Evaluation of urea hydrolysis for MICP technique applied in recycled aggregate: Concentration of urea and bacterial spores

Aggregate is an indispensable raw material for concrete. Microbial induced carbonate precipitation (MICP) is an environmentally friendly aggregate modification method, which could modify recycled aggregate (RA) to replace natural aggregate for the preparation of concrete in the future. The urea hydrolysis is the key to modified RA by MICP. Biological calcium carbonate is generated during mineralization reaction in modification solution to fill the weak part of aggregate, which will enhance the performance of RA. The bacterial solution and urea solution, as the sources of CO32- in biological CaCO3, jointly affect the modification effect of RA. In this paper, the different ratios of bacterial solution concentration (×108 cells/mL) to urea concentration (mg urea/mL) are used for exploring the improvement effect and the cooperative enhancement mechanism of recycled coarse aggregate (4.75 mm-9.5 mm). The results show that the bacterial mineralization efficiency is most improved when bacterial solution concentration/urea concentration (B/U) is in the optimal interval of 5–6 because 1 mg urea provides a moderate number of bacteria during the modification period. In the interval, the precipitation amount of biological CaCO3 and effective CaCO3 that could modify RA is larger, and the pores of the attached mortar of RA are better filled and blocked. Therefore, the macro performance of RA is greatly improved in the internal. When the B/U is outside the optimal interval, the modification effect is poor because the amount of biological CaCO3 and effective CaCO3 is too less to fill pores or too more to enter pores.