PORTLAND CEMENT

PORTLAND CEMENT

Portland cement is the type of cement commonly and generally used around the World because of mortar, stucco, concrete and most non-specially grout as a basic ingredient. It is a fine powder produced by grinding Portland cement clinker (more than 90%) a limited amount of calcium sulfate which control the set time and up to 5% minor constituents. The two cements which are not group of Portland cement which are used in construction are;

• Super sulphate cement
• High alumina cement

For the purpose of this project the discussion is limited to Portland cement, which is the most widespread in use. The most commonly used Portland.

 

Types of ordinary Portland cement

• Rapid Hardening Portland cement
• Extra Rapid Hardening Portland cement
• Low Heat Portland cements
• White Portland cement
• Portland Pozzolana cement
• Portland Blast Furnace cement
• Sulphate Resistant Portland cement

Production and uses of Portland cement

Portland cement is manufactured from a mixture of limestone (calcium carbonate) and clay (silica) alumina and iron dioxide burnt in a rotary kiln at temperature of approximately 1300°C -1400°C, when the material sinters it practically fuses into a solid mass known as clinker. The clinker is cooled and ground to a fine powder in a mill, a little gypsum being added at this to control the setting time of the cement.

Portland cement is used for mortars, concrete and stabilization of loose materials in highway construction. Cement if not stored properly shows and it should be kept on raised platforms to avoid contact with the floor so as to avoid the cement from being moist and damp.

Setting time of cement

Immediately water is poured in the Portland cement hydration begin to occur, when Portland cement is mixed with sufficient water to make a soft paste, it gradually becomes less plastic and finally becomes stiff and hard. When the paste has lost its plasticity and becomes sufficiently rigid so that it can withstand on arbitrary defined pressure, it is said to have set. This period is divided into two parts; the initial and the final set. After the cement paste has attained a final set, it further increases in rigidity and strength. This last process is called Hardening.

Tests for fineness of cement

The BS -450 specifications has two tests for fineness. In addition to the sieving test on BS 170 mesh which determine the percentage of coarse material. The second test records the surface areas of the articles of cement. This is used when a measurement of the finer particle is required.

 Soundness of cement

This means the ability of cement to maintain a constant volume. It is essential that the cement paste does not undergo an appreciable change in volume once it has set. Unsound cement will expand slowly over a long period. Its effects may not become apparent immediately, but it is capable of causing severe cracking of concrete and eventually failure. Unsoundness is caused by the presence of excess gypsum or to high proportion of free lime or magnesia. With the hydration of lime encased within cement, this protective film prevents the immediate hydration of the free lime. However, when the moisture finally reaches the lime after the cement has set and since lime expands manifold and with considerable force when hydrated, units delayed hydration may rapidly disrupt the mass.

The Le-Chateliers test which detects unsoundness due to free lime is usually employed according to BS 12:1989 procedure.

Strength of cement

This would be affected if it is disturbed during the period between initial and final strength of cement in position as soon as possible to avoid loss of strength. Ordinary Portland cement hardens slowly, but rapid hardening Portland cement hardens quickly and in 3 days the strength is usually as high as ordinary Portland cement at 7 days.

Compressive strength of cement

The compressive strength of concrete is taken as the maximum compressive load it can carry per unit area. Concrete strengths of up to 8ON/mn² can be achieved by selective use of the type of cement, mix proportions, method of compaction and curing conditions.

Concrete structures, except for road pavements, are normally designed on the basis that concrete is capable of resisting only compression, the tension being carried by steel reinforcement. The standard method described in BS 1881: Part 116 requires that the test specimen should be cured in water at 20⁰ C and crushed by loading it at a constant rate of stress, which increases between 12 and 24N/mm2 per min, immediately after it has been removed from the curing tank.

References

ASTMC 192-92 (1992) Test for the Time Setting for Hydraulic Cement in Cement in American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pp 19103, U.S.A.

 

Beeby, A. W. (1994). Concrete in the oceans, cracking and corrosion. Technical Report. No. 1. Cement and Concrete Association, 7(2), 1-75.

Orchard, D.F. (1973). Concrete Technology Vol. I and ii. Applied Science Publishers ltd, London. U.K.

Wikipedia (2009). Pozzolans http//en.wikipedia.org/wiki/pozzolane. Retrieved 13^th March 2009.

 

Wikipedia(2009). Concrete.http://wiipedia.org/wiki/concrete. Retrieved 2^nd January 2010.