A study of thermal decomposition of phases in cementitious systems using HT-XRD and TG

•This study suggests correct decompositional temperatures of cementitious phases.•DTG peaks of a phase can be affected by the presence of other phases.•In TG, when Ca(OH)2 plentifully exists, the weight of CaCO3 can be overestimated.•The gas type in TG (N2 vs. air) had no significant influence on TG below 800 °C.•Different heating rates and sample fineness can alter the TG results. Significant variations have been reported on the temperature range of thermal decomposition of cementitious phases. Thus, this study identified temperature ranges on the phases in actual cementitious systems (portland cement (OPC) pastes, blended pastes of ground granulated blast furnace slag (GGBFS) with OPC, and Ca(OH)2-activated GGBFS) by simultaneously using thermogravimetry (TG) and high-temperature X-ray diffraction (HT-XRD) as follows: (1) 81°–91 °C for dehydration of ettringite, (2) ∼80°–240 °C for major dehydration of C-S-H, (3) ∼241°–244 °C for hydrogarnet, (4) ∼129°–138 °C for Al2O3-Fe2O3-mono phase (AFm), (5) ∼411°–427 °C for Ca(OH)2, and (6) ∼648°–691 °C for CaCO3. The CaO layers and SiO2 chains of C-S-H likely started to decompose from 615°–630 °C, and eventually transformed to new crystalline phases. This study also demonstrated that (a) the quantity of calcite could be overestimated due to additional carbonation when Ca(OH)2 is plentifully present in samples, and (b) the quantification of phases would be greatly affected by sample particle size when GGBFS is used in the system.