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 o f the alkaline hydroxide that activates the solid mix. In order to study th ese reaction products. a series of MK + calcium hydroxide mixes were activa ted in 1:1 proportion. with different sodium hydroxide concentrations: 5, 1 0, 12, 15 and 18 M at 45 degreesC during 24 h. The activation steps were ex amined through isothermal conduction calorimetry and the reaction products characterisation was carried out by means of chemical analysis and instrume ntal techniques (XRD, FTIR and nuclear magnetic resonance (NMR)). When the activator concentration is 5 M or less, MK activation in the conditions pre viously described occurs only to a minor extent and the main reaction produ ct 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 seco ndary reaction product is also observed. (C) 2001 Elsevier Science Ltd. All rights reserved.