One inconvenience presented by the thermal activation of kaolinite-based wastes is their 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 metakaolinite/lime (MK/CaO) system cured at 1, 7, 28, 90 and 360 days reaction time. To do so, activation temperatures between 550ºC and 650ºC were analyzed as well as percentages of ZnO between 0.05 and 3.0% by weight of sample.
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 90 days in the pozzolan/lime system. To do so, activation temperatures of between 550ºC/650ºC 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 characterizations. Mineralogical composition of the bulk samples was determined by XRD. The mineralogical quantification was performed with the Rielveld method. Morphological observation and microanalyses of the samples were carried out by SEM/EDX. A confocal Renishaw Invia Raman microscope equipped with a Leica microscope and an electrically refrigerated CCD camera was used for sample analysis. The FTIR analyses determined the spectra in the range of compressed frequencies of between 4000 and 400 cm-1.
The results showed that the incorporation of ZnO inhibited 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, albeit with some exceptions, in the samples activated at 550ºC and 650ºC with 0.5% of chemical activator. In none of the cases under analysis was the chemical activator able to improve the properties of the metakaolinite in comparison with the properties of the reference sample activated only with temperature. Thermal activation (550ºC/650ºC) without the addition of ZnO, maintained a very similar mineralogy. The hydrated phases that appeared in the pozzolanic reaction were tetracalcium aluminate hydrate, stratlingite, 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.
This research has been supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund Project BIA-2015-65558-C3-1,2,3-R) (MINECO/FEDER) and as well as the Spanish Training Program, co-financed by the European Social Fund (MINECO/FSE) (BES-2016-078913 and BES-2016-078454). The authors are also grateful to the S.A. Hullera Vasco-Leonesa, SIKA (Spain) and to IECA for their assistance with this research
Interaction and chemical compatibility with other materials and structures , New products, applications and machinery , Development of testing methods and standardization
