Comparison of Different Approaches for Quantification of Amorphous Phase in Hydrated Cement Paste by XRD

One of the most important tasks in the investigation of hydrated cement paste is the determination of the amorphous phase content, which is mostly cementitious gel. The determination of the cementitious gel content is important for the evaluation of the hydration process. However, this task is not trivial and it is hard to achieve high accuracy. The major approach for determining the content of the amorphous phase is by the use of an internal or external standard in the x-ray diffraction (XRD) analysis. This study compared two external standards, as well as two procedures of internal standard intermixing on amorphous phase determination accuracy. In addition, a novel method of calibrating the HKL phase for the Partial Or Not Known Crystal Structure method of evaluation of the amorphous phase using an internal standard was applied and evaluated. For this purpose, cement pastes with water to cement ratios of 0.30, 0.35, and 0.40 were prepared, and hydration was stopped using the solvent exchange at the ages of 7 and 28 days. The hydrated cement paste samples were examined using XRD and the hydrates assemblage and amorphous phase content were analyzed using Rietveld refinement. The bound water in hydrated cement paste was studied using thermal analysis. For comparison, the degree of hydration was determined by XRD, thermal analysis, and isothermal calorimetry. The experimentally determined amorphous phase content was compared with theoretical calculations based on the degree of hydration and the recommendations for the best amorphous phase quantification procedure were given.