Influence of various sodium salt species on formation mechanism of lightweight aggregates made from coal fly ash-based material

Abstract

To save energy in sintering to produce construction and building materials, alkali compounds are always recognized as a good flux component. We added various sodium salts, to coal fly ash/glass mixture, to investigate the effect of sodium salt species type on formation mechanism of lightweight aggregates. TGA/DTA, and XRD are used in this study. Results indicate that, despite the great similarity in melting point of the salt additives, a change of additive from one sodium salt to another leads to considerably different lightweight aggregate formation mechanism, thereby affecting the aggregate properties. Thermal behavior of sodium salt additives, as revealed in their TGA/DSC curves, might play a critical factor in influencing lightweight aggregate formation. All sodium salts can indirectly, following two-step sequential reactions, form sodium silicate with SiO2 present in coal fly ash/glass mixture. Sodium silicates are suggested as major components of viscous layer during sintering; they can envelope bloating gases to expand the size of lightweight aggregates. The solubility of coal fly ash/glass mixture in melt NaCl during sintering, is not as much as in melt Na2O and Na2SO4, thus requiring higher sintering temperature to prepare lightweight aggregates.As compared with the lightweight aggregates prepared in industry, all sodium salt additives used in present study can effectively reduce sintering energy, from the aspects of lightweight aggregate particle density. Effectiveness for the sodium salt additives to reduce sintering temperature for successful lightweight aggregate preparation is, in increasing order, sodium carbonate < sodium sulfate < sodium chloride. Sintering temperature can be potentially reduced to ≤1050 °C by proper addition of the sodium salts.