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 h ydroxide content of the metakaolin-blended OPC pastes observed by DTA are i ndicative of the pozzolanic reaction of metakaolin. An increase in the capa city of metakaolin-blended OPC pastes to exclude chloride ions from the por e electrolyte phase, via solid phase binding, has been reported. It is prop osed 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 gehle nite and other AFm phases. The accelerating effect of replacement additions of metakaolin has been shown by Si-29 NMR and was denoted by a comparative increase in the intensity of resonances arising from Q(1) and Q(2) species compared with that of Q(0) species for metakaolin-blended specimens. The p rimary reactive centres of the pozzolan have been shown to be the 5-coordin ate aluminium and amorphous silica. The spreading of the Q(4) resonance of the amorphous silica of metakaolin through the Q(3) and into the Q(2) and Q (1) regions of the NMR spectrum during pozzolanic reaction has been observe d. (C) 2000 Kluwer Academic Publishers.