The Stonemason’s Gospel According to Ian Cramb


dsc_0004Lime Explained by Andrew deGruchy (pg. 143)

Limestone, the material calcium carbonate, has never changed from its beginning up to and including now. There have always been two classifications, ‘pure’ and ‘impure.’ Today it is classified as pure (high calcium) and two levels of impure lime based on the magnesium content, Dolomitic and Magnesian.

What has changed over the course of time, especially in more recent years, is how the calcium carbonate stones have been prepared by firing them in the kiln to produce quicklime. There is a most simple way of burning limestone in a vertical kiln using wood for fuel and keeping the temperature between 1650F and 2000F and then cooking it slowly over a few days. This has been done for centuries.  The proof that this method of cooking the stone has extreme merit is evidenced by the very old buildings throughout the world which still stand that utilized this method of preparing lime. ‘Lime’ is what limestone is called when it is cooked and slaked to make a putty that is incorporated into making building mortars, plasters and paints. The technical chemistry was unknown to old lime burners and masons. They just knew what worked and kept using the time-honored methods of preparing the lime.

When burned limestone has water reintroduced to it, called slaking, it then blooms into a beautiful white putty-like material. The volume of putty produced is double that of what was once the condensed rock. This ‘lime putty’ will draw carbon dioxide out of the air for a very very long time and slowly convert back closely to a limestone again. Lime putty has its initial set over a six week period by exposure to air. However it will attract carbon dioxide almost to a point of being completely ‘carbon neutral’ over time in regard to the embodied energy first required to produce the lime.  Through lime’s interconnected pores it even knits minor fissures together by moving about some of the not fully burned ‘free lime’ which creates more surface area to draw in the carbon dioxide.

Early masons knew that some limestone deposits produced limes that set quicker and became harder sooner. So, unlike simple air-setting lime putty, hydraulic limes were used throughout the world and in the United States to build with when the impure raw material had reactive silica or certain clays naturally found in the stone. These impurities were cooked along with the calcium carbonate stone. The term ‘hydraulic’ means to set with water and under water. Portland cement is hydraulic lime. The reason it is overall strongly suggested not to be used for masonry building conservation is that the synthetically added materials used to make Portland cement become intensely hydraulic also make the whole lot detrimental by various degrees of incompatibility with original porous building components. Two of those detrimental characteristics are that Portland cement is brittle and does not accommodate movement and secondly it reacts with sulfates. But a great incompatibility and detriment to historic masonry buildings is the increased densification of mortar that consequently occurs with every increment of additional Portland cement added to make the mortar become very hard. Densification does not allow the building to remain ‘breathable’ through the mortar joints but instead allows water to become held back and sometimes trapped into absorptive inner bedding joints. This phenomenon forces the wetting and drying cycles of the building to occur through the porous historic units and this is what greatly contributes to accelerated deterioration of the irreplaceable bricks and stone used to originally build a building.

In Ian’s first book he used and suggested mortar mixes that I and every other mason has typically used. These mixes gauge-in some Portland cement into high-lime (Type S lime) containing mortars. The reason we all did this is because readily available Type S Hydrated Builder’s Lime and cement were what we had to work with prior to the commercial availability of natural hydraulic limes now sold in the US. If Type S lime was blended with sand alone we discovered it would not hold up to the freeze-thaw cycles in northern climates. Why this occurs when nothing has changed about the limestone itself puts the spotlight on the cooking procedures. Too hot and too fast of a burn can cause the limestone to become ‘dead-burned’ and loose its ‘reactive’ nature which allows it to closely convert back to a hard and durable limestone again. A durable mortar made from reactive lime which maintains vapor permeable pores and has a desired malleable nature to accommodate minor building movement is the best for vertical, above grade work. Pure air- setting limes that remain reactive because they are burned at a low temperature can be obtained in the US too. However, due to the six week set time the cost for building with these limes goes up exponentially. So in this book the mortar mixes are more clearly defined from Ian’s first book as being mixes that use a binder of hydraulic lime but not the hydraulic lime that is Portland cement. I hope this helps you in designing appropriate mortar mixes for certain corresponding applications. It is a labor of love and worth understanding in order to realize the greatest long-term service life which can be obtained for repairing a vintage building and its components. I hope my contribution of this knowledge into what makes one lime better than another brings about a higher degree of excellence in the historic building conservation work you endeavor to do.

Sincerely,

Andrew deGruchy

 

P. S. Ian passed away in 2013 and has left his legacy in print.  You can purchase this Ian Cramb  book from LimeWorks.us at the on-line store.

 

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Join us for IRON HILL MUSEUM ARCHAEOLOGY & HERITAGE FESTIVAL

Sunday, May 3, 2015
12:00 PM until 4:30 PM
Cost: $5.00
View map
Iron Hill Museum
1355 Old Baltimore Pike
Newark, DE
The Delaware Academy of Science (Iron Hill Museum) is pleased to announce the 2015 Archaeology & Heritage Festival. This event will feature professionals in the fields of Archaeology, History, Mineralogy, and Natural Sciences. Come to the Iron Hill Museum and see the past come alive.

Andy deGruchy from LimeWorks.us will be there demonstrating the slaking of lime  –  join us and learn about the process! 

Slaking 

Andy is also building a kiln on site and continuing the burning of oyster shells he started a few days earlier in another kiln. He is having kids weave some wattle and then after slaking the lime and mixing it with clay and sand, daub up the wattle and demonstrate some plastering circa 1660 in Colonial America and circa 4500 BC worldwide.

The 2015 Archaeology Festival will be held from Noon to 4:30 p.m. on Sunday May 3th on the grounds of the Iron Hill Museum–1355 Old Baltimore Pike.

Admission is $5.

Scouts in uniform are free. Children under 4 years old are free.

Additional Information
Sponsor: Delaware Academy of Science
Phone: 302-368-5703
Contact name: Maureen Zieber

Contact email:
director@ironhill-museum.org
The Museums Website: http://www.ironhill-museum.org
County: New Castle County
Neighborhood: Newark, DE

We make every effort to ensure the accuracy of this information. However, you should always call ahead to confirm this information.

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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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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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Phone: 215-536-6706

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