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1. What is high alumina cement?

High alumina cement is a type of cement that is very different from ordinary Portland cement in its composition and certain properties. Therefore, its structural use is very limited. In order to solve the erosion problem of gypsum water on Portland cement concrete structures, Jules Bied of France discovered high alumina cement. Alumina cement is, as the name suggests, about 40% alumina, 40% lime and up to 8% silica along with some ferrous and ferric oxide. The oxide composition is shown in Table 3.10 below. However, the alumina should be below 32% by weight and the alumina/lime ratio should be between 0.85 and 1.3.

High alumina cement refers to a fast-hardening, high-strength, heat-resistant and corrosion-resistant cementitious material. The main characteristics are high early strength, high temperature and corrosion resistance. High alumina cement is mainly used for projects with urgent construction period, such as pressure prevention, road and special emergency repair projects, etc., and can also be used for winter construction projects. Compared with Portland cement, little is known about the composite composition of high alumina cement, and there is no simple calculation method. The main gelling compound is low alkalinity calcium sulfoaluminate cement. Other compounds are present in a few percent, but free lime is not present. Therefore, there is no problem that the high alumina cement is not firm.

2. Hydration of high alumina cement

During the setting process of high alumina cement, important reactions are the formation of monocalcium aluminate decahydrate, dicalcium aluminate octahydrate and alumina gel. These aluminates exhibit high strength in high alumina cement concrete, but the aluminate cement is unstable and gradually convert to the more stable alumina tricalcium hexahydrate and gibbsite. Changes in the composition of high alumina cement lead to loss of strength.

The change in crystal form from hexagonal to cubic releases a large amount of water, which leads to an increase in concrete porosity. The way these changes occur depends on the water/cement ratio, temperature and chemical environment. The change in composition results in a loss of strength and the process by which the crystal form changes from hexagonal to cubic is called conversion.

The decomposition in the chemical reaction of monocalcium aluminate decahydrate to tricalcium alumina hexahydrate and alumina hydrate is affected by temperature. The higher the temperature, the faster the conversion. It is also affected by higher concentrations of lime or increased alkalinity. Furthermore, the higher the water/cement ratio, the higher the conversion. This reaction releases the water needed for the conversion process to continue. Therefore, conversion is a function of temperature and water-cement ratio. Since the overall dimensions of the grout or concrete specimen remain unchanged, the transformation reaction results in a decrease in solid volume and an increase in porosity.

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