Quantitative analysis of the amorphous phase in binary blends of siliceous fly ash and hydrated cement is presented using the Partial Or No Known Crystal Structure (PONKCS) method. In binary blends, the total amorphous phase contains contributions from the glassy phase in fly ash and amorphous components of hydrated cement. A single pseudo-Voigt function is found to adequately fit the broad hump in the X-ray diffraction pattern resulting from diffuse scattering produced by the glassy phase in siliceous fly ash. The weight fractions of the amorphous component of hydrated cement and the glassy phase of siliceous fly ash in a binary blend are determined using the fitted profiles for the X-ray diffraction (XRD) patterns. For low lime content of less than 5%, the peak characteristics of the fitted function for the broad hump in the XRD pattern of fly ash are nominally identical for different oxide compositions of the glassy phase. The PONKCS method is shown to provide good accuracy in determining siliceous fly ash content in binary blends. © 2016 American Society of Civil Engineers.