Concrete Incorporating a Spent CO₂ Absorbent: Comprehensive Assessment of Microstructure, Strength, and Durability

Recycling spent CO₂ absorbents generated from direct air capture (DAC) processes is important for improving the sustainability of carbon capture technologies. This study investigates the feasibility of using a spent alkaline CO₂ absorbent as a partial replacement of mixing water in cementitious materials and evaluates its effects on microstructure, strength, and durability. Mortar and concrete mixtures were prepared with replacement ratios of 0-40%. Microstructural and phase evolution were analyzed using scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis, while mechanical performance was assessed through compressive and flexural strength tests. Durability was evaluated by freezing-thawing resistance, chemical resistance in acidic environments, and accelerated carbonation tests. The results show that low replacement ratios (10-20%) improve early-age strength due to CaCO₃-induced micro-filling and nucleation effects, while maintaining comparable long-term strength to the reference mixture. In contrast, higher replacement ratios (≥30%) cause excessive carbonation, C-S-H decalcification, increased micro-porosity, and strength reduction. Overall, spent CO₂ absorbents can be effectively recycled in cementitious materials within a 10-20% replacement range.