Composition and structure of C-S-H in white Portland cement-20% metakaolin pastes hydrated at 25 °C

The microstructure and composition of water- and alkali-activated hardened pastes of white Portland cement-20% metakaolin blends have been studied using solid-state NMR spectroscopy and analytical TEM. The results show that after hydration for 1 day nearly half the cement had reacted in the water-activated paste but very little, if any, of the metakaolin; by 28 days two-thirds of the cement had reacted and most of the metakaolin. In contrast, whilst alkali-activation again led to about half the cement reacting by 1 day, about a quarter of the metakaolin had also reacted; and whilst most of the metakaolin had again reacted by 28 days, there had been no further reaction of the cement. The high degree of reaction of the MK in both pastes at 28 days resulted in long-chain highly aluminous C-S-H, with most of the bridging sites occupied by Al³⁺ rather than Si⁴⁺. The data for the C-S-H in the water-activated paste are consistent with both the tobermorite/jennite (T/J) and tobermorite/calcium hydroxide (T/CH) models for the nanostructure of C-S-H - although very little J- or CH-like structure is needed to account for the observed compositions - whilst those for the alkali-activated paste can only be accounted for on the T/CH viewpoint.