Uva dhevun- making lime for dwelling

(*Information provided by Mohamed Ali Didi, Fasiya, Gn.Fuvahmulah)

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Lime burning in an Asian country

In early days, cement was not available for construction work. People of Fuvahmulah produced uva (u’a) or lime. Uvadhavaameehun, Lime-burners, collect akiri (pebbles) from the beach and carry it near the uva vado- the klin. Uva vado is made by digging the ground and was in the form of a crater. The klin’s depth is normally 4 to 8 feet; and 5 to 15 feet in its width. If more uva had to be produced, they used deeper and wider uva vado.

At the bottom of the uva vado, they spread lighter fire woods. Uvadhavaameehun fill the bottom of the kiln with the driest wood possible – and then the men lay alternating layers of fuel and limestone. A narrow opening of about 1 to 2 feet is made from the bottom to the top, along the side of the uva vado so that fire can be ignited through the window of this passage. Once the bottom layer is made, they put akiri till the uva vado is completely full.

The uva vado had to be filled carefully, with precisely measured amounts and materials (akiri and dharo or woods in the desired proportion) – if the akiri did not bake at a high enough temperature for long enough, the akiri would not transform into uva and the work would be in vain.

Once the uva vado is filled, the dried wood, which is at the bottom of the uva vado, by that little door – will be ignited. And in turn, lit the fuel through the rest of the structure, setting the whole uva vado on fire. The burning process lasts two to three days. People stay near the uva vado to make sure no interruption happens. When the aikiri turns into anguru (red hot), water is poured into the burning akiri.

Limetstone burning in other countries

According to elderly people, water is poured to extinguish the fire at that stage so that the “perfect” uva will be produced.  When the fire dies down, the uva is collected and put into bags. The lime is then transferred to fallavaa, special place to hold lime, located in households. And used by raavaa meehun, masons.

Typically an uva vado took a day to load, two to three days to fire, two days to cool and a day to unload. The whole process continue for a week.

Quicklime and slaked lime (from bbc.co.uk)

For your exam, you need to know how quicklime and slaked lime are obtained from limestone.

Making quicklime

If limestone is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is also called quicklime. It is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate right facing arrow with heat calcium oxide + carbon dioxide

CaCO3right facing arrow with heat CaO + CO2

This is a thermal decomposition reaction.

Making slaked lime

Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

Here are the equations for this reaction:

calcium oxide + water → calcium hydroxide

CaO + H2O → Ca(OH)2

A lot of heat is produced in the reaction, which may even cause the water to boil.

Summary

Using common names instead of chemical names, this is what happens:

limestone right facing arrow with heat quicklime + carbon dioxide

quicklime + water → slaked lime

Uses of limestone

Limestone, quicklime and slaked lime are all used to neutralise excess acidity – which may be caused by acid rain – in lakes and in soils.

Limestone is used as a building material, and to purify iron in blast furnaces. It’s also used in the manufacture of glass, and of cement (one of the components of concrete).

The main uses of limestone and its products

  • Limestone (CaCO3) can be used as a building material and in the manufacturing of iron.
  • Glass – heated with sand and soda (sodium carbonate).
  • Cement – heated with clay in a kiln.
    • Concrete – mixed with sand, water and crushed rock
    • Mortar – mixed with sand and water
  • Quicklime – heated.
    • Slaked lime (Calcium Hydroxide Ca(OH)2) – mixed with water
      • Lime motar – mixed with water

Glass

Glass is made by melting sand and then cooling it. Flat sheets of glass for windows are made by floating molten glass on a layer of molten tin.

Glass manufacturers add sodium carbonate to sand during the manufacturing process, to reduce the melting temperature of the sand and so save energy. The sodium carbonate decomposes in the heat to form sodium oxide and carbon dioxide, but this makes the glass soluble in water. Calcium carbonate (limestone) is therefore also added, to stop the glass dissolving in water. The calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide. About 90 per cent of glass is soda-lime glass, or bottle glass.

Advantages and disadvantages of various building materials

Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or “pointed”. Pollution from burning fossil fuels makes the rain more acidic than it should be, and this acid rain makes these problems worse.

Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.

Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, buildings with lots of glass can be too hot in the summer.

Lime in Roman concrete (from Wikipedia)

The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered an exothermic chemical reaction. The lime was hydrated – incorporating water molecules into its structure – and reacted with the ash to cement the whole mixture together.

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