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1. The minerals in calcium aluminate cement

The main minerals in calcium aluminate cement are CA, CA2, Cl2A7, and C2AS.

CA is the main mineral of pure calcium aluminate cement. However, pure calcium aluminate cement may also contain a small amount of CA2 and C12A7.

It is generally acknowledged that with the characteristics of the normal setting and rapid hardening, CA has a high hydraulic activity and is the main source of cement strength. The strength improvement of cement with higher CA content is mainly in the early stage, and the strength development in the later stage is not significant.

The hydration and hardening of CA2 are slow, and the early strength is low, but the later strength improves rapidly. When it contains too much CA2, the fast hardening performance of cement will be affected.

The coordination between aluminum and calcium in C12A7 is extremely irregular, and there are a large number of pore spaces in the crystal structures, which makes the hydration and condensation very fast and the strength not as high as that of CA. When containing a large amount of C12A7, fast setting, strength reduction, and heat resistance decline of the cement will occur. However, if controlled properly, a small amount of C12A7 in some cement can actually accelerate the setting and increase the early strength.

In general, C2AS has no hydraulicity. However, if the SiO2 in the high aluminium cement can be managed to form CaO-Al2O3-SiO2 glass, the SiO2 in the high aluminium cement may participate in hydration, and the performance of the high aluminium cement may be further improved.

2. The hydration of calcium aluminate cement

Calcium aluminate cement will hydrate when it meets with water.

After meeting with water, cement minerals were dissolved in the water, and the concentration of some ions such as Ca2+ and Al(OH)4 -  get higher in the solution, which makes the conductivity rise rapidly. Subsequently, the concentration of ions reaches saturation, the concentration of ions in the liquid phase is no longer increased, the crystalline phase of the hydrate forms slowly, and the slurry gradually loses its flowing ability. Then large quantities of hydration reactions occur at the ID stage. The temperature of the cement paste and the content of the bound water increase, the ion concentration decreases, and the paste begins to harden and develop the strength. The main sources of calcium aluminate cement strength are all kinds of Cx AHy and AH3.

3. The transformation of calcium aluminate cement hydrate

CAH10 and C2AH8 are metastable minerals. With the increase of the temperature and the extension of time, CAH10 and C2AH8 will become stable mineral C3AHS. CAH10 and C2AH8 are hexagonal flake crystals, with the respective density of 1.72 g/cm3 and 1.95 g/cm3. C3AH6 is a kind of isometric system material, the density of which is 2.52 g/cm3. So after realizing the transformation of CAH10 and C2AH8 into C3AH6, the density of hydrate increases and water content decreases. The strength decreases significantly with the decrease of the hydrate volume.

4. The transformation of calcium aluminate binding phase after heating

When heated, there will be a more complex transformation in the refractory cement. As is reported by Roesel: the stable temperature ranges of CAH10, C2AH6, and C3AH6 are respectively 0-20℃, 20-60℃, and 0-350℃. In 200-350 ℃, AH3 will transform into Al2O3; C3AH6 will transform into CaO and C12A7. In 600-1000 ℃, C12A7 will react with CaO and produce CA; In 1000-1300 ℃, CA will react with A and produce CA2; In 1400-1600 ℃, and CA2 will react with aluminium oxide and produce CA6.

After the dehydration of the hydrate, the hydration bond will be destroyed, but the ceramic bond has not been formed, so the bonding force of the materials is quite low. Therefore, the strength of traditional refractory castables decreases by more than 50% after the heat process at 1100℃, which seriously affects the life of refractories. Because there are great expansion effects after the solid-phase reaction carried out in the 1000℃ environment, some people believe that the volume effect which is accompanied by the solid-phase reaction leads to the loose structure and low strength of traditional refractory castables at medium temperature.

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