The effect of temperature on the hydration rate and stability of the hydration phases of metakaolin-lime-water systems

The paper presents the results of a study carried out to determine the effect of curing temperature on the kinetics of reaction of a metakaolin (MK)/lime mixture. MK and analytical grade Ca(OH)₂ were mixed in a ratio of 1:1 by weight and with a water/binder ratio of 2.37. Specimens were cured at 20 and 60 °C. In the first case, the curing time varied from 2 h up to 180 days and, in the second case, from 2 h up to 123 days. A mathematical model was applied to calculate the rate constant for the hydration reaction. The identity and the amount of the phases present were determined from thermal analysis (TG and DTA) data. The results showed that the rate constant for the samples cured at 60 °C was 68 times greater than the rate constant at 20 °C for the same curing period (up to 9 days). At 20 °C, the sequence of appearance of the hydrated phases was C-S-H, C₂ASH₈ and C₄AH₁₃; while at 60 °C, the sequence was C-S-H, C₂ASH₈, C₄AH₁₃ and hydrogarnet (C₃ASH₆). There is no evidence of further C₂ASH₈ and C₄AH₁₃ transformation into hydrogarnet in the mixture studied for 123 days at 60 °C.