Portland cement is by far the most common type
of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium
oxide, or quicklime, which then chemically combines
with the other materials that have been included in the mix to form calcium
silicates and other cementitious compounds. The resulting hard substance,
called 'clinker', is then ground with a small amount of gypsum into a powder to make 'ordinary Portland cement',
the most commonly used type of cement (often referred to as OPC). Portland
cement is a basic ingredient of concrete, mortar and
most non-specialty grout. The
most common use for Portland cement is in the production of concrete. Concrete
is a composite material consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in
almost any shape desired, and once hardened, can become a structural (load
bearing) element. Portland cement may be grey or white.
When water is mixed with Portland cement, the
product sets in a few hours, and hardens over a period of weeks. These
processes can vary widely, depending upon the mix used and the conditions of curing of the product, but a typical concrete sets in
about 6 hours and develops a compressive strength of 8 MPa in 24 hours. The strength rises
to 15 MPa at 3 days, 23 MPa at 1 week, 35 MPa at
4 weeks, and 41 MPa at 3 months. In principle, the strength
continues to rise slowly as long as water is available for continued hydration,
but concrete is usually allowed to dry out after a few weeks and this causes
strength growth to stop.
White Portland cement or white ordinary
Portland cement (WOPC) is similar to ordinary, grey, Portland cement in all
respects, except for its high degree of whiteness. Obtaining this colour
requires high purity raw materials (low Fe2O3 content), and some modification to the method of
manufacture, a.o. a higher kiln temperature required to sinter the clinker in
the absence of ferric oxides acting as a flux in normal clinker. As Fe2O3 contributes to decrease the melting point of the
clinker (normally 1450 °C), the white cement requires a higher sintering
temperature (around 1600 °C). Because of this, it is somewhat more
expensive than the grey product. The main requirement is to have a low iron
content which should be less than 0.5 wt.% expressed as Fe2O3 for white cement, and less than 0.9 wt.% for
off-white cement. It also helps to have the iron oxide as ferrous oxide (FeO)
which is obtained via slightly reducing conditions in the kiln, i.e., operating
with zero excess oxygen at the kiln exit. This gives the clinker and cement a
green tinge. Other metallic oxides such as Cr2O3 (green), MnO (pink), TiO2 (white), etc., in trace content, can also give colour
tinges, so for a given project it is best to use cement from a single batch.