Effect of an activating agent on the transformation of coal waste into a supplementary cementing material: Kinetic parameters and heat evolution

Abstract

Over the last years, an important research line based on the recycling of kaolinite-based wastes has arisen worldwide as an alternative to the production of metakaolinite. This recycled metakaolinite could be used as a supplementary cementing material for the manufacture of innovative and eco-efficient cements, which could substitute the ones that are currently being marketed. So far, this type of industrial wastes (paper sludge, coal mining waste or sewage sludge) have shown good results in blended cement matrices, comparable to those obtained with pure MK. According to previous works, chemical reactivity of MK can be enhanced by using a chemical activator (ZnO) during thermal treatment of kaolinite. Nonetheless, its effect on the kaolinite-based wastes remains unknown. In the present work, we have assessed the effect of ZnO (0.5 and 1%) on the reactivity of a recycled MK from a Spanish coal waste, which has been activated at 600 oC during 2h in a furnace. The resulting metakaolinite-based pozzolan has been studied in a pure pozzolan/lime system and the kinetic parameters of the pozzolanic reaction between MK and Ca(OH)2 have been assessed. Subsequently, its influence in the heating and hydration heat evolution of standardized mortars with low clinker content (up to 50% level of substitution) has been analysed. The results show that incorporation of the activating agent during thermal treatment of the industrial waste negatively affects the kinetic parameters. This is especially pronounced in the rate of reaction k (h-1), since it is delayed in two orders of magnitude (10-5) with respect to the same pozzolan without the activating agent (10-3). Accordingly, heating and hydration heat evolution are more delayed with increasing percentages of chemical activator and pozzolan. This would be explained by the combination of two negative effects: inhibition of the pozzolanic reaction by accumulation of ZnO particles on the surface of the activated pozzolan and delay in hydration of the cement particles due to the formation of calcium zincate.