Mortar (masonry)

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This article describes the use of the term "mortar" in masonry. For other meanings, see mortar.

Mortar holding weathered bricks.

Mortar is a material used in masonry to fill the gaps between blocks in construction and bind the blocks together. The blocks may be stone, brick, breezeblocks (cinder blocks), etc. Mortar is a mixture of

sand,
a powdered adhesive such as cement,
and water

and is applied as a paste which then dries hard.

Contents

1 Gypsum mortar
2 Cement mortar
3 Lime mortar
4 Dating lime mortar
5 Pozzolana mortar
6 References

Gypsum mortar

The earliest known mortar was used by the ancient Egyptians and was made from gypsum. This form was essentially a mixture of plaster and sand and was quite soft.

Cement mortar

Cement mortar is created by mixing Pozzolana or Portland cement with sand and water and is harder than gypsum mortar.

Though cement was first invented by the Egyptians (see cement), the first known use of cement mortar is found in the Roman Empire. However, the use of cement mortar did not become widespread in Europe until the 18th century and did not fully replace lime mortar until about 1930.

Lime mortar

Lime mortar is created by mixing sand and quicklime, or quicklime and cement mortar, and water.

The earliest known use of lime mortar dates to about 4000 BC in Ancient Egypt. Lime mortars were used throughout the world, notably in Roman Empire buildings throughout Europe and Africa.

Quicklime is made by the following simple process:

CaCO₃ + heat → CaO + CO₂.

Calcination: heat limestone or marble, both of which contain primarily calcium carbonate (CaCO₃), to 900°C.
The excessive heat then causes carbon dioxide, CO₂, to be released as a gas.
The result is quicklime, CaO (calcium oxide).

When the quicklime is then mixed with water it forms calcium hydroxide (Ca(OH)₂), also called hydrated or slaked lime. As the mixture dries the slaked lime reacts with atmospheric carbon dioxide (CO₂) to form back into solid calcium carbonate (CaCO₃), releasing yet more (evaporating) water in the process. Though slaked lime is normally fairly plastic and easy to work with, it is nevertheless toxic with overexposure (see slaked lime).

Lime mortar dries very slowly. In some buildings, lime mortar made of quicklime and sand alone may take several years or even decades to completely solidify as this process relies solely on the re-absorption of atmospheric carbon dioxide back in the mortar to form calcium carbonate. [Dating Ancient Mortar]. Page 3. American Scientist Online. Even when quicklime is mixed with cement, the same properties still hold; though it may seem to dry faster (because of the quicker-drying cement present).

Despite the fact that pure lime mortar is slow setting, the speed of set can be increased by using a pozzolanic material in the mix. Some sources of inpure limestone have a high level of acid insoluble material (clays, etc) which act as pozzolans. Lime made from these impure limestone sources is called hydraulic lime.

Nevertheless, limestone-free cement mortar is not as strong as lime mortar (see cement). Analogous to the greater strength and lower porosity associated with modern Pozzolana mortar when compared with Portland mortar alone (see Pozzolana), lime mortar is stronger than cement mortar alone (see cement). This is because the components of slaked lime and slag (both consisting primarily of calcium) act analogously as the silicates and alumina -- in the form of silica fume and the primary component of fly ash -- that are added to modern cement mortars to create a stronger mortar. [Dating Ancient Mortar]. Page 5. American Scientist Online. Note too that Portland cement used today already contains finely ground limestone for some added strength.

Lime mortar was an integral part of how traditional buildings "worked". Lime mortar, unlike cement mortar, is porous. Water entering walls from the ground or above could escape through the mortar, rather than leaving the wall waterlogged, which would lead to increased dampness inside. Many old buildings have been damaged by being repointed in modern harder cements, which are not porous and change the water handling profile in an undesirable way. Lime mortar is also more flexible than Portland cement, so building movement is less likely to crack the mortar. Traditional buildings lack the expansion joints found in modern buildings.

Because lime mortar is slow-drying and slightly more hazardous to work with, it is less often used for new buildings, which take a different approach to damp prevention. The strength of many walls made today with modern bricks derives primarily from the stronger bricks.

Dating lime mortar

Because lime mortar absorbs atmospheric carbon dioxide as it dries, samples of lime mortar taken at archaeological sites may be carbon-dated to determine approximate age. [Dating Ancient Mortar]. Page 2. American Scientist Online.

Pozzolana mortar

References

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