Synthesis and characterization of BaAl2Si2O8 using mechanically activated precursor mixtures containing coal fly ash

BaAl2Si2O8 (BAS) was synthesized by solid state reaction, using coal fly ash [CFA, containing (in wt.%) 75-78% silico-aluminous glassy phase, 14.6% mullite, 5.2% quartz, 1.9% magnetite, plus other minor phases] as main raw material. A BaCO3-CFA-Al2O3 powder mixture of stoichiometric BAS composition was mechanically activated in an attrition mill for up to 12 h and then sintered at 900-1300 °C. The monoclinic BAS phase (Celsian) was desired because it has better mechanical and thermal properties than the hexagonal BAS polymorph (Hexacelsian), but this tends to form first, remaining frequently metastably at low temperatures; besides, the Hexacelsian to Celsian conversion is sluggish and difficult to occur. The reaction rate, the apparent density and the mechanical properties of the synthesized materials increased with increasing milling time and sintering temperature. The mineralizing effect caused by some of the CFA impurities produced Hexacelsian to Celsian conversions higher than those previously reported for mechanically activated BAS materials.