A direct decomposition method of X-ray diffraction pattern associated with amorphous phases in a binary blend containing hydrated cement and siliceous fly ash is developed. The method relies on identifying patterns associated with individual components within the overall intensity pattern produced by the total amorphous phase. The intensity pattern associated with the total amorphous phase in a binary blend of siliceous fly ash and hydrated cement is obtained using the Pawley intensity refinement method. The features of the intensity patterns associated with the amorphous reaction products in hydrated cement and the glassy content of siliceous fly ash extracted by decomposition of the overall intensity pattern produced by the combined amorphous phase are shown to match the features obtained directly from the pure phases. The direct decomposition method allows for quantification of amorphous components, which can be applied to study the evolving phases in a hydrating binary system. © 2016 Informa UK Limited, trading as Taylor & Francis Group.