Carbonation of recycled concrete fines as a treatment for promoting their recycling as supplementary cementitious material: influence of initial moisture on CO2 uptake and reactivity.

Construction and demolition waste (CDW) is a major global waste stream. Coarse aggregates from CDW are widely reused, but recycled concrete fines (RCf, <80 µm) remain underutilised, often ending up in landfills or used in low-value applications. Carbonation is a promising treatment to valorise RCf as a supplementary cementitious material (SCM), converting reactive calcium-bearing phases into calcium carbonate while permanently sequestering CO2. Hence, this study explores the semi-dry accelerated carbonation of RCf generated in a concrete recycling facility in the Netherlands, focusing on the influence of the initial moisture content, varied from 0 to 50 wt%. Pre-wetting the RCf prior to carbonation significantly influenced calcium carbonate formation, with higher moisture levels promoting calcite formation. Samples preconditioned with 10-50 wt% moisture demonstrated enhanced CO2 uptake and a pronounced increase in specific surface area, from ∼ 7 m2/g for raw RCf to up to ∼ 20 m2/g for carbonated samples at intermediate moisture content. CO2 uptake increased with increasing moisture, whereas reactivity did not follow the same trend. The 50 wt% condition achieved the highest CO2 uptake but the lowest R3 reactivity among the carbonated samples, while intermediate moisture (∼20 wt%) led to the highest specific surface area and favourable R3 reactivity and was selected for paste validation. Selected carbonated and uncarbonated RCf were subsequently used to replace 20% of the cement in paste mixtures and were compared against a limestone reference. RCf carbonated at 20 wt% moisture achieved a 13.4% increase in 28-day compressive strength compared with the limestone reference. These results demonstrate that RCf, often considered low-value waste, can be valorised as an SCM through moisture-controlled carbonation. Their use supports CO2 utilisation in cementitious systems. It contributes to more circular construction practices, while offering a critical perspective on the application of mineral carbonation to materials produced in full-scale waste treatment plants.