Development of Hydration Heat

Development of Hydration Heat

When mixed with water, cement begins to react chemically. This is called hydration of the cement.

The chemical process of hardening is the foundation for the formation of the hardened cement paste and therefore of the concrete. The chemical reaction with the mixing water produces new compounds from the clinker materials —->hydration.

Viewing under an electron microscope shows three distinct phases of the hydration process, which is strongly exothermal, i.e. energy is released in the form of heat.

Hydration phase 1

Generally up to 4 to 6 hours after production

The gypsum in the plastic cement paste binds the tricalcium aluminates (C3 Al) to form trisulphate (ettringite), a water-insoluble layer which initially inhibits the conversion process of the other components. The gypsum addition of 2–5% therefore has a retarding effect.

The longer “needles” which are created in this phase bind the separate cement particles together, causing the concrete to stiffen.

Hydration phase 2

Generally between 4 to 6 hours after production and up to one day

After a few hours comes the start of vigorous hydration of the clinker materials, particularly the tricalcium silicate (Ca3Si), with the formation of intertwined long-fiber calcium silicate hydrate crystals which further consolidate the structure.

 Hydration phase 3

From about one day

The structure and microstructure of the cement matrix are initially still open. As hydration progresses, the interstices are filled with other hydration products and the strength is further increased.