Low-cement castables are widely used due to their advantages, such as low water requirement, high density, excellent volume stability, and high strength. However, their relatively poor gas permeability can lead to issues like explosive spalling, surface peeling, or flaking during the initial drying and firing processes. In severe cases, this can result in the complete structural failure of the castable lining, causing significant losses for the user. Consequently, improving the explosion-proof (anti-spalling) performance of low-cement castables is of great importance for their practical application and further development.
Currently, the primary method for enhancing the explosion-proof performance of castables is the addition of anti-spalling fibers, which serve to increase the material’s gas permeability. This paper investigates the effects of anti-spalling fiber content and silicon carbide (SiC) content on the explosion-proof performance of low-cement castables. Furthermore, it examines the influence of anti-spalling fiber content on the porosity of these castables, as well as the impact of silicon carbide content on their thermal shock stability.
Raw Materials for Low-Cement Castables
The castables used in the experiments were low-cement castables. The primary raw materials included 85-grade homogeneous bauxite clinker, 97-grade black silicon carbide, α-Al₂O₃ micropowder, 95-grade silica fume, and Secar 71 cement. Polypropylene fibers were used as anti-explosion fibers.
Specimen Preparation and Performance Testing
The ingredients were proportioned according to the designed formulation. The mixture was dry-mixed in a mixer for 1 minute, followed by the addition of a specific amount of water and wet-mixing for 3 minutes. The resulting material was cast into specimens measuring 40 mm × 40 mm × 160 mm and 230 mm × 114 mm × 65 mm. After curing at 20°C for 24 hours, some specimens were placed directly into a preheated furnace; they were held at the target temperature for 15 minutes and then observed for signs of spalling (explosive spalling). Other specimens underwent heat treatment—at 110°C for 24 hours, 200°C for 24 hours, or 1400°C for 3 hours—and were subsequently tested for apparent porosity and thermal shock resistance. Apparent porosity was measured in accordance with the YB/T 5200 standard, while thermal shock resistance was evaluated using the water-quenching method (in accordance with the YB/T 376.1 standard).
Experimental Results and Discussion
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Influence of Anti-Explosion Fiber Content on the Spalling Resistance and Porosity of Castables
The results of the spalling tests on low-cement castable specimens with varying anti-explosion fiber contents indicate that specimens without these fibers spalled at 600°C. As the content of anti-explosion fibers increased, the spalling temperature of the castable specimens rose to 700°C, 900°C, and above 1000°C, respectively. Furthermore, an increase in the anti-explosion fiber content led to a notable rise in the apparent porosity of the specimens after heat treatment at 200°C for 24 hours. Relevant literature indicates that when anti-explosion fibers are incorporated into castables, they shrink and melt during the heating and drying process. This creates slender, open-ended channels within the material, which increases its gas permeability and accelerates the escape of water vapor. Consequently, this enhances the castable’s resistance to explosive spalling and improves its overall anti-explosion performance.
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Effect of Silicon Carbide Content on the Spalling Temperature and Thermal Shock Stability of Castables
With the content of anti-explosion fibers held constant, the influence of varying silicon carbide (SiC) concentrations on the spalling temperature and thermal shock stability of low-cement castables was investigated.
Experimental results regarding the spalling resistance of castable specimens with different SiC contents indicate that as the SiC concentration increases from 0% to 13%, the spalling temperature of the specimens decreases from 900°C to 800°C, showing a downward trend.
This reduction is primarily attributed to the formulation adjustments: as the SiC content increases, the content of 85-grade homogeneous bauxite decreases (while the combined total of SiC and 85-grade homogeneous bauxite remains constant). Since the thermal conductivity of SiC is higher than that of 85-grade homogeneous bauxite, the overall thermal conductivity of the castable increases as the SiC content rises and the bauxite content falls. Consequently, higher transient steam pressure is generated within the castable at elevated temperatures; once this steam pressure exceeds the tensile strength of the material, spalling occurs. The experimental data show that while the spalling temperature of the low-cement castables exhibits a downward trend as the SiC content increases, the magnitude of this decrease is relatively small.
Regarding thermal shock stability, the test results for specimens heat-treated at 1400°C for 3 hours show a significant improvement: the number of thermal shock cycles the material can withstand increases from 4 to 25 as the SiC content rises. This trend is both pronounced and substantial. There are two primary reasons for this improvement.
- First, there is a difference in the coefficients of thermal expansion (CTE) between the 97-grade SiC and the 85-grade homogeneous bauxite clinker; the 97-grade SiC has a lower CTE, whereas the 85-grade homogeneous bauxite clinker has a higher CTE. This mismatch in the coefficient of thermal expansion leads to the formation of microcracks within the castable. As the silicon carbide content increases—accompanied by a corresponding decrease in the content of the 85-grade homogenized bauxite—the overall coefficient of thermal expansion of the castable is reduced. This, in turn, lowers the thermal stress experienced by the material and improves its thermal shock stability.
- Furthermore, increasing the silicon carbide content while reducing the 85-grade homogenized bauxite leads to a gradual rise in the overall thermal conductivity of the castable. This increase in thermal conductivity plays a significant role in enhancing the material’s thermal shock stability; specifically, higher thermal conductivity helps to reduce the internal thermal stress generated when the castable is subjected to thermal shock, thereby improving its overall thermal shock resistance.
In summary, the following conclusions can be drawn:
- First, increasing the content of explosion-proof fibers significantly contributes to raising the apparent porosity of low-cement castables and improving their explosion-proof performance.
- Second, reducing the silicon carbide content helps improve the spalling resistance of low-cement castables, though the effect is not very pronounced; conversely, increasing the silicon carbide content significantly enhances the thermal shock stability of low-cement castables after heat treatment at 1400°C.
Thermal-Shock-Resistant Low-Cement Castable
Definition: A thermal-shock-resistant low-cement castable is a high-performance refractory material. By optimizing the mix design—reducing the cement content to 4%–8%—it achieves significantly enhanced thermal-shock resistance (the ability to withstand rapid temperature fluctuations), while also offering high strength, low porosity, and excellent high-temperature stability.
Outstanding Thermal-Shock Resistance
Low-Cement Matrix: Minimizes the formation of low-melting-point phases (such as anorthite) caused by CaO at high temperatures, thereby preventing material spalling or cracking during thermal cycling.
Micro-crack Mechanism: Ultrafine powder fillers create a dense structure, where localized micro-cracks effectively absorb thermal stress (with the critical micro-crack propagation depth being less than or equal to the depth of the thermal stress impact).
Optimized Aggregate Grading: Utilizes low-expansion aggregates—such as corundum or mullite (with a coefficient of thermal expansion ≤ 8×10⁻⁶/℃)—that are matched to the thermal expansion characteristics of the matrix, significantly reducing interfacial stress.
Key Aspects of Low-Cement Technology
Cement Substitution: Employs α-Al₂O₃ ultrafine powder (particle size < 1 μm) and silica fume (reactive SiO₂) to form a hydrated bonding network, effectively reducing the risk of high-temperature shrinkage and cracking.
Water Requirement: Requires only 5%–7% water (compared to 10%–15% for conventional castables), which lowers porosity to 13%–18% and substantially increases material density.
Typical Performance Indicators of Thermal-Shock-Resistant Low-Cement Castable
| Items | Typical Value |
| Cement Content | 4%-8% |
| Bulk density | ≥2.8 g/cm³ |
| Flexural Strength at 110℃ | 10-15 MPa |
| Flexural Strength at 1500℃ | ≥25 MPa |
| Thermal Shock Cycles (1100℃ Air-Cooled) | ≥50 cycles without damage (standard materials only last 20–30 cycles) |
| Linear Change rate (1600℃×3h) | ±0.2% |
Thermal-Shock-Resistant Low-Cement Castable Application Scenarios
Environments subject to rapid thermal cycling: Ladle linings, RH vacuum refining vessel covers, firing zones of lime kilns, and linings for hazardous waste incinerators.
High-temperature, high-wear components: Cyclone separators in circulating fluidized bed (CFB) boilers and coal-injection pipes at the discharge ends of cement rotary kilns.
Performance Comparison: Offers a service life more than twice that of conventional high-cement castables (containing 12% CaO) and reduces high-temperature linear shrinkage by 40%.
Key Considerations for Thermal-Shock-Resistant Low-Cement Castable Installation and Curing
Moisture-proof Storage: Ultrafine powders are prone to moisture absorption and clumping; once opened, the material must be used within 3 days.
Water Quality Control: The chloride ion (Cl⁻) content in the mixing water must be <50 ppm to prevent high-temperature corrosion.
Drying/Heating Schedule: Maintain at 110°C for 24 hours → gradual temperature increase to 350°C (at a rate of ≤15°C/h) → final temperature of 600°C (total duration ≥96 hours). Strict adherence is required to prevent cracking caused by rapid dehydration.
