Fundamental Studies on Enzyme Induced Carbonate Precipitation

Enzyme-induced carbonate precipitation (EICP) is a biogeotechnical soil improvement method that involves the precipitation of calcium carbonate via hydrolysis of urea (ureolysis) catalyzed by free urease enzyme in a calcium chloride solution. When this reaction takes place in the pore space of a sand, the precipitated calcium carbonate may bind soil grains together, thereby improving strength. Three studies on EICP are presented in this dissertation. In the first study, chemical equilibrium modeling via PHREEQC is used to develop a method for evaluating urease activity from electrical conductivity (EC) measurements in a closed reactor containing urea and urease. It is shown that a commonly used correlation to estimate urease activity from EC measurements overestimates the initial urea hydrolysis rate (thereby overpredicting the urease activity as well). In the second study, the crystal structure and mechanical properties of calcium carbonate minerals formed by EICP are studied. It is shown that a “modified” precipitate synthesized by the inclusion of nonfat dry milk in the EICP solution is more ductile than a “baseline” precipitate synthesized from an EICP solution without nonfat milk. Additionally, in sands biocemented using the modified EICP solution, precipitation occurs preferentially at the grain contacts. This may contribute to relatively high unconfined compressive strengths at low carbonate contents in some EICP-treated sands. The third study discusses the role of some sand characteristics on the strength following modified EICP treatment. Three batches of Ottawa 20-30 sand from different sources were treated identically using the modified EICP solution. Subsequent testing showed large differences in their unconfined compressive strengths. It is shown that this variation in unconfined compressive strength is due to differences in the surface microtexture and surface mineralogy of the sands.The fundamental studies presented in this dissertation provide a deeper understanding of some aspects of the EICP process.