To Carbonate or Not to Carbonate? An experimental study on Synthetic and Industrial Recycled Cement Paste

Mineral carbonation has gained attention as a carbon utilization strategy, offering a way to permanently sequester CO2 through its reaction with materials bearing metal oxides, such as calcium and magnesium, forming stable carbonates. Within the field of cement production, Recycled Cement Paste (RCP) offers a carbon utilization pathway, where the carbonated RCP (cRCP) can be used as a Supplementary Cementitious Material (SCM) in composite cements. The latter is vastly reported through studies conducted on synthetic Hydrated Cement Paste (HCP). To broaden the scope of cRCP use in cement, this study investigates and industrially-sourced RCP (RCP-i), while utilizing a laboratory-synthesized samples of HCP as a reference. Both materials undergo mineral carbonation at ambient temperature under wet conditions. Thermogravimetry and LECO analyses were used to measure carbonation uptake, and the R3 test to assess the pozzolanic reactivity of the carbonated products. Also, blended cement pastes were prepared with Ordinary Portland Cement (OPC) and HCP or RCP-i (before and after carbonation) at a 20% replacement by weight of OPC –– to assess the impact on hydration characteristics using isothermal calorimetry. For the tested materials, results show that a) the carbonated variants perform similarly to their uncarbonated counterparts as cement replacements, and b) the actual CO2 uptake of the RCP-i is less than one-third of that of the synthetic HCP. For this particular RCP-i, the conclusion seems to be not to carbonate.