Study On Compressive And Permeability Properties Of Cement Soil Mixed With Mineral Powder And Fly Ash

Soil-cement is widely used in various projects such as foundation treatment and seepage prevention of reservoir dam for its advantages of convenient construction and good durability.However,with the development of engineering construction,the performance requirements are becoming higher and higher,soil-cement also has problems such as low strength and large deformation in the later period.Under the condition of not changing the cement content,selecting various appropriate admixtures to improve the performance of soil-cement has become the key to solve the problem.When blast furnace ironmaking produces waste slag containing silicate,aluminosilicate and other components,which can be prepared into mineral powder after drying and grinding fine.Fly ash produced after coal combustion is a kind of harmful solid pollutant discharged from coal-fired power plants.In this paper,two kinds of industrial wastes,mineral powder and fly ash,are used as admixtures to explore the influence law and significance degree of their addition on unconfined compressive strength and permeability of soil-cement by means of compound admixture and single admixture.The microstructure characteristics of soil-cement were studied by scanning electron microscopy,and the mechanism of improving the properties of soil-cement with admixtures and the difference of microstructure under different conditions were analyzed combined with the test results.The research contents and conclusions are as follows:(1)Through the orthogonal test,the influence law of cement,mineral powder and fly ash on the compressive strength of soil-cement is explored.The test results show that cement can greatly improve soil-cement strength with the increase of cement content under the two curing ages of 7d and 28 d.However,the strength increase tends to be stable when the cement content reaches 18% of soil sample mass,and the maximum values are 4.15 MPa and 5.08 MPa respectively.The cement strength increases first and then decreases.When the mineral powder content reaches 16%,the cement strength reaches the maximum,which are 4.11 MPa and 4.91 MPa,respectively.Fly ash will first decrease and then significantly increase soil-cement strength.The inflection point content is 6%,and the maximum strength of soil-cement is 4.29 MPa and 4.90 MPa respectively.At the age of 7 days,the cement content plays a significant role in enhancing the influence on the compressive strength of soil-cement,followed by fly ash and mineral powder.At the age of 28 days,the order is cement > mineral powder > fly ash.Under the curing age of 7 days and 28 days,the intensity influence curve of the three groups has a high consistency.The strength of soil-cement will gradually increase with the extension of curing age,but with the further growth of curing age,the strength curve gradually becomes stable and the change is no longer significant.(2)Through the control variable method,the mineral powder and fly ash are mixed into soil-cement in the way of single mixing to explore the influence of cement and their mixing amount on soil-cement permeability.The test results show that under different cement contents(12%,15%,18%,21%),the impermeability of soil-cement increases first and then decreases with the increase of mineral powder.When the content of mineral powder is 16%,the soil-cement permeability coefficient curve reaches the lowest point.Under 7d curing,the minimum permeability coefficients were 1.85×10-5cm/s,1.33×10-5cm/s,1.45×10-5cm/s and 2.09×10-5cm/s,respectively.At the age of28 days,they are 1.74×10-5cm/s,1.22×10-5cm/s,1.31×10-5cm/s and 2.00×10-5cm/s,respectively.With the increase of fly ash,the change of soil-cement permeability coefficient curve is consistent with that of mineral powder,and the impermeability of soil-cement reaches the best when fly ash content is 12%.At the age of 7 days,The minimum permeability coefficients were 2.96×10-5cm/s,2.77×10-5cm/s,1.82×10-5cm/s,3.02×10-5cm/s,and 2.24×10-5cm/s,1.68×10-5cm/s,1.56×10-5cm/s and 2.31×1at 28 d,respectively 0-5cm/s.(3)The impermeability of soil-cement increases first and then decreases with the increase of cement content.Under the curing period of 7 days,the impermeability of the cement content of 18% can be optimized,and when the curing period is extended to 28 days,the cement content of 15%,the impermeability is optimized.When fly ash content remained unchanged,the impermeability of soil-cement increased first and then decreased with the increase of cement content.When the cement content is 18%.The curve of soil-cement permeability coefficient reaches its lowest point.It can be seen that the continuous increase of cement content will not always improve the impermeability of soil-cement,but the high cement content will deteriorate the impermeability of soil-cement.The curve slope of plain soil-cement and fly ash soilcement is highly consistent,and the slope drops fastest when the age increases from 7days to 28 days.However,the slope of the permeability coefficient curve of cement doped with mineral powder is basically unchanged during the whole test period,and the decline of the curve is relatively gentle,with little difference between the permeability coefficient of 7d and 60 d.(4)SEM test and soil-cement curing mechanism analysis show that the curing process of soft soil is relatively complex,among which the main changes include the following: hydration reaction of cement itself,chemical interaction between soil and cement hydration products and physical and chemical interaction produced by admixtures.Among them,the main effect of cement hydration products on the strength of soil-cement is tricalcium silicate and dicalcium silicate.The positive effects of mineral powder and fly ash on soil-cement properties are mainly due to volcanic ash and microaggregate effects.