Effect of some additives on the development of spinel-based glass-ceramic glazes for floor-tiles

The feasibility of developing spinel-based glass-ceramic glazes from a glass with composition in the system ZnO–MgO–B2O3–Al2O3–SiO2 was examined. To do it additional fluxes and/or nucleants were added to a parent glass before melting. Pressed pellets of powdered glasses were submitted to standard thermal treatments up to 1200 °C. The crystallization path and the microstructural development at several temperatures were followed by several experimental techniques. The results showed that additions of TiO2 as nucleant or additional B2O3 as flux to the chosen glass favored the crystallization of cordierite or mullite as main crystalline phase. Glasses which also contained minor amounts of Na2O and K2O, as additional fluxes, crystallized rapidly to spinel (Mg,Zn)Al2O4 using a single relatively fast heating ramp without a previous nucleating heat treatment step. Microstructural examination revealed that a uniform, fine-scale phase separation preceded crystallization. Very small, well-shaped crystals of spinel with octahedral morphology were formed. Slips of these glasses on conventional tile supports thermally treated under single heating ramps developed similar microstructures that the ones raised from pellets. This feature allows to suggesting these spinel-based glass-ceramic glazes as good candidates to improve the mechanical properties of conventional glazes.