Accelerated carbonation of recycled concrete aggregates: Binding of CO2 and decalcification of calcium-silicate-hydrate

Recycled concrete aggregates (RCA) are a valuable resource for reuse as aggregates in concrete and asphalt. However, the porous mortar adhering to the original aggregates and microcracks both in the mortar and at the interfaces result in increased water absorption and reduced mechanical strength of RCA. Carbonation of the RCA reduces its porosity, binds atmospheric CO 2 , and decalcifies the calcium silicate hydrate (C-S-H), rendering it reactive when used in concrete. In this study, RCA of various size fractions obtained from construction demolition waste in Qatar were subjected to accelerated carbonation for 1–5 days under controlled conditions. This paper discusses the mineralogical, microstructural, and chemical transformations of the aggregates, as well as their CO 2 sequestration, drawing on a comprehensive set of analytical techniques including XRF, XRD, TGA-DTG, FTIR, SEM-EDS, and solid-state ²⁹Si NMR. Carbonation led to the depletion of portlandite and ettringite, calcite precipitation, and the formation of silica-rich gels through C-S-H decalcification. The fine RCA (0–5 mm), paste-rich fractions exhibited the highest reactivity, with additional CO 2 uptake due to accelerated carbonation reaching up to 4.1% by mass relative to the uncarbonated RCA. ²⁹Si NMR analysis confirmed the structural transformation of C-S-H, showing a clear transition from Ca-rich Q¹ -Q² silicate environments in uncarbonated RCA to highly polymerized Q³ -Q⁴ structures. These findings confirm the benefits of RCA carbonation in terms of CO 2 sequestration and indicate the potential for enhanced pozzolanic reactivity, particularly in the fine fraction.