Thermal Behavior of Celsian Ceramics Synthesized from Coal Fly Ash
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Thermal Behavior of Celsian Ceramics Synthesized from Coal Fly Ash Jorge López-Cuevas1, David Long-González2, Carlos A. Gutiérrez-Chavarría1 Cinvestav-Saltillo, Carr. Saltillo-Monterrey Km 13.5, Ramos Arizpe, Coahuila, México, 25900 2 RHI REFMEX, S.A. de C.V., Carr. Saltillo-Monterrey Km. 9, Coahuila, México, 25900 1
ABSTRACT Celsian with a chemical composition of Ba0.75Sr0.25Al2Si2O8, is synthesized by using coal fly ash (byproduct of a Mexican coal-burning power plant, composed mainly by SiO2 and Al2O3) as main raw material. The thermal behavior of the synthesized material is evaluated by differential (DTA) and gravimetric (TGA) thermal analyses as well as by heating microscopy; its coefficient of linear thermal expansion (CTE) is also determined. Heating microscopy shows that cylinders of compacted powdered Celsian start sintering at ~1140 ºC, which is associated with a considerable contraction occurring up to 1500 ºC. The mean CTE value of the material in the temperature range of 30-1100ºC is slightly affected by the synthesis conditions employed. Synthesis at 1400 or 1300 ºC during 10 h, with a pre-calcination step at 900 ºC/5h in both cases, produce mean CTE values of 5.15 x 10-6 and 5.43 x 10-6 ºC-1, respectively. On the other hand, Celsian synthesized at 1400 ºC/10 h, without the pre-calcination step, has a mean CTE value of 5.25 x 10-6 ºC-1. Lastly, the DTA/TGA analysis of the synthesized material shows that a slight weight gain takes place from room temperature to 1100ºC, which is followed by a slight weight loss up to 1300ºC. This is attributed to oxidation and evaporation of some of the impurities present in the material. INTRODUCTION Barium-strontium aluminosilicate solid solutions (Ba1-xSrxAl2Si2O8) are interesting materials for structural applications at high temperatures, especially in their monoclinic polymorphic form known either as Celsian or as Monocelsian. These materials exhibit high refractoriness, low thermal expansion coefficient, good resistance to oxidation and to slag attack, as well as good thermal shock resistance [1,2]. These solid solutions are formed when BaO is partially substituted by SrO in the crystalline structure of barium aluminosilicate (BaAl2Si2O8, BAS); this process constitutes an effective way to promote the formation of the monoclinic BAS form corresponding to the so-called Celsian or Monocelsian phase [3]. Without the addition of SrO, or of other mineralizing agents that cause a similar effect [4], a hexagonal polymorphic form of BAS (Hexacelsian) is very likely to be formed instead of Celsian, and it frequently remains in a metastable state at temperatures at which the latter is the equilibrium phase (from room temperature to 1590°C) [5]. Hexacelsian is undesirable due to its relatively high thermal expansion coefficient and because it undergoes a conversion into an orthorhombic form at ~300°C, which is associated with a volume change of ~3-4%. This causes microcracking in the material, negatively affecting its final mechanical properties [2,6,7]. In this work, Celsian
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