Carbon dioxide mineralisation of field-collected recycled concrete powder and its role in cement paste

The effects of carbonation treatment on recycled concrete powder (RP) derived from demolished concrete and its impact on the mechanical properties of cement paste were explored. Carbonation significantly enhanced the pozzolanic reactivity of the RP through mineralogical transformations, including an increased content of calcium carbonate and the formation of polymerised silica gel. These changes lead to a refined pore structure with increased specific surface area, reduced pore size and a densified microstructure. Incorporating carbonated recycled powder (CRP) into cement paste improved the compressive and flexural strengths as compared with the addition of non-carbonated RP, particularly at the 15% replacement level, where the strengths approached those of the plain cement paste by 28 days. This enhancement is attributed to improved particle packing, intensified nucleation and growth of hydration products (notably additional calcium silicate hydrate), and stronger interfacial bonding between the CRP and the cement matrix. However, increasing the CRP content reduced the flowability of the paste due to the higher water demand driven by finer particles and the affinity of silica gel for water. These findings demonstrate that carbonation is as an effective strategy to upgrade RP for sustainable cementitious applications.