Microscale Visualization of Microbial-Induced Calcium Carbonate Precipitation Processes

Microbial-induced calcium carbonate (${\mathrm{CaCO}}_3$) precipitation (MICP) has been explored for its potential engineering applications such as soil stabilization, but current understanding of the fundamental MICP processes at the microscale is limited. In this study, real-time in situ microscale experiments were conducted using glass slides and microfluidic chips (synthetic porous media that simulate soil matrices to model the conditions similar to actual MICP treatments) to visualize the ${\mathrm{CaCO}}_3$ precipitation process. The results of this study show that irregularly shaped ${\mathrm{CaCO}}_3$ precipitates initially emerged on bacterial aggregates and subsequently dissolved with time as regularly shaped ${\mathrm{CaCO}}_3$ crystals started growing; less stable and smaller ${\mathrm{CaCO}}_3$ crystals may dissolve at the expense of growth of more stable and larger ${\mathrm{CaCO}}_3$ crystals. The time-dependent phase transformation of ${\mathrm{CaCO}}_3$ precipitates makes the size of the crystals formed during MICP highly dependent on the time interval between cementation solution injections during a staged-injection procedure. When the injection interval was 3–5 h, a larger number of crystals (200–1,000 per ${10}^6{\mathrm{\mu m}}^3$) with smaller sizes (5–10 μm) was produced. When the injection interval was longer (23–25 h), the crystals were larger (10–80 μm) and fewer in number (5–20 per ${10}^6{\mathrm{\mu m}}^3$). The direct observation of MICP processes in this study improves the understanding of MICP fundamentals and the effect of MICP processes on the properties of ${\mathrm{CaCO}}_3$ crystals formed after MICP treatment. These observations will therefore be useful for designing future MICP treatment protocols that improve the properties and sustainability of MICP-treated samples.