Mortar (masonry)
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and is applied as a paste which then dries hard.
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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. But 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 (see quicklime).
- 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 (potentially toxic). As the mixture dries the slaked lime reacts with atmospheric carbon dioxide (CO2) to re-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 [1] (http://www.americanscientist.org/template/AssetDetail/assetid/17236/page/3) as this process relies solely on the re-absorption of atmospheric carbon dioxide back in the mortar to form calcium carbonate. 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).
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 ([2] (http://www.americanscientist.org/template/AssetDetail/assetid/17236/page/5)) to create a stronger mortar. Note too that Portland cement used today already contains finely ground limestone for some added strength.
Because lime mortar is slow-drying and potentially toxic, lime mortar is no longer used today. Also, because most modern bricks are stronger than earlier bricks, the strength of lime mortar is no longer needed. 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. See American Scientist: Dating Ancient Mortar (http://www.americanscientist.org/template/AssetDetail/assetid/17236/page/2).
Pozzolana mortar
See Pozzolana.