Effect of heat treatment temperature on strength of pure calcium aluminate cement-bonded castable
Pure calcium aluminate cement combined with castable has good high temperature performance. As a fast repair material, the service strength cannot meet the expectation after maintenance in some 600-900 ℃ temperature areas of hot blast stove, which is mainly affected by the dehydration and phase transformation of calcium aluminate hydration products under medium and low temperature conditions.
In order to study the effect of heat treatment junction temperature on the strength of pure calcium aluminate cement binding castable, the following tests were carried out.
High bauxite clinker (Al2O3 ≥ 80%) is used as raw material, activated alumina micropowder and silica micropowder are added, and pure calcium aluminate cement is used as binder to prepare castable samples with pure calcium aluminate cement content of 5%. The samples are heat treated at 110 ℃ x24 h, 300 ℃ x3 h, 500 ℃ x3 h, 700 ℃ x3 h, 900 ℃ x3 h, 1100 ℃ x3 h and 1300 ℃ x3 h respectively. The normal temperature compressive strength of the sample at different heat treatment temperatures is shown in Figure 1.
Fig.1 Normal temperature compressive strength of castable at various heat treatment temperatures
As can be seen from fig. 1, the normal temperature compressive strength of the castable after heat treatment at 700 ℃ x3 h is the lowest, which is 33% lower than that after heat treatment at 110 ℃ x3 h. The reasons are analyzed as follows:
Figure 2. Hydration products of pure calcium aluminate
The hydration products produced during the hydration of pure calcium aluminate cement vary with the curing, baking and use temperature of the castable. The hydration products are mainly CAH10 when curing below 21 ℃, C2AH8 and aluminum glue (AH3) when curing at 21~35 ℃, C3AH6 and aluminum glue (AH3) when curing above 35 ℃, as shown in Fig. 2. The hydration products CAH10 and C2AH8 of pure calcium aluminate cement at normal temperature are medium stable products, and they will be converted into stable C3H6 when the temperature exceeds 35 ℃.
Figure 3. Phase transformation of pure calcium aluminate cement hydration products during heating process
The hydration products of calcium aluminate cement will change during heating, as shown in Figure 3. When the sample is heated above 300 ℃, the hydration product C3AH6 will dehydrate and decompose into CaO and C12A7, while the aluminum glue will dehydrate to form amorphous Al2O3. When heated to above 600 ℃, part of amorphous Al2O3 will react with CaO to form CA. When heated to above 940 ℃, the remaining part of Al2O3 reacts with C12A7 to form CA or CA2. Between 600 ℃ and 900 ℃, due to the destruction of the hydration bonds of calcium aluminate hydration products in the process of dehydration and decomposition, water vapor is discharged, leaving voids, and at the same time, high density hydrates are generated from low density hydrates, and the pores are enlarged. Therefore, the strength of the castable combined with pure calcium aluminate cement after medium temperature treatment is obviously reduced. Due to the proportion of active alumina and silica powder in the composition, the castable sample forms a ceramic bond at about 900 ℃, which improves the strength of the sample.
Therefore, when the amount of pure calcium aluminate cement is 5%, the strength of the castable will be significantly reduced when it is used at 600 ℃ ~ 900 ℃. It is recommended to use low cement or cement-free castable.