Reliability of the X-ray diffraction quantitative determination of the fraction unhydrated in the hardened paste of Portland cement

The data from several studies concerning the non-evaporable water hydration criterion (as a reference criterion) and the fraction unhydrated in the hardened paste of Portland cement determined by the X-ray diffraction quantitative analysis are analysed in order to investigate the reliability of the X-ray method. The residue unhydrated in the hardened paste of Portland cement or of the major individual phases determined by the X-ray direct method does not agree with the residue estimated on the basis of the non-evaporable water criterion. The results of this study point to the unreliability of the X-ray diffraction quantitative analysis for determining the correct fraction hydrated in the hardened paste of Portland cement.