- Calcium silicate hydrate (C-S-H) is a complex and poorly crystalline compound that forms as the primary hydration product when Portland cement reacts with water.
- It is not a single compound with a fixed chemical formula, but rather a family of materials with varying calcium-to-silicon (Ca/Si) ratios, typically between 1.2 and 2.1, and variable water content. C-S-H is the main binding phase in cement and concrete, responsible for most of the mechanical strength, durability, and dimensional stability of hardened cement paste.
- Structurally, calcium silicate hydrate resembles naturally occurring tobermorite and jennite minerals but is largely amorphous or nanocrystalline in form. It consists of chains of silicate tetrahedra linked by calcium ions, with water molecules and hydroxide groups located between layers. This structure forms a gel-like matrix that fills the spaces between cement particles, adhering them together and creating a solid mass. The high surface area and ability to bind water contribute to the unique rheological and mechanical properties of cement-based materials.
- C-S-H begins to form within hours after cement is mixed with water and continues to develop over weeks or even months, contributing to the long-term strength gain of concrete. The gradual growth of C-S-H crystals fills pores and capillaries, reducing porosity and increasing compressive strength. It also plays a major role in durability, as it helps protect steel reinforcements from corrosion by maintaining a highly alkaline environment (pH >12.5).
- From a chemical engineering and construction materials perspective, the composition and morphology of C-S-H can be influenced by various factors including cement composition, water-to-cement ratio, additives (like fly ash, silica fume, or slag), and curing conditions. Modern research focuses heavily on modifying and optimizing C-S-H formation to enhance concrete’s performance, such as improving resistance to sulfate attack, reducing shrinkage, or increasing toughness.
- In fireproofing and insulation, synthetic forms of calcium silicate (though more crystalline) are manufactured for use in thermal insulation boards, refractory linings, and pipe coverings. These materials share some compositional similarities with C-S-H but are tailored for lightweight, thermally stable, and non-combustible applications in industrial environments.
- C-S-H also plays an emerging role in carbon capture and storage (CCS) research. Due to its high surface area and reactivity, it can chemically bind carbon dioxide through carbonation processes, forming calcium carbonate and silica gels. This has implications in sustainable construction practices, where cementitious materials are modified to absorb CO₂ from the atmosphere.
