Study on preparation of supplementary cementitious material using microbial CO2 fixation of steel slag powder

Mineral CO2 sequestration, a vital way to reduce CO2 emission, has been paid extensive attention. Industrial residues are considered a promising material for CO2 sequestration because of their low cost and secondary utilization. However, there are few methods to jointly consider CO2 emission reduction and efficient waste residue utilization, which are vital for the sustainable development of industries. In this paper, we propose a collaborative technology to prepare high-quality supplementary cementitious material (SCM) that can be used to substitute cement. Specifically, the microorganism is utilized to improve CO2 fixation and modification of steel slag powder, then its characteristics and the effects of SCM in cement-based materials are further studied. The results show that microbial mineralization can improve the stability and reactivity of steel slag powder by promoting the phase conversion of free oxides and silicates. The free CaO and MgO of modified steel slag powder decrease from 1% and 1.9% to 0% and 1%. The flexural and compressive strength reactivity index of 28-day cured samples increased from 81.1% and 78.5% to 86.8% and 87.1%, respectively. Moreover, more calcium carbonate and pozzolanic-active amorphous gel phases produced in mineralized steel slag powder improve its reactivity. The results on the enhancement mechanism show that bio-CaCO3 with high crystallinity and small particle size attach the surface of steel slag powder, which act as nucleation sites to promote cement hydration; And bio-CaCO3 participates in the cement hydration to form new products, such as hydrated calcium carboaluminate, improving the compactness of samples. Microbial mineralization increases the CO2 fixation of steel slag powder to 11.25%. When the mineralized steel slag powder is used as SCM, the sustainable efficiency of the mixed binder reaches 0.0384 MPa/(kg/m3), which is 27.15% higher than that of pure cement. This technical route is conducive to carbon neutralization and sustainable development of the construction industry.