Effect of regulating urease activity on the properties of bio-CaCO₃ precipitated on recycled aggregates

The high porosity of attached mortar on recycled aggregates leads to high water absorption and low strength, which hinders its application efficiency in concrete production. Microbial induced carbonate precipitation is a promising approach for enhancing recycled aggregates. The difficulty at present is that the low bonding between biogenic CaCO₃ and aggregates leads to low strengthening efficiency. The cohesion among CaCO₃ and its bonding with aggregates have been found to be closely influenced by the precipitation rate. While in the ureolytic process, urease activity is the key to control the precipitation process. In this study, the precipitation process was controlled by regulating the release rate of urease activity and its effect on the mechanical properties of the deposited CaCO₃ was studied. The results showed that controlling the initiation time of induced bacterial urease and the concentration of precipitated precursors (calcium and urea) could effectively slow down the release rate of initial urease activity, thus regulating the precipitation rate. It was found that the bacteria cultivated without substrate urea can slowly release urease activity, and the urea decomposed process can be sustained until 14 d, and the best cohesion of biogenic CaCO₃ could be obtained at 0.5 M of calcium and urea. The aggregates treated by the bacteria cultivated with urea showed a mass loss of 1.88% and water absorption reduction of 8.9%. While the ones treated by the bacteria cultivated without urea showed a mass loss of 1.70% and water absorption reduction of 29.83%, and the ones treated by the pellets centrifuged by the bacteria cultivated without urea showed a mass loss of 1.44% and water absorption reduction of 22.46%. It proved that the method of bacterial cultivation without urea can efficiently enhance the bonding strength between biogenic CaCO₃ and aggregates, thus further improving the properties of aggregates, which was of great significance to improve the application efficiency of recycled aggregate in concrete.