For the cement-clay slurry commonly used in dynamic water grouting, consider adding coal ash to optimize the performance of cement-clay slag composite slurry and discuss the reaction mechanism of the slurry through microchemical element analysis; the orthogonal test was used to study the influence of various factors on material setting time, solidification ratio, water segregation rate, and the optimized ratio of the slurry that was obtained by integrating the unconfined compressive strength of grouting concretion body and slurry configuration cost. The results showed that the water-solid ratio had the greatest influence on the comprehensive performance, followed by the amount of coal ash admixture. The best performance of the composite slurry was obtained with a water-solid ratio of 0.8:1 and a cement:coal ash:clay:quicklime:sodium sulfate:water mass ratio of 1:0.45:0.20:0.05:0.07:1.32. Finally, by comparing the mechanical properties of the optimized slurry and the grouting concretion body, it is proved that the optimized slurry has superior performance to meet the general grouting project requirement.
With the implementation of China’s western development strategy, underground projects such as tunnel construction and mine developments are gradually shifting to the central and westerly regions of the country. Meanwhile, the more complex geologic environment and frequent geological disasters come with it [
Since the emergence of grouting technology, grouting materials have been constantly changing with their development. The properties of the grouting material directly affect the retention of the slurry in the water and the strength or stability of the grouting concretion body after hardening, which ultimately determines the quality of the grouting projects. For a successfully grouting process, it is necessary to consider the properties of the grouting material. At present, the slurry can be subdivided into organic grouting materials with urea-formaldehyde resin, acrylamide, lignin, and so on as the main raw materials and inorganic grouting materials based on cement according to its composition [
In summary, many experts and scholars have conducted in-depth investigations on cement-clay slurry configuration or performance, and the research results have promoted the rapid development of the grouting discipline. By analyzing these studies, it can be noted that the existing researches are mostly focused on one type of clay addition, and the configuration was expensive, while the research on multiple clay slurry is few. Depending on this, this paper configures a low-cost clay slag composite slurry with ordinary Portland cement, coal ash, and clay as the main raw materials and quicklime and sodium sulfate as activators. The influence of slurry components on the properties of setting time, unconfined compressive strength, flexural strength, and the solidification ratio was studied through the orthogonal test. Moreover, the optimal design ratio of the composite slurry was finally obtained. The research conclusion has reference value for future compound slurry proportioning design and research.
According to the mechanism of cement hydration reaction, lime hydrolysis produces calcium hydroxide. Ca2+ and OH− have a stimulating effect on coal ash. H+ can destroy the Si-O and Al-O bonds of coal ash. Reactive silicon oxide, alumina, and calcium hydroxide react to produce an adsorption system of indeterminate components and then form hydrated calcium silicate and hydrated calcium aluminate. At the same time, calcium hydroxide and alumina react with sulfate in sodium sulfate to generate tricalcium aluminate hydrate and calcium sulfate, which forms calcium alumina in aqueous solution. Ca2+ in solution is combined with aluminate, sulfate, and silicate activated by coal ash in the excitation agent to generate hydrated calcium aluminate, and the result in Ca2+ is greatly reduced, which accelerates the hydration of cement clinker. The main chemical reaction formulae are as follows:
Under the action of the excitation agent, the hydrated calcium aluminate, hydrated calcium sulfate, and hydrated calcium silicate generated by the reaction in solution gradually become crystallized and accumulate. The crystalline mesh structure is formed between the crystals through strong chemical bonds, which makes the clay and hydration products interlinked and grow the strength and water stability of the crystalline body [
Qinling brand P.O. 32.5 ordinary Portland cement was used for the experiments; its chemical composition is shown in Table
Chemical composition of cement.
Composition | SiO2 (%) | CaO (%) | Fe2O3 (%) | Al2O3 (%) | MgO (%) |
---|---|---|---|---|---|
Content | 20.2 | 41.7 | 4.7 | 7.2 | 4.38 |
The coal ash is obtained from local commercial coal ash in Xi’an, and the main chemical components are shown in Table
Chemical composition of coal ash.
Composition | SiO2 (%) | Al2O3 (%) | Fe2O3 (%) | CaO (%) | MgO (%) | Na2O (%) | K2O (%) |
---|---|---|---|---|---|---|---|
Content | 50.6 | 27.1 | 7.1 | 2.8 | 1.2 | 0.5 | 1.3 |
The clay is kaolin produced by Shanghai Fengcheng Reagent Factory, acquired by the laboratory, showing white color, and the technical conditions are shown in Table
Technical conditions of ceramic clay.
Ignition loss | |
Carbonate inspection | Eligible |
Arsenic | |
Heavy metal | |
Iron salt inspection | Eligibility |
Chloride | |
Soluble matter in acid | |
Sand quality inspection | Eligible |
The quicklime used in the experiment was produced by Tianjin Kaitong Chemical Reagent Company, and its calcium oxide content was in accordance with the current standard JC/T479-1992 “Building Quicklime” of China’s building materials industry, and the technical conditions are shown in Table
Technical conditions of quicklime.
Calcium oxide content | |
Acetic acid insoluble matter | |
Chloride (Cl) | |
Nitrate | |
Heavy metal | |
Ammonia precipitates | |
Clarity test | Eligibility |
Loss on ignition | |
Sulfate (SO4) | |
Iron | |
Alkali metal and magnesium |
In the experiment, the effect of five experimental factors on the slurry performance was mainly considered: “A – water-solid ratio (the ratio of water to the total mass of cement, coal ash, and clay),” “B – mass percentage of coal ash to cement,” “C – the mass percentage of clay to cement,” “D – mass percentage of quicklime to cement,” and “E – mass percentage of sodium sulfate to cement,” and each factor had four levels, so orthogonal table L16 (45) was used for the orthogonal test. The experimental scheme is shown in Tables
Orthogonal factors and levels.
Factors and levels | A | B | C | D | E |
---|---|---|---|---|---|
1 | 0.5 : 1 | 15 | 10 | 2 | 1 |
2 | 0.8 : 1 | 25 | 15 | 3 | 3 |
3 | 1 : 1 | 35 | 20 | 5 | 5 |
4 | 1.5 : 1 | 45 | 25 | 7 | 7 |
Orthogonal experimental design.
Experiment number | Factor-level distribution | A | B | C | D | E |
---|---|---|---|---|---|---|
1 | A1B1C4D3E2 | 0.5 : 1 | 15 | 25 | 5 | 3 |
2 | A1B2C3D2E3 | 0.5 : 1 | 25 | 20 | 3 | 5 |
3 | A1B3C1D4E4 | 0.5 : 1 | 35 | 10 | 7 | 7 |
4 | A1B4C2D1E1 | 0.5 : 1 | 45 | 15 | 2 | 1 |
5 | A2B1C1D1E3 | 0.8 : 1 | 15 | 10 | 2 | 5 |
6 | A2B2C2D4E2 | 0.8 : 1 | 25 | 15 | 7 | 3 |
7 | A2B3C4D2E1 | 0.8 : 1 | 35 | 25 | 3 | 1 |
8 | A2B4C3D3E4 | 0.8 : 1 | 45 | 20 | 5 | 7 |
9 | A3B1C3D4E1 | 1 : 1 | 15 | 20 | 7 | 1 |
10 | A3B2C4D1E4 | 1 : 1 | 25 | 25 | 2 | 7 |
11 | A3B3C2D3E3 | 1 : 1 | 35 | 15 | 5 | 5 |
12 | A3B4C1D2E2 | 1 : 1 | 45 | 10 | 3 | 3 |
13 | A4B1C2D2E4 | 1.5 : 1 | 15 | 15 | 3 | 7 |
14 | A4B2C1D3E1 | 1.5 : 1 | 25 | 10 | 5 | 1 |
15 | A4B3C3D1E2 | 1.5 : 1 | 35 | 20 | 2 | 3 |
16 | A4B4C4D4E3 | 1.5 : 1 | 45 | 25 | 7 | 5 |
When the slurry is configured, the content of each component in the slurry is first calculated through the orthogonal experiment table; then the cement, fly ash, lime, and water are stirred with a mixer for 5∼10 min, mixed with the preconfigured sodium sulfate solution later after thorough mixing, and stirred again for 5∼10 min until the composite slurry is free from obvious segregation.
The water separation rate, setting time, and solidification rate are essential for the study of slurry performance. The water separation rate and setting time directly affect the spreading distance and hardening time of the slurry in the injected rock, while the solidification rate affects the volume and strength of the slurry after hardening. Therefore, the experiment focuses on the results of these three indicators to analyze the performance of each proportion of slurry. The water separation rate test is with reference to the “Technical Specification for Construction of Cement Grouting for Hydraulic Buildings” (SL62-2014) and the final setting time test is with reference to the “Test Method for Water Consumption, Setting Time, and Settlement of Cement Standard Consistency” (GBT1346-2011). The results of the experimental tests are shown in Table
Results of the orthogonal experiment.
Experiment number | Density | Viscosity | Setting time (min) | Water separation rate (%) | Solidification rate (%) | |
---|---|---|---|---|---|---|
Initial | Final | |||||
1 | 1.80 | — | 7.50 | 12.83 | 0.2 | 99.5 |
2 | 1.80 | — | 6.58 | 12.42 | 0.5 | 99.3 |
3 | 1.80 | — | 7.00 | 12.25 | 1.0 | 98.3 |
4 | 1.79 | — | 7.58 | 13.17 | 1.0 | 98.8 |
5 | 1.58 | 32 | 11. 75 | 20.92 | 4.2 | 95.0 |
6 | 1.56 | 33 | 12.08 | 21.25 | 2.5 | 93.3 |
7 | 1.50 | 29 | 12.50 | 21.75 | 3.1 | 93.3 |
8 | 1.53 | 31 | 13.25 | 23.08 | 4.2 | 93.3 |
9 | 1.44 | 25 | 19.58 | 30.08 | 7.0 | 86.3 |
10 | 1.43 | 24 | 20.25 | 31.08 | 5.1 | 91.3 |
11 | 1.45 | 23 | 19.08 | 29.25 | 15.1 | 88.3 |
12 | 1.43 | 22 | 20.75 | 31.25 | 13.0 | 87.5 |
13 | 1.32 | 20 | 30.75 | 47.92 | 37.4 | 68.8 |
14 | 1.32 | 20 | 31.25 | 48.58 | 30.5 | 71.8 |
15 | 1.32 | 18 | 31.08 | 48.17 | 43.6 | 66.3 |
16 | 1.34 | 18 | 31.58 | 49.17 | 30.5 | 72.0 |
Table
Flocculation structure diagram of cement slurries with different water-solid ratios [
Meanwhile, the data in Table
The data acquired were analyzed by the method of range analysis, and the primary and secondary relationships of the influence of each factor on slurry viscosity, water precipitation rate, initial setting times, and final setting times were obtained, as shown in Tables
Range analysis of slurry viscosity.
Analysis index | Factors | ||||
---|---|---|---|---|---|
A | B | C | D | E | |
+ | 25.71 | 24.43 | 24.83 | 24.71 | |
31.37 | 25.74 | 25.64 | 23.88 | 24.38 | |
23.66 | 23.40 | 24.67 | 24.64 | 24.37 | |
18.95 | 23.80 | 23.91 | 25.63 | 25.19 | |
Range | + | 2.34 | 1.73 | 1.75 | 0.82 |
Primary and secondary factors | A > B > D > C > E |
Range analysis of slurry water separation rate.
Analysis index | Factors | ||||
---|---|---|---|---|---|
A | B | C | D | E | |
0.68 | 12.20 | 12.18 | 13.48 | 10.40 | |
3.50 | 9.65 | 14.00 | 13.50 | 14.82 | |
10.05 | 15.70 | 13.82 | 12.50 | 12.58 | |
35.50 | 12.28 | 9.72 | 10.25 | 11.92 | |
Range | 34.82 | 6.05 | 4.28 | 3.25 | 4.42 |
Primary and secondary factors | A > B > E > C > D |
Range analysis of the initial setting time of slurry.
Analysis index | Factors | ||||
---|---|---|---|---|---|
A | B | C | D | E | |
7.17 | 17.40 | 17.69 | 17.67 | 17.73 | |
12.40 | 17.54 | 17.50 | 17.65 | 17.85 | |
19.92 | 17.42 | 17.62 | 17.77 | 17.21 | |
31.17 | 18.25 | 17.92 | 17.52 | 17.81 | |
Range | 24 | 0.85 | 0.42 | 0.25 | 0.64 |
Primary and secondary factors | A > B > E > C > D |
Range analysis of the final setting time of slurry.
Analysis index | Factors | ||||
---|---|---|---|---|---|
A | B | C | D | E | |
12.67 | 27.94 | 28.25 | 28.34 | 28.40 | |
21.75 | 28.33 | 28.06 | 28.34 | 28.38 | |
30.42 | 27.86 | 28.44 | 28.44 | 27.94 | |
48.46 | 29.17 | 28.71 | 28.19 | 28.58 | |
Range | 35.79 | 1.31 | 0.65 | 0.25 | 0.64 |
Primary and secondary factor | A > B > C > E > D |
It can be concluded from Tables
In summary, the slurry with a water-solid ratio of 0.8:1 was initially selected as the best mix ratio. Moreover, a water-solid ratio of 0.8:1 was selected for the next mechanical test as shown in Table
Strength is the most important mechanical property of the grouting concretion body. Mechanical tests of the grouting concretion body in this section include unconfined compressive and flexural strengths. According to the orthogonal test design table, a composite slurry with different material ratios and a water-solid ratio of 0.8:1 is configured. After the slurry is fully mixed, it is immediately poured into a triple cement test mold with the size of 40 mm × 40 mm × 160 mm, and the mold is released after the slurry has gelled. Finally, the test blocks are placed in the water with a curing temperature of 20 ± 5°C for curing, and the strength of the blocks is tested for curing cycles of 3 d, 7 d, 14 d, and 28 d. The molded sample is shown in Figure
Molding sample.
The flexural and compressive strengths of the grouting concretion body were measured by DKZ-5000 electric flexural testing machine (Figure
Strength test equipment: (a) DKZ-5000 electric flexural testing machine and (b) NYL-300C pressure testing machine.
The mechanical test results of grouting concretion bodies at different curing ages are drawn on a graph, and the final result is shown in Figure
Mechanical properties of slurry with a water-solid ratio of 0.8:1 at different curing ages: (a) curve of unconfined compressive strength with time and (b) curve of flexural strength with time.
It can be observed from Figure
The above analysis results are synthesized, while taking into account the economic benefits brought by different coal ash content; No. 8 (the mass ratio of cement:coal ash:clay:quicklime:sodium sulfate:water is 1:0.45:0.20:0.05:0.07:1.32) was finally selected as the best ratio for the composite slurry. In the optimized composite slurry (No. 8), a certain amount of water glass was added (the Baume degree of water glass was 35°B, and the volume ratio of water glass to cement composite slurry was 0.6:1), and the initial setting time of the slurry was measured to be about 10 s, indicating that the addition of water glass solution can effectively regulate the setting time of the composite slurry. By analyzing the reasons, the existence of water glass solution accelerates the consumption of cement hydrate calcium hydroxide to generate hydrated calcium silicate (CaO·nSiO2·mH2O). The specific reaction equations are as follows [
Due to the rapid reaction of the water glass solution with calcium hydroxide, the gelation time of the cement-water glass slurry will be reduced substantially. Therefore, the amount of water glass added can be determined according to the geological conditions to control the gelling time of the slurry, so that the grouting can be controlled.
To further evaluate the engineering practicality of the optimized slurry, it is necessary to conduct a comparison test of the mechanical properties between the optimized slurry and ordinary cement slurry. The specific test methods are as follows: two types of slurry were prepared before the experiment; one group of slurry is mixed with the best ratio obtained above (No. 8), and the other group is mixed with a pure cement slurry with a water-cement ratio of 0.8:1. After mixing evenly, the two slurries are poured into two identical standard molds (100 mm × 100 mm × 100 mm) and then we vibrate and compact the slurries to make them dense. The specimens were left to solidify for 24 h before release from the mold and then placed in a curing box under standard conditions for curing. Specimens were taken out with curing ages of 7 d, 14 d, and 28 d for the compressive strength test. Finally, the results of the compressive strength tests of the specimens at different curing ages are plotted in a graph, as shown in Figure
Comparison of strength between pure cement slurry and optimized slurry in different ages.
It can be observed from Figure
The performance of the cement-clay-slag composite slurry was investigated by orthogonal tests. The results show that the comprehensive performance of slurry is the best when the water-solid ratio is 0.8 : 1. Combining the cost of slurry configuration and mechanical properties of the grouting concretion body, group 8 slurry (the mass ratio of cement:coal ash:clay:quicklime:sodium sulfate:water is 1 : 0.45 : 0.20 : 0.05 : 0.07 : 1.32) was selected as the optimized composite slurry.
The range analysis of orthogonal test results shows that the water-solid ratio is the primary factor affecting the performance of composite slurry, and the coal ash is the second. In the process of slurry configuration, the influence of water-solid ratio and coal ash content on the comprehensive performance of slurry should be considered first.
The experiment shows that water glass can effectively adjust the setting time of composite slurry. Therefore, when designing the slurry ratio in the early stage of a project, a certain amount of water glass can be considered to be added to control the hardening time so that the grouting work can be controlled.
By comparing the physical strength of the grouting concretion body at different curing ages of optimized slurry and cement slurry with a water-cement ratio of 0.8, the results show that the strength of optimized slurry is lower than that of pure cement slurry, but the difference in strength is small. Moreover, the strength growth trend of composite slurry during hardening is as stable as that of pure cement slurry, indicating that composite slurry has good performance.
The data supporting this research article are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support of the 2020 Open Fund of Xi’an Key Laboratory of Geotechnical and Underground Engineering (no. XKLGUEKF20-03) and the Shaanxi Natural Science Basic Research Program (no. 2018JM5126).