Utilization of Biomineralized Steel Slag in Cement Mortar to Improve Its Properties

Steel slag, an industrial waste generated in the production of steel, has cementitious properties due to high contents of free CaO. In the present investigation, different bacteria were isolated from steel slag and tested for their ability to produce the enzyme urease, which is involved in CaCO3 precipitation. One of the bacterial isolates, Staphylococcus pasteurii SS9, was selected further, based on its high urease activity to treat the steel slag for stabilization. Bacterial treated steel slag at 30% and 50% was used as a replacement to aggregate in cement mortar and cured in bacterial solution. The mortar specimens without bacterial treatment as well as without steel slag served as controls. After 7-day and 28-day curing, the compressive strength and permeability properties of the mortar specimens were evaluated. The 30% slag–modified mortar specimens treated with bacteria improved the strength by 15.9% and 12.8% at 7 and 28 days compared to the specimen without bacterial treatment. The compressive strength of 50% slag–modified mortar specimens with bacterial treatment showed only 6.3% and 1.8% improvement compared to the specimen without bacteria at 7 and 28 days, respectively. About eight times less water was absorbed in 30% and 50% slag–modified mortar specimens treated with bacteria than the control specimens. Scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray diffraction analysis confirmed the mineralization of CaCO3 on the surface of bacterial treated mortar specimens. Bacterial treated steel slag can effectively be used as a partial replacement of fine aggregates in cement mortar in the construction industry, and the current study provides an efficient way to utilize this waste material in cement mortar.