STRENGTHENING OF SAND CEMENTED WITH CALCIUM PHOSPHATE COMPOUNDS USING PLANT-DERIVED UREASE

Abstract

The effect of plant-derived urease enzyme to induce the precipitation of calcium phosphate compounds (CPCs) and hence, to improve the unconfined compressive strength (UCS) of sand was examined as a novel, eco-friendly ground improvement method. Initially, Toyoura sand test pieces were cemented only from CPC solution. Furthermore, another sand test pieces were cemented by different concentrations of urea, concentration fixed plant seeds extract (Watermelon) to obtain optimal cementation, and different concentrations of CPC solutions made from calcium and phosphate stock solutions. All test pieces were cured up to 28 days in an airtight container at high humidity at 25 o C. The UCS tests and scanning electron microscope (SEM) observations of sand test pieces were carried out. The UCS of test pieces cemented with CPC and plant extract were significantly higher than that of test pieces cemented without plant extract and increases with time. The best ratio of Ca: P in CPC solution was 0.75 M: 1.5 M, reaching a maximum UCS of 125.6 kPa after 28 days of curing. In addition, pH concentration was measured after UCS test and it has increased with time. Results indicate that the best pH for optimal cementation is 8.0. A specific crystal structure could not be identified from SEM observations in the segments of the test pieces cemented with CPC in all cases in this study. These results suggest that the addition of plant extract to CPC significantly enhances the mechanical properties of sand.