Influence of Cement Dosage on Strength Increase of Refractory Castables After Heating Above 800 °C
Calcium Aluminate Cement (CAC) exhibits excellent rheological properties and high early strength, making it the most widely used binder in castables. During the hydration reaction, the hydrates form an interlocking network structure, which gives the castables high early strength. At different curing temperatures, the hydration products of CAC include CAH₁₀, C₂AH₈, C₃AH₆, and AH₃.
During the heating process of CAC-bonded castables in baking, the hydration products undergo dehydration and decomposition reactions, which damage the hydration bonds. Meanwhile, the low-density hydrates are converted into new high-density phases, leading to a reduction in molar volume and an increase in porosity. As a result, the strength of CAC-bonded castables significantly decreases after medium-temperature heat treatment. Different hydration products decompose at different thermal decomposition temperatures: the hydration product C₃AH₆ dehydrates and decomposes between 210–300℃, forming the amorphous aluminate mineral phase C₁₂A₇; AH₃ gel dehydrates and decomposes into AHO(OH) between 210–300℃, and AHO(OH) further dehydrates and decomposes into amorphous alumina at 678℃.
It is generally believed that the dehydration and decomposition of hydration products result in a decrease in the room-temperature strength of calcium aluminate cement-bonded castables after heat treatment at 300–900℃. Compared with low-cement castables or ultra-low cement-bonded castables, high-cement-bonded castables generate more hydration products. However, no research has been conducted on whether the dehydration and decomposition of more hydration products in high-cement-bonded castables lead to a more severe strength loss.
Research results show that the quantity of cement hydration products increases with the rise in cement dosage. After heat treatment at 300℃ for 5 hours, due to the dehydration and decomposition of C₃AH₆ into C₁₂A₇ and AH₃ into AHO(OH), the strength of the castables is lower than that after drying at 110℃. Nevertheless, since the structural morphology of the decomposed hydration products does not change significantly compared with that of C₃AH₆ and AH₃ before dehydration and decomposition, castables with higher cement content maintain relatively high strength after calcination at 300℃.
Compared with calcination at 300℃, the strength of CAC-bonded castables decreases significantly after treatment at 800℃ because AHO(OH) further dehydrates and decomposes into amorphous alumina at 678℃. Even so, the strength of high-cement-bonded castables remains higher than that of low-cement castables after calcination at 800℃. To date, there have been no research reports exploring the fundamental reasons for the strength reduction of castables after calcination at 800℃ from the perspective of the structure of hydration products after decomposition. Moreover, no definitive conclusion has been reached regarding whether higher or lower cement content is beneficial or detrimental to the medium-temperature strength of castables. Therefore, this study investigates the influence of different cement mass fractions on the mechanical properties of castables after calcination at 800℃ for 5 hours, aiming to explore the relationship between the strength of castables after calcination at 800℃ and the phase composition and structural morphology of hydration products.
1. Experiment
1.1 Raw Materials
The raw materials used in the test were as follows: tabular corundum with w(Al₂O₃) ≥ 99% (particle sizes: 6–3, 3–1, 1–0.5, ≤0.5, and ≤0.045 mm); α-alumina micropowder with w(Al₂O₃) ≥ 99% (d₅₀ = 2.14 μm); calcium aluminate cement (d₅₀ = 13.6 μm) as the binder; and a composite water reducer composed of ADS₃ and ADW1.
1.2 Sample Preparation
The raw materials were weighed according to the proportions in Table 1, placed in a plastic bag for premixing for 1 minute, then transferred to a cement mortar mixer for dry mixing for 1 minute. Next, water was added, and the mixture was wet-mixed for 3 minutes. The mixture was vibration-molded on a vibrating table into bar samples with dimensions of 40 mm × 40 mm × 160 mm. The samples were naturally cured for 24 hours before demolding, then dried at 110°C for 24 hours, and heat-treated at 800°C for 5 hours. Castable matrix samples were prepared by the same method, following the mix ratios in Table 2.
1.3 Property Testing
The bulk density and apparent porosity of the fired samples were measured in accordance with GB/T 2997-2000. The cold modulus of rupture and cold crushing strength of the fired samples were tested in accordance with GB/T 3001-2007 and GB/T 5702-2008, respectively. The phase composition and microstructure of the fired samples were analyzed using an X-ray diffractometer (Bruker-AXS D8-Focus) and a field emission scanning electron microscope (FESEM, SIGMA HD, Zeiss, Germany).
2 Results and Discussion
Figure 1 shows the room-temperature strength, apparent porosity, and bulk density of the samples after drying at 110°C for 24 hours and calcination at 800°C for 5 hours. It can be seen that for the samples containing 5 wt% CAC, compared with those dried at 110°C, the room-temperature strength of the samples calcined at 800°C decreases significantly: the modulus of rupture drops from 20.4 MPa to 8.6 MPa, and the cold crushing strength decreases from 107.1 MPa to 58.5 MPa. The apparent porosity increases from 4.46% to 13.86%, while the bulk density decreases from 3.24 g·cm⁻³ to 3.18 g·cm⁻³.
Figure 2 shows the XRD patterns of the castable matrix samples after drying at 110°C for 24 hours and calcination at 800°C for 5 hours. As seen in Figure 2, the main phases of the samples dried at 110°C for 24 hours are α-Al₂O₃, C₃AH₆, and AH₃. After calcination at 800°C for 5 hours, the diffraction peaks of C₃AH₆ and AH₃ disappear, and the diffraction peaks of C₁₂A₇ emerge. This is because C₃AH₆ dehydrates and decomposes into amorphous C₁₂A₇ at 210–300°C; as the heat treatment temperature increases, the amorphous C₁₂A₇ gradually transforms into crystalline C₁₂A₇. The AH₃ gel dehydrates and decomposes to form AHO(OH), and AHO(OH) further dehydrates and decomposes into amorphous Al₂O₃ at 678°C. The dehydration and decomposition of these hydration products destroy the hydration bonding structure inside the castables, accompanied by the release of crystal water, which leads to an increase in the porosity of the samples. Moreover, the ceramic bonding structure of the castables has not yet formed at 800°C. Therefore, compared with the samples dried at 110°C, the samples calcined at 800°C for 5 hours exhibit a significant decrease in room-temperature strength, an increase in apparent porosity, and a reduction in bulk density.
Figure 3 shows the microstructure photograph of castable matrix sample SJ5 after drying at 110 °C. It can be seen that C₃AH₆ presents a granular morphology, while AH₃ exhibits a fine acicular morphology.
Figure 4 shows the microstructural photograph of castable matrix sample SJ5 after calcination at 800°C for 5 hours. It can be seen that obvious microcracks and nanoscale pores (10–30 nm) have appeared on the surface of dehydrated and decomposed C₃AH₆ particles. In contrast to the granular C₃AH₆ particles after drying at 110°C (see Figure 3), the particles have undergone significant collapse, resulting in a substantial decrease in strength.
For the samples dried at 110°C, as the cement content increased from 5 wt% to 12 wt%, the modulus of rupture and cold crushing strength increased (see Figure 1). Meanwhile, the diffraction peaks of C₃AH₆ and AH₃ became increasingly intense (see Figure 2), indicating an increase in the yield of hydration products C₃AH₆ and AH₃. This, in turn, enhanced the bonding between the aggregates and matrix in the samples, resulting in improved strength. In addition, as the cement content increased from 5 wt% to 12 wt%, the bulk density of the samples increased while the apparent porosity decreased. This was because the elevated cement content led to the formation of more hydration products, which filled the pores in the samples.
For the samples heat-treated at 800°C for 5 hours, the modulus of rupture and cold crushing strength also increased as the cement content rose from 5 wt% to 12 wt% (see Figure 1), demonstrating that increasing cement content was beneficial for improving the strength of the samples after firing at 800°C. At the same time, as the CAC content increased, the diffraction peak of C₁₂A₇ became more intense (see Figure 2), suggesting that more hydration products C₃AH₆ and AH₃ underwent dehydration and decomposition to generate greater amounts of C₁₂A₇ and amorphous products. Although C₁₂A₇ and amorphous products formed a porous structure, as shown in Figure 4, they could still partially contribute to the strength of the samples. Therefore, the samples with higher cement content exhibited higher strength after calcination at 800°C.
3 Conclusions
(1) Compared with the samples dried at 110°C, the samples calcined at 800°C for 5 hours showed an increase in apparent porosity, a decrease in bulk density, and a significant reduction in strength. The hydration product C₃AH₆ collapsed and was damaged during calcination at 800°C, leading to a marked decrease in the strength of the samples.
(2) With the increase in CAC content, more hydration products were generated. After heat treatment at 800°C for 5 hours, more hydration products, including C₃AH₆ and AH₃, dehydrated and decomposed to form a greater amount of C₁₂A₇ and amorphous products. Although their structural morphology changed significantly, they could still impart partial strength to the samples, thus increasing the overall strength.