Abstract: The plant-derived urease-induced calcium carbonate precipitation (EICP) can significantly improve the engineering mechanical properties of sand. However, there is no corresponding specification for the parameter value in the specific operation, and the reinforcement effect needs to be improved. Based on soybean urease, the effects of temperature, urease concentration, urea concentration, calcium concentration, pH, calcium source type and other variables on urease activity and calcium carbonate precipitation were studied, and the tests on SEM and XRD of precipitated calcium carbonate crystals were carried out. On this basis, the unconfined compressive strength and curing effect of soybean urease-cured sand were tested. The results showed that urease activity increased linearly with the increase of urease concentration, but there was a temperature threshold value. When the temperature exceeded the threshold, urease activity was completely inactivated, and the threshold decreased with the increase of urease concentration. Urea concentration and pH affected urease activity together, and there was an optimal combination of them, that is, the optimal pH is 7 when urea concentration is 0.1-1 mol/L, and it is 8 when urea concentration is1.0-1.5 mol/L. Urease is the catalyst of a precipitation reaction. The higher the urease concentration was, the more complete the reaction was, and the higher the precipitation rate of calcium carbonate was. For urea and calcium solution, the dosage mainly affected the precipitation of calcium carbonate, and the dosage ratio should be 1:1. The concentration and pH of urea and calcium solution can affect the precipitation of calcium carbonate by affecting urease activity. Different calcium sources had little influence on the precipitation amount of calcium carbonate. The composition and density of precipitated calcium carbonate crystals from different calcium sources were basically the same, but the crystal structure was very different. The calcium chloride precipitated calcium carbonate crystals are mainly massive, with spherical and spheroidal crystals on the surface and large cementation surface, which can be used as an ideal calcium source in EICP technology. The unconfined compressive strength of the sand solidified with urease from soya beans and calcium chloride as calcium source was about 6 times that of the sand mixed with coal fly ash. SEM images show that the precipitated calcium carbonate crystals wrap and bond the sand into a whole, and the curing effect is ideal.

Key words: soybean urease, urease activity, calcium carbonate precipitation, solidified sand, experimental study