Calorimetric study of alkaline activation of calcium hydroxide-metakaolin solid mixtures

The products originated in the alkaline activation reaction of metakaolin (MK) in calcium hydroxide presence vary as a function of the concentration of the alkaline hydroxide that activates the solid mix. In order to study these reaction products. a series of MK + calcium hydroxide mixes were activated in 1:1 proportion. with different sodium hydroxide concentrations: 5, 10, 12, 15 and 18 M at 45 degreesC during 24 h. The activation steps were examined through isothermal conduction calorimetry and the reaction products characterisation was carried out by means of chemical analysis and instrumental techniques (XRD, FTIR and nuclear magnetic resonance (NMR)). When the activator concentration is 5 M or less, MK activation in the conditions previously described occurs only to a minor extent and the main reaction product is CSH gel. If activator concentration is 10 M or higher, MK dissolution is very fast and the main reaction product is an aluminosilicate with high mechanical performance. In all cases, the formation of CSH gel as the secondary reaction product is also observed. (C) 2001 Elsevier Science Ltd. All rights reserved.