Mortar (masonry)


Mortar (masonry)
Mortar holding weathered bricks

Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder such as cement or lime, and water. Mortar can also be used to fix, or point, masonry when the original mortar has washed away.[1]

Contents

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 ziggurat of Sialk in Iran, built of sun-dried bricks in 2900 BC.[2] 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 BC, the limestone blocks were bound by mortar of mud and clay, or clay and sand.[3] In later Egyptian pyramids, the mortar was made of either gypsum or lime.[4] 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.[5][6]

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 BC.[7] 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 (Pozzuoli) near Naples, Italy. The Romans later improved the use and methods of making what became known as pozzolanic mortar and cement.[4] 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.[8]

Hydraulic mortar was not available in ancient China, possibly due to a lack of volcanic ash. Around CE 500, 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. [9][10]

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 can be degraded by contact with water, many structures suffered from wind blown rain over the centuries.

Laying bricks with portland cement mortar

Portland cement mortar

Portland cement mortar (is very often known simply as cement mortar) and is created by mixing Ordinary Portland cement (OPC), hydrated lime, and aggregate (or sand) with 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, by 1930 it had 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 less skilled workers. However, as a general rule, Portland cement 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.

Mortar mixed inside a 5 gallon bucket using clean water and mortar from a bag. When it's the right consistency as in the photo (trowel stands up) it's ready to apply.

In the United States (and other countries), one of five standard types of mortar (available as a dry premixed product) 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 (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. For an example see 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.[11]

Mortar Analysis

I. Mortar analysis is the scientific investigation of a variety of cementitious materials such as mortar, stucco, chinking, concrete, and plaster. For the purposes of this discussion all are referred to as “mortar” even though they differ from each other in very significant ways. Mortar analysis is the effort to determine the components of mortar. Unfortunately, no known form of analysis is able to determine relative proportions of the components with any precision. There are two primary types of analysis being used at the present time, as follow:

A. Acid digestion. Acid digestion is a technique that dates from at least the mid-nineteenth century. It is a relatively simple procedure in which hydrochloric acid is used to dissolve the binding component of the mortar which is typically lime and/or Portland cement. Insoluble components such as sand and fines (dirt associated with the sand and/or coloring agents) remain. The sand can then be sieved to determine ratios of sand grain sizes.

Some individuals have (unsuccessfully) attempted to calculate exact proportions of the components using this technique. At this time there is no accurate means of determining proportions, although experienced mortar analysts can provide general recommendations, especially concerning the probable components.

Advantages to this technique include the following: 1.) It is simple and, therefore, inexpensive relative to other techniques. 2.) It provides an excellent set of data regarding the sand component of the mortar as well as an actual sample of the sand. 3.) It provides an excellent sample of any coloring agents, whether they are fines or actual granular components which appear in the sand sample. 4.) In comparative analysis the sand serves as a type of DNA so that samples which have sand which is closely similar in gradation and color to other samples can be reliable understood to be of the same mortar. Contrariwise, if the sand does not match between samples then one can reasonably conclude that the mortars were applied at differing times.

Disadvantages to this technique include the following: 1.) It can be time-consuming as it is a wet process requiring filtering of the fines. Portland cement mortar samples can take upward of a week to filter. Lime mortar samples typically require two days to process. 2.) If the sand is composed of calcium carbonate or other acid-soluble particles (which is quite unusual) the results will be skewed. 3.) If the binder is not acid-soluble, as with gypsum in many plasters, the gypsum will appear typically in the fines or in the finer elements of the sand. 4.) If there is no binder in the mortar, as in many mud chinkings, then the dirt, or mud, will appear in the fines. 5.) It does not provide an accurate ratio of binder to sand. 6.) Precise identification of the mineral content of the sand is impossible.

B. Instrumental techniques. A vast array of highly technical analytical instruments are currently available to the construction industry, man of which are routinely applied to mortar analyses. Commonly employed instrument include atomic absorption spectrometers, which measure elemental composition, and X-ray diffractometers, which identify mineralogical components. These machines are capable of producing very precise data with excellent resolution, but there is no analytical instrument that can identify mortar components and determine proportions. This information can only be arrived at through interpretation by an experienced materials scientist.

II. Mortar analysis is performed to determine as best possible the significant aspects of the mortar. Typically, the desire is to replicate mortar to match other mortar (typically the original mortar) in order to accurately match that mortar both visually and functionally. This is critical for projects such as spot repointing. It may also be done for mortars which have not performed well so that adjustments in the mixture can be recommended to provide a similar mortar with greater longevity and which will not negatively impact the adjacent masonry units. The significant aspects include the following:

A. Sand content. This includes such things as sand grain sizes, sand color, mineral types. It also may be used in a comparative determination to ascertain the relative similarity and ages between mortar from varying locations in the building.

B. Binder content. This includes determination of binder types (lime, Portland cement, natural cement, gypsum, or other) and approximate ratios of binder to sand.

C. Presence of other binding materials such as hair or fiber binders typically used in plaster and identification of composition of these binders.

D. Colorants. Frequently mortar, especially pointing mortar, is colored. It is essential to identify the colorants used to achieve the final color in order to achieve a mortar which will match the existing mortar both at the present time and long into the future.

III. Who does mortar analysis? There are several types of venues where mortar analysis is performed, as follow:

A. Independent architectural conservators. These individuals typically employ the acid digestion technique for their analysis. This is the technique used by David Arbogast, Architectural Conservator (www.mortaranalysis.biz). Their experience does vary considerably and it is recommended that before choosing one, a resume of projects and experience be requested. The advantage to using an architectural conservator is that it tends to be quite cost-effective and answers the basic questions concerning mortar. As noted above, however, the acid digestion technique does have its limitations.

B. Independent material testing laboratories. There are a number of independent testing laboratories which have the equipment and personnel to perform the highly technical analysis described above. As with architectural conservators, they provide clear and unbiased results. Sometimes, however, as with any such reports, the average layman may find them difficult to interpret and understand so that care should be taken on the part of the laboratory to write the reports in understandable prose. One disadvantage to using these techniques, as noted above, is the relatively high cost.

C. Industry laboratories. Both the lime industry and the Portland cement industry have testing facilities and most of the current lime suppliers are able to provide mortar analysis using the acid digestion technology. They then use the results to produce new mortar for the customer. The advantage to this approach is that it tends to be faster than working with independent laboratories because the results are produced in-house. The disadvantage is that there frequently is a strong bias toward identifying mortar as containing the product of the manufacturer. For example, if a historic mortar contained both lime and Portland cement as the binders, it is not unreasonable that a lime manufacturer will simply use lime as the binder of the new mortar and vice versa for a Portland cement supplier. Another disadvantage is that most manufacturers of lime putty mortar have a standard stock of sand on hand and very rarely go to the trouble to actually match the historic sand of the mortar.

See also

References

  1. ^ tuckpointing masonry
  2. ^ http://www.presstv.ir/detail.aspx?id=37364&sectionid=3510304
  3. ^ [1]
  4. ^ a b [2]
  5. ^ O. P. Jaggi, History of science and technology in India, Volume 1, Atma Ram, 1969, http://books.google.com/books?id=Qm3NAAAAMAAJ, "... In some of the important-looking buildings, gypsum cement of a light gray colour was used on the outside to prevent the mud mortar from crumbling down. In a very well constructed drain of the Intermediate period, the mortar which was used contains a high percentage of lime instead of gypsum. Bitumen was found to have been used only at one place in Mohenjo-daro. This was in the construction of the great bath ..." 
  6. ^ Abdur Rahman, History of Indian science, technology, and culture, Oxford University Press, 1999, ISBN 9780195646528, http://books.google.com/books?id=4bnaAAAAMAAJ, "... Gypsum cement was found to have been used in the construction of a well in Mohenjo-daro. The cement was light grey and contained sand, clay, traces of calcium carbonate, and a high percentage of lime ..." 
  7. ^ [3]
  8. ^ [4]
  9. ^ ""Revealing the Ancient Chinese Secret of Sticky Rice Mortar"Science Daily". http://www.sciencedaily.com/releases/2010/05/100530093704.htm. Retrieved 23 June 2010. 
  10. ^ Fuwei Yang, Bingjian Zhang, and Qinglin Ma, ‘’ Study of Sticky Rice−Lime Mortar Technology for the Restoration of Historical Masonry Construction’’, Acc. Chem. Res., 2010, 43 (6), pp 936–944
  11. ^ Dating Ancient Mortar - American Scientist Online vol. 91, 2003

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