Portland cement


Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, and then [|grinding] the clinker with the addition of several percent gypsum. Several types of Portland cement are available. The most common, historically called ordinary Portland cement, is grey, but white Portland cement is also available.
The cement was so named by Joseph Aspdin, who obtained a patent for it in 1824, because, once hardened, it resembled the fine, pale limestone known as Portland stone, quarried from the windswept cliffs of the Isle of Portland in Dorset. Portland stone was prized for centuries in British architecture and used in iconic structures such as St Paul's Cathedral and the British Museum.
His son William Aspdin is regarded as the inventor of "modern" Portland cement due to his developments in the 1840s.
The low cost and widespread availability of the limestone, shales, and other naturally occurring materials used in Portland cement make it a relatively cheap building material. At 4.4 billion tons manufactured, Portland cement ranks third in the list of manufactured materials, outranked only by sand and gravel. These two are combined, with water, to make the most manufactured material, concrete. This is Portland cement's most common use.

History

Portland cement was developed from natural cements made in Britain beginning in the middle of the 18th century. Its name is derived from its similarity to Portland stone, a type of building stone quarried on the Isle of Portland in Dorset, England. The development of modern Portland cement began in 1756, when John Smeaton experimented with combinations of different limestones and additives, including trass and pozzolanas, intended for the construction of a lighthouse, now known as Smeaton's Tower. In the late 18th century, Roman cement was developed and patented in 1796 by James Parker. Roman cement quickly became popular, but was largely replaced by Portland cement in the 1850s. In 1811, James Frost produced a cement he called British cement. James Frost is reported to have erected a manufactory for making of an artificial cement in 1826. In 1811 Edgar Dobbs of Southwark patented a cement of the kind invented 7 years later by the French engineer Louis Vicat. Vicat's cement is an artificial hydraulic lime, and is considered the "principal forerunner" of Portland cement.
The name Portland cement is recorded in a directory published in 1823 being associated with a William Lockwood and possibly others. In his 1824 cement patent, Joseph Aspdin called his invention "Portland cement" because of its resemblance to Portland stone. Aspdin's cement was nothing like modern Portland cement, but a first step in the development of modern Portland cement, and has been called a "proto-Portland cement".
William Aspdin had left his father's company, to form his own cement manufactury. In the 1840s William, apparently accidentally, produced calcium silicates which are a middle step in the development of Portland cement. In 1843, he set up a manufacturing plant at Rotherhithe, southeast London, where he was soon making a cement that caused a sensation among users in London. In 1848, William further improved his cement. Then, in 1853, he moved to Germany, where he was involved in cement making. William made what could be called "meso-Portland cement". Isaac Charles Johnson further refined the production of "meso-Portland cement", and claimed to be the real father of Portland cement.
In 1859, John Grant of the Metropolitan Board of Works, set out requirements for cement to be used in the London sewer project. This became a specification for Portland cement. The next development in the manufacture of Portland cement was the introduction of the rotary kiln, patented by Frederick Ransome in 1885 and 1886 ; which allowed a stronger, more homogeneous mixture and a continuous manufacturing process. The Hoffmann "endless" kiln which was said to give "perfect control over combustion" was tested in 1860 and shown to produce a superior grade of cement. This cement was made at the Portland Cementfabrik Stern at Stettin, which was the first to use a Hoffmann kiln. The Association of German Cement Manufacturers issued a standard on Portland cement in 1878.
Portland cement had been imported into the United States from England and Germany, and in the 1870s and 1880s, it was being produced by Eagle Portland cement near Kalamazoo, Michigan. In 1875, the first Portland cement was produced in the Coplay Cement Company Kilns under the direction of David O. Saylor in Coplay, Pennsylvania, United States. By the early 20th century, American-made Portland cement had displaced most of the imported Portland cement.

Composition

C219 defines Portland cement as:
The European Standard EN 197-1 uses the following definition:
.
Clinkers make up more than 90% of the cement, along with a limited amount of calcium sulphate, and up to 5% minor constituents as allowed by various standards. Clinkers are nodules of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The key chemical reaction distinguishing Portland cement from other hydraulic limes occurs at these high temperatures as belite combines with calcium oxide to form alite.

Manufacturing

Portland cement clinker is made by heating, in a cement kiln, a mixture of raw materials to a calcining temperature of above and then a fusion temperature, which is about for modern cements, to sinter the materials into clinker.
The four mineral phases present in the cement clinker are alite, belite, tricalcium aluminate and tetracalcium alumino ferrite. The aluminium, iron and magnesium oxides are present as a flux allowing the calcium silicates to form at a lower temperature, and contribute little to the strength. For special cements, such as low heat and sulphate resistant types, it is necessary to limit the amount of tricalcium aluminate formed.
The major raw material for the clinker-making is usually limestone mixed with a second material containing clay as a source of alumino-silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of these limestones can be as low as 80%. Secondary raw materials depend on the purity of the limestone. Some of the materials used are clay, shale, sand, iron ore, bauxite, fly ash, and slag. When a cement kiln is fired with coal, the coal ash acts as a secondary raw material.

Cement grinding

To achieve the desired setting qualities in the finished product, a quantity of calcium sulphate is added to the clinker, and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the 'specific surface area', which is the total particle surface area of a unit mass of cement. The rate of initial reaction of the cement on the addition of water is directly proportional to the specific surface area. Typical values are 320–380 m2·kg−1 for general purpose cements, and 450–650 m2·kg−1 for 'rapid hardening' cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for one to 20 weeks of production, depending upon local demand cycles. The cement is delivered to end users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In industrial countries, 80% or more of cement is delivered in bulk.
ClinkerCCNMass
Tricalcium silicate 3 · SiO2C3S25–50%
Dicalcium silicate 2 · SiO2C2S20–45%
Tricalcium aluminate 3 · Al2O3C3A5–12%
Tetracalcium aluminoferrite 4 · Al2O3 · Fe2O3C4AF6–12%
Gypsum CaSO4 · 2 H2OCS̅H22–10%

CementCCNMass
Calcium oxide, CaOC61–67%
Silicon dioxide, SiO2S19–23%
Aluminium oxide, Al2O3A2.5–6%
Ferric oxide, Fe2O3F0–6%
Sulphur oxide, SO3S̅1.5–4.5%

Setting and hardening

Cement sets when mixed with water by way of a complex series of chemical reactions that are still only partly understood. A brief summary is as follows:
The clinker phases—calcium silicates and aluminates—dissolve into the water that is mixed with the cement, which results in a fluid containing relatively high concentrations of dissolved ions. This reaches supersaturation with respect to specific mineral phases: usually first ettringite, and then calcium silicate hydrate which precipitate as newly formed solids. The interlocking of the C-S-H and the ettringite crystals gives cement its initial setting, converting the fluid into a solid, and chemically incorporating much of the water into these new phases.
Gypsum is included in the cement as an inhibitor to prevent flash setting; if gypsum is not present, the initial formation of ettringite is not possible, and so hydrocalumite-group calcium aluminate phases form instead. The premature formation of AFm phases causes a rapid loss of flowability, which is generally undesirable because it renders the placement of the cement or concrete very difficult.
Hardening of the cement then proceeds through further C-S-H formation, as this fills in the spaces between the cement grains with newly formed solid phases. Portlandite also precipitates from the pore solution to form part of the solid microstructure. Some of the initially formed ettringite may be converted to AFm phases, releasing part of the sulfate from its structure to continue reacting with any remaining tricalcium aluminate.