Study on Proportion Optimization of Magnesium Oxychloride Cement-Stabilized Clayey Soil Based on the Response Surface Methodology

Zhang, Huzhu; Wang, Kai; Hu, Bing; Zheng, Yueqing

doi:https://doi.org/10.1155/2023/3054786

Advances in Materials Science and Engineering

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Abstract Introduction Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 3054786 | https://doi.org/10.1155/2023/3054786

Study on Proportion Optimization of Magnesium Oxychloride Cement-Stabilized Clayey Soil Based on the Response Surface Methodology

Huzhu Zhang,¹Kai Wang,¹Bing Hu,²and Yueqing Zheng¹

Academic Editor: Shengwen Tang

Received17 Oct 2022

Revised20 Jan 2023

Accepted20 Jan 2023

Published06 Feb 2023

Abstract

The energy-saving and green environmental protection magnesium oxychloride cement (MOC) is introduced into the pavement base and subbase to improve the shortcomings of CO₂ emissions and high industrial energy consumption in the production process of traditional cementitious materials such as lime and Portland cement. Box−Behnken design of design-expert is employed for experiment arrangement, in which MgO/MgCl₂ molar ratio, MOC content, and fly ash content are influencing factors, while response values are 7d unconfined compressive strength (USC) and 1d softening coefficient (SC) of solidified soil. The response surface methodology (RSM) is used to optimize the ratio of three additives, and the effects of various factors on the response value are investigated by response surface model analysis and interaction analysis. The results show that the MOC content has the most excellent effect on 7d unconfined compressive strength, and the mutual influence for the MOC content and fly ash content are significant, respectively. However, the influential factor is the fly ash content for the 1d softening coefficient. It is predicted by the RSM analysis that the optimum balance of USC and SC is 8.61 for the MgO/MgCl₂ molar ratio, 18% for the MOC content, and 20.36% for the fly ash content. With the additives in the optimal ratio, the actual unconfined compressive strength and softening coefficient of stabilized soil are 2.56 MPa and 0.76, respectively. It is confirmed that the response surface methodology plays an important part in optimizing the proportion of MOC-stabilized clayey soil.

1. Introduction

Due to their good physical and mechanical properties, inorganic binder stabilized materials are extensively used in pavement bases and subbases. With the rapid development of the economy and society, the steady implementation of the “Belt and Road Initiative,” and the continuous advancement of urbanization, China’s transportation infrastructure construction will maintain a strong development trend. In the future, the demand for Portland cement and lime as binders for pavement base and subbase materials will continue to be high. However, traditional cementitious materials have the disadvantages of CO₂ emissions, noxious gas emissions, and high industrial energy consumption [1–3], which brings severe challenges to energy-saving, environmental protection, and the strategic goal of “Emission Peak and Carbon Neutrality.” Thus, the research on low-carbon environmental protection of new cementitious materials for road engineering construction is of great significance.

Magnesium oxychloride cement (MOC), a green cementing material, is composed of magnesium oxide (MgO), magnesium chloride hexahydrate (MgCl₂·6H₂O), and some water. On the one hand, the calcination temperature and CO₂ emission of MgO as the main component in the production process are lower than those of Portland cement. On the other hand, MgCl₂·6H₂O is a by-product of salt production in coastal or salt lakes and is widely available. MOC is restricted to engineering applications because of poor water resistance and other issues [4, 5].

In recent years, with the continuous development of new cementitious materials, relevant scholars began to use MOC in curing and achieved some achievements. Wang et al. [6–9] used environment-friendly MOC for sludge disposal, explored the mechanical properties and microscopic mechanism of MOC-based curing sludge, and provided preliminary exploration and theoretical support for the proportion design and optimization of MOC-based cementitious materials and their application in the field of solidified sludge. Zhang et al. [10] reported that fly ash-modified MOC may solidify drilling wastes and achieve excellent test results. Li et al. [11, 12] researched the impact of material ratio on the USC and compaction properties of MOC-solidified gravel soil with magnesium oxychloride cement as a binder and gravel soil as the curing object. Wang et al. [13] studied the influence of different mineral admixtures on the resistance to water of MOC curing macadam base; it is concluded that the resistance to water is obviously improved by fly ash, and the best fly ash content is 25%. Xiao et al. [14, 15] added MOC concrete to construct pavement, studied its mechanical properties, analyzed its microstructure, and proved the engineering application prospect of MOC concrete.

MOC is composed of a MgO-MgCl-H₂O ternary composite system. The influence of MgO properties on the performance of MOC is pronounced, which is similar to magnesium oxysulfate cement (MOS), magnesium phosphate cement (MPC), and other magnesium cements. Huang et al. [16, 17] found that a high molar ratio of MgO is beneficial to the formation of main hydration products in MOS and the reduction of porosity and fractal dimension D_f, while MPC is the opposite. In addition, MgO also has a definite effect on the pore structure and frost resistance of cement concrete [18]. The performance of MOC is controlled by the amount and concentration of MgCl₂, except for MgO [19]. So, the MgO/MgCl₂ molar ratio is highly significant for MOC. At the same time, the modified agent is needed to extend the resistance to water of MOC curing soil. Thus, the composition of the mixture is slightly complicated. Although the number of experiments could be reduced by the traditional ratio optimization methods such as orthogonal design, it is impossible to clarify the functional relationship between each component content and the response index. Therefore, it is arduous to accurately determine the optimal response value of different elements in a mixture ratio.

In contrast, as a statistical method that can simultaneously consider the effect of various factors to find the optimal value, the response surface methodology (RSM) not only could establish the relationship between influencing factors and response indexes, but also examine the interaction law between different influencing factors. The response surface methodology mainly includes experimental design, model fitting, and process optimization. The typical design methodology mainly includes central composite design (CCD) and Box−Behnken design (BBD), a step-by-step continuous fitting process. It is different from the traditional orthogonal test methodology [20]. The Box−Behnken design requires fewer tests and has higher efficiency and better economy than the central composite design when the influencing factors are the same, and the prediction accuracy of the central composite design is slightly higher than that of the Box−Behnken design [21]. Explicit function expressions and geometric figures of the experimental results could be obtained with the assistance of the response surface methodology. The test results became concise and easy to understand. This statistical method has been studied in mixed proposition optimization designs [22–25].

In summary, MOC is selected as the basic cementing material, and fly ash is a modifier. The dosage range of each additive was determined by the existing literature and research [26–31]. The optimal proportion of MOC-stabilized clay soil is determined by the Box−Behnken design because of the long experimental period. Subsequently, the influence of different factors on the response index and the interaction between various factors are analyzed by the RSM. Finally, the curing mechanism is attempted to be interpreted. The experimental results and consequences give impetus to the practical highway engineering application of MOC-stabilized soil in pavement bases and subbases.

2. Experimental Materials and Methods

2.1. Materials

The experimental soil was taken from a road engineering site located in Erdao District of Changchun City, China. It was brown in color for raw soil. The initial water content of the soil is 17.6%, which is measured by the drying method in the oven at 105°C. The liquid limit, plastic limit, and plasticity of soil are 34.6%, 18.9%, and 15.7%, respectively, by the boundary moisture content test. The particle gradation of soil was obtained by the screening method and the density meter method, which is shown in Figure 1. The soil belongs to low liquid limit clay containing sand. Based on the heavy compaction test, the maximum dry density and optimum water content are 1.96 g/cm³ and 11.0%, respectively. The major properties of the soil are illustrated in Table 1.

The vital materials of MOC are magnesium oxide (MgO) and magnesium chloride hexahydrate (MgCl₂·6H₂O), which were bought from a refractory materials factory in Yingkou city, China. The active MgO content is about 62.2% by hydration method [32]. The MgO is made from magnesite as raw materials, calcined at 750°C–800°C in the boiling furnace of the reflective kiln, and then finely ground. And the particle gradation of MgO is also shown in Figure 1. The MgCl₂ content of MgCl₂·6H₂O is provided by the factory. The chemical components of two vital materials are shown in Tables 2 and 3.

The fly ash was chosen as a modifier for MOC-stabilized soil and purchased from a mineral processing corporation in Handan city, China. And the product grade of fly ash reached the first grade. The main components are illustrated in Table 4.

2.2. Sample Preparation and Test Methods

According to the existing research achievements, the conditions of the influence factors were a MgO/MgCl₂ molar ratio of 6–10, MOC content of 10%–18%, and fly ash content of 10%–30%. And fly ash was added into MOC by an internal mixing method to replace MgO with an equal mass.

The air-dried soil was broken through the specified sieve and classified, and then dried in the oven at 110°C. And it was sieved with 4.75 mm and 2.36 mm diameter sieves. The optimum water content under different ratios was obtained in the light of the heavy compaction test, and the additional water content for making standard specimens could be calculated clearly. The heavy compaction test of MOC-solidified soil and the optimum water content test are shown in Figure 2.

The quality of each test material required to make the samples, including MgO, MgCl₂·6H₂O, and fly ash, was determined based on the maximum dry density and optimum water content of MOC-solidified soil under different mix proportions. After that, the exact mass of MgCl₂·6H₂O was added to a certain amount of water; the glass rod was thoroughly stirred to produce a magnesium chloride solution. Then, fly ash and part of the magnesium chloride solution (about a third of the total mass) were sequentially added to the soil and fully mixed. The mixtures were sealed in a plastic bag for 16–24 hours to facilitate complete contact of the MgCl₂ and water with the soil. The predetermined amount of MgO and the remaining magnesium chloride solution were added to the mixture and stirred evenly again within 1 hour before the samples were formed. The compaction degree of MOC-FA-soil pastes was greater than or equal to 96%. Then, the pastes were transferred to every single cylindrical metal mold three times, and the samples were prepared by the static compaction method with the help of a pressure testing machine. Finally, the samples were stripped by the stripping device. The diameter and height of the standard samples are both 100 mm.

Afterward, the samples were demolded and weighed, wrapped in preservative films, and put in the standard curing boxes at 20°C ± 2°C and a relative humidity of 95%. Some samples were immersed in a constant temperature water tank for 1d when they were maintained to the predetermined curing age. The unconfined compressive strength of samples after standard curing and immersion was measured by a pavement material test system at 1 mm/min, which was calculated as the representative value of 7d unconfined compressive strength and 1d softening coefficient in the light of theoretical equations. The formula for calculating the softening coefficient is as follows:

In the formula: is the unconfined compressive strength of the specimen after standard curing for 7 days and soaking for 1 day and R₀ is the unconfined compressive strength of the specimen after standard curing for 7 days.

To enhance the accuracy of testing data, there were entirely six samples in each group. Samples and standard curing boxes are shown in Figure 3.

2.3. Experimental Design

The 7d unconfined compressive strength and 1d softening coefficient of samples were selected as response indexes in this study. MgO/MgCl₂ molar ratio, MOC content, and fly ash content were chosen as influence factors, and the Box−Behnken design experiment was designed.

X₁, X₂, and X₃ were the corresponding coding values of the three factors. The values of the test factors and levels are shown in Table 5, where X_i was coded according to the following formula:

In the formula: x_i is the actual value of the influence factor, x₀ is the value of the center level of the effect factor, and Δx is the change step of the influence factor.

In addition, according to Reference of Li et al. [21], the number of three-factor Box−Behnken design experiments was 12 + n_c, where n_c is the number of central point test groups. The design accuracy could be improved by selecting the appropriate number of central points in RSM. For the three-factor Box−Behnken design test, the number of central point test groups was recommended to be 3–5 groups [33]. Therefore, the number of center points n_c was 4 in this experiment. Factors and their code levels for Box−Behnken design are shown in Table 5.

3. Results and Discussion

3.1. Test Results and Establishment of Multiple Regression Model

The results of unconfined compressive strength, softening coefficient, and standard deviation (SD) are shown in the following table.

The quadratic equation is used to describe the relationship between independent variables and response indexes.where Y is the response index; X is the independent variable; β₀ is the intercept; β_i is the linear coefficient; β_ii is the square coefficient; β_ij is the interaction coefficient.

These experimental data were processed by Design-Expert. In V8.0.6.1, the coefficients of the quadratic equation and variance analysis are shown in Tables 7 and 8. It can be seen that a “ value” less than “α” indicating model terms are significant [21]. In this case, α = 0.05. And the correlation coefficient of the model was R² > 0.95, denoting that the model is well fitted. The multiple regression model can optimize the ratio of MgO, MgCl₂, and fly ash.

For the 7d unconfined compressive strength, the linear effect of all the factors is significant on the strength of MOC-solidified soil, and the interaction effect between MOC content and fly ash content is noticeable. Similarly, the interaction of the 1d softening coefficient among MgO, MgCl₂, and fly ash could be obtained.

Design-Expert was used to remove the nonobvious items in the regression equation. The adjusted regression equations for compressive strength and softening coefficient are as follows:

To illustrate the credibility of the RSM, the model fitting accuracy could be judged by comparing the predicted values with the actual values. The predicted values of 16 response indexes and 4 setting values were calculated by the adjusted regression equation. Four sets of setting values were used to verify the generalizability of this method. The distribution of the predicted values and actual values of unconfined compressive strength and softening coefficient are shown in Figure 4.

From Figure 4, the predicted numeral values and the actual numeral values are linear distributions. The actual values are evenly distributed near the predicted value. These show that the predicted values of the response index are close to the actual values. The response surface model has high accuracy, which can be used for the analysis and forecast of unconfined compressive strength and softening coefficient.

3.2. Analysis of the RSM Model

According to the test results obtained by the model, the 3D diagram and contour plots of MOC-solidified soil under the interaction of 7d unconfined compressive strength and 1d softening coefficient with different factors were plotted by Design-Expert software. The models of 7d unconfined compressive strength and 1d softening coefficient were analyzed, respectively, where the red points in the figure represent the design points in the Box−Behnken design, and the lines in the figure represent the values of the response indicators.

3.2.1. 7d Unconfined Compressive Strength

Based on the Chinese industry standard [26], the 7d unconfined compressive strength is the main index for the construction quality control of inorganic binder-stabilized materials.

As shown in Figure 5, the 7d unconfined compressive strength rises first and reduces with the growth of the MgO/MgCl₂ molar ratio, which has a maximum value. And the unconfined compressive strength gradually becomes large with the growth of the MOC content. It is elementary to explain this phenomenon. Obviously, the MOC content is more significant than the MgO/MgCl₂ molar ratio in the 7d unconfined compressive strength. We can get the conclusion from Figure 5 and the table of variance analysis for the RSM model.

Two pictures in Figure 6 show the 3D diagram and contour picture of the influence of the MgO/MgCl₂ molar ratio and fly ash content on the 7d unconfined compressive strength. The tendency of the MgO/MgCl₂ molar ratio is similar to the previous one when the MOC content is constant, but the rate of change becomes smaller. And the unconfined compressive strength tended to decrease with the rise in fly ash, which is owing to the decrease of the MgO content and molar ratio caused by the diminution of the MgO/MgCl₂ molar ratio. So, fly ash is unfavorable to the unconfined compressive strength of curing soil. In addition, in Figure 6, the distribution of contour lines is more in the abscissa than in the ordinate, illustrating that the effect of the MgO/MgCl₂ molar ratio is more excellent than fly ash content on the 7d unconfined compressive strength of MOC curing soil.

From Figure 7, the unconfined compressive strength presents the opposite trend with the alteration of MOC and fly ash contents. Furthermore, the unconfined compressive strength is better with less fly ash content. The response surface is distorted in 3D images, demonstrating that the interaction effect between MOC content and fly ash content on unconfined compressive strength is significant.

Therefore, the influence degree of X₁, X₂, and X₃ on the 7d unconfined compressive strength of the response value can be obtained as follows: MOC content > MgO/MgCl₂ molar ratio > fly ash content, which is combined with the 3D response surface, the contour plots, and the previous variance analysis table.

3.2.2. 1d Softening Coefficient

The softening coefficient indicates the strength of resistance to water of the material. The larger the value, the higher the softening coefficient of the material. Since the base and subbase of the pavement are affected by factors such as groundwater and atmospheric precipitation in the use process and the water resistance of MOC is poor, it is prominent to consider the softening coefficient.

As illustrated in Figure 8, the MgO/MgCl₂ molar ratio has a certain effect on the unconfined compressive strength of MOC curing soil when the fly ash content is not changed. It shows that only MgO/MgCl₂ molar ratio is in a reasonable range, phase 5 and phase 3 can be produced by MgO, MgCl₂, and water, which will accrue the resistance to water of solidified soil. Then, the fluctuation of the MOC content has little effect on the softening coefficient, so the impact of the MgO/MgCl₂ molar ratio on the softening coefficient is higher than that of the MOC content.

In Figure 9, fly ash content plays an essential role in the water resistance of solidified soil. The softening coefficient will become prominent with a suitable amount of fly ash. And the optimum addition of fly ash is approximately 20%. The softening coefficient shows the same trend with the change in MgO/MgCl₂ molar ratio and fly ash content. The maximum value can be taken in this test range based on the downward 3D model. Besides, the contour map is approximately circular, illustrating the interaction between MgO/MgCl₂ molar ratio and fly ash content is not remarkable in the softening coefficient of solidified soil. Similarly, the distribution of contour lines is more in the abscissa than in the ordinate in the contour line diagram, which indicates that the influence of fly ash on the softening coefficient is more potent than that of the MOC content. Moreover, the impact of fly ash content on the softening coefficient is more incredible than that of the MOC content, owing to the rate of change of diagrams.

Finally, the 3D image and contour plot of the influence of fly ash content and MOC content on the softening coefficient are evident in Figure 10 when the molar ratio remains unchanged. Comparably, it is not hard to recognize that the fluctuation of fly ash content on the softening coefficient is much sharper than that of the MOC content.

In conclusion, with the 3D response surface, the contour plots, and the “F” value of the previous variance analysis table, the influence degree of X₁, X₂, and X₃ on the 1d softening coefficient of the response value can be obtained: fly ash content > MgO/MgCl₂ molar ratio > MOC content.

3.3. Analysis of the Curing Mechanism

The hydration process of MOC is analogous to other magnesium cements [16, 17]. Active magnesium oxide, magnesium chloride, and the proper amount of water can be hydrated to form the MgO-MgCl₂-H₂O ternary compound. The main components of MOC are 5MgO·MgCl₂·8H₂O and 3MgO·MgCl₂·8H₂O [34, 35]. The leading chemical reaction equations are as follows:

MOC has excellent bond performance. The loose soil particles can be combined with their main components, including phases 5 and 3. And the contact area of soil particles increases, resulting in less porosity and compressibility of soil. Finally, the strength and impermeability of solidified soil have improved. These are the primary reasons for increasing the unconfined compressive strength of MOC curing soil.

With the aid of industrial waste fly ash, the porosity of materials such as soil and concrete could be reduced, and the frost resistance is improved [18]. In addition, lots of active Al₂O₃-SiO₂ substances are in fly ash, which has potential alkali-activated activity. Alkali-activated aluminosilicate reactions can generate cementitious materials with marvelous water resistance and chemical stability [36, 37]. The pore structure could be changed by these new materials, and water would be prevented from entering the internal channels of the solidified body. Therefore, the resistance to water of MOC curing soil is improved.

For the unconfined compressive strength, the change of MOC content had the most influence by observing the results of the 7d unconfined compressive strength experiment. With the growth of the MOC content, the unconfined compressive strength of solidified soil increases significantly, but the crack resistance of solidified soil might be poor, similar to that of Portland cement when the MOC content is too excessive [38, 39]. Meanwhile, the economy of MOC-solidified soil is not excellent. The MOC content can be appropriately reduced to promote the crack resistance and economy of solidified soil under the condition of ensuring strength. Furthermore, the MgO/MgCl₂ molar ratio plays a vital role in the strength of solidified soil, too. Vast amounts of phases 5 and 3 can be produced in large quantities without a reasonable MgO/MgCl₂ molar ratio, enhancing the unconfined compressive strength of MOC-solidified soil. Finally, fly ash cannot react directly with soil to enhance its strength [37]. Not only that, the MgO/MgCl₂ molar ratio was lower than that of the rational range with the addition of fly ash due to the equivalent substitution of fly ash for magnesium oxide by the internal mixing method. The strength of stabilized soil is further decreased by a low MgO/MgCl₂ molar ratio.

As for another corresponding index, fly ash content is the foremost factor affecting the softening coefficient of MOC-solidified soil. There are many small gaps in solidified particles, even after sufficiently compacting, which could be filled by the tiny particles in fly ash. The generated cementitious materials, including M-S-H gel, will effectively bond the wrapped soil particles and fill the pores of the particles [40]. The water resistance of the active fly ash-MOC-solidified soil will be improved. The MgO/MgCl₂ molar ratio also has a certain effect on the softening coefficient. According to the previous analysis of MOC, MgO, and MgCl₂ can fully react without the formation of a large amount of Mg (OH)₂ or the phenomenon of antihalogen when the MgO/MgCl₂ molar ratio is in a reasonable range [8], which is also essential to raise the water resistance of MOC-solidified soil. In the end, the variation in the MOC content cannot have a substantial influence on the resistance to water of MOC, even if we already know that it is necessary for the unconfined compressive strength.

3.4. Optimal Prediction Results and Verification

The 7d unconfined compressive strength and 1d softening coefficient of MOC-solidified clay soil are used as response values, and the MgO/MgCl₂ molar ratio, MOC content, and fly ash content are optimized by Box−Behnken design in the RSM. As a result, the optimal balance of MOC-solidified clay soil is as follows: the MgO/MgCl₂ molar ratio is 8.61, the MOC cement content is 18%, and the fly ash content is 20.36%. Then, the samples are made according to the optimal ratio for actual measurement, and the predicted date is compared with the actual data to verify the accuracy of the response surface model. The test data demonstrate that the measured values of 7d unconfined compressive strength and 1d softening coefficient are 2.56 MPa and 0.76, which are close to the predicted results. Thereby, the results were in excellent agreement with the predicted testing data. It is possible to optimize the proportion of MOC-solidified clay soil by using the response surface methodology.

4. Conclusions

In this work, the energy-saving and green environmental protection magnesium oxychloride cement were used for pavement base and bottom base stability. Based on the Box−Behnken design in response surface methodology, the optimal parameters of MOC-stabilized clay soil were determined by unconfined compressive strength and water resistance experiments, and the curing mechanism was analyzed. The major conclusions are as follows:(1)According to the analysis of variance, the regression model of two response indexes is significant and the fitting degree is also excellent, which accurately reflects the relationship between the influencing factors and the response indicators.(2)The response surface model analyses show that the MOC content has the most significant effect on unconfined compressive strength. There exists an optimal value of MgO/MgCl₂ molar ratio to make unconfined compressive strength and softening coefficient have desirable performance. Fly ash could boost the resistance to water of MOC-solidified soil, but it is unfavorable to the unconfined compressive strength.(3)The optimum parameters of magnesium oxychloride cement-solidified clay soil are the MgO/MgCl₂ molar ratio of 8.61, MOC content of 18%, and fly ash content of 20.36%. The results show that the predicted value obtained by the response surface methodology is close to the actual value, and the error is within a reasonable range.(4)The influence of various factors of MOC-solidified soil on the response index can be analyzed by the response surface methodology effectively. This optimization method provides a reliable basis for the practical highway engineering applications of the magnesium oxychloride cement solidification field.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The work was supported by the Science and Technology Research Planning Project belonging to the Education Department of Jilin Province (JJKH20220291KJ).

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Copyright © 2023 Huzhu Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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