The hydration of Portland cement

Abstract

Electron microscopy and conduction calorimetry have been employed to study the hydration of Portland cement. In situ studies of wet cement pastes in an environmental cell in the high voltage microscope confirm that the reaction involves two stages: (a) the rapid initial formation of gelatinous hydrate coatings around the cement grains and, (b) after a dormant period, the growth from these coatings of fine fibrillar calcium-silicate-hydrate (C-S-H) gel material into a reticulate network between the cement grains. Detailed electron microscopy indicates that the individual fibres are not solid but consist of fine hollow tubes. Attention is drawn to the striking visual analogy between the sequence of hydration of cement and the tubular growth forms that are obtained in 'silicate gardens'. The latter are known to depend on the development of osmotic pressure as a driving force and, in spite of obvious differences in the scale and rate of growth, the basic parallels are close enough to suggest that a similar osmotic mechanism probably applies to the hydration of cement. On this basis, a model is proposed which explains the two-stage nature of the hydration process and provides a viable mechanism for the transport of the silicate material during growth of the secondary, fibrillar C-S-H product.