Effect of water glass-enhanced biocementation in sand: Early strength enhancement and mechanistic insights

Microbially induced carbonate precipitation (MICP) is considered an effective way to reinforce the ground. To improve the early strength of biocemented sand, in this paper, water glass (Na₂O-mSiO₂-nH₂O) was incorporated into MICP as an additive. A series of tube tests were employed to quantitatively investigate the impact of water glass concentration on precipitation rate and amount under varying urease activities. Experimental observations revealed that, in general, MICP was hindered by an increase in Na₂O while being promoted by SiO₂, both effects were controlled by urease activity. For SiO₂, optimal concentrations are 2 %, 3 %, and 8 % at urease activities of 25 KU/L, 15 KU/L, and 10 KU/L respectively. Then, the sand column experiments were conducted to quantify the strength enhancement. Compared with the control tests, the unconfined compression strength (UCS) was enhanced by 32.3 % after a single treatment and by 26.5 % after three treatments, respectively, while their secant modulus (E₅₀) rose by 37.8 %, 33.8 %, and 38.1 % following single, two, and three treatments. at a urease activity of 10KU/L. Simultaneously, based on the microscopic and compositional analyses, three distinct stages were summarized during the reaction period. In stage 1, free Ca²⁺ was precipitated and calcium silicate hydrate (C-S-H) served as potential nucleation sites. In stage 2, the intralayer calcium of C-S-H was decalcified, providing sustained calcium for MICP. In stage 3, bacteria were coated and the reaction reached stabilization. This study proposed a potential method and provided reference insights for enhancing the reinforcement effect of MICP.