This study was conducted based on the calcium carbonate precipitate from microbially induced calcite precipitation (MICP) to determine the optimal conditions for the highest precipitation. The Bacillus subtilis QN7 strain was tested by changing certain parameters, including (1) bacterial density, (2) pH concentration, (3) culture temperature, (4) urea concentration, (5) calcium source, and (6) calcium concentration. From that, the urease activity of this strain was determined. The results showed that the most effective calcium source for calcite precipitation was calcium chloride supplemented with 150-200 mM CaCl2 and 250 mM urea at 40 °C, pH 8
the precipitate formed reached 10.21 g/L and the urease enzyme activity obtained was 703.86 U/L. These initial research results have determined the optimal conditions for the highest calcium precipitation of the B. subtilis QN7 strain, which is the premise for further research on potential applications in crack healing, self-healing, and bio-cement production in Vietnam's climatic and environmental conditions