Content provided by the Mineral Information Institute, © 2002 www.mii.org
Cement as we know it was first developed by Joseph Aspdin, an enterprising 19th-century British stonemason, who heated a mix of ground limestone and clay in his kitchen stove, then pulverized the concoction into a fine powder.
The result was the world's first hydraulic cement: one that hardens when water is added. Aspdin dubbed his creation Portland cement due to its similarity to a stone quarried on the Isle of Portland, off the British coast. In 1824, this brilliant craftsman obtained a patent for what would prove to be the world's most ubiquitous building material, laying the foundation for today's global Portland cement industry.
Portland cement - a combination of calcium, silica, aluminum and iron - is the fundamental ingredient in concrete.
Producing a calcium-silicate Portland cement that conforms to specific chemical and physical specifications demands careful control of the manufacturing process.
First, the raw materials - limestone, shells or chalk along with shale, clay, sand or iron ore - are mined from a quarry that's usually near the manufacturing plant. Before leaving the quarry these materials are reduced in size by two sets of crushers. The primary set crushes the stone to about five inches (125 mm) in diameter and the secondary set pulverizes it to just 3/4 inch (19 mm). Then the raw materials are sent to the manufacturing plant, where they are proportioned to create cements with specific chemical compositions.
Portland cement is manufactured using two methods: wet and dry.
In the dry method, dry raw materials are proportioned before being ground into a fine powder, blended, then fed dry into a kiln.
In the wet method, a slurry is created by adding water to properly proportioned raw materials prior to them being ground, blended and fed into the upper end of a tilted and rotating cylindrical kiln, where their rate of passage is controlled by the kiln's slope and rotational speed.
Burning fuel - usually powdered coal or natural gas - is then forced into the kiln's lower end, heating the raw materials to 2,600-3,000 degrees F (1,430-1,650 degrees C). At 2,700 degrees F (1,480 degrees C), several chemical reactions fuse the raw materials, creating what are called cement clinkers: grayish-black pellets the size of marbles.
The red-hot clinkers are discharged from the lower end of the kiln and transferred into various types of coolers to reduce their temperature so they can be handled safely. Now cooled, the clinkers are combined with gypsum and ground into a gray powder so fine that it can pass through a 75-micron - or number 200 mesh - sieve.
This fine gray powder is Portland cement.
The flexibility of Portland cement is evident in the different types, which are manufactured to meet various physical and chemical requirements.
The American Society for Testing and Materials (ASTM) Specification C-150 provides for eight individual types of Portland cement.
When architectural considerations require white or colored concrete or mortar, Portland cement can adapt with the manufacture of white Portland cement, just one of a number of special-purpose hydraulic cement types available.
White Portland cement is identical in composition to the traditional gray-colored product, except in color. This is made possible during the manufacturing process by selecting raw materials containing only negligible amounts of the iron and magnesium oxides that give Portland cement its gray color.
Blended hydraulic cements, designed to conform to the special requirements of the ASTM C595 or C1157 standards, are produced by mixing Portland cement, ground and granulated blast-furnace slag, fly ash, natural pozzolans and silica fume. These cements may also designed as air-entraining, moderate sulfate-resistant or with moderate or low heat of hydration, depending on the need.
The ASTM C1157-compliant cements can also be designated for low reactivity (option R) with alkali-reactive aggregates. There are no restrictions on the composition of C1157 cements. Manufacturers can optimize ingredients, such as pozzolans and slags, to achieve a particular set of concrete properties.
Of all the blended cements available throughout the world, Types IP and IS are the most common. While Europe and Asia currently use more blended cements than the United States, environmental and energy concerns, in addition to consumer demand for cements with specific properties, may alter this situation.