Factors influencing softening temperature and hot-strength of geopolymers

This paper presents the effects of cation type, silicate concentration, compositions of low-calcium fly ash and test load on softening temperature and hot-strength (loaded while at high temperature) of fly ash-based geopolymers. It was found that softening temperature (Ts) of sodium (Na) based-geopolymer remained the same (610 °C ± 20 °C) regardless of the silicate concentration, fly ash composition, and test load. However, when the cation was changed from sodium to potassium (K), the Ts increased to 800 °C. Further, when Na/K is mixed, the Ts dropped to 570 °C. Significant increases in hot-strengths (at 530 °C and 730 °C) were found in all geopolymers. This increase in strength was observed to be in conjunction with heat release, indicating an exothermic reaction. Since the reaction between fly ash and activators is exothermic, this reaction is believed to be the reason for the strength gain at high temperatures.