Cement, Lime And Silicates PDF

Cement, Lime and Silicates  Cement Cement, in general, adhesive substances of all kinds, but, in a narrower sense, the binding materials used in building and civil engineering construction. Cements of this kind are finely ground powders tha...

Cement, Lime and Silicates  Cement Cement, in general, adhesive substances of all kinds, but, in a narrower sense, the binding materials used in building and civil engineering construction. Cements of this kind are finely ground powders that, when mixed with water, set to a hard mass. Setting and hardening result from hydration, which is a chemical combination of the cement compounds with water that yields submicroscopic crystals or a gel-like material with a high surface area. Because of their hydrating properties, constructional cements, which will even set and harden under water, are often called hydraulic cements. It is the most commonly used construction material Definition: “Cement is a crystalline compound of calcium silicates and other calcium compounds having hydraulic properties” (Macfadyen, 2006). In BS EN 197-1, ‘cement’ is defined as: “ … A hydraulic binder, i.e. a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydraulic reactions and processes and which, after hardening, retains its strength and stability even under water.” 2  History The term cement, derives from the Latin word caementum, which meant stone chippings such as were used in Roman mortar—not the binding material itself Lime and clay have been used as cementing material on constructions through many centuries. Romans are commonly given the credit for the development of hydraulic cement, the most significant incorporation of the Roman’s was the use of pozzolan-lime cement by mixing volcanic ash from the Mt. Vesuvius with lime. In 1824, Joseph Aspdin, a British (Leeds) stone mason, obtained a patent for a cement he produced in his kitchen. The inventor heated a mixture of finely ground limestone and clay in his kitchen stove and ground the mixture into a powder create a hydraulic cement-one that hardens with the addition of water. Aspdin named the product Portland cement because it resembled a stone quarried on the Isle of Portland off the British Coast. With this invention, Aspdin laid the foundation for today's Portland cement industry. 3  Chemical Composition Lime 63% Silica 22% Alumina 06% Iron oxide 03% Gypsum 01 to 04%  Function of the manufacturing constituents (i) Lime (CaO): Lime forms nearly two-third (2/3) of the cement. Therefore sufficient quantity of the lime must be in the raw materials for the manufacturing of cement. Its proportion has an important effect on the cement. Sufficient quantity of lime forms di- calcium silicate and tri- calcium silicate in the manufacturing of cement. Lime in excess, causes the cement to expand and disintegrate. (ii) Silica (SiO2): The quantity of silica should be enough to form di-calcium silicate and tri-calcium silicate in the manufacturing of cement. Silica gives strength to the cement. Silica in excess causes the cement to set slowly. (iii) Alumina (Al2O3): Alumina supports to set quickly to the cement. Lowers the clinkering temperature. Alumina in excess, reduces the strength of the cement. (iv) Iron Oxide (Fe2O3): Iron oxide gives colour to the cement. (v) Magnesia (MgO): It also helps in giving colour to the cement. Magnesium in excess makes the cement unsound. (vi) Calcium Sulphate (or) Gypsum (Ca SO4) : At the final stage of manufacturing, gypsum is added to increase the setting of cement.  Types of cement Cements used in construction are usually inorganic, often lime or calcium silicate based, which can be characterized as non-hydraulic or hydraulic respectively, depending on the ability of the cement to set in the presence of water. ❑ Non-hydraulic cement does not set in wet conditions or under water. Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting. ❑ Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. 8  World production is about four billion tonnes per year, of which about half is made in China. If the cement industry were a country, it would be the third largest carbon dioxide emitter in the world with up to 2.8 billion tonnes, surpassed only by China and the United States. The initial calcination reaction (at or above the thermal decomposition temperature) in the production of cement is responsible for about 4% of global CO2 emissions. The overall process is responsible for about 8% of global CO2 emissions, as the cement kiln in which the reaction occurs is typically fired by coal or petroleum coke due to the luminous flame required to heat the kiln by radiant heat transfer. As a result, the production of cement is a major contributor to climate change. 9  Types of Cement Cements are considered hydraulic because of their ability to set and harden under or with excess water through the hydration of the cement’s chemical compounds or minerals There are two types: Those that activate with the addition of water And pozzolanic that develop hydraulic properties when the interact with hydrated lime Ca(OH)2 Pozzolanic: any siliceous material that develops hydraulic cementitious properties when interacted with hydrated lime. HYDRAULIC CEMENTS: Hydraulic lime: Only used in specialized mortars. Made from calcination of clay-rich limestones. Natural cements: Misleadingly called Roman. It is made from argillaceous limestones or interbedded limestone and clay or shale, with few raw materials. Because they were found to be inferior to portland, most plants switched. (http://pubs.usgs.gov/of/2005/1152/2005-1152.pdf)  Hydraulic cement By far the most common type of cement is hydraulic cement, which hardens by hydration of the clinker minerals when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides. The four main mineral phases of the clinker, abbreviated in the cement chemist notation, being: The silicates are responsible for the cement's mechanical properties — the tricalcium aluminate and Tetra Calcium Alumino Ferrite are essential for the formation of the liquid phase during the sintering (firing) process of clinker at high temperature in the kiln. 11  The chemistry of these reactions is not completely clear and is still the object of research. First, the limestone (calcium carbonate) is burned to remove its carbon, producing lime (calcium oxide) in what is known as a calcination reaction. This single chemical reaction is a major emitter of global carbon dioxide emissions. 12  Non-Hydraulic cement A less common form of cement is non-hydraulic cement, such as slaked lime (calcium oxide mixed with water), hardens by carbonation in contact with carbon dioxide, which is present in the air (~ 412 vol. ppm ≃ 0.04 vol. %). First calcium oxide (lime) is produced from calcium carbonate (limestone or chalk) by calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure: CaCO3 → CaO + CO2 The calcium oxide is then spent (slaked) mixing it with water to make slaked lime (calcium hydroxide): CaO + H2O → Ca(OH)2 Once the excess water is completely evaporated (this process is technically called setting), the carbonation starts: Ca(OH)2 + CO2 → CaCO3 + H2O This reaction is slow, because the partial pressure of carbon dioxide in the air is low (~ 0.4 millibar). The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non- hydraulic cement and cannot be used under water. This process is called the lime cycle. 13  Portland cement Portland cement, a form of hydraulic 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 1,450 °C (2,640 °F) in a KILN, in a process known as calcination that liberates a molecule of carbon dioxide from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials 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 nonspecialty grout. The most common use for Portland cement is to make concrete. Concrete is a composite material made of cement, aggregate (gravel and sand), and water. As a construction material, concrete can be cast in almost any shape, and once it hardens, can be a structural (load bearing) element. Portland cement may be grey or white. 14  Manufacturing of cement Producing a cement that meets specific chemical and physical specifications requires careful control of the manufacturing process. The first step in the Portland cement manufacturing process is obtaining raw materials. Generally, raw materials consisting of combinations of limestone, shells or chalk, and shale, clay, sand, or iron ore are mined from a quarry near the plant. At the quarry, the raw materials are reduced by primary and secondary crushers. Stone is first reduced to 5-inch size (125-mm), then to 3/4-inch(19 mm). Once the raw materials arrive at the cement plant, the materials are proportioned to create a cement with a specific chemical composition. 15  GEOLOGY (RAW MATERIALS) The fundamental chemical compounds to produce cement clinker are: Lime (CaO) Silica (SiO2) Alumina (Al2O3) Iron Oxide (Fe2O3) Raw materials used in the production of clinker cement Fly ash: by-product of burning finely grounded coal either for industrial application or in the production of electricity (Macfadyen, 2006) (Hoffman, 2006) 17 18 19 20  Manufacturing of cement Type of Manufacturing Wet Process Dry Process - 74% of cement produced Preheater/Precalciner Process 21 22 23 24  Silicates Silica, SiO2, is a hard, rigid network solid. It is insoluble in water and occurs naturally as quartz and sand, which consists of small fragments of quartz, usually colored golden brown by iron oxide impurities Minerals based on silica and silicates, such as sandstone and granite, are used when a strong, durable, corrosion-resistant construction material is required. Quartzite 25  Silica gets its strength from its covalently bonded network structure. In silica itself, each Si atom is at the center of a tetrahedron of O atoms, and each corner O atom is shared by two Si atoms. Hence, each tetrahedron contributes one Si atom and 4×1/2= 2 O atoms to the solid, which has the empirical formula SiO2. Quartz has a complicated structure; it is built from helical chains of SiO4 units wound around one another, giving a net composition of SiO2 when the sharing of O atoms between units is taken into account. 26  Different Silicates There are many different silicates, which have various arrangements of tetrahedral oxoanions of silicon. The Si-O bond has considerable covalent character. The differences in properties between the various silicates are related to the number of negative charges on each tetrahedron, the number of corner O atoms shared with other tetrahedra, and the manner in which chains and sheets of the linked tetrahedra lie together. Most glasses are primarily mixtures of silicates and ionic compounds Quartz itself is useful in spectroscopy because it is transparent to both ultraviolet and visible radiation. The simplest silicates, the orthosilicates, are built from SiO4-4 ions. They are not very common but include the mineral zircon, ZrSiO4, which is used as a substitute for diamond in costume jewelry. The pyroxenes consist of chains of SiO4 units in which two corner O atoms are shared by neighboring units, the repeating unit is the metasilicate ion, SiO3-2. 27  Electrical neutrality is provided by cations regularly spaced along the chain. The pyroxenes include the gemstone jade, NaAl(SiO3)2 Chains of silicate units can link together to form ladderlike structures that include tremolite, Tremolite is one of the fibrous minerals called asbestos, which can withstand high temperatures. Their fibrous quality is due to the way in which the ladders of SiO4 units lie together but can easily be torn apart. Because of their resistance to fire, asbestos fibers were once widely used for heat insulation in buildings. However, these fibers can lodge in lung tissue, where fibrous scar tissue forms around them, giving rise to asbestosis and a susceptibility to lung cancer. In some minerals, the SiO4 tetrahedra link together to form sheets. An example is talc, a hydrated magnesium silicate, Mg3(Si2O5)2(OH)2. Talc is soft and slippery because the silicate sheets can slide past one another. 28 29 30