Cordierite-based ceramics with coffee husk ash addition: I –microstructure and physical properties

Cordierite is one of the compounds in the MgO–Al2O3–SiO2 system which has the lowest thermal expansion coefficient. Despite this advantage, which results in high resistance to thermal shock, monophasic cordierite is difficult to prepare due to its narrow sintering temperature range and the existence of secondary phases that increase its thermal expansion coefficient. Fluxing agents are generally used to stimulate the sintering of cordierite precursor powders as well as to modify the microstructures and properties of these ceramics. In this context, this work aims at studying the structural, microstructural and physical properties of cordierite-based ceramics with small amounts of coffee husk ash residue as fluxing agent. Sintered samples were prepared from a standard formulation (containing clay, alumina and talc) with controlled additions of coffee husk ash residue (a natural source of K2O) aiming to investigate the effects of the sintering temperature and amount of residue on the phase composition and properties. Quantitative phase analysis of samples sintered between 1350 and 1450 °C was determined by Rietveld refinement of XRD data. Physical properties were estimated by the Archimedes methodology. The microstructure of sintered samples was inspected by scanning electron microscopy. The results indicated that the addition of residue amounts as low as 1 wt% accelerates the reaction between cristobalite and spinel, originating the cordierite phase. The formation of liquid phase with increasing the residue content explained the decrease in the apparent porosity and water absorption