Unshaped refractory castables and castable preforms have been widely used by more and more high-temperature industrial kilns due to their advantages of convenient, fast construction and low labor intensity.
Unshaped refractory castables and castable preforms have been widely used by more and more high-temperature industrial kilns due to their advantages of convenient, fast construction and low labor intensity.
Chalking occurs during storage of preforms made of cement-containing refractory castables. Some of them grow "white hair" on the surface, commonly known as white alkali, which causes powdering on the surface of the castable. In severe cases, the strength of the castable decreases.
1. Carbonation of Castables
Carbonation of high-alumina refractory cement castables is often considered to be one of the main reasons that lead to the decrease of castable structure and spalling. The high alumina cement-bonded refractory castable is exposed to the air, and the hydration products therein react with the CO2 gas in the air to undergo carbonation reaction. Moreover, refractory castables are more likely to undergo carbonation under high humidity conditions, and compared with dense castables, light castables with high water content are easier to pulverize, which shows that under the conditions of water participation, hydration products and CO2 The acid gas interacts and the PH value of the liquid in the single pore of the phase drops sharply, which makes all the new hydration organisms in the refractory castable become unstable.
2. Sulfation of Castables
Compared with the carbonation of high alumina cement hydration products, the sulfation phenomenon has not attracted enough attention from the construction party and the user. In the atmosphere, in addition to the CO2 acid gas, there are also SO2, H2S and other gases. It also has a strong corrosive effect on the hydration products of the matrix part of the high-alumina cement castable, and eventually causes the castable to powder and peel off.
In addition to being sulphated by SO2 and H2S in the air, high-aluminum cement hydrates may also interact with SO42- or H+ in the water source. As the activity and acidity of the two increase, three types of high-alumina The stability of cement hydration products is also drastically reduced.
3. Salt precipitation phenomenon of Castable
The salt precipitation on the surface of refractory castables is another important factor that aggravates the structural damage of the castables. During the curing or natural drying process of refractory castable, as the water evaporates, some soluble carbonate, sulfate, chloride, phosphate and other salt compounds focus on the surface of refractory castable and crystallize out. This phenomenon is often It is called salting out phenomenon.
As the water evaporates from the surface of the refractory castable, these ions migrate and aggregate with the water molecules from the inside to the surface of the castable, or react with acid gases in the air on the surface of the castable, and the salts formed after the hydrate reaction Focusing on the surface, these salt compounds finally crystallize out due to their solubility in water.
In fact, all soluble salts containing crystal water, including chlorides, sulfates, carbonates, etc., have a process of crystal water removal. With the change of crystal form and volume change, these are caused by the castable When curing in the air, it is one of the reasons that aggravate the surface structure pulverization, peeling, and falling.
Solution:
(1) Dry the castable as soon as possible after construction to remove the water and eliminate the carrier of sodium salt in the cast, so that the alkali stays in the construction body and no longer migrates to the surface;
(2) If the construction conditions cannot reach timely drying, the pouring material must be sealed and the surface covered with plastic film to prevent moisture from migrating to the surface with alkali.
(3) Use refractory materials with low alkali content for refractory castables to increase the viscosity and rapid hardening of refractory castables, which can reduce sodium salt migration speed, strengthen quality management, and strictly control the amount of water added in construction.
