Efficiency of bacteria based self-healing method in alkali-activated slag (AAS) mortars

Alkali-activated slag (AAS) is a promising alternative to ordinary Portland cement (OPC) as a sole binder for concrete structures and offers advantages such as superior acid resistance and higher strength development. However, AAS basically suffers from a high rate of drying shrinkage causing more cracks in AAS concrete compared to the OPC concrete. The Microbially Induced Calcium Carbonate precipitation (MICP) is a self-healing technology whose effects on AAS mortars is not thoroughly investigated. Therefore, it would be highly desirable to impart the self-healing process into AAS concrete. In the present paper, the effect of using Bacillus pasteurii with three different cell concentrations of 103, 105, and 107 cells/ml as a microbial self-healing agent on the strength parameters and microstructures of AAS mortars was investigated. In order to investigate the strength parameters of specimens, the bacteria were directly added to the mix water of the mortars. Then, the compressive and flexural strength tests were performed at the ages of 7 and 28 days. In addition, scanning electron microscopy (SEM), X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were conducted to analyse the microstructures of the AAS mortars specimens. The obtained results indicated that Bacillus pasteurii produced calcium carbonate precipitation and probably remained viable to decompose urea in highly alkaline environment of AAS mortars. In addition, the activity of bacteria not only healed the cracks but also increased the strength of the AAS mortars up to 24.67%.