Effect of Maintenance Temperature on the Medium-Temperature Properties of Corundum Castables
Calcium aluminate cement (CAC) is widely used in refractory castables due to its early strength development, which provides high demoulding strength, as well as its excellent resistance to erosion and thermal shock. The hydration properties of CAC are influenced by several factors, such as temperature, humidity, water-cement ratio, micronutrients, and additives. Among these, temperature not only affects the type of CAC hydration products formed but also impacts the hydration process (i.e., the dissolution–precipitation mechanism), which in turn may influence the properties of the castables.
During heating, cement hydration products undergo dehydration and decomposition reactions, which damage the internal structure of the castables and lead to significant strength loss between 400–1000 °C. Due to the temperature gradient within the castables during the drying and service processes, the castable lining is prone to spalling between 400–1100 °C. Additionally, since castables are installed throughout the year, construction temperatures can vary widely.
At present, there is a lack of research on how construction at different ambient temperatures affects the medium-temperature performance of castables. In this study, the effect of curing temperatures (5, 10, 25, 40, and 50 °C) on the properties and phase composition of calcium aluminate cement-bonded corundum castables after medium-temperature heat treatment was investigated.
Test
The main raw materials for specimen preparation test are: plate corundum particles(6～3,3～1,1～0.5和≤0.5mm,w(Al₂O₃)≥99.5%),Plate corundum fines (≤0.045 mm, w(Al₂O₃) ≥99.1%), reactive α-Al₂O₃ micropowder (d50 = 2.41 μm, w(Al₂O₃) ≥99.8%); the binding agent was calcium aluminate cement (Secar71), and the water reducing agent was FS10.
According to the material dosages listed in Table 1, the raw materials were first placed into plastic bags and hand pre-mixed. The pre-mixed materials and water were then placed in a constant temperature and humidity curing chamber at different temperatures (5, 10, 25, 40, and 50 °C) for 24 hours.
After initial curing, the materials were dry-mixed for 1 minute using an NRJ-411A-type cement mortar mixer, followed by wet mixing for 4 minutes after the addition of water. The resulting mixture was poured into stainless steel molds measuring 25 mm × 25 mm × 150 mm, and formed by vibration using a vibration table.
The molded specimens were then placed into an HWS-350X-type constant temperature and humidity curing chamber at different temperatures (5, 10, 25, 40, and 50 °C) and 100% relative humidity for 3 days. After demolding, the samples were dried at 110 °C for 24 hours, followed by heat treatment at 800 °C and 1000 °C for 3 hours, respectively, for subsequent testing.
To facilitate phase composition analysis of the castable matrix, the aggregate was removed from the formulation, and the matrix specimens were prepared under the same conditions.
Performance testing was conducted as follows: the apparent porosity of the specimens was measured according to GB/T 2997-2000; the room-temperature flexural strength was tested according to GB/T 3001-2007; and the room-temperature compressive strength was determined in accordance with GB/T 5072-2008. The phase composition of the matrix specimens was analyzed using a Bruker AXS D8 Focus X-ray diffractometer.
Results and analysis
1.Physical phase composition of matrix after treatment at different temperatures
The XRD patterns of matrix specimens maintained at different temperatures after drying at 110°C are shown in Fig. 1.
It can be seen that there are still obvious residues of CaO–Al₂O₃ (CA) in the matrix specimens after curing at 5, 10, and 25 °C. The intensity of the CA diffraction peaks decreases significantly with increasing curing temperature. In the matrix specimens cured at 40 and 50 °C, no diffraction peaks of CA or CaO·2Al₂O₃ (CA₂) were observed after drying at 110 °C. Instead, distinct diffraction peaks corresponding to 3CaO·Al₂O₃·6H₂O (C₃AH₆), Al₂O₃·3H₂O (AH₃), and C₃AHₓ were present. This is because the hydration of CAC is a dissolution–precipitation process, and the degree of hydration increases as the rate of dissolution and precipitation accelerates with higher curing temperatures, thereby promoting the hydration of CAC.
Figure 2 shows the XRD patterns of matrix specimens after heat treatment at 1100 °C for 3 hours under different curing temperatures. It can be observed that after curing at 5, 10, and 25 °C, the main phases in the matrix specimens are corundum and CA₂, with weak diffraction peaks of CA still present. In contrast, after curing at 40 and 50 °C, the matrix specimens contain only corundum and CA₂, with no observable CA diffraction peaks.
As shown in Figure 1, the degree of CAC hydration increases with higher curing temperatures. Since the hydration of CAC is a dissolution–precipitation process, the presence of alumina micropowder provides nucleation sites for hydration products, promoting CAC hydration. Additionally, it may reduce the particle size of residual CAC, leading to a more uniform distribution of hydration products.
The increased contact area between hydration products and Al₂O₃ promotes the in situ formation of CA₂ from the reaction between Al₂O₃ and CaO (the calcium source), following the dehydration and decomposition of hydration products during heat treatment. Therefore, higher curing temperatures likely contribute to the reduction in residual cement particle size, enhance the uniformity of hydration product distribution, and facilitate the in situ formation of CA₂
2.Physical properties of castable specimens after different temperature treatment
Figure 3 illustrates the apparent porosity of the specimens after different temperature treatments.
It can be seen that:
The apparent porosity of the castable specimens after baking at 110 °C decreases with increasing curing temperature.
The apparent porosity of the specimens heat-treated at 800 °C and 1100 °C is higher than that of those baked at 110 °C. Furthermore, the apparent porosity of the specimens heat-treated at both 800 °C and 1100 °C increases as the curing temperature rises. The apparent porosity of the specimens treated at 800 °C is slightly higher than that of those treated at 1100 °C.
The apparent porosity of the specimens heat-treated at 800 °C is higher than that of those heat-treated at 1100 °C.
With increasing curing temperature, the degree of CAC hydration increases (see Fig. 1), producing less dense hydration products and AH₃ that fill the pores, thereby reducing apparent porosity. Therefore, the apparent porosity of the dried specimens decreases with increasing curing temperature.
During baking, the decomposition of hydration products generates pores and microcracks (corresponding to volume reductions of 43% and 60% after dehydration of C₃AH₆ and AH₃, respectively), which increases the apparent porosity of the castables. Consequently, the apparent porosity of specimens heat-treated at 800 °C and 1100 °C is higher than that of specimens baked at 110 °C.
More hydration products form at elevated curing temperatures, and their decomposition during heat treatment produces additional porosity and microcracks. Thus, the apparent porosity of the castable specimens after heat treatment at 800 °C and 1100 °C increases with curing temperature.
The apparent porosity of the specimens after heat treatment at 1100 °C decreases slightly compared to those treated at 800 °C, possibly due to the formation of CA₂, which is accompanied by a volume expansion of approximately 13.6%.
Figure 4 shows the room-temperature flexural and compressive strengths of the castable specimens after curing and heat treatment at different temperatures. It can be seen that:
The demolding strength and strength after heat treatment of the castables increase with increasing curing temperature. Between curing temperatures of 5 and 25 °C, the increase in both demolding strength and strength after heat treatment is not very significant. However, castable specimens cured at 40 °C and 50 °C show a much greater increase in strength compared to those cured at lower temperatures.
The strength of the castable specimens decreases as the heat treatment temperature increases from 110 °C to 800 °C, and continues to decrease when the heat treatment temperature increases from 800 °C to 1100 °C.
With increasing curing temperature, the degree of CAC hydration and the amount of hydration products increase, which improves the cementation between components in the castables. As a result, the demolding strength and drying strength of the castables increase with higher curing temperatures. However, the decomposition of hydration products during heat treatment produces pores and microcracks, causing the strength of specimens heat-treated at 800 °C to decrease compared to that of specimens after drying.
The in situ formation of CA₂ in specimens heat-treated at 1100 °C enhances the bonding between components. According to Figure 2, it is hypothesized that as the curing temperature increases, the amount of hydration products increases and their distribution becomes more uniform, while the size of residual CAC particles decreases. The CA and CA₂ generated by the in situ reaction between calcium sources (CaO), decomposed from hydration products during heat treatment, and Al₂O₃ from the matrix and the edges of the aggregates increase the bonding strength within the matrix and between the matrix and aggregates. This leads to improved strength at medium temperatures.
Therefore, the medium-temperature strength of the castable specimens increases with curing temperature.
The schematic diagram illustrating the effect of curing temperature on the distribution of hydration products and the in situ synthesis of CAC is shown in Figure 5. It can be seen that when the casting material is cured at a lower temperature, the amount of calcium aluminate cement hydration is small, and the hydration products are poorly dispersed. When the casting material is cured at a higher temperature, the amount of hydration products increases and they are more uniformly distributed throughout the matrix and along the edges of the aggregates through the dissolution–precipitation process. This reduces the residual particle size of the CAC and improves the demolding and drying strength of the castables. During baking, the hydration products undergo dehydration and decomposition and then react in situ with Al₂O₃ in the matrix and at the edges of the aggregates to form CA and CA₂. This enhances the bonding strength between the aggregates and the matrix, thereby increasing the medium-temperature strength of the castables.
CONCLUSION
The amount of hydration products of calcium aluminate cement increases with rising curing temperature. Due to the dissolution–precipitation process of CAC hydration, this promotes the uniform distribution of hydration products and reduces the residual particle size of CAC.
As the curing temperature increases, the demolding strength and drying strength of the castables improve, indirectly reflecting the increased hydration degree of calcium aluminate cement and the more uniform distribution of hydration products.
The increase in curing temperature promotes the formation and uniform distribution of CA₂ during heat treatment, which leads to an increase in the medium-temperature strength of the castables.