Optimization of Calcium Conditions for Microbial Induced Carbonate Precipitation on Recycled Mixed Aggregate

Recycling of construction and demolition wastes for the production of new concrete is a sustainable approach to avoid disposal while saving natural resources. However, the replacement of natural aggregate with recycled mixed aggregates is often challenging, especially with grain size smaller than 2 mm. The brick content and the attached mortar on the aggregate surface results in high water absorption that can influence the workability of fresh mortar. Microbial induced calcium carbonate precipitation (MICP) is a novel approach to this issue. It acts as a surface treatment to fill the pores of recycled aggregate with calcium carbonate precipitate, thus lowering water absorption capacity, improving the usability of recycled concrete. In this study, different calcium sources for MICP were investigated to optimize this bio-treatment. The MICP was applied to expanded glass granulate using a vacuum suction method. Calcium acetate was found to most remarkably reduce the water absorption of the expanded glass granulate and precipitated calcium carbonate content, with further repetitions of treatment enhancing this effect. The optimized bio-treatment conditions were tested on recycled mixed aggregates in a second phase of the study. Promising results were shown in the reduction of water absorption of 47.6% and the improved workability of the fresh mortar, compared to untreated recycled mixed aggregates, which is accompanied by a higher compressive strength of 15.6%. These findings help to improve the MICP method further, which can lead to more acceptance for the practical use of recycled mixed aggregates.