An overview of utilizing CO2 for accelerated carbonation treatment in the concrete industry

Accelerated carbonation treatment as an effective approach of reducing the carbon footprint in the concrete industry is being investigated extensively. Such carbonation treatment converts gaseous CO2 into carbonate minerals (or mineral CO2 sequestration). The present review provided a state-of-the-art summary on the relevant carbonation strategies that have been investigated, with a special focus on identifying the remaining industrial challenges and critical research gaps that need further investigation. Key parameters affecting pre-carbonation treatments of recycled concrete aggregates, fly ash, and slag were examined systematically, including chemical compositions and particle sizes of the materials, temperature, pressure, and CO2 concentration. The CO2 uptake capacity and efficiency of the materials were compared with each other, and the environmental impact of pre-carbonation treatment of fly ash and slag was approximated. After carbonation treatment, the incorporation of those carbonated materials in cement-based composites was assessed. It was found that the rising CO2 uptake of ingredient materials subjected to pre-carbonation treatment does not necessarily benefit the mechanical properties of the cement-based composites with the carbonated materials. For a specific material, there may exist a CO2 uptake threshold over which high CO2 uptake tends to be deleterious, e.g. for the mechanical properties. In terms of the influence of accelerated carbonation curing on the compressive strength of cement-based composites at early ages, both positive and negative influences were identified. Corresponding mechanisms behind such contrary influences were critically discussed. It was noted that over-intensified early-age carbonation curing could compromise the strength properties of cement-based composites both in the short and long terms.