Composition, silicate anion structure and morphology of calcium silicate hydrates (C-S-H) synthesised by silica-lime reaction and by controlled hydration of tricalcium silicate (C₃S)

The main product of Portland cement hydration is C-S-H. Despite constituting more than half of the volume of hydrated pastes and having an important role in strength development, very little is known about the factors that determine its morphology. To investigate the relationship between the chemical composition, silicate anion structure and morphology of C-S-H, samples were synthesised via silica-lime reactions and by the hydration of C₃S under controlled lime concentrations and with/without accelerators. The silicate anion structure of the samples was studied by ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and the morphology and chemical composition by TEM and SEM. All samples prepared via silica-lime reactions with bulk Ca/Si up to 1.5 were foil-like. The hydration of C₃S at fixed lime concentration yielded foil-like C-S-H for [CaO]<22 mmol L^- and fibrillar C-S-H for [CaO]>22 mmol L^-1. A relationship between the silicate anion structure and the morphology of C-S-H was found for the samples fabricated with accelerators.