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Quality of high-purity calcium aluminate cement

Time:

2017-04-22

Source:

network

Alcoa produces four grades of 70% cement for different applications. CA-14 cement is a traditional 70% cement, but CA-270 is a 70% cement with different mineral structures and particle size distribution, which needs to be developed into a cement with low water demand and high thermal strength. The development of CA-270 requires accurate control of clinker production under very stable conditions. The existing knowledge of Alcoa Transformation can improve the technological process of clinker and accurately control the performance of CA-14 cement on working time and settling time. The CA-14 provided now has three strictly controlled levels, and their working hours are different, as shown in Figure 3. The three grades are named CA-14S (S stands for summer grade, and cement settlement is relatively slow), CA-14M (M stands for medium grade) and CA-14W (W stands for winter grade, and cement settlement is relatively fast). The range of test properties is more stringent than previously mentioned, requiring good consistency between cements. CA-14 performance is controlled by different relative ratios of cement phase without any chemical additives [3].

In order to clarify the differences between grades, three independent standard refractory formulas were used to test CA-14: Alcoa Nortab test clinker, aluminum silicate low cement vibrating castable, and tabular corundum gravity flow castable.

Because the standard test methods are often used to compare the type and grade of Portland cement, develop Nortab test clinker to standardize the test methods of Alcoa Laboratories and our customers to evaluate the quality of calcium aluminate cement. Alcoa also uses the test of castable thermal development (exothermic) to study the development of reaction heat generated by cement during settlement and hardening. There are two points on the time curve of thermal development that are particularly important. The beginning of heat release is the time when the temperature of castable starts to rise, corresponding to the end of the working time. Exothermic is the time when the temperature rises to a higher level, corresponding to hardening, and the castable has enough strength to demould [4]. To demonstrate the concept of special raw materials, Alcoa has developed test materials that are very close to the industrial formulation of refractories. The two formulations used here (MulcoaVIB and SFL204) are typical of these test materials and are used in Alcoa's global application laboratories.

The type of cement used to control the performance of a single mix. In general, the performance of the mixture depends on the type of cement. For example, when the added cement changes from CA-14W (fast) to CA-14S (slow), the performance of the mixture will be similarly changed. The importance of this control cannot be underestimated when refractory manufacturers have to manufacture castables for use in very variable environmental conditions (hot summer and cold winter).

The purity of materials is very important to the control of properties, especially the thermal properties (creep and high temperature bending strength). CA-14 and CA-270 are designed with high purity raw materials while maintaining the actual cost. The comparison between CA-14 and CA-270 and other common 70% cements shows that even a small amount of impurities will have a great impact on the high temperature strength (even for low cement castables containing only 5% CA cement). In terms of expansion, it will have an impact on all high temperature properties, as shown in Figure 7. The flexural strength at high temperature decreases about 80% when the impurity content increases by 1%.

Key words: calcium aluminate powder