Microbial-induced calcium carbonate precipitation (MICP) is an innovative ground improvement technique which can enhance the strength and stiffness of soils and can also control their hydraulic conductivity. These engineering properties of MICP-treated soils are affected by the particle-scale behaviour of the precipitated calcium carbonate – namely, the composition, amount and distribution, which are controlled by the MICP process occurring at the particle scale. In this study, a microfluidic chip was designed and fabricated to improve understanding of MICP at the particle scale by observing the behaviour of bacteria and calcium carbonate crystals during this process. It was found that bacteria became evenly distributed throughout the microfluidic chip after the injection of bacterial suspension, grew during bacterial settling and detached during the injection of cementation solution. Bacteria aggregated during the cementation solution injection, and calcium carbonate crystals formed at narrow pore throats or open pore bodies either during or after the cementation solution injections.

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.

Protocols for microbially induced carbonate precipitation (MICP) have been extensively studied in the literature to optimise the process with regard to the amount of injected chemicals, the ratio of urea to calcium chloride, the method of injection and injection intervals, and the population of the bacteria, usually using fine- to medium-grained poorly graded sands. This study assesses the effect of varying urease activities, which have not been studied systematically, and population densities of the bacteria on the uniformity of cementation in very coarse sands (considered poor candidates for treatment). A procedure for producing bacteria with the desired urease activities was developed and qPCR tests were conducted to measure the counts of the RNA of the Ure-C genes. Sand biocementaton experiments followed, showing that slower rates of MICP reactions promote more effective and uniform cementation. Lowering urease activity, in particular, results in progressively more uniformly cemented samples and it is proven to be effective enough when its value is less than 10 mmol/L/h. The work presented highlights the importance of urease activity in controlling the quality and quantity of calcium carbonate cements.

Microbial-induced carbonate precipitation (MICP) has been explored for more than a decade as a promising soil improvement technique. However, it is still challenging to predict and control the growth rate and characteristics of ${\mathrm{CaCO}}_3$ precipitates, which directly affect the engineering performance of MICP-treated soils. In this study, we employ a microfluidics-based pore-scale model to observe the effect of bacterial density on the growth rate and characteristics of ${\mathrm{CaCO}}_3$ precipitates during MICP processes occurring at the sand particle scale. Results show that the precipitation rate of ${\mathrm{CaCO}}_3$ increases with bacterial density in the range between $0.6{\times 10}^8$ and $5.2{\times 10}^8\mathrm{cells}/\mathrm{mL}$ . Bacterial density also affects both the size and number of ${\mathrm{CaCO}}_3$ crystals. A low bacterial density of $0.6{\times 10}^8\mathrm{cells}/\mathrm{mL}$ produced $1.1{\times 10}^6$ crystals/mL with an average crystal volume of 8,000 ${\mu \mathrm{m}}^3$ , whereas a high bacterial density of $5.2{\times 10}^8\mathrm{cells}/\mathrm{mL}$ resulted in more crystals ($2.0{\times 10}^7\mathrm{crystals}/\mathrm{mL}$ ), but with a smaller average crystal volume of $450\mu {\mathrm{m}}^3$ . The produced ${\mathrm{CaCO}}_3$ crystals were stable when the bacterial density was $0.6{\times 10}^8\mathrm{cells}/\mathrm{mL}$ . When the bacterial density was 4–10 times higher, the crystals were first unstable and then transformed into more stable ${\mathrm{CaCO}}_3$ crystals. This suggests that bacterial density should be an important consideration in the design of MICP protocols.