The influence of environmental factors and precipitation precursors on enzyme-induced carbonate precipitation (EICP) process and its application on modification of recycled concrete aggregates

Recycled concrete aggregates are greatly limited in applications due to the defects in the adhered mortar. In this study, a novel bio-deposition method, enzyme-induced carbonate precipitation (EICP) technology, was applied to improve the quality of recycled concrete aggregates. Firstly, the influence of environmental factors (temperature and pH) and precipitation precursors (urea and Ca²⁺) on urease activity and EICP process were investigated. The results showed that the urease activity and deposition rate of CaCO₃ were increased with the increase in temperature, and pH of 7–10 had little effect on them. However, when the concentration of CO(NH₂)₂–Ca(NO₃)₂ was more than 0.5 M, Ca²⁺ could not be utilized due to the deactivation of urease at high salt concentration. After obtaining the suitable conditions of EICP process, 0–0.5 M CO(NH₂)₂–Ca(NO₃)₂ and self-made urease solution were used to modify recycled concrete aggregates. The mass of recycled concrete aggregates increased by 1%–3.5% after EICP treatment. The saturated water absorption of the modified recycled concrete aggregates was decreased by a maximum of 20.9% after 0.5 M CO(NH₂)₂–Ca(NO₃)₂ treatment. After ultrasonic pulse treatment, the loss of the formed precipitation was 36.02% when the initial CO(NH₂)₂–Ca(NO₃)₂ was 0.2 M. This study has expanded the application field of EICP technology, and the application value of recycled concrete aggregates can be greatly improved.