Porcellanite-CaCO3 high temperature reaction: phases, microstructure, microhardness

The characteristics of reaction phases, microstructure and microhardness of thermally fused powder mixtures of porcellanite and limestone batched in various proportions have been studied after firing at 1200 °C. X-ray diffraction and scanning electron microscopy were employed in phase and microstructure investigation, respectively. Formed as thermo-chemical reaction products, morphologically flat, thin, randomly orientated plates of 3CaO·Al2O3·2SiO2 and 2CaO·Al2O3·SiO2 embedded in molten 3CaO·2SiO2 characterized the microstructure of compositions originally containing 60 and 70% porcellanite. In contrast, and consistent with XRD phase identification, the microstructures of the mixtures originally containing 75% and higher porcellanite featured predominantly 3CaO·2SiO2-bonded particulate and/or dendritic and fibrous mullite. Retained in compositions originally containing ≥50% porcellanite, remnant unreacted free silica featured microstructurally as fine dispersed rounded particles. For the compositions which fused sufficiently strongly and homogeneously to facilitate indentation hardness testing (50-90% porcellanite), Vickers microhardness increased through maximum then gradually decreased with increasing porcellanite content.