Study on Properties of Magnesium Oxychloride Cement Solidified Soil

Due to the poor performance of ordinary Portland cement (OPC) as a solidied soil road and the large pollution in the production process, environment-friendly magnesium oxychloride cement (MOC) was used as the soil curing agent to prepare the solidied soil, exploring the optimal ratio of various raw materials when MOC is used as a curing agent. Analyzing the properties of MOC solidied soil in the application of road subgrade. is paper tests compaction, mechanical properties, and durability of the MOC solidied soil, simulates the development trend of 7 days unconned compressive strength of MOC solidied soil, and then analyzes the hydration process and strengthens the formation mechanism of MOC in solidied soil. e study found that the addition of MOC as a curing agent to the soil can eectively improve the compaction andmechanical properties of the soil. Matlab simulation found that when the MgO content is 5.5% to 6% and the ratio of raw materials MgO, MgCl2, and H2O is 2.45 :1 :14 to 6.3 :1 :14, the performance of MOC solidied soil is excellent. Fitting UCS data, it is found that MOC solidied soil has early strength characteristics. e excellent compaction and mechanical properties of MOC solidied soil are due to the formation of a small amount of phase 5 and layered Mg(OH)2 by the hydration of MOC, and the formation of amorphous gel with SiO2 in the soil. is reaction improves soil compaction and reduces internal porosity from a microscopic perspective. e strength loss rate of MOC solidied soil is higher after immersion in water at the initial stage of curing, but it is still better than that of traditional cement-based solidied soil. Poor performance after immersion in water is associated with disruption of the network-like structure. As an environment-friendly soil curing agent, MOC can be used in engineering practice with low environmental humidity.


Introduction
Soft soil can lead to potential safety hazards caused by poor bearing capacity during construction; high soil expansion will generate subgrade shrinkage, resulting in pavement cracking and reduced service life [1][2][3]. Soil stabilization is one of the soil improvement methods in geotechnical engineering. In addition, cement and lime stabilizations are two of the most commonly used methods in recent years [4,5]. e addition of traditional curing agents such as ordinary Portland cement (OPC) and lime can improve soil mechanical properties and durability [6][7][8]. Further, the study found that after adding MgO to the OPC stabilized soil, the pH of the soil increased with the increase of magnesium slag. During the hydration process, calcium aluminate hydrate (C-A-H) and calcium silicate hydrate gel (C-S-H) were produced, which improved soil conditions [9,10]. However, traditional OPC curing agents have problems such as limited mechanical strength, poor durability, poor volume stability, high pollution in the production process, and high CO 2 emissions [11][12][13][14].
Magnesium oxychloride cement (MOC) is an environment-friendly magnesia cementitious material prepared by lightly burning magnesium oxide, magnesium chloride and water [15]. Due to its fast-setting speed, high mechanical strength, good wear resistance, salt resistance and halogen corrosion, simple production process, and low cost, it is widely used in construction materials, biological materials, and other fields [16][17][18]. e salt lakes in the Qinghai area of China have sufficient reserves of magnesium salts and high grades [19]. Part of the waste MOC can be reused after recycling and processing, such as superhydrophobic magnesium oxychloride cement [20]. Waste building materials and industrial residues can also be used to prepare MOC or improve the performance of MOC, such as waste gypsum, ammonia alkali residue, and fly ash [21][22][23]. Studies have shown that when MOC is used as a soil stabilizer, it can form an amorphous gel product covering the surface of soil particles, effectively cementing soil particles and filling interparticle voids. It also greatly improves soil mechanical properties and reduces harmful ion leakage. e optimum dosage of MOC as a soil stabilizer is 10% [24][25][26]. At present, the environment-friendly soil stabilizer MOC has not attracted enough attention, and there are few related researches and experiments. e optimal raw material ratio and maintenance method of MOC as a soil stabilizer still need to be studied. Various properties of soil after curing with MOC have yet to be tested, and the microscopic mechanism remains to be explored. is paper analyzes and simulates the optimal raw material ratio of MOC as a curing agent through a macro compaction test, mechanical test, durability test, and MATLAB software. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) are used to observe the microstructure of MOC solidified soil. e microscopic mechanism of MOC reinforced fine-grained soil has been explored. It is hoped that this study can provide a valuable reference for the subsequent use of MOCs as soil stabilizers.

Materials.
e test soil was fine-grained soil with a plasticity index of 9.1, which has low liquid limit clay. Technical indexes are shown in Table 1. e raw materials for the preparation of MOC were lightly burned magnesia and industrial magnesium chloride. MgO was a light-burned magnesite powder produced in Haicheng, Liaoning Province. e content of MgO was more than 85%, and the content of active MgO was 60%. e specific surface area of the light-burned magnesia powder was 340 m 2 /kg. e physical properties and chemical composition are shown in Table 2. e particle size distribution of MgO is shown in Figure 1. MgCl2 was produced by Qinghai Golmud. e content of magnesium chloride was more than 45%.

Experimental Methods.
Mechanical properties and durability tests were used to evaluate the performance of MOC stabilized soil. Test flow is shown in Figure 2. e following is a detailed description of the mechanical properties and durability tests.

Compaction Test.
e compaction test (Method A) was adopted JTG-E51-2009. e size of the tube used in the test was 10.0 cm × 12.7 cm (inner diameter × height) and the volume was 997 cm . Five different water contents were set, with a difference of 2.0% in sequence. e ratio of the compaction test is shown in Table 3.

Mechanical Property Analysis.
e differences in mechanical properties of OPC and MOC solidified soils were compared. OPC consisted of 5% 42.5 cement and 11% water content. e effects of different ratios of raw materials on the mechanical strength of MOC solidified soil were discussed. e MOC raw material ratio is shown in Table 4. e    specimens were prepared and cured in an indoor environment for 7 days, and the unconfined compressive strength (UCS) was tested. e variation trend of 7 days UCS of MOC solidified soil was simulated by MATLAB.

Durability Analysis.
e durability of MOC solidified soil was evaluated by volume shrinkage test, wet-dry cycle test, water stability test, and infiltration conductivity test. e specimen size was Φ50 × 50 mm cylindrical specimens.    Advances in Materials Science and Engineering 3 e volume shrinkage test adopted the ratio of B2, C2, and D2, and the curing temperature was 20 ± 2°C. e original height and diameter of specimens were measured with a vernier caliper, then continued to test the data for 1 to 7 days of curing, and calculated the volume change.
e wet-dry cycle test was prepared according to the C2 ratio, the static pressure method was compacted and the demoulding curing was carried out for 7 days. e number of wet-dry cycles was set to 0, 1, 3, and 5 times. e water stability test applied C2 ratio and the specimen standard curing for 48 h in 20 ± 2°C. After the specimen had been formed, the UCS was measured after immersing in water for 24 h on the third and seventh day, respectively. Water stability coefficient of the specimens was calculated according to water stability coefficient � UCS of immersing water 24h UCS of standard care e ratio of B2, C2, and D2 were set for the water resistance test, and the curing temperature was 20 ± 2°C. After curing for 7 days, the specimens were completely immersed in water, and the amount of water in each group was strictly controlled to be exactly the same. After immersing for 6 h, the conductivity changes of water were measured. e instrument used is a DDS-11A digital conductivity meter as shown in Figure 3.

Micro Tests.
Scanning electron microscope was used to observe the microscopic morphology of MOC solidified soil, and X-ray diffractometer was used to test the chemical composition of MOC solidified soil. e sample design scheme of the microscopic analysis test is shown in Table 5. Figure 4 shows the effect of different raw material ratios on the maximum dry density of MOC solidified soil. e maximum dry density can reflect the compaction of MOC solidified soil well. As shown in Figure 4, the change of MgO content has a great influence on the maximum dry density of MOC solidified soil. In Figure 4(a), when the content of MgO is 5%, the maximum dry density reaches 2.008 g/cm 3 compared with 3% MgO, which is increased by 15.33%. e curing effect improved continuously with the increase of MgO content. In Figure 4(b), the curing effect has little correlation with the ratio of MgCl 2 to H 2 O. Figure 5 illustrates 7 days UCS of MOC solidified soil specimens with different ratios. e strength of C2, D1, D2, E1, and E2 samples are higher than OPC solidified soil. e strength of D2 reaches 1.76 MPa, which is composed of 6% MgO and the ratio of MgCl 2 to H 2 O is 1:15. Compared with OPC solidified soil, D2 is increased by 30.4%. Compared with traditional cement-based soil stabilizers [27,28], such as OPC and calcium sulfoaluminate cement (CSA), 7 days UCS value of MOC stabilized soil is the highest. erefore, in order to make the strength of the MOC solidified soil higher, MgO should not be less than 5% and the ratio of MgCl 2 to H 2 O is about 1 to 10∼15.

Mechanical Properties of MOC Solidified Soil.
In order to explore the effect of the raw material ratio on the 7 days UCS of MOC solidified soil, MATLAB was used to fit the ratio of MgO, MgCl 2 , and H 2 O on the surface. ree fitting surfaces are shown in Figure 6. e x-axis is the content of MgO (a%). e y-axis is the molar ratio of MgCl 2 to H 2 O (1:b), and z-axis is the 7 days UCS. e functional equations obtained by the fitting of Models I, II, and III are sequentially shown in e statistical parameter lists of the three models were compared, as shown in Table 6. Statistical parameters include error term degrees of freedom (f ), error sum of squares (SSE), root mean square (RMSE), coefficient of determination R-square, and adjusted R-square.
e SSE and RMSE of Model II tend to be closer to 0 and the R-square is closer to 1. It can be seen that the fitting degree of Model II for MOC solidified soil is better. In addition, from the models in Figure 6 and Table 6, it can be seen that Model II is matched only when the MgO content is greater than 5%, and the fitting degree of the data is better under the same condition. When the MgO content is lower, the strength predicted in Model II increases, but the actual strength decreases. Model II is the opposite of the actual situation, so it is not adopted. Taking into account comprehensively, Model I is suitable for characterizing the 7 days UCS of MOC solidified soil with different proportions. e shaded part of Figure 7(b) indicates that 7 days UCS of MOC solidified soil is higher than that of OPC solidified soil. It can be seen from Figure 7 that the optimum content of MgO is 5.5%∼6% and the ratio of MgCl 2 to H 2 O is 1:14. At this dosage range, the strength reaches more than 1.6 MPa, and the effect of constantly increasing the dosage of MgO on improving the strength is not significant. Combined with the experimental data, 7 days UCS of C2, D1, D2, E1, and E2 are better than that of OPC solidified soil, and the strength of D2 reaches 1.76 MPa, which is 30.4% higher than that of OPC solidified soil. 7 days UCS of MOC solidified soil is more  Table 7, it can be seen that it meets the requirements of the sub-base of secondary and lower highways under medium and light traffic loads.
In order to study the effect of curing age on UCS of MOC solidified soil, different curing times were taken and UCS tests were carried out. Figure 8 shows the UCS change rate of specimens after different curing aging. UCS development trends of MOC solidified soil and OPC solidified soil are shown in Figure 9. Figures 8 and 9 demonstrate that the UCS of MOC solidified soil develops faster than that of OPC solidified soil. 3 days strength reaches 90% of 7 days strength and the 28 days strength is significantly higher than that of the OPC solidified soil. It shows that MOC added to fine-grained soil as a curing agent can effectively improve the mechanical properties of the solidified soil. Figure 10 represents the volume shrinkage of MOC solidified soil as the ratio change of MgCl 2 and H 2 O. After the MOC solidified soil was formed, a slight volume expansion occurred on the first day, then a certain degree of shrinkage came up. When MgO content is 4%, 7 days change rate is 2.3%. When MgO content is 6%, the change rate is 1.27%. Overall, the volume change rate of MOC solidified soil was slight.

Durability of MOC Solidified Soil.
Under the natural environment, the early soil is easy to absorb water and swell. With the extension of time, the water in the soil gradually volatizes and shrinks in volume, which is easy to produce a large volume change rate. Nevertheless, MOC can play a role in consolidating and MgO has a certain expansion, which offsets part of the soil autogenous shrinkage and reduces the volume change rate of MOC solidified soil.
After the wet-dry cycle test, the strength loss rate and volume shrinkage rate of MOC solidified soil (C2) are shown in Figure 11. Wet-dry cycles have little effect on the volume shrinkage of MOC solidified soil, but have a greater effect on    Advances in Materials Science and Engineering UCS. After 5 cycles, the volume change rate of MOC solidified soil is only 1.98%, and the strength loss rate is as high as 41.92%. By simply fitting the data, it can be seen that the strength loss rate decreases significantly before the 6th time, and becomes flat at the 9th time. e volume shrinkage rate of C2 fluctuates sharply only before 3 times, and fluctuates in a small range since then. Figure 12 shows the strength change rate of the MOC solidified soil specimens before and after immersion in water. After curing for 48 hours, improving with the age, the UCS of the immersed specimens decreased by 80.4% and 36.5% compared with the unimmersed specimens. After immersion in water for 24 h on the seventh day, the strength of the CSA solidified soil was only 0.6 MPa [27]. e strength of the MOC solidified soil is still as high as 1.26 MPa.
To sum up, MOC solidified soil has poor water stability and is sensitive to moisture and environmental humidity. With the increase of age, the strength loss rate of the     Advances in Materials Science and Engineering specimens after immersing in water gradually decreases. In particular, during the initial maintenance, contacting with water should be avoided or the ambient humidity should be reduced. However, compared with other cement-based solidified soils, it still has better water resistance. As illustrated in Figure 13, the electrical conductivity of the samples with different ratios in the infiltration solution is compared. As the ratio of MgCl 2 to H 2 O increases, the electrical conductivity gradually decreases, and it enhances continuity with the increase of MgO content. e change of conductivity is more correlated with the ratio of MgCl 2 to H 2 O, and the effect of MgO content is less. When the MOC solidified soil was soaked in water, free Cl − ions played a conductive role, and Mg 2+ ions might be transformed into Mg(OH) 2 and MgCO 3 .
Because MOC is an unsaturated porous, multiphase material, its main strength phase (phase 5) is an unstable crystalline phase, which is prone to hydrolysis in water environment. After immersing in water, the needle-like dense structure was transformed into a loosely structured Mg(OH) 2 . erefore, the density of MOC decreases and the internal voids increase, which led to poor water resistance and mechanical properties.

Microstructure.
e XRD patterns of the four samples are shown in Figure 14. Sample 1 contained more phase 5, a small amount of MgO and SiO 2 phases. It indicated that the reaction of active MgO was relatively complete, and it reacted with SiO 2 in the soil to form an amorphous gelling substance. Calcium was derived from fine-grained soils and impurities in raw materials.
After sample 2 absorbed water, the Mg 2+ concentration in the system decreased. is phenomenon led to the instability of the presence of alkaline hydrates on the surface, and phase 5 hydrolysis reaction occurred to generate Mg(OH) 2 . In addition, free Mg 2+ was carbonized to form MgCO 3 and MgCO 3 ClOH·2H 2 O. MgCO 3 ClOH·2H 2 O was   Advances in Materials Science and Engineering not stable, and would continue to react in the solidified soil under the condition of sufficient water to generate Mg(ClO 2 ) 2 ·(H 2 O) 6 [29]. e hydrolysis reaction is indicated in formula (5), and the carbonization reaction is presented in MgO e spectrum of sample 3 was similar to that of sample 1. Sample 3 has a higher content of phase 5 and less SiO 2 . It showed that the curing reaction of sample 3 was more complete. e SiO 2 content of sample 4 was very high, and phase 5 began to appear. A small amount of MgO could still be seen in the sample. It illustrated that the curing reaction was still continuing when curing under standard conditions for 6 h. Figure 15 indicates the SEM of the four samples. In sample 1, it can be observed that the surface of soil particles is covered with a large number of amorphous gelling products, lamellar Mg(OH) 2 , and a few needle-like phase 5. e gelling product and the lamellar Mg(OH) 2 are closely connected, and a small amount of phase 5 fills the voids in the soil to form a network structure and improve the compaction and mechanical properties of the soil.
Sample 2 was soaked in water after the hydration product was formed. On the one hand, phase 5 hydrolysis reaction occurred to produce loosely packed triangular pyramid Mg(OH) 2 , which distributed along the soil voids.

Advances in Materials Science and Engineering
And part of Mg(OH) 2 reacted with CO 2 dissolved in water to form MgCO 3 , attaching to the unhydrolyzed hydration product in the form of massive grains. On the other hand, the soil expands after the flood, causing the network structure to be destroyed. erefore, larger voids could be seen, and the density of MOC solidified soil decreased. After increasing the active MgO content and the ratio of MgCl 2 , the amorphous gel, lamellar Mg(OH) 2 , and needlelike phase 5 were superimposed on each other to form a network structure, and soil particles could hardly be seen. e reaction greatly increases soil density, it is consistent with the above compaction degree and mechanical property analysis.
For sample 4 had smaller size amorphous gels as well as layered Mg(OH) 2 . Because of the short curing time, the activated magnesium oxide had not fully reacted, the strength mainly depended on the consolidation of soil particles by the gel tissue produced by the previous hydration. At the same time, it also reflects that a small amount of the phase 5 appeared in MOC solidified soil at the beginning of curing for 6 h, which is consistent with the fastsetting characteristics of MOC.
rough the above microscopic experiments, it can be concluded that the process of using MOC as a curing agent to solidified soil includes: active MgO, H 2 O, and silicon in soil form M-S-H gel, which is wrapped on the surface of soil; then Cl − in dissolved MgCl 2 reacted with the gel layer to form the main strength phase (phase 5). Phase 5 continued to grow to fill the gaps between soil particles and crosslinked to form a network structure to consolidate the soil particles.
e reason for the poor performance of MOC solidified soil after immersing is that the phase 5 was hydrolyzed to form layered Mg(OH) 2 , and part of the Mg(OH) 2 was carbonized to form MgCO 3 , which destroys the network structure formed by the phase 5 and soil particles.

Conclusions
In this paper, MOC stabilized soil was studied through laboratory experiments and mathematical modeling, and the feasibility of MOC as an environmentally friendly soil-stabilizing agent was described and discussed. rough a series of mechanical properties, durability and microstructure test, engineering properties, main hydration products and microstructure characteristics of MOC stabilized soil were explored, and the optimal raw material ratio of MOC as a soil curing agent was proposed. e precautions and application fields of MOC as a soil stabilizer in engineering practice are given. According to the experimental data and model, the following conclusions are drawn: (1) Compared with existing cement-based soil stabilizers, such as OPC and CSA, the MOC curing agent can effectively improve soil compaction and mechanical properties. Combined with the specification requirements under the action of different grades of highways and traffic loads, MOC solidified soil is suitable for the sub-base of secondary and lower highways under light traffic load. (2) In-depth research was carried out on the strength development law of MOC solidified soil with different ratios of raw materials. e test found that when the content of MgO is 6% and the ratio of MgO, MgCl 2 , and H 2 O is 3.68 : 1 : 15, the properties of MOC solidified soil are the best. en the relevant mathematical model was established according to the test data, and the optimal ratio of raw materials was obtained as MgO content in 5.5%∼6% and the ratio of MgCl 2 to H 2 O is 1:14. After the test data was fitted to analyze the strength change trend, it was found that the MOC solidified soil had the characteristics of early strength, and the construction time should be controlled in practice.
(3) e strength loss rate of MOC solidified soil after water immersion in the initial stage of curing is higher, but it is still better than that of traditional cement-based solidified soil. MOC solidified soil should be avoided in coastal areas, areas with sufficient rainfall, and the sub-base of permeable pavement. Water erosion should be minimized in the initial stage of maintenance. (4) According to mechanical test and durability test combined with microscopic morphology, the hydration process of MOC in soil was analyzed, and the strength formation mechanism of MOC that used as a soil stabilizer was explained: MgO, H 2 O, and SiO 2 formed M-S-H gel to wrap soil particles. It further reacts with MgCl 2 to form the main strength phase (phase 5) and lamellar Mg(OH) 2 , and the soil particles form a cross-linked network structure with phase 5 and lamellar Mg(OH) 2 , thereby forming a high-strength solidification soil. e poor water resistance is due to the destruction of the network structure, resulting in the formation of loose triangular pyramid-shaped Mg(OH) 2 and MgCO 3 . (5) In this paper, optimal raw material ratio range of MOC curing agent is given. Experiments and microscopic analysis are conducted to investigate the process that MOC enhanced soil. Subsequent experiments can be refined with reference to the given ratio range and mathematical model. Moreover, the test found that the water stability and water resistance of MOC solidified soil were poor. Follow-up research could focus on improving the water resistance of MOC solidified soil.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.