Microbially induced calcite precipitation (MICP) is a bio-mediated ground improvement technique designed to cement particle-to-particle contacts in low density soil deposits. While significant research has studied the process, most of the work has focused on testing silica-rich sands at lab-scale. The objective of the present study is to investigate the physicochemical effects of a controlled carbonate mineral phase on the MICP process. For this purpose, carbonate sand mixtures were prepared by combining Iceland spar particles (pure calcite mineral phase) with each of the three silica sands, Ottawa 20/30, F-75, and F-110 (~99% quartz) in 1:3 proportion. Chemical (pH, urea, ammonium, and calcium) measurements, shear-wave velocity (Vs) monitoring using bender elements, and scanning electron microscopy were performed during MICP treatment to assess pore fluid chemistry, evolution of soil stiffness, and microstructural analysis of bio-cemented samples. The MICP column tests showed that the inclusion of 25% Iceland spar led to an increase in precipitated calcite content by approximately 17% for Ottawa 20/30 and F-110 sands, and by 25% for F-75 sand, when compared to the experiments performed with pure silica sands. Despite this increase in calcite content, the carbonate–sand mixture columns exhibited lower shear-wave velocities than the pure silica sand columns. Microstructural SEM investigation into these aspects revealed the continuous growth of pre-existing calcite crystals across the surface of Iceland spar particles during the MICP process, and the formation of cementation bonds between Iceland spar particles required higher precipitated calcite content than Ottawa sand particles. This was due to greater surface area required to cement the flat particles, which explains the measured decrease in shear-wave velocity. These results suggest that carbonate minerals present in soil could act as preferential sites for calcite crystal growth during the MICP process, resulting in more widespread and higher calcite precipitation. This study also highlights the role of soil particle shape and mineralogy in the formation of cementation bonds and on the physicochemical impacts of the MICP process.
Shivaprakash et al. (Sun,) studied this question.