An internal standard method based on Rietveld/XRD whole-pattern fitting analysis of fly ash is used to assess
the quantitative accuracy to determine its crystalline and amorphous phases under various conditions such
as internal standards (types, SiO2 or Al2O3 and dosages, 10–50%), incident X-rays (laboratory or
synchrotron) and refinement software (GSAS or TOPAS). The results reveal that the quantitative stability is
quite sensible to minor phases, identical to the internal standard, in fly ash. Errors positively correlate
with the weight fraction of that minor phase and negatively correlate with the dosage of an internal
standard and amorphous phase content in fly ash. The original equation for the amorphous phase
calculation is not applicable for a case with a higher inherent quartz content (>2.5%) in fly ash while the
dosages of the internal standard is lower than 20%. The original equation is modified as proposed. Based
on it, the quantitative results of five different patterns report a good reproducibility with the arithmetic
mean errors and the standard errors of identified main phases of around 1%.
