Lime Concrete – Mix Proportions, Uses and Properties

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Lime concrete is composite mixture of lime as binding material, sand as fine aggregate and gravel as coarse aggregate in appropriate proportions. Lime concrete mix ratio depends on the type of construction, but in general, it can be taken up to 1:2 ratio for lime, sand respectively and up to 1:3 ratio for lime, coarse aggregate respectively.

Lime concrete had wide applications in construction before the trend of using cement as binding material. Now-a-days Lime concrete is also used as a capillary break by laying on top of insulation base which is vapor permeable.

It is well known from 5000 years and was widely used for the construction works all over the world before the replacement of lime with cement. Romans prepared concrete using lime and volcanic rock to build underwater structures.

The seawater reacts with the lime through water molecules hydrating the lime and reacting with volcanic ash binding them all together.

What is Lime?

Lime is great binding material. Calcium carbonate (CaCo₃) when heated to a temperature of 1100^oC gives quick lime (CaO) and carbon dioxide (CO₂). Quick lime is a very pure form of lime and highly reactive in nature. Thus, it cannot be stored for more than a week.

Hence, slacking of lime is done by adding a small quantity of water to quick lime. This gives hydrated lime which is stable in nature and can be easily stored. Slacking of lime is an exothermic chemical reaction and causes a significant volume increase.

Industrial Classification of lime

Fat lime:

• Lime with clay impurities less than 5% is called Fat lime. i.e, 95% pure lime. Hence, slacking of this type of lime is highly vigorous and volume increase is up to 2-3 times.
• Fat lime cannot be used in the preparation of lime mortar or lime concrete due to its increase in volume.
• This lime can be used for filling thin joints, whitewashing and in industries like steel, cement etc.
• Bright white in color.

Hydraulic lime:

• Lime with clay impurities less than 30%.
• This lime has maximum hydraulicity. (The ability of lime to set and harden even underwater is known as Hydraulicity)
• Slacking of this lime is moderate and the volume increase is less and hence it is used for the preparation of lime concrete.
• This lime is used for construction underwater, filling thick joints in dam locations.

Poor lime:

• Lime with clay impurities greater than 30% and brownish white in color.
• Slacking of this lime is very slow and the volume increase is very less.
• Due to very low compressive strength, it is only used for construction of temporary structures.

Proportioning of lime concrete

Generally 1:1 to 1:3 lime and coarse aggregate by volume; 1:1 to 1:2 lime and sand by volume are taken to prepare the concrete mix using lime as binding material.

Mixing

First, sand and lime are mixed to form mortar by adding required amount of water in it. After that, coarse aggregates and prepared mortar are mixed thoroughly in the mechanical mixer by adding a proportionate amount of water in batches until a good consistency of the mix is formed i.e, mortar should not tend to separate from the mortar.

Properties of Lime Concrete

• Lime concrete provides good workability and has desired plasticity.
• It prevents subsoil dampness due to certain level of waterproofing property.
• It resists weathering effects.
• Bases constructed with lime concrete bears sufficient loads.
• It needs more time for setting.
• It attains very less compressive strength at the initial stage of setting.
• Lime is cheaper and available easily.
• Lime concrete is environment-friendly as it requires less energy for production of lime and emits less carbon dioxide through carbonation process compared to conventional Portland cement.

Carbonation: Lime mortar hardens by absorbing carbon dioxide when exposed to atmosphere to become calcium carbonate again. Hard and strong hardened lime mortar is formed due to this carbonation process by lime returning to its original state i.e., calcium carbonate.

Lime Concrete Uses in construction

• Lime concrete can be used for the construction of temporary structures or unimportant structures which are built for the ease of construction of permanent structures like tunnels, bridges, retaining walls, dams etc.
• It has low thermal conductivity used for flooring at the ground level of old buildings.
• Lime concrete mixed with an insulating material like lightweight clay or pumice is used for flooring purpose.
• Masonry before the 20^th century was done using lime concrete due to elasticity of the lime mortar which allowed expansion or contraction of the walls without damaging masonry units.
• It adjusts well when coming into the contact of any surface due to its flexibility and hence forms a good bottom base and upper base construction for cement base.
• Foundation base of load-bearing walls can be constructed using Lime concrete.

Use of lime became obsolete over time due to its inconsistent quality, long initial setting time but lime has been rediscovered to be a more environmentally sustainable binding material. The use of the lime has been evolved over time and used for temporary constructions, insulation purpose, underwater constructions to some extent when mixed with volcanic ash.