Calcium silicate hydrate
Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.
Calcium silicate hydrate (also shown as C-S-H) is a result of the reaction between the silicate phases of Portland cement and water. This reaction typically is expressed as:
Synthetic C-S-H can be prepared from the reaction of CaO and SiO2 in water or through the double precipitation method using various salts. These methods provide the flexibility of producing C-S-H at specific C/S ratios. The C-S-H from cement phases can also be treated with ammonium nitrate in order to achieve desired C/S ratio.
C-S-H is a nano sized material with some degree of crystallinity as observed by X-ray diffraction techniques. The underlying atomic structure of C-S-H is similar to the naturally occurring mineral Tobermorite. It has a layered geometry with calcium silicate sheet structure separated by an interlayer space. The silicates in C-S-H exist as dimers, pentamers and 3n-1 chain units  (where n is an integer greater than 0) and calcium ions are found to connect these chains making the three dimensional nano structure as observed by DNP surface enhanced NMR. The exact nature of the interlayer remains unknown. One of the greatest difficulties in characterising C-S-H is due to its variable stoichiometry.
The SEM micrographs of C-S-H does not show any specific crystalline form. They usually manifest as foils or needle/oriented foils.
Synthetic C-S-H can be divided in two categories separated at the Ca/Si ratio of about 1.1. There are several indications that the chemical, physical and mechanical characteristics of C-S-H varies noticeably between these two categories.
- Other C-S-H minerals:
- Other calcium aluminium silicate hydrate, (C-A-S-H) minerals:
- Mechanisms of formation of C-S-H phases:
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