Influence of Soil Particle Size, Shape, and Mineralogy on Microbially Induced Calcite Precipitation

Microbially induced calcite precipitation (MICP) has gained increasing attention as a potential sustainable ground improvement technique. This study investigates the influence of soil particle size, shape, and mineralogy on the MICP process and on the evolution of shear-wave velocity (Vs) in biocemented soil columns. MICP experimental soil column tests were conducted using three silica sands (Ottawa 20/30, F-75, and F-110), a naturally carbonate-rich silty sand (Blessington soil), and artificially graded carbonate–silica sand mixtures (75% silica sand and 25% Iceland spar). Vs and chemical measurements were performed during MICP experiments, and biocemented samples were preserved for scanning electron microscopy (SEM) analyses. Results showed that the amount of precipitated calcite in carbonate-rich Blessington soil columns was up to twice that observed in the silica sand columns, highlighting the strong influence of soil mineralogy on the MICP process. The Vs profiles showed an overall trend of decreasing Vs with an increase in median particle size tested, which indicated the dominant effect of the number of particle contacts on the evolution of Vs with cementation. Furthermore, carbonate–sand mixtures exhibited increased calcite precipitation, but reduced Vs compared to their pure silica sand counterpart columns, demonstrating the combined effects of soil mineralogy and particle shape on both calcite precipitation and stiffness development. SEM observations revealed a distinct change in the morphology of precipitated calcite on the surfaces of carbonate soil particles (Blessington and Iceland spar) forming closely packed, uniform calcite crystals resembling the growth of a preexisting carbonate phase, which suggested that carbonate soil particles offered a lower energy barrier that facilitated a more extensive calcite precipitation compared to silica sands. Furthermore, the nature of cementation bonds evolved from contact cementation to cementation matrix and ultimately to pore filling as the median particle size of the tested soils decreased, implying that the formation, morphology, and extent of cementation bonds are governed by the specific surface area of soil. Based on these insights, a new model is proposed that relates Vs to normalized calcite content, which effectively captures the variation of Vs with cementation across different soil types tested in the present study, as well as within a broader dataset compiled from the literature.