Mortar is a workable paste used to bind building blocks such as stones, bricks, and concrete
masonry units together, fill and seal the irregular gaps between them, and sometimes
add decorative colors or patterns in masonry walls. In its broadest sense mortar includes
pitch, asphalt, and soft mud or clay, such as used between mud bricks. Mortar comes from
Latin mortarium meaning crushed. Cement mortar becomes hard when it cures,
resulting in a rigid aggregate structure; however the mortar is intended to be weaker
than the building blocks and the sacrificial element in the masonry, because the mortar
is easier and less expensive to repair than the building blocks. Mortars are typically
made from a mixture of sand, a binder, and water. The most common binder since the early
20th century is Portland cement but the ancient binder lime mortar is still used in some new
construction. Lime and gypsum in the form of plaster of Paris are used particularly
in the repair and repointing of buildings and structures because it is important the
repair materials are similar to the original materials: The type and ratio of the repair
mortar is determined by a mortar analysis. There are several types of cement mortars
and additives.
Ancient mortar The first mortars were made of mud and clay.
Because of a lack of stone and an abundance of clay, Babylonian constructions were of
baked brick, using lime or pitch for mortar. According to Roman Ghirshman, the first evidence
of humans using a form of mortar was at the Mehrgarh of Baluchistan in Pakistan, built
of sun-dried bricks in 6500 BCE. The ancient Harappa city in Pakistan was built in about
2600 BCE with kiln-fired bricks and a gypsum mortar. Gypsum mortar, also called plaster
of Paris, was used in the construction of the Egyptian pyramids and many other ancient
structures. It is made from gypsum which requires a lower firing temperature so is easier to
make than lime mortar and sets up much faster which may be a reason it was used as the typical
mortar in ancient, brick arch and vault construction. Gypsum mortar is not as durable as other mortars
in damp conditions. In early Egyptian pyramids constructed about
2600–2500 BCE, 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.
In the Indian subcontinent, multiple cement types have been observed in the sites of the
Indus Valley Civilization, such as the Mohenjo-daro city-settlement that dates to earlier than
2600 BCE. Gypsum cement that was "light grey and contained sand, clay, traces of calcium
carbonate, and a high percentage of lime" was used in the construction of wells, drains
and on the exteriors of "important looking buildings." Bitumen mortar was also used at
a lower-frequency, including in the Great Bath at Mohenjo-daro.
Historically, building 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 BCE. Pozzolanic mortar is a lime
based mortar, but is made with an additive of volcanic ash that allows it to be hardened
underwater; thus it is 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
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, introducing aluminum oxide and silicon dioxide into the mix. This
mortar was not as strong as pozzolanic mortar, but, because it was denser, it better resisted
penetration by water. Hydraulic mortar was not available in ancient
China, possibly due to a lack of volcanic ash. Around 500 CE, sticky rice soup was mixed
with slaked lime to make an inorganic−organic composite mortar that had more strength and
water resistance than lime mortar. It is not understood how 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
can be degraded by contact with water, many structures suffered from wind blown rain over
the centuries.
Ordinary Portland Cement Mortar Ordinary Portland Cement Mortar, commonly
known as OPC Mortar or just plain 'Cement Mortar' is created by mixing powdered Ordinary
Portland Cement, Aggregate and Water. It was invented in 1794 by Joseph Aspdin and
patented on 18 December 1824, largely as a result of various scientific efforts to develop
stronger mortars than existed at the time. It was made popular during the late nineteenth
century, and owing to the First World War, it had by 1930 superseded lime mortar for
new construction. The main reasons for this were that Portland cement sets hard and quickly,
allowing a faster pace of construction, and requires fewer skilled workers. However, as
a general rule, Portland cement should not be used for the repair or repointing of older
buildings constructed in lime mortar, which require the flexibility, softness and breathability
of lime if they are to function correctly.
In the United States, one of five standard types of mortar are generally used for both
new construction and repair. The ratio of cement, lime, and sand included in each mortar
type produces different strengths of mortar. The formulations for each type are specified
by the ASTM standards organization. These premixed mortar products are designated by
one of the five letters M, S, N, O, and K, with Type M mortar being the highest strength
and Type K the weakest. These type letters are taken from the alternate letters of the
words "MaSoN wOrK". Polymer Cement Mortar
Polymer cement mortars 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. For an example see MagneLine.
Lime Mortar
The speed of set can be increased by using impure limestone 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 no 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. Re−pointing or rendering
an old wall with cement mortar stops this evaporation and can cause problems associated
with moisture behind the cement. Pozzolanic 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 Romans learned
that pozzolana added to lime mortar allowed the lime to set relatively quickly and even
under water, and 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. Pozzolana has since become a generic term for any siliceous and/or aluminous additive
to slaked lime to create a hydraulic cement. Finely ground and mixed with lime it a hydraulic
cement, like Portland cement, and makes a strong mortar that will also set under water.
Firestop mortar Firestop mortars are mortars most typically
used to firestop large openings in walls and floors required to have a fire-resistance
rating. They are passive fire protection items. Firestop mortars differ in formula and properties
from most other cementitious substances and cannot be substituted with generic mortars
without violating the listing and approval use and compliance.
Firestop mortar is usually a combination of powder mixed with water, forming a cementatious
stone which dries hard. It is sometimes mixed with lightweight aggregates, such as perlite
or vermiculite. It is sometimes pigmented to distinguish it from generic materials in
an effort to prevent unlawful substitution and to enable verification of the certification
listing.
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. See also
References Technical data sheets, Mortar Industry Association,
www.mortar.org.uk