Difficulties in Determining the Pozzolanic Activity of Thermally Activated Lower-Grade Clays

Thermally activated clays (TACs) have been identified as possible supplementary cementitious materials (SCMs). To find a suitable clay and to optimise the activation process, it is necessary to determine its pozzolanic activity. However, the nature of clays is different from that of conventional SCMs. Therefore, the results of commonly used methods may differ; in some cases, they can even be misrepresented and misleading. This article aims to assess their applicability to TAC. Four direct and four indirect methods were compared by determination of the pozzolanic activity of three different clays calcined at varying temperatures. The isothermal calorimetry with lime combined with the mechanical strength’s development was identified as an ideal combination. Contrarily, the lime saturation test was inapplicable. For the Frattini method, it was found to be beneficial to assess the change in activity due to the thermal treatment rather than the strict comparison with a calcium hydroxide saturation curve.