Archive for October, 2011

The Lime Cycle

Lime has been used for thousands of years for building construction as an ingredient in mortar and plasters and limewash. The conversion process by which the material claims its name is from the lime cycle. It is described in most books as a clock face with the corresponding chemical reactions and changes to limestone that take place as it goes through the various cycles.

The Lime Cycle

At the noon position on the clock face we have limestone [calcium carbonate – CaCO3]. As we move around the clock-face toward the three o’clock position we introduce heat. The heat needed to convert limestone to quicklime [calcium oxide – CaO] is 1,650F.  At that temperature the CO2 is driven off, water is vaporized into a gas at a much earlier stage in the firing process at 212F. From the three o’clock position moving toward the six o’clock position we introduce water to the quicklime.

This converts the quicklime into lime putty while giving off an exothermic reaction causing the water to boil in a process called lime slaking. The lime putty [calcium hydroxide – Ca(OH)2] settles down into a consistency of thick Philadelphia cream cheese before its ready for use. The quicklime naturally takes the amount of water it needs and drains off the rest, so in a sense you over soak the quicklime during slaking and the material finds its natural balance as it settles down into a putty under the water – a process that generally takes 60 to 90 days to complete if left undisturbed.

Moving now from the six o’clock position to the nine o’clock position we introduce sand, and mix the lime mortar or plaster into a cohesive mixture usually in a volumetric ratio of 1 part lime putty to 2.5 parts sand. We end up at the nine o’clock position with our mixture ready for installation.

From the nine o’clock position back to the noon position we introduce water to the walls [by spray misting] in a series of wetting and drying cycles to encourage carbonation.  Carbonation is defined as the process by which lime cures – or converts, back to limestone from which it originated. We suggest a minimum of nine (9) wetting and drying cycles to initiate this process after installation. And that’s it!  That is the process of the Lime Cycle. We take limestone apart using fire, mix it back up with water and sand and we have lime-stone mortar in the end – a very durable long-lasting material.

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The Peter H. Brink Award for Individual Achievement

Pamela Bates received the 2011 the Peter H. Brink Award for Individual Achievement from the National Trust for Historic Preservation – announced in the Preservation Magazine November/December  page 46. I was inspired to read her commitment to the Lowell’s Boat Shop in Massachusetts, the oldest continuously operating boat shop in the country founded in 1793.

Lowell's Boat Shop

Through its different ownerships and threats by big developers – for the prime waterfront property, the building survived after Bates assembled a coalition called Lowell’s Maritime Foundation which purchased the landmark. The nonprofit took ownership in 2007 and has operated Lowell’s ever since.

What I think is most dynamic about the preservation plan is it includes building wooden boats, a well-kept secret, in a working museum setting. People can actually go there and watch boat building in progress.  I believe the best museums are the working museums – full of life, just like the old days when business was booming in the 1700s.

Today, I acknowledge Pamela Bates for her inspiration, patience, and preservation energy. Congratulations Ms. Pamela Bates and to the National Trust for selecting a great candidate for the Peter H. Brink Award for Preservation.  I hope to someday visit this property in the future and hope you will too.

Lowell’s Boat Shop:


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Mortar Analysis – Limitations

Mortar analysis can be done various ways with several different approaches to identify the original mortar ingredients of a formulation. However, there are limitations and replacement mortar specifications should not be based solely on laboratory analysis. Analysis requires interpretation, and there are important factors which affect the condition and performance of the mortar that cannot be established through laboratory analysis. These may include: the original water content, rate of curing, weather conditions during original construction, the method of mixing and placing the mortar, and the cleanliness and condition of the sand (Pres. Brief 2 pg. 2).

Mortar can be evaluated by simple wet-chemistry of using hydrochloric acid and water to dissolve out the binder components (calcium carbonate) leaving only the sand particles behind. The ratio of binder to sand can be determined by drying the sample first then weighing it before and after the wet-chemistry process. The problem occurs when calcium carbonate is part of the sand component which would give you a false reading of the ratio. There is also x-ray diffraction, and petrographic analysis by microscope, as well as thin-section technology, where small samples of mortar are cut into very thin sections and dies are injected into the sample showing the different components of the mortar. In addition, ASTM C1324 is a test method to determine components of hardened mortar samples.

Sand particle shape from an analysis - Historic Scotland

The most useful information that can come from a laboratory analysis is the identification of the sand by gradation and color. This allows the color and the texture of the mortar to be matched with some accuracy because sand is the largest ingredient by volume.

A simple non-technical evaluation of the masonry units and the mortar can provide information concerning the relative strength and permeability of each-critical factors in selecting the repointing mortar – while visual analysis of the historic mortar compared to the new replacement repointing mortar can be made. It’s important to match the un-weathered portions of the historic mortar in case the building will be cleaned in the future, or cleaning should be taken into account before the sample is matched.

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Repointing – Lime Putty Mortar Placement

Lime putty mortar repointing, ca. 1850 after cement ribbon joints were removed

As I sit and write this blog I realize I am writing about style and approach to a specific task that many of you have been doing for many years. So I will start with this disclaimer – I will share my experiences with you on many successful repointing projects that involved lime putty mortar and most required no washing after wards except an occasional vinegar quick rinse to remove the white film from red brick units. I have made mistakes along the way and worked over the years to attempt to figure out what is the best practice approach to this task of mortar replacement called repointing.

Repointing, unlike tuckpointing, requires the full preparation of the joints to a depth of 2 to 2-1/2 times the width as discussed in yesterdays post. The American version of tuckpointing is only a skim coating of cement-based mortar over the top of existing mortar joints without the removal process (Chicago). The British version of tuckpointing is where a mortar joint is actually made to the same color as the units and a grapevine line is established in the center of the joint and tucked with a different color and mixture of mortar – true tuckpointing. This true tuckpointing is remarkably difficult to learn but in the end it makes brickwork appear straighter and rubble stonework look like it was laid up in ashlar units.

Mixing lime putty mortar is straight forward. You mix the putty into the sand placing the sand in the wheelbarrow first. Create a hole in the sand for the putty and place the putty into the hole and start to twist and knead the material together. What is interesting is that you do not need additional water during the mixing process; there is enough water in the lime putty to give you a good brown-sugar consistency for repointing. If you are laying brick you can add a small amount of water to create a spreadable mortar. A good repointing mortar should not be able to be spread with a trowel, if it is, than you have too much water in the mixture. If this occurs all is not lost, simply set the material aside on a dry sheet of plywood and allow the excess water to run off (be absorbed into the wood) until you get the desired consistency. This may take several days.

Pre-soaking the joints with water prior to mortar placement

Pre-soak the wall with water. Be sure to get the water between the units and back into the existing original mortar joints. I have had projects where the water pre-soaking process never made it back into the joints – the sprayer was held too far away from the wall surface and while the unit faces received water the joints remained dry – especially the tighter joints. To ensure that you are getting the water back into the joints run the sprayers right on the walls and into the joints filling them with water. Allow the water to absorb into the wall materials and become Saturated Surface Dry (SSD) – no standing water to the touch, no drips, no glistening or shining if viewed from an angle – usually takes 8 to 10 minutes depending on the rate of absorption. You are now ready to start placing the mortar material.

It is most important that your repointing tools fit between the masonry units to enable you to compress the mortar back to the original material. If your repointing tools are too wide you will stain the face of the masonry as you attempt to get the material back into the joint and you will not get the necessary compression required for a good job. You may need to alter your tools by using a bench grinder to create thinner repointing tools. Most big box stores only carry 3/8 inch size even though ¼ inch sizes are available. Work from a hawk. If you are right-handed, work from right to left, on a slight angle, leading with the tip of the repointing tool. Overfill the mortar joints past the surface of the masonry units at least ¼ inch. Allow to dry and become thumbprint hard. Scrap away the excess mortar using a margin trowel and follow the contours of the masonry edges if required. Match the mortar joint profile of the original work. Stipple brush finish the joints by beating the faces with a churn brush – opening up the surfaces to expose the aggregate and create a texture that will encourage prompt evaporation of water and rapid carbonation as the mortar cures.

Protect the mortar for the first 24 hours after placement from wind driven rain and direct sunlight. Keep the material moist – spraying the entire wall with water three times per day for the first three days after application. The material carbonates as it goes through wetting and drying cycles – a minimum of at least nine (9) cycles. Do not allow the mortar to dry out too quickly. Repointing into hot or warm masonry units (south elevation) during summer months is not recommended. The masonry units simply will draw out the water from the mortar to quickly as the temperature rises reducing the chances for proper curing and carbonation to take place.

Repointing tools are available at:

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Repointing – Joint Preparation

After a careful evaluation and clear understanding of why the mortar joints have deteriorated (or not, in the case of removing hard portland cement mortars) it’s time to repoint the wall. First, is the question of how deep to cut the old mortar out from the joints in preparation to receive the new replacement mortar. The Preservation Brief 2 “Repointing Mortar Joints in Historic Masonry Buildings 1998 published by the U.S. Department of the Interior’s National Park Service , Heritage Preservation Services gives us a good place to start.

Measuring mortar joint removal depth from the face of the wall

Old mortar should be removed to a minimum depth of 2 to 2-1/2 times the width of the joint to ensure an adequate bond and to prevent mortar “popouts”. For most brick joints, this will require removal of the mortar to a depth of approximately 1/2 to 1 inch; for stone masonry with wide joints, mortar may need to be removed to a depth of several inches. Any loose or disintegrated mortar beyond this minimum depth also should be removed”(page 9).

I like that the Brief advises on a range of mortar depths in correlation with the width of the joints.  It makes sense to approach the removal in this way.  Often I see contractors bidding repointing projects calling for the depth of the removal at ¾ inch. For most mortar joints that are the thickness of your little finger, about 3/8 inch, this is not deep enough. It does not cost the contractor any more money to remove another 1/4 inch of material during preparation and it makes for a better job.

Luckily for the projects requiring the removal of hard portland cement mortars from old historic lime mortar walls, the contractors of years past, did not follow this quality protocol of 2 to 2-1/2 times, otherwise the portland cement mortars would be much more difficult to remove. Instead, we most often find these projects only skim coated with the harder material. It is important as a mason contractor to know what you are getting into prior to bidding a project that has been pointed in portland cement. Questions you should be asking are; How deep is the non-original portland pointing?,… How hard is the mortar?,… and how difficult will it be to remove it without causing damage to the surrounding masonry units? Sometimes the only way to really know for sure about the answers to these questions is to commission a test panel prior to bid.

Thin diamond blade - turbo type (top)

A four-step approach to removing mortar joints in historic masonry buildings has been the industry method and best practice approach now for the past 15 years or so. First, the use of thin diamond-blade (turbo-type) grinders has been successful in cutting down the center of the horizontal (bed) joints for the removal of hard portland cement mortars. Second, followed by hand hammer and chisels or pneumatic chisels, to remove the excess mortar from the top and bottom of the masonry units. The vertical (head) joints are removed by chisel and hammer once the bed joints are removed. The third step is to use a caulking cutter with a diamond sickle type blade to clean the top and bottom of the masonry units and create a square cut back to the original lime mortar.

In the fourth and final step, mortar debris from the process should be removed by compressed air or a vacuum system. We do not use large amounts of water to flush out the debris during the cooler fall or spring months as it takes a long time for the walls to dry out especially the north facing elevations that do not benefit from the direct sunlight. We have unintentionally had efflorescence become an issue on some projects – waking up salts that lie deep within the masonry wall system due to excess water flushing.

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Moisture – Part Four, Solutions

Part of the solution to the problem of moisture migration is allowing the water to have its way. In above grade walls, that means letting it go through the wall, then redirecting it through flashing and weep holes if possible, and most importantly, using a breathable mortar that is more porous than the brick or stone.

Below grade, keep water from resting on the outside of the foundation walls in saturated soil conditions. Create a drainage system, a way for the water to move away from the foundation, perhaps installing perforated foundation tile at the base footing of the wall with gravel fill. Again, check gutters and downspouts to ensure they are clean and take water away from the wall, extending downspouts at least three feet past the elbow at grade level is a good idea. Grade the soil and pavement materials around the building to encourage water runoff and avoid collecting and pooling near walls.

A digital moisture meter in use

Detecting trouble in advance – The use of a moisture meter can sometimes be helpful in determining a baseline for acceptable moisture content in a historic masonry walls. Because not all masonry walls are created or built equally, all have varying levels of moisture depending on conditions. What is important in establishing a baseline is looking for the wall sections that are performing well.  In these areas, take readings to compare to areas with deteriorating brick or stone. This will put you on a specific path toward understanding what to expect in the future.

Also consider choosing breathable mortar materials like lime putty or hydraulic lime blended with coarse aggregate particles – often the vary material that has turned to dust over the past 100 years. Do not try to make it stronger or better. Just match the old lime mortar and move onto the next project. If the original historic mortar has turned into dust or is falling out of the wall, it is likely a lime mortar. It has done the hard work of absorbing more water than the brick over and over again and now needs replacement. The brick or stone is generally preserved in these cases.

The new lime mortar replacement mixture should match the old mortar and perform as the old mortar did – it to will turn to dust and fall out of the wall in the next hundred years, giving the next generation something to fix.

Lime Putty Suppliers in the US:

U.S. Heritage Group, Inc.

Virginia Lime Works

Hydraulic Lime Suppliers in the US:

U.S. Heritage Group, Inc.

Virginia Lime Works

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Moisture – Part Three, Rising Damp

Rising damp condition on a building in Europe - M. Machnicki

Capillary suction in foundations is often called rising damp. Evidence of the condition is commonly found inside the basement, and in more advanced cases, outside too. Rising damp will often leave behind a tidemark on the wall surface. In areas subject to freezing, the condition can cause spalled masonry units and failed mortar joints. Broken downspouts, clogged gutters and underground sprinklers contribute to the condition, as will soil heavy with clay.

In the late 1800s builders and architects generally would select masonry materials that had a lower porosity for the first few courses of foundations in an effort to manage moisture from rising. Many government buildings in the United States have granite incorporated into the design of the building foundation, and then sometimes a course of slate tiles in the bed joints between the masonry units to offer additional protection. This design would mitigate the rise and force the water back down into the foundation wall from where it came. Under total saturated conditions, with nowhere to go the water could find its way out the face of the masonry units and mortar joints either indoors or outside, deteriorating the materials over time.

Water can cause trouble by itself, such as when it is exposed to freezing temperatures. But when water is combined with salt the trouble is magnified many times. Pressures build inside the wall system when salts try to dry out and crystallize, making for an eruption of spalled and deteriorated masonry. Osmotic pressure is caused by expanding salt crystals trapped in the body of masonry formed during and after the evaporation of water on the surface. It is these salt crystals that appear as a white powder on the wall. They can potentially build up to 3,000psi, stressing the masonry. This condition can be compounded up to 5,000psi when past attempts to control the moisture by repairing with hard mortar and applying spray-on waterproofing materials are involved.

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Moisture – Part Two, Capillary Suction

As promised from yesterdays post we were going to take a look at how water enters a building. We know water can enter a building in many ways, through masonry walls, roofs, windows, and saturated soil surrounding the foundation just to name a few. There is also interior sources of water, such a condensation from cooking, cleaning, showering – generally occupying the home. However, this is not the whole story when it comes to historic masonry walls. What we don’t talk about much is the “embodied water,” the water that remains in the walls at all times. Old historic masonry walls are moist in the center, due to the porosity of the masonry units, the lime mortars, and the thickness of the wall – it’s just hard for them to dry out completely. They really never do completely dry out, especially if they are foundation walls. We must consider this condition and account for moisture already present within the wall, because if we don’t, we could unintentionally trap moisture even when the outside of the wall feels normal and dry.

Capillary suction of water at different widths

Load-bearing historic masonry walls (16 inches to 24 inches in thickness) are porous, capable of wicking large amounts of water great distances due to capillary suction. The smallest cracks and pores found in mortar, brick and stone can bring on water in a big way. The action of wicking is energized by the horsepower of smallness. The smaller the pores or cracks in the wall, the more powerful the draw of the wicking action. If you have given blood, you may recall the very small glass tube the nurse uses to take a droplet of blood from your fingertip with a seemingly invisible vacuum cleaner. The blood droplet instantly goes up the tube – effortlessly, capillary suction at work.

The horsepower of smallness regarding the capillary action of water should be cause for concern, because architects and contractors are often focused on repairing large cracks in buildings while leaving the small ones untreated. Don’t be deceived. The small cracks, even the hairline cracks, are where the suction power is. One way to slow down the power of capillary suction is to reduce the surface area of the material that comes in contact with the water source. For small hairline cracks, injecting dispersed hydrated lime (DHL) into the crack with a syringe will sometimes do the trick. Unlike epoxy or cement, DHL remains flexible after it cures and maintains good vapor transmission, allowing the wall to breathe while at the same time halting the water – pulling the plug on the vacuum of capillary suction.

Dispersed Hydrate Lime (DHL) is a product imported from Germany and has been used successfully on many historic masonry structures here in the United States for over 10 years. Information on the product can be obtained by contacting the U.S. Heritage Group based in Chicago. Other vender choices I’m sure are available, but this is the product we have specified and are most farmiliar with using.

SPC Training video on DHL injection:


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Moisture – Part One, Watertight Envelope Theory

Stone decay from hard cement repointing - Historic Scotland

The new term on the streets used by industry consultants to describe the details of how a building takes on water and then (hopefully) sheds it is “water management.” The longevity of historic masonry walls relies heavily on how water is managed in and around them. I am personally not yet convinced we can control water. I can work to manage where it goes, and possibly how long it stays – by redirecting it, but in the end it goes where it wants, the easiest way. When you attempt to fight water it usually wins. The ways water impacts a building depends on how long it stays – which is directly correlated to its architectural design, geographic location, topography, soil, the water table, the type of brick, stone or mortar, and whether the building has recently been restored.

Sometimes the understanding of historic load-bearing masonry walls built with lime mortar materials is not established, or respected, prior to a restoration project being undertaken. While the joints may look like they are in need of repointing due to deterioration, it’s important to know why they deteriorated in the first place. The cause is most likely from water saturation – then freezing and thawing or extreme temperature variations. One of the challenges is understanding that a building can, and does, breathe though its mortar joints as well as its masonry units.

The shear thickness of most load-bearing masonry walls keep the water out. The original building materials made for quick evaporation of the water on the surface of the walls and kept the inside dry, but this breathability does takes its toll on old lime mortar joints and they need to be repointed in high moisture areas every 75 to 100 years or so. Problems start when an architect specifies a replacement mortar that is harder than the original (in an effort to make it last longer) than potentially traps moisture inside the wall system. The effort in the restoration repairs is totally focused on keeping water out from coming in through the exterior side of the wall. The problem is that old masonry walls contain a certain amount of moisture already and often do not perform very well with harder/stronger mortar joints surrounding them.

When the goal of the restoration project is to create a Watertight Envelope you’d better run the other way – fast.  “Watertight Envelope” and “Historic Load-bearing Masonry” should not be used in the same sentence. Keeping water on the outside would seem to be an honorable goal for any restoration project, but observing the current condition of some masonry buildings restored in the past 10 years tells us a much different story, a troubling one. Basically, the buildings subjected to this watertight-envelope theory are not doing very well.

Where waterproofing and harder cement-based mortars are applied we find decay patterns that are surprising – in just a decade after application. Instead of the mortar surfaces wearing, there is a new pattern of brick and stone decay. Strong osmotic and hydrostatic pressures build up in brick and stone that are subjected to these hard, strong, and water resistant materials.

Tomorrow we will discuss how water enters a building.


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The Lime Revival

J. Speweik during on-site masonry inspection

As a young boy growing up in a family of stone masons mixing mortar was like brushing my teeth…I did it every day, at least during the summer months when school was out. Who would have thought that in the age of technology, speed and convenience that my great great grandfather’s 1846 mortar formulation would return. The trend seems to be one that is sweeping across Scandinavia, Europe and Canada as architects and heritage masons work together to preserve their country’s historic masonry properties built hundreds and often thousands of years ago.  They call it the “Lime Revival” It’s been 30 years for Sweden, 20 years for England, 10 years for Canada….its America’s turn now.

The oldest archaeological sites in the world are, of course, masonry. As early as 2450 B.C., masons began using lime and sand for mortar. Lime is made from limestone (calcium carbonate) which has been heated to temperatures exceeding 1,650F where the heat drives off the carbon dioxide and water turning the limestone into quicklime (calcium oxide). Traditionally this quicklime (sometimes called lump lime or hot lime) was delivered fresh to construction sites or made on-site in a temporary kiln just for the job. The quicklime was mixed with damp sand and stacked up into piles for slaking into a hydrate powder (calcium hydroxide) and run through a screen or the quicklime was combined with water in the ground, formed into a putty (also calcium hydroxide), and mixed with the sand at a later time depending on the project needs. Either way, the mixtures were left to mature or rest for a time before use, due to the expansion of the lime particles during slaking.

The lime was generally mixed with local sand in a ratio of 1 part lime putty to 3 parts sand by volume. Other ingredients like crushed brick, clay, lamp black, and natural cement were sometimes found in smaller quantities before 1870; however, the basic lime putty/quicklime sand mortar formulation has remained unchanged for centuries.

Portland cement was first manufactured in America in 1871, but did not become truly widespread until the 20th century. As late as 1883 there were only three portland manufacturing plants in the U.S. Up until the turn of the last century portland cement was considered an additive, or “minor ingredient” to help accelerate mortar set time. By the 1930s, most masons were using equal parts of portland cement and lime putty or quicklime. Thus, masonry structures built between 1871 and 1930 might be pure lime and sand mixes or a wide range of lime and portland combinations.

What we do know about lime, and the reason for its come-back, is its incredible performance characteristics, and versatility as a time-tested building material – and not just as a masonry mortar either, but also as paint, (limewash/whitewash) exterior stucco/render, and interior plaster as well. Lime, when properly combined with clean, sharp, well graded sand can perform for many centuries in masonry applications. Lime has the ability to handle water without trapping it within a wall structure. It is breathable, flexible, obtains high bond strength to masonry units, it is truly sustainable (less energy is required to heat a ton of lime as compared to a ton of cement) and it has autogenious healing capabilities, often referred to as “self-healing” where hairline cracks do develop over time water combines with the lime again to re-knit the cracks back together. Limes durability comes through a process of what’s called carbonation. Carbonation is a process by which lime turns back to limestone by reabsorbing the CO2 back from the atmosphere though wetting and drying cycles. You can say that the material interacts with nature on a daily basis.

Portland cement mortar "Cover-up"

As portland cement became more widely used many lime sand mortars were being “covered-up” during repair projects. Exterior masonry buildings suffered badly from hard portland cement mortars (1940s until today) which did not accommodate for movement or stresses within the wall systems, and as a result, many historic brick and stone units got damaged by this un-sacrificial material. When cracks did occur, in the portland cement mortars, water would migrate into the wall cavities and not be able to escape or evaporate back out as they once had done with the lime sand combination mortars.

But times are changing. We are seeing signs of the “Lime Revival” hitting the shores of the United States. Mortar manufacturing companies are now offering lime mixes now for restoration and a few specialty companies offer traditional lime putty, quicklime and imported hydraulic lime for sale.

Lime mortar materials, that I am currently aware of, are available from the following U.S. companies listed in alphabetical order for your convenience.  Be sure to ask questions about each of the company’s offerings, as they differ. Some still use portland cement in their lime mortars. It’s best to know what you need first – then go out and find a supplier that can meet that need.

Cathedral Stone Products

Edison Coatings



U.S. Heritage Group

Virginia Lime Works

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