A recent study published in Engineering has unveiled a novel approach to addressing two critical challenges in the construction materials sector: the efficient utilization of steel slag and the reduction of CO₂ emissions from cement production. The research, led by Professor Chunxiang Qian from Southeast University in China, explores the use of microbial technology combined with a rotary kiln process to accelerate the carbonation of steel slag, thereby fixing CO₂ from cement kiln flue gas and producing supplementary cementitious materials.

Cement production is a significant contributor to global CO₂ emissions, accounting for approximately 8% of the total. Steel slag, a byproduct of the steelmaking process, is an alkaline solid waste with over 400 million metric tons produced globally each year. Despite its potential use in construction, the comprehensive utilization rate of steel slag is less than 30%, leading to substantial accumulation and environmental concerns.

In this study, a combination of microbial technology and a rotary kiln process was employed to expedite the carbonation of steel slag for CO₂ fixation from cement kiln flue gas. This approach resulted in a significant increase in the CO₂-fixation rate, with a CO₂-fixation ratio of approximately 10% achieved within 1 hour and consistent performance across different seasons throughout the year. Investigation revealed that both the CO₂-fixation ratio and the particle fineness are pivotal for increasing the soundness and reactivity of steel slag. When the CO₂-fixation ratio exceeds 8% and the specific surface area is at least 300 m²/kg, the soundness issue of steel slag can be effectively addressed, facilitating the safe utilization of steel slag. Residual microbes presented in the carbonated steel slag powder act as nucleating sites, increasing the hydration rate of the silicate phases in Portland cement to form more hydration products. Microbial regulation results in the biogenic calcium carbonate having smaller crystal sizes, which facilitates the formation of monocarboaluminate to increase the strength of hardened cement paste. At the same CO₂-fixation ratio, microbial mineralized steel slag powder exhibits greater hydration activity than carbonated steel slag powder. With a CO₂-fixation ratio of 10% and a specific surface area of 600 m²/kg, replacing 30% of cement clinker with microbial mineralized steel slag powder yields an activity index of 87.7%. This study provides a sustainable solution for reducing carbon emissions and safely and efficiently utilizing steel slag in the construction materials sector, while expanding the application scope of microbial technology.

The paper “Microbial-Enhanced Steel Slag Fixation of CO₂ from Cement Kiln Flue Gas for the Production of Supplementary Cementitious Material,” authored by Chunxiang Qian, Yijin Fan, Yafeng Rui, Xiao Zhang, Yangfan Xu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.03.024. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.