Types of Masonry Binders

Lime and Cement Cycle

by  Jessica (Focht) Aquiline, MSHP, LimeWorks.us Conservation Specialist

Binders are materials that act as a bonding agent that when mixed with aggregate and water form mortar, which is used to bond various masonry units together playing a structural and decorative role in a building. There are four main binders that have been used throughout masonry history, lime, hydraulic lime, natural cement, and Portland cement, all of which are derived from limestone. Binders affect the physical and chemical properties of the mortar including its strength, how quickly it hardens or sets, and how it reacts with surrounding materials. Below is a brief history of each binder type, the chemical reaction of their production, and their physical properties.

Lime

The history of the use of lime in an architectural application dates to the fourth millennium BCE in Anatolia and Palestine where it was used as a medium to paint walls. The earliest surviving known example of lime used as a binder in mortars is found in the Knossos palaces of the Minoan age, around 1700 BCE, were it was applied as a plaster. Lime mortar used as a structural component is not documented prior to the third century BCE in Rome, which coincides with the addition of pozzolanic materials modifying the chemistry of the mortar.1

Lime mortar is derived from limestone, composed primarily of calcium carbonate (CaCO3), which is fired in a kiln at temperatures above 700°C (calcination process), and is slaked with water to produce lime, which is then mixed with sand to make mortar. During calcination the limestone decomposes, losing carbon dioxide and 40% of its weight, producing quicklime (CaO).

CaCO3 ￿ CaO + CO2 (g)

Quicklime is then added to water during the slaking process, resulting in an exothermic reaction which produces calcium hydroxide (Ca(OH)2) known as slaked lime.

CaO + H2O  Ca(OH)2 + heat

This process was traditionally carried out in a pit dug in the ground where the quicklime was left to mature, allowing the calcium hydroxide to break down slowly and thoroughly to achieve the characteristic smoothness, workability and stickiness of fine lime putty.2 Today slaking is preformed by blowing steam over the quicklime resulting in a powder known as hydrated lime.

At this point the slaked lime is combined with sand in a 1:2-3 v/v ratio to produce a lime mortar that can then be used in the laying of masonry units or as a plaster or stucco. Water must be added if hydrated lime powder is used, however, the volume of water should not largely exceed the volume of lime. Lime mortar sets by contact with carbon dioxide that is present in the air through a process known as carbonation, converting back to calcium carbonate.

Ca(OH)2 + CO2  CaCO3 + H2O

Lime mortars are typically classified as air-setting mortars. As the water in the fresh mortar evaporates, air can enter into the now open pores allowing CO2 to react with lime inside the mortar achieving complete hardening. Since lime mortars require CO2 to set and harden there are some limitations as to where they can and cannot be used. They do not harden properly in very damp environments because the water does not leave the pores open for air penetration. They also cannot be used in bulk or in the core of thick walls because carbonation would not occur in a reasonable time allowing the mortar to harden. Unreacted Ca(OH)2 is frequently found in the core of ancient walls.3

There are several benefits to using a lime mortar in a masonry system. They have higher vapor permeability allowing the system to breath, keeping moisture from becoming trapped, and making the system more durable. Lime mortar provides flexibility to the masonry system allowing it to accommodate movements resulting from environmental and structural loading. The low strength of the mortar ensures that any structural movement occurs along the joints between the masonry units, protecting them from cracking and breaking. Lime mortars are also considered to be autogenous or self-healing. Cracks and fissures are healed through a process of dissolution, transport and re-precipitation of calcium compounds, CaCO3 and Ca(OH)2, within the mortar. Water allows calcium bearing compounds to go into solution and then transports them from a binder rich zone to voids and cracks that are present in the mortar. Re-precipitated calcium compounds may then fill thin cracks.4

Hydraulic Lime

A binder is considered hydraulic when it can set and develop strength through a chemical interaction with water. Hydraulic limes are produced from mixtures of limestone with clays, which can occur naturally as in impure limestone (natural hydraulic limes, NHL) or be achieved artificially (hydraulic lime, HL) through the addition of clay and other materials to calcium hydroxide. Impure or clay contaminated limestone contains silica and alumina and often other materials that can provide hydraulicity.5 These impurities form materials similar to those found in Portland cement, such as dicalcium silicate, aluminate and ferric phases. Hydraulic lime mortars are stronger and set faster then lime mortars while still being breathable, allowing moisture to escape the masonry system, and are able to set under water.

The reaction of the silica and alumina of the clay with heat, water and lime are what provide the hydraulic component to the binder. There are two principal types of hydraulic components, alite (tricalcium silicate, C3S) and belite (dicalcium silicate, C2S). Alite is only produced at firing temperatures above 1260°C and is therefore not present in hydraulic lime, where the initial material is burned between 600 and 1200°C. Alite is the main hydraulic component found in Portland cement. Belite forms at temperatures between 900 and 1200°C, which falls within the firing range of limes.6 Analysis has shown that hydraulic lime was used in medieval structures before the modern discovery of the process as a result of clay-rich limestone being fired at adequate temperatures to produce belite, resulting in a natural hydraulic lime.7

Natural hydraulic lime is produced from limestone (calcium carbonate, CC) containing 5-20% clay (marliacous limestone) that when fired at a high temperature (1000-1100°C) results in a silica-lime reaction producing belite or dicalcium silicate (C2S), lime (calcium oxide, C), alumina (A) and carbon dioxide (C).

CC + AS  C2S + C + A + C

Since there is more calcium carbonate present in the limestone than clay, firing produces a sizeable amount of quicklime (CaO). The burnt stone is then slaked with a calculated amount of water breaking it into a powder, as seen in the reaction above.

Hydraulic lime sets initially by the reaction of dicalcium silicate with water (H) at room temperature forming hydrated calcium silicate (CSH) and some free lime (calcium hydroxide, CH).

C2S + H  CSH + CH

As with lime, hydraulic lime also undergoes carbonation. Carbon dioxide from the atmosphere penetrates into the mortar after it has dried transforming the hydrated lime into calcium carbonate and splitting the hydrated calcium silicate into calcium carbonate and amorphous silica (SH).

CSH + CH + C  CC + SH + H

During the hardening process the binder undergoes some shrinkage and the addition of a non-shrinking inert filler, sand, is needed to reduce the shrinkage and improve the binder’s mechanical properties. The typical ratio for hydraulic lime mortar by volume is 1 part hydraulic lime powder to 1 to 3 parts sand to 1/3 to ½ part water.

Natural Cement

During the eighteenth century there were substantial developments in the understanding of cementitious materials, the first since the time of the Romans. In 1796, a patent was granted to Rev. James Parker for his invention of “Roman cement”, natural cement, which was notable for having a rapid set. Many other types of natural cement then began to appear on the market, all with varying characteristics. Natural cements are produced from argillaceous limestone, such as marls and septaria that have a clay content higher then 25%. They are classified as natural because all of the necessary materials needed are already present in the limestone. The limestone is fired in a kiln at the same low temperatures, 1000-1100°C, which are used for firing hydraulic lime. The calcium in the limestone combines with the alumino-silicates in the clay to form hydraulic minerals.8 After firing the calcined rock is ground into a fine powder, unlike lime, natural cement cannot be slaked.

Natural cement is a hydraulic binder with rapid setting due to the production of calcium aluminate hydrates.9 As a binder, natural cement has a high compressive strength compared to lime mortars but is still water vapor permeable. Rapid setting and the hydraulic properties of natural cement made it a popular mortar choice for civil engineering projects as well as general construction during the nineteenth century until the arrival of Portland cement in the mid nineteenth century. The properties of natural cements are a direct result of the amount and composition of the clay present in the limestone.

Portland Cement

Portland cement was patented by Joseph Aspdin in 1827, who claimed that his invention could produce an artificial stone as good as Portland stone. However, his invention was not yet comparable to what is used today. A comparable material to present day cement was produced by I. C. Johnson in 1845 by firing limestone and clay at such high temperatures that the final product was a vitrified mass.10 As kiln technology advanced during the nineteenth century they were able to fire at higher temperatures for longer periods of time allowing for complete vitrification of the silicates present in the clay.

Portland cement is manufactured by firing a mixture of limestone (CC) and clay (AS), around 22%, at high temperatures (1450°C) where almost complete melting occurred, transforming the limestone clay mixture into their hydraulic mineral species, resulting in a clinker after cooling. The clinker is then finely ground into a powder and mixed with up to 5% gypsum, which is required to reduce the speed of setting that starts when the powder is combined with water. Firing of the original product at this temperature results in the production of tricalcium silicate (C3S, alite), dicalcium silicate (C2S, belite, the only active compound in hydraulic lime), tricalcium aluminate (C3A), and calcium alumino-ferrite (C4AF).

CC + AS  C3S + C2S + C3A + C4AF

Water (H) is then added to the products resulting in the formation of hydrated calcium silicate (CSH), hydrated calcium aluminate (CAH) and free lime, calcium hydroxide (CH). This reaction is what causes the cement to harden and gives it its hydraulic properties as well as its high strength.

C3S + C2S + C3A + H  CSH + CAH + CH

As the hardened material ages and undergoes carbonation the free lime converts back into calcium carbonate and converts the hydrated calcium silicate and aluminate into amorphous silica and alumina. Carbonation reaction is very negligible and does not impair the mechanical strength of the cement mortar.

CSHCAHCH + C  CC + SH + AH

The physical properties of Portland cement are primarily dictated by tricalcium silicate (C3S). C3S is what gives Portland cement its fast hardening time and high strength. During setting C3S will hydrate to produce hydrated calcium silicate (CSH), just as dicalcium silicate (C2S) will, but C3S will produce over three times more calcium hydroxide (CH) then C2S does. The formation of calcium hydroxide begins as soon as water is added to the powdered clinker and will crystallize in the pores of the mortar altering the pore structure.11 This results in a poor void structure within the mortar making it quite dense and reducing the vapor permeability to the point where it is four times less vapor permeable then Natural Hydraulic Lime. Crystallization of calcium hydroxide also alters the elasticity of the mortar, stiffening it, which puts the mortar at higher risk of long-term cracks forming.

The binder is an integral part of a masonry system, bonding the structure together. The type of binder used dictates the physical and chemical properties of the mortar. In a properly engineered masonry system the mortar is meant to be compatible with the masonry units used and to be sacrificial so that the masonry units do not become damaged as a result of the binder that is present in the mortar. Each of the binders discussed provide different properties that are more suitable for specific applications. When choosing a mortar for a masonry project keep in mind the properties of the masonry units that are being used and choose a mortar that will be compatible and in best service to the building.

1 Torraca, Giorgio. Lectures on Materials Science for Architectural Conservation. (Los Angeles: Getty Conservation Institute, 2009). 50.

2 Brocklebank, Ian. Building Limes in Conservation. (Shaftesbury: Donhead, 2012). 23.

3 Torraca. 53.

4 Lubelli, B., T.G. Nijland, and R.P.J. Van Hees. “Self-healing of Lime Based Mortars: Microscopy Observations N Case Studies.” HERON 56.1/2 (2011): 76.

5 Brochleband. 48.

6 Brocklebank. 24.

7 Torraca. 58.

8 Lowry, Richard M. P. “In Defense of Natural Cement: A Critical Examination of the Evolution of Concrete Technology at Fort Totten, New York.” (Thesis. Columbia University, 2013) 6.

9 Brocklebank. 11.

10 Torraca. 61.

11 “Mineralogy of Binders and the Effects of Free Lime Content and Cement Addition in Lime Mortars.” Test and Research for Natural Hydraulic Lime Products from St. Astier UK. (St. Astier, 2006). 8 Nov. 2013. <http://www.stastier.co.uk/nhl/testres/mineralogy.htm>.

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Masonry Pore Structures

Lime putty for mortar has been in known use for over 6000 years. Modern Portland cement, lime and sand mortars have dominated for less than 100 years with many known failures. Here are what some modern scientific investigations have found about what worked so well about lime mortars and why so many modern mortars may not be appropriate for historic restoration. This information is offered so that you can make an informed choice on which binder to use for your projects.

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1 LIME PUTTY TO 3 PARTS OF SHARP SAND

This cross section cut away of a traditional lime putty mortar demonstrates via the blue dyed epoxy resin, which fills the open pore structure, that historic mortar based solely on lime and sand have a tremendously high and desirable liquid and vapor permeability.

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1 NATURAL HYDRAULIC LIME (3.5) TO 3 SHARP SAND

Note the high porosity of this mortar formulation. The wall can breathe by allowing moisture to enter and exit the system, encourage the crystalline bridging phenomenon (also known as the autogenous or self-healing properties of lime mortar) by dissolving free lime in the mix and re-depositing it to close larger fissures. Excess moisture then quickly evaporates back into the atmosphere. The open pore structure allows the carbon dioxide, needed for carbonation during curing, to be delivered deeper into the mortar.

pic2
1 NATURAL HYDRAULIC LIME (5) TO 3 SHARP SAND

Note that the pore structure is still open but finer and more dense than the Natural hydraulic lime 3.5. This mortar is suitable for copings, parging and pointing in extremely wet conditions including sea driven rain with high salt content.

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1 PORTLAND CEMENT TO 3 PARTS SHARP SAND

Note the dense fabric and the greatly reduced porosity, with only the presence of shrinkage cracks. The shrinkage cracks are a portal for moisture to get into the absorptive bedding mortar (usually of lime and sand.) Normally, the sun will draw moisture back out of a lime joint. However, Portland cement based mortars trap moisture within the wall system because their dense pore structure does not always allow it to escape through evaporation. The saturated wall of trapped moisture can lead to moisture being driven much further into the building when heat inside draws moisture through the wall, or trapped moisture evaporates out through the softer historic brick or stone accelerating its deterioration.

Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Stirling, Scotland

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Lime as a Green Build Material

Here’s How St. Astier Natural Hydraulic Lime (NHL) Plays An Important Role In Construction As A Green Build Material.

St. Astier is a 100% natural product and does not contain any additives. It is one of the greenest materials used in construction. This is due to its purity, its calcium carbonate composition, its longevity and potential for allowing the materials to be reused or recycled, and the result of a low energy production process.

The amount of energy used at the production stage is a fraction of what is needed to produce cement. Consequently, the release of CO2 into the atmosphere is reduced considerably. Furthermore, contrary to cement, NHL reabsorbs most of the CO2 during the curing process, while cement reabsorbs none.

NHL has received the LABELVERT EXCELL, or “Green Label”, in France. This label guarantees the total absence of contaminants and any risk of pollution. It also authorizes the use of this product in chemically sensitive areas such as living spaces, wine cellars, etc. Other attributes listed below, prove over and over NHL outperforms modern day Portland cement.

  • The absence of detrimental chemicals like tri-calcium aluminate, potassium and sodium oxides (which are ever-present in cement), protect NHL mortars from chemical reactions such as sulfate or alkali attacks.
  • Very rapid evaporation of moisture from NHL mortars ensures that the drying cycle is faster than cement mortars and subsequently the healing requirements are lower.
    Material used in construction with NHL may be reused or recycled. In addition, the NHL mortar itself may be recycled in a number of ways, such as an aggregate for new lime mortars, fertilizer (NHL is calcium carbonate), or it can be used for water purification to adjust pH levels.
  • Breathability, elasticity, plasticity, gradual development of strength, low shrinkage, longevity, CO2 absorption, self-healing through the presence of free (or available) lime in crystalline bridging to close minor fissures, are all highly desirable. These traits, with sustainability and “greenness”, are only some of the qualities of St. Astier Natural Hydraulic Lime.
  • The change-of-use of older buildings through adaptation or preservation and restoration maximizes the need for the environmental recovery of materials. It is essential to ensure the long term survival of these structures with compatible materials. Some buildings have been in use for centuries; there is no logical reason that this cannot continue. Preservation, adaptation and restoration can have significant environmental advantages over new construction. Aside from the environmental impact, there is the aesthetic value in preservation. Natural Hydraulic Limes have a significant part to play in the process.
  • Material longevity is unsurpassed when applied and maintained correctly and its life will span over several generations. The manufacturer’s warranty subsequently extends for 50 years.

 

CO2 Emissions Chart
CO2 Emissions Chart

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Limelight on Historic Brownstone Restoration – Alfred’s Victorian

Alfred’s Victorian Restoration Story –  with Andy deGruchy

Alfred's Victorian Staff
Alfred’s Victorian Staff

 

This beautiful late 19th century Hummelstown brownstone was recently restored by deGruchy Masonry Restoration, the Technical Install/Training Team of LimeWorks.us. Using historically appropriate, breathable Natural Hydraulic Lime based materials for repointing the brickwork and repairing the brownstone, this iconic building is now put into an excellent state of conservation. It remains a testament to excellent stewardship of our built heritage thanks to the owner, and lifelong resident of Middletown, PA, Robin Pellegrini.

Taking an architectural conservator’s approach, the team of masons repaired the broken and missing pieces of historic sandstone and lime mortar with environmentally friendly Ecologic® Mortar and Lithomex Brick and Stone repair material. The team retained as much of the historic fabric as possible by repairing what could be salvaged with these specialty materials. These materials allow the building envelope to process water out naturally through the lime and sandstone because of their effective liquid/vapor transfer properties over any patch material based on Portland cement.

Please take a look at our other videos for the full extent of this remarkable restoration:

True Sustainable Development in Historic Restoration  – Alfred’s Victorian –  Randy Ruth

Restoring Historic Alfred’s Victorian Brownstone  –  Randy Ruth

Natural Hydraulic Lime Mortars for Historic Preservation and Their Impact on the Environment –  Randy Ruth

 

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How do Natural Hydraulic Lime Mortars Compare to Common Type-O Mortars Containing Portland Cement? by Randy Ruth

by Randy Ruth

For well over 30 years it has been common practice to prescribe the use of a Type-O masonry mortar for use on the conservation of masonry buildings. The most common formula of a Type-O mortar is 1:2:9, a blend of 1 part white Portland cement, 2 parts Type-S hydrated lime (most often dolomitic lime) and 9 parts sand by volume. In recent years there has been an increasing amount of research on similar mix designs and how they compare to Natural Hydraulic Lime (NHL) based mortars used in Europe to repoint historic masonry buildings. Just recently, a professor and two graduates of Columbia University published a research study on the same topic here in the United States. The results of this study can be found in the Association for Preservation Trades International bulletin Vol. XLIII. Their innovative approach to allow a real comparison between different mortar types used in conservation reveals some interesting results.
Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Striling, Scotland

Petrographic thin section images courtesy of William Revie of The Construction Materials Consulting Group; Striling, Scotland

The innovative approach the research team had developed with their curing protocol of various binder types, established a relatively level field of comparison for various mortars in regards to the way each mortar uniquely cures. With this testing detail established, the 11 commonly used repointing mortars were tested at various stages on their splitting tensile strength, compressive strength, water absorption and water-vapor transmission.

The tests reveal that for a splitting tensile strength the NHL/sand mortars are most comparable to traditional pure lime/sand mortars made of High calcium lime and dolomitic lime, while Type-O mortars were more than twice the splitting strength of NHL mortars. Although anecdotal, the cases where historic pure lime mortar has been used to repoint soft brickwork and has eroded causing a need for repointing, Type-O mortars could be consequently be too rigid for use a repointing mortar.

Compressive strength data shows that although NHL mortars have higher values than that of high-calcium lime mortars (with the exception of NHL 2 being less in strength than dolomitic lime) when compared to Type-O mortars their values are nearly half.

When mortars were measured for their water absorption, initially all NHL mortars significantly out-performed other mix designs. The most comparable mix was that of the Type-O mortars that ended up with similar characteristics to NHL mortars and still outperformed both pure lime mortars.

Water-vapor transmission results indicate that all NHL mortars process water vapor at much higher rates than dolomitic and both Type-O mortars, with values comparable to High-calcium lime mortars.

Although the results from the laboratory study are not entirely representative of values that are obtained in field work, they do represent what specifiers and conservation masons are attempting to achieve in real world situations. By eliminating variables, that can give anecdotal results and margins of error, the data suggests that NHL\’s are indeed appropriate when specifying a historic repointing mortar or a new construction mortar used for masonry mortar, plaster or stucco applications.

In conjunction with this study and the new ASTM C1713Standard Specification for Mortars for the Repair of Historic Masonry, Natural Hydraulic Lime clearly proves its self as an alternative to mortar mix designs that have been used widely in the United States for many decades and have shown, for one reason or another premature failure.

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“Soft Reds and Hard Tans” Exploring Historic Masonry Architecture in the French Quarter

by Randy Ruth

New Orleans. It’s quite the town especially during the weeks leading up to Mardi Gras, known as Carnival. Technically beginning on January 6th and concluding on the day before Ash Wednesday (aka Fat Tuesday), the long celebration is concentrated on the final few weeks, giving rise to more frequent parades and events.  Granted I had never been there before but what I expected was much different from my experience. As a Philadelphia native, I had assumed that it’s nothing but one insane party similar to events that might occur during the Mummers parade on New Year’s Day in Philly, and be only on Fat Tuesday. What I had experienced was by far richer in both culture and architectural significance. I don’t mean this to down play the heritage of historically significant Philadelphia. It’s just that New Orleans has a charm that I believe to be facilitated by it organic architectural style, originating in function over form.

Spending most of my time in the oldest neighborhood in New Orleans also known as the French Quarter or as some may traditionally refer to it as Vieux Carré, I felt a certain type of warmth from the people and its buildings. The symbiotic relationship felt I found to be rooted in the architecture bound mostly by French, Spanish and African influence. This dynamic create a true Creole culture. Perhaps this is why nobody can exactly pinpoint the exact year that Carnival festivities began, a very real unique place and people whose origins can be traced to their roots but only studied in regard to its complex organic creation. The trend can be translated to local architecture in the French quarter however it is a bit easier to study.

A New Orleans local showed me around the French Quarter explaining all the influences of Spanish, French, African and local climate on many of the buildings architecture. Probably the most iconic would be the balconies and galleries extending over the sidewalks. Just to be clear balconies are cantilevered into the building with no supporting posts extending halfway over the sidewalk, while galleries typically extend to the edge of the street. Unbeknownst to me I found that the reason for this is threefold.

First, has to deal with the influence of the Spanish and French architects designing buildings since New Orleans beginnings. However, more often than not these buildings needed to be protected from the weather. Wind driven rain during hot summers created the need to have large open windows to permit air flow but something needs to prevent the buildings from getting wet on the inside, hence the need for balconies to act as an eve to prevent rain from entering the interior. The same principle also protects the inferior masonry walls. New Orleans suffers from poor clay local clay for making bricks. The result being bricks called “soft reds &amp; hard tans” that in most cases must be protected with a stucco or render of lime.

New-Orleans-1

 

 

What is currently known as the Napoleon house was first occupied by the mayor of New Orleans from 1812 to 1815. Although Napoleon never came he was offered this residence during his exile in 1822.

 

 

 

 

New-Orleans-2

 

 

Detailing of deteriorated “Soft red” brick rendered with lime stucco as a Trompe L’oei to imitate limestone blocks on the Napoleon House.

 

 

 

Another defining characteristic of architecture in the French Quarter is the entresol, also known as mezzanine. Entresol’s were needed for storage in commercial buildings due to the high local water table.  Looking at the exterior of a building one can clearly notice the well defined tall window front that help hide the storage floor between the 1st and 2nd floors.

 

New-Orleans-3

 

This store front exemplifies the use of an entresol commonly found in the French Quarter.

 

 

 

Due to New Orleans high water table, rising damp is a big issue for many of the building. Because the majority of the historic Buildings located in the Vieux Carré district were built before the advent of Portland cement, lime was used and accommodated moisture very well.  However, today pure lime technology is used to restore the local historic properties, which results in fewer lost or damaged cultural resources.

The time I spent in New Orleans was certainly too limited. There was far too much to see in the few days I was there. Its impact left me forever moved by its places and people. For anyone who has not been there. GO! The treasure hidden in the city is everywhere, waiting for people to experience it and take a piece of it home. Perhaps the next time I visit I will see you there.

 

 

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Damaged Bricks, The Quick Moving Culprit? FAQ by Randy Ruth

by Randy Ruth

Bricks can hollow back quite profusely in some cases. What are some of the causes?

Don’t forget vermin. Accelerated wetting and drying cycles near the bottom of the brick building cause an increase in the decay of the soft bricks. However, where there is water and calcium rich lime mortar there can be ants and bugs nesting in and behind the wythe. Masonry Bees are found in some parts of the country and will burrow. Sparrows and Starlings will take the calcium bits from lime mortar and like a cuttle bone use the grains for digestion. Some very deep holes in the brickwork are from wood peckers who sense the presence of the bugs and they hone in on certain areas in an attempt to get to them. Here’s one example that may have been caused by a combination of water and animals.

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Lime Mortars vs. Cement FAQ with Randy Ruth

Shall I use Lime or Cement?

Q: Could I perhaps use cement and sand? What is the advantage of using lime on old stone structures

A: You should not use only cement and sand for a variety of reasons, the first being that in today’s mortars, lime adds workability and plasticity to the Portland cement mortars. Without lime in the mix or proprietary additives, the cement and sand mortar will have extremely poor workability. Secondly, modern-day cement is much different than early cement or lime, it is very hard, dense, vapor impermeable, and brittle. Cement can trap moisture inside the wall and erode the mortar behind the repair mortar, this can cause further unseen deterioration and masonry unit (stone, brick, terracotta) deterioration, thus resulting in a delayed and much larger/costly masonry repair.

The advantage of using an appropriate lime mortar on old stone structures deals with compatibility. There is a rule of thumb when approaching a restoration project and that is to repair in kind with like materials. By following this approach an individual can avoid unforeseen problems associated with trying something “new and improved” when there is such a well over 2,000 years of lime building history. There have been a number of studies done around the world on historic structures that conclude that even a small amount of Portland cement added to a lime mortar mix, can cause detrimental damage to the adjacent masonry and historic bedding mortar.

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