Bacteria Turn Steel Waste Into Carbon-Capturing Cement todayheadline

Chinese researchers have developed a microbial system that transforms steel industry waste into useful construction materials while simultaneously capturing carbon dioxide from cement plant emissions. The technology addresses two major environmental challenges with a single biological solution.

The study, published in Engineering, demonstrates how bacteria can accelerate the conversion of steel slag—a massive industrial byproduct—into supplementary cement materials that actively remove CO2 from the atmosphere. Over 400 million metric tons of steel slag accumulate globally each year, with less than 30% finding productive use.

Microbial Engineering at Work

The research team, led by Professor Chunxiang Qian from Southeast University, employed Bacillus mucilaginosus bacteria in a rotating reactor system that processes cement kiln flue gas. The microorganisms accelerate chemical reactions that would normally require high temperatures and pressures, making the process energy-efficient and scalable.

Within just one hour, the microbial system achieved a CO2 fixation ratio of approximately 10%—nearly double the rate achieved without bacterial assistance. The process remained consistent across different seasons and varying flue gas compositions, demonstrating remarkable stability for industrial applications.

Solving Multiple Problems

Steel slag presents significant challenges for the construction industry. Its expansion-prone compounds make it unsafe for widespread use, while its low reactivity limits its effectiveness as a cement substitute. The microbial carbonation process addresses both issues simultaneously.

“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,” the researchers found.

Key Performance Metrics

• CO2 fixation rates doubled compared to non-microbial processes
• Consistent performance across four different steel slag batches and seasonal variations
• Activity index of 87.7% achieved when replacing 30% of cement clinker
• Reaction transition zones 50% deeper than chemical-only processes
• Reduced porosity in final cement products by up to 15%

The Biological Advantage

The bacteria don’t just speed up reactions—they fundamentally change the final product’s properties. Microbial action produces calcium carbonate crystals that are significantly smaller than those formed through purely chemical processes, measuring just 30.7 nanometers compared to 61.1 nanometers for chemically synthesized alternatives.

These smaller crystals create denser, stronger cement structures. The bacterial cells themselves remain in the final product, acting as nucleation sites that continue to enhance the material’s performance during construction applications.

Analysis revealed that microorganisms consumed 71.40% of dicalcium silicate and 68.25% of tricalcium silicate in steel slag after 48 hours—rates 63% higher than chemical processes alone. This enhanced reactivity stems from the bacteria’s ability to accelerate ion dissolution and carbonate precipitation.

Industrial Implementation

The team tested their system using actual cement kiln flue gas containing 22-31% CO2, along with various pollutants including sulfur dioxide and nitrogen oxides. The bacterial system proved resilient to these harsh conditions, maintaining effectiveness across temperature ranges from 48°C to 67°C.

The rotating reactor design, measuring 1.1 meters in diameter by 3 meters long, processes materials continuously rather than in batches. This approach facilitates large-scale production while ensuring thorough mixing between steel slag powder and CO2-rich gases.

Environmental Impact

Cement production accounts for approximately 8% of global CO2 emissions, while steel slag accumulation has exceeded 1 billion metric tons in China alone. The microbial technology offers a pathway to address both problems while creating valuable construction materials.

The process transforms problematic waste streams into high-performance cement substitutes that meet first-level standards specified in Chinese construction guidelines. With proper optimization, the technology could significantly reduce both industries’ environmental footprints while creating economic value from waste materials.

The research demonstrates how biological systems can enhance industrial processes in ways that purely chemical approaches cannot match. As the construction industry seeks sustainable alternatives, microbial engineering may provide solutions that are both environmentally beneficial and economically viable.