Pozzolanic Reactivity of Calcareous Low-Grade Calcined Clays

Abstract

Clays have played a fundamental role in human development, from early use in pottery and construction to modern applications in advanced ceramics, pharmaceuticals and environmental remediation. Their properties, combined with their abundance, make them an indispensable material for meeting today's engineering challenges, reinforcing their importance for sustainable development and technological innovation. With the increasing urgency of carbon emission reduction, the use of calcined clays to reduce the carbon footprint of the cement sector has become a near-term focus. It is well known that heating kaolinitic clays to certain temperatures increases their reactivity, making them suitable for cement and concrete applications, such as the use of metakaolin in high performance concrete and limestone calcined clay cements for reduced carbon intensity. Of particular interest, however, is the calcination of under-utilized low-grade clays that are not used in such production processes and do not meet the stringent requirements of high-quality ceramics, paper coatings or refractories. When properly processed, these low-grade clays may also exhibit pozzolanic reactivity, enhancing the durability and performance of cementitious systems while simultaneously reducing environmental impact and offering economic advantages. This study evaluated the pozzolanic reactivity of eight different clays, actively employed and readily sourced by Turkish cement plants from their existing deposits, following thermal activation at 600°C and 800°C. The relationship between the presence of kaolinite, montmorillonite, illite, and calcite, and the resulting pozzolanic reactivity was analyzed. Thermogravimetric analysis/differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), X-ray fluorescence (XRF), and particle size distribution (PSD) analyses were performed on the clays for characterization. The results showed that samples with higher clay mineral content exhibited lower strength in the absence of calcite. In other words, it was ultimately demonstrated that even clays with lower clay mineral content can be effectively utilized as SCMs through thermal activation, with the presence of calcite further enhancing their reactivity.

Keywords

• Clay calcination
• clay composition
• calcareous clays
• strength activity index
• sustainable cement production

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