Metakaolin as a pozzolanic microfiller for high-performance mortars

Superplasticized mortars containing metakaolin (MK) as 15% replacement of cement and with a water/binder ratio of 0.33 have been characterized. Four commercially available MK samples have been studied and compared to silica fume. Three of the four MK samples give, at early ages, a compressive strength enhancement that is more pronounced than that of silica fume. The fourth and coarser sample gives a lower strength at early ages, but at 90 days and later the difference is reduced. Compressive stength enhancement differences between the specimens with microfillers and the control decrease after 28 days, as the control shows a slighter slowdown of the hydration rate. The DTA calcium hydroxide peak heights show that three of the MK samples have a remarkable pozzolanic activity, comparable to that of silica fume even if slightly lower at longer ages. A 24-h treatment at 90°C of the mortar specimens, after a 7-day moist room curing, causes a loss of water whose extent is different for all types of specimens and can be related to the fineness of the microfiller for the MK-containing specimens.