Refractory castables often experience a significant decrease in strength at intermediate temperatures (typically 800℃-1000℃ or higher). This is mainly due to the dehydration, recrystallization, and physical shrinkage of hydrates in the binder, leading to a porous structure. To improve the intermediate-temperature strength of refractory material castables, one can focus on several core dimensions: raw material proportioning, binder system optimization, additive usage, and construction techniques. The following are specific improvement strategies:
I. Optimizing Raw Materials and Compensating for Chemical Reactions
This is the most direct and effective method. The core is to utilize the volume expansion generated by the chemical reaction to offset the shrinkage during sintering.
1. Adding Al₂O₃ Fine Powder: Adding an appropriate amount of Al₂O₃ (alpha alumina) fine powder to aluminate refractory castables is crucial. At intermediate temperatures, it undergoes an expansion effect chemical reaction, compensating for the strength decrease caused by volume shrinkage. Especially when the binder is CA-70 high-alumina cement, adding this fine powder can even increase the intermediate-temperature strength instead of decreasing it.
2. Introduction of active fillers: Pure aluminate cement is combined with silica fume. At 800-1200℃, the silica fume reacts with calcium oxide to form an anorthite reinforcing phase, which can effectively increase the intermediate-temperature strength by about 20%.
II. Addition of sintering agents and expanding agents
By introducing specific mineral raw materials, the sintering behavior or volume stability of the material at intermediate temperatures can be changed.
1. Addition of soft clay (sintering agent): Adding 3%-6% soft clay can promote sintering of the castable at lower temperatures, changing the microstructure and thus increasing the intermediate-temperature strength, even exceeding the oven-dried strength.
2. Utilizing andalusite (high-temperature reinforcement): Although andalusite primarily functions at high temperatures (above 1300℃), if the formulation is properly designed (added in fine powder form), the mullite and excess SiO₂ generated during its decomposition at high temperatures can form secondary mullitization, which is very helpful in maintaining strength after crossing the intermediate temperature range.
3. Using boron carbide: Boron carbide softens at high temperatures and adheres to the particle surface, contributing to densification. The B₂O₃ oxide film formed on its surface provides oxidation resistance, while the columnar crystals generated reduce porosity and improve intermediate temperature strength.
III. Improving the bonding system:
The binder is the "skeleton" of the refractory castables. Choosing a suitable binder can fundamentally change the weakness in intermediate temperature strength.
1. Using high-performance cement: Pure calcium aluminate cement (CA-70 or higher grade) should be used whenever possible. Compared to ordinary CA-50 cement, it has a better strength retention rate in the intermediate temperature stage.
2. Composite Binders: Cement is combined with chemical binders (such as phosphates), or cohesive binders (such as silica sol and alumina sol) are used. These bonding methods form a stable network structure at intermediate temperatures, unlike pure hydration binders which are prone to collapse due to dehydration.
IV. Microstructure and Particle Size Optimization:
Physical methods are used to make the internal structure of the material more compact and reduce defects.
1. Reasonable Particle Size Distribution: Optimize the particle distribution of aggregates (such as corundum and mullite), following the principle of closest packing to reduce internal porosity.
2. Introduction of Micropowder Technology: Add appropriate amounts of activated alumina micropowder or silica micropowder, utilizing the filling effect of the micropowder to reduce apparent porosity, increase material density, and thus improve strength.
V. Construction and Curing Control:
Even with the best refractory castables material formulation, improper construction will significantly reduce strength.
1. Strict Control of Water Addition: Excessive water addition will significantly increase porosity and reduce density. The water addition amount must be strictly followed according to the manufacturer's recommended amount during mixing.
2. Standardize the baking process: When heating at the medium temperature stage (especially 900℃-1200℃), sufficient holding time must be ensured to allow the hydrates to fully dehydrate and recrystallize, avoiding cracking or loose structure due to excessive heating.
