Biocementation of soils of different surface chemistries via enzyme induced carbonate precipitation (EICP): An integrated laboratory and molecular dynamics study

Biocementation is a ground improvement technique that involves precipitating a mineral (commonly calcium carbonate, CaCO₃) in the soil pore space to bind soil particles, in turn increasing the strength and reducing the permeability of the soil. Ureolysis (i.e. hydrolysis of urea) is the most researched calcium carbonate precipitation mechanism, which can be induced through either a microbial (MICP) or enzymatic (EICP) process. While laboratory tests and field trials have provided strong evidence of the efficacy of biocementation in strengthening granular materials, the role of the precipitate–grain interface and the surface chemistry of soil grains in biocementation are largely unknown. This study aims to address this gap. To this end, two geotechnically similar sand samples differing considerably in the amount of iron oxide and iron sulfate on grain surface are biocemented via EICP and tested for unconfined compressive strength (UCS). The biocemented sample containing a high concentration of iron oxide and iron sulfate exhibits almost 50% lower UCS than the other sample. To investigate whether surface chemistry can explain this considerable difference, interactions of CaCO₃ with quartz (SiO₂), hematite (Fe₂O₃), and marcasite (FeS₂) as polymorphs of silicon dioxide, iron oxide, and iron sulfide, respectively, are simulated using molecular dynamics. The influence of water content at the precipitate–grain interface is also considered. Simulation results indicate that in dry conditions, CaCO₃ has almost two times stronger affinity for SiO₂ than Fe₂O₃ and FeS₂, suggesting that biocementation is most effective for clean sands. It is also shown that water reduces the precipitate–grain adhesion.