Soybean urease-based EICP stabilization of washed recycled sands derived from demolition wastes cured at low temperatures

This study evaluated the feasibility and efficiency of stabilizing washed recycled sand (RS) derived from construction and demolition wastes when treated by soybean-based enzyme-induced carbonate precipitation (EICP), cured at a low temperature of 4 °C. Two types of RS were studied, being fine recycled sand (FRS) and coarse recycled sand (CRS). Urease extracted from soybeans was used to catalyze the urea hydrolysis reaction, which led to the precipitation of calcium carbonate, which acts as a soil binder. Such a low-temperature curing condition was found to effectively minimize surface clogging at the same treatment cycles when compared to room-temperature curing due to a comparatively low urease activity. The excess amount of CaCO3 precipitated on the surface was found to block the effective EICP solution from flowing through the sample and did not contribute to strength improvement. The unconfined compressive strength (UCS) results on CRS depicted a slight strength reduction at low-temperature curing, but the improvement in bio-clogging indicated the potential of involving extra treatment cycles which could further develop the strength. SEM, EDS and XRD analyses further established the existence of the biocement gel triggered by soybean-extracted urease cured at low temperatures within the both CRS and FRS particles. As such, soybean-based EICP solution was found to be an effective green binder for washed RS in terms of the noticeable stabilizing effect, hence was found to be a suitable as a geotechnical fill material for ground improvement works.