What is Portland Cement?

It is the generic term for fine, gray or white powder manufactured using high temperature to produce calcium silicates that, in the presence of water, will undergo hydration producing a product that will bring aggregates together to produce mortar, stucco or concrete. back to top

Portland cement is made when materials that contain proper amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are usually materials such as limestone, marl, shale, iron, ore, clay, and fly ash.
The kiln or the furnace resembles a large horizontal pipe with a diameter of 10 to 15 feet (3 to 4.1 meters) and a length of 300 feet (90 meters) or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials moves slowly toward the lower end. Flame jets are located at the lower end. All the materials in the kiln are heated to high temperatures that range between 2700 and 3000 Fahrenheit (1480 and 1650 Celsius). This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For every ton of material that goes into the feed end of the kiln, two thirds of a ton that comes out the discharge end, called clinker. The clinker is in the shape and appearance of marble sized pellets. The clinker is very finely ground to produce Portland cement. To control the cement’s rate of hardening, a small amount of gypsum is added during the grinding process.

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Portland cement and blended cement. back to top

All Portland cement is mainly similar but there are eight types of cement that are manufactured to meet various physical and chemical requirements for specific applications:

• Type I is a general purpose Portland cement that is suitable for most uses.
• Type II is used for constructions in water or soil that contains moderate amounts of sulphate, or when heat build-up is a concern.
• Type III cement provides and supplies high strength at an early state, usually in a week or less.
• Type IV moderates heat produced by hydration that is used for massive concrete surfaces such as dams.
• Type V cement resists chemical attack by soil and water high in sulfates.
• Type IA, IIA and IIIA are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, but with the exception of possessing small quantities of air-entrained materials shared and combined with them.

White Portland cement is made from raw materials which contain little or no iron or manganese. These are the substances that give conventional cement its gray color.
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Blended hydraulic cements are produced by intimately and uniformly intergrinding or blending two or more types of fine materials. The primary materials are portland cement, ground granulated blast furnace slag, fly ash, silica fume, calcined clay, other pozzolans, hydrated lime, and pre-blended combinations of these materials.
What are the types of blended cement?
Type IS-Portland blast furnace slag cement
Type IP and Type P-Portland-pozzolan cement
Type I(PM)-Pozzolan-modified portland cement
Type S-Slag cement
Type I(SM)-Slag-modified portland cement. back to top

The two main raw materials used in making Portland cement are calcareous substances like chalk, limestone, marl or shells and argillaceous elements like clay and shale that are rich in silica. back to top

Cement is actually an ingredient of concrete. Concrete is a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Concrete is a hard, strong construction material consisting of sand, conglomerate gravel, pebbles, broken stone, or slag in a mortar or cement matrix. Concrete gets stronger as it gets older. Portland cement is the generic for the type of cement used in virtually all concrete. Cement compromises from 10 to 15 percent of the concrete mix, by volume. In the process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years that it brings the idea that concrete gets stronger and tougher as it gets older.
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Curing is a very important step in concrete construction. When concrete has been cured properly, it greatly increases its strength and durability. Through the result of hydration, concrete hardens. It is the chemical reaction between cement and water. Hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period which is from five to seven days after the placement for conventional concrete, the concrete surface needs to be kept moist to allow the hydration process. The new concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture. back to top

Gypsum plays a very important role in controlling the rate of which hardening of the cement develops. It must be controlled within the limits to make the cement a useful product in construction. Small amount of gypsum are added to the clinkers at the grounding period. back to top

Portland cement is hydraulic cement. It means that it sets and hardens due to a chemical reaction with water. back to top

The compounds present in Portland cement react with water to form a cementitious crystalline structure that adheres to the sand and aggregate. This helps in binding the mass together and increases its strength till it becomes very hard. back to top

When concrete hydrates, it hardens and gains strength. This hydration process continues over a long period of time. It occurs quickly at the outset and slows down as time passes. It would require a wait of several years before the strength of concrete can really be measured. Since this is impractical, a time period of 28 days was introduced by specification-writing authorities as the time where all concrete should be tested. In this period, a substantial percentage of the hydration has already taken place. back to top

How do you control the strength of concrete?

In order to add strength to concrete, one has to add cement. The ratio of water to cement in the paste that binds the aggregates together plays a big role in controlling the strength of concrete. The higher the ratio, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water. back to top

No. There is no universal international standard for Portland cement because every country has their own standard. The United States uses the specification prepared by the American Society for Testing and Material (ASTM C-150) Standard Specification for Portland cement. A few other countries have also adopted this, however, there are countless other conditions. Unfortunately, they do not use the same criteria for measuring properties and defining physical characteristics so they can be virtually “non-translatable.” The European Cement Association located in Brussels, Belgium, publishes a book entitled “Cement Standards of the World.” back to top

When water is mixed with cement, a smooth paste is produced that remains plastic for a short time. During this period, the paste can be disturbed and remixed without injury. As the reaction between water and cement continues, the plasticity of the cement paste is lost. This early period in the hardening of cement is known as `Setting of Cement'.
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Hardening of cement is caused by some reactions described by the process of liberation of heat. This is referred to as Heat of Hydration. In mass concreting jobs like dams where dissipation of heat is not possible, the temperature of concrete rises. On subsequent cooling, cracks develop. Hence, it is very necessary to use cement with low heat of hydration in massive structures. back to top

There are some people who are more sensitive to the free lime than other people. It may cause some irritations if it comes in contact with the eyes. It also possible that cracked skin will leave you more susceptible to dermatitis. back to top