Carbonation of recycled cement paste specimen: influence of paste particle size, temperature, and pressure on physical properties

This study proposes a novel approach for recycling waste concrete fines, a by-product of concrete crushing, in a resource- and energy-efficient manner. Cement paste ground to 300 μm or less was used as a substitute for waste concrete fines, compacted at low pressure (0.6 MPa), and subjected to carbonation curing under controlled temperature (30–60 °C) and CO₂ pressure (1 or 3 MPa) to obtain hardened specimens. The compressive strength, CO₂ uptake, and porosity of the specimens after carbonation curing were evaluated. Powders ≤ 150 μm achieved the highest performance, with compressive strength up to 42 N/mm² and > 90% of theoretical CO₂ uptake after 6 h at 60 °C and 3 MPa. Scanning electron microscopy confirmed that calcium carbonate precipitation on particle surfaces filled interparticle voids and enhanced binding. This low-energy compaction and CO₂ curing technique offers a promising pathway to recycle waste concrete fines while simultaneously achieving CO₂ reduction in construction. By enabling effective reuse of waste concrete fines, the method also supports the transition toward a circular economy in concrete-based construction.