04《Peeling resistance》
CaO has good creep characteristics, good high temperature toughness and low elasticity. Therefore, in the actual use of magnesium-calcium refractory materials, it has good stress relaxation, strong thermal shock resistance and thermal spalling resistance. Moreover, the dihedral angle of the MgO-CaO combination is larger than that of the MgO-MgO combination, which is beneficial to improve the resistance to slag penetration and slag erosion.
05 “Abundant resources and low cost”
my country has very rich reserves of natural dolomite and limestone, and they are widely distributed. MgO-CaO refractories are cheap, only equivalent to 1/3 to 1/2 of MgO-C bricks. From the perspective of environmental protection, it does not contain chromium which is harmful to the environment.
06《Hydrability of CaO》
At this stage, the biggest problem facing magnesium-calcium refractories is the easy hydration of CaO. CaO and MgO have the same crystal structure—NaCl type crystal structure, and their structural constants are 4.8Å (CaO) and 4.20Å (MgO) respectively. Face-centered coordination structure, that is, a unit cell contains four molecules. However, Mg2+ has a smaller volume and is completely surrounded by O2-, and O2- is in contact with each other; while Ca2+ has a larger ion radius than Mg2+, and its polarization ability is weaker than that of O2-, and it is slightly pushed out of the lattice by O2-.
The CaO hydration reaction equation is as follows:
CaO(s)+H2O(g)→Ca(OH)2(s)
ΔG0=-109.35+0.14TkJ (1)
MgO(s)+H2O(g)→Mg(OH)2(s)
ΔG0=-80.88+0.15TkJ (2)
If the hydration reaction of CaO is based on its reaction with liquid water:
CaO(s)+H2O(l)→Ca(OH)2(s)
ΔG0=-70.00+0.05TkJ (3)
From the comparison of the two formulas (1) and (2), it can be seen that at the same temperature, CaO is easier to hydrate than MgO, so CaO is mainly responsible for the hydration of magnesium-calcium refractory materials. Comparing the temperature (T) when ΔG0 in formulas (1) and (3) is equal, it can be concluded that when T<120°C, CaO is easier to hydrate in steam than in liquid water, and after T>120°C, the theoretical Liquid water is more likely to hydrate CaO than water vapor, and the hydration temperature extends to 1150°C. However, due to the high saturated vapor pressure of water at high temperatures, Gu Huazhi and other scholars believe that it is mainly water vapor before 470°C that promotes the formation of CaO water. change.
The study of hydration kinetics shows that when CaO is distributed in a continuous phase, its hydration rate is controlled by the chemical reaction rate, resulting in intense hydration; while when CaO is distributed in a dispersed phase, its hydration rate appears to be controlled by the hydration reaction first. Controlled by the characteristics of diffusion rate control, the two alternately, so that the hydration process is gentle. Therefore, the hydration resistance of CaO distributed in dispersed phase is stronger than that distributed in continuous phase. In addition, a small amount of impurities can improve the hydration resistance of magnesia-calcium sand, mainly because impurities can react with CaO to form hydration-resistant minerals on the surface of CaO grains, such as Ca3SiO5, Ca2Fe2O5, etc. These minerals also play a role in CaO. The role of waterproof protective film.
Similar to the hydration of polycrystalline MgO, the structural unit of CaO disintegrates during hydration to generate four Ca(OH)2 structural units, and the size of the structural unit changes from 4.80 Å of CaO to 3.52 Å of Ca(OH)2. The four structural units occupy much larger positions than the original units of CaO. This hydration of CaO not only releases a large amount of heat, but also causes harmful volume expansion in the direction. It is calculated that the volume of CaO increases by 96.5% when hydrating, which leads to the complete pulverization of CaO refractories into powder.