Method for improving coastal saline soil by lime-metakaolin and application

Abstract

The invention discloses a method for improving coastal salinized soil by lime-metakaolin and application of the method, and the performance of the salinized soil is remarkably improved by adding lime and metakaolin into to-be-improved salinized soil in a composite manner. The method comprises the following steps: mixing salinized soil with lime, adding metakaolin, fully mixing, adding water to the optimal water content, compacting and molding, and finally curing. Wherein the mass ratio of the salinized soil to the lime to the metakaolin is (100: 7)-(10: 0.53), the optimal water content is 12-18%, the compaction degree is 96%, and the curing conditions are that the temperature is 20 + / -2 DEG C, the relative humidity is greater than 95% and the time is 7-60 days. According to the method, the strength, the water stability and the durability of the salinized soil are remarkably improved, the defects of single lime improvement are overcome, and meanwhile the lime consumption is reduced. The method is suitable for improvement of the chlorine weak salinized soil, and has wide application prospects in the engineering fields of roadbed fillers, slope protection and the like.

Description

Technical Field

[0001] The invention relates to soil improvement technology, in particular to a method for improving coastal saline soil with lime-metakaolin, and application of the method in roadbed engineering. Background Art

[0002] Coastal saline soil is a special type of soil widely distributed in coastal areas, formed under the influence of natural factors such as seawater infiltration, evaporation and geological action. The high concentration of soluble salts in this type of soil, especially chloride salts, has a significant impact on its engineering properties. In the field of civil engineering, the poor engineering properties of saline soil often lead to problems such as dissolution, mudslides and salt swelling, which seriously affect the quality and safety of the project and increase the difficulty of construction and maintenance costs. Although the traditional lime improvement method can improve the mechanical properties of the soil through hydration reaction and carbonization, it has limitations such as large shrinkage, easy cracking, and poor water stability, and its effect is limited in high-salt environments. To overcome these defects, researchers began to explore new improvement methods.

[0003] As a special material calcined at high temperature, metakaolin has high volcanic ash activity and gelling properties, making it a potential material for improving coastal saline soil. The hydration products generated by its reaction with alkaline substances can improve soil strength and stability. In addition, the fine particles and large specific surface area of ​​metakaolin help fill soil pores and improve density. Its good adsorption and ion exchange capacity can effectively fix salt in the soil. However, the use of metakaolin alone for saline soil improvement has limitations such as slow reaction speed and high cost. Therefore, the lime-metakaolin composite improvement method has become a new research direction, aiming to give full play to the advantages of the two materials, overcome the shortcomings of single material improvement, improve the engineering performance of saline soil, and reduce the improvement cost. In practical applications, this method still faces many challenges, such as determining the optimal ratio, controlling reaction conditions, and evaluating long-term stability, which need further research. In addition, the engineering application of improved saline soil still needs more practical verification to ensure its reliability and durability.

[0004] In summary, the method of improving coastal saline soil with lime-metakaolin provides a new idea for solving saline soil engineering problems, which has important theoretical significance and practical application value. In-depth research on this method will not only help improve the quality of engineering construction in coastal areas, but also provide reference for the improvement of other special soils. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a method and application of improving coastal saline soil with lime-metakaolin, so as to improve the mechanical properties and road performance of saline soil, solve the problems of large shrinkage, easy cracking, poor water stability, etc. existing in the traditional lime improvement method, and at the same time reduce the amount of lime used and improve the stability and durability of the improvement effect.

[0006] The technical solution adopted by the present invention is a method for improving coastal saline soil with lime and metakaolin, wherein the method comprises adding lime and metakaolin in combination into the saline soil to be improved, stirring and compacting the soil, and then curing the soil, and specifically comprises the following steps:

[0007] (1) mixing the saline soil to be improved with lime to form a uniform solid mixture;

[0008] (2) adding metakaolin to the mixture of step (1), and mixing thoroughly to form improved soil;

[0009] (3) Add water to the improved soil and stir evenly;

[0010] (4) Add the improved soil into the mold, compact it into shape, demould it, and cure it.

[0011] Preferably, the mass ratio of the saline soil to be improved, lime and metakaolin is 100:7 to 10:0.53.

[0012] Preferably, the saline soil to be improved is weakly chloride saline soil with a salt content of 0.3-1.3%.

[0013] Preferably, the lime is quicklime, and the CaO content is not less than 95%.

[0014] Preferably, the main components of the metakaolin are SiO2 and Al2O3, wherein the content of SiO2 is 40-50%, and the content of Al2O3 is 35-45%.

[0015] Preferably, in step (3), the water content of the water is 12-18%.

[0016] Preferably, in step (4), the compaction degree of the compaction is 96%.

[0017] Preferably, in step (4), the curing temperature is 20±2° C., the relative humidity is greater than 95%, and the curing time is 7 to 60 days.

[0018] The above-mentioned method of improving coastal saline soil with lime-metakaolin is applied in engineering fields such as roadbed filling and slope protection.

[0019] Principle of the invention: The present invention optimizes the lime improvement method according to the characteristics of coastal saline soil. By adding metakaolin, not only the improvement effect is improved, but also the problems existing in the traditional lime improvement method are solved. It provides an efficient improvement method suitable for coastal saline soil and provides technical support for engineering construction in coastal areas. Specifically, the improvements include the following points:

[0020] First, the ratio of lime to improve coastal saline soil is optimized. The traditional lime improvement method usually uses lime alone, but the present invention introduces kaolin as an auxiliary material. The mass ratio of lime to kaolin is controlled between 14:1 and 3.33:1. This ratio can not only give play to the rapid solidification effect of lime, but also utilize the volcanic ash activity of kaolin to form a more stable hydration product.

[0021] Second, the mixing method of lime-metakaolin and saline soil is improved. The present invention adopts a step-by-step mixing method, firstly mixing lime and saline soil thoroughly, and then adding metakaolin. This method ensures that lime can fully react with the salt in the saline soil, and at the same time, metakaolin can be evenly distributed in the soil, avoiding the occurrence of agglomeration.

[0022] Third, the curing conditions of the improved soil are optimized. The present invention uses a temperature of 20±2°C and a relative humidity of more than 95% for curing for 7 to 60 days. Such curing conditions are conducive to the full hydration reaction of lime and metakaolin to form a stable cementing material.

[0023] The synergistic effect of lime and metakaolin brings unexpected technical effect improvement. The rapid hydration reaction of lime provides initial strength, while the slow pozzolanic reaction of metakaolin continuously improves soil performance. The combination of the two not only improves the strength of the improved soil, but also significantly improves its durability. The addition of metakaolin reduces the shrinkage and cracking problems of lime-improved soil and improves water stability. In addition, the fine particles of metakaolin fill the pores of the soil and work together with the hydration products formed by lime to form a denser soil structure, effectively reducing the dissolution and salt expansion problems of saline soil. At the appropriate addition amount, the synergistic effect of lime and metakaolin is more significant. When the lime content is 7.1% and the metakaolin content is 0.53%, the improvement effect is the best. This is because within this range, lime can provide a sufficient alkaline environment to stimulate the activity of metakaolin, and metakaolin can fully react with the lime hydration products to form more CSH gel and CAH crystals, further improving the strength and stability of the soil.

[0024] Beneficial effects: (1) The improvement method of the present invention significantly improves the strength and stability of saline soil through the synergistic effect of lime and kaolin, and solves the problems of shrinkage cracking and poor water stability existing in the traditional lime improvement method; (2) The mixing method of the present invention ensures the uniform distribution of the improvement material in the saline soil, forms a denser soil structure, and improves the consistency of the improvement effect; (3) The improvement method of the present invention promotes the full progress of the hydration reaction by optimizing the curing conditions, improves the durability of the improvement effect, and is particularly suitable for application in engineering construction in coastal areas. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described below in conjunction with the embodiments. The test materials used in the embodiments can be purchased through conventional channels. The basic parameters of the materials are shown in Table 1.

[0026] Table 1. Basic properties of materials

[0027]

[0028] Example 1

[0029] The method for improving coastal saline soil with lime-metakaolin of the present invention comprises the following steps:

[0030] (1) mixing the saline soil to be improved with lime accounting for 9% of the mass of the soil to form a uniform solid mixture;

[0031] (2) adding 1% by weight of kaolin to the mixture of step (1), and mixing well to form improved soil;

[0032] (3) Add 15% of the optimum water content to the improved soil and stir evenly;

[0033] (4) Pour the improved soil into In the standard test mold, compaction and demoulding are carried out according to the compaction degree of 96%;

[0034] (5) Place the specimens in a standard curing box for curing at a temperature of 20±2°C and a relative humidity greater than 95%. The curing time is 7 days, 14 days, 28 days and 60 days.

[0035] Example 2

[0036] The method for improving coastal saline soil with lime-metakaolin of the present invention, compared with Example 1, changes the amount of lime and metakaolin added, and comprises the following steps:

[0037] (1) mixing the saline soil to be improved with lime accounting for 8.5% of the mass of the soil to form a uniform solid mixture;

[0038] (2) adding 1.5% by weight of kaolin to the mixture of step (1), and mixing well to form improved soil;

[0039] (3) Add water with an optimum moisture content of 16% to the improved soil and stir evenly;

[0040] (4) Pour the improved soil into In the standard test mold, compaction and demoulding are carried out according to the compaction degree of 96%;

[0041] (5) Place the specimens in a standard curing box for curing at a temperature of 20±2°C and a relative humidity greater than 95%. The curing time is 7 days, 14 days, 28 days and 60 days.

[0042] Example 3

[0043] The method for improving coastal saline soil with lime-metakaolin of the present invention, compared with Example 1, changes the amount of lime and metakaolin added, and comprises the following steps:

[0044] (1) mixing the saline soil to be improved with lime accounting for 7% of the mass of the soil to form a uniform solid mixture;

[0045] (2) adding 3% by weight of kaolin to the mixture of step (1), and mixing well to form improved soil;

[0046] (3) Add water with an optimum moisture content of 17% to the improved soil and stir evenly;

[0047] (4) Pour the improved soil into In the standard test mold, compaction and demoulding are carried out according to the compaction degree of 96%;

[0048] (5) Place the specimens in a standard curing box for curing at a temperature of 20±2°C and a relative humidity greater than 95%. The curing time is 7 days, 14 days, 28 days and 60 days.

[0049] Comparative Example 1

[0050] A method for improving saline soil, which only uses lime for improvement compared with the embodiment, comprises the following steps: mixing the saline soil to be improved with lime accounting for 10% of the mass of the soil, adding water with an optimum water content of 14%, and stirring evenly. Pour the uniformly improved soil after stirring into In the standard test mold, compaction, demoulding and curing are carried out according to a compaction degree of 96%. The curing temperature is 20±2℃, the relative humidity is greater than 95%, and the curing time is 7 days, 14 days, 28 days and 60 days.

[0051] Comparative Example 2

[0052] The method for improving coastal saline soil with lime-metakaolin of the present invention, compared with Example 1, changes the amount of lime and metakaolin added, and comprises the following steps:

[0053] (1) mixing the saline soil to be improved with lime accounting for 6% of the mass of the soil to form a uniform solid mixture;

[0054] (2) adding 4% by weight of kaolin to the mixture of step (1), and mixing well to form improved soil;

[0055] (3) Add water with an optimum moisture content of 18% to the improved soil and stir evenly;

[0056] (4) Pour the improved soil into In the standard test mold, compaction and demoulding are carried out according to the compaction degree of 96%;

[0057] (5) Place the specimens in a standard curing box for curing at a temperature of 20±2°C and a relative humidity greater than 95%. The curing time is 7 days, 14 days, 28 days and 60 days.

[0058] The improved saline soil prepared in the embodiment and the comparative example was subjected to a performance test, comprising the following steps:

[0059] (1) For different curing times Standard specimens were subjected to unconfined compressive strength tests;

[0060] (2) After 28 days of maintenance The standard specimens were subjected to water stability tests with immersion times of 1 day, 3 days, 5 days, 7 days and 9 days;

[0061] (3) After 28 days of maintenance The standard specimens were subjected to dry-wet cycle tests with the number of cycles being 1, 3, 5, 8 and 12 times.

[0062] The relevant standards are: the basic properties of soil are tested with reference to the "Highway Geotechnical Test Code" (JTG 3430-2020), and the unconfined compressive strength test, water stability test and dry-wet cycle test are tested with reference to the "Highway Engineering Inorganic Binder Stabilized Material Test Code" (JTG E51-2009). The improvement effect of the method of improving coastal saline soil with lime-metakaolin in the present invention was tested by unconfined compressive strength test, water stability test and dry-wet cycle test, and the results are shown in Tables 2, 3 and 4.

[0063] Table 2 Unconfined compressive strength of saline soil under different improvement methods (MPa)

[0064]

[0065]

[0066] Table 3 Water stability coefficient of saline soil under different improvement methods

[0067]

[0068] Table 4 Retention rate of compressive strength of saline soil under different improvement methods (%)

[0069]

[0070] It can be seen from the above comparative data that the method of improving coastal saline soil with lime-metakaolin in the present invention can effectively improve the strength of saline soil, and Examples 1, 2, and 3 all show good improvement effects. Compared with the simple lime improvement in Comparative Example 1, the compressive strength of the saline soil in the example can be increased by 1.5-2 times. In Example 3, when the lime content is 7% and the kaolin content is 3%, the improvement effect is the best, and the 60-day compressive strength reaches 1.33MPa. The results of the water stability test show that the water stability coefficient of the improved saline soil with the addition of kaolin is significantly higher than that of the saline soil improved with simple lime. The water stability coefficient of Example 3 after 9 days of immersion in water is 0.88, which is 22% higher than that of Comparative Example 1, indicating that the addition of kaolin significantly improves the water stability of the improved saline soil.

[0071] The results of the dry-wet cycle test show that the improved saline soil with the addition of kaolin has less strength loss after multiple dry-wet cycles. After 12 dry-wet cycles, the compressive strength retention rate of Example 3 is 82%, while that of Comparative Example 1 is only 62%, indicating that the addition of kaolin significantly improves the durability of the improved saline soil. The amount of kaolin added in Comparative Example 2 is 4%, which exceeds the maximum amount added in the examples, and its improvement effect is slightly lower than that of Example 3. This is because excessive kaolin may affect the hydration reaction of lime, resulting in a slight decrease in the improvement effect. At the same time, the addition of excessive kaolin will increase engineering costs and reduce the economic efficiency of the improvement.

[0072] In summary, the lime-metakaolin composite improvement method proposed in the present invention significantly improves the strength, water stability and durability of saline soil by optimizing the ratio of lime and metakaolin. This method not only overcomes the shortcomings of single lime improvement, but also reduces the amount of lime while improving the improvement effect, and has good engineering application prospects.

Claims

1. A method for improving coastal saline soil by using lime-metakaolin, characterized in that: The method comprises the following steps: adding lime and metakaolin in combination into the saline soil to be improved, stirring and compacting the soil, and then curing the soil. (1) mixing the saline soil to be improved with lime to form a uniform solid mixture; (2) adding metakaolin to the mixture of step (1), and mixing thoroughly to form improved soil; (3) Add water to the improved soil and stir evenly; (4) Add the improved soil into the mold, compact it into shape, demould it, and cure it.

2. The method for improving coastal saline soil with lime-metakaolin according to claim 1, characterized in that: The mass ratio of the saline soil to be improved, lime and metakaolin is 100:7 to 10:0.

53.

3. The method for improving coastal saline soil with lime-metakaolin according to claim 2, characterized in that: The saline soil to be improved is weakly chloride saline soil with a salt content of 0.3-1.3%.

4. The method for improving coastal saline soil with lime-metakaolin according to claim 3, characterized in that: The lime is quicklime, and the CaO content is not less than 95%.

5. The method for improving coastal saline soil with lime-metakaolin according to claim 4, characterized in that: The main components of the metakaolin are SiO2 and Al2O3, wherein the content of SiO2 is 40-50%, and the content of Al2O3 is 35-45%.

6. The method for improving coastal saline soil with lime-metakaolin according to claim 5, characterized in that: In the step (3), the water content of the water is 12-18%.

7. The method for improving coastal saline soil with lime-metakaolin according to claim 6, characterized in that: In the step (4), the compaction degree is 96%.

8. The method for improving coastal saline soil with lime-metakaolin according to claim 7, characterized in that: In the step (4), the curing temperature is 20±2° C., the relative humidity is greater than 95%, and the curing time is 7 to 60 days.

9. An application of the method for improving coastal saline soil by using lime-metakaolin as claimed in any one of claims 1 to 8, characterized in that: The method is applied in engineering fields such as roadbed filling and slope protection.

Images