Study On Mechanical Properties And Microstructure Of Lignin Fiber-cement Modified Soil

Lignin fiber-cement modified soil is a mixture composed of plain soil,lignin fiber,cement and water.Following upon the improvement,the microstructure of the soil has greatly changed,of which the physical properties,strength properties and deformation been improved.In order to explore the specific influence of various admixture materials,the effects of lignin fiber content,cement content and curing on the physical and mechanical properties of improved soil were studied through unconfined compressive strength test,confined compression test and the direct shear test.The impact mechanism of various admixture materials on the strength and deformation characteristics of improved soil was analyzed through microscopic scanning of the sample.The main research contents and achievements of this paper are as below:(1)The effect of lignin fiber and cement content on permeability and thermal conductivity of improved soil was discussed.The results show that: the permeability coefficient of improved soil was positively correlated with lignin fiber content and had a negative correlation with cement content.The thermal conductivity of the improved soil is in a negative relation with the content of lignin fiber,which can be regarded as an open downward quadratic function relationship with the cement content.The thermal conductivity reaches its maximum with the cement content being 2%.(2)The unconfined compressive strength UCS of the improved soil increases first and then declines with the increase of lignin fiber content.When the content is 4%,the UCS reaches the maximum 180 k Pa,and the soil strength increases by about 80%.With the increase of cement content,the maximum strength is about 700 k Pa.In addition to enhancing the strength of the sample,the existence of lignin fiber can also contribute to improve the toughness of the sample.When the sample is damaged,the fracture length of the sample with higher content is shorter,which is particularly noticeable after the addition of cement.Moreover,when lignin fiber and cement are mixed together,the hydration reaction rate of cement is faster,and the initial strength of the sample is improved rapidly.(3)The compression performance of improved soil under lateral confinement was tested,from which it was found that the compression strain ε and pore change rate of improved soil decreased with the increase of cement content,and increased with the increase of lignin fiber content.The trend was more obvious with lignin fiber content greater than4%.With the normal stress in a range of 0.2 ～ 1.6 MPa,the stress-strain relationship of the improved soil changes linearly in the semi-logarithmic coordinate system.In the e-lgp curve,the void ratio of improved soil decreases linearly with the increase in vertical stress,and its compression coefficient Cc is close to a constant,which keeps constant with the change of normal stress,lignin fiber and cement content.(4)The effects of lignin fiber and cement on the shear properties of improved soil were investigated.Independent on the contents of lignin fiber and cement,the shear strength and normal stress of improved soil are in linear relation with the shear strength law of cohesive soil.The cohesion c and internal friction angle φ increased first and then decreased with the increase of lignin fiber content,and the optimal content was 2% ～ 4%.c and φ also increase with the increase of cement content and curing age.The shear strength of the sample reaches the maximum 99 k Pa when the lignin fiber content is 4% and the cement content is 4%.(5)The effects of lignin fibers and cement on the microstructure of modified soil were studied.Qualitative analysis shows that: After the cement is added,obvious hydration products are produced between soil particles and lignin fibers,and the lignin fibers will be cemented with soil particles.Small fracture surfaces.Quantitative analysis showed that: the orientation of the pores after compression of the modified soil weakened with the incorporation of lignin fibers and increased with the incorporation of cement;the orientation of particles increased with the incorporation of lignin fibers,and increased with the incorporation of cement.The incorporation of first increased and then decreased.The average diameter of pores gradually concentrates in the range of particle size less than 1 μm with the addition of lignin fibers,and the proportion of pore diameters of 1～2 μm and 2～4μm increases with the increase of cement content;the average diameter of particles increases with the increase of cement content.With the increase of lignin fiber and cement content,the proportion of particles less than 1 μm gradually reduced.The abundance of pores is mostly concentrated in the range of 0.2～0.5 with the increase of lignin fiber and cement content;the abundance of particles is mainly concentrated in the range of 0.1～0.6 with the addition of lignin fiber,particle distribution presents a "mountain" shape.The area proportion of pores firstly decreased and then built up over the increase of the content of lignin fibers.It reaches its minimum with the content of 4%,and decreases with the increase in cement content.