Mesoporous silica nanoparticles loaded urea for enhancement of the cohesion of biogenic CaCO₃ and its adhesion with recycled concrete aggregates

The advantage of the biogenic CaCO₃ formed through Microbially Induced Carbonate Precipitation (MICP) lies in its cohesion, which plays a crucial role in the regain of mechanical strength of the materials. The cohesion among CaCO₃ particles is significantly affected by the precipitation kinetics. Especially precipitation rate (yield of CaCO₃ per unit time), which relates with the instantaneous concentration of the precipitation precursors in the surroundings. The lower the concentration of the precursors, the lower the precipitation rate. Therefore, in this study, an innovative slow-release strategy was used to control the real-time concentration of urea in the solution and to slow down the precipitation rate, thereby improving the cohesion of CaCO₃. To this end, mesoporous silica nanoparticles (MSN) were used as carriers for the urea, and the release behavior was investigated and optimized, achieving a 40 wt% loading capacity with 6 g/L MSN and 1 M urea. The urea release from MSN demonstrated a significantly prolonged release period, extending by approximately 20 times compared to free urea, and lasting up to 5 days. These urea-releasing MSN were then applied to provide a controlled supply of urea in the MICP process, as well as in the MICP-based treatment of recycled concrete aggregates. The cohesion of the precipitates and their adhesion to the concrete aggregates were evaluated, revealing the use of MSN-loaded urea significantly improved the cohesion of CaCO₃ by 25 %, while its adhesion to recycled concrete aggregates was increased by 37 %. This enhancement contributed to the improved MICP strengthening efficiency of recycled concrete aggregates.