Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part II: Carbonated Phases

The effects of carbonation of mechanochemically prepared C–S–H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X‐ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO 2 (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S≥0.67 and aragonite at C/S≤0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium‐rich phases polymerize rapidly to yield Q 2 silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C–S–H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q 2 silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short‐range order of poorly crystalline materials is ideal for investigations of C–S–H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C–S–H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO 2 gel in the aged samples.