Improvement of the quality of recycled fine aggregates by microbial induced calcium carbonate precipitation

The limited availability of natural resources, combined with the expanding construction industry and the resulting construction and demolition waste (CDW), has become a major global concern, and has led to the search for sustainable approaches in the construction industry. In recent years, research on green construction practices has gained substantial momentum and among the most popular of these researches is the enhancement of the quality of recycled aggregates from CDW for use in new concrete production. Microbial induced calcium carbonate precipitation method (MICP) has been investigated by researchers as a biobased approach to improve the surface properties of recycled coarse aggregates. However, studies on the improvement of recycled fine aggregates are scarce. Considering the significant fraction of recycled aggregates consists of fine aggregates, it is necessary to develop approaches for improvement of recycled fine aggregates facilitating their integration into the circular economy. The thesis study aimed to improve the properties of recycled fine concrete aggregates (fRCA) through MICP, incorporating both urea hydrolysis and nitrate reduction metabolic pathways. The effects of the MICP treatment on surface and mechanical properties of fRCA were investigated by utilizing two different types of fRCA. Following the application of MICP to six different size fractions of fRCA, a decrease in water absorption across all aggregates sizes was observed, accompanied by an increase in weight. The MICP method was further optimized for the enhancement of fRCA sand mixes. In this method, bacterial activity was confirmed by urea hydrolysis. Bacteria could hydrolyse 20 g of the urea in the biomineralization media demonstrating the 82% urea hydrolysis efficiency. The method resulted in precipitation of deposition of CaCO3 on aggregates' surface, which was quantified as 4.6% weight increase in the MICP-treated fRCA sand mix. Same optimized treatment led to 3.8% weight increase in commercially available fRCA sand mix. In this setup, the bacterial activity was confirmed by 39% nitrate reduction and 88% urea hydrolysis. Finally, the study achieved significant improvements in the mechanical properties of mortar specimens with MICP applied fRCA. The 28-day compressive strength and flexural strength of mortar specimens with MICP treated fRCA increased by up to 95% and 75%, respectively, compared to mortar containing untreated fRCA. It was supported by SEM, EDX and FTIR analyzes that the increase in the strength quality of the mortars was due to microbial CaCO3 precipitates. Keywords: recycled fine concrete aggregates (fRCA); biomineralization; urea hydrolysis; nitrate reduction; green concrete; circular economy