Differential thermal analysis to establish the stability of aluminum-fly ash composites during synthesis and reheating

The major constituents of fly ash are various oxides of silicon, aluminum, and iron present in complex glass and ceramic forms. Thermodynamic analysis indicates the possibility of exothermic reduction of silica and iron oxides by molten aluminum; however, no data is available on the thermodynamics and kinetics of these reactions in a complex glass ceramic particles like fly ash, and this makes predictions about their stability difficult. In view of this, differential thermal analysis was used to establish the influence of processing and reheating on the stability of the aluminum-fly ash composite system using aluminum-fly ash composite-aluminum couples. During cooling of the sample after reheating in DTA equipment, several exothermic peaks were recorded, suggesting that rapid chemical reactions occur between aluminum and fly ash after pressure infiltration at 700°C. After 1 h of holding at 850°C, the DTA showed less than 1/2 of the exothermic peaks indicating rapid reaction during that period. After 10 h of holding at 850°C, a similar DTA plot showed no exothermic peaks except that for aluminum phase transformation. This suggests that the reactions between aluminum and fly ash were close to completion. The liquidus temperature of the once-pure aluminum matrix was seen to decrease from 655 to 644°C because of solute enrichment from reduction and dissolution of the silicon and iron oxides in the fly ash. The results of the present DTA study indicate that pressure infiltrated aluminum–40 vol.% fly ash composite made in this study was chemically stable after holding for 10 h at 850°C, demonstrating the usefulness of DTA to establish the chemical stability of aluminum-fly ash composites during synthesis and reheating.