(vii) Diffuse bands at ≃ 2.1 and ≃ 2.4 Å become resolved into separate lines in preparations that are better crystallized. (vi) There is often a peak at ≃ 5.3 Å that varies markedly in intensity from very, very weak to moderate. (v) Most other peaks can be assigned hk0 indices, but perhaps should be regarded as hk band heads. (iii) The basal spacing depends on both the Ca/Si and H 2O/Si ratios. (ii) Preparations tend to have a single, broadened basal reflection that has a maximum between 9 and 14 Å this was interpreted as being due to mixtures of hydrates that have different layer spacings, randomly interstratified in sheets normal to c. (i) C-S-H(I) can be considered to be a structurally imperfect tobermorite. They noted the following points, which have been modified slightly with information from other early work (Taylor, 1950 Bernal et al., 1952 Grudemo, 1955 Taylor & Howison, 1956 Kalousek & Prebus, 1958 Taylor, 1964, 1969 ): An early summary of X-ray powder patterns for C-S-H(I) phases is given by Heller & Taylor (1956 ). Solutions of an alkali silicate and a soluble calcium salt (usually nitrate) are often used or a reactive form of silica is mixed with Ca(OH) 2 or anhydrous C 3S or β-C 2S. Powder X-ray diffraction data for C-S-H(I)Ĭ-S-H(I) can be prepared that has a Ca/Si ratio between 2/3 and ≃1.4 by mixing calcium and silicate ions in dilute aqueous suspension at temperatures below ≃ 333 K. The models were derived using crystal-chemical and geometrical reasoning, which necessitated a review of general aspects of the crystal chemistry of calcium silicate hydrates and related phases.Ģ. The purpose of this paper is to present a review, collation and new interpretation of the most important data for C-S-H(I) and to provide structural models that account for the observed trends. Despite much research attention there are currently no structural models available for either phase that can account comprehensively for the experimental observations, which are rather numerous for C-S-H(I). C-S-H(I) has been considered to be a structurally imperfect form of 14 Å tobermorite (Taylor, 1964, 1997 ) and C-S-H(II) to be related in a similar way to jennite (Gard & Taylor, 1976 ). The latter include C-S-H(I) for Ca/Si ratios less than about 1.4 and C-S-H(II) for higher values. The crystallinity of synthetic C-S-H preparations varies considerably: some have poor powder X-ray diffraction patterns that are similar to those of the C-S-H that is present in most cement pastes, whilst others are quite highly ordered. Researchers have as a consequence looked for compositional and structural similarity with natural crystalline calcium silicate hydrates – most commonly 14 Å tobermorite (C 5S 6H 9) and jennite (C 9S 6H 11 Taylor, 1986 Richardson & Groves, 1992 a ) – and have attempted to synthesize single-phase C-S-H in the laboratory that is similar to the phase that forms in concrete. This C-S-H is virtually X-ray amorphous, compositionally and structurally very variable, and generally finely intermixed with other phases, all of which make it difficult to study. The principal binding phase in all of this concrete is a calcium silicate hydrate phase. The models account for experimental observations, including variations in Ca/Si, H 2O/Si, (alumino)silicate anion structure and layer spacing.Įvery year over seven billion cubic metres of Portland cement-based concrete are manufactured worldwide (Gartner, 2004 ). A number of models that represent different mean chain lengths are developed using crystal-chemical and geometrical reasoning. Crystal-chemically plausible models are developed that are based instead on clinotobermorite. of the type that has been used in all previous studies. It is not possible to generate a structural model for a dimer that is crystal-chemically consistent with known calcium silicate hydrates if the starting structure is an orthotobermorite, i.e. Preparations that have Ca/Si greater than about 1.4 include an intermixed Ca-rich phase. It is shown that there are no interlayer calcium ions when the silicate chains are of infinite length and that one is added for each tetrahedral `bridging' site that is vacant. New structural–chemical formulae are presented for single- and double-chain tobermorite-based phases and equations are provided that can be used to calculate a number of useful quantities from 29Si NMR data.
It is a structurally imperfect form of 14 Å tobermorite that has variable composition and length of (alumino)silicate anions.
C-(A)-S-H(I) is a calcium silicate hydrate that is studied extensively as a model for the main binding phase in concrete.