Carbon utilization for low-carbon concrete: prospects for carbonated steel slag and recycled concrete

The urgent need to decarbonize concrete has driven interest in CO₂ mineralization as a dual-purpose strategy for permanent carbon storage and industrial by-product valorization. This review examines the accelerated carbonation of steel slag and recycled concrete, two underutilized by-products with potential as supplementary cementitious materials (SCMs) and aggregates in low-carbon concrete. CO₂ mineralization alters their physicochemical properties, improving phase stability, microstructural densification, and interfacial bonding. Depending on the carbonation method, CO₂ uptake ranges between 5 % and 40 %. Moderate carbonation levels offer an optimal balance between CO₂ uptake and pozzolanic reactivity as SCM, enhancing strength and durability. The review also compares the microstructural and performance characteristics of these carbonated materials with conventional SCMs and aggregates, offering insights not addressed in previous literature. Furthermore, environmental assessments show that carbonated SCMs can achieve materials with net-negative emissions (−4 to −77 kg CO₂-e per ton of recycled concrete and steel slag powder, respectively). However, most of the carbon reduction stems from the magnitude of cement replacement by carbonated SCM in concrete. Techno-economic analyses suggest cost-competitiveness, particularly when processing is co-located with CO₂ sources and powered by renewable energy. Despite encouraging progress, key challenges remain: variability in feedstock composition, scalability of carbonation technologies, and effective use of diluted industrial CO₂ streams. Addressing these issues through targeted R&D is essential to unlock the full potential of utilizing carbonated materials in concrete, positioning it as a practical pathway toward circular, low-carbon construction.