Effects of CO2 and temperature on the structure and chemistry of C–(A–)S–H investigated by Raman spectroscopy

Abstract

Calcium (alumino)silicate hydrate (C–(A–)S–H) is the critical binding phase in modern Portland cement-based concrete, yet the relationship between its structure and stoichiometry is not completely understood. In this study, Raman spectroscopy is used to analyze the effects of varying Ca/Si molar ratio (0.6–1.6), Al/Si molar ratio (0.0–0.1), and synthesis temperature (7–80 °C) on the chemical composition and atomic configuration of C–(A–)S–H. The experimental results indicate that increasing Ca/Si molar ratio produces less cross-linked C–(A–)S–H structures, while the addition of Al into the system increases the long-range order of its chain-like structure. Furthermore, increasing the synthesis temperature leads to the formation of more polymerized structures, especially in the Al-containing samples. The Raman spectra also suggest the formation of vaterite in C–S–H samples synthesized at low temperatures. Finally, this study reveals that uptake of atmospheric CO₂ in C–S–H and C–A–S–H favors the formation of long-range ordered chain-like structures.