Chapter 5: Calcium silicate hydrates: natural minerals

Over 4 bn tonnes of Portland cement are currently manufactured annually worldwide (CEMBUREAU, 2018; US Geological Survey, 2019) from which an estimated 2·1–2·3 m³ or 4·8–5·5 tonnes per capita of cement-based materials are produced (Scrivener et al., 2016). The principal binding phase in all Portland cement-based materials is a calcium silicate hydrate (C–S–H) phase. 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. Researchers have consequently looked for compositional and structural similarity with natural crystalline calcium silicate hydrates – most commonly 14 Å tobermorite (C₅S₆H₉) and jennite (C₉S₆H₁₁) – and have attempted to synthesise a single-phase C–S–H in the laboratory that is similar to the phase that forms in concrete. The results of studies of the C–S–H that forms in hardened pastes of tricalcium silicate and β-dicalcium silicate are discussed in detail in Chapter 7. The natural minerals are the subject of this chapter and the laboratory-synthesised material is the subject of Chapter 6. Since diffraction techniques are of only limited utility in studies of the C–S–H that forms in real cements (see §7.3.5), there is increasing interest in using atomistic modelling techniques, which offer the prospect of significant advances in understanding its nanostructure. The results of such studies should of course be consistent with the results of the experimental studies that are described in the aforementioned chapters, as well as with crystal chemical principles. While atomistic modelling studies have usually started with the structure of 14 Å tobermorite (see §6.6.2), inspection of the structures of all of the crystalline calcium silicate hydrate phases is necessary to establish the crystal-chemical principles; the results of such an inspection (Richardson, 2014) are also reiterated in this chapter.