Augmenting Pozzolanic Stabilization Using Novel Microbial Carbonate Precipitation in a Soft Soil

To reduce carbon dioxide (CO₂) emissions from the use of cement and lime in civil engineering applications, microbially induced calcite precipitation (MICP) is one of the sustainable technologies being explored. MICP utilizes the metabolic activities of bacterial species to produce chemical environments that are conducive for calcite formation, which either plugs the pore spaces and/or bonds the soil particles together to reduce permeability and increase the strength and stiffness of soils. This study examined the reduction in use of chemical stabilizers by employing microbially induced carbonate cementation (MICC) along with magnesium oxide (MgO) stabilization. A soft soil [unconfined compressive strength (UCS) = 20 kPa] was prepared in the laboratory by mixing equal-weight proportions of kaolinite (50%) and sand (50%) at a high (21%) water content. Simultaneous pozzolanic and microbial activity was induced in the soft soil by the addition of MgO and cattle manure (CM). Degradation of the CM by native microbes produced CO₂, which dissolved in the water to form bicarbonate ions $({\mathrm{HCO}}_3^{-})$. The alkaline pH generated by the addition of MgO transformed the ${\mathrm{HCO}}_3^{-}$ ions to carbonate ions, and these precipitated as calcium and magnesium carbonates. The alkaline pH (11.73 to 8.89) facilitated by the addition of MgO also induced pozzolanic reactions. The deposition of carbonate precipitates and pozzolanic reaction products at aggregate contacts transformed the soft soil into a hard (UCS > 400 kPa) soil. For every tonne of chemical required in the field for ground improvement, augmenting pozzolanic stabilization using the proposed MICC method has the potential to reduce CO₂ emissions by 0.78 t.