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.