What happens when limestone is heated
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1 Chapter name 7 The transformation of substances by humans 112 7.14 Building materials For thousands of years, people have endeavored to protect themselves from weather and temperature fluctuations. Most houses are built from bricks and similar materials. In this chapter we do not understand building materials as the bricks themselves, but rather the binding agents with which the building is made, i.e. substances that are mixed with water to form an easily workable paste and then harden. The binders used today are lime, cement and gypsum. Lime The oldest of these three building materials is lime. It occurs as calcium carbonate (CaCO 3) forming mountains and is mined in quarries. The aim is now to chemically change the lime so that it becomes malleable, but in the end it is again as solid and hard limestone. Lime burning In the first step, lime is burnt, i.e. heated to around 1000 ° C. We already know this burning from carbonate ores. In the process, carbon dioxide is split off (experiment 112.1) and calcium oxide is formed, which is also known as burnt lime. CaCO 3 → CaO + CO 2 Lime burning Lime slaking This burnt lime is then reacted with water. The calcium oxide becomes calcium hydroxide in an exothermic reaction. Only enough water is used to produce a dry powder. It is called slaked lime. CaO + H 2 O → Ca (OH) 2 Lime slaking Setting of the lime mortar This slaked lime is mixed with sand and water and forms the mortar for walls and plastering. Since mortar contains the strongly basic calcium hydroxide (Experiment 112.1), it is corrosive. Mortar splashes in the eyes can cause permanent damage. The mortar sets after processing, which means that the substrate soaks up the water and the mortar becomes harder. In this state, however, it can still be deformed by re-moistening. This way, plaster is smoothed after it has been applied and set. The subsequent hardening takes weeks to months. Calcium hydroxide absorbs carbon dioxide from the air and becomes lime again with a similar strength to the limestone at the beginning. When it hardens, water is produced, which is why freshly plastered interiors are damp for weeks. Ca (OH) 2 + CO 2 → CaCO 3 + H 2 O setting of lime mortar therefore needs the carbon dioxide in the air to harden. It therefore does not become hard under water and is unsuitable for underwater structures (Experiment 113.1). Leftovers can be covered with water and the next day poured off the water, stirred up the mortar again and used again. A disadvantage of the lime mortar is the shrinkage, ie. that it contracts when it hardens. It is therefore essential to add sand. Despite the sand, cracks appear when it hardens. For this reason, two layers of lime mortar are used for plastering. The thin layer of fine plaster closes the shrinkage cracks and gives a beautiful surface. Fig. 112.1: From rock lime to building lime Fig. 112.2: Use of building lime in wall construction Bricks Hardened mortar Coarse plaster Fine plaster Limestone quarry Lime burning CaCO 3 CO 2 Ca (OH) 2 Lime slaughtering Sand H 2 O CaO H 2 O CO 2 CaCO 3 Mortar Heating W&L Lime Lime Lime Lime Weigh a piece of lime and heat it to glow with crucible tongs in a gas burner. Cradle again. Then put some water from a squeeze bottle on the piece of lime and check the pH value with universal indicator paper. Experiment 112.1 Lime burning and extinguishing For test purposes only - property of the publisher öbv
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