Explain the following types of cement and elaborate on their properties and usage
- Ordinary Portland Cement
- Super Sulphate Cement
- High Alumina Cement
1. Ordinary Portland Cement
This is a low cost general- purpose cement recommnded for all type of structures, such as columns, beams, floor stabs and other general construction purposes of develops strength sufficiently rapidly for most cement work.
The setting and hardening of ordinary portland cement paste is accompanied by heat liberation. Thus in mass concrete construction such as in dams, where it is difficult for this heat to escape, it becomes necessary to avoid using this type of cement and instead use low-heat portland cement.
For ordinary Portland cement practically all the cement particles will pass 150 um (micron) aperture (BS410 No 100 sieve) and over 95% will pass a 75 mm no 200 siene. Particle size has a bearing on the rapidity of hardening.
The strength which the concrete is made with cement is determined by compression tests on 100mm concrete cubes. For ordinary portland cement, the minimum strength requirements are.
3-day conpressive strength > 8n/mm2
7-day compressive strength > 14n/mm2
Ordinary portland cement concretesnate of hardening may be accelerated by warmth and retarded by cold its resistance to attack by sulphates is generally low its rate of gaining strength, heat of hydration, dujms shrunkage and resistanie to cracking are moderate
2. Super Sulphate Cement
Super sulphate cement is made from well granulated blast furnace slag (80 to 85%), calcium sulphate (10 to 15%) and portland cement (1 to 2%) and is ground finer than portland cement. Thus it has a modified chemical composition.
One of its most important properties is its low total heat of hydration. It is therefore very suitable for construction of dams and other mass concreting work. Concrete made from super sulphate cement may expand y cured in water and may shrink if the concrete is cured in air. Another big advantage of super sulphate cement is its comparatively high resistance to chemical attack by sulphate solutions which are found in some soils.
Super sulphate cement, unlike normal portland cement harden slowly.
3. High Alumina Cement
High alumina cement is a specially manufactured high early strength cement. High alumina cement is used in small quantities as compared to Portland cement. Its technical importance originates from the special properties such as high early strength. The term high alumina cement refers to the class of hydralic cements, the essential constituents of which are calcium aluminates. High alumina cement is much more expensive than portland cement.
The raw material customarily used for the manufacture of ordinary high alumina cements are limestone and bauxite (rocks of high aluminum content)
High alumina cements are greatly sperior to Portland cements in sulphate resistance, a property that is attributed to the absence of free calcium hydroxide in the hydrated high alumina cements.
Early strength development of high alumina cement (low-silico type) is very evident. The superiority of the strength developing ability of high alumina cements is even more conspicuous when the applied curing temperature is close to the freezing point.
Hydration of tri-calcium alumina does not contribute any strength to the concrete. It is mainly responsible for setting action of the cement. High alumina cement has been banned for structured use in some countries like the UK due to some well publicized structural failures.
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