Dual eco-friendly application of silica fume and scoria in cement-based materials through the enhancement of microbially-induced carbonate precipitation

The partial replacement of cement or natural aggregates with industrial wastes and using microbially-induced carbonate precipitation to heal cracks are two developing methods to make more sustainable concrete. In this study, a combination of these two methods was used for the first time: Partial replacement of cement with silica fume to economically create a more favorable pH for bacterial activity and substituting natural sands partially with scoria aggregates as a novel carrier and protector for bacterial spores against the harsh and alkaline conditions of cement mortar. Preliminary investigations identified scoria as a suitable carrier since the activity of immobilized cells did not decrease considerably. Thermogravimetric and X-ray diffraction analyses confirmed that spores immobilized in scoria are capable of precipitating calcium carbonate. Different mortar mixtures were made with and without scoria immobilized healing agents and silica fume. In the absence of silica fume, adding biological healing agents to mortar mixture enhanced flexural and compressive strengths by 13% and 10%, respectively, and by 18% and 15% in the silica fume amended mortars. Healing agents lowered sorptivity coefficients by 49% when samples lacked silica fume and 63% when specimens included it. The presence of silica fume resulted in a more favorable pH environment for bacterial activity and increased precipitation of calcium carbonate, as confirmed by the scanning electron microscope. Replacing a part of cement with silica fume -as an industrial byproduct- has multiple ecological benefits: protecting the environment via waste reuse, reducing demand for energy and natural resources, decreasing greenhouse gas emissions, lowering cement manufacturing costs, and finally improving the performance of microbial carbonate precipitation leads to the production of more sustainable concrete.