Optimization of EICP-Based Treatment Process to Improve the Properties of Recycled Aggregates

Recycled aggregates (RA) from construction and demolition waste have many shortcomings such as high porosity and low strength due to adhered mortar and defects inside. If the defects (micropores and microcracks) of RA were repaired, the quality of RA could be improved greatly and its application could be further enlarged. Our previous study has proposed a new modification method, enzyme-induced carbonate precipitation (EICP), to repair the internal defects of RA. In this study, the efforts were focused on the optimization of the EICP treatment. It was found that the two-step immersion method, consisting of preimmersing in CO(NH2)2-Ca(NO3)2 solution for 24 h, then adding urease solution at once with single treatment duration of 5 days and cycling two treatments, was the optimal treatment. Compared with the untreated RA, the water absorption and crush value of treated recycled concrete aggregates (T-CA) were decreased by 7.01% and 9.91%, respectively, and 21.59% and 14.40% for treated recycled mixed aggregates (T-MA), respectively. By use of the optimized EICP-treated RA, the compressive strength of concrete increased by 6.05% (T-CA concrete) and 9.23% (T-MA concrete), and the water absorption of concrete decrease by 11.46% (T-CA concrete) and 18.62% (T-MA concrete). This indicates that the optimized EICP treatment could reduce the porosity and improve the strength of aggregates, thus enhancing the mechanical properties and impermeability of recycled concrete.