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High Alumina Cement (also known as bauxite cement or refractory cement) is a hydraulic cementitious material with the code CA, made from bauxite and limestone as raw materials. After high-temperature calcination, it produces a clinker mainly composed of calcium aluminate, which is then finely ground to make the cement. It is the main variety of aluminate-based cement.
High Alumina Cement's main mineral components are monocalcium aluminate CaO·Al2O3, abbreviated as CA, and dicalcium aluminate (CaO·2Al2O3, abbreviated as CA2), in addition to a small amount of dicalcium silicate and other aluminates.
The hydration process of high alumina cement is mainly the hydration of monocalcium aluminate. Generally, it is considered that its hydration reaction differs with temperature. When the temperature is below 20°C, the main hydration product is hydrated monocalcium aluminate hydrate (CaO·Al2O3·10H2O, abbreviated as CAH10). At 20°C to 30°C, the main hydration product is dicalcium aluminate hydrate (2CaO·Al2O3·8H2O, abbreviated as C2AH8).
When the temperature exceeds 30°C, the main hydration product is tricalcium aluminate hydrate (3CaO·Al2O3·6H2O, abbreviated as C3AH6). Additionally, there is also aluminum oxide hydrate gel (Al2O3·3H2O). The hydration reaction of CA2 is similar to that of CA, but its hydration speed is very slow. Dicalcium silicate produces calcium silicate hydrate gel.
Hydrated monocalcium aluminate and dicalcium aluminate form flaky or needle-like crystals. They interlock and overlap to create a strong crystallized skeleton structure, while the formed aluminum oxide hydrate gel fills the skeleton spaces, creating a denser structure and giving the cement stone very high strength. After 5-7 days of hydration, the amount of hydration products increases very little, and the strength tends to stabilize. Thus, High Alumina Cement shows rapid early strength growth, with less significant strength increase afterward. The amount of dicalcium silicate is minimal, and it does not play a significant role in the hardening process.
Rapid setting and early strength, with 1-day strength reaching over 80% of the maximum strength, classifying it as a fast-hardening cement. When using High Alumina Cement, it is essential to control the hardening temperature. The optimal hardening temperature is around 15°C, generally not exceeding 25°C. Excessive temperature can cause dicalcium aluminate hydrate to transform into tricalcium aluminate hydrate, reducing strength. Under hot and humid conditions, the strength reduction is even more severe. The heat of hydration is high, with concentrated heat release, discharging 70%-80% of the total heat of hydration within one day, compared to 25%-50% for Portland cement.
Strong sulfate resistance but poor alkali resistance. High Alumina Cement hydrates without liberating calcium hydroxide, and its hardened structure is dense, giving it excellent sulfate and seawater corrosion resistance. It also shows good stability against carbonated water and dilute hydrochloric acid. However, once the crystals transform into stable tricalcium aluminate hydrate, the porosity increases, reducing corrosion resistance. High Alumina Cement cannot resist alkali erosion, so alkali corrosion should be avoided.
Good heat resistance. High Alumina Cement concrete retains 70% of its original strength at 900°C and about 53% at 1300°C. This remaining strength results from the solid-phase reactions between the cement stone components, forming a ceramic matrix.
The long-term strength tends to decrease. Over time, CAH10 or C2AH8 gradually transform into the more stable C3AH6, releasing free water and increasing the pore volume. Additionally, C3AH6 itself has lower strength, hence the long-term strength of the cement stone decreases.
