Five main factors affecting the rheological properties of refractory castables
author: XINTAI
2025-08-19
01. Particle size distribution and particle shape
In castable materials, coarse particles flow along with the slurry and have poor fluidity, while fine powder and fine particles have better fluidity. That is, the role of coarser particles in the castable material is an obstructive effect, especially the obstructive effect of coarser particles is greater.
The most authoritative theory on the particle size gradation of unshaped refractory materials is Andreassen's particle size distribution theory. It can be expressed by the following formula:
CPFT=(d/D)q×100%(1)
In the formula: CPFT - the cumulative percentage (volume) less than a certain particle size; d- Minimum Particle size; D- Maximum particle size: q- Distribution coefficient.
The particle size distribution is calculated based on the volume percentage of the multi-component system. If the Andreassen distribution is plotted as a logarithmic curve, the particle size distribution tends to be a straight line, and the slope of the straight line is represented by q. For castable materials, in order to achieve the best stacking method, the q value should be between 0.2 and 0.3. By using a lower q value, there will be more fine powder in the material. These fine powders, as fillers and lubricants, can reduce the friction between coarse particles, thereby achieving good fluidity. The particle size composition, especially the matrix part, plays an important role in improving the fluidity of castable materials. At a low shear rate (2 revolutions per second), when q=0.21, the torque value of the castable is 0.2N.m and the self-flow value is 114%. When q=0.26, the torque value is 0.2N.m and the self-flow value is 60% of q=0.21. When q=0.31, the torque value is 1.6N.m. With the increase of q value, the shear stress of the castable increases and the flow value decreases.
In low-cement bauxite-based self-flowing castables, as the particle size distribution decreases (q values range from 0.29, 0.26 to 0.23), the shear stress, flow resistance and shear viscosity of the samples all show an upward trend, and the particle size distribution significantly affects the rheological properties of the castables.
The research on the flow characteristics of corundum-spinel self-flowing castable shows that the proportion of coarse, medium and fine particles in the aggregate should be appropriate. When the ratio of coarse: medium: fine is 10:30:20, the fluidity is better. The ratio of aggregate to powder has a significant impact on fluidity. When the ratio of aggregate to powder is 55:45, the fluidity is better. In self-flowing materials, if there is a large amount of less than 1mm, the fluidity is good, while if there is too much powder, the water demand will be high. Therefore, increasing the particle composition by 1 to 0mm is beneficial for improving fluidity. The particle size composition range with good fluidity is: >1mm35~50%, 1 ~ 0mm15~30%, <0.088mm35~50%.
In castable materials, coarse particles flow along with the slurry and have poor fluidity, while fine powder and fine particles have better fluidity. That is, the role of coarser particles in the castable material is an obstructive effect, especially the obstructive effect of coarser particles is greater.
The most authoritative theory on the particle size gradation of unshaped refractory materials is Andreassen's particle size distribution theory. It can be expressed by the following formula:
CPFT=(d/D)q×100%(1)
In the formula: CPFT - the cumulative percentage (volume) less than a certain particle size; d- Minimum Particle size; D- Maximum particle size: q- Distribution coefficient.
The particle size distribution is calculated based on the volume percentage of the multi-component system. If the Andreassen distribution is plotted as a logarithmic curve, the particle size distribution tends to be a straight line, and the slope of the straight line is represented by q. For castable materials, in order to achieve the best stacking method, the q value should be between 0.2 and 0.3. By using a lower q value, there will be more fine powder in the material. These fine powders, as fillers and lubricants, can reduce the friction between coarse particles, thereby achieving good fluidity. The particle size composition, especially the matrix part, plays an important role in improving the fluidity of castable materials. At a low shear rate (2 revolutions per second), when q=0.21, the torque value of the castable is 0.2N.m and the self-flow value is 114%. When q=0.26, the torque value is 0.2N.m and the self-flow value is 60% of q=0.21. When q=0.31, the torque value is 1.6N.m. With the increase of q value, the shear stress of the castable increases and the flow value decreases.
In low-cement bauxite-based self-flowing castables, as the particle size distribution decreases (q values range from 0.29, 0.26 to 0.23), the shear stress, flow resistance and shear viscosity of the samples all show an upward trend, and the particle size distribution significantly affects the rheological properties of the castables.
The research on the flow characteristics of corundum-spinel self-flowing castable shows that the proportion of coarse, medium and fine particles in the aggregate should be appropriate. When the ratio of coarse: medium: fine is 10:30:20, the fluidity is better. The ratio of aggregate to powder has a significant impact on fluidity. When the ratio of aggregate to powder is 55:45, the fluidity is better. In self-flowing materials, if there is a large amount of less than 1mm, the fluidity is good, while if there is too much powder, the water demand will be high. Therefore, increasing the particle composition by 1 to 0mm is beneficial for improving fluidity. The particle size composition range with good fluidity is: >1mm35~50%, 1 ~ 0mm15~30%, <0.088mm35~50%.
02. Cement addition amount
The amount of cement added has a significant impact on the fluidity of the castable. Calcium aluminate cement requires an appropriate amount of water during the formation of hydration products. When the amount of cement added is the same, a higher amount of cement added will inevitably reduce the amount of free water and thus lower the fluidity of the castable. However, adding too little cement will affect the strength of the castable at low temperatures. Therefore, under the premise of ensuring the strength of the castable, the amount of cement should be appropriately reduced. In ultra-low cement castables, cement mainly serves as a delayed setting accelerator. The study on the rheological properties of the corundum-spinel-calcium aluminate suspension shows that with the increase of the addition amount of calcium aluminate cement, both the yield stress and plastic viscosity of the suspension show an increasing trend.
The amount of cement added has a significant impact on the fluidity of the castable. Calcium aluminate cement requires an appropriate amount of water during the formation of hydration products. When the amount of cement added is the same, a higher amount of cement added will inevitably reduce the amount of free water and thus lower the fluidity of the castable. However, adding too little cement will affect the strength of the castable at low temperatures. Therefore, under the premise of ensuring the strength of the castable, the amount of cement should be appropriately reduced. In ultra-low cement castables, cement mainly serves as a delayed setting accelerator. The study on the rheological properties of the corundum-spinel-calcium aluminate suspension shows that with the increase of the addition amount of calcium aluminate cement, both the yield stress and plastic viscosity of the suspension show an increasing trend.
03. Types and dosages of micro-powders
The micro-powder in the castable is prone to form micelles with a double charge layer when it comes into contact with water. Due to the dispersion effect of the electrolyte and surfactant, the particles do not agglomerate with each other. After adding the dispersant, the ξ potential is increased through ion exchange, making the micelles larger. Thus, under the same water consumption, the fluidity of the castable can be improved, while maintaining the same fluidity will reduce the amount. Therefore, the use of micro-powder reduces water consumption, lowers the porosity of the castable sample, and enables the castable to obtain a more uniform and dense microstructure. SiO2 micro-powder is amorphous silica, featuring a large specific surface area and surface energy, as well as a small particle size. It can more effectively fill the voids of particles and displace free water, which plays a lubricating role. In addition, under the same conditions, SiO2 micro-powder forms micelles with a double charge layer in water. Under the same conditions, compared with α-Al2O3 micro-powder, it has a larger ξ potential (-69mV), and the corresponding ξ potential of α-Al2O3 micro-powder is. Therefore, SiO2 micro-powder has a better water-reducing effect and fluidity.
The micro-powder in the castable is prone to form micelles with a double charge layer when it comes into contact with water. Due to the dispersion effect of the electrolyte and surfactant, the particles do not agglomerate with each other. After adding the dispersant, the ξ potential is increased through ion exchange, making the micelles larger. Thus, under the same water consumption, the fluidity of the castable can be improved, while maintaining the same fluidity will reduce the amount. Therefore, the use of micro-powder reduces water consumption, lowers the porosity of the castable sample, and enables the castable to obtain a more uniform and dense microstructure. SiO2 micro-powder is amorphous silica, featuring a large specific surface area and surface energy, as well as a small particle size. It can more effectively fill the voids of particles and displace free water, which plays a lubricating role. In addition, under the same conditions, SiO2 micro-powder forms micelles with a double charge layer in water. Under the same conditions, compared with α-Al2O3 micro-powder, it has a larger ξ potential (-69mV), and the corresponding ξ potential of α-Al2O3 micro-powder is. Therefore, SiO2 micro-powder has a better water-reducing effect and fluidity.
04. Types of dispersants and their dosages
When powder is mixed with water, some grid structures will be formed, which is unfavorable for fluidity. The possible reasons for the emergence of grid structures might be:
(1) Due to the different charges carried by cement during the hydration process, it is formed by the mutual attraction of opposite charges:
(2) Due to the movement of charged particles in the solution, they collide, adsorb and attract each other, causing:
(3) It is caused by the Van derWaals force potential energy between particles and the repulsive force potential energy of the double electric layer.
After the addition of the dispersant, the dispersant dissociates anionic groups, which interact with the charged particles in the mud, increasing the electrostatic repulsive potential energy between the particles and reducing the VanderWaals gravitational potential energy between the particles. On the other hand, when the particles in the dispersant adsorption layer approach each other, the overlap of the adsorption layers will generate a new repulsive potential energy, which prevents particle aggregation and destroys the grid structure, thereby improving the fluidity of the castable.
The types and dosages of dispersants have varying degrees of influence on the rheological properties of mud suspensions. The macroscopic manifestation is that the shear stress and viscosity vary with the type and dosage of the dispersant. The most commonly used water reducers are surface-active substances, which are divided into ionic and nonionic water reducers. Ionic water reducers ionize into anionic groups in water and adsorb onto the surface of micro-powder colloidal particles. Nonionic water reducers do not ionize into ionic groups in water. Their hydrophilic groups are mainly composed of a certain number of oxygen-containing groups (generally ether groups and hydroxyl groups), which have a strong active effect. They adsorb onto the surface of colloidal particles through physical adsorption, especially on the surface of ultrafine powder, changing the electric potential of the colloidal particle surface and increasing the repulsive force between the colloidal particles. Due to the repulsive force, the aggregates of the ultrafine powder and the network structure formed between the cement particles are disrupted, thus releasing the encapsulated free water. Excessive addition of dispersant can have adverse effects. Excess dispersants will form micelles, increasing the viscosity of the solution. At the same time, they ionize to produce cations that compress the double electric layer, making it thinner, reducing the ξ potential, weakening the repulsive force between particles, and re-forming the flocculation structure, with free water being encapsulated again. Therefore, excessive dispersants not only fail to exert their dispersion effect but may even introduce impurities, affecting the physical and chemical properties of the castable.
For different systems, the types of dispersants applicable are not the same. It is necessary to conduct comparative studies through experiments to select the appropriate dispersant.
When powder is mixed with water, some grid structures will be formed, which is unfavorable for fluidity. The possible reasons for the emergence of grid structures might be:
(1) Due to the different charges carried by cement during the hydration process, it is formed by the mutual attraction of opposite charges:
(2) Due to the movement of charged particles in the solution, they collide, adsorb and attract each other, causing:
(3) It is caused by the Van derWaals force potential energy between particles and the repulsive force potential energy of the double electric layer.
After the addition of the dispersant, the dispersant dissociates anionic groups, which interact with the charged particles in the mud, increasing the electrostatic repulsive potential energy between the particles and reducing the VanderWaals gravitational potential energy between the particles. On the other hand, when the particles in the dispersant adsorption layer approach each other, the overlap of the adsorption layers will generate a new repulsive potential energy, which prevents particle aggregation and destroys the grid structure, thereby improving the fluidity of the castable.
The types and dosages of dispersants have varying degrees of influence on the rheological properties of mud suspensions. The macroscopic manifestation is that the shear stress and viscosity vary with the type and dosage of the dispersant. The most commonly used water reducers are surface-active substances, which are divided into ionic and nonionic water reducers. Ionic water reducers ionize into anionic groups in water and adsorb onto the surface of micro-powder colloidal particles. Nonionic water reducers do not ionize into ionic groups in water. Their hydrophilic groups are mainly composed of a certain number of oxygen-containing groups (generally ether groups and hydroxyl groups), which have a strong active effect. They adsorb onto the surface of colloidal particles through physical adsorption, especially on the surface of ultrafine powder, changing the electric potential of the colloidal particle surface and increasing the repulsive force between the colloidal particles. Due to the repulsive force, the aggregates of the ultrafine powder and the network structure formed between the cement particles are disrupted, thus releasing the encapsulated free water. Excessive addition of dispersant can have adverse effects. Excess dispersants will form micelles, increasing the viscosity of the solution. At the same time, they ionize to produce cations that compress the double electric layer, making it thinner, reducing the ξ potential, weakening the repulsive force between particles, and re-forming the flocculation structure, with free water being encapsulated again. Therefore, excessive dispersants not only fail to exert their dispersion effect but may even introduce impurities, affecting the physical and chemical properties of the castable.
For different systems, the types of dispersants applicable are not the same. It is necessary to conduct comparative studies through experiments to select the appropriate dispersant.
05. Add water volume
Castable materials are particularly sensitive to the amount of water added. If the amount of water added is insufficient or the flow value is too small, the material cannot flow and fill normally, and the bubbles cannot be discharged. Excessive water addition can cause the aggregates to sink, leading to the separation of the aggregates from the matrix. Moreover, excessive moisture can affect the density and strength of the material, as well as its erosion resistance and wear resistance, etc.
Castable materials are particularly sensitive to the amount of water added. If the amount of water added is insufficient or the flow value is too small, the material cannot flow and fill normally, and the bubbles cannot be discharged. Excessive water addition can cause the aggregates to sink, leading to the separation of the aggregates from the matrix. Moreover, excessive moisture can affect the density and strength of the material, as well as its erosion resistance and wear resistance, etc.
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Henan Xintai Refractory Materials Co., Ltd. manufactures monolithic refractory materials that are constructed using ramming (manual or mechanical) and hardened by heating above room temperature. These materials are composed of graded refractory aggregate, powder, binder, and admixtures, mixed with water or other liquids. Ramming materials include high-alumina, clay, magnesia, dolomite, and zirconium.
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Admixtures are substances that, when added in appropriate amounts to the batching of refractory materials, can improve the physical properties, formability or construction performance, and service performance of refractory materials. They are also known as additives.
Common admixtures for unshaped refractory materials