RESUMEN
Crushing waste concrete and using it directly as RAs has the disadvantages of high porosity and high water absorption. To achieve the reuse of resources, the researchers use microbial mineralization methods to further reinforce RAs. In this paper, the effect of the microbial carbonic anhydrase mineralization method on the water absorption of RAs was investigated, and the macroscopic analysis was performed by determining the indexes of water absorption and apparent density of RAs before and after the modification, and the microscopic analysis of RAs by using the methods of SEM, XRD, DSC, and EDS as well. According to the microscopic analysis, the mineralization products of microorganisms are calcium carbonate crystals, and with the increase in microbial liquid concentration, the water absorption rate of RAs shows a trend of decreasing and then increasing, and it can be found through the microscopic morphology that abundant mineralization products attached to the surface of the aggregate lead to the surface of the aggregate becoming rougher and more porous. The method of soaking the RAs in 3% bacterial solution and 0.1 mol/L calcium acetate solution followed by carbonation with 20% CO2 resulted in a 4.85% reduction in water absorption.
RESUMEN
Cement-based Materials have been widely used in the world, and bacteria have a capacity to induce mineral precipitation and can be applied to heal crack and resist efflorescence of cement-based materials. In this paper, Bacillus mucilaginosus was used for inducing mineralization by fixing CO2 from the air. The calcium-containing substances in cement-based materials that participating in the bio-mineralization were analyzed. Among them, the reaction between C3S and CO2 in the air can be ignored, and the mineralized products of Ca(OH)2 were mainly calcite, while the mineralized products of C-S-H were mainly aragonite. The results of XRD and SEM indicated that products obtained by reaction between C3S paste and CO2 were calcite, so the calcium-containing substances participating in bio-mineralization was mainly Ca(OH)2. The effect of bacteria cells on the formation of carbonate ions was analyzed. The pH value and the concentration of ions indicated that the absorption of CO2 was accelerated when the bacteria were involved, and more carbonate ions were supplied to bio-mineralization. The experiments of Zeta potential and conductivity showed that bacteria cells had an adsorption effect on Ca2+ because of its negatively charged surface. The analysis of nucleation kinetics indicated that calcium carbonate nucleated on the surface of bacteria cells, and its followed heterogeneous model, the rate of nucleation was 3.36 × 10-4 s-1-3.38 × 10-3 s-1. Analysis of every mineralization step showed that the enzyme-catalyzed reaction rate was the minimum when microorganisms existed, and it was the control step.
