The effect of surface tension/viscosity ratio of melts on the sintering propensity of amorphous coal ash slags

Abstract

Mechanical strength development in 20 amorphous coal ash slags sintered in air was examined. The sintering parameter expressed as the surface tension/viscosity ratio ( γ η ) of slags and determined above the temperature of critical viscosity ( T cv ) was of basic importance. This parameter gave information on the ash densification propensity. The γ η ratio increased linearly with the base/acid ratio calculated from the ash composition. The compressive strength of sintered coal ash slags at 1000 and 1100 °C decreased significantly with increasing γ η ratio in the range 0–1 cm s −1. This was linked with the increase of creep deformation due to the movement of grains in the non-Newtonian liquid phase or the liquid-like second phase. The molecular volumes and quantity of the crystalline second phases, such as anorthite and gehlenite precipitated in the grain boundaries during the sintering process, were also important factors controlling the high strength of coal ash slags. The compressive strength increased with the γ η ratio in the range 1–8 cm s −1, owing probably to the variation of elastic and plastic contributions of the amorphous-crystalline interface. At this point, the deposit microstructures showed very low closed porosity, ~ 4%. The compressive strengths of amorphous coal ashes were significantly affected by pores on sintering at 900 °C for a constant time.