Comparison of rapid and slow sintered pulverised fuel ash

This study has investigated the properties of sintered PFA using conventional (20°C/min) and rapid heating rates. Rapid heating was achieved by directly placing pressed PFA samples into a furnace preset at the sintering temperature. Slow sintered PFA samples show an increase in shrinkage and fired density and a reduction in water absorption as the sintering temperature increases, with maximum density obtained at around 1250°C. At higher temperatures samples show a bloating effect associated with the enlargement of closed porosity. This peak in density is not observed in rapidly sintered PFA. Rapid sintering at temperatures between 1150°C and 1300°C resulted in constant values of shrinkage (6.5%), fired density (1.4g/cm3) and water absorption (15%). During rapid sintering the residual carbon in the PFA is present in the sample at the sintering temperature whereas conventional sintering removes the carbon at lower temperatures before sintering occurs. Rapid sintering also retains significant carbon in the core of the sintered sample. This carbon is believed to act as rigid inclusions that inhibit PFA sintering, limiting shrinkage and densification. Rapid sintering is associated with black coring which is also characteristic in lightweight aggregate manufactured from PFA sintered on a sinter strand. The rapid heating rate experienced by PFA pellets on a sinter strand and the associated inhibition of sintering by carbon is essential for producing sintered PFA products with properties appropriate for use as lightweight aggregate.