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 25oC. 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.