Effects of phosphorous-based additive on flow properties of high silicon-aluminum coal ash

•Effects of Ca3(PO4)2 addition on the fluidity of Huainan coal ash were revealed.•Phosphorus decreased ash fusion temperatures and temperature of critical viscosity.•Phosphorus contributed to the formation of slag during the ash fusion process.•Polymerization of slag structure was enhanced by phosphorus at high temperatures.•Crystallization in the slag during cooling was restricted by phosphorus.