Bio-mediated and bio-inspired improvement of granular soils via microbially induced CaCO₃ precipitation (MICP), enzyme induced CaCO₃ precipitation (EICP), and microbially induced desaturation and precipitation (MIDP), techniques sometimes referred to bio-grouting, have attracted an increasing amount of attention in the past decade. MICP and EICP are techniques which rely upon the hydrolysis of urea as catalyzed by the enzyme urease. MIDP is a technique that relies upon dissimilatory reduction of nitrate and also desaturates the soil via biogas generation. MICP and MIDP are bio-mediated process, as they rely on living organisms. EICP may be considered a bio-inspired technique as, while it employs biologically generated free urease, no living organisms are involved in the precipitation reaction. Bio-grouting has the potential to serve as a sustainable technique for a variety of soil improvement applications. While bio-grouting has not been commercialized, promising results have been obtained in the laboratory and limited field trials.

The mechanical properties of calcium carbonate minerals formed by enzyme-induced carbonate precipitation (EICP) were studied using nanoindentation. Two types of precipitates were considered: (i) a “baseline” precipitate, synthesized via urea hydrolysis in an aqueous solution of urease enzyme, urea, and calcium chloride; and (ii) a “modified” precipitate, synthesized from a similar solution, but with the inclusion of nonfat dry milk. While both precipitates predominantly comprised calcite, X-ray diffraction and Raman spectroscopy indicated broader peaks in the modified precipitate, implying differences in the crystal structure of the two precipitates. Both precipitates were polycrystalline and had a higher average indentation hardness (H) and a lower indentation modulus (M) compared with the values for single calcite crystals reported in the literature. The ductility of the precipitates was quantified by the ratio M/H. The modified precipitate had a higher average M/H, implying greater ductility. The increased ductility of the modified precipitate results in higher resistance to crack propagation. In sands biocemented using the modified EICP solution, the increased ductility of the precipitate, in addition to preferential precipitation at interparticle contacts, may contribute to relatively high unconfined compressive strengths at low carbonate contents.