The refractory castable is a disordered composite consisting of refractory aggregates, ultrafine powder, and binder, which is produced using self-flow/vibration molding processes. It has high-temperature stability at 1600℃+, and is suitable for rapid construction of the lining in metallurgical kilns, with both thermal shock resistance and resistance to slag erosion characteristics.
Aggregates Mixing Proportions
For various particle sizes of aggregate materials, mixing is carried out according to the principle of the closest packing.
Since castables are mostly used to form various large cross-section structures and large blocks, the maximum particle size of granular materials can be correspondingly increased. However, to avoid excessive expansion differences between coarse particles and cement stone during heating, which may destroy the bond between them, apart from selecting aggregates with low expansion, the maximum particle size should also be appropriately controlled. It is generally believed that for vibration molding, it should be controlled below 10-15mm; for machine pressing, it should be less than 10mm, and for large products or integral structures, it should not exceed 25mm; all should be less than 1/5 of the smallest cross-sectional dimension.
The general mixing ratio of aggregates is usually 3 to 4 grades, with the total content of granular materials accounting for 60% to 70%.
Highly fine powdered admixtures that are stable in volume at high temperatures, especially those containing a portion of ultrafine powder, have a positive effect on the normal and high-temperature properties of the castables, and should be proportioned appropriately. It is generally considered that the amount of fine powder should be between 30% to 40%.
Binder and Setting Accelerant Determination
The type of binder depends on the requirements for the properties of the structure or product and should correspond to the selected granular and powdered materials, as well as relate to the construction conditions.
For castables made from non-alkaline granular materials, cement is generally used as the binder. When using cement as the binder, consideration should be given to the requirements for both the normal and high-temperature properties of the hardened body, preferably selecting fast-hardening, high-strength cement with a low content of fusible substances, and the dosage should be appropriate, generally not exceeding 12% to 15%. To avoid the reduction of the medium-temperature strength of the hardened body and to improve its high-temperature performance, the amount of cement should be minimized and replaced with ultrafine powder admixtures.
If phosphoric acid or phosphates are used as binders, the concentration of the diluted phosphoric acid or phosphate solution should be selected based on the properties required for the hardened castable and the construction characteristics. A phosphoric acid concentration of about 50% is considered suitable, with the additional amount generally controlled between 11% to 14%. If aluminum phosphate is used as the binder, with an Al2O3/P2O5 molar ratio of 1:3.2 and a relative density of 1.4, the additional amount should be controlled at around 13%. The castables made with this kind of binder will have slow setting and low strength before heat treatment, so a small amount of slightly alkaline material is often added to promote its curing. If ordinary high-alumina cement is used as a setting accelerator, the general additional amount is 2% to 3%.
If water glass is used, its modulus and density should be controlled. When the modulus is between 2.4-3.0 and the relative density is between 1.36-1.40g/cm³, the general dosage is 13% to 15%. If sodium silicate is used as a hardening agent, its amount is usually 10% to 12% of the water glass.
Other binders and their dosages are determined based on the characteristics of the lean materials, the properties required for the hardened body, and the construction requirements.
Water Content
Various castables contain moisture corresponding to the amount of binder. Water can be added after the binder and lean materials have been mixed, as is often the case with calcium aluminate cement that hydrates and sets quickly; or it can be mixed in advance with the binder to form a solution or sol added, as is mainly the case with binders that require pre-hydrolysis to have bonding properties. When the binder does not deteriorate after reacting with water, it is often mixed with water in advance to ensure even distribution in the castable, as is the case with phosphoric acid and water glass mentioned earlier.
For refractory castables made of silicon-aluminate clinker bonded with ordinary high-alumina cement, the water-cement ratio is generally between 0.4 to 0.65. For those using vibration molding, a ratio of 0.5 to 0.65 is often used, with the mixture's moisture content at 8% to 10%; for machine pressing, a ratio of around 0.4 is common, with the mixture's moisture content at 5.5% to 6.5%. To reduce the moisture in the castables and improve the density of the hardened body, an appropriate amount of plasticizer and water reducer should be added to the castables.
Curing
After the castable has been plasticized, it is necessary to take appropriate measures for curing according to the hardening characteristics of the binder to promote its setting. Calcium aluminate cement should be cured under appropriate temperature and humidity conditions. Specifically, ordinary high-alumina cement should be covered at a lower temperature (less than 35℃) first, then watered or soaked in water after setting for 3 days; low-calcium high-alumina cement should be cured for 7 days or steam-cured for 24 hours. Some inorganic salts require drying and roasting. For those bonded with water glass, they should be stored in an air temperature of 15-25℃ for 3-5 days without moisture, and can also be roasted at below 300℃; however, they must not be cured in潮湿 conditions or watered, as the silica gel swells with water absorption, loses its bonding ability, and strength will also decrease sharply after water leaching.
For phosphates, they can be cured in air above 20℃ for more than 3 days and then roasted at 350-450℃; they must not be exposed to moisture or water before roasting. The curing system for refractory concrete is shown in Table 1.
When castables form the lining and body of thermal equipment, they should generally be roasted before first use to gradually remove the physical and crystalline water, to achieve a stable volume and certain properties at the operating temperature, to a certain degree of sintering. The appropriateness of the sintering system greatly affects the service life. The basic principle for establishing a roasting system is to ensure that the heating rate is compatible with possible dehydration and other phase changes and deformation. During certain temperature stages where these changes occur rapidly, the heating should be slowed down or even held at a constant temperature for a period.
The roasting speed of the hardened body varies depending on the binder and the cross-sectional size of the structure. For cement-based castables, the process can be roughly divided into three stages:
(1) Removal of free water: heated at a rate of 10-20℃/h to 110-150℃, and held for 24-48h.
(2) Removal of crystalline water: heated at a rate of 15-30℃/h to 350℃, and held for 24-48h.
(3) Uniform heating stage: heated at a rate of 15-20℃/h to 600℃, and held for 16-32h; then heated at a rate of 20-40℃/h to the working temperature. For larger cross-section structures, the lower heating rate and longer holding time should be chosen, and vice versa for smaller cross-sections.
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