Rongsheng, as one of the low-cement castable manufacturers, produces castable refractory castables with low cement content that rely on the addition of micro-powders or sols to achieve cohesion and bonding. Rongsheng low-cement refractory castables use oxide or synthetic compound micro-powders or sols with the same chemical composition as the main castable material as the binder, resulting in low impurity content. Without reducing the castable’s refractoriness and resistance to slag erosion, it can self-bond during long-term use, effectively improving the high-temperature structural strength.
Bonding Mechanism of Low-Cement Refractory Castables
There are various bonding methods for low-cement refractory castables. Currently, common methods include silica micropowder (dust silica) bonding, clay bonding, silica sol bonding, and silica-alumina sol bonding. Manufacturers can also choose bonding methods suitable for customer requirements. The setting and hardening mechanism of Rongsheng low-cement castables is as follows: The castable, when mixed with water, first achieves a certain degree of fluidity (or thixotropy) through the addition of dispersants (deflocculants or anti-flocculators) and delayed-acting accelerators. After self-flowing or vibration molding, the castable sets and hardens due to the delayed-acting accelerators.
Low-cement castables can use oxide micropowders alone as binders, or silica sols and alumina sols as binders, or a combination of oxide micropowders and sols as binders. The choice of binder depends on the chemical composition of the aggregates used. For example, corundum castables should use reactive alumina, or alumina micropowder combined with silica micropowder as binders. Aluminosilicate castables can use silica micro powder or silica sol as binders.
Hardening of Low-Cement Refractory Castables
The hardening of low-cement refractory castables requires the addition of a delayed-setting accelerator. This accelerator is a type of agent that slowly hydrolyzes and ionizes in water, releasing counterions with the opposite charge to the surface charge of the micro-powder or colloidal particles. When the adsorbed counterions on the particle surface reach the “isoelectric point,” the particles aggregate and harden through drying.
Performance Characteristics of Low-Cement Refractory Castables
Compared to calcium aluminate cement-bonded castables, low-cement refractory castables have a slower setting and hardening rate, and slightly lower strength after room temperature curing. They are suitable for direct casting into monolithic linings on-site. Their long-term operating temperature is higher than that of sol-bonded castables of the same material. Low-cement castables can be used as linings for high-temperature vessels under more demanding operating conditions, such as monolithic linings for induction furnaces and steel ladles.
Analysis Report on Thermal Shock-Resistant Low-Cement Castables
This experiment utilized ultrafine silica fume and high-efficiency water-reducing agent technology to reduce the amount of cement in refractory castables. This resulted in a dense structure with low porosity, improved mid-temperature strength, and excellent erosion and abrasion resistance. Adding SiC micron powder to low-cement castables already incorporating silica fume significantly impacted the material’s strength and thermal shock resistance.
To investigate the effect of SiC micron powder on the performance of low-cement castables already incorporating silica fume, samples A, B, C, and D were prepared. All four samples used the same aggregate material, dosage, and particle size distribution, with a total fine powder (including micron powder) content of 30%. The fine powder in sample A has a chemical composition of CaO/11.12%, Al2O3/60.16%, and SiO2/28.72%, and is composed of high-alumina cement, grade I bauxite clinker powder, and silica fume. Other samples B, C, and D, while maintaining the same total amount of fine powder, gradually replaced the fine powder in sample A with 325-mesh SiC micro-powder; the specific composition is shown in Table 1. Different amounts of high-efficiency water-reducing agent were added to each sample to ensure the same amount of water was used during molding. Each sample was made into a 4cm × 4cm × 16cm specimen, vibrated, and cured at 40℃ for 24 hours. The number of thermal shock tests after firing at 1450℃ was then measured. The results were compared by directly immersing each sample from 1450℃ to 20℃ cold water without cracking. The experiment also measured the apparent porosity, bulk density, and linear change rate of each sample after calcination at 1450 ℃ for 4 h, and compared the fine powder of each sample.
SiC has good thermal conductivity, and its introduction into low-cement, high-alumina castables is beneficial for improving thermal stability. However, the amount of SiC micropowder added is not large enough to be the main reason for a significant improvement in thermal shock resistance. The thermal shock resistance of a material largely depends on its microstructure. The oxidation of SiC micropowder in the samples increases the porosity of the material. Since the SiC micropowder is uniformly dispersed in the fine powder, the pore distribution formed in the material matrix during calcination is also uniform, which is equivalent to a large number of microcracks uniformly distributed in the matrix. The Hasseman theory states that the more microcracks there are, the shorter the final length reached by crack propagation under the critical temperature difference, and smaller cracks can propagate in a quasi-static manner, avoiding catastrophic fracture. Therefore, the addition of SiC micro powder to the fine powder improves the thermal stability of the castable matrix, which in turn greatly enhances the overall thermal shock resistance of the material.
Rongsheng Low-Cement Castable Manufacturer
The simultaneous addition of SiC micropowder to low-cement high-alumina castables containing silica fume and high-efficiency water-reducing agents significantly improves the material’s thermal shock resistance due to the oxidation of SiC, which causes the fine powder portion of the castable to form a microstructure beneficial to thermal stability at high temperatures. However, the increased porosity delays the sintering process, resulting in a slight decrease in the compressive strength of the SiC-doped samples after firing at 1450 °C. The formation of more needle-like mullite somewhat compensates for this strength reduction.
