Soil improvement using plant-derived urease-induced calcium carbonate precipitation

This study addresses a soil improvement technique using plant-derived urease-induced calcium carbonate (CC) precipitation (PDUICCP) as an alternative to microbially induced carbonate precipitation (MICP). A crude extract of crushed watermelon (Citrullus lanatus) seeds was used as the urease source along with calcium chloride (CaCl2) and urea (CO (NH2)2) for CC precipitation. Test specimens (φ = 2.3 cm, h = 7.1 cm) made from commercially available Mikawa sand (mean diameter, D50 = 870 µm) were cemented, and estimated unconfined compressive strength (UCS) of several kPa to MPa was obtained by changing the concentration of CaCl2- urea, urease activity, curing time, and temperature. The increase of curing time and that of the CaCl2-urea concentration from 0.3 M to 0.7 M caused an increase in estimated UCS value. The average estimated UCS obtained after 14 days’ curing time for 0.7 M CaCl2-urea and 3.912 U/mL urease was around 3.0 MPa and for 0.3 and 0.5 M CaCl2-urea and 0.877 U/mL urease, it was around 1.5–2.0 MPa at 25 °C. By changing each of the abovementioned parameters, it may be possible to apply this method for strength improvement of loose sand, to mitigate the liquefaction, protection and restoration of limestone monuments and statuaries, and artificial soft rock formations. Crude urease from crushed watermelon seeds has the potential to replace commercially available urease for carbonate precipitation and for use as a low environmental impact type soil improvement method.