Thermal activation and pozzolanic reactivity assessments of Estonian clays for cementitious applications

Abstract This study evaluates the pozzolanic reactivity of locally sourced Estonian clays as supplementary cementitious material (SCM) alternatives. Two selected clays, Arumetsa (ARC) and Aseri (ASC), were calcined in a rotary tube furnace at three distinct temperatures (750 °C, 825 °C, and 900 °C) to investigate the influence of thermal activation on reactivity. To isolate intrinsic chemical reactivity of calcined clays (without cement, superplasticizers, etc.), model systems were employed: (i) mortars composed of slaked lime, calcined clay, and siliceous sand for compressive and flexural strength testing, and (ii) binary slaked lime–clay pastes for residual Ca(OH) 2 quantification. Reactivity was further characterized through a combination of analytical and mechanical approaches. Both tested clays demonstrated pozzolanic activity, as all mortar samples exceeded the 7-day compressive strength threshold of 4 MPa required for natural pozzolans, confirming their suitability as potential SCMs. Calcination temperature plays a decisive role in governing the reactivity of illite-rich clays. Elevated temperatures enhance the Si leachability, while concurrently suppressing Al solubility, necessitating a careful balance in thermal treatment. The higher kaolinite content in ARC relative to ASC provides greater reactive alumina, as evidenced by the quantitative XRD results and more pronounced calcium aluminate silicate hydrate (C–(A)–S–H) and calcium aluminate hydrate (C–A–H) peaks in the TGA/DTG profiles of ARC, reflecting enhanced pozzolanic interaction with calcium hydroxide in the paste.