Calcium silicate hydrates (CSH or C-S-H) are the main products of the hydration of

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

and are primarily responsible for the

strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...

of cement-based materials. They are the main binding phase (the "glue") in most

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

. Only well defined and rare natural crystalline minerals can be abbreviated as CSH while extremely variable and poorly ordered phases without well defined

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

, as it is commonly observed in hardened cement paste (HCP), are denoted C-S-H.

Preparation

When water is added to cement, each of the compounds undergoes hydration and contributes to the final state of the concrete. Only calcium silicates contribute to the strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, only contributes to late strength. 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: : also written in cement chemist notation, (CCN) as: : 2 + 7 H → + 3 CH + heat or, tricalcium silicate + water → calcium silicate hydrate + calcium hydroxide + heat The

of C-S-H in

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

paste is variable and the state of chemically and physically bound water in its structure is not transparent, which is why "-" is used between C, S, and H. 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 (Ca/Si, or CaO/SiO₂) ratios. The C-S-H from cement phases can also be treated with an

ammonium nitrate Ammonium nitrate is a chemical compound with the formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly us ...

solution in order to induce calcium leaching, and so to achieve a given C/S ratio.

Properties

C-S-H is a nano sized material with some degree of crystallinity as observed by

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

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 dynamic nuclear polarisation surface-enhanced

nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

. The exact nature of the interlayer remains unknown. One of the greatest difficulties in characterising C-S-H is due to its variable

. The scanning electron microscope micrographs of C-S-H do not show any specific crystalline form. They usually manifest as foils or needle/oriented foils. Synthetic C-S-H can be schematically divided into two categories, depending on whether their Ca/Si molar ratio is below or above a threshold value of 1.1. There are several indications that the chemical, physical and mechanical characteristics of C-S-H vary noticeably between these two categories.

References

{{Reflist Calcium compounds Cement Concrete Hydrates Inorganic compounds Silicates Phyllosilicates

Preparation

Properties

See also

References