POZZOLANIC ACTIVITY OF MINERAL ADDITIVE BASED ON THERMOACTIVATED KAOLIN
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Abstract
Abstract. The research is related to an important topical issue of studying the processes of kaolinite transformation into metakaolin during heating, which allow supplementing the existing provisions and patterns in terms of reducing the energy intensity of the thermal activation process and the pozzolanic activity of the resulting additive, on the other hand. The chemical composition of the original kaolin clay was determined. On the base of the results of laser diffraction, differential thermal and X-ray phase analysis, as well as infrared spectroscopy, the effect of kaolinite firing temperature on the granulometric composition, degree of dehydroxylation and amorphization of metakaolin was established. The authors determined the pozzolanic activity of the metakaolin mineral additive obtained by thermal activation of kaolin clay in the temperature range of 450-900 °C, when metakaolin was used as a partial replacement of Portland cement. The greatest increase in the compressive strength of the cement paste, both at the early and grade age is achieved by replacing part of the cement with a mineral additive - thermally activated kaolin at a temperature of 650 ºС. An increase in the water demand of the composite cement with the addition of the obtained metakaolin is noted. Higher values of the strength index of cement paste with the addition of metakaolin, obtained by heat treatment of kaolin clay at a temperature of 650 ºС, are most likely explained by achieving the optimal temperature regime for firing kaolinite. In this case incomplete dehydroxylation, but the maximum degree of amorphization that determines the pozzolanic activity of the additive in terms of strength is achieved. The results obtained may be of interest for the practical implementation of the problem of reducing carbon dioxide emissions, resource and energy conservation, improving the construction and technical properties of materials obtained on the basis of a composite cement with the addition of metakaolin.
Subject: regularities of the process of thermal activation of kaolin clay in the temperature range of 450-900 °C, pozzolanic activity of metakaolin additive in the formulation of composite binder with partial replacement of Portland cement.
Materials and methods: the following materials as initial components for the experimental studies were used: OPC CEM I 42.5 N, Uglegorsk-Cement; kaolin clay of the Vladimir deposit, DPR. Kaolin clay was preliminarily thermally activated at temperatures of 450-900 ºС (step 50 ºС) to obtain metakaolin and subsequent use as a mineral additive for cement. The process of kaolinite transformation into metakaolin was studied using a set of methods: differential thermal and X-ray phase analysis, infrared spectroscopy. The physical-mechanical properties of the binder were studied using standard methods.
Results: according to the laser diffraction data, it was found that the granulometric composition of the particles of metakaolin samples changes towards an increase in the content of particles in the range of 0.7-4 μm by 4-6 % relatively to the original kaolin, which may indicate a phase transition process of kaolin during thermal activation with its partial destruction. Based on the results of a set of methods: differential thermal and X-ray phase analysis, infrared spectroscopy it can be concluded that when firing kaolinite in the temperature range of 600-850 °C, an amorphized material with varying degrees of dehydroxylation is formed. At the same time, the maximum indicator of pozzolanic activity in terms of strength is achieved when firing kaolinite at a temperature of 650 °C. This is probably due to the maximum degree of amorphization of the material.
Conclusions: the obtained results allow obtaining a highly reactive mineral additive for Portland cement with minimal energy costs. In the future, the research will be aimed at studying the transformation of the coordination bonds of the aluminum cation during the heating of kaolinite and their effect on the pozzolanic activity of metakaolin.
Keywords: kaolinite, metakaolin, thermal activation, dehydroxylation, amorphization, pozzolanic activity.
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