Study On The Improvement Of Expansive Soil With Calcium Bicarbonate From Pseudo-Karstification

Being as a special soil,expansive soil is composed of montmorillonite and illite,and it has caused olsses to engineering construction because of the special engineering properties such as water swelling and dehydration shrinkage.Aiming at the special properties of expansive soil,lime and cement are widely used,but there also exists some problems not such as environmental pollution and being difficult to be well mixed.Therefore,it is necessary to find a new soil improvement material which is environmental-friendly.Based on the formation mechanism of calcareous nodules in natural stalactites and loess,this research proposes an environmental-friendly method for improving expansive soils,that is,Calcium Bicarbonate from Pseudo-Karstification（Calcium Bicarbonate from Pseudo-Karstification,CFPK）.The swelling and shrinkage performance,mechanical properties,surface adsorption,calcium carbonate content and microstructure of the modified expansive soil of imitating karst calcium bicarbonate were studied.The research conclusions are as follows:（1）Calcium Bicarbonate from Pseudo-Karstification can significantly reduce the expansion and contraction of expansive soil.When the liquid-solid ratio is 6:1,the free expansion rate was reduced to less than 60%,and the unloaded expansion rate was reduced from 15%to 8%,the linear shrinkage rate was reduced from 8%to 6.5%.From the perspective of economy and improvement effect,recommend the optimal liquid-solid ratio is 6:1.（2）The physicochemical and mechanical properties of modified soil improved.After improvement,the alkalinity and the calcium carbonate content increased,but the exchangeable Na⁺,K⁺and Mg²⁺content adsorbed by the soil changed small,the exchangeable Ca²⁺content and the total exchangeable cation content decreased.The fine particle content decreased,but the content of coarse particles increased.Shear strength of the soil improved to a certain extent,but the cohesive force and internal friction angle changed little.（3）The results of scanning electron microscopy and energy spectrum analysis showed that the granular cementation of modified soil was significantly enhanced.The surface curvature and wrinkles disappeared,and the internal pores were reduced.The section became smoother,and the structure was more dense.Calcium carbonate in modified soil increased significantly.（4）The mechanism of Calcium Bicarbonate from Pseudo-Karstification improving expansive soil is that the solution contains H⁺,Ca²⁺,HCO₃^-,CO₃²⁺,and saturated carbon dioxide gas.Through ion exchange,the H⁺in the solution replaced the exchangeable Ca²⁺adsorbed by the soil particles which reduced the thickness of the electric double layer.Through karstification,the newly formed calcium carbonate had the functions of sedimentation,cementation and filling,and the bonding strength and granulation between particles were enhanced which contributed to reduce the expansion and contraction of the soil.