XRD analysis in combination with the Rietveld method enables the quantification of hydrated cementitious systems containing cement, blast-furnace slag and fly ash. The aim of the analysis approach is to determine all occurring crystalline phases and to distinguish between the amorphous contents originating from blast-furnace slag, fly ash and amorphous or ill-crystalline phases formed during the hydration reaction. The investigation of the time dependent decrease of the amorphous blast-furnace slag and fly ash content enables to follow the reaction of these materials and to determine the overall degree of reaction. In order to quantify the different amorphous contents, internal and external standard methods as well as hkl-approaches can be applied.