Salting-out Effect On The Surface Of Refractory Castable

Salting-out effect on the surface of refractory castable
During the curing or natural drying process of refractory castables, as components evaporate, some soluble salts such as carbonates, phosphates, and sulfates gather, crystallize, and precipitate on the surface of the refractory castables, a phenomenon known as salting-out. The appearance of "white fuzz" on the surface of refractory castables is a direct reflection of the salting-out phenomenon. The salting-out 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 accumulate towards the surface of the refractory castables through the capillary pores as the water evaporates. When the concentration exceeds the solubility of these salt compounds, crystals form on the surface of the refractory castables.
2. As mentioned above, salts formed by the reaction of calcium aluminate cement hydration products with gases such as CO2, SO2, and H2S in the atmosphere accumulate on the surface of refractory castables. When the concentration exceeds the solubility of these salt compounds, crystals also form on the surface of refractory castables.
Most of these salts are compounds containing crystal water. When the ambient temperature exceeds 50°C, the crystal water gradually loses its presence, accompanied by significant volume changes. For example, Na2SO4·10H2O belongs to the monoclinic system, with a theoretical density of 1.446g/cm3, while its dehydrated product, Na2SO4, belongs to the orthorhombic system, with a theoretical density of 2.68g/cm3. Therefore, when Na2SO4·10H2O loses its water, the volume contraction can reach 45.4%. This may be one of the main reasons for the intensified pulverization and spalling of the surface structure of refractory castables. This is a physical process.
Of course, there is another type of salts that do not contain crystal water, such as NaCl. The salting-out of these salts does not cause crystal transformation or volume changes, and has little impact on the surface strength of refractory castables. It only affects the surface quality of refractory castables.