During the curing or natural drying process of refractory castables, as the components evaporate, some soluble salts such as carbonates, phosphates, sulfates, etc. aggregate, crystallize, and appear on the surface of refractory castables, which is called salting out. The appearance of "white hairs" on the surface of refractory castables is a direct reaction to salt precipitation phenomenon. The salt precipitation phenomenon is mainly caused by the following reasons:
1. The ions contained in refractory castables, such as SO2-4, Cl -, Ca2+, Fe2+, Na+, PO3-4, CO2-3, etc., gradually migrate and aggregate towards the surface of refractory castables through capillary pores as water evaporates. When the concentration exceeds the solubility of these salt compounds, crystallization occurs on the surface of refractory castables.
2. As mentioned above, the hydration products of calcium aluminate cement react with gases such as CO2, SO2, H2S in the atmosphere to form salts that aggregate on the surface of refractory castables. When the concentration exceeds the solubility of these salt compounds, crystallization also occurs on the surface of refractory castables.
Most of these salts are compounds containing crystalline water. When the ambient temperature exceeds 50 ° C, the crystalline water will gradually lose, accompanied by significant volume changes. Like Na2SO4? 10H2O belongs to the monoclinic crystal system with a theoretical density of 1.446g/cm3, while the dehydrated product Na2SO4 belongs to the orthorhombic crystal system with a theoretical density of 2.68g/cm3 °. Therefore, when Na2SO4? After losing water, the volume shrinkage of 10H2O will reach 45.4%. This may be one of the main reasons for exacerbating the powdering and peeling of the surface structure of refractory castables. This belongs to a physical process.
Of course, there is also a type of salt that does not contain crystal water, such as NaCl. The salt precipitation of these salts does not cause crystal transformation or volume change, and does not have a significant impact on the surface strength of refractory castables. It only affects the surface quality of refractory castables.
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