Kinetic study of mullite growth in sanitary-ware production by in situ HT-XRPD. The influence of the filler/flux ratio

The mullitisation kinetics in a sanitary-ware-like precursor system is here investigated by means of high-temperature X-ray powder diffraction, as a function of the filler/flux ratio. We used a blend based on kaolinite (50 wt%), quartz (10-28 wt%) and Na-feldspar (22-40 wt%). The results show that the content of feldspar boosts the formation of mullite as proven by the apparent activation energy values determined, ranging from 394 to 1111 kJ/mol, and giving a dEa/dxfeldspar ~ -23 kJ/mol/wt (xfeldspar = feldspar weight fraction). The mullitisation temperature has also been observed to depend on the Na-feldspar content, inasmuch as the sample bearing the smallest amount of feldspar flux exhibits a mullite growth onset between 1100 and 1150 oC, that is at a temperature about 50 oC higher than the one observed in the richer blends. The mullitisation kinetic process is in this work described as a one-mechanism transformation, satisfactorily formalised by Avrami-Erofeyev equation.