Lime is the starting point for the production of pure lime binders, as well as cements. It is also an important ingredient in glass. It is used in the production of iron in blast furnaces, and also in the production of aluminium from nephelin. Here a great deal of lime is used, which becomes Portland cement as a by-product.
Most places on Earth have deposits of lime, either as chalk deposits or as coral and sand formed from disintegrated seashells. The purity of the lime is the decisive factor as far as the end product is concerned. For pure lime binders there has to be a purity of 90%, preferably over 97%. Lime in Portland cement can be less pure. Chalk is a white or light grey lime originating from the shell of Foraminifera organisms.
The production of lime binder from lime ore starts with a combustion process, usually called calcination:
This dividing reaction is endothermic and continues as long as the temperature is maintained at 800 to 1000 °C. In production of Portland cement a temperature between 1400 and 1500 °C is needed. The above reaction illustrates how carbon dioxide is the inevitable by-product, and the high temperature also means that the process is energy intensive. In the production of Portland cement approximately 50% of the emissions of carbon dioxide is from the chemical process, and 50% from burning fuel.
For calcination there are a number of kiln types. Many are simple both to build and use, and production rates of 30 to 150 tons over 24 hours can be achieved in small local plants (Figure 6.2). There are even mobile units that can be used for very small lime deposits (Figure 6.3). Wood is the best fuel, since the flames are longer and create a more even burning of the limestone than other fuels. Also in the calcination of Portland cement wood chips can be used in large
Local scale calcination plant with shaft kilns. Source: Ellis, 1974.
quantities, but mineral coal is still the dominating fuel. In the calcination of lime, natural gas and mixed organic waste - such as used car tyres - are also sometimes used. Wood fuel is looked upon as climate neutral whilst the natural gas gives slightly less emissions of carbon dioxide than mineral coal. The emissions from the burning of organic waste depend on the constituents. Plastics and synthetic rubber have emissions similar to fossil fuels.
Calcinated lime can be used directly to make lime sandstone (see Table 13.2) and pozzolana cements. During the production of Portland cement calcination occurs after the necessary extra constituents are added, see page 69.
Lime has to be slaked so that it can be used, without any additives, for renders, mortars and concrete. The slaking process starts by adding water to the lime on a slaking bench. Figure 6.4 shows a very simple
Mobile calcination plant with rotating kiln. Source: Spence, 1976.
version. The principles are the same regardless of the size of the system. The reaction is exothermic:
A part of the energy needed for combustion is now released as heat. The lime swells up quickly and breaks up during a strong 'explosion' of heat. The lime milk is drained into a hollow and covered with sand. The lime is re-slaked and after a week it is usable for mortar, while lime for rendering needs two to three months storage in the hollow.
The lime is supposed to get stronger and harder if the ground moisture performs the slaking process. In that case, storage has to take from three to seven years, anaerobically, at a depth below frost level.
The technique of dry slaking has become more widespread recently. It is an industrial process where the exact amount of water needed is added. The product is called 'hydrated lime'. Whilst ordinary slaked lime is usually mixed with sand and water, hydrated lime is in powder form.
Small scale production plant for lime mortar. Source: Jessen, 1980.
This has the advantage of lower transport costs and easier handling on site, where it is mixed with sand. Waste from demolition does not cause any problems. Lime products can, in principle, be material recycled by re-burning.
Dolomite usually has a finer grain than lime but otherwise similar properties. The content of magnesium is too high for use in Portland cement, but it has a certain potential as an alternative to lime in pozzo-lana cements. The methods for calcination and slaking are approximately the same as for lime.
Gypsum is an aqueous calcium sulphate which is a natural part of stone salt deposits, precipitated in seawater or in lakes. Anhydrite is a white, translucent material that forms gypsum when water is added. Anhydrite and gypsum are used in the production of plasterboard, sheeting, mortars and as constituents in Portland cement. In recent years industrial gypsum by-products have made up a large proportion of the total volume of gypsum produced (see Industrial gypsum, page 185).
In order to cast moulds with gypsum, the raw material has to be calcined. A temperature of not more than 160-200°C is needed, which entails relatively low energy consumption. In the process most of the chemically bound water in the gypsum is evaporated and the calcination is complete when the vapour smells like rotten eggs.
Waste from demolition and building sites can develop sulphurous pollution from the breaking down by microbes, but this can be avoided by adding lime to the waste. Waste gypsum can be recycled, but these products are heavy and, therefore, may need much energy in terms of transport.
Silicon dioxide is usually used in the form of quartz sand. It has an important role in several cements and in the production of glass and silicone (polysiloxane).
Silicone is the only common plastic that is not mainly based on carbon. The molecule consists of silicon and oxygen atoms, but needs hydrocarbons and copper to initiate the process which takes place at 400°C.
Besides quartz there is also usually a high concentration of silicon dioxide in most types of clay. Fossil meal, diatomite, is a type of earth which is rich in silicon dioxide. It consists of petrified and closed shells from silicious algae. Fossil meal is used as pozzolana, or as insulation against very high temperatures, alone or as an ingredient in brick or mortars.
Perlite is a volcanic type of earth with a high content of silicon dioxide and is usually expanded for use in insulation. The largest deposit is situated in Iceland.
Potassium chloride and sodium chloride are extracted from salt water and used to produce the two important base materials potash and soda, which in turn are the starting point for the manufacture of glass and waterglass.
Potassium waterglass is produced by melting potash and quartz at a temperature of more than 1700 °C. Potash (K2CO3) was once produced from the ash of deciduous trees. It is now mainly produced from potassium chloride.
Sodium waterglass is produced by allowing soda to replace potash in a combination with quartz. The soda is made by passing carbon dioxide and ammonia through a concentrated solution of sodium chloride salt.
Chlorine is produced electrolytically from a solution of sodium chloride. It is very important in the production of chlorinated hydrocarbons for the plastics industry. Hydrochloric acid is made industrially by combusting hydrogen and chlorine gas and is used in the production of polyvinyl chloride PVC.
Sulphur occurs in a natural state, as in Iceland, and can be used for casting by melting and then pouring into a mould. Sulphur is also used in the production of ammonium sulphate and aluminium sulphate. Ammonium sulphate is used as a flame retardant and aluminium sulphate as a fungicide in many plant based insulation materials. Sulphur is most relevant to use when it is an industrial by-product (see Sulphur, page 185). The natural reserves are very limited.
Mica consists of aluminium silicates and is used as a heat resistant transparent material, for example in the windows of oven doors. Ver-miculite is a form of mica that can be expanded by heating to make a granular insulation material. It is mainly used for high temperature insulation. Some deposits of vermiculite have been shown to contain asbestos fibres (US Geological Survey, 2007).
Kaolin is a clay mineral with a high content of silicon dioxide. It is one of the most common minerals and is used as filler in plastics and paints. Metakaolin is obtained from calcination of kaolin clays at temperatures in the range of 700 to 800 °C and is used in lime pozzolana cements.
Montmorillonite is found mainly in bentonite clay. Its most important use is as a waterproofing membrane. By adding water, the clay expands to up to twenty times its own volume. There are many sources on the European continent, but the USA is the main producer.
Phosphates are found in phosphate rock, mainly apatite. Ammonium phosphate which is much used as fire retardant in plant based insulation materials, is produced in a mixture with ammonia which in turn is produced from natural gas.
Borax is extracted mainly from kernite which is relatively common. Boric acid is produced through a reaction with sulphuric acid. Borax and boric acid are used as fungicides and fire retardants in building materials such as insulation made of cellulose fibre, and for timber impregnation. Boron substances are moderately poisonous, and in larger concentrations they affect plants and fish in freshwater.
Asbestos is a fibrous material that was used as reinforcement for ceramics as early as the Stone Age. As a building material it was widely used from around 1935 as reinforcement in different types of concrete, plastic and plaster products, and as insulation, in particular against fire. Use collapsed after the 1960s as it became very clear that asbestos is highly carcinogenic. Products containing asbestos are now banned in most countries.
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