The influences of curing temperature and alite to belite ratio on the C-S-H structure characterized by X-ray diffraction

This paper introduces a general method for extracting the profile of C-S-H with Ca/Si ratios at and beyond 1.7 from X-ray diffraction (XRD) patterns of synthetic cement composed of alite and belite at 6 ratios (100/0, 80/20, 65/35, 50/50, 35/65, and 0/100) hydrated between 5℃ and 90℃. This method integrates the initial construction of a C-S-H model via asymmetric least squares (ALS) and partial or no known crystal structure (PONKCS) techniques, followed by iterative refinement using Rietveld full-pattern fitting and an external standard K-factor approach until convergence criteria are met, with thermogravimetric analysis employed as a cross-check to validate model accuracy. Deconvolution-based peak profile analysis reveals that C-S-H structurally resembles low-symmetry defect-bearing tobermorite, with low-temperature (≤30°C) C-S-H resembling anomalous type and high-temperature (≥50°C) C-S-H resembling normal type. Insights from crystal defect, main peak positions and intensities, and fingerprint zone reveal that the XRD patterns/structures of each C-S-H exhibit their unique characteristics, mainly as functions of cement composition and curing temperature. Elevated curing temperatures, high-belite contents, and low relative humidity were more favorable to yield C-S-H interlayer spacing data (11 ± 0.6 Å). The crystallinity of C-S-H improves as the temperature increases (excluding pure belite system). The morphology of C-S-H crystallites ranges from stable rods in most cement systems to metastable bricks formed from pure alite or belite, and occasionally to platelets, with all crystallites having a maximum dimension of 24.2 nm. Distinct from pure C-S-H, the XRD patterns of C-S-H formed in the hydration of synthetic cement obtained in this study provide reference data for quantitative analysis and molecular modeling of C-S-H in various types of calcium silicate cement. • Methods for extracting/analyzing C-S-H XRD patterns in hardened cement were proposed. • The average crystallite size of C-S-H determined by XRD pattern analysis is 3.6 ± 1.4 nm. • The structure of C-S-H in hardened cement resembles low-symmetry defect-bearing tobermorite.