Posts Tagged lime
I encourage the next generation of future masons to always look for opportunities to embrace mentors in our trade. And to the older masons I say make a difference in the world of a young person looking for a trade and openly share your professional experience in masonry knowledge in the preservation of important historic architecture. We each bring to the table a skill set that can, and will, make a difference – if, or course, we are given the chance and awarded the projects we seek to secure.
Many of you may not know about my background and experience in masonry preservation – so I will provide you with just a brief overview of how I ended up where I am today with the title of: “Historic Masonry Preservation Specialist” I grew up in the masonry construction business working for my father, uncles, and grandfather in a family owned business located in Toledo, Ohio. This opportunity to be birthed into a family of working brick and stonemasons was not by choice but to keep me out of trouble in my teenage years. And yes, it did work. There is something about carrying brick and stone, building scaffolding, mixing mortar, and cleaning out the toolbox that keeps one honest. I think I was just to tired after working all day to have the energy to get into trouble as I learned the trade of masonry construction is most demanding on the body.
As it turns out I developed a lower back injury that kept me from working in the trade actively – so I decided to go back to school and study architecture. It was in college that I began to appreciate the art of design and the process of construction as it related to historic and traditional masonry architecture. If I could no longer lift and set the stone – I could learn about how to preserve its original condition – and more importantly to do this in the means and methods of the original builders. This meant I needed to be willing to learn about traditional masonry construction tools, methods and materials. It also meant I needed to find other masons that understood these aspects of my new desire. The year was 1990 – 26 years ago. I researched my own family of origin in the masonry trade which dates back to 1870 in Posen, Prussia. So I was the 5th generation in my family to be involved in the masonry business. The problem; however, was that the oldest living relative that would had known about traditional masonry construction methods, workmanship, and materials was my great grandfather and he died in 1951. My grandfather died in 1979; my father in 1985.
As luck would have it I secured a job at a lime manufacturing company in 1991. It was during my employment I discovered an enthusiasm for historic mortar materials, which of course are based upon lime, and have been for thousands of years. Working with several conservators, architects, and a masonry contractor based in Toronto, Canada I began offering lime putty for use as a binder (without Portland cement) combined with sand at the jobsite. It was not long after; however, that I realized the need to train masons on the jobsite to use the Portland-free mix design and assist them in delivering the best quality possible. I traveled to England, Ireland, and Scotland over the next several years to work alongside other masons who generously mentored me in preserving historic masonry using lime mortars and traditional methods on castles. Then I brought that information back to the United States to assist in our training efforts here. That was 1998.
We made mistakes; we learned from our mistakes, we improved our methods, tools, equipment and materials. We did not give up. Encouraged from my mentor masons in Europe and Canada – I completed one project after another across the United States monitoring the progress as I went along. Writing, speaking and communicating with industry professionals I stayed focused. I wish to gratefully thank the pioneers in the masonry preservation movement in Europe that encouraged and personnally helped me like; R.H. Bennett, MBE, Winchester, England; Dr. Gerard Lynch, London, England; Mr. Douglas Johnston, Glasgow, Scotland; Mr. Patrick McAfee, Dublin, Ireland; Ms. Pat Gibbons; Charleston Fife, Scotland; the late Mr. John Ashurst, London, England; Mr. John Fidler, York, England; Mr. Colin Burns, Manchester, England; Mr. Stafford Holmes, London, England; Mr. Tim Meek, Charleston Fife, Scotland; Mr. Michael Wingate, England; Mr. Sam Trigila, Toronto, Ontario, Canada; Scottish Lime Centre; and English Heritage and the Society of the Protection of Ancient Buildings (SPAB) in London, England.
Since the early 90s I have been privileged to assist in the effort to establish (or best said, re-introduce) the lost art of true traditional masonry preservation with the use of lime mortars leading the way. As we continue this effort in 2016 we will actively be searching for architects and historic building owners that seek to preserve the architectural history and character of their properties by supporting the masons by offering onsite historic masonry training that is project specific. I strongly believe that by understanding our past and acknowledging the workmanship, trade practices, techniques, and tools used by the original masons in the process of the original construction we will have a better chance at success in the authentic preservation of historic masonry.
If we go into historic masonry preservation projects expecting that masons today should know all the details that are vital to the success of a masonry preservation project – I think we are asking for too much – especially in a low-bid environment in which many design professionals must deliver their services. Let us expect the best, write excellent specifications to support quality assurance – but we must be realistic in the understanding of what that actually means to the mason working at the site. In many cases these men and women have never worked with a straight pure lime mortar before so there is a natural learning curve that must be acknowledged. But we can not, and should not, let masonry contractors figure it out on the jobsite without proper guidence. This is why I started my company to do just that – mentor and help the masons through training. The same way I learned in Europe.
Speweik Preservation Consultants are not masonry contractors. We use our hands-on historic masonry experience to provide the necessary technical consulting services in: condition assessment, material testing, specification assistance and masonry contractor training at the project site. We strive to support the efforts of the Architects and Historic Building Owners in meeting the US Department of the Interior’s Secretary Standards for Rehabilitation in Division 4 and to protect the historic integrity of the architecture under repair consideration.
Our team last week installed a repointing test panel on a project in St. Louis. Being the first week of December in the Midwest we knew we were taking a chance with the weather. Question: Are we going to have enough time for the mortar to cure before frost set in? We prepared for winter protection and had the wall and working area at 75F during the installation of the hydraulic lime mortar and left the temperature at that level for five hours after installation.
If you have not seen a frozen surface of a newly installed mortar joint well now you have! As it turns out the weather was below freezing for the critical 4 days after our installation and thus the surface of the mortar froze. I have only received images at this point on the condition so my involvement is somewhat limited as to the severity of the problem. The solution to fix frost bitten mortar is to reinstall the material if the damage is justifiably deep and out of the specification requirement for the joint profile finish. We do have an open stipple finish profile which may come to our rescue, but we must be certain the bond between the mortar and stone has not been jeopardized in any way.
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.
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.
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-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: http://ushg.macusa.net/heritagedetail1.1.php?Current_Name=Restoration%20Pointing%20Iron
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.
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
U.S. Heritage Group
Virginia Lime Works
Most people don’t realize that lime particles are said to be some 500 times smaller than portland cement particles. This might explain, in part, why some lime mortar applications for repointing get so messy with mortar on the faces of the stonework – and yes, very difficult to remove especially the longer the lime mortar sits on the stone. The key in delivering a clean project, one that needs minimal cleaning (just with a small brush and water at the edges of the stone) is water content of the repointing mortar when being applied.
The consistency should be like that of stiff compacted brown sugar like you find in the kitchen cabinet. Yep, just like that. The feeling, the moisture content, and the compaction power. Mortar made to this consistency will not stick to the surfaces of the stonework and cannot be dragged along the top causing a stain from the hawk being pressed against the wall. You get a cleaner wall and the chances of shrinkage cracks are reduced as the mortar cures.
But moisture is important when repointing a wall. Bond strength is delivered during the application when a thoroughly soaked wall (with water) is allowed to partially dry-out and become surface-saturated-dry or (SSD). The SSD condition gives the dry mortar a bond potential with the advancement of each masons pressurized push against the material to the back of the joint. This is why it is so important that the masons repointing tool be sized to fit within the joint to allow for this compression.
I have seen a trend in recent years to rely on washing down the repointed walls with a light solution of an acid-based cleaner to remove the mortar stains from the stone surfaces. Problem is that the cleaner also cleans the mortar and dissolves the binder paste from the surfaces. While some in the industry call this aging the mortar, because it exposes the aggregate and gives the appearance of an old mortar joint. Well, you would get that appearance anyway if you waited 10 years as the lime paste naturally wears off the surface of the aggregate particles.
So, in fact, what you are doing in washing the wall down is giving the customer a used wall – a clean wall, but a wall that has been exposed to accelerated weathering is how I look at it. I figure you rip off at least 10-15 years of life cycle performance from the face of a mortar joint by washing it down with an acid-based cleaner. Seems the evidence is clear that the lime mortars do not withstand a cleaning as well as portland cement-based mortar mixtures. What makes things much worse is that lime mortar is very absorbent to water by its natural ability to transfer water in and out through evaporation which often causes the cleaners to penetrate deeper into the joint surface weakening the material even further.
The story I tell in my masonry seminars is a fun one to illustrate the point. It would be like selling someone a brand new set of tires for their truck and make them pay full price for them, but just as they are ready to drive away, you tell them, “Let me use your tires for say 20,000 miles first, then I will give them back to you” – essentially selling them used tires for the price of brand new ones. Don’t sell used lime mortar.
Best way I know of to match a historic mortar is to first identify whether you are dealing with a cement-based binder or a straight lime binder. You can do this by dissolving a sample of original historic mortar in a solution of hydrochloric acid and some water. Watch the reaction of the material as the solution makes its first contact. It’s best to place the material into the solution rather than the solution into the material for best results. If it is a lime binder the material will break down quickly and form foam at the top of the material, bubbling and hissing as the calcium breaks down. If the solution just sits there with no reaction, only a few bubbles – but know foam or hissing action you have a portland cement mortar.
Allow the materials to soak in this solution until all the binder materials are gone. You can check and see this visually by looking at what is left – the aggregate of the mortar. It should be clean and free of any particles of binder still attached. Lime binder mortar can dissolve quickly, sometimes in a matter of hours. Cement binders can take up to several days to dissolve down. Now comes the fun part – identifying the aggregate or sand in the mortar.
Drain off the solution through filter paper to collect the fines. Dry the material in an oven at around 200F on a hot tray. Weigh the material into an even gram amount then run the sand through the series of ASTM E11 sieves specified through ASTM C144 and calculate the percentage of grain particles on each sieve. Then create a sand gradation chart which depicts to sand particle size, shape and color to make for an easy way to match the material.
Chances are if you match the sand and binder materials in a historic mortar you should not need to assistance that oxide pigments can provide. However, these materials are used in the industry to assist masons in matching mortars regularly. We typically stick with a 1 part binder to 2.5 parts sand in our formulas as this was the standard in the industry for the exception of butterjoint brickwork which is often a mixture of one part binder to one part sand by volume.
The performance characteristics of a historic mortar; bond strength, flexibility, breathability, vapor permeability, and compressive strength (in that order of importance) will typically fall into place if the necessary time and testing of the original material is carried out. Most historic lime mortars are very durable and well carbonated and worth replicating. It is not best practice to trump a historic mortar formulation and go to the next higher mortar type, i.e., Type L to a Type O for example. It is best to fix the problem of why the historic mortar deteriorated in the first place – more than likely a water related issue.
Testing masonry materials for durability and performance has been going on for some time. It is important to study and test materials prior to construction of an actual wall to prevent wasted material, time and labor. Some of the earliest recorded tests performed on masonry mortar ingredients were carried out by the U.S. Army Corps of Engineers in the construction of fortifications during the early part of the 19th century.
The durability of masonry buildings relied heavily upon the past performance of the actual structures and the master mason’s experience with the individual materials. Much of the heritage knowledge of making good mortar was passed down from generation to generation through the trades. Testing mortar ingredients historically involved the masons working with the architects in a team approach for common understanding. However, signs of change began to appear as early as the 1890s.
Uriah Cummings writes in his book, “American Cements,” which was first published in 1898. “With their former teaching and experience on the one hand, and the testing machine on the other, the question was not long in doubt. The machine was victorious, and henceforth all judgment founded on experience was laid aside and they became blind believers in the tensile strain tests. What matter though they were continually befogged by the frequent, unreasonable, and capricious pranks of the machine, they had found a god, and were determined to worship it. And so it came to be established as a fixed belief among engineers and architects that the best cement was the one which tested the highest, and the manufacturer had no alternative but to strive to make his product test as high as possible.”
Seems from the tone of Mr. Cummings writing that he knew the industry was going in the wrong direction toward high compression. Is it high compression that destroys historic masonry? Well indirectly it does. Most mortars that are very high in compressive strength are very low in vapor permeability. The ability a mortar has to capture and release water easily through evaporation. What tends to happen in a historic masonry wall is moisture infiltrates by various ways; rising damp, poor roof/parapet/flashing details; driving rain; capillary action through cracks among other ways. The water needs to escape from inside the walls through the mortar joints ideally keeping everything dry from the inside out. Hard, high strength mortar prevents water from escaping thus trapping it inside the wall potentially causing damage to the masonry units of brick and stone as well as terra cotta over the course of time. It’s always better to insist on a lower compressive strength lime mortar that readily breathes with the masonry allowing quick evaporation of water, and in addition, provides the natural flexibility needed for traditional load-bearing masonry walls to perform at their best.
Let me save you some time and trouble if you are considering specifying lime putty (ASTM C1489-01) for your next historic masonry restoration project. Forget about the standard way of mixing mortar with a gas-powered paddle mixer or drum type machine used in new masonry construction. These machines require the mortar ingredients to have a high rate of flow by adding enough water into the mixer to keep everything moving and mixing thoroughly. Not so with lime putty. This material is generally 50 percent water and 50 percent solid (looks like thick cream cheese) and requires a mixer that provides pressure or a kneading action to evenly incorporate the sand particles into the material.
Mixing lime putty and sand together works well when mixed by hand with a mortar hoe and shovel as you can place pressure into the mix by pressing down during the process. Ramming rods made from wood with handles also work well to beat the mortar into submission forcing the sand particles into the lime putty.
What is interesting about mixing lime putty mortar, if you have never had the pleasure to do so, is that it requires no additional water once properly mixed. There is enough water in the lime putty to create a good workable mixture that can be used for repointing. For years we have used a vertical shaft mixer that whips the material into form from the outside-in once all the ingredients are in the shaft mixer. So whatever you decide to do on that next historic masonry restoration project, if it involves lime putty, be ready for some good-old-fashion hand mixing or get ready for some buckshot of lime putty balls coated with sand!
The next time you come across a historic masonry building take a close look at the surface of the mortar joints. Yes, I know they often get over-looked in competition with the brick or stone, but trust me on this one. The first thing you should notice is the sand. The sand is the largest part of the mortar by volume and is the material that gives the joint its color, texture and cohesiveness. The next thing you should notice is white specs or small chunks of carbonated lime putty. If this evidence is identified you’ve got yourself a truly historic lime putty mortar. No need to hire a fancy consultant or pay for an expensive mortar test, you can with confidence declare your finding.
Mortars that display lime inclusions were typically mixed using quicklime and sand mixed on the jobsite with a shovel or mixing hoe by hand and with a lot of hard work I might add. Often, the moisture would be added to the sand first then the quicklime added to the damp material. The quicklime would slake first into a hydrate of lime then into putty if more water was added to the mixture.
The batch of mortar would be tossed and turned until the masons yelled out “MUD!” then the material would find its way onto the laborers back then unloaded onto the boards. The mortar would be placed in the wall as construction proceeded. Mortar consistency might certainly vary from batch to batch with this serve as you go system in place. There might be a time when a laborer catches up with the demand for mortar and has more time to mix a particular batch-thus breaking up the lime inclusions into smaller pieces and even dissolving them altogether.
If it is your desire to match these inclusions you have a couple of options. Use a mortar mixture made from damp sand and quicklime (hot lime mortar mix- allow 24 hrs before use), or make lime inclusions from straight lime putty by allowing the material to air dry then running the harden pieces through a series of aggregate sieves to match the inclusion size you specify. Then simply add the inclusions to your lime putty mortar just before application taking care not to over mix. The inclusions in the image above were added to the masons hawk just before installation and were protected from the initial mixing of the lime putty and sand to keep them from breaking apart.