Thermal activation of natural clays containing kaolinite

Pozzolanic properties of pure kaolinitic clays are widely proved, but natural clay deposits present several impurities: illite, quartz, anatase and feldspar, and the prediction of its pozzolanic properties are poorly investigated.
The purpose of this investigation is to study the thermal activation of natural kaolinitic clays from different regions of Argentine. Five clays with different kaolinite content, different impurities and different degree of crystallinity were analyzed. The samples were tested by X-ray diffraction (XRD) and Fourier transformed infra-red spectrometry (FTIR). FTIR spectra provide immediate qualitative and quantitative information about the order or disorder of the structure. The clays were calcined at 700 ºC and grounded to a particle size lower than 45 micron. Structural changes in the clays after calcinations were studied by FTIR and XRD, while the reactivity was assessed using the method of Fratini and lime consumption test. Among clays tested, four of them can be classified as pozzolanic by the Fratini test. The compressive strength of cement pozzolan mortars was tested at 7, 28 and 90 days. The pozzolanic reactivity of the clays showed good correlation with the content of kaolinite, and with the inverse of crystallinity determined by FTIR. Thus, the potential of the clays for use as pozzolans could be simply determined.
This study confirms that common clays from different regions of Argentina other than pure kaolinite can be interesting as pozzolanic materials when they are thermally activated.
Author: A. Tironi, M. Trezza, A. Scian, E.F. Irassar

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