Cement

I

Introduction

Cement, any material that hardens and becomes strongly adhesive after application. The term “cement” is often used interchangeably with “glue” and “adhesive“; in engineering and building construction the term usually refers to a finely powdered, manufactured substance consisting of gypsum plaster, or Portland cement, that hardens and adheres after being mixed with water.

Cements are used for various purposes, such as binding sand and gravel together with Portland cement to form concrete, for uniting the surfaces of various materials, or for coating surfaces to protect them from chemical attack. Cements are made in many compositions for a wide variety of uses. They may be named after the principal constituents, such as calcareous cement, which contains silicon oxide, and epoxy cement, which contains epoxy resins; after the materials they join, such as glass or vinyl cement; after the object to which they are applied, such as boiler cement; or after their characteristic property, such as hydraulic cement, which hardens under water, acid-resisting cement, or quick-setting cement. Cements used in construction are sometimes named after their commonly reported place of origin, such as Roman cement, or for their resemblance to other materials, such as Portland cement, which produces a concrete resembling the Portland stone used for building in Britain. Cements that resist high temperatures are called refractory cements.

Cements can set, or harden, by one of several mechanisms: the evaporation of the plasticizing liquid such as water, alcohol, or oil; internal chemical change; hydration; the growth of interlacing sets of crystals; or reaction with oxygen or carbon dioxide in the atmosphere.

II

Portland Cement

Typical Portland cements are mixtures of tricalcium silicate (3CaO · SiO₂), tricalcium aluminate (3CaO · Al₂O₃), and dicalcium silicate (2CaO · SiO₂), in varying proportions, together with small amounts of magnesium and iron compounds. Gypsum is often added to slow the hardening process.

These active compounds in cement are unstable, and when water is added they rearrange their structure. The initial hardening of the cement is caused by the hydration of tricalcium silicate, which forms jelly-like hydrated silica and calcium hydroxide. These substances ultimately crystallize and bind together the particles of sand or stone, which are always included in a mortar or concrete mixture, into a hard mass. Tricalcium aluminate acts in the same way to produce the initial setting, but does not contribute to the ultimate hardening of the mixture. The hydration of dicalcium silicate proceeds similarly but far more slowly, hardening gradually over a period of years. The process of hydration and setting of a cement mixture is known as curing; during this period heat is evolved.

Portland cement is manufactured from lime-bearing materials, usually limestone, together with clays, shales, or blast-furnace slag containing aluminium oxide and silicon oxide, in the approximate proportions of 60 per cent lime, 19 per cent silicon oxide, 8 per cent aluminium oxide, 5 per cent iron, 5 per cent magnesium oxide, and 3 per cent sulphur trioxide. Some rocks, called cement rocks, are naturally composed of these elements in approximately suitable proportions and can be made into cement without the use of large quantities of other raw materials. In general, however, cement plants rely on mixed materials.

In the manufacture of cement the raw materials are ground together, the mixture is heated until it fuses into a clinker, and the clinker is ground into a fine powder. The heating is usually accomplished in rotary kilns more than 150 m (500 ft) long and 3.7 m (12 ft) or more in diameter. The kilns are slightly tilted from the horizontal, and the raw material is introduced at the upper end, either in the form of a dry rock powder or as a wet paste composed of ground-up rock and water. As the charge progresses down through the kiln, it is dried and heated by the hot gases from a flame at the lower end. As it comes nearer the flame, carbon dioxide is driven off, and in the area of the flame itself the charge is fused at temperatures between 1540° and 1600° C (2800° and 2900° F). The material takes approximately six hours to pass from one end of the kiln to the other. After it leaves the kiln, the clinker is cooled quickly and ground, and then conveyed by a blower to packing machinery or storage silos. The amount thus produced is so fine in texture that 90 per cent or more of its particles will pass through a sieve with 6,200 openings per sq cm (40,000 per sq in).

In a modern kiln, 45 kg (about 100 lb) of raw material will make 27 to 30 kg (about 59 to 66 lb) of cement. The weight lost is largely carbon dioxide and water. Kilns usually burn coal in the form of powder and consume about 450 g (1 lb) of coal for about every 900 g (2 lb) of cement produced. Oil and gas are also used.

A number of tests are used to check the quality of the cement. A common one is to use a mortar specimen of 1 part cement and 3 parts of sand and measure its tensile strength after a week in air and under water. A good cement will show a tensile strength of 19.4 kg per sq cm (275 lb per sq in) under these conditions.

III

Special Cements

By varying the percentage of its normal components or adding others, Portland cement can be given various desirable characteristics, such as rapid hardening, low production of heat during hydration, and resistance to alkalis. Rapid-hardening cements, sometimes called high-early-strength cements, are made by increasing the proportion of tricalcium silicate or by finer grinding, so that up to 99.5 per cent will pass through a screen with 16,370 openings per sq cm (105,625 per sq in). Some of these cements will harden as much in a day as ordinary cement does in a month. They produce much heat during hydration, however, which makes them unsuitable for large structures where such heat may cause cracks. Special low-heat cements, which usually have a large proportion of dicalcium silicate, are generally used for massive pourings. Where concrete work must be exposed to alkaline conditions, which attack concretes made with ordinary Portland cement, resistant cements with a low aluminium content are generally employed. Cements for use under salt water may contain as much as 5 per cent iron oxide, and those with as much as 40 per cent aluminium oxide are used to resist the action of sulphate-bearing waters.

IV

History

Although various types of mineral-based hydraulic cement are of ancient origin, hydraulic cements have been used only since the middle of the 18th century. The term “Portland cement” was first used in 1824 by Joseph Aspdin, a British cement-maker, because of the resemblance between concrete made from his cement and Portland stone, which was commonly used in building in Britain. The first modern Portland cement, made from lime and clay or shale materials heated until they formed cinders (or clinkers) and then ground, was produced in Britain in 1845. At that time cements were usually made in upright kilns where the raw materials were spread between layers of coke, which was then burnt. The first rotary kilns were introduced about 1880. Portland cement is now almost universally used for structural concrete.