Lime

Pozzolanic materials are siliceous or aluminous materials that have little cementing value themselves, but in finely divided form and in the presence of moisture react chemically with calcium hydroxide (lime) at ordinary temperature to form compounds with cementing properties. Examples: fly ash, volcanic ash, surkhi, burnt clay, rice husk ash and silica fume.

Lime is a cementing material obtained by burning limestone (calcium carbonate). It is classified as follows (IS 712).

Other forms are quick lime (CaO), slaked lime (Ca(OH)₂) and hydrated lime. Magnesium lime (dolomitic) contains over 5% magnesia and is used in plaster.

Hydraulic limes are also used in structural work and for lime-surkhi mortar, while fat lime is mainly used for finishing.

Calcination

Calcination is the burning of limestone at 900 to 1000 °C in a kiln, which drives off carbon dioxide and gives quicklime.

Slaking (hydration)

Slaking is the addition of water to quicklime. It reacts violently, gives out heat, swells to 2 to 2.5 times its volume and turns into slaked lime (calcium hydroxide) in powder or paste form.

Pozzolanic materials are added to lime (and cement) for the following reasons.

1. They react with free lime to form insoluble calcium silicates and aluminates, giving hydraulic properties to fat lime so that it can set under water and in damp places.
2. They increase the strength and durability of mortar and concrete, and improve resistance to sulphate and chemical attack.
3. They give low heat of hydration, useful in mass concrete like dams.
4. They reduce permeability, and the cost, since fly ash, surkhi and ash are cheap by-products.
5. They improve workability and reduce bleeding.
6. They save cement and help the environment by using waste material.

Properties

• Fat lime is white, hardens slowly and has high plasticity.
• Slakes with large heat and expansion; hydraulic lime slakes slowly.
• Sets in air (fat lime by carbonation) and under water (hydraulic lime).
• Good workability and cohesion, shrinks little and is adhesive.
• Resists weathering moderately, and is easily available and cheap.

Uses

1. Whitewashing and distempering of walls.
2. Plastering and mortar for masonry, with sand or surkhi.
3. Lime concrete in foundations and floors.
4. Manufacture of cement, lime-sand bricks and as a soil stabiliser.
5. Making putty, and use in water softening and sugar industry.

Quick lime is calcium oxide (CaO), a white lumpy material obtained by burning limestone at about 900 to 1000 °C. It is very reactive and slakes violently with water to form slaked lime.

Lime is manufactured by burning limestone (CaCO₃) in the following steps.

1. Quarrying and selection: pure limestone is quarried and broken into pieces of size 50 to 75 mm.
2. Calcination: the stone is burnt in a clamp or kiln (intermittent or continuous kiln) at 900 to 1000 °C. Fuel is wood, coal or oil.

3. Cooling and sorting: the quicklime is drawn from the kiln, cooled and sorted from the unburnt and over-burnt pieces.
4. Slaking: quicklime is treated with water to get slaked lime, which can be used or stored.

Continuous (vertical) kilns give better output than clamp kilns. Burning below the correct temperature leaves unburnt core and over-burning makes dead lime.

Properties of pozzolanic materials:

1. Rich in reactive silica and alumina (over 70% SiO₂ + Al₂O₃ + Fe₂O₃).
2. Very fine in grain size, often finer than cement; fineness gives more reaction surface.
3. Do not harden by themselves, but react with lime in the presence of water at normal temperature.
4. Form insoluble cementing compounds that give strength and impermeability.
5. Impart hydraulic character to lime and reduce the heat of hydration.
6. Give good resistance to sulphates, chemical attack and alkali–aggregate reaction.
7. Have a slow strength gain, but higher long-term strength.
8. Low specific gravity and low cost.

Fat lime (high calcium, rich or pure lime) is obtained by burning nearly pure limestone.

Composition

Contains over 95% calcium oxide (CaO) with less than 5% of clay and other impurities.

Properties

• Pure white colour.
• Slakes vigorously with water, producing much heat and expanding to 2 to 2.5 times its volume.
• Sets and hardens slowly in air by absorbing CO₂ (carbonation); does not set under water.
• High plasticity and good workability.
• Low strength compared to hydraulic lime, and soluble in water.

Uses

1. Whitewashing and colour washing.
2. Plastering walls and as mortar for ordinary masonry in dry places.
3. Mixed with pozzolana (surkhi) for hydraulic mortar.
4. Making bricks, putty, and in the manufacture of cement and glass.

Fat lime

Pure quicklime with over 95% CaO.

• White colour, slakes with great heat and swelling.
• Sets slowly in air by absorbing CO₂, not under water.
• Low strength, high plasticity.
• Used for whitewashing and plaster.

Hydraulic lime

Limestone burnt with 5 to 30% clay.

• Greyish colour, slakes slowly with little heat and expansion.
• Sets under water and in damp places through hydration of silicates and aluminates, and gains good strength.
• Classified as feebly, moderately and eminently hydraulic.
• Used for mortar in foundations, damp places and masonry below ground.

The slaking nature is a field test of the lime class.

1. Take a small lump of quicklime in a container and add water.
2. Observe the reaction.
   • Fat lime: slakes quickly and violently, producing great heat and steam, swelling to 2 to 2.5 times its volume and forming a fine white paste.
   • Hydraulic lime: slakes slowly with little heat and little swelling and leaves a gritty residue.
   • Poor lime: hardly slakes and gives little heat.

The time taken and the heat evolved, together with the volume expansion, show the nature of the lime. A thermometer can record the maximum temperature rise.

Hydraulicity of lime

Hydraulicity is the property of lime that allows it to set and harden under water and in damp conditions, due to the reaction of calcium oxide with the clay (silica and alumina) present in it. It depends on the clay content, burning temperature and grinding.

Lean lime

Lean (poor) lime contains over 30% clay and other impurities. It slakes slowly, sets slowly, gives a thin paste of low strength and dirty grey colour. It is used for inferior mortar and plastering in dry conditions.

Hydraulic lime is lime that contains clay (silica and alumina), usually 5 to 30%, so that it sets and hardens even under water.

Manufacture

Impure (clayey) limestone is burnt at 1000 to 1100 °C. Silica and alumina combine with lime to form calcium silicates and aluminates, which give hydraulic properties.

Classification (by clay content)

• Feebly hydraulic: 5 to 15% clay.
• Moderately hydraulic: 15 to 25%.
• Eminently hydraulic: 25 to 30%.

Properties

• Grey or yellowish colour.
• Slakes slowly with little heat and small expansion.
• Sets in damp and underwater conditions, within 2 to 3 days, with higher strength than fat lime.
• Less affected by atmosphere.

Uses

Mortar for masonry in foundations, damp places and thick walls, plastering of exposed surfaces, lime concrete and lime-surkhi mortar.

Setting is the stiffening of lime mortar or paste into a hard mass. It happens in two ways.

Fat lime (non-hydraulic): setting by carbonation

Slaked lime paste first loses water by evaporation and then absorbs CO₂ from the air to turn back into calcium carbonate, which binds the sand particles.

The action is slow and starts from the surface inwards, so thick walls take years to harden fully. It does not happen under water or in damp inside.

Hydraulic lime: setting by hydration

The calcium silicates and aluminates formed during burning react with water to form insoluble hydrates which crystallise and interlock, giving strength within a few days, even under water. Later, carbonation of the free lime adds strength.

Lime sets slower and with less strength than cement.

Hydraulic lime is classified by the clay content, which decides its hydraulicity.

IS 712 also classes lime as Class A (eminently hydraulic), B (semi-hydraulic), C (fat lime), D (magnesium lime) and E (kankar lime).

Fat lime reacts with moisture and CO₂ in the air, so it must be stored and handled with care.

1. Quick lime: store in a dry, airtight shed with a raised, waterproof floor, away from water and rain. Use as soon as possible, because it air-slakes.
2. Slaked lime: keep in covered pits or tanks and cover with a layer of sand or water to prevent carbonation. It improves with ageing in the pit for a few weeks.
3. Handling: use gloves, goggles and a mask. Quicklime is caustic and generates heat with water, so protect skin, eyes and lungs. Keep it away from flammable material.
4. Do not mix different lime batches, and transport in bags or covered vehicles.