Influence of mechanical activation on the synthesis of Sr-Celsian employing a precursor mixture containing coal fly ash

Strontium aluminosilicate, SrAl2Si2O8 (SAS), was synthesised by a solid state reaction using coal fly ash (CFA) as the main raw material. A precursor mixture of SrCO3, CFA and Al2O3 was mechanically activated for times of up to 12 h using an attrition mill, and subsequently sintered at temperatures of 900-1300 C. The CFA and the mechanical activation were employed with the aim of promoting transformation from the hexagonal (Sr-hexacelsian) into the monoclinic (Sr-celsian) polymorphic form of SAS, since the latter phase is associated with better physical and mechanical properties but the former tends to be the first one to appear. The mean particle size, crystallite size and temperature at the end of the curve of weight loss (T(f)) decreased, while the specific surface area and the degree of amorphisation increased, with increasing milling time. Samples milled for at least 4 h and then sintered at 1100 C achieved full transformation into Sr-celsian. The same result was obtained for sintering temperatures > 1100 C, independently of milling time. An increment in both the milling time and the sintering temperature improves the densification and the mechanical properties of the synthesised materials.