The refractory cement is a type of newly developed material that has excellent heat-resistant performance and is widely used in the industrial field. The most common refractory cement is calcium aluminate cement (or high alumina cement), which is a kind of hydraulic cementitious material made by grinding the aluminate cement clinker.
The manufacturing process of the aluminate cement clinker is much similar to that of the ordinary portland cement clinker, including steps of raw material crushing & pre homogenization, raw meal pre-heating & calcination. The key to distinguish refractory cement from ordinary cement and endow it with fire resistance performance lies in the raw materials used in production. The calcium aluminate cement clinker is made by burning bauxite and limestone, with calcium aluminate as the main component and alumina content of about 50%.
The main minerals in calcium aluminate cement are calcium aluminate (CaO · Al2O3) and other aluminates, and a small amount of dicalcium silicate (2CaO · SiO2). The outer surface of aluminate cement often presents yellow or brown, and also presents gray in some cases. The aluminate cement can maintain a firm role in high temperature, so it is widely used as a high-quality additive material in thermal furnaces or wall lining binder.
Refractory Cement Categories
The calcium aluminate refractory cement can be further divided into two types: low-calcium aluminate refractory cement and calcium magnesium aluminate cement.
Low-Calcium Aluminate Refractory Cement
Low-calcium aluminate refractory cement is made of high-quality bauxite and limestone in proper proportion after sintering and grinding. Its mineral composition is mainly calcium aluminate, accounting for 60% – 70%, and the refractoriness is above 1650 ℃.
The proportion of various compounds in the composition is: alumina (Al2O3) 70%, calcium oxide (CaO) 19% to 23%, silica (SiO2) < 4%, ferric oxide (Fe2O3) < 1.5%.
| Alumina (Al2O3) | Calcium oxide (CaO) | Silica (SiO2) | Ferric oxide (Fe2O3) |
| 70% | 19%~23% | < 4% | <1.5% |
Low-calcium aluminate refractory cement has following features:
- High alumina content and low calcium oxide content;
- Low early strength but high fire resistance.
Applications:
1) It can be combined with refractory materials with refractoriness above 1770℃, such as calcined bauxite, high alumina brick fragments, etc., to make refractory concrete or refractory mortar, and used in certain high-temperature kilns as refractory linings and some high-temperature projects which are long-time exposed to the atmosphere or rain.
2) It can be made into heat insulation concrete and heat resistant concrete with lightweight aggregate.
3) It can be compounded with asbestos into asbestos cement products with insulation and heat resistance.
Calcium Magnesium Aluminate Cement
Calcium magnesium aluminate refractory cement is made of high-quality bauxite and dolomite in proper proportion after sintering and grinding. In its mineral composition, calcium aluminate and calcium dialuminate account for 45% to 60%, and spinel accounts for 35% to 50%. The refractoriness is above 1650 ℃.
The proportion of various compounds in the composition is: alumina (Al2O3) 66% to 74%, calcium oxide (CaO) 13% to 18%, magnesium oxide (MgO) 10% to 13%, trace amounts of silica (SiO2) and ferric oxide (Fe2O3).
| Alumina (Al2O3) | Calcium oxide (CaO) | Magnesium oxide (MgO) | Silica (SiO2) and Ferric oxide (Fe2O3) |
| 66%~74% | 13%~18% | 10%~13% | <1% |
Calcium magnesium aluminate cement has following features:
- It has strong resistance to chemical erosion of slag and molten metal;
- It has high refractoriness because of the spinel in its composition;
- With less mixing water, the porosity of hardened cement is low and the calcination shrinkage is small;
- Its strength loss is small when heated to 1100 ℃.
Applications:
The concrete made of calcium magnesium aluminate cement and fused white corundum can be used as lining of cement rotary kiln and other industrial kilns.
Refractory Cement Standards
- The fineness grade: finer cement particles means larger specific surface area and faster hydration reaction for the cement, and gives the cement higher early and late strength. In China, the specific surface area should be more than 300m2/kg, otherwise, it is unqualified.
- The setting time: in order to ensure that there is sufficient time to complete mixing, transportation, molding, and other processes during construction, the initial setting time of the refractory cement should not be too short; after the construction, it is hoped that the cement can harden as soon as possible to provide strength, so the final setting time should not be too long. The initial setting time of refractory cement should not be earlier than 45 minutes, and the final setting time should not be later than 390 minutes.
- Cement volume stability: the uniformity of volume change of cement paste in the process of setting and hardening is called the cement volume stability. If the volume changes unevenly, that is, the volume stability is poor, it is easy to deform and crack, reduce the engineering quality, and even cause accidents.
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