The air permeability of refractory castables is only related to its own structure, that is, to the interconnected pores in the structure. Particle size distribution, moisture content, binding agent, water reducing agent, explosion-proof agent, construction method and maintenance conditions will all have a certain impact on its structure, thereby affecting its air permeability.
(1) Effect of particle size on air permeability
There are two main theoretical models for the particle size ratio of castables: one is the classic continuous accumulation theory, the Andreasen equation; the other is the distribution and accumulation theory, the Dinger-Funk equation.
The particle accumulation in dense refractory castables is continuous and is usually described by the Andreasen distribution equation.
In the 1970s, Dinger and Funk introduced finite minimum particle size into the distribution, modified the Andreasen equation, and obtained the Dinger-Funk equation.
In formula (2), D is the particle size (mm); DL is the maximum particle size (mm); Ds is the particle size of the smallest particle (mm), CPFT is the cumulative percentage of particles with a diameter smaller than D (%); q is the distribution. index.
Distribution index affects MTP and close packing, which in turn affects air permeability. Yin Yucheng and others studied the effect of particle size composition on the pore characteristics of A12O3-SiC-C castables, and prepared A12O3-SiC-C castables with different particle size compositions by changing the distribution index q. . The results show that when q increases from 0.21 to 0.25, the apparent porosity and air permeability of the sample first decrease and then increase. To study the effect of PSD on the air permeability of castables, refractory castings with q of 0.21, 0.26, and 0.31 were prepared for air permeability testing. The results showed that the air permeability increased significantly. Dinger-Funk also conducted three-dimensional and two-dimensional computer simulations of the particle accumulation of the continuous system, and proposed that in the three-dimensional case, the densest packing of continuously distributed spheres occurs when the distribution index is 0.37; In the two-dimensional case, the closest arrangement occurs when the distribution index is 0.56. It can be seen that when other conditions are the same, increasing the distribution index q can improve the breathability.
(2) Effect of moisture content on air permeability
To study the effect of PSD on the air permeability of castables, when the samples have the same particle size distribution and are cured and dried under the same conditions, an increase in moisture can cause an increase in air permeability, which is due to the migration of water molecules in the structure. The voids are distributed throughout the caster body, and even if the moisture content is appropriate during the pouring process, small, uniform pores may be formed in the matrix particles. Data obtained using the drainage method confirm that increasing moisture changes the structure. porosity rather than its pore structure.
(3) Effect of binding agent on breathability
The influence of binders on air permeability is mainly due to the different types of binders and the changes in binders during the baking process. Murilo D.M.innocentini was studying the effects of various hydration binders on the air permeability of high-aluminum castables, and compared the effects of aluminate cement and hydrated alumina on the air permeability of two binders. The air permeability measurement test was conducted on the dried samples and it was found that the air permeability constant k1 of the aluminate cement bonded castable was 6 times that of the hydrated alumina bonded castable, and the air permeability constant k2 was 115 times that of the hydrated alumina bonded castable. This larger gap arises from the characteristics of the binder. The main components of the hydration product of hydrated alumina are bayerite and a large amount of gel. The gel can effectively fill the matrix voids and reduce air permeability. In addition to their effect on gas permeability channels in the matrix, hydration binders may also have an effect on the thickness of the aggregate-matrix interface. Compared with aluminate cement, hydrated alumina particles are smaller, and more alumina gel is formed during the hydration process. The gel wraps the aggregate-matrix interface area more densely, resulting in its air permeability being significantly lower than that of aluminate cement. Breathability of salt cement bonded castables.
(4) Effect of explosion-proof agent on breathability
At present, in order to improve the burst resistance, several admixtures commonly used at home and abroad include metal aluminum powder, azodicarbonamide, aluminum lactate and organic fibers. The purpose is to increase the open pores or fine cracks inside the dense castable. To improve the breathability and allow the water vapor in it to be easily discharged, thereby improving the anti-burst performance.
When metal aluminum powder is added to the castable and mixed with water, the following reaction occurs, accompanied by a large amount of heat, to produce aluminum hydroxide gel and hydrogen gas.
This reaction is slow at room temperature, but if it coexists with a base, complex ions are generated, which are soluble in water, so the reaction can be rapid and almost complete.
During the reaction between metal aluminum powder and water, a large amount of gas will be produced, which can be discharged through certain channels.
The generated H2 forms a large number of open pores while escaping, which improves the air permeability and provides a channel for the drying and drainage of the castable.
