Effects of metakaolin, water/binder ratio and interfacial transition zones on the microhardness of cement mortars

Variations in the microhardness of the hydrated cement matrix component of model mortars have been investigated as functions of the distance from the aggregate surfaces for specimens in which the binder was Portland cement or a blend of Portland cement and metakaolin. Microhardness measurements were made using a Knoop indenter at distances of up to 120 mum from the aggregate. The microhardnesses of the paste-aggregate interfacial transition zones (ITZs) were found to be between 14% and 22% lower than those of the corresponding bulk cement pastes at the lower water/binder ratios investigated, i.e. 0.4 and 0.5 for samples prepared with Portland cement and 0.4 for samples prepared with a binder comprising Portland cement and metakaolin. Metakaolin increased the mean microhardness of specimens prepared at the higher water/binder ratios of 0.5 and 0.6 by 13% and 54%, respectively. (C) 2002 Published by Elsevier Science Ltd.