Difference in pozzolanic behaviour of Tunisian clays with lime and cement

Two local clays, one from the region of Tabarka and the other from Medenine, were tested as pozzolanic materials. The characterisation of the raw and the calcined clays included X-ray diffraction (XRD), differential thermal analysis, thermogravimetry and infra-red spectroscopy. The pozzolanic activity was evaluated by mechanical tests, performed on calcined clay–lime and on calcined clay–cement mini-cylinders, and by physical method (dosage of calcium hydroxide (Ca(OH)₂) by differential scanning calorimetry). The identification of the hydrated phases was done by XRD, differential scanning calorimetry and scanning electron microscopy. Kaolinitic clay from Tabarka was found to have the highest pozzolanic activity, compared with the illito-kaolinitic clay from Medenine containing some haematite. It was demonstrated that the haematite contained in the last clay did not contribute to the pozzolanic reaction in the presence of calcium hydroxide while in the presence of cement, the pozzolanic reactivity is improved. The haematite contained in Medenine clay contributes to the reaction with cement to form iron substituted ettringite and iron-substituted C–S–H. Furthermore, a new hydrated ferritic phase (Ca₃Fe₂(OH)₁₂) is formed. The major hydraulic products such as C–S–H, C₂ASH₈ and C₄AH₁₃ were identified.