Water glass-enhanced biocementation: Crystal precipitation evidence and environmental dependence analysis

Water glass (Na 2 O- m SiO 2 - n H 2 O) modified microbially induced calcium carbonate precipitation (MICP) shows promise for rapid sand treatment by reacting with Ca 2+ in cementation solution to form predominantly calcium silicate hydrate (C-S-H) gel. However, the interfacial regulatory role of C-S-H remains unclear. This study systematically investigated the effect of C-S-H coating mass, initial Ca/Si ratio, pH, and urease activity on precipitation and morphology of calcium carbonate using glass slide experiments combined with multiscale characterization techniques. Results demonstrate that C-S-H mass dominates precipitation enhancement through optimized substrate adsorption and nucleation site density. 75.3% average enhancement in precipitation was observed as the C-S-H mass increased from 0.2 g to 0.6 g, substantially surpassing the effect of Ca/Si ratio, which yielded enhancements of less than 10%. The combined effect of pH and urease activity significantly enhanced precipitation, lowering pH induced C-S-H decalcification to generate more available silanol groups that adsorb prenucleation clusters of calcium carbonate, while increasing in urease activity accelerates urea hydrolysis to create supersaturated nucleation conditions. Crystal analysis revealed that pH exerted dominant influence: increasing pH triggered a morphological transformation from rosette to ellipsoids, whereas enhanced urease activity promoted the formation of discrete particles. Microscopy revealed a three-layer architecture: glass slide substrate, silica-bonded ellipsoids composed with nanoparticles, and micron-scale calcite surface layer. Meanwhile, a C-S-H-mediated mechanism for calcium carbonate precipitation comprising decalcification, silanol-assisted adsorption, and structural templating was proposed, advancing deeper understanding of C-S-H on MICP mineral-microbe interactions.