Effect of Biochemical Parameters on Biomineral Formation and Soil Strength Development in Microbially Induced Calcite Precipitation

Microbially induced calcite precipitation (MICP) provides a low-carbon alternative to traditional soil stabilization methods. However, the coupled impact of key input biochemical parameters, namely biomass concentration, chemical reagent dosage, and initial pH, on this biocementation process remains largely unexplored, which in turn influences the precipitation pathway and crystal characteristics, such as quantity, size, and mineralogy, ultimately affecting the overall strength gain. The study conducts laboratory experiments using the Sporosarcina pasteurii bacterium with varying biomass concentrations, ranging from an optical density of 0.25 to 1.00, cementation reagent concentrations varying from 0.25 M to 1.00 M, and initial pH values changing from 7 to 9. This is followed by an optimization scheme aimed at achieving maximum strength gain. Urea hydrolysis and calcite precipitation were monitored through the release of ammonium amount and the concentration of dissolved calcium ions in the cementation solution, respectively. The precipitated biomineral was analyzed for microstructural and mineralogical attributes. Following this, soil biocementation experiments were conducted to arrive at optimized biochemical parameters using statistical regression analysis. Results show that higher biomass accelerates ureolysis, while final calcite quantity mainly depends on reagent availability. Yet, soil strength is not primarily dependent on biomineral quantity; instead, crystal size and morphology are decisive, which are strongly influenced by the coupled interaction of biochemical parameters. A lower biomass concentration, combined with an increased reagent amount, promotes crystal growth. However, an increase in the amount of cementation reagent becomes detrimental to crystal size at higher biomass levels. Moreover, lower pH provides some lag time to the reaction but can also accelerate bacterial growth, thereby altering the crystal size. Furthermore, stable calcite mineral is found to precipitate at lower biomass cementation due to the inhibition of bacterial enzymatic activity. Soil biocementation results revealed that larger crystals bridging the soil pores significantly increase strength, up to 10 MPa from 0.17 MPa, compared to abundant but small-sized crystals. Thus, reaction conditions that favour rapid precipitation can be mechanically ineffective without effective pore bridging, emphasizing that biocementation should focus not only on producing large amounts of biominerals but also on the size of the precipitated crystals. By identifying biochemical thresholds that promote stronger, more interlocked crystals, this work offers guidelines for achieving maximum strength gain with optimised biochemical parameters.