Activation of kaolinite based coal waste: Chemical and thermal activation and their mineralogy

Abstract

One inconvenience presentad by the thermal activation of kaolinite(K)-based waste is its low content of metakaolinite, a highly pozzolanic product listed in current standards for the manufacture of commercial cements. The addition of a chemical activator during the thermal activation process is a priority line of research to increase the reactivity of the recycled metakaolinite. The objective of the present investigation is to analyze the combined effect of the activation of a K-based coal waste on pozzolanic reactivity and on the evolution of mineralogical phases in a metakaolínitellime system cured at 1, 3, 7, 28, 90 and 360 days of reaction time. To do so, activation temperaturas between 550°C and 650°C were analyzed as well as percentages of ZnO between 0.05 and 3.0% by sample weight. In this paper, an additional chemical activator, ZnO, is studied and its effect on both pozzolanic properties and the evolution of mineralogical phases in the thermal activation of coal waste with a reaction time of up to 360 days in the pozzolanllime system. To do so, activation temperaturas of between 550°C and 650°C were selected and additions of chemical activator (ZnO) in percentages of 0.0%-3.0% by weight of coal waste, because it is an activator with a positive effect on a 100% natural kaolinite. Different techniques were used for the material characterization. Mineralogical composition of the bulk samples was determinad by XRD. The mineralogical quantification was performed with the Rietveld method. Morphological observation and microanalyses of the samples were carried out by SEM/EDX. A confocal Renishaw lnvia Raman microscope equipped with a Leica microscope and an electrically refrigerated CCD camera was used for sample analy sis. The FTIR analyses determinad the spectra in the range of compressed frequencies of between 4000 and 400 cm-1. The results showed that the incorporation of ZnO highly reduces the reactivity of the recycled metakaolinite and in consequence, the capacity of the metakaolinite to react with the surrounding lime; even more so when the content of added chemical activator was raised. In none of the cases under analysis was the chemical activator able to significantly improve the properties of metakaolinite in comparison with those of the reference. Thermal activation (550°C-650°C) without the addition of ZnO maintained a very similar mineralogy. The crystalline hydrated phases that appeared in the pozzolanic reaction were tetracalcium aluminate hydrate, stratlingite, ettringite, monosulfoaluminate hydrate and LDH (phyllosilicate/carbonate). The laminar morphological compounds were formed to the detriment of the C-S-H gels that acted as a substrate for crystallization and were found interspersed with them, forming aggregates.