Carbonation Behavior of Early-Age Cement Paste Conditioned with Laboratory-Prepared CO2-Loaded HEPZ and MDEA Solutions

This study evaluated cement paste conditioned at 1 day by 15- or 30-min immersion in laboratory-prepared CO2-loaded deionized water (DW), 1-(2-hydroxyethyl)piperazine (HEPZ), and N-methyldiethanolamine (MDEA) solutions. Solution pH and potentiometric acid titration characterized acid−base responses, while literature-constrained PHREEQC calculations estimated post-bubbling CO2 loading and species distributions. At post-bubbling pH values of 9.35 and 9.55, estimated CO2 loadings were 0.340 and 0.105 mol CO2 mol−1 amine for HEPZ and MDEA, respectively. TG/DTG and XRD-Rietveld analysis showed solution-dependent phase evolution. HEPZ-conditioned specimens generally showed greater portlandite depletion and higher 28-day crystalline carbonate volumes (10.11–11.21 cm3 per 100 g of dry specimen) than MDEA-conditioned specimens (5.90–6.02 cm3 per 100 g). DW15 and DW30 also showed non-identical portlandite, thermogravimetry-derived carbonate, and crystalline carbonate trends. Most conditioned specimens showed lower early strength followed by continued strength development. The results associate contrasting solution states with different carbonation-related responses, while showing that strength development does not follow crystalline carbonate accumulation alone.