Innovative CO₂ utilization by carbonation curing of lightweight concrete made with Portland limestone cement

The atmospheric concentration of carbon dioxide gas has immensely increased with escalation in the world's greenhouse gas (GHG) emissions. Through CO₂ recovery, storage and utilization, emission induced global climate changes can be alleviated. The use of Portland limestone cement (PLC) as binder in lightweight concrete can effectively improve net gains in carbon emission reductions by partial replacement of cement with limestone. To further mitigate CO₂ discharge, carbonation curing was examined as a potential curing mechanism of lightweight concrete made with PLC. Slab samples were cast and initially cured at 25°C and 50% relative humidity for durations up to 18 hours. Carbonation was then carried out for 4 hours in a sealed chamber at an absolute pressure of 1 bar. The degree of reactivity was assessed by the CO₂ uptake based on PLC content and excluding preexisting limestone. The resulting uptake of fresh lightweight concretes without initial curing and after 18 hours of initial curing reached 6% and 18% respectively, corresponding to 13% and 39% reaction efficiency. The effect of carbonation curing on the mechanical properties, microstructure and reaction products of concrete was then investigated. A comparison between the optimum carbonated sample and hydrated reference was performed. While both concretes showed similar early-age compressive strength, late strength of carbonated sample was only comparable if water compensation through surface spray was performed. Hydration products were characterized as well crystalline calcium carbonates with the transformation of carbonate polymorphs into more stable calcite. In the process, PLC could replace OPC in carbonation curing of lightweight concrete while maintaining similar carbon sequestration potential.