  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 CaCO3
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 CaCO3 precipitates during MICP processes occurring at the sand particle
scale. Results show that the precipitation rate of CaCO3 increases with
bacterial density in the range between 0.6e8 and 5.2e8 cells/ml. Bacterial
density also affects both the size and number of CaCO3 crystals. A low
bacterial density of 0.6e8 cells/ml produced 1.1e6 crystals/ml with an average
crystal volume of 8,000 um3, whereas a high bacterial density of 5.2e8 cells/ml
resulted in more crystals (2.0e7 crystals/ml) but with a smaller average
crystal volume of 450 um3. The produced CaCO3 crystals were stable when the
bacterial density was 0.6e8 cells/ml. When the bacterial density was 4-10 times
higher, the crystals were first unstable and then transformed into more stable
CaCO3 crystals. This suggests that bacterial density should be an important
consideration in the design of MICP protocols.
