Experimental Materials

The soil was collected from a construction site in Taiyuan, Shanxi Province, China. The soil particles were passed through a 2.0 mm sieve. Table 1 lists the particle-size distributions. The coefficients of uniformity (C_u) and curvature (C_c) are 0.76 and 3.65, respectively, which is poorly graded sandy soil according to the Chinese GB50021-2001 standard [29]. The ordinary Portland cement (OPC) was produced in Taiyuan, China. The 28 d compressive strength of the cement is 42.5 MPa, and the particle size distribution range is 5 μm~10 μm. Table 2 lists the chemical composition of the cement. The CMK was obtained from Shanxi Bright Kaolin Technology Co., Ltd., China. Particles smaller than 2 μm account for more than 90% of the total mas. Table 3 lists the chemical composition of the CMK. The sodium sulfate reagent was analytically pure anhydrous Na₂SO₄ with a purity of more than 99%, produced by the Tianjin Reagent Factory Co., Ltd., China. The pH of the Na₂SO₄ reagent was maintained between 5 and 8 at room temperature.

Sample Preparations

Prior to sample preparation, sandy soil was dried in an oven at 105ºC for 24 h. Table 4 lists the mix proportions for the preparation of the CMK cement soil. During the sample preparation, the sandy soil, CMK, cement, and distilled water were weighed, as listed in Table 4. All mixtures were poured into a mixer for two min to ensure uniform stirring of the slurry. The slurry was then added in two steps to a 70.7 mm × 70.7 mm × 70.7 mm steel mold. After adding one-third of the slurry, the steel mold was vibrated on a vibration table for one min to ensure that all air was exhausted. The sample surface was scraped flat using a scraper, and the steel mold was wrapped in plastic. After 24 h, the mold was removed. The samples were cured in a constant-temperature water tank for 7 d. Finally, all the CMK cement-soil samples were removed and placed in solutions of different Na₂SO₄ concentrations to continue curing for 28 d. The concentrations of the Na₂SO₄ solutions were 0, 4.5 g/L, 9 g/L, 18 g/L and 36 g/L, respectively, according to the Chinese GB50021-2001 standard [29]. The samples in the five different environments were denoted as NS0, NS1, NS2, NS3, and NS4.

Testing Methods

The compressive strengths of the CMK cement-soil samples at 28 d were tested using a WDW-100 electronic universal testing machine at a loading rate of 0.3 kN/s. Phase composition tests of the CMK cement-soil samples were performed at 28 d using a Shimadzu Suzhou XRD-6000 X-ray diffractometer, the scanning angle ranging from 2°~60° and the speed being 6°/min. Microscopic images of the CMK cement-soil samples at 28 d were captured using a Hitachi TM-3000 scanning electron microscope, at magnifications of 500× and 5000×.

After the test, the XRD peaks were analyzed using MDI Jade 6.0 software, to determine the phase composition of the CMK cement soil. Additionally, Image-ProPlus6.0 analysis software was used to process and analyze the SEM images at 500× magnification to obtain the pore distribution of the CMK cement soil.

Compressive Strength

Fig. 1 shows the compressive strength of the CMK cement soil cured in different Na₂SO₄ solution concentrations at 28 d. In general, compared with the CMK cement soil cured in clear water (Na₂SO₄ concentration of 0), the compressive strength of the CMK cement soil increases to differing degrees with increasing Na₂SO₄ solution concentration. When the Na₂SO₄ solution concentration is less than 18 g/L, the compressive strength of the CMK cement soil increases with increasing Na₂SO₄ solution concentration. When the Na₂SO₄ concentration is greater than 18 g/L, the compressive strength of the CMK cement soil decreases with increasing Na₂SO₄ solution concentration. Within the scope of the experiment designed in this study, when the Na₂SO₄ solution concentration is 18 g/L, the strength of the CMK cement soil reaches its maximum value, indicating that a moderate Na₂SO₄ solution concentration has a crucial enhancing effect on the CMK cement soil strength. It should be noted that the relationship between the compressive strength of the CMK cement soil and the CMK content has been previously reported [30, 31]. When the CMK content is 3%, the strength of the CMK cement soil reaches its maximum value. Consequently, the CMK cement soil with 3% CMK content cured in different concentrations of Na₂SO₄ solution was selected for microscopic analysis.

XRD Analysis

Fig. 2 shows the XRD patterns of the CMK cement soil with 3% CMK content cured in different concentrations of Na₂SO₄ solution at 28 d. The main phases of the CMK cement soil are quartz, ettringite, calcium aluminate hydrate, calcium hydroxide, calcium aluminate silicate hydrate, albite, calcium silicate hydrate, calcite, and gypsum. The CMK cement soil is reinforced mainly through the hydrolysis and hydration of MK and cement, the interaction between the soil particles and hydration products, and carbonation [32]. Moreover, the interaction between the soil particles and hydration products manifests as an ion-exchange agglomeration reaction and a pozzolanic reaction. Additionally, Na⁺ and SO₄^2- participate in chemical reactions in the CMK cement soil. The chemical reactions between Na⁺ and the CMK cement soil can be expressed as follows [33]:

Fig. 1.

Compressive strength of CMK cement soil at 28d in the Na₂SO₄ solution environment.

Fig. 2.

XRD patterns of CMK cement soil at 28d in the Na₂SO₄ solution environment.

Ca(OH)₂ + Na₂SO₄∙10H₂O = CaSO₄∙2H₂O∙2NaOH

+ 8H₂O 3CaO∙2SiO₂∙3H₂O + 4NaOH = 3Ca(OH)₂ + 2Na₂SiO₃

+ 2H₂O 3CaO∙Al₂O₃∙6H₂O + 2NaOH = 3Ca(OH)₂ + Na₂O∙Al₂O₃

+ 4H₂O

Na₂O∙Al₂O₃ + 6SiO₂ = Na₂O∙Al₂O₃∙6SiO₂

The reaction equation shows that calcium hydroxide (Ca(OH)₂) is the product of the Na⁺ reaction. As shown in Fig. 2, the peak value of calcium hydroxide increases as the reaction progresses – that is, as the Na₂SO₄ solution concentration increases, more calcium hydroxide is produced. Moreover, the hydration reactions of cement produce large quantities of calcium hydroxide [34].

As shown in Fig. 1, when the Na₂SO₄ solution concentration is less than 18 g/L, the compressive strength of the CMK cement soil increases with increasing Na₂SO₄ solution concentration. This is because the production of calcium hydroxide continues to improve the overall CMK cement-soil strength in three ways: First, Na₂O∙Al₂O₃ reacts chemically with SiO₂ to produce albite (Na₂O∙Al₂O₃ ∙6SiO₂) owing to the presence of Na⁺. Moreover, Na⁺ has a larger ionic radius in albite, which can cause an ion exchange with Ca²⁺ in the calcium hydroxide [35]. All calcium hydroxides are produced by the hydration of cement and CMK, as well as by the reaction between Na⁺ and the cement soil. After ion exchange, the diffusion layer of the electric double layer in the cement-soil system weakens. Consequently, the volume of bound water is reduced, thereby increasing the direct cohesion of soil particles and improving the strength of the soil [36]. Additionally, the generated Ca²⁺ chemically reacts with active SiO₂ and Al₂O₃ in the sandy soil and CMK to produce calcium silicate hydrate (3CaO∙2SiO₂∙3H₂O) and other products in alkaline environments [37]. These hydration products bond with the soil particles to form a cemented network structure, which continues to increase the strength of the cement soil. Finally, part of the free calcium hydroxide is carbonized with carbon dioxide in air and water to produce calcite (CaCO₃), which also enhances the strength of the cement soil [38].

The chemical reactions between SO₄^2- and CMK cement soil can be expressed as follows [39]:

CaSO₄ + 2H₂O = CaSO₄∙2H₂O

3CaSO₄ + 4CaO∙Al₂O₃∙19H₂O + 14H₂O

= 3CaO∙Al₂O₃∙ 3CaSO₄∙32H₂O + Ca(OH)₂

3CaSO₄∙3CaO∙Al₂O₃∙CaSO₄∙18H₂O + 14H₂O

= 3CaO∙Al₂ O₃∙3CaSO₄∙32H₂O

Here, SO4^2- reacts with the cement soil to form gypsum (CaSO₄∙2H₂O), which has an expansion effect on the cement soil [40]. Additionally, gypsum reacts with calcium aluminate hydrate (4CaO∙Al₂O₃∙19H₂O) and monosulfur calcium aluminate hydrate (3CaO∙Al₂O₃∙CaSO₄∙18H₂O) to form ettringite (3CaO∙Al₂O₃∙3CaSO₄∙32H₂O). Ettringite is characterized primarily by its filling and expansion effects [41]. When the generation of ettringite is low during the early stages, the space network structure formed by the intersecting ettringite crystals and calcium silicate hydrate fills the gaps in the cement-soil particles. This reduces the porosity and increases the strength. However, during the later stages, with an increase in the number of ettringites, the expansion effect is not conducive to cementation between the soil particles, contributing to a decrease in the cement-soil strength.

As shown in Fig. 1, when the Na₂SO₄ solution concentration is greater than 18 g/L, the compressive strength of the CMK cement soil decreases with increasing Na₂SO₄ solution concentration. This is because large quantities of Na⁺ and SO₄^2- participate in the reaction. Sodium hydroxide (NaOH) undergoes a chemical reaction with the calcium silicate hydrate produced by the hydration of cement and CMK, leading to poor cementation between the soil particles. This is reflected in the reduction in the cement-soil strength. Furthermore, the gypsum and ettringite generated by excessive SO₄^2- exhibit expansion effects, generating a large swelling force. This increases the volume of the soil particles and damages its internal structure, causing a reduction in the cement-soil strength.

SEM Analysis

Fig. 3 shows SEM images of the CMK cement soil with 3% CMK content cured in different concentrations of Na₂SO₄ solution at 28 d. As shown in Fig. 3a), the soil particles of the CMK cement samples cured in clean water are bonded through flocculent cementitious products. However, some pores remain between soil particles. The strength of the flocculent cementitious products is low, and they are relatively loosely arranged. Once the soil particles are disturbed by external forces, their internal microstructures can be easily destroyed.

Fig. 3(b-d) shows SEM images of the CMK cement-soil samples cured in different concentrations of Na₂SO₄ solutions. When the Na₂SO₄ solution concentration is less than 18g/L, more and more flocculent cementitious products are generated from the hydration reaction of the CMK cement soil with increasing Na₂SO₄ solution concentration. This flocculent cementitious product wraps the soil particles from all sides, promoting firmer soil particles. With increasing Na₂SO₄ solution concentration, an increasing number of needle-shaped and rod-shaped ettringites are generated by the hydration reaction of the cement soil. The pores between the soil particles are effectively filled with ettringite, thereby reducing the number of pores. The structural units of the soil particles are granulated to ensure that the soil structure is stable.

As shown in Fig. 3e), when the Na₂SO₄ solution concentration exceeds 18 g/L and reaches 36 g/L, a large quantity of ettringite crystals are generated during the hydration reaction of the CMK cement soil. In addition to filling the pores between the soil particles, these ettringite crystals are attached to the surface of the soil particles, forming a part of the acicular soil particle structure. Because of the expansion effect produced by ettringite crystals, the volume of the CMK cement soil expands rapidly, and the pores become larger. When the expansion effect produced by ettringite is greater than the cementation effect between the soil particles, the soil is damaged, leading to a decrease in the cement-soil strength [42].

Fig. 3.

SEM images at 5000× magnification of CMK cement soil at 28d in the Na₂SO₄ solution environment.

Pore Distribution Analysis

To obtain the pore distribution of CMK cement soil cured in different concentrations of Na₂SO₄ solution, SEM images of the CMK cement soil at 500× magnification were processed using Image-ProPlus 6.0 analysis software. Fig. 5 shows the original SEM images and binary images of the CMK cement soil with 3% CMK content cured in different concentrations of Na₂SO₄ solution at 28 d. The size of the pores was calculated using the black and white binarized images, the classification and statistical parameters of which are listed in Table 5.

From the statistical results of the sample porosity in Table 5, the porosity of the CMK cement soil in Na₂SO₄ solution is considerably lower than that in clean water. Moreover, with an increase in the Na₂SO₄ solution concentration, the porosity first decreases before increasing. When the concentration of the Na₂SO₄ solution is 18 g/L, the porosity reaches its minimum value of 26.52%. This is because the hydration products generated by the reaction between the CMK cement soil and Na₂SO₄ solution play a far greater role in filling than the hydration products generated by the CMK cement soil. Consequently, the CMK cement-soil porosity decreases, and the CMK cement-soil strength increases in the Na₂SO₄ solution. When the Na₂SO₄ solution concentration is too high, the gypsum and ettringite generated by the CMK cement soil and Na₂SO₄ solution contributes to the expansion. Consequently, the porosity of the CMK cement soil increases and the CMK cement-soil strength decreases in the Na₂SO₄ solution [43].

Based on the size of the average pore diameter, the microscopic pores of the sample can be divided into the following five categories – that is, micro pores (<2 μm), small pores (2 μm~5 μm), medium pores (5 μm~20 μm), macro pores (20 μm~50 μm), and holes (>50 μm) [44, 45]. From the statistical analysis of the pore distribution shown in Table 5, it is evident that the pore distribution of the CMK cement soil changes owing to the influence of the Na₂SO₄ solution. The distribution proportions of micro pores, small pores, and medium pores all initially increase before decreasing with increasing Na₂SO₄ solution concentration – that is, the proportion of all the pores in sample NS3 reaches its maximum. By contrast, the distribution proportion of macro pores first decreases before increasing with increasing Na₂SO₄ solution concentration – that is, the proportion of NS3 reaches its minimum value. Moreover, the proportion of holes decreases with increasing Na₂SO₄ solution concentration, reaching a minimum in sample NS4. This indicates that the Na₂SO₄ solution environment changes the pore distribution of the CMK cement soil, accounting for the change in porosity, and ultimately leading to a difference in strength.

Fig. 4.

Original SEM images and binarized images at 500×magnification processed by Image-ProPlus6.0 analysis software of CMK cement soil at 28d.