The use of N-(N-butyl)-thiophosphoric triamide to improve the efficiency of enzyme induced carbonate precipitation at high temperature

Enzyme induced carbonate precipitation (EICP) is widely studied as a promising technique for soil stabilization and solidification. Most related studies are carried out in a room temperature. However, some application scenarios exist at a high environmental temperature. Both the higher urease activity and quick decay at a high temperature hinder the application of EICP. In this study, the N-(n-butyl)-thiophosphoric triamide (NBPT) was used to address the problem of EICP solidification at higher temperatures. The appropriate addition way of NBPT was investigated, where urease activity and productive rates for calcium carbonate (CaCO₃) were measured under different temperatures and NBPT concentration. The influences of NBPT concentration and temperature on the solidification efficiency of sand columns were studied via permeability test, ultrasonic testing, unconfined compressive strength, and CaCO₃ contents. Finally, the microstructure characteristics of precipitation were also investigated. Results demostrated that adding NBPT to the urease solution was a more appropriate and efficient way to control urease activity. The urease activity and precipitation rates for CaCO₃ significantly decreased with the NBPT addition; with increased temperature, the urease activity, however, gradually recovered to the level without NBPT. The needed concentration of NBPT was higher at a higher temperature. Moreover, the NBPT addition was beneficial to mitigate the plugging and improve the homogeneous distribution of CaCO₃, which achieved a higher strength for the solidified sands. Additionally, NBPT addition did not affect the microscopic morphology of CaCO₃ at 65 °C, and the CaCO₃ crystals were mainly vaterite. These results demonstrate that the NBPT addition can significantly improve the solidification homogeneity and increased the strength of solidified sands at high temperatures, presenting a promising potential for future application of EICP at high temperatures.