Real Causes of Refractory Castable Pulverization and White Fuzz: Complete Guide
Refractory castable refers to a hydraulic-bonded refractory material. It is composed of refractory aggregates, fine powders, and various admixtures. After mixing with water, it forms an unshaped material with a certain degree of hardness and strength. During application, formwork is required, and construction is carried out by pouring and vibrating. It is a widely used refractory material in high-temperature industries.
If the refractory castable is not dried in time after formwork removal, white deposits may appear on its surface during storage, followed by gradual sanding and pulverization. Obvious sand loss can be felt when touched by hand. This phenomenon is known as alkali efflorescence.
Causes of White Fuzz and Pulverization on the Surface of Refractory Castable
This phenomenon is referred to as efflorescence in refractory castables. The main cause is that the raw materials used in refractory castables contain alkali metal oxides such as Na₂O, K₂O, and Fe₂O₃. When these alkali metal oxides are mixed with water, the castable sets and hardens, and free water migrates to the surface, carrying loose, gelatinous white precipitates. This phenomenon occurs because unhydrated cement particles on the surface of the refractory castable form a saturated solution when exposed to water, which then precipitates alumina gel and calcium aluminate hydrates. As a result, alkali metal oxides react with carbon dioxide and other substances in the air, leading to efflorescence.
The raw materials of refractory castables include refractory aggregates, refractory fine powders, calcium aluminate cement used as a binder, silica fume, and various additives such as water reducers, accelerators, and anti-explosive agents. These materials usually contain small amounts of alkali metal oxides. Part of the alkalis can dissolve in water, especially those from dispersants, which almost completely dissolve in water. The dissolved alkali metals migrate through capillary pores to the surface of the hardened castable, where they react with CO₂ in the air to form carbonates, resulting in efflorescence. After efflorescence occurs, white deposits or frost appear on the surface. Although this phenomenon does not affect the overall performance of the refractory castable, it causes a rough and uneven surface, impairing the appearance.
When surface pulverization occurs in aluminate-based refractory castables, there are two main causes: one is surface pulverization caused by alkali impurities, and the other is surface pulverization caused by carbonation reactions. As long as cement hydration products exist, the following reactions will proceed cyclically.
Causes of Low-Temperature Shrinkage in Refractory Castables
An excessive linear change rate in refractory castables can cause serious damage to the lining, easily leading to structural spalling and shortening its service life. Therefore, technical measures such as adding admixtures should be adopted to minimize the linear shrinkage of the material.
Low-temperature shrinkage of refractory castables generally occurs under two conditions:
(1) volume change during oven drying after casting, formwork removal, and curing, when the combined water and free water in the refractory castable evaporate and are removed;
(2) linear change caused by chemical reactions of the castable during high-temperature service.
Both conditions result in shrinkage of refractory castables. This phenomenon is caused by an inappropriate formulation of the refractory castable. The material composition needs to be improved by introducing metallic silicon into the matrix to optimize the matrix structure, which can reduce shrinkage, enhance high-temperature resistance, and endow the castable with excellent resistance to high-temperature linear change, good thermal shock resistance, and a high softening temperature under load.
Adding various admixtures to refractory castables, such as metallic silicon, andalusite powder, and kyanite powder, can improve high-temperature anti-shrinkage performance to a certain extent. Adding metallic silicon powder can effectively eliminate the problem of high-temperature shrinkage at 1400°C, offset high-temperature linear shrinkage, and reduce shrinkage cracks during high-temperature service. Its effect is superior to that of kyanite and andalusite. However, since metallic silicon is relatively expensive, reducing its addition amount plays a key role in cost control.
How to Improve the Penetration Resistance of Basic Refractory Castables?
Another major problem with magnesia refractory castables among basic refractory castables is their low resistance to slag penetration. To address this issue, studies have been conducted by adding SiO₂, Al₂O₃, Cr₂O₃, ZrO₂·SiO₂, etc., to improve the slag resistance of the material. Results show that ZrO₂·SiO₂ provides the highest slag resistance but suffers from severe corrosion.
Combining Al₂O₃ and TiO₂ in MgO-rich basic refractory castables forms a magnesium–titanium spinel solid solution as the bonding phase. Adding fine spinel powder helps improve slag penetration resistance; the finer and more uniform the added particles are, the more effectively slag penetration can be restricted.
Another method to control slag penetration is by increasing slag viscosity. Both fine SiO₂ powder and Al₂O₃ can increase slag viscosity. However, considering the reactivity of basic refractory castables with molten metal and their resistance to basic slag attack, the addition of SiO₂ must be limited. This is especially true for basic refractory castables using ultrafine SiO₂ as a binder: although slag penetration is reduced, corrosion is increased.
In this case, ultrafine Al₂O₃ can be used to balance the corrosion resistance and penetration resistance of the material. Experiments show that when the content of fused SiO₂ (medium particles) is 4% and the content of ultrafine Al₂O₃ is not less than 5%, magnesia refractory castables achieve excellent corrosion resistance and penetration resistance.