Material characteristics and pollutant diffusion simulation of cutoff walls made of alkali activated slag/bentonite

Abstract

To prevent heavy metal pollution, vertical cut-off walls made of cement soil are often used as barriers. This article developed a cementitious material with a “low-carbon footprint” (alkali activated slag) to replace cement. The performance of alkali activated slag soil and cement soil was comprehensively explored through unconfined compressive strength, hydraulic conductivity, Zn2+ adsorption, and chloride ion diffusion tests. And the micro pore structure and micro morphology characteristics of the cutoff wall were explored through nuclear magnetic resonance testing and scanning electron microscopy testing. Finally, considering the normal distribution of pollutants, the diffusion process of pollutants in the transition layer, cut-off wall and aquifer is simulated by using the COMSOL Multiphysics 5.6. Research has found that alkali activated slag soil meets the requirement of low permeability in the early stage of maintenance, making up for the shortcomings of cement soil maintenance in the early stage. After adding bentonite to alkali activated slag soil, it was found that the hydraulic conductivity and effective diffusion coefficient showed a decreasing trend, while the adsorption rate of Zn2+ was significantly increased. Due to the double layered structure of bentonite and the hinge of C-A-S-H generated by alkali activated slag, it promotes an increase in the proportion of small pores and the generation of free water with poor fluidity, thereby improving the blocking effect of the cut-off wall on pollutants. Through simulation results, it was found that the breakthrough time of alkali activated slag soil as a cutoff wall is 3.07 times that of cement soil. After adding 6% and 12% bentonite to the alkali activated slag soil, the breakthrough time of the cut-off wall increased by 1.74 times and 4.25 times, respectively.