Abstract
Microbial-induced carbonate precipitation (MICP) has outstanding characteristics in solidifying soil, such as good fluidity, ecological environmental protection, adjustable reaction, etc., making it have a good application prospect. As a typical silty clay, the composition of loess is fine, and the microstructure is quite different from that of sand. Previous research has found that the unconfined compressive strength of loess cured by MICP can be increased by nearly four times. In this paper, by comparing the changes of structural characteristics of undisturbed loess before and after MICP solidification, the mechanism of strength improvement of loess after MICP solidification is revealed from the microscopic level. Firstly, the microstructure of loess before and after solidification is tested by scanning electron microscope, and it is found that the skeleton particles of undisturbed loess are granular, and the soil particles coexist in direct contact and indirect contact, and the pores in soil are mainly overhead pores compared with the microstructure of solidified loess, it is found that the surface contact between aggregates increases obviously, and calcium carbonate generated by MICP is adsorbed around the point contact between aggregates, which makes the contact between soil particles change from point contact to surface contact. Then, Pores (Particles) and Cracks Analysis System (PCAS) is used to quantitatively analyze the pores of loess before and after solidification. The results show that the total pore area, the maximum total pore area and porosity of soil samples decrease, and the total number of pores decreases by 13.2% compared with that before MICP solidification, indicating that a part of calcium carbonate produced by MICP reaction accumulates in tiny pores, thus reducing the number of pores. One part is cemented between soil particles, which increases the contact area of particles. Therefore, some pores of loess solidified by MICP are filled and densified, the contact area between soil particles is increased, and the strength of loess under load is obviously improved.
Highlights
Loess is accumulated for a long time under specific climate and natural conditions [1], and is widely distributed in China
It can be seen from the above table that the total pore area, the maximum total pore area and porosity of soil samples are reduced after Microbial-induced calcium carbonate precipitation (MICP) solidification, which indicates that calcium carbonate precipitates generated after microbial solidification fill the pores and serve as the connecting cement between soil particles
(2) In the loess solidified by MICP, the cementation substances produced are mainly cementation at both sides of particle contact points, which makes the point contact between particles gradually evolve into surface contact, and the flocculent cementation substances produced on the surface of skeleton particles increase, increasing the cohesion of soil
Summary
Loess is accumulated for a long time under specific climate and natural conditions [1], and is widely distributed in China. The microscopic structure of soil samples is quantitatively analyzed by the image processing software Pores (particles) and Cracks Analysis System (PCAS) developed by Nanjing University. It can be seen from the above table that the total pore area, the maximum total pore area and porosity of soil samples are reduced after MICP solidification, which indicates that calcium carbonate precipitates generated after microbial solidification fill the pores and serve as the connecting cement between soil particles. The porosity probability entropy of loess decreases by 0.012, which is negligible, that is, the order of voids and pores before and after solidification is relatively poor
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