Effect of Mechanical Activation on the Synthesis of Ba-Celsian and Sr-Celsian using Precursor Mixtures Containing Coal Fly Ash

BaAl 2 Si 2 O 8 and SrAl 2 Si 2 O 8 were synthesized by solid-state reaction of stoichiometric mixtures of either BaCO 3 or SrCO 3 with coal fly ash and Al 2 O 3 . The mixtures were mechanically activated in an attrition mill for up to 12 h and then reaction-sintered at 900-1300 °C, aiming to promote the formation of BaAl 2 Si 2 O 8 and SrAl 2 Si 2 O 8 as well as the conversion from their hexagonal (Hexacelsian) into their monoclinic (Celsian) forms, which is associated with improved mechanical properties in the sintered materials. Especially in the case of SrAl 2 Si 2 O 8 , the formation of Celsian was favored at relatively low sintering temperatures by increasing milling time. Although only the SrAl 2 Si 2 O 8 composition was fully converted into Celsian, the Hexacelsian to Celsian conversions obtained for the mechanically-activated BaAl 2 Si 2 O 8 composition were significantly higher than those previously reported in the literature for this compound. This could be attributed to the use of coal fly ash as raw material, which contains mineralizers that promote the mentioned conversion.