Cement

What is cement?

Portland cement is an extremely fine grey powder manufactured from some of the earth’s most common minerals.

  • Limestone -CaCO3
  • Alumina source -Al2O3
  • Iron ore -Fe2O3
  • Silica source -SiO2

Mixed together in the correct proportions and heated up to ± 1400°C, it creates a binder (clinker) with hydraulic properties. This is the ‘glue’ that binds sand and gravel together into the rock-like mass we know as concrete.

Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete. Cement comprises 10 to 15% of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass.

So, there is no such thing as a cement floor, or a cement mixer; the proper terms are concrete floor and concrete mixer.

Materials that contain appropriate amounts of oxides of lime, silica, alumina and iron are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.

The kiln resembles a large horizontal pipe with a diameter of 3 to 4 metres and a length of 90 metres or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures of about 1400 and 1450 degrees 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 each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. A small amount of gypsum is added during the milling process in order to to control the cement’s set or rate of hardening.

Though all portland cement is basically the same, five types of cement are manufactured to meet different physical and chemical requirements for specific applications:

  • CEM I  portland cement with a maximum of 5% minor additional constituents
  • CEM II portland cement containing varying additions of secondary materials, i.e. fly ash, pozzolana, slag, silica fume, or limestone.
  • CEM III blast furnace cement
  • CEM IV pozzolanic cement
  • CEM V composite cement

With the wide range of secondary product addition amounts allowable under the South African standard for common cements (SANS 50197-1), there are now potentially 27 products in the family of “common cements”. The Concrete Institute in South Africa supplies a downloadable leaflet on the full composition details of these different cements. Alternatively, a full copy of the specification can be obtained from the South African Bureau of Standards.

White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its grey colour. White cement is used primarily for decorative purposes.

Each country has its own standard for portland cement, so there is no universal international standard. South Africa uses the specifications prepared by the South African Bureau of Standards:

SANS 50197-1 Cement: Part 1 Composition, specifications and conformity criteria for Common Cements and this is supported by SANS 50197-2 2000 Cement:

Part 2:Conformity evaluation

SANS 50413-1: 2004 Masonry cement. Part 1: Composition, specifications and conformity criteria. SNAS 50413-2 covers the test methods which apply to masonry cements only.

 

There are a few other countries that also have adopted their own standard. Unfortunately, many do not use the same criteria for measuring properties and defining physical characteristics so they are virtually “non-translatable.” The European Cement Association located in Brussels, Belgium, publishes a book titled “Cement Standards of the World.”