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construction science and materials

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Construction science and materials: Assessment No 4: TASK 1 Production and manufacturing processes: LIME: Lime is made by heating chalk or limestone (both calcium carbonate, CaCO3) to a temperature of 900� in a kiln. The heating breaks down the limestone to produce quicklime (calcium oxide, CaO) and carbon dioxide gas (CO2). The chemical reaction is shown below: CaCO3 Heat CaO CO2 + Limestone 900�C quick lime carbon dioxide When quicklime is added to water the quicklime combines with the water to form hydrated lime (calcium hydroxide Ca (OH) 2.) This process is known as slaking. The chemical reaction, which takes place, is shown below: CaO H2O Ca (OH) 2 + Quicklime Water Hydrated lime GYPSUM PLASTER: Gypsum plaster is made from gypsum, whish is naturally occurring rock (calcium sulphate dihydrate, CaSO4 .2H 20). The gypsum is quarried, crushed and ground to a fine powder. The gypsum is then heated to drive off some or all of the water. If the gypsum is heated to 150�C, then only some of the water is lost and the gypsum converts to plaster of Paris (calcium sulphate hemihydrate, CaSO4.1/2 H20) as shown below CaSO4 .2H 20 heat CaSO4.1/2 H20 Gypsum 150�C plaster of Paris Plaster of Paris sets rapidly (within 10-20 minutes). Because this is too fast for most plastering work, a retarder (e.g. keratin) is often added to slow down the setting. If the gypsum is heated to 650�C, then all the water is driven off and the gypsum converts to anhydrous gypsum (calcium sulphate, CaSO4). CaSO4 .2H 20 heat CaSO4 Gypsum 650�C anhydrous gypsum PORTLAND CEMENT: Portland cement is made by heating a mixture of limestone (or chalk) and clay (or shale) to 1,500�C in a kiln. This heating causes the material to form a hard granular material called clinker. The clinker is allowed to cool and then ground to a fine powder to produce Portland cement. ...read more.


Both dry and wet rots can attack hardwood and softwood and whilst dry rot will not grow in saturated conditions neither will grow in a dry environment. Fungi usually attack untreated wood because it is easier to penetrate. It develops extensive root systems (mycelium) to remove nutrients and oxygen from the cells. This in turn destroys the chemical structure of the timber. The minute airborne spores will germinate if they land on a damp substrate such as wood. The germinating spores produce this thread like hyphae, which collectively from a mycelium. The hyphae making up the mycelium continue to grow and extend through the timber cells where the timber then begins to disintegrate. The presence of fungi is usually a warning of more serious dampness problem further down the structure and should always be investigated. Insect attack: Common furniture beetle "Woodworm" is the most common cause of insect attack of softwoods in buildings, and is often to be found in structural timbers, roofs, floors and joists. The female adult beetle lays her eggs onto the susceptible timbers, and they hatch into larva which in turn burrows into the timber, gradually weakening it. This process can take at least three years, with the larvae growing to a length of 2.5mm. After the pupal stage, the adult beetles emerge from the timber through a 2mm flight (exit) hole. Deathwatch beetle The deathwatch beetle causes deterioration in structural hardwoods such as oak, elm and chestnut, which have already been partly decayed by wet rot. This pest is more of a threat to large timbers in older buildings and occurs mainly in the Southern and Central areas of England and Wales. It has yet to be recorded in Scotland. Powder post beetle This beetle attacks the sapwood of larger-pored hardwoods, and is more commonly found in flooring, plywood and furniture. Wood-boring weevils After the Furniture Beetle, wood-boring weevils are probably the most common timber pest. ...read more.


a partial vacuum is created within the lumen of the cells, causing the preservative to be drawn into the wood. Some penetration occurs during the hot baths, but most of it takes place during the cold baths. This cycle is repeated with a significant time reduction compared to other steeping processes. Each bath may last 4 to 8 hours or in some cases longer. The temperature of the preservative in the hot bath should be between 60 to 110 �C (140 to 225 �F) and 30 to 40 �C (85 to 105 �F) in the cold bath (depending on preservative and treespecies). The average penetration depths achieved with this process ranges from 30 mm to 50 mm (1 to 12/3 in.). Both preservative oils and water-soluble salts can be used with this treatment. Due to the longer treatment periods, this method finds little use in the commercial wood preservation industry today. PRESSURE IMPREGNATION Pressure processes are those in which the treatment is carried out in closed cylinders with applied pressure and/or vacuum. These processes have a number of advantages over the non-pressure methods. In most cases, a deeper and more uniform penetration and a higher absorption of preservative is achieved. Another advantage is that the treating conditions can be controlled so that retention and penetration can be varied. These pressure processes can be adapted to large-scale production. The high initial costs for equipment and the energy costs are the biggest disadvantages. These treatment methods are used to protect ties, poles and structural timbers and find use throughout the world today. The various pressure processes that are used today differ in details, but the general method is in all cases the same. The treatment is carried out in cylinders. The timbers are loaded onto special tram cars, so called "buggies," and into the cylinder. These cylinders are then set under pressure often with the addition of higher temperature. As final treatment a vacuum is frequently produced to extract excess preservatives. These cycles can be repeated to achieve better penetration. Sabeen Choudhary 1 of 17 ...read more.

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