Potential Use of Clays from Zemlet El Beidha (Southern Tunisia) as Aluminosilicate Precursors for Geopolymer Synthesis

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. Tau he raw materials are predominantly illitic, although kaolinite remains a main reactive phase. Samples BHG2 and BHG5 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 BHG2 being rich in hematite and BHG5 being rich in calcite. This selection allowed for a comparative analysis of how different impurities affect geopolymerization. The samples were calcined at 550 degrees C, 750 degrees C and 950 degrees 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 (similar to 8 MPa) was achieved with BHG5 calcined at 750 degrees 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.