1、 Portland cement Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It is a fine powder produced by grinding Portland ce
2、ment clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as the European Standard EN197.1 ASTM C 150 defines portland cement as hydraulic cement (cement that not only hardens by reacting with
3、water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition. Clinkers are nodules (diameters： 0.2-1.0 inch 525 mm) of a sintered mate
4、rial that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of the limestone, shales, and other naturally occurring materials make portland cement one of the lowest-cost materials widely used over the last century thro
5、ughout the world. Concrete becomes one of the most versatile construction materials available in the world. Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 C for modern cements. The aluminium oxide and iron
6、 oxide are present as a flux 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 (3CaO.Al2O3) formed. The major raw material for the clinker-making is usually limestone (CaCO3)
7、 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%. Second raw materials (materials in the rawmix other than limestone) depend on the purity of the
8、limestone. Some of the second raw 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 History Portland cement was developed from natural cements made in Britain in the early part of the
9、nineteenth century, and its name is derived from its similarity to Portland stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England. The Portland cement is considered to originate from Joseph Aspdin, a British bricklayer from Leeds. It was one of his employees (I
10、saac Johnson), however, who developed the production technique, which resulted in a more fast-hardening cement with a higher compressive strength. This process was patented in 1824. His cement was an artificial cement similar in properties to the material known as Roman cement (patented in 1796 by J
11、ames Parker) and his process was similar to that patented in 1822 and used since 1811 by James Frost who called his cement British Cement. The name Portland cement is also recorded in a directory published in 1823 being associated with a William Lockwood, Dave Stewart, and possibly others. Aspdins s
12、on William, in 1843, made an improved version of this cement and he initially called it Patent Portland cement although he had no patent. In 1848 William Aspdin further improved his cement and in 1853 he moved to Germany where he was involved in cement making. Many people have claimed to have made t
13、he first Portland cement in the modern sense, but it is generally accepted that it was first manufactured by William Aspdin at Northfleet, England in about 1842. The German Government issued a standard on Portland cement in 1878. Production There are three fundamental stages in the production of Por
14、tland cement: 1. Preparation of the raw mixture 2. Production of the clinker 3. Preparation of the cement To simplify the complex chemical formulae which describe the compounds present in cement, a cement chemist notation was invented. This notation reflects the fact that most of the elements are pr
15、esent in their highest oxidation state, and chemical analyses of cement are expressed as mass percent of these notional oxides Rawmix preparation The raw materials for Portland cement production are a mixture of minerals containing calcium oxide, silicon oxide, aluminium oxide, ferric oxide, and mag
16、nesium oxide, as fine powder in the Dry process or in the form of a slurry in the Wet process. The raw materials are usually quarried from local rock, which in some places is already practically the desired composition and in other places requires the addition of clay and limestone, as well as iron
17、ore, bauxite or recycled materials. The individual raw materials are first crushed, typically to below 50 mm. Formation of clinker The raw mixture is heated in a cement kiln, a slowly rotating and sloped cylinder, with temperatures increasing over the length of the cylinder up to a peak temperature
18、of 1400-1450 C . A complex succession of chemical reactions takes place (see cement kiln) as the temperature rises. The peak temperature is regulated so that the product contains sintered but not fused lumps. Sintering consists of the melting of 25-30% of the mass of the material. The resulting liqu
19、id draws the remaining solid particles together by surface tension and acts as a solvent for the final chemical reaction in which alite is formed. Too low a temperature causes insufficient sintering and incomplete reaction, but too high a temperature results in a molten mass or glass, destruction of
20、 the kiln lining, and waste of fuel. When all goes according to plan, the resulting material is clinker. On cooling, it is conveyed to storage. Some effort is usually made to blend the clinker because, although the chemistry of the rawmix may have been tightly controlled, the kiln process potentiall
21、y introduces new sources of chemical variability. The clinker can be stored for a number of years before use. Prolonged exposure to water decreases the reactivity of cement produced from weathered clinker. The enthalpy of formation of clinker from calcium carbonate and clay minerals is about 1500 to
22、 1700 kJ/kg. However, because of heat loss during production, actual values can be much higher. The high energy requirements and the release of significant amounts of carbon dioxide makes cement production a concern for global warming. Cement grinding In order to achieve the desired setting qualitie
23、s in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum or anhydrite) 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
24、 broad particle size range, in which typically 15% by mass consists of particles below 5 m diameter, and 5% of particles above 45 m. 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 (up
25、to 24 hours) of the cement on addition of water is directly proportional to the specific surface area. Typical values are 320380 m2kg1 for general purpose cements, and 450650 m2kg1 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 120 weeks 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 customers silo. In industrial countries, 80% or more of cement is delivered in bulk
