Influence of mineralogy of Tunisian clays on pozzolanic activity—assessment by different methods

Five local clays were characterised by several physicochemical methods to predict their pozzolanic reactivity. The studied samples were treated thermally at 600, 700 and 800°C. The heat-treated samples were characterised by X-ray diffraction. The pozzolanic activity was assessed by chemical (Chappelle test), mechanical (compressive strength at 28 days) and physical (differential thermal analysis/thermogravimetry) methods. It was found that under the chosen conditions, the kaolinitic clay fraction was transformed to an amorphous phase that was reactive with lime, whereas the illitic phase remained in a crystalline form and was consequently unable to produce pozzolans. The montmorillonitic fraction started to gain pozzolanic activity above a calcination temperature of 700°C. It was also demonstrated that the pozzolanic activity of the clays calcined at 700°C was directly related to the percentage of kaolinite in the crude samples. The hydration products were C–S–H and C₄AH₁₃, and C₃AH₆ and C₂ASH₈ were also detected in samples that were rich in alumina.