Portland cement is the most common type of cement in general use around the world, used as a basic ingredient of concrete, mortar, stucco, and most non-speciality grout. It developed from other types of hydraulic lime in England in the mid 19th century and usually originates from limestone. It is a fine powder produced by heating materials in a kiln to form what is called clinker, grinding the clinker, and adding small amounts of other materials. Several types of Portland cement are available with the most common being called ordinary Portland cement (OPC) which is grey in color, but a white Portland cement is also available.
Portland cement is caustic so it can cause chemical burns, the powder can cause irritation or with severe exposure lung cancer, and contains some toxic ingredients such as silica and chromium. Environmental concerns are the high energy consumption required to mine, manufacture, and transport the cement and the related air pollution including the release of greenhouse gases (e.g., carbon dioxide), dioxin, NOx, SO2, and particulates.
The low cost and widespread availability of the limestone, shales, and other naturally occurring materials used in Portland cement make it one of the lowest-cost materials widely used over the last century throughout the world. Concrete is one of the most versatile construction materials available in the world.
History
Portland cement was developed from natural cements made in Britain beginning in the middle of the 18th century, and 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 (sometimes called ordinary or normal Portland cement) began in 1756 when John Smeaton experimented with combinations of different limestones and additives including trass and pozzolanas relating to the planned 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 1843, Aspdin's son William improved their cement, which was initially called "Patent Portland cement", although he had no patent. In 1818, French engineer Louis Vicat invented an artificial hydraulic lime considered the "principal forerunner" of Portland cement and "...Edgar Dobbs of Southwark patented a cement of this kind in 1811."
Portland cement was used by Joseph Aspdin in his cement patent in 1824 because of the cements' resemblance to Portland stone. The name "Portland cement" is also recorded in a directory published in 1823 being associated with a William Lockwood, Dave Stewart, and possibly others.However, Aspdins' cement was nothing like modern Portland cement, but was a first step in the development of modern Portland cement, called a 'proto-Portland cement'.William Aspdin had left his fathers company and in his cement manufacturing apparently accidentally produced calcium silicates in the 1840s, a middle step in the development of Portland cement. In 1848, William Aspdin further improved his cement; in 1853, he moved to Germany, where he was involved in cement making.William Aspdin made what could be called 'meso-Portland cement' (a mix of Portland cement and hydraulic lime).Isaac Charles Johnson further refined the production of 'meso-Portland cement' (middle stage of development) and claimed to be the real father of Portland cement.John Grant of the Metropolitan Board of Works in 1859 set out requirements for cement to be used in the London sewer project. This became a specification for Portland cement. The next development with the manufacture of Portland cement was the introduction of the rotary kiln patented by German Friedrich Hoffmann called a Hoffmann kiln for brick making in 1858 and then Frederick Ransome in 1885 (U.K.) and 1886 (U.S.) which allowed a stronger, more homogeneous mixture and a continuous manufacturing process.The Hoffman "endless" kiln which gave "perfect control over combustion" was tested in 1860 and showed the process produced a better grade of cement. This cement was made at the Portland Cementfabrik Stern at Stettin, which was the first to use a Hoffman kiln.It is thought that the first modern Portland cement was made there. The Association of German Cement Manufacturers issued a standard on Portland cement in 1878.
Portland cement had been imported into the United States from Germany and England and in the 1870s and 1880s it was being produced by Eagle Portland cement near Kalamazoo, Michigan, and in 1875, the first Portland cement was produced by Coplay Cement Company under the direction of David O. Saylor in Coplay, Pennsylvania.By the early 20th century American made Portland cement had displaced most of the imported Portland cement.
Manufacturing
ASTM C150 defines Portland cement as "hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers which consist essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition."and the European Standard EN 197-1 as:
Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaO·SiO2 and 2 CaO·SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.
(The last two requirements were already set out in the German Standard, issued in 1909).
Clinkers make up more than 90% of the cement along with a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents (fillers) as allowed by various standards. Clinkers are nodules (diameters, 0.2–1.0 inch [5–25 mm]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The key chemical reaction which defines Portland cement from other hydraulic limes occurs at these high temperatures (>1,300 °C (2,370 °F) and is when the belite (Ca2SiO4) combines with calcium oxide (CaO) to form alite (Ca3SiO5).
Portland cement clinker is made by heating, in a cement kiln, a mixture of raw materials to a calcining temperature of above 600 °C (1,112 °F) and then a fusion temperature, which is about 1,450 °C (2,640 °F) for modern cements, to sinter the materials into clinker. The materials in cement clinker are alite, belite, tri-calcium aluminate, and tetra-calcium 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 (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3 CaO·Al2O3) formed. The major raw material for the clinker-making is usually limestone (CaCO3) mixed with a second material containing clay as 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 (materials in the rawmix other than limestone) 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 by coal, the ash of the coal acts as a secondary raw material.
Setting and hardening
Cement sets when mixed with water by way of a complex series of chemical reactions still only partly understood. The different constituents slowly crystallise and the interlocking of their crystals gives cement its strength. Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble calcium carbonate. After the initial setting, immersion in warm water will speed up setting. Gypsum is added as an inhibitor to prevent flash setting.
Use
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. Concrete can be used in the construction of structural elements like panels, beams, road furniture, or may make cast-in situ concrete for building superstructures like roads and dams. These may be supplied with concrete mixed on site, or may be provided with "ready-mixed" concrete made at permanent mixing sites. Portland cement is also used in mortars (with sand and water only) for plasters and screeds, and in grouts (cement/water mixes squeezed into gaps to consolidate foundations, road-beds, etc.).
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.
