Castable vs Plastic Refractory: Which Performs Better in Heating Furnaces?
Plastic refractory is a type of monolithic refractory material. It is a cohesive refractory material in a plastic, mud-like or dough-like form, produced by thoroughly mixing refractory aggregates, fine clay powder, water, and chemical binders in fixed proportions. It is applied by ramming, vibrating, and similar methods.
Classification of Plastic Refractories
High-alumina plastic refractory: Suitable for furnace roofs, walls, burner blocks, and other parts operating at long-term service temperatures above 1400°C.
Fireclay plastic refractory: Suitable for furnace roofs, walls, burner blocks, and other parts operating at long-term service temperatures below 1400°C.
Plastic patching mix: Used for repairing various parts of heating furnace linings.
At present, plastic refractories are mainly used as lining and roof materials for various furnaces and kilns in the iron and steel industry. This paper compares and analyzes the differences and advantages between plastic refractories and castables, and systematically examines their applications in heating furnaces.
Properties of Plastic Refractories
01. "Zero" Expansion Design
Plastic refractories adopt the "zero" expansion design principle, resulting in very low internal stresses, such as mutual "tension" or "compression," at high temperatures. The furnace lining remains stable and exhibits excellent integrity.
In contrast, PVC expansion joint boards must be embedded during castable construction, which leads to poor integrity of the furnace lining or furnace floor. The relationship between expansion joint width and temperature over a length of 1500 mm is shown in Figure 1.
It can be seen from Figure 1 that the expansion joint width of castables is zero at 800 °C. When the temperature exceeds 800 °C, compressive stress develops inside the castable refractory, which is detrimental to the overall stability of the material.
In comparison, the expansion joint width of plastic refractory is zero at 1300 °C, a temperature very close to the normal operating temperature of metallurgical heating furnaces. Therefore, the use of plastic refractory ensures minimal internal stress within the refractory material during normal operation of the heating furnace.
02. Excellent Thermal Shock Resistance
Plastic refractory can withstand rapid temperature fluctuations in furnaces and flame impact. It allows frequent furnace shutdowns and reheating without concern for lining spalling. Its thermal shock resistance is 3 to 6 times that of ordinary castables.
Accordingly, heating furnaces with fully plastic refractory linings permit rapid heating and cooling during daily operation, whereas castables may suffer from spalling and similar failures. A comparison photograph of the 1300 °C water-cooling test for thermal shock resistance between ordinary low-cement castables and plastic refractories is shown in Figure 2.
03. Rapid Curing & Heating
Plastic refractories are suitable for construction in all seasons. No curing is required after installation, and heating can be started immediately. The heating rate is 50 °C per hour, and the furnace temperature required for production can generally be reached within 24 hours, greatly shortening the heating time and reducing heating costs.
In contrast, castables require a waiting period for setting and curing after construction, followed by a lengthy heating process. They are not suitable for winter construction, and strict frost protection measures must be observed. A comparison of the heating curves of castables and plastic refractories is shown in Figure 3.
04. Good Chemical Corrosion Resistance
Plastic refractories do not contain cement and use neutral materials as binders, offering strong resistance to both acids and alkalis. Therefore, they place no special requirements on fuel types.
Figures 4 and 5 show a comparison of the corrosion resistance of furnace roofs made of castables (left) and plastic refractories (right) after one year of service.
05. Reasonable Construction Technology
Plastic refractories adopt construction techniques involving preloading and form-pulling during installation.
Especially in furnace roof construction, preloading and form-pulling can ensure that deformation of the furnace steel structure is essentially synchronized with that of the furnace lining, so that stress on anchor blocks is uniform, avoiding possible breakage and roof collapse during service.
In contrast, castables are constructed with formwork support. After formwork removal in furnace walls and roofs, secondary deformation occurs, resulting in uneven stress on anchor blocks and easily leading to collapse and other accidents during operation.
A comparison of the stress distribution of castables and plastic refractories during construction is shown in Figures 6 and 7.
05. Good Construction Performance
Plastic refractories exhibit excellent construction performance, and the formwork can be removed immediately after installation is completed.
Especially in the construction of complex furnace roofs, good construction performance is a prerequisite for achieving long service life of the furnace roof.
The construction of anchor blocks and the ramming process of plastic refractories are shown in Figures 8 and 9.
07. Excellent Stability of Construction Quality
Compared with castables, plastic refractories exhibit better quality stability.
Plastic refractories can be used directly out of the package on site. Their mixing ratio is strictly controlled during production, and human factors have little impact on construction quality during repeated ramming operations, so the quality of the furnace lining can be easily ensured.
In contrast, castables require on-site control of multiple processes, including the addition of additives, dry mixing, wet mixing, and vibration, each of which can affect the quality of the castable furnace lining.
Construction procedure of plastic refractory:
Unpacking and direct use → Formwork pulling → Ramming → Formwork removal → Grinding → Direct furnace drying
Construction procedure of castable:
Castables + additives + water → Proportional batching control → Formwork erection → Mixing time control → Casting → Vibration → Curing → Formwork removal → Furnace drying
08. Good Economic Efficiency in Application
(1) The service life of plastic refractories is generally longer than that of castables, typically exceeding 10 years.
(2) Compared with castables, the bulk density of plastic refractories after drying is approximately 10% lower, which makes the furnace roof lighter and improves the safety of the steel structure.
(3) Plastic refractories have low thermal conductivity and good thermal insulation performance, resulting in energy savings.
Conclusion
Compared with castables, plastic refractories require a relatively long construction period. However, they offer outstanding advantages such as excellent spalling resistance, strong resistance to rapid temperature fluctuations, and long service life, making them particularly suitable for furnace roof applications.
Some large-scale continuous heating furnaces in China, such as those in the hot rolling mills of WISCO and Baosteel, use plastic refractories for their furnace roofs. Plastic refractories are also used for the furnace walls of the 1580 hot rolling line heating furnace at Baosteel.