Different Effects of Water Addition, Construction Temperature, and Maintenance Time on the Performance of Cement-Bonded Castables
Cement-bonded castables have been widely used in the iron and steel, cement, petrochemical, and other industries in recent years due to their simple production process and convenient construction. These castable materials exhibit high early strength, stable high-temperature performance, and good overall service life. As a type of monolithic refractory material, they are mainly used in environments subject to wear, erosion, or complex structural requirements.
However, due to the presence of matrix components such as silica micropowder, alumina micropowder, and alumina cement, frequent reactions occur among these constituents. The performance and service life of cement-bonded castables are not only dependent on their intrinsic properties and quality but are also significantly influenced by environmental and construction factors-particularly the amount of water added, the curing environment, and the maintenance duration.
This study investigates the impact of construction conditions on the performance of cement-bonded castables. It compares key performance indicators such as bulk density, apparent porosity, flexural strength, compressive strength, and flow value. The objective is to identify critical parameters that must be controlled during on-site construction to ensure the long-term stability and service life of the castables.
01 Test Process
1. Raw Materials and Proportioning
To reduce performance fluctuations in the castables caused by impurities in the raw materials, this test focuses on high-purity, high-alumina cement-bonded white corundum castables as the research subject. White corundum and brown corundum are used as the main raw materials, with appropriate amounts of aluminate cement, alumina micropowder, and silica micropowder added. The raw material specifications are shown in Table 1.
2. Test Method
The base mixture consisted of 80% (white corundum + brown corundum) aggregate and powder material, 5% aluminate cement, 15% alumina powder and silica powder, and 0.2% water-reducing agent and retarder. The prepared slurry was thoroughly mixed and cast into specimens of 160×40×40 mm and 70×70×70 mm. These specimens were cured at room temperature for 24 hours, demoulded, and then heat-treated at different temperatures as required for subsequent performance testing.
According to the standards GB/T 2997-2000, GB/T 3001-2007, GB/T 5072-2008, GB/T 5988-2007, and YB/T 4117-2003, the specimens were tested for bulk density, apparent porosity, flexural strength, compressive strength, permanent linear change after firing, and resistance to spalling. The workability (fluidity) of the castables was measured using the "jump table method."
02. Results and Discussion
By comparing the comprehensive performance of castables with water additions of 4%, 5%, 6%, 7%, and 8%, the influence of water content on the construction performance of cement-bonded castables was analyzed. This helped to determine the appropriate water addition range to be controlled during on-site construction.
From Figures 1 to 3, it can be observed that when the water content is 4%, the slurry is too dry and unsuitable for construction, while 5% is more appropriate. When the water content exceeds 5%, the fluidity of the castables increases with additional water. However, this is accompanied by a decrease in bulk density, an increase in apparent porosity, and a rapid decline in both flexural and compressive strength. For every 1% increase in water content, the structural strength of the castables decreases by an average of 10%.
This is mainly because the increase in water content enlarges the gaps between the particles and fine powders, reducing the packing density. These gaps are filled with water, which, upon high-temperature sintering, evaporates and forms numerous pores. This results in a decrease in structural strength and reduces the service life of the material.
Additionally, as water content increases, the drying time of the castables becomes longer, necessitating extended curing periods. The water of crystallization cannot be discharged in a short time, increasing the risk of cracking during the baking process.
In summary, although increased water improves the castable's fluidity and speeds up construction, it significantly reduces structural strength and increases the risk of cracking, thereby shortening service life. Therefore, for cement-bonded white corundum castables, 5% water addition is optimal. For other high-alumina cement-bonded castables, the water content should not exceed 7%. To maintain performance, water should not be arbitrarily increased for the sake of construction speed. The water content used must strictly follow the specifications for each castable type to ensure optimal performance.
2. The Influence of Maintenance Temperature on the Performance of Castables
With a constant water addition of 5%, the physical and chemical property changes of the basic formula castables were compared under different maintenance temperatures. This analysis evaluates the impact of maintenance temperature on the construction performance of the castables (see Table 2).
From the data in Table 2, it can be seen that under both low temperature (5 ℃) and high temperature (40 ℃) conditions, the flow value decreases rapidly. The flow value tends to stabilize after 45 minutes and does not change after 60 minutes, indicating that both low and high temperatures affect the construction performance of the castables.
After 30 minutes of curing at 5 ℃, the aggregate and mortar tend to separate during vibration. This is mainly due to the high viscosity of the castable matrix slurry at low temperature and poor dispersion of the micropowder. At 40 ℃, after 45 minutes of curing, accelerated hydration of the cement causes increased water loss. As a result, the mixture begins to partially agglomerate, and the surface of the cast body gradually hardens, slowing the rate of decrease in flow value.
As can be seen from Fig. 4, under experimental conditions of constant humidity, the higher the curing temperature, the greater the strength both before and after baking. Since aluminate cement releases part of its hydration heat during the hydration process, if the curing temperature is too high, the initial and final setting times of the castables become too short. The released hydration heat cannot be fully dissipated in a short time, which adversely affects the construction quality of the castables.
At around 20℃, the initial and final setting times of the slurry are suitable, which improves the workability of the castables and allows the hydration heat to dissipate fully during setting and hardening. Compared with curing at room temperature (20℃), specimens maintained at low temperatures (5℃ and 10℃) have lower flexural and compressive strengths after demoulding. In contrast, specimens cured at higher temperatures (30℃ and 40℃) show little change in strength, but their drying rate is accelerated.
This is because the nucleation and precipitation processes of aluminate cement slow down at low temperatures, inhibiting cement hydration. The resulting calcium aluminate hydrate contains a high water content and low density, leading to lower early strength of the castables. The study results indicate that curing at around 20℃ yields more stable construction performance.
In conclusion, maintenance temperature significantly affects the construction performance of cement-bonded castables, and both low and high temperatures are unfavorable for on-site construction. For high-temperature environments, adding a retarder can extend the working time of the castables. For low-temperature environments, measures such as adding coagulants or early strength agents, or covering the cast body with insulation, can help improve curing temperature and ensure early strength.
3. The Effect of Maintenance Time on the Performance of Castables
Under the same curing temperature and humidity conditions, with 5% water addition, the changes in the physical and chemical properties of castables were compared after demoulding at different maintenance times.
As shown in Figure 5, there is no significant difference in bulk density and apparent porosity of specimens dried at 110°C after 6, 12, and 24 hours of curing. After high-temperature calcination, the bulk densities of the specimens are very similar. However, due to the higher degree of hydration in the specimen cured for 24 hours, its apparent porosity after calcining at 1100°C is greater than that of the specimens cured for 6 and 12 hours.
As the calcination temperature continues to increase, the bulk density and apparent porosity of all three specimens show little change. Therefore, as long as the curing time exceeds 12 hours, the bulk density and apparent porosity of the castables remain relatively stable.
At the same time, after complete removal of water through high-temperature calcination, the linear shrinkage of the specimens decreases with increasing curing time.
As can be seen from Fig. 6, the curing time has little effect on the flexural and compressive strengths of the specimens after drying at 110°C. However, after high-temperature calcination, the flexural and compressive strengths of the specimens cured for 24 hours are slightly higher than those cured for 6 and 12 hours. This indicates that prolonging the in-mould curing time within a certain range does not significantly improve the strength of the castables after drying, but it does have a positive effect on improving the structural strength of the specimens after calcination.
At the same time, burst resistance tests were conducted on specimens maintained for 6, 12, and 24 hours at 400°C, 500°C, and 600°C. The results show that the specimens cured for 6 hours had the most unstable burst resistance: minor bursting occurred at 400°C, while complete bursting occurred at 500°C and 600°C, with less than 20% of the specimen integrity remaining after bursting. Specimens cured for 12 hours showed improved but still unstable burst resistance: no bursting occurred at 400°C, but bursting occurred at 500°C and 600°C, with about 50% specimen integrity remaining. Specimens cured for 24 hours did not burst at 400°C and 500°C, and only slight corner peeling was observed at 600°C.
Due to the fast production pace in most enterprises, the baking process of castable materials may not always allow sufficient low-temperature firing time. This can lead to incomplete discharge of crystallization water within the castable structure and increase the risk of bursting during the baking process. Therefore, the curing time should not be less than 24 hours to ensure stable burst resistance of the castables.
In summary, curing time has little influence on the performance of castables after drying, but it does promote structural strength after calcination. Maintaining the castables for more than 24 hours results in more stable physical and chemical properties. To ensure the baking quality of the construction body and reduce the risk of bursting, the curing time should be extended as much as possible.
03 Conclusion
A water addition of 5 to 7 percent is optimal for cement-bonded castables. Increasing the amount of water significantly reduces the structural strength and service life of the construction body and increases the risk of baking cracks. Under the premise of ensuring that the castables have sufficient fluidity to meet construction requirements, the amount of water should be minimized as much as possible.
Maintaining a room temperature of about 20°C is ideal. Both low and high temperature curing environments are unfavorable for on-site construction of castables. For different ambient temperatures, measures such as adding coagulants, retarders, early strength agents, or covering the cast body with insulation should be taken to ensure sufficient construction time and early strength development.
A maintenance time of more than 24 hours can ensure the stability of the physical and chemical properties of the castables. Prolonging the maintenance time can improve the resistance to cracking during baking and enhance the safety and durability of the castables in use.