Alum-Enhanced MICP for Sand Stabilization: Mechanisms and optimization

Microbial-induced calcium carbonate precipitation (MICP) is an environmentally friendly novel ground improvement technology. This study proposes an innovative alum-enhanced MICP technique to address critical challenges in traditional MICP applications for sand reinforcement, including prolonged construction duration, low treatment efficiency, and poor uniformity. Through systematic aqueous solution tests and sand column experiments, this study investigated the pH regulation mechanism of alum in MICP processes and its dual effects on calcium carbonate precipitation patterns and hydration product formation. Advanced characterization techniques including scanning electron microscope, X-ray diffraction, and fourier transform infrared spectroscopy were employed to elucidate the microstructural evolution. The results show that the addition of appropriate alum (3 mmol/L) significantly enhances the MICP effect through three synergistic mechanisms: (1) alum induced a shift in calcium carbonate crystallization from vaterite to calcite, resulting in a 180 % increase in calcite content. (2) analysis of variance results showed that alum addition significantly improved the spatial distribution uniformity of the precipitate, and (3) hydrocalumite, aluminum hydroxide, and calcium carbonate formed a framework cementation structure, which significantly improved the degree of cementation and strength of the specimens. Under these mechanisms, the addition of 3 mmol/L alum enabled the sand column to achieve stabilization after a single treatment cycle, reducing treatment time and cementation solution usage by over 70 % compared to conventional MICP. After five treatment cycles, the sand's strength showed an 851 % increase compared to the control group. The demonstrated techno-economic advantages highlight the potential of alum-enhanced MICP as a sustainable solution for sand ground improvement.