14. Other clinker phases

alite

It is the most important part of all conventional cement clinkers; its content is 50-70%. It trёhkaltsievy silicate 3SaO * SiO2 (abbreviated C3S), the composition and structure of which are modified due to placement in the lattice of foreign ions, especially of Mg2 +, Al3 + and Fe3 +. Alite relatively quickly reacts with water and normal phases of all cements plays the most important role in the development of strength; daily for 28 strength contribution of this phase is particularly important.

Belit

Content for normal belite cement clinker is 15-30%. This dicalcium silicate 2SaO * SiO2 (C2S), modified by introducing into the structure of the foreign ions and usually completely or predominantly present in a β-modification. Belit slowly reacts with water, thus affecting the strength weak during the first 28 days, but significantly increases the strength at a later date. A year later, the strength of pure alite and belite net under comparable conditions are about the same.

aluminate phase

Aluminate phase content is 5-10% for most normal cement clinkers. It tricalcium aluminate 3SaO * Al2O3, significantly modified the composition and sometimes the structure, due to foreign ions, especially Si4 +, Fe3 +, Na + and K +. Aluminate phase reacts rapidly with water and may cause undesirably rapid setting, if no seizure supervising added reagent commonly gypsum.

Alyumoferritnaya phase

Ferrite phase (CaAlFe) 5-15% of a conventional cement clinker. It - chetyrёhkaltsievy alumina ferrite 4CaO * Al2O3 * Fe2O3, the composition of which varies considerably when changing Al / Fe ratio in the structure and placement of foreign ions. The rate at which the ferrite phase is reacted with water, may vary somewhat due to differences in the composition or other characteristics, but usually, it is high in the initial period and is intermediate between the speed of alite and belite at a later date.

The clinker is typically present in small amounts of several other phases such as alkali sulfates and calcium oxide.

In Portland cement clinker is often 1-2% free CaO (free lime). The reasons may be: lack of training materials (roughly chopped or non-homogeneous raw meal), lack of temperature and firing time, resulting CaOnepolnostyu associated with other oxides, too slow cooling, when the partial decomposition of C3S and C3A, or too high saturation coefficient KH> 0,96-0,98). The polished sections of clinker free lime in the etching is visible in the form of colored particles from turquoise to blue. Often one can observe that the free lime occurs in the nests (Figure 1.2). This is due in most cases to poor homogeneity of the raw meal. Free lime is undesirable because it may cause (for example, in amounts of> 2.5%) non-uniform change in volume of cement, hardened cement stone can expand and cause cracks. Lime cracks occur due to increased volumes of approximately 2-fold when hydrated CaO with conversion to Ca (OH) 2. Saturated MgO clinker may contain free MgO - periclase. Since approximately 2,0-2,5% MgO as well as other ions can be introduced into the clinker phases, part of MgO can remain free and form periclase. The standardized cement according to EN 197 German industrial standard can contain a maximum of 5,0% MgO, a maximum of 2.5-3.0% of free periclase. The content of MgO, which binds to the other phases, depending on the chemical composition of clinker and its manufacturing conditions. The polished sections of MgO clinker in most cases occur in the hexagonal form. It is necessary to distinguish between emerging pink colored crystals of periclase from tricalcium silicate. Along with the different colors by etching it detects smaller size crystals, common jacks. They are somewhat uplifted and polished areas are extracted from the clinker as MgO has a higher hardness than the other phases of the clinker. Periclase is undesirable, since a higher content of hazardous magnesium (similar free lime) may occur. Harmful magnesia insidious, since the destruction of concrete products may appear only after many years. The finely crystalline and evenly distributed periclase expansion phenomenon is significantly less than in the coarse MgO or "nests" MgO. It is also typical for a free free lime.