What is alumina cement?
Alumina cement (also called calcium aluminate cement or CAC) is a type of hydraulic cement made primarily from limestone and bauxite, in which calcium aluminates — rather than calcium silicates — are the main binding compounds. It is known for its rapid strength gain, high heat resistance and excellent performance in aggressive chemical environments.
How is it made?
Alumina cement is produced by fusing or sintering a mixture of:
- Limestone (calcium carbonate) — provides the calcium
- Bauxite (aluminium ore) — provides the alumina (Al2O3)
The raw materials are heated to approximately 1,450-1,600 degrees C in a rotary kiln or electric arc furnace, then cooled and ground into a fine powder.
Key properties
| Property | Alumina cement | Ordinary Portland cement (OPC) |
|---|---|---|
| Main compounds | Calcium aluminates (CA, CA2) | Calcium silicates (C3S, C2S) |
| Setting time | 3-5 hours | 2-4 hours (initial) |
| Strength at 24 hours | 40-60 MPa | 10-15 MPa |
| Heat resistance | Up to 1,300-1,800 degrees C | Up to ~300 degrees C |
| Chemical resistance | Excellent (acids, sulphates) | Moderate |
| Colour | Dark grey to black | Light grey |
| Cost | Significantly higher | Standard |
Advantages
- Rapid hardening — reaches near-ultimate strength within 24 hours, ideal for urgent repairs
- Refractory performance — withstands very high temperatures, suitable for furnace linings and fireplaces
- Chemical resistance — resists sulphate attack, acidic conditions and seawater exposure
- Low alkalinity — reduces the risk of alkali-silica reaction with certain aggregates
Disadvantages
- Conversion — over time (months to years), the initial hydrated compounds can convert to a more stable but weaker form, leading to strength loss and increased porosity
- High cost — several times more expensive than Portland cement
- Heat of hydration — generates significant heat during curing; thick sections can overheat
- Not for structural concrete — due to conversion risk, many building codes prohibit its use in structural elements
Common applications
- Refractory concrete — kiln linings, furnace floors, chimney flue linings
- Rapid repair work — patching roads, runways, factory floors where fast turnaround is essential
- Sewer infrastructure — pipes and manholes exposed to hydrogen sulphide (H2S) attack
- Chemical-resistant flooring — in factories, laboratories and agricultural buildings
- Specialty grouts — anchoring bolts, machinery bases
Important precautions
- Do not mix alumina cement with Portland cement — the combination can cause flash setting and severe strength loss
- Keep concrete sections thin to manage heat of hydration
- Cure at temperatures below 30 degrees C to minimise conversion
Related terms
- Portland cement
- Mortar
- Concrete
- Refractory material
- Reinforcement
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