Hydration and carbonation modelling of moderate-heat cement and Portland–slag cement

Concretes incorporating moderate-heat Portland cement (MHC) and Portland–slag cement (PSC) are widely used in hydraulic engineering due to their low temperature rise and excellent long-term strength. However, understanding of the hydration and carbonation behaviours of these cements is incomplete. In this investigation, long-term calorimetry was combined with thermodynamic modelling to quantify hydration degrees, pore-solution evolution and phase transformations. A predictive model for the hydration degree of MHC was established, giving a stable-state value of 87.9%. Thermodynamic simulations revealed that the saturation indices (SIs) of different calcium silicate hydrate end-members correlate positively with the portlandite SI and with the concentrations of OH− and silicon, but negatively with calcium concentration. The addition of slag led mainly to the consumption of portlandite, which was completely depleted at a slag content of 60%. Lower slag contents promoted calcium silicate hydrate (C-S-H) with a higher calcium/silicon ratio (CSR), whereas higher slag contents reduced the C-S-H content, the CSR and the pH of the pore solution. Carbonation directly lowered the paste alkalinity and caused decalcification of portlandite, C-S-H gel and ettringite to form calcium carbonate, amorphous silica and microcrystalline aluminium hydroxide. Although PSC offers considerable potential for low carbon dioxide and mass concrete applications, its lower alkalinity may impair carbonation resistance. When the pH of the paste pH was reduced to 9, MHC exhibited 31.8% superior carbonation buffering capacity compared with PSC.