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

Abstract

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/Sr0.50. 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.G. Scherer-contributing editor

Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics, microstructure changes and moisture properties

“…As a matter of fact, C-S-H carbonation may significantly contribute to this evolution. C-S-H carbonation has been studied by many authors [13,26,27,28,29]. All agree on a complex decalcification-polymerisation process of the C-S-H and the formation of amorphous silica gel according to the following reaction: C x S y H z + xC −→ xCC + ySH t + (z − yt)H…”

“…Furthermore, the provided assessment of C S remains global given that decalcified C-S-H (represented by Q 1 and Q 2 peaks in 29 Si MAS-NMR diagrams) is not distinguished here from Ca modified silica gel whose C S can be lower than 0.67 which corresponds to the allowed minimum value for decalcified tobermorite (represented by Q 3 and Q 4 peaks in 29 Si MAS-NMR diagrams according to [68]). As a result, this global way to quantify the C S of our cementitious systems justifies the possibility to get C S below 0.…”

“…It is essential to keep in mind that the assessment of the C S provided in case of non-carbonated and fully-carbonated systems is based on a molar balance of calcium and silicon, and not on a Q 29 Si MAS-NMR for a non-carbonated system. Furthermore, the provided assessment of C S remains global given that decalcified C-S-H (represented by Q 1 and Q 2 peaks in 29 Si MAS-NMR diagrams) is not distinguished here from Ca modified silica gel whose C S can be lower than 0.67 which corresponds to the allowed minimum value for decalcified tobermorite (represented by Q 3 and Q 4 peaks in 29 Si MAS-NMR diagrams according to [68]).…”

“…The polymorphism of formed CC (calcite, vaterite, aragonite, and even amorphous calcium carbonate [8,9,10,11]) has been analyzed according to CO 2 concentration [12,13,1,14] and to the C S ratio of the initial C-S-H [29] (vaterite, and aragonite in particular, are formed when C-S-H of low C S is exposed to CO 2 ). Additionally, Black et al [29] observed amorphous calcium carbonate as the first carbonation products.…”

Supercritical Carbonation of Lightweight Aggregate Containing Mortar: Thermal Behavior

“…Under these conditions, carbonation of cement pastes takes hours instead of years [9]. In that sense, supercritical carbonation can act as a pore reducing agent in cement paste with the conversion of calcium hydroxide into the more stable calcium carbonate, yielding to changes in the pore structure of cement matrices [10][11][12][13][14]. Nonetheless, very few studies address the issue of interfacial transition zone between cement paste and aggregates, especially taking into consideration the nature of the porosity of each lightweight aggregate.…”