Effects of phosphorus on the fusion characteristics of synthetic ashes with different Al2O3/CaO ratios

In this paper, five series of synthetic ashes with different Al2O3/CaO ratio mass ratios were prepared, and the ash fusion temperatures (AFTs) of the samples with different percentages (5-20 wt%) of phosphorus pentoxide (P2O5) added were measured under a mild reducing atmosphere to study the effect of phosphorus on the fusion characteristics of the ashes in different compositions. X-ray diffraction (XRD), thermodynamic equilibrium calculations, and high-temperature heating stage microscopy (HHSM) were used to investigate the mechanism of phosphorus in changing the fusion characteristics of the ash. The results showed that the addition of P2O5 produced the least effect on AFTs of samples with an Al2O3/CaO ratio of 6:4. For lower Al2O3/CaO ratios (classified as peralkaline), the addition of P2O5 caused a decrease-increase-decrease trend in AFTs, while for higher Al2O3/CaO ratios (classified as peraluminous), the AFTs of samples were decreased by phosphorus. The stronger affinity of P2O5 for CaO and Al2O3, compared to that of SiO2, was noticed and led to a shift of main minerals in the slag from silicates to phosphates and to the isolation of SiO2. In addition, the phosphorus-bearing samples followed the same "melt-dissolution mechanism" as the phosphorus-free samples.