Performance of self-healing mortar containing bacteria immobilized in alginate coated alkali activated lightweight aggregate

Among possible bacteria carriers available on the market, lightweight aggregate (LWA) shows great potential, as it is compatible with the mortar matrix and can retain high amounts of bacteria in the pores. However, the high energy needs to produce commercial LWA is a point of consideration in terms of producing sustainable mortar. Furthermore, the need of increasing the use of fly ash in countries, such as Indonesia, that rely for their electricity on coal power plants is still high. Therefore, LWA generated by cold bonding of fly ash was introduced here as an alternative bacteria carrier. Due to the high open porosity, a coating is needed to protect the bacteria and prevent leakage. Sodium alginate was applied as a coating on expanded clay and fly-ash based LWA containing cells or spores of Bacillus Sphaericus. The viability of spores and cells after encapsulation, and the compressive strength, the healing efficiency and the sealing efficiency of resulting mortar were investigated. The results show that the cells and spores of B. sphaericus are still viable and can actively decompose urea after encapsulation into both LWA types coated with sodium alginate. The strength decreased up to 11% in all mortar samples containing LWA coated with sodium alginate compared to mortar containing LWA without alginate coating. The sodium alginate coating improved the healing efficiency of mortar when cracks were created at 90 days. However high variability occurred for the sealing efficiency, due to the non-uniform crack geometry. In conclusion, fly ash based LWA as a bacterial carrier provides adequate healing performance, similar to commercial expanded clay, but slightly decreased the mechanical properties of resulting mortar.