Quantitative analysis was performed by X-ray diffraction/Rietveld refinement with the Reference Intensity Ratio (RIR) method serving as the basis for determining the halo (background) intensity characteristic of the amorphous content. Although quantification by Rietveld analysis based on RIR is a standard-less method, there are few examples of this application in the literature. Since no RIR value exists for the amorphous phase, it was necessary to empirically determine the amorphous RIR value by gravimetrically preparing a set of simulated samples from blast-furnace slag and other reagents. The RIR value of the amorphous phase was determined to be 3.60, as calculated from the minimum root-mean-square error (RMSE) between known and analyzed values of crystalline and amorphous phases by a Rietveld refinement with various RIR values. Subsequently, this RIR value was applied to analyze the contents of amorphous phases in two commercial cement sample analytical results of the amorphous and crystalline phases in the cement samples by the RIR method were in good agreement with analytical results by the internal standard method (using silicon). It was thus demonstrated that Rietveld refinement, using the RIR value as the basis for the halo intensity, was an effective method for the quantitative analysis of the amorphous phase in cement.