Cement

In engineering and building construction, cement 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 course and fine aggregates together with Portland cement to form concrete. When used with sands alone the purpose is generally to produce some form of plaster or mortar used in stucco or masonry mud. Cements are made in a wide variety of compositions for a wide variety of uses. Special blends of manufactured cement are named for the principal constituents, such as calcareous cement, which contains silica, and epoxy cement, which contains epoxy resins; for the materials they are intended to join, such as hydraulic cement, which hardens underwater, or acid-resisting cement, or quick-setting cement.

History

Although mineral-based hydraulic cements have ancient origins, hydraulic cements have been used only since 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 much used in building in England. The first modern manufactured Portland cement, was produced in England in 1845. The first rotary kilns were introduced about 1880. Portland cement is now almost universally used for structural concrete.

Portland Cement

There are five principal types of manufactured cements as specified by the American Society for Testing and Materials (ASTM) under ASTM C-595. These are:

In Canada cement types are labeled Types 10, 20, 30, 40, and 50 respectively. The most commonly manufactured is Type I (10 CAN.) which is used by most concrete producers.

Cement n (14c.) 1. A powder of alumina, silica, lime, iron oxide, and magnesium burned together in a kiln and finely pulverized and used as an ingredient of mortar and concrete also: CONCRETE 2. A binding element or agency.

Cements set, or harden, by the evaporation of the plasticizing liquid such as water, alcohol, or oil, by internal chemical change, by hydration, or by the growth of interlacing sets of crystals. Other cements harden as they react with the oxygen or carbon dioxide in the atmosphere.

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 added to slow or control the hardening process.

The active compounds in cement begin process called hydration when water is added. Initial hardening of the cement is caused by the hydration of tricalcium silicate, which forms jellylike hydrated silica and calcium hydroxide. These substances crystallize, bind together the of sand and stone within the hardening cement paste in a mortar or concrete mixture. Tricalcium aluminate in the same way produces the initial set, but does not contribute to the ultimate hardening of the mixture. 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 natural limestone, together with clays, shales, or blast-furnace slag containing alumina and silica, in the approximate proportions of 60% lime, 19% silica, 8% alumina, 5% iron, 5% magnesia, and 3% sulfur trioxide.

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. Raw materials pass through a slightly tilted from the horizontal kiln either in the form of a dry rock powder or as a wet paste composed of ground-up rock and water. The components become dried and heated by the hot gases from a flame at the lower end of the kiln. Near the open flame of the kiln, carbon dioxide is driven off, and in the area of the flame itself the fuses components to become clinker at temperatures between 1540° and 1600° C (2800° and 2900° F). The material takes approximately 6 hours to pass from one end of the kiln to the other. Exiting the kiln, the clinker is cooled quickly, ground, and then conveyed to packing machinery or storage silos in its’ finished form, Portland cement.

To check cement quality a mortar specimen of one part cement, and three parts of sand is prepared to measure its tensile strength after a week in air and underwater. Good cement will show a tensile strength of 19.4 kg/cm2 (275 lb/in2) under these conditions.

Special Cements

By varying normal components or adding others, Portland cement can be given various desirable characteristics, such as rapid hardening (high-early-cement), low heat during hydration, and resistance to alkalis or sulfates. Some of these cements will set within minutes or harden as much in a day as ordinary cement does in a month. Special low-heat cements (high in dicalcium silicate), are generally used for massive concrete placements (bridge supports, dams). Where concrete exposed to alkaline conditions a resistant cement (low aluminum content) is employed. Cements for use under salt water may contain as much as 5 percent iron oxide, and those with as much as 40 percent aluminum oxide are used to resist the action of sulfate-soils.

Aggregates

Not considered here are the infinite combination of aggregate sources for concrete mixes or use of natural and man made pozzolans for use in diverse purpose-made material mixtures.

FACTORS AFFECTING READY-MIXED CONCRETE PERFORMANCE

An increase in:	Will:
WATER	Increase slump and reduce strength.
AIR	Increase slump and reduce strength.
SLUMP	Extend set time*, and increase shrinkage/ cracking.
MIXING TIME	Reduce strength, increase temperature, and increase air.
TEMPERATURE	Reduce strength, slump, and set time.
	* Except where slump is Increased by use of admixtures for performance purposes.

Other factors that play a large role in the quality or integrity of hardened concrete are site preparation, discharge time, placement, consolidation, and finishing processes. The best concrete mix available can not make up for poor building practices.

PROBLEMS THAT START FROM WITHIN

Alkalies (Arabic al-qili, "ashes of the saltwort plant"), originally the hydroxides and carbonates of potassium and sodium, leached from plant ashes. As they apply to cements and concrete, in high enough concentration to destroy flesh; for this reason they are called caustic alkalies.

Alkali Metals, series of six chemical elements in group 1 (or Ia) of the periodic table. They are soft compared to other metals, have low melting points, and are so reactive that they are never found in nature uncombined with other elements. They are powerful reducing agents, that is, they give up an electron easily, and react violently with water to form hydrogen gas and hydroxides, or strong bases. The alkali metals are, in order of increasing atomic number, lithium, sodium, potassium, rubidium, cesium, and francium. Francium exists only in a radioactive form.

LINKS:

• www.aci-int.org
• www.astm.org
  
• www.portcement.org
  
• www.worldofconcrete.com