Metakaolin—lime hydration products

Four mixtures were prepared of weight ratios 80:20, 70:30, 60:40 and 50:50 of china kaolin/pure Ca(OH) 2. Each mix was activated for 2 h at 800°C and then paste hydrated at room temperature and 100% RH for up to 28 days. The hydration products were studied by XRD as well as DTA and TG techniques. The chemically-combined water and Ca(OH) 2 contents were quantitatively determined from the TG curves. The results illustrate the formation of gehlenite hydrate (C 2ASH 8) as the main hydration product; its amount increases with the curing time. Hydrogarnet is also formed at the early stages of hydration in the mixes of lower lime content (e.g., mix 80:20) and increases on prolonged hydration; this is probably due to the lower pH of the reaction medium. CSH gel could not be easily detected by XRD because of its low degree of crystallinity. However, this phase was identified by DTA and gives the characteristic losses on the TG curves. Ca(OH) 2 is consumed gradually as the hydration proceeds and disappears nearly completely after long periods (after 28 days).