly maintained what they were after compaction, which is confirmed by the ESEM scanning. Thus, the strength of the mixture is principally dependent on the strength of the soil structure, which is controlled by the average thickness of the water membrane (DDL and free water). As initial water content increases, the average water membrane is thicker. Consequently, the UCS decreases linearly with the increase of IWC, as shown in Figures 10 and 11. However, as water content increases, there is a bigger probability to form continuous water passage, which facilitates the cation exchange, dissolution, and diffusion process of the soil–water–stabilizer reaction system and thus, the increase rate of UCS in the first 7 days is higher for the specimens with higher IWC. The UCS value does not change significantly with the variation of IWC at the curing age of 7 days. After that, as the pores are gradually filled by reaction products and free water is lacking, the increase rate for all the specimens decreases. At 28 days curing age, for specimens with lower IWC, there are more large pores and some of them cannot be fully filled and bonded (Figure 15a,c); while in the specimens with higher IWC, though a more homogeneous structure is observed (Figure 15b,d), higher IWC excessively destroys the soil structure and more free water remains in the water membrane of the soil particles. This may be why the UCS first increases and then decreases with the increase of IWC at 28 days curing age. For the specimens cured in moist air, the water was continuously supplied by an external water source until the specimens were saturated. Therefore, the inherent aggregates and agglomerations of the soil can be totally wetted during the curing process and the strength contributions of the soil structure are weakened. As a result, at the same compaction degree, the UCS of specimens cured in moist air significantly reduced compared to that of specimens cured without external water. Under different compaction degrees at the same curing age, the UCS variation with IWC is different (Figure 11) and this may be attributed to the different original structures after compaction. It can be speculated that there must be an optimum original mixture structure corresponding to certain initial water content under different compaction degrees that is beneficial to the strength development, which needs further investigation. From the discussion above, it can be concluded that the determination of IWC in soil stabilization practice should consider three issues: compaction, curing environment, and the targeted value of specific engineering properties; rather than only compaction. It is recommended that when a soil is stabilized by a stabilizer that can react with water, for a given dry density and a given curing environment, a series of IWCs should be tested to find the appropriate compaction energy and the optimum IWC. 5. Conclusions Initial water content is an important factor affecting engineering properties such as strength, stiffness, and compressibility of the soil stabilized by the calcium-based stabilizer in the research. The following conclusions can be drawn from the study:
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• •
In the range of initial water content studied, both the compaction energy and the maximum compaction force decrease linearly with the increase of initial water content, and the largest reduction is 53.4% and 45.9%, respectively. As the initial water content increases from 11% to 19%, there are less soil aggregates or agglomerations, and a smaller proportion of large pores in the mixture structure after compaction. For specimens cured without external water supply, regardless of the compaction degree, after a curing age of 28 days, the highest unconfined compressive strengths were acquired at initial water content of 15%, which is equal to the optimum water content derived from the modified Proctor compaction test. Higher initial water content enlarges the increase rate of the unconfined compressive strength in the first 7 days; after that, the increase rate first increases then decreases, and acquires maximum at initial water content of 15%.
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For specimens cured in moist air, the optimum initial water contents are 11% under 100% compaction degree, and 15% under 96% compaction degree, respectively. As the initial water content increases, the percentage of the oxygen element tends to increase in the reaction products of the calcium-based stabilizer. In the curing ages studied, the initial water content did not notably change the primary mineral composition of the soil-stabilizer mixture.
Author Contributions: The background research for this publication was carried out by all authors. C.Y., W.Z. and Z.H. conceived and designed the experiments and wrote this manuscript. C.Y., W.Z., and X.J. performed the experiments. All the authors participated in the analysis of experiments results. Funding: This research was funded by the Shaoxing (Zhejiang, China) Science and Technology Planning Project with Grant No. 2017702. Conflicts of Interest: The authors declare no conflict of interest.
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