Assessment of pozzolanic potential in lime–water systems of raw and calcined kaolinic clays from the Donnigazza Mine (Sardinia–Italy)

Raw and calcined kaolinic clays from a mid-western Sardinia (Italy) sector were investigated in order to evaluate the feasibility of lime-activated materials. The great reserves of clays in this mining district, which are not well suited to traditional ceramics because of their low kaolinite content, excess of quartz, iron oxides, alunite, and the open pit exploitation, suggest an alternative use for these materials. The kaolinic clay samples and a reference kaolin were thermally activated in a laboratory oven at 530, 630 and 800 °C temperatures. Pastes of thermally activated and non-activated materials with lime, in the ratio of 1:1 by weight and water / solid ratio 1: 2, were prepared and cured at different times. The reactions that took place in the metakaolin-lime-water systems were monitored using thermogravimetric analysis, differential thermal analysis and X-ray diffraction analysis. The results of pozzolanic tests and laboratory analyses were combined and offered very interesting property relations compared with a high-grade kaolin used as reference sample, even though a slower hydration kinetics. The large availability of the Donnigazza kaolinic clay reserves encourages the use of these materials in the binders sector and particularly in the Cultural Heritage sector as pozzolanic additives for restoration mortars.