Portland Cement

The use of cement has long been the basis for development of society and for the welfare of the people. For generations concrete, which is made from cement, has been the ultimate material for construction of harbours, roads, bridges, dams, houses, schools.

History & Manufacture of Portland Cement

Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove. By this crude method he laid the foundation for an industry which annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water.

Cement is so fine that one pound of cement contains 150 billion grains.

WHAT IS CEMENT?

Portland Cement is a fine powder mixture of various minerals. It is produced by mixing calcareous minerals such as chalk, limestone containing silica and alumina and heating it to 1450 deg C when it cools a clinker is formed. This clinker is then finely ground and mixed with small quantity 3-5% of gypsum which has calcium sulphate in it. The amount of gypsum controls the physical properties of the eventual concrete. Sometimes very minute quantities of pigments are also added. Cement producing processes do however vary.Lime and silica make up about 85% of the mass. Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.

An industry with a homogeneous product

Although produced from natural raw materials which vary from plant to plant, cement can be considered a standard product - there are only a few classes of cement and in each class, products from different producers can generally be interchanged. Therefore, price is the most important sales parameter next to customer service; quality premiums exist but are rather limited.

A heavy product

Land transportation costs are significant and it used to be said that cement could not be economically hauled beyond 200 or at most 300 km. The price of long road transportation may even be higher than the cost price. Bulk shipping has changed that, however, and it is now cheaper to cross the Atlantic Ocean with 35 000 tonnes of cargo than to truck it 300 km. However, in large countries transportation costs normally cluster the markets into regional areas, with the exception of a few long-distance transfers (where, for example, sea terminal facilities exist).

A mature product

Demand for cement (which was first produced in the early 1800s) increased considerably in the 20th century, reflecting the development of industry and growing urbanisation. Consumption in the industrialised countries multiplied 6 to 8 times following World War II. Other than a few ups and downs in both the United States and Europe in the intervening years, growth continued until the 1975 oil crisis - with a subsequent decline of 20 to 40 percent in mature markets.

However, over the last 25 years, some European countries have doubled or even tripled their consumption (Greece, Portugal and Spain ) since these countries have experienced significant growth over the last 10 years.

Market parameters

Consumption of cement is closely linked to both the state of economic development in any given country or region and to the economic cycle.

In mature markets, such as in Europe, where cement consumption per capita still varies considerably from one country to another, cement sales are dependent on evolution and habits in the construction sector, a sector that is itself following very closely (usually after a brief delay) the evolution of the economy in general.

World Resources: Although individual company reserves are subject to exhaustion, cement raw materials, especially limestone, are geologically widespread and abundant, and overall shortages are unlikely in the future.

Cement and clinker importation/exportation is generally cyclical. Typically smaller amounts of cement are imported during recessions — perhaps less that 5% of total national consumption — but during boom times, imports can increase to 20% or more of a typical total national consumption.

Substitutes: Virtually all portland cement is used either in making concrete or mortars and, as such, competes in the construction sector with concrete substitutes such as aluminum, asphalt, clay brick, rammed earth, fiberglass, glass, steel, stone, and wood.

Types of Portland Cement

- OPC Ordinary Portland Cement

- Rapid Hardening Portland Cement

- Sulphate Resisting Portland Cement

- Low Heat Portland Cement

- White Portland Cement

- Other Cements based on Portland:

-Waterproof & water repellent Cement

-Masonry Cement

-Hydrophobic Cement

-Blast-furnace Cement

Difference between European and ASTM cement specifications

In some instances, project designed by engineering firms from other countries refer to cement standards other than those locally are specified. For example, the European cement standard, EN 197, sometimes appears on project specifications. EN 197 cement Types CEM I, II, III, IV, and V do not correspond to the cement types in ASTM C 150, nor can ASTM cements be substituted for EN specified cement without the designer’s approval. EN 197 Type CEM I is a portland cement and CEM II through V are blended cements. EN 197 also has strength classes and ranges (32.5, 42.5, and 52.5 MPa). There is no direct equivalency between ASTM and other cement standards of the world because of differences in test methods and limits on required properties. EN 197 cements are usually not available in the United State.

Different types of portland cement are manufactured to meet various physical and chemical requirements. The American Society for Testing and Materials (ASTM) Specification C-150 provides for eight types of portland cement.Type I portland cement is a normal, general-purpose cement suitable for all uses. It is used in general construction projects such as buildings, bridges, floors, pavements, and other precast concrete products. Type IA portland cement is similar to Type I with the addition of air-entraining properties. Type II portland cement generates less heat at a slower rate and has a moderate resistance to sulfate attack. Type IIA portland cement is identical to Type II and produces air-entrained concrete. Type III portland cement is a high-early-strength cement and causes concrete to set and gain strength rapidly. Type III is chemically and physically similar to Type I, except that its particles have been ground finer. Type IIIA is an air-entraining, high-early-strength cement. Type IV portland cement has a low heat of hydration and develops strength at a slower rate than other cement types, making it ideal for use in dams and other massive concrete structures where there is little chance for heat to escape. Type V portland cement is used only in concrete structures that will be exposed to severe sulfate action, principally where concrete is exposed to soil and groundwater with a high sulfate content.

Portland cements can also be made to ASTM C1157 and include the following: Type GU hydraulic cement for general construction, Type HE-high-early-strength cement, Type MS-moderate sulfate resistant cement, Type HS-high sulfate resistant cement, Type MH-moderate heat of hydration cement, and Type LH-low heat of hydration cement. These cements can also be designated for low reactivity (option R) with alkali-reactive aggregates.

White Portland Cement
In addition to the eight types of portland cement, a number of special purpose hydraulic cements are manufactured. Among these is white portland cement. White portland cement is identical to gray portland cement except in color. During the manufacturing process, manufacturers select raw materials that contain only negligible amounts of iron and magnesium oxides, the substances that give gray cement its color. White cement is used whenever architectural considerations specify white or colored concrete or mortar.

Proportioning

The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.

A properly designed concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.


CEMENT Consumption

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