# Calcium silicate hydrate

Calcium silicate hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.

## Preparation

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:

${\displaystyle {\mathsf {{\ce {{2Ca3SiO5}+7H2O->{3CaO.2SiO2.4H2O}+{3Ca(OH)2}}}+173.6\ kJ}}}$

The stoichiometry of C-S-H in cement 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.[1]

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.

## Properties

The crystal structure of C-S-H in cement paste has not been fully resolved yet and there is still ongoing debate over its nanostructure.[2]

The SEM micrographs of C-S-H does not show any specific crystalline form. They usually manifest as flakes or dendrites/fibrils.

Synthetic C-S-H can be divided in two categories separated at the C/S 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. [3][4]