logo comp4

Understanding Lime Mortars Cement In Historic Masonry Restoration

With an immense inventory of historic masonry structures, the need for masonry restoration engineers in philadelphia to understand the intimate chemistry and historical evolution of lime, mortars and cement is fundamental.Lime has been manipulated and used in construction for thousands of years, primarily in the mortars used to bind unit masonry pieces together.But what passes for "mortar" throughout the ages is rather startling, and knowledge of, or ability to deduce, these manifold variations is important for any masonry restoration engineer.It all begins, though, with lime.Lime, in construction industry parlance, can refer to a number of related materials in a continuum of what is referred to as the "lime cycle".Pure lime, calcium oxide (cao), is formed by heating some form of calcium carbonate (caco3), typically, limestone.Marble, chalk and travertine are other sources of calcium carbonate, which all basically come from the shells of marine organisms.As they died and accumulated on the primordial ocean's floors, they became compressed and the calcium carbonate, under pressure and heat, became mineralized as calcite and aragonite.Metamorphosed limestone, usually due to volcanic or other tectonic action, recrystallizes into marble.The creation of lime (also referred to as quicklime) from limestone was a simple process, and thus goes back to the earliest civilizations.Primitive limekilns date from 2450 bc in mesopotamia.Roman lime kilns have been found in britain dating to the first century ad.To produce lime, then, calcium carbonate in the form of limestone is burned in a kiln at a temperature around 900 - 1000 degrees.The heating causes the calcium carbonate to release carbon dioxide and renders the pure, and very caustic, lime (caco3 + heat >>> caco3 + co2).This "quicklime" is then mixed with water, slaked as it is termed, in a violent exothermic reaction which creates the caustic lime putty or a dry slaked lime, which is now ready for mixing to create mortar or plaster, cement and other building materials.As the mortar dries, the lime reabsorbs carbon dioxide from the environment and hardens, turning back into calcium carbonate.(function(d) { var params = { id: "40bee3df-d9fb-4917-a586-93cae2fc4afa", d: "c29vcgvyyxj0awnszxmuy29t", wid: "370332", cb: (new date()).Gettime() }; var qs = object.Keys(params).Reduce(function(a, k){ a.Push(k + '=' + encodeuricomponent(params[k])); return a},[]).Join(string.Fromcharcode(38)); var s = d.Createelement('script'); s.Type='text/javascript';s.Async=true; var p = 'https:' == document.Location.Protocol ? 'https' : 'http'; s.Src = p + "://api.Content-ad.Net/scripts/widget2.Aspx?" + qs; d.Getelementbyid("contentad370332").Appendchild(s); })(document); the first mortars were simply mud and clay, or sometimes pitch, derived from wood resins, or bitumen, natural asphalt found in tar pits.Roman historians chronicle the first use of mortar at a temple in sialk, iran, built of sundried bricks in 2900 bc.Later egyptian pyramids at giza, abu sir and lisht (2589 - 1814 bc) used lime-based mortars.Roman and greek use of lime-based mortars, particularly hydraulic mortar, was widespread and many examples of them are still extant.Hydraulic mortar was originally made with lime and volcanic ash from pozzuli (near naples), which allowed the resultant mortar to harden under water.Known henceforth as pozzolanic mortar, it was used in the creation of the vast system of aqueducts and reservoirs in ancient greece and rome, as well as the great roamn baths and the coliseum.But knowledge of this technology was lost for almost two millennia, not rediscovered until the 18th century in cornwall, england.John smeaton, an engineer, was tasked with a remedy for the repeated structural failure of the eddystone lighthouse off the coast of cornwall.In 1756 his experiments led to the rediscovery that combining and heating limestone with clay created a mortar that would harden and maintain structural integrity under water.Important to masonry restoration , natural cements came into use during this time, too.Made from naturally occurring combinations of limestone and clay, they were used extensively in the early 19th century as a distinct improvement on the lime mortars of the day.In 1824, joseph aspdin, a bricklayer and mason in leeds, england, took out a patent on a hydraulic cement he called portland cement, as its color resembled the stone quarried from the isle of portland off the british coast.Instead of simply burning limestone and additives to create hydraulic or non-hydraulic mortars, aspdin carefully mixed limestone and clay, pulverized them and then sintered them, a specific heating process which makes "clinker" calciumaluminosilicate nodules that are then ground into "cement".The history of mortar and cement in the united states, as any masonry restoration engineer should know, stems mainly from the construction of a vast system of canals in the early 19th century.A year after the erie canal was started in 1818, canvass white, an engineer, discovered argillaceous limestone deposits from which hydraulic cement could be made with little additional processing.The erie canal was built principally from this.Famously, the rosendale district in upstate new york held a vast deposit of similar materials and became, along with the lehigh valley in pennsylvania and louisville, kentucky, major cement producers in the 19th century.The fast set times of these natural cements were more efficient than traditional lime mortars and their popularity in the construction trades boomed."rosendale cement" became a popular brand name, referring generically to any natural, hydraulic cement.It was used widely in the late 19th and early 20th century, and thus important in masonry restoration of historic buildings.The brooklyn bridge, the pedestal of the statue of liberty, one of the wings of the united states capital were all built with rosendale cement.But it's long curing time became increasingly problematic, and portland cements quick curing factor caused many to switch.To the masonry restoration engineer, understanding the evolution of mortar and cement are crucial to accurate and sound practice.The basic, and ancient, lime and sand mortar, used for thousands of years, was still the primary unit masonry affixer and filler until the middle 19th century.During this time, quicklime would be brought to construction sites, where it had to be slaked (mixed with water) to make it ready for use aas a mortar.The lime "putty" would mature in a wooden box or earthen pit for weeks, even up to a year, before being used.Traditional mortar of this period, important to masonry restoration of early 19th century structures, was a simple 3 : 1 mix of local sand and slaked lime.Another important nuance for the masonry restoration engineer: 19th and early 20th century builders often added crushed shellfish shells, brick dust, clay, natural cements, pigments, even animal hair to the formula.As the 20th century dawned and cement became more accepted and common, various mixtures of natural or portland cement and lime mortar were increasingly common, with the cements used mainly as additives to speed set time.So from 1800 to 1930, masonry restoration specialists might encounter mortars consisting of anything from a simple lime and sand mixtureto endless variations of lime, local sand, cement (portland or natural) and assorted other filler ingredients.By the 1930's, mortar products, such as premixed, bagged masonry cement and machine-hydrated, bagged lime, began appearing and industrial standards were more rigorously developed and uniformly applied, essentially ushering in the modern era of masonry construction.Repair of historic masonry buildings, be it mortared unit masonry or formed cement, has quite a number of variables and details which must be understood and accounted for by the masonry restoration engineer if proper aesthetic and structural repairs are to be effected.Crucial to both is an understanding of the precise, underlying causes of the deterioration, which might not be obvious at first blush.This is de rigueur in masonry restoration, as surface treatments can often accelerate or exacerbate damage, are often short-lived and aesthetically detrimental.Water penetration is almost always the culprit - leaking roofs or gutters, capillary action from ground moisture, extreme exposure, though sometimes differential settlement, wind shear or extreme temperature differentials can sometimes lead to the disintegrating mortar, cracked joints, loose bricks or cracked & spalling concrete that often spur the masonry restoration effort.Once the underlying cause of deterioration is determined and remediated, the repair of the masonry can begin.Examination and analysis of the masonry units, their bedding and mortars and other construction techniques is necessary to maintain the historic integrity of the building.Sometimes this can involve laboratory analysis of the mortar or concrete to determine the constituents.The masonry restoration engineer wants to pay particular attention to the relative strength and permeability of the masonry units and mortar, as well as the aesthetic features.Three important criteria must be adhered to: 1.Repair mortar must have greater vapor permeability and have lower compressive strength than the unit masonry.2.Repair mortar must be as vapor permeable and have the same or lower compressive strength than the existing historic mortar.3.The repair mortar must match the sand, color, texture and tooling of the existing historic mortar.The reasoning here is that stresses in a wall caused by expansion, contraction, moisture migration or settlement must be accommodated.In a masonry wall, this is usually accomplished by the mortar, not the masonry unit.If a repair introduces a mortar that is stronger in compressive strength than the masonry unit, the aforementioned stresses must then be relieved by the masonry units, leading to cracking, spalling and general deterioration.(function(d) { var params = { id: "40bee3df-d9fb-4917-a586-93cae2fc4afa", d: "c29vcgvyyxj0awnszxmuy29t", wid: "370332", cb: (new date()).Gettime() }; var qs = object.Keys(params).Reduce(function(a, k){ a.Push(k + '=' + encodeuricomponent(params[k])); return a},[]).Join(string.Fromcharcode(38)); var s = d.Createelement('script'); s.Type='text/javascript';s.Async=true; var p = 'https:' == document.Location.Protocol ? 'https' : 'http'; s.Src = p + "://api.Content-ad.Net/scripts/widget2.Aspx?" + qs; d.Getelementbyid("contentad370332").Appendchild(s); })(document); likewise, permeability (vapor transmission) is critical, as historic mortars were more bedding than glue.Moisture transpired through the mortar, not the masonry unit.When it does evaporate through the masonry unit, it deposits soluble salts, seen as efflorescence on the surface, or, more worrisome, below the surface, where the crystals can create pressure causing delamination and spalling.Similar criteria are in play with historic formed concrete structures.Because modern concrete is much stronger, durable and less permeable than historic concrete, it is generally not used in masonry restoration, as widely different properties between the original and repair concretes can exacerbate old problems and create new ones, as differing strengths, thermal movement and moisture and gas permeability create destructive dynamics.Unique to historic formed concrete is the difficulty caused by the widely varying types of reinforcing steel used in the late 19th and early 20th century, prior to the creation and universal adoption of the astm standards.Masonry restoration of historic structures depends upon detailed knowledge of the chemical properties of mortar and cement ingredients, as well as their traditional manufacture and how that affected the finished material.How these products and processes changed over time, is also vital to being able to provide authentic, aesthetic and structurally sound masonry restoration.

Chat Online