X-ray diffraction (XRD) is a prominent technique to characterise cement-based materials. The combination of the Rietveld refinement with the Partial Or No Known Crystal Structure (PONKCS) approach now enables the quantification of both crystalline phases and amorphous contribution of SCMs. This paper describes the application of Rietveld-PONKCS to determine the amount of reacted glass powder (GP) in blended cement pastes. The accuracy and precision of the method were compared to the results of independent methods such as selective acid dissolution, thermogravimetric analysis (TGA) combined to energy-dispersive spectroscopy (EDS) or inductively coupled plasma (ICP) applied to GP-lime mixtures. For blended cement, the consistency of the method was internally checked using the standard addition method. Overall, an average precision of 1.6 wt.% and accuracy better than 1.5 wt.% were found for Rietveld-PONKCS applied to GP containing systems.