Pozzolanic reactivity and specific surface area of thermally treated clay materials: comparative study of smectite, illite and kaolinite-based compositions

Abstract

Ten clay mineral samples from currently exploited deposits in Poland were examined to determine how mineral composition and thermal transformations affect specific surface area and pozzolanic reactivity. The raw materials exhibited wide mineralogical variability, including clays dominated by illite, smectite, kaolinite, and halloysite, along with minor crystalline phases rich in Ca, Fe, Mg, and alkalis. Phase composition and its evolution during firing were determined using X-ray diffraction (XRD), while changes in specific surface area were measured by the Brunauer–Emmett–Teller (BET) method. Pozzolanic activity was assessed through dissolution tests in alkaline solutions. Firing caused dehydroxylation, amorphization, and structural collapse of layered minerals, leading to the formation of highly reactive aluminosilicate phases. The highest reactivity was observed in materials containing kaolinite and smectite. In kaolinite, it resulted from the formation of metakaolinite – a highly reactive pozzolanic phase. In smectite-rich materials, the activity was attributed to dehydroxylation and amorphization of the 2:1 layers and to the presence of amorphous silica (opal-A), which enhances CaO fixation. Additionally, fine particle size increased the specific surface area and accessibility of reactive tetrahedral layers. However, the results showed that surface area alone did not determine reactivity – the key factors were chemical composition and the nature of thermally transformed phases. This study elucidates the interrelations between mineralogy, microstructural evolution, and pozzolanic properties, providing insights for optimizing clay raw materials as sustainable pozzolanic additives and functional components in ceramic and environmental technologies.