Technological properties and structural change of a fired carbonatic clay from Ourzazat (Morocco)

Abstract
Firing technological properties ( linear shrinkage, apparent density, open porosity and bending strength) of a raw clay from Ourzazat (Morocco) were measured for 850 T 1125C and correlated to the microstructure changes. The slight increase of porosity and the feeble reduction of density observed for T < 1000 C, are linked to the escape of CO₂, resulting from the decomposition of dolomite. The almost constancy of linear shrinkage is related to the opposite effects due to the neocristallization of pyroxenes and olivine, and the onset of the sintering process. The latter magnesian phases arise from the decomposition of fibrous minerals and dolomite. For higher temperatures, the drastic increases of density, linear shrinkage and bending strength, which are intended by a sharp reduction of porosity, is interpreted in terms of a quantitative development of melt. On the other hand, it is observed that the colour of shards changes likely because of the concomitant disappearance of hematite and formation of pyroxenes.