Study of the reaction stages of alkali-activated cementitious materials using microcalorimetry

Abstract

The roles of sodium hydroxide, calcium hydroxide and temperature on heat flow evolution and its relation to the reaction stages of alkali-activated coal fly ash were evaluated. The response surface method was used to analyse data. Both the heat flow and kinetic parameters were determined by the deconvolution of microcalorimetric signals at 25, 35 and 45°C. The studied factors were found to have a strong influence on the dissolution of ash rich in aluminium units, on the diffusion–nucleation processes and its growth, both initial and final. A sodium hydroxide concentration of 8 M guaranteed optimal reorganisation of the dissolved units to form the reaction products. Calcium hydroxide altered the aluminium distribution in the paste, acting as nucleation spots and intervening in the formation of initial and final reaction products. High temperatures were found to favour the efficiency of the alkali activation, increasing the rate constant and simplifying the expiration of energy barriers. Knowledge of the heat flow evolution in these kinds of cementitious materials will allow manipulation of the characteristics of the mixtures in both the fresh and hardened state.