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High alumina cement refers to cement mainly composed of calcium aluminate. The clinker with about 50% aluminum oxide content and mainly composed of calcium aluminate is ground to become a water-hardening binder called high alumina cement. Calcium aluminate cement is an amorphous refractory material, especially the basic binding agent for casting refractory materials. It is rarely used in the production of refractory bricks.
The chemical composition of high alumina cement is mainly Al2O3 and CaO, and some also contain a considerable amount of Fe2O3 and SiO2. The mineral composition includes the following five minerals: calcium monoaluminate CaO·Al2O3 (CA), calcium dialuminate CaO·2Al2O3 (CA2), and calcium aluminum heptadecaoxide 12CaO·7Al2O3 (C12A7), as well as calcium feldspar with inclusions 2CaO·Al2O3·SiO2 (C2AS), and calcium iron aluminate 4CaO·Al2O3·Fe2O3 (C4AF).
The main mineral composition of high alumina cement, calcium monoaluminate, has a high water-hardening activity, and although its setting is not very quick, it hardens rapidly. This is the main source of the strength of high alumina cement, especially early strength. Calcium dialuminate hydrates slowly, with low early strength but high later strength. Calcium aluminum heptadecaoxide (12CaO·7Al2O3) has the characteristic of rapid hydration and rapid setting, but low strength. In ordinary high alumina cement containing inclusions, C4AS plays a role in making the cement set early, while in advanced high alumina cement, C12A7 plays a hardening role. As for C2AS, it basically does not produce a hydration reaction, so it cannot show its strength.
Calcium aluminate cement contains five calcium aluminate minerals, and these calcium aluminate salts will show intense heat release with the progress of hydration reaction. The peak temperature of ordinary high alumina cement at 5-8 hours after thickening reaches 80-90℃; the advanced high alumina cement containing 70% Al2O3 reaches peak temperature at 10-12 hours after thickening, and that containing 80% Al2O3 reaches peak temperature at 8-10 hours, which are 80℃ and 70℃, respectively. The heat release temperature and the time to reach the highest temperature of a single type of high alumina cement pouring body are determined by the heat release characteristics and content of calcium aluminate salts contained.
C12A7 and C4AF, which are quick-hardening, have higher heat release. Therefore, the heat release of high alumina cement will also vary depending on the content of the aluminum salt containing them. If too much high alumina cement with C12A7 or C4AF as the main component is added to the refractory material casting, the concentrated heat release will cause the rapid evaporation of moisture in the hardened material, leading to cracking and surface peeling. Therefore, the selection of cement dosage and quality is very important.
During the transportation and storage of high alumina cement, special attention should be paid to moisture-proofing to prolong the storage period and avoid affecting product performance; during the transportation, storage, and use of high alumina cement, avoid contact with other types of cement (such as silicate cement) and harmful impurities (such as lime). When using high alumina cement, the mixing water should be pure to avoid possible harmful impurities affecting product performance.
