Crack self-healing in alkali-activated slag composites incorporating immobilized bacteria

Decarbonizing concrete manufacturing calls for sustainable construction materials since cement production contributes about 8% of global anthropogenic CO2 emissions. Alkali-activated materials (AAMs), usually derived from the activation of industrial by-products, have demonstrated great potential as alternatives to ordinary portland cement (OPC) concrete. However, AAMs are susceptible to cracking due to their inherent brittleness, which may compromise their service life and severely hinder wider acceptance and applications of AAMs. While bacteria-based crack self-healing has attracted enormous research interests in OPC-based materials, only limited research has explored such self-healing in AAMs. Accordingly, this study aims at investigating the self-healing efficiency of alkali-activated slag-based composites using bacteria immobilized in expanded glass granules made from recycled glass. The effect of different calcium sources on the reaction kinetics, mechanical properties, capillary water absorption, and final self-healing compounds was investigated. The results reveal that the biobased specimens attained a remarkably enhanced crack healing ratio, while no obvious self-healing was observed in abiotic control samples. Furthermore, the sorptivity coefficient decreased significantly by incorporating bacteria. Scanning electron microscopy with energy dispersive X-ray (SEM-EDX) analysis and Raman spectroscopy confirmed that calcite was the dominant self-healing product. This study suggests that using calcium lactate as the calcium source is advantageous owing to its economic and sustainability considerations.