The calcium silicate hydrate phase, abbreviated as CS-
H, makes up 50 to 60 percent of the volume of solids in a completely hydrated
portland cement paste and is, therefore, the most important phase determining
the properties of the paste. The fact that the term C-S-H is hyphenated signifies
that C-S-H is not a well-defined compound; the C/S ratio varies between 1.5 and
2.0 and the structural water content varies even more. The morphology of C-SH
also varies from poorly crystalline fibers to reticular network. Due to their
colloidal dimensions and a tendency to cluster, C-S-H crystals could only be
resolved with the advent of electron microscopy. In older literature, the material
is often referred to as C-S-H gel. The internal crystal structure of C-S-H also
remains unresolved; previously it was assumed to resemble the natural mineral
tobermorite and that is why C-S-H was sometimes called tobermorite gel.
Although the exact structure of C-S-H is not known, several models have
been proposed to explain the properties of the materials. According to the
Powers-Brunauer model,1 the material has a layer structure with a very high
surface area. Depending on the measurement technique, surface areas on the
order of 100 to 700 m2/g have been proposed for C-S-H, and the strength of the
material is attributed mainly to van der Waals’ forces. The size of gel pores, or
the solid-to-solid distance,∗ is reported to be about 18Å. The Feldman-Sereda
model2 visualizes the C-S-H structure as being composed of an irregular or
kinked array of layers which are randomly arranged to create interlayer spaces
of different shapes and sizes (5 to 25 Å).