9.8. Alumina cement

Alumina cement is a high-early-strength binding agent, which is produced by the fine milling of clinker which consists mainly of low basic calcium aluminate.

The bases of alumina cement, as well as Portland cement, are four main oxides - CaO, Al₂O₃, SiO₂, Fe₂O₃. However, numerical correlations of these oxides are principally different. Alumina cements contain mostly Al₂O₃ (30-50%) and CaO (30-45%). These oxides are provided by the principal raw materials - bauxite and limestone.

The clinker of alumina cement is produced by two methods: melting of raw meal, or its burning to sintering.

The degree of fineness of the cement must be such that not less than 90% by weight of the test material passes when sifting through a sieve № 008. The quality of alumina cement is determined mainly by the amount of calcium aluminate. The water demand of alumina cement is the same as Portland cement (23-28%), setting. but it is more sensible to control the water content during mixing to avoid premature

Initial setting time must begin not early than 30 minutes, and the final set - not later than 10 hour

After 5-6 hours of hardening, the strength of alumina cement reaches about 30% of ultimate strength. After 3-days hardening alumina cement usually achieve a compressive strength of 40-60 MPa.

Alumina cement is considerably more expensive than Portland cement in connection with the deficit of bauxite as raw material. It is practical to apply it, if its advantages are rationally used: at emergency works, at the speed of concrete construction building, especially at a temperature below zero, for heat-resistant concretes.

Gypsum-alumina cement and waterproof expanded cements are produced on the basis of alumina cements. Gypsum-alumina cement is produced by the compatible grinding of clinker of alumina cement and gypsum in the ratio 70: 30 (by weight), and waterproof expanded cement - by a compatible grinding or mixing of 73-76% alumina cement, 20-22% gypsum and 10-11% of the high-basic calcium hydroaluminates. Hydroaluminates are produced by the treatment of mixture of alumina cement with slaked lime. Gypsum-alumina cement expand during hardening in a moist environment, and waterproof expanded cement – also in the air conditions. The feature of this cements are short setting time, high watertightness of mortars and concretes even at early age.

Self-Assessment Questions

1. Discuss the nature and properties of mineral binders.

2. Explain the nature of mineral non-hydraulic binders, their classification and value in building.

3. Distinguish between the features of a building and alpha - gypsum?

4. Describe the technical properties of gypsum binders.

5. What types of lime are used in building construction?

6. What are the features and properties of building lime?

7. Explain the manufacture and properties of liquid glass?

8. Provide a classification of hydraulic binding agents.

9. Discuss the nature and properties of hydraulic lime.

10. Describe the process of manufacture of Portland cement clinker.

11. Describe the chemical-mineralogical composition of Portland cement clinker and its influence on the properties of Portland cement.

12. What are the basic properties of Portland cement ?

13. Explain the processes of the cement hydration?

14. Describe the corrosive environments which destroy a cement stone.

15. Describe the chemical reactions characteristic of the process of cement stone corrosion? What are the methods of preventing chemical corrosion?

16. Outline the varieties of cements based on Portland clinker.

17. What are the features of alumina cement and binders based on it?

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