Influence of Treatment Temperature Conditions on the Performance of Enzyme-Induced Cemented Sand

Enzyme induced calcite precipitation (EICP), as a bio-cementation technique, has been used to improve the strength and stiffness of unstable soils. In this paper, the influence of key treatment conditions for EICP soil treatment such as curing temperatures (at 5°C, 25°C, 30°C, and 50°C) as well as post-treatment conditions such as “oven-dried” (at 60°C) or “wet” (at room temperature, ~22°C) on the strength of EICP-treated sand were evaluated. The results from this study indicate that unconfined compressive strength (UCS), splitting tensile strength (STS), CaCO₃ content (C_C), the mass of precipitated CaCO₃ (${\mathrm{M}}_{{\mathrm{CaCo}}_3}$), and CaCO₃ precipitation ratio (PR) increased with increasing curing temperatures during EICP soil treatment possibly due to the increase in urease enzyme activity at high temperatures. For similar C_C, the “oven-dried” samples showed a significantly higher strength (i.e., UCS and STS) compared to the “wet” samples, possibly due to the transformation of unstable vaterite to a more stable form (i.e., calcite) during oven-drying. Scanning electron microscopy images and X-ray powder diffraction analysis showed that the “oven-dried” sample mainly produced calcite crystals, while the “wet” sample produced a mixture of calcite and vaterite crystals. The results indicate that thermodynamically unstable vaterite crystals formed at low temperatures may be transformed into a more stable polymorph of CaCO₃ (e.g., calcite) at high temperatures (~40°C to 60°C), an indication of the higher strength achieved for the “oven-dried” samples.