Mortar (masonry)
Mortar is a material used in masonry to fill the gaps between blocks in construction. 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 that then sets hard.
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.
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 It was popularised during the nineteenth century and it had superseded lime mortar by 1930 for new construction.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.
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Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and most non-specialty grout. It is a finely-ground powder produced by grinding Portland cement clinker (more than 90%), a maximum of about 5% gypsum which controls the set time, and up to 5% minor constituents (as allowed by various standards). As defined by the European Standard EN197.1, Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium content (MgO) shall not exceed 5.0% by mass. (The last two requirements were already set out in the German Standard, issued in 1909). Portland cement clinker is made by heating, in a kiln, an homogenous mixture of raw materials to a sintering temperature, which is about 1450 °C for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as Low Heat (LH) and Sulfate Resistance (SR), it is necessary to limit the amount of tricalcium aluminate (3CaO.Al2O3) formed. The major raw material for the clinker-making is limestone (CaCO3). Normally, an impure limestone which contains SiO2 is used - the CaCO3 content can be as low as 80%. Secondary raw materials depend on the purity of the limestone. Some of the secondary raw materials used are: clay, shale, sand, iron ore, bauxite, fly ash and slag. When a cement kiln is fired by coal, the ash of the coal becomes a secondary raw material.
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Lime mortar
Lime mortar is created by mixing sand, slaked lime and water.
The earliest known use of lime mortar dates to about 4000 B.C.E. in Ancient Egypt. Lime mortars have been used throughout the world, notably in Roman Empire buildings throughout Europe and Africa. The vast majority of pre-1900 masonry buildings in Europe and Asia are built from lime mortar.
The process of making lime mortar is simple. Limestone is burnt in a kiln to form quicklime. The quicklime is then slaked (mixed with water) to form slaked lime, either in the form of lime putty or of hydrated lime powder. This is then mixed with sand and water to form mortar.
This kind of lime mortar, known as non-hydraulic, sets very slowly through reaction with the carbon dioxide in air. A very thick wall made of lime mortar may take centuries to completely set and harden. This is normal and not problematic.
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.
Pozzolana mortar
Pozzolana is a fine, sandy volcanic ash, originally discovered and dug in Italy at Pozzuoli in the region around Vesuvius, but later at a number of other sites. 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. Also see pozzolanic ash for more information.
Finely ground and mixed with lime it acts like Portland cement and makes a strong mortar that will also set under water.
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Pozzolana, also known as pozzolanic ash, is a fine, sandy volcanic ash, originally discovered and dug in Italy at Pozzuoli in the region around Vesuvius, but later at a number of other sites. 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 creates a hydraulic cement and can be used to make a strong mortar that will also set under water. It transformed the possibilities for making concrete structures, although it took the Romans some time to discover its full potential. Typically it was mixed two-to-one with lime just prior to mixing with water. The Roman port at Cosa was built of Pozzolana that was poured underwater, apparently using a long tube to carefully lay it up without allowing sea water to mix with it. The three piers are still visible today, with the underwater portions in generally excellent condition after 2100 years.
Pozzolana is a siliceous and aluminous material which reacts with calcium hydroxide in the presence of water to form compounds possessing cementitious properties at room temperature. This allowed it to be used in the Roman Empire to make cement by combining with lime and water. Pozzolanic ash occurs naturally in volcanic deposits from Pozzuoli and Naples.
Modern pozzolanic cements are a mix of natural or industrial pozzolans and Portland cement. In addition to underwater use, the pozzolana's high acidity makes it especially resistant to common forms of corrosion from sulphates. Once fully hardened, the Portland cement-Pozzolana blend may be stronger than Portland cement due to its lower porosity, which also makes it more resistant to water absorption and spalling.
Some industrial sources of materials with pozzolanic properties are: Class F (silicious) fly ash from coal fired power plants, silica fume from silicon production, rice husk ash from rice paddy-fields (agriculture), and metakaolin from oil sands operations. Metakaolin, a powerful pozzolan, can also be manufactured, and is valued for making white concrete.
Other industrial waste products used in Portland composite cements include Class C (calcareous) fly ash and ground granulated blast furnace slag.
ReferencesISBN links support NWE through referral fees
- McCann, A.M. (1994). "The Roman Port of Cosa",(273 B.C.E.), Scientific American, Ancient Cities, pp. 92-99, by Anna Marguerite McCann. Covers, hydraulic concrete, of "Pozzolana mortar" and the 5 piers, of the Cosa harbor, the Lighthouse on pier 5, diagrams, and photographs. Height of Port city: 100 B.C.E.
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