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, alumi num, and iron present in complex glass and ceramic forms. Thermodynami c 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 By ash, and this makes predictions about their stability difficult. In view of this, differential thermal analysis wa s used to establish the influence of processing and reheating on the s tability of the aluminum-fly ash composite system using aluminum-fly a sh composite-aluminum couples. During cooling of the sample after rehe ating in DTA equipment, several exothermic peaks were recorded, sugges ting that rapid chemical reactions occur between aluminum and fly ash after pressure infiltration at 700 degrees C. After 1 h of holding at 850 degrees C, the DTA showed less than 1/2 of the exothermic peaks in dicating rapid reaction during that period. After 10 h of holding at 8 50 degrees C, a similar DTA plot showed no exothermic peaks except tha t for aluminum phase transformation. This suggests that the reactions between aluminum and By ash were close to completion. The liquidus tem perature of the once-pure aluminum matrix was seen to decrease from 65 5 to 644 degrees C because of solute enrichment from reduction and dis solution 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 degrees C, demonstrating the usefulness of DT A to establish the chemical stability of aluminum-fly ash composites d uring synthesis and reheating. (C) 1998 Elsevier Science S.A.