LIME Q and A

Real Questions & Answers about Lime

1. Why use high-calcium lime as opposed to S-type or other mason’s lime?

– Steve, a contractor from Maryland

S-type limes are almost always dolomitic limes, meaning that they contain magnesium as well as calcium. If you are talking about working with a pure lime mortar for re-pointing an eighteenth century building, then the S-type has several drawbacks. The smooth working properties of S-type lime are due to its small particle size, which increases its effectiveness when used with a Portland cement. Unfortunately the S-type is also “hardburned” meaning it is calcined (fired) at a higher temperature that reduces the reactivity. In other words, it is made to be less chemically active than a “soft burned” lime so that its rate of cure is secondary to the quick hydraulic set of the Portland. In contrast, a pure lime and sand mortar derives its strength from the carbonation of the lime cement portion binding the sand particles together. This carbonation is the chemical reaction of the calcium hydroxide [Ca(OH)2] with carbon dioxide in the air. Calcium hydroxide made from soft-burned lime will react more quickly with atmospheric carbon dioxide [CO2] and therefore set quicker and harder.

In a dolomitic lime, the magnesium portion takes much longer to carbonate. The magnesium portion of cement does not contribute significantly to the cementitious qualities, but instead should be considered unstable aggregate. Magnesium in a lime-only mortar is mainly just a filler.

When you are using lime-and-sand-only mortars, your best bet for lasting, high-quality work is a soft-burned, high-calcium lime that has been calcined to have a large surface area. This will ensure that it is highly reactive and can be properly slaked into putty with a small particle size. This material will give you superb working qualities and a rapid set.

2. How are increased surface area and strength of mortar related?

-Scott, a cemeteries conservator from Georgia

Increased surface area of the particles of lime translates into quicker and more complete carbonation. In carbonating, the lime changes from the hydroxide form into a hard carbonate like the stone from which it originally came. More complete carbonation means greater compressive strength sooner.

3. What do you mean by “surface area,” and how does it affect the quality of lime mortar?

-David, a mason from Virginia

To understand this it is necessary to know a bit about the carbonate cycle. This is the way in which limestone is converted into a cementitious paste before being combined with sand. This lime putty and sand is then used as a mortar between masonry units where it hardens by reacting with atmospheric carbon dioxide [CO2].

Initially the calcium carbonate [CaCO3] from limestone, oyster shells or chalk is heated (calcined) to a high temperature of about 900 degrees C. This drives out the CO2 and water over several days leaving calcium oxide, or quicklime [CaO].

These lumps of quicklime are slaked in water in a controlled manner to create a slippery paste, calcium hydroxide [Ca(OH)2]. This cementitious paste will remain soft so long as it is kept wet and away from carbon dioxide. When it is mixed with an aggregate and used between masonry units or as a plaster or stucco, the water begins to evaporate or is drawn out of the mortar into the porous surrounding masonry. As the water is drawn out, air enters and CO2 in the air combines with the calcium hydroxide to re-form calcium carbonate.

So the hardening or curing of mortar depends on adsorption of atmospheric carbon dioxide by the lime particles. The greater the surface area of the lime, the more rapidly this chemical process takes place. When the limestone is calcined, great care and skill are necessary to heat it enough to drive out the CO2 and leave a porous matrix, but not so much that the lime is melted and the pores are closed off. I have seen scanning electron microscope (SEM) images of over-fired (hard-burned) calcium oxide particles that look like melted bars of soap. In contrast, soft-burned lime that has been properly calcined has a more porous surface.

For example, an S-type lime that is deliberately hard-burned may have a surface area of about 10-11 m2/g (square meters per gram) where a soft-burned lime for use in a lime-only mortar can easily have a surface area of 25-35 m2/g. The lower surface area lime is less reactive. If this type of lime were used to make a lime mortar it would have low strength and a much longer set time. The high surface area lime has many more surfaces open to reaction with atmospheric CO2 and is called a high-reactivity lime. This lime will begin to set rapidly and will ultimately develop higher strength.

The surface area of the calcium oxide can also be improved or degraded during the slaking process, so it is important to understand exactly how to handle this material if you are slaking it yourself. I proved this for myself empirically by treating the same high-quality calcium oxide to two extremes of improper slaking technique to compare the results. One half was dumped directly into a full tub of cold water so the heat of hydration was interrupted. The other half was slowly spritzed with a hose so the high heat of the reaction was maintained, but the oxide was starved to a minimum of moisture. Both resulting putties had very large particle sizes and limited plasticity.

While both techniques I used were extremes, the result appears similar to the large clumps of lime and limited plasticity that ancient texts reference. In the past they dealt with this by storing the putty for five years or longer to increase the workability and reactivity. Research at the Getty* suggests that the process of storage breaks down the size of the particles, thereby increasing both plasticity and reactivity. Modern technology gives us more control over the hydration process (both amount of water and pressure) and can shorten the wait for a high quality lime putty.

Calcium Hydroxide Crystal Evolution upon Aging of Lime Putty, C. Rodriguez-Navarro, E. Hansen, and W. Ginell of the Getty Conservation Institute, J. Am. Ceram. Soc., 81 [11] 3032-34, 1998.

4. How are vertical and rotary lime calcining different and which is better?

– Bill, a mason from Virginia

The goal in calcining is to heat the limestone or shells (mainly calcium carbonate and magnesium carbonate) to a high enough temperature for a long enough time that the carbon dioxide is driven off, leaving calcium and magnesium oxide. This material is then slaked (combined with water) to form the cementitious paste calcium hydroxide that is mixed with sand to form mortar, or a dry hydrate that after soaking in water can be combined with sand to make mortar.

The difficulty in calcining lime has always been controlling the process – getting the temperature you want and maintaining it for as long as necessary.

The earliest method of firing lime was to stack lime and fuel in alternate layers and set it all on fire. Vertical kilns were an improvement on this ancient practice. In a modern vertical kiln the lime is fed in through the top and fills the upper portion of the kiln. The fuel source of wood, coal or gas heats from below with the combustion gases rising up through the limestone pieces. Many factors must be observed and controlled. The rock at the very bottom will get hotter quicker, the stone above less so, and near the top the stone is really only being pre-heated but not calcined. To fully calcine the lime (drive off all CO2) it must be heated to at least 1650 degrees F. Since the outer surface of the rock may reach this temperature while the interior is still cooler, the minimum temperature must be held and distributed long enough and evenly enough to allow complete CO2 removal.

The size of the rocks is an important factor in the success of this process. If the rocks are all one size, the spaces between them are so large and numerous that much of the heat passes right through and is lost. Too many smaller pieces and the heat cannot flow efficiently and some parts are overheated while others receive too little heat. As they say, calcining is an art as well as a science.

The calcining process is observed through ports on the perimeter of the kiln. The supervisor’s job is to watch the process and make adjustments – more heat, less heat, increased draft, etc.

As the process continues, the supervisor may pull pieces of glowing limestone from the kiln through the ports and examine them to gauge the condition and determine when to remove the lower layers of lime that have now been adequately fired and let the stone above drop down onto the firing chamber.

There are very modern vertical kilns that are easier to control and can produce high-quality limes, but most firing today is done in kilns that rotate so all of the stone is being continually mixed and exposed to the heat more evenly. Rotary kilns are usually long tubes of at least six feet in diameter with several walls running from one end to the other, creating chambers that hold the mixed-size rock and allow the hot gases to pass through as the rocks are tumbled. Another modern rotary kiln looks like a large hollow donut with the limestone moving around the interior on a conveyor belt with gas jets shooting across the surface.

Back to the initial question: Good, high-surface-area, soft-burned lime can come from any type of kiln, but since the greatest degree of control during manufacturing produces the best lime products, a rotary kiln run by experienced hands with constant monitoring of temperatures is best.

5. I’ve heard that if you calcine a limestone that has clay in it, it will produce a hydraulic lime. Is this true?

-Julia, an architect from New York

The answer is maybe and maybe not. Hydraulicity is dependent on many factors. “Hydraulic” generally means able to harden under water or set under local conditions. Hydraulic lime undergoes an internal chemical reaction that forms cements that set in the presence of water. In contrast, ordinary lime mortar cures and hardens first by losing its liquid or free water and then by absorbing CO2 from the atmosphere to form calcium carbonate.

In order for the lime to have hydraulic properties it must have a clay that contains silica, and to a lesser extent alumina and even iron. Silica is the important mineral. In firing the limestone, the silica is chemically combined with the calcium to form calcium silicate. It can then form calcium silicate cement when it is combined with water.

So in order for the lime to have hydraulic properties it must contain the right type of clay that has the minimum silica content and be fired at the correct temperature. But there are other factors that determine hydraulicity. We can discuss these later when we talk about pozzolans. For now just think of hydraulic lime as a type of natural cement that in terms of strength is between pure lime and Portland-type cements.

6. What is the difference between dolomitic and high-calcium limes, and what does this mean for lime mortars?

-Matt, a stone mason from Colorado

High-calcium limestones are properly defined as those with 95% calcium content, although you should check with the supplier because many say “high-calcium” simply because the calcium content is more than half.

The majority of commercially processed limestones have 5% or more magnesium. The degree of magnesium content will chemically define them as either magnesium limestone or dolomitic limestone, but typically the term “dolomitic” is used to describe any limestone with a higher than 5% magnesium content. The range can easily be from 5-45% magnesium content – and the amount of magnesium does make a difference.

For lime-only mortars, the calcium hydroxide will react with atmospheric carbon dioxide [CO2] to produce calcium carbonate (essentially returning to its limestone state), which gives the mortar strength. However, the magnesium portion of a dolomitic lime will never carbonate to form a structural material—it simply remains as aggregate and does not contribute to the cementitious qualities of the mortar.

Sometimes the case is made that if a dolomitic limestone was originally used on an old building, then dolomitic limes should be used in its repair. Although I’ve yet to see a mortar analysis followed by an authentic formula that called for dolomitic lime, I suppose this case for a dolomitic lime could be made in historic building work. Again, the degree of magnesium content can make considerable difference in the durability of the resulting mortar.

When working on historic buildings, we should consider the fact that losses occur every time we repair a brick building. Original material will be lost even in the most experienced hands. I think that it is best to use the highest quality lime possible for any repair, on the theory that it will outlast lower-quality materials and there will be less need for intervention in the future. And the best material for lime mortar is soft-burned, high surface area lime putty with the highest possible calcium content.

7. Are there weather-related restrictions to working with lime-only mortars?

– Drake, a mason from Maryland

Many masons are reluctant to use lime-only mortar, often because of questions about setting time and fear of not having enough strength when compared to Portland cement mortars. The working properties of the two mortars are very different, especially in terms of water usage, and this takes a bit of getting used to for many masons. But one of the most common questions is about working temperatures.

While most trades recognize that very cold weather can limit what they are able to accomplish (for example, in northern areas like Minnesota it is not unusual for the contractors to tent and heat portions of buildings so that work can continue all winter), the modern masonry field is increasingly geared toward mortars that can be set even in winter. This generally requires the addition of salts and/or antifreeze products, both of which can cause problems later. We all know that chemical processes typically slow down as the temperature drops, and I don’t advise laying up unprotected brick walls in winter. However, pointing a wall with lime mortar in the late fall or early winter with can be done if certain precautions are taken.

Plenty of heat is usually retained in masonry walls even as outside temperatures drop. Remember that because heat flows to cold, heat from the inside is flowing to the outside surfaces. If the weather forecast calls for freezing temperatures at night and temperatures back up into the 40s or higher the next few days, I don’t give it a thought. If colder weather is expected to last, then I might tape foam insulation board over the areas of pointing to slow down heat loss from within the wall against the surface for a few days, and most importantly to keep wind off these damp surfaces.

I once deliberately pointed with a fairly wet mix on a December day near the corner of a building on the waterfront, just to see what would happen. I knew that the Maryland nighttime temperatures would be in the teens and that there would be a brisk wind. When I returned several days later, some of the joints near the corner had expanded slightly and were crumbling. It appeared that the formation of ice crystals had probably torn the surface apart, but the damage was slight and limited to the top 1/16” of the joint. Later that year I checked the remaining mortar, and it had carbonated and was quite sound.

I don’t think the problem is as great as some have made it out to be. I would not, however, lay up a freestanding brick wall with lime mortar- or any mortar- and leave it out in freezing weather uncovered. The main consideration is that unless there is a great deal of liquid water near the surface, formation of ice crystals is unlikely.

One final note: while many feel the need to protect freshly mortared walls from rainwater for long periods, this precaution is only necessary with inferior limes. My experience from years of small-scale, unscientific samples indicates that, in fact, lime mortar cures more rapidly and deeply when exposed to rainwater in the first year or two. I am about to run a series of controlled tests on samples to verify the validity of this idea, but at the very least I think the fears of damaging mortar with rainwater early in the carbonation process are overstated. With high-calcium, high surface area lime, surface carbonation is rapid, and within two or three days the walls can tolerate rainfall.

In short, the use of relatively inferior limes (or those meant for other uses, e.g. Portland cement, agriculture, etc.) has led to the development of a great many unnecessary practices. These include over-wetting the masonry, covering it and continually dampening it, applying many thin coats of limewash, and letting each shallow lift of stucco or plaster surface carbonate before the next is applied, which in fact makes the material weaker.

8. How does the sand content of a lime-only mortar affect the project?

– John, a plasterer from New York

Most people think, as I once did, that whatever sand is at the mason’s supply house is fine for their project and might even be properly graded- and after all, it’s just sand. The precise grades sometimes specified on projects are usually for highly critical projects such as high rises, bridges, tunnels, etc., although this sometimes occurs on large restoration jobs. In ordinary residential work it is often left up to the mason, or pre-mixed, bagged mortars are used with pigments added as specified by the architect to meet a certain aesthetic.

When I first started working with masonry I thought the sand at the supply house was carefully graded for masonry work, but after using it many times I rather doubt it. More likely, it is only washed to remove most of the dirt and clay. Usually the only description of recommended sand is that it should be sharp and clean. This was as true in the 18th century as it is now, although from my analysis of many 18th and 19th century mortars, it is clear that there are different degrees of “clean” sand.

But when we think about sand, we need to remember that as the aggregate in the mortar, sand is there both to create strength and to reduce shrinkage. If you have been on a project with routine mortar analyses, then you know that most mortar analyses contain sand sieve information. This information is shown as both the weight of the sand as a whole and the weight of each portion that is retained on a series of screens, each about half the size opening of the preceding one (ex: 4, 8 16, 30, 50, 100, 200, and 325). This information can be used to create a replica sand with the same ratio and distribution of particle sizes, and can even “fingerprint” a sand for comparison with others. In my experience, however, this sieve information is rarely used at all.

In a well-made mortar, a range of sand particle sizes pack together tightly with many points of contact while still retaining enough void space for the cementitious portion to fill in and hold these particles together. This packing arrangement is very important: a well-made mortar has the correct ratio of cement to aggregate, and sufficient interlocking contact between a range of particle sizes. A mortar with mostly large aggregate has less strength and needs more cement to fill the voids, and has poor working qualities. A mortar with too many fines shears easily, and, in the case of a pure lime mortar, this dense mix is slow to take in CO2. Too much cement and the particles are held apart with fewer points of contact and an overall loss of strength. Too little cement and the particles are not bound together.

Every project on historic structures these days seems to require a mortar analysis. While most people probably believe this process necessary for the appearance of the new mortar to match that of the original, a good mortar analysis should address much more than aesthetics.

My priorities in mortar analysis begin with trying to relate the overall condition of the building with the mortar and looking closely at areas that contain original sound mortar, as well as areas that have failing original mortar. Second to this is a general mapping of later repair mortars, to begin to understand some of the sequence of failures and repair attempts. I look closely at the best of the original mortar taken from a protected area and note if the building masonry was set in one mortar and pointed in another.

My first concern when analyzing mortar is to learn all that I can about what is in it. The replica mortar must be of a high quality- maybe even better than the original- and must not be harder than the sound mortar already in the building or the masonry units (e.g. bricks, stone). Next in importance is matching the appearance, color and texture. This is not so difficult if you make an effort to locate a matching sand or several different sands that can be separated by particle size and color and then recombined. It is also important to collect the fines and silt from the original that often contribute to the color and determine if this material is available naturally from the sand. In rare cases where the amount or type of fines in the original mortar could weaken it, I substitute stable, natural iron oxide pigments. But pigments alone can never be a substitute for getting the sand color range right.

I have found it not only possible but, with a little practice, quite easy to find local sands for use in matching building sands from the past. When I’m having trouble, our county soil geologists can often look at a sample of the sand from a historic mortar and suggest areas where similar sand can be found today. In the worst case, the larger aggregate suppliers usually stock a variety of sands that can be sieved and recombined to imitate the desired mix.

9. When is it best to add some Portland to lime and when are all-lime or all-Portland mixes best?

Andy, a conservator from Pennsylvania

The Brick institute of America states that the softest mortar that can be used for a given unit of masonry is always the best choice. What does this mean in practical terms? The mortar should be able to support the load placed on it with plenty of allowance and should be weaker than the individual masonry units it connects.

How much compressive strength does a mortar need on a fifty foot tall brick building from the 1860s? Calculate 50 x 150 lbs/ft3 (an average weight for this type of masonry) and you come up with 7,500 lbs. Take that and divide by 144 inches in a square foot and you have about 53 lbs/in2. Add in the loads of the floors, roof structure, furniture and occupants and an additional safety allowance and call it 200 psi (pounds per square inch). The bricks themselves will probably have a compressive strength of at least 500 psi. So the mortar for any relaying or re-pointing on this building should be in a range of 200-400 psi.

Lime mortar should be used when repairing any structure that was originally built with lime mortar. I am tempted to say that when building a new structure, say an addition on my house or a new garage, I would use a stronger mortar like Portland, but I’m not so sure. Is there any real reason? I work in buildings every day that are one or two hundred years old and they are holding up just fine, unless the original compression masonry system has been undermined or water is pouring into them.

When I started in construction in the 1960s, the masons normally just used the mixture from the bag, but I remember that a few would always add several extra shovelfuls of gray Portland cement into the pan saying that they liked to make the mortar stronger so it would really last. The only comment I ever heard from others at the time was that this was foolish because it was costing the mason more money. Even today I meet masons who add extra Portland, feeling that it gives insurance against possible failure. Of course, I now realize that in doing this the mason is making the concrete more brittle and increasing the amount it will shrink during curing.

Properly-made, high-quality, lime-only mortar is nice in that it is not so brittle as Portland-based mortars and, because it has little or no shrinkage, it is less likely to crack and allow water to enter. A well-made lime-sand mortar has a porosity that allows water that enters to return to the outside and evaporate easily, while Portland cement usually slows or prohibits this evaporation. And in the process of allowing water back to the surface, lime-only mortars have the additional benefit of autogenous healing. Slightly acidic rainwater carries hydroxide from the interior to the surface, where the hydroxide carbonates and closes up cracks and figures.

Clearly as soon as you begin adding Portland cement to lime mortar you run the risk of making a mortar that is not only harder than the original mortar, but maybe harder than the masonry units it is joining. Adding Portland will likely increase the amount of shrinkage, which is not desirable. Also it is clear that just adding lime to Portland in the belief that it will make it less hard is only part of the story. In fact, different types and qualities of lime would each behave differently, some acting as little more than additional aggregate, others perhaps remaining as a very soluble hydroxide and later dissolving in rainwater and forming calcite deposits on the surface.

10. What should I expect from a mortar analysis and what do I do with this information?

-Joy, a building owner in Illinois

There is analysis and then there is analysis. It is possible to analyze mortar from a building and learn a great deal, including what compounds were in the original mortar, how it was made, why the mortar is failing (poor quality, salt, acidic attack, chemical reactions with other materials or new compounds introduced as preservation treatments, etc.), as well as other deterioration processes occurring in the building. In addition, careful observation of mortar characteristics can be most helpful in understanding the sequence of building, alterations, and repairs. Unfortunately, this sort of analysis rarely happens.

The term “analysis” suggests a scientific process. Most mortar analysis is merely a description or a characterization of mortar. This can be useful, but very often the wrong information is emphasized, and the most useful information for making a true replica mortar is not presented at all. This is because these so-called analyses are typically performed by people who have an academic interest in historic buildings and materials, have little practical building experience of value, and know almost nothing about the nature of the materials, the underlying chemistry, or the working properties. Usually they are merely repeating a simple process they learned from a preservation course or a book.

There are a few masons who know enough from long experience, thoughtful observation, and reading on the history and practical aspects of their craft to make very good replica mortars. Unfortunately, few masons are interested enough to go a bit further and learn about the underlying chemistry of the materials they use. This combination of real skill developed over decades with an understanding of the nature of the materials themselves is a necessity if we are to come anywhere close to making repairs to historic buildings that are equal in quality to the original and do not cause more harm.

These days it is de rigueur to order a mortar analysis. It’s one box on a checklist. I am regularly asked by clients to review mortar analyses they have commissioned. These range from very simple to absurdly complex and lengthy. Usually the author describes the mortar in general terms, compares it to a Munsell color chart (sometimes wet, sometimes dry, never specifying), and then weighs the sample and immerses it in hydrochloric acid. This last step dissolves the binder, the cement. When this reaction is described as vigorous, it is given as proof that the binder was lime or mostly lime. Now the time for a real analysis has just begun and the most important component, the binder, is dissolved into a bubbling froth of gas and water.

The sand is then dried and weighed. The difference between the mortar sample the weight of the sand alone after digestion gives you the volume of lime, and you can calculate the ratio of lime to aggregate Some reports describe a few of the minerals in the sand, and lately there seems to be a trend for very long-winded descriptions of mineral types running several paragraphs. The sand is then passed through a series of screen sieves that are supposed to begin with a #4 and continue with 8, 16, 30, 50, 100, 200 and sometimes 325. But often the sequence varies with a more random selection, jumping from a 4 to a 10, then maybe a 20, then a 50 or 60.

The weight of the sand retained on each screen is noted and sometimes its percentage of the total is listed. Usually this is followed by a recommended formula for a replica mortar. These recipes may specify sand such as Schofield #145 that may or may not exist anymore, but typically just refer to the sand as a yellow-tan or white builder’s sand. Almost always, the recommended binder consists of 2 parts lime to 1 part Portland cement followed by 9 parts sand. These days various ASTM guidelines are inserted for the type of cement or lime and aggregate, and are usually irrelevant to the task at hand: matching a historic mortar. More than once I have seen ASTM guideline #270 used for specifying the type of lime to be used in matching a historic lime mortar. This standard is for S-type lime that is hard-burned and finely ground for use with Portland cement. It is completely unsuitable for making a high-quality lime mortar.

C-270 is specified because the mortar being recommended is really a Portland cement mortar with added lime. Often pigments are specified to create an approximation of the color of the original mortar. The more lengthy reports may include some thin section information, all kinds of USA Today-type pie charts and graphs, and, most disturbingly to me, many pages of instructions to the site workers on proper techniques for making, storing, and using the mortar, stucco, or plaster.

You are right to wonder about the value of these reports. What do you learn that is useful? You do not learn much about the components of the original mortar. But then again, you are being given a formula that contains very different materials from the original anyway. What is the purpose of the analysis? Why do these reports state that matching the original mortar qualities is so important and then provide no information to do this? What is the purpose of the sand sieve information?

All good questions, and later I will try to provide some better descriptions of onsite preparations and methods for determining much of this information yourself. For now, stick to learning about and finding good quality lime and begin collecting sand samples from your area.

11. Once I’ve got the right replica mortar mix, how do I make the joints blend in with older work?

– Mike, a bricklayer from Pittsburgh

Presumably a mason with considerable experience has spent a lot of time looking at old masonry in many states of preservation, repair, and disrepair, paying attention to how the joints were struck and tooled. For example, masons in the past must have had many tools similar to ours, but also many that were different. The trick in all of this work is to see clearly, a real skill that takes years to develop.

The first thing to realize when we are repairing older work is that we will be attempting to imitate an appearance that was created in a way we may not be able to precisely duplicate. The original mason was working with a freshly laid wall where he was mixing mortar to the consistency required for laying brick, mortaring, and setting the brick in place. Depending on his judgment, the joints were struck in certain ways and at certain times to achieve the finished appearance.

Often our repairs consist of removing some deteriorating mortar or previous repair to a depth of an inch or so, packing the new mortar in, and tooling it straight away. We need to adjust the consistency of the mortar, the moisture content in the joint, the pressure of our tools, and the sequence and timing of work in order to imitate the appearance of the best preserved of the original work. Then we must complete the process with more tooling, brushing, washing, etc., in order to soften the sharp appearance and create something that approximates the weathering of the adjacent original mortar.

By careful examination of well-preserved early masonry, we can determine the shape and size of some tools, the sequence of tooling, and consistency of the mortar (which may be different on the surface than deeper in the joint). Examining old texts on the masonry arts and making replica tools is also essential. And finally, there is no replacement for lots of practice.

12. I’ve had a mortar analysis done and I think I’ve gotten lucky finding a good sand match right at the local masonry supply store. Last weekend I slaked a Lime hydrate to a thick cream consistency and now that I’ve finished removing the portland cement pointing, I want to begin re-pointing this weekend with my lime mortar. My concern is setting, or the likely potential for shrinkage and cracking without the use of a pozzolanic additive. So the question is gauging. What to gauge with? I have no known source of pozzolans in this area. Am I then left to use white portland? If so, how much? What about crazing dangers?

– Mike, a stone conservator in Kentucky

This answer may seem long-winded, but I think in order to get at the heart of your concerns I must first run through a bit of information about quality lime for use in lime mortar, getting and mixing the right materials, and application. Well-calcined high-quality lime is usually very white. Some lime putties on the market these days are touting their off-white, almost tan color. Often these lime putties are from poor quality limestone and are improperly fired.

I wouldn’t worry too much about the aggregate/lime ratio from a standard mortar analysis. (More about this another time.) For the moment, stick with one lime to three sand unless this is a special pointing mix. Is the original surface mortar different than the mortar deeper in the joint?

After acid digestion, look closely at the sand and the fines. These components will largely create the color of the mortar. The fines give it an overall shade or hue and the sand contributes to the weathered color and texture. Sometimes you will be lucky enough to find a commercial sand that matches exactly, but I have found that even when I can’t find a match, by selectively sieving several sands I can sometimes pull the appropriate range of both particle sizes and colors out of sands that otherwise would be the wrong color.

When calcium oxide or quicklime is slaked it becomes calcium hydroxide, or in the case of your  Lime it’s the hydrate (has molecular water added, but needs free water to make the putty). Therefore you are not slaking, but rather soaking the hydrate. It is good to mix it a few times, but then you want to let it sit. Our hydrate is only lime I would consider to be of high enough quality that you could feel sure of a good product just after a few days of soaking. Using hydrates for mortar to point with is perfectly adequate. Not so when plastering or stuccoing.

I was surprised when you called the consistency of your soaked lime “thick cream,” as this sounds more like you are making limewash. Lime putty should not have much excess water or be runny-think more along the lines of Philadelphia cream cheese. Your fears of shrinkage will be realized if you have too much extra water in the mix.

A nice putty has the water it needs already bound up in the matrix; you release the water for your needs through mixing the mortar. Very little additional water will need to be added to your mix. Trust me on this one: elbow grease is the answer. Case in point: Gerard Lynch, in his first Brickwork book, references a French proverb about allowing the only additional moisture to come from the sweat of a laborer’s brow.

When you first start mixing in the sand you won’t believe that you can add in three or four parts of sand because the first part already looks like too much. But as you mix, the lime will appear to get wetter. Keep this in mind also as you are applying the lime mortar. Often during the summertime, the mortar on your trowel will appear to dry out rapidly, but with a little beating the mortar is usually ready again without more water. For years we mixed our mortar in a mason’s plastic mortar pan with a hoe. You’ll work up a sweat, but it’s doable. Beat the lime well before adding any sand and just stay with it until everything is well mixed (again, don’t just add water to make your job easier).

I don’t recommend the use of pozzolans for pointing, and Portland cement will only increase the shrinkage. Properly prepared and beaten mortar will have virtually no shrinkage. It is preferable to make the mortar before the day you use it since the longer sand and lime are in contact, the better the working qualities will be. As long as these are kept moist, we have found that the working qualities keep improving over months, and even years. By using our Lime, using our mixing advice, properly wetting the joints well in advance (not just immediately before adding mortar as water can easily be drawn out by the surrounding masonry and undermine the bond -water an hour ahead, then again a bit before, but save yourself messy brick faces and ensure the surrounding brick faces themselves are not wet when you point), and preparing the joints properly, you should not have problems with shrinkage or cracking. If there is a limited amount of micro-cracking, you are lucky to be using lime and not Portland, because of lime’s ability to “heal itself” by re-depositing lime into those cracks.You should not have problems with this unless you make the mortar too wet.

Preparing the joints: I don’t know how big the joints are. Could you send us an emailed picture? Until then, I’ll assume they are variable in size and depth. The rule of three times as deep as the joint width is okay, but not to be taken to extremes. For instance, if the joint is 1/2″ wide, you still only need to scrape it out an inch, 1-1/4″ at most. It is a good idea to work across a wall filling the deepest cavities first, before coming back and building out in lifts or layers of no less than 1/2″ each. Ignore the standard advice about letting each lift set up (carbonate) before you apply the next one. It’s a recipe for disaster. Think of it this way: you want to achieve a mortar that is as densely packed as possible. Leave your last lift of mortar for 1/2 hr to a few hours (weather dependent) until the free water has evaporated and/or been drawn into the surrounding masonry. This allows you to compress the mortar (minus extra water) with your tool or a hammer and wooden wedges before applying the next lift. In other words, for a joint that is one inch deep, I would easily be able to fill it with two lifts and tool and finish the joint in a single day.

One more word of caution here: many people overwork the surface of their mortar by slicking it too much. Compression does not mean slicking, because over-tooling creates a condition of too much lime on the surface and a plane of aggregate just below the surface that has been starved of its cement binder. This can create a weak plane below the surface that has the potential to fail later for a variety of reasons, but in particular due to freezing. You will also have an appearance that certainly won’t match the rest of the building, particularly on exterior stucco. Usually, we will slightly age mortar within a day or so of applying it by hitting it moderately with a stiff bristle brush to open the surface a bit and expose the aggregate. (The exception to all I’ve just told you is decorative pointing with a graded sand and higher putty content. That is something we can talk about another day).

The mortar we have been discussing here should be adequate for all the locations on this building. The key to good cement-aggregate ratios is that a range and distribution of particle sizes fills in nearly all the voids. Imagine a box filled with tennis balls, golf balls, marbles, and lead shot, with the idea that there is much point-to-point contact between all of the aggregate particles and the minimum of cement necessary to bind the particles together.

Make sure that the work is covered from weather – hot sunlight, wind, and rain – for a few days until you get surface carbonation (light color and hard surface). Remember lime mortar carbonates as carbon dioxide moves in at the same time the water is moving out, so carbonation won’t occur if the mortar dries too rapidly. After the initial surface carbonation, rain will actually increase the rate of carbonation beneath the surface, so further protection of the mortar at that point is counter-productive.

© Preservation Science 2008
Used with permission.
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