Portland cement (hereinafter “cement”) belongs to a group of man-made hydraulic binders. It is an inorganic material formed by firing clay and limestone together with a mix of corrective raw materials, mostly sources of iron oxide (Fe₂O₃), aluminium oxide (Al₂O₃) and silicon dioxide ), and subsequent grinding into a fine powder. The adjective “hydraulic” means that after mixing with water, cement can set and harden even underwater – as opposed to aerial binders that require air to set and harden, such as lime or gypsum.

During the firing in a cement kiln, the raw materials are melted at temperatures of up to 1400 °C, and for white cement even up to 1600 °C. Out of these broken-down building blocks, new minerals emerge. The four main clinker (the main component of cement) minerals are tricalcium silicate (also alite, C₃S, content 50 to 60%), dicalcium silicate (belite, C₂S, content 20 to 25%), tricalcium aluminate (C₃A, content 6 to 10%) and tetra calcium aluminate ferrite (C₄AF, content 6 to 10%). Before the final grinding of the clinker to the cement, a portion of the gypsum is added, whose main task is inhibiting the rapid hydration reaction of C₃A, otherwise causing a premature loss of workability of the concrete – so-called false setting.

Hydration generally refers to the reaction of anhydrous material with water and its product is a new substance – hydrate. By mixing cement with water, the resulting cement paste sets and hardens due to hydration reactions. It is a process in which a liquid suspension passes to a solid material at room temperature and without the need for additional energy. Cement hydration is a very complex physicochemical process. For simplicity, it can be divided into 5 distinct stages:

Stage 1: Immediately after mixing

The second cement particles come in contact with mixing water, their surface starts to dissolve and an aqueous solution of calcium and silica ions is formed.

Stage 2: Minutes after mixing

The ion solution becomes supersaturated and nuclei of the first hydration products begin to precipitate. These are mostly calcium silicate hydrate (C-S-H) gels, the main building blocks of hydrated cement stone.

Stage 3: Hours after mixing

The formation of cement stone is at full speed. Ions are coming out of the solution and form new hydration products like calcium silicate hydrate, portlandite, ettringite and others. To balance the concentration, new ions are being released from yet undissolved parts of cement grains.

Stage 4: Days after mixing

As the insoluble cement stone grows and matures, the diffusion becomes slower and further precipitation is limited by decreasing surface area. As a result, the hydration process slows down significantly.

Stage 5: Up to 28 days*

The degree of hydration is about 80% and most of the remaining unhydrated material is very-slow-reacting belite C2S.

*In reality, the chemical reactions inside concrete never really stop, they just become infinitely slower and slower.er.