Mortar (masonry)

Mortar is a workable paste formed by mixture of cement, water and fine aggregate masonry to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, breeze blocks (cinder blocks), etc. Mortar is a mixture of sand, a binder such as cement or lime, and water and is applied as a paste which then sets hard. Mortar can also be used to fix, or point masonry when the original mortar has washed away.

Ancient mortar

The first mortars were made of mud and clay. Because of a lack of stone and an abundance of clay, the Babylonian constructions were of baked brick, using slime or pitch for mortar. According to Roman Ghirshman, the first evidence of man using a form of mortar was at the ziggurat of Sialk in Iran, built of sun-dried bricks in 2900 BC. The Chogha Zanbil Temple in Iran was built in about 1250 BC with kiln-fired bricks and a strong mortar of bitumen.

In early Egyptian pyramids constructed about 2600-2500 B.C., the limestone blocks were bound by mortar of mud and clay, or clay and sand. In later Egyptian pyramids, the mortar was made of either gypsum or lime. Gypsum mortar was essentially a mixture of plaster and sand and was quite soft.

Historically, building skills with concrete and mortar next appeared in Greece. The excavation of the underground aqueduct of Megara revealed that a reservoir was coated with a pozzolanic mortar 12 mm thick. This aqueduct dates back to c. 500 BC. Pozzolanic mortar is a lime based mortar, however it is made with an additive of volcanic ash that allows it to be hardened underwater, thus known as hydraulic cement. The Greeks obtained the volcanic ash from the Greek islands Thira and Nisiros, or from the then Greek colony of Dicaearchia (Pozzuoli) near Naples, Italy. The Romans later improved the use and methods of making what became known as pozzolanic mortar and cement. Even later, the Romans used a mortar without pozzolana using crushed terra cotta, which introduced aluminum oxide and silicon dioxide into the mix. This mortar was not as strong as pozzolanic mortar, but, because it was more dense, it better resisted penetration by water.

It is not understood why the art of making hydraulic mortar and cement, which was perfected and in such widespread use by both the Greeks and Romans, was then lost for almost two millennia. During the Middle Ages when the gothic cathedrals were being built, the only active ingredient in the mortar was lime. Since cured lime mortar could be degraded by contact with water, many structures suffered from wind blown rain over the centuries.

Portland cement mortar

Portland cement mortar (often known simply as cement mortar) is created by mixing Portland cement with sand and water.

It was invented in the mid-nineteenth century, as part of scientific efforts to develop stronger mortars than existed at the time. It was popularized during the late nineteenth century, and by 1930 it had superseded lime mortar for new construction. The main reason for this was that it sets hard and quickly, allowing a faster pace of construction. However, as a general rule, it should not be used for the repair of older buildings constructed in lime mortar, which require the flexibility, softness and breathability of lime if they are to function correctly.

The Portland cement mortar is the basis for concrete, a mixture usually composed of this particular mortar with the addition of aggregate.

Polymer cement mortar

Polymer cement mortars (PCM) are the materials which are made by partially replacing the cement hydrate binders of conventional cement mortar with polymers. The polymeric admixtures include latexes or emulsions, redispersible polymer powders, water-soluble polymers, liquid resins and monomers. It has low permeability, and it reduces the incidence of drying shrinkage cracking, mainly designed for repairing concrete structures. Example: MagneLine

Lime mortar

The speed of set can be increased by using impure limestones in the kiln, to form a hydraulic lime that will set on contact with water. Such a lime must be stored as a dry powder. Alternatively, a pozzolanic material such as calcined clay or brick dust may be added to the mortar mix. This will have a similar effect of making the mortar set reasonably quickly by reaction with the water in the mortar.

Using Portland cement mortars in repairs to older buildings originally constructed using lime mortar can be problematic. This is because lime mortar is softer than cement mortar, allowing brickwork a certain degree of flexibility to move to adapt to shifting ground or other changing conditions. Cement mortar is harder and allows less flexibility. The contrast can cause brickwork to crack where the two mortars are present in a single wall.

Lime mortar is considered breathable in that it will allow moisture to freely move through it and evaporate from its surface. In old buildings with walls that shift over time, there are often cracks which allow rain water into the structure. The lime mortar allows this moisture to escape through evaporation and keeps the wall dry. Repointing or rendering an old wall with cement mortar stops this evaporation and can cause problems associated with moisture behind the cement.

Pozzolana mortar

Pozzolana is a fine, sandy volcanic ash, originally discovered and dug in Italy at Pozzuoli in the region around Mount Vesuvius, but later at a number of other sites. The ancient Roman architect Vitruvius speaks of four types of pozzolana. It is found in all the volcanic areas of Italy in various colours: black, white, grey and red.

Finely ground and mixed with lime it acts like Portland cement and makes a strong mortar that will also set under water.

Radiocarbon dating

An international team headed by Åbo Akademi University has developed a method of determining the age of mortar using radiocarbon dating. As the mortar hardens, the current atmosphere is encased in the mortar and thus provides a sample for analysis. One major challenge is various factors that affect the sample and raise the margin of error for the analysis.

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