Potential use of clays from Zemlet El Beidha (southern Tunisia) as aluminosilicate precursors for geopolymer synthesis

Abstract This study investigates the potential of six illite-rich clay samples from the Lower Cretaceous outcrops of Zemlet El Beidha (southern Tunisia) as precursors for geopolymer synthesis. Τhe raw materials are predominantly illitic, although kaolinite remains a main reactive phase. Samples BHG 2 and BHG 5 were specifically selected for further investigation due to their relatively high kaolinite content compared to the remaining samples and due to their distinct secondary mineralogy, with BHG 2 being rich in hematite and BHG 5 being rich in calcite. This selection allowed for a comparative analysis of how different impurities affect geopolymerization. The samples were calcined at 550°C, 750°C and 950°C to determine the optimal dehydroxylation temperature. Alkali activation was systematically performed using 10, 12 and 14 M NaOH solutions to assess the influence of alkalinity on the polycondensation process. The 12 M NaOH was most effective for the dissolution of aluminosilicate phases without the detrimental effects of excess sodium. Structural characterization (X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy) confirmed that the greatest compressive strength (∼8 MPa) was achieved with BHG 5 calcined at 750°C. Although this strength is lower than that of kaolinite-based geopolymers, it meets the requirements for non-structural applications, such as thermal insulation or lightweight masonry units. This research demonstrates a viable pathway for valorizing abundant illitic-kaolinitic Tunisian clays into sustainable construction materials.