The solid state chemistry of metakaolin-blended ordinary Portland cement

The hydration of ordinary Portland cement (OPC) pastes containing 0 and 20% metakaolin was monitored by differential thermal analysis (DTA) and solid state magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The presence of hydrated gehlenite and a relative reduction in calcium hydroxide content of the metakaolin-blended OPC pastes observed by DTA are indicative of the pozzolanic reaction of metakaolin. An increase in the capacity of metakaolin-blended OPC pastes to exclude chloride ions from the pore electrolyte phase, via solid phase binding, has been reported. It is proposed that this increase in chloride binding capacity could be attributed to the participation of calcium aluminate species in the formation of Friedel's salt which would otherwise be engaged in the formation of hydrated gehlenite and other AFm phases. The accelerating effect of replacement additions of metakaolin has been shown by 29Si NMR and was denoted by a comparative increase in the intensity of resonances arising from Q1 and Q2 species compared with that of Q0 species for metakaolin-blended specimens. The primary reactive centres of the pozzolan have been shown to be the 5-coordinate aluminium and amorphous silica. The spreading of the Q4 resonance of the amorphous silica of metakaolin through the Q3 and into the Q2 and Q1 regions of the NMR spectrum during pozzolanic reaction has been observed.