Reviving Lime Mortar: A Stone Foundation's Best Friend

I am coming to the end of a long-term project involving the stone foundation that supports our early-1890’s home.  Looking around the city where I live, many 19th Century era stone foundations, and many brick homes and chimneys as well, are in dire need of repair, ours being no exception.  Originally bound together with a mixture of locally-sourced fired-limestone, sand and water, the joints between the stones had been long neglected and/or the sorry recipient of a common “repair” method involving cement-based mortar lain over the original lime mortar mix.  This post examines my ventures into the ancient world of lime mortar and the rekindling of a largely forgotten practice. 

 

Why our Foundation Declined

Contemporary masonry is focused largely upon mortars designed for concrete masonry units and modern super strength brick.  However, only a hundred years ago, mortars made from locally available limestone were the norm.   If given the proper attention, joints comprised of this fantastic material can last almost indefinitely.  However, if left to its own devices, weak lime-based mortar, like that used long-ago in our foundation, eventually turns to dust, degrading back into its component parts.  That’s what happened in our case – the natural process of decline dramatically accelerated thanks to three separate factors.

First, a lack of gutters on the building has for a long while permitted large amounts of rain water, concentrated by large sloping roofs, to fall more than two stories and splash back on the foundation, eroding the masonry in relatively short order.  [This deficiency has since been corrected.]

Secondly, in the intervening years since the building’s construction, few of the various owners of the property probably ever went through the effort of applying annual ‘whitewashes’ (a mixture of limestone and water) to the foundation to protect the underlying mortar – a necessity for the upkeep of weak lime mortar joints.  

And lastly, though well-intentioned, earlier work by some owner(s) or laborer(s) to shore up the masonry joints served to only exasperate the condition.  Soft stone (or brick) and cement-based mortars do not mix well.  Cured cement is very hard and very stable, while natural stone tends to (a) absorb water and (b) expand and contract along with rising and falling temperatures.  The incompatibility of these materials has not been widely known or necessarily understood by masons or homeowners, nor widely advertised by the cement industry.  This is simply evidenced by the fact that one cannot drop into their local hardware store and pickup a bag of anything but cement-based mortar.  The decline of the use of lime mortar is inversely proportional to the invention of Portland cement in the early to mid 1800’s.  Check out the Wikipedia entries for Portland cement and lime mortar for more on the history of each material. 

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Before and After of North Wall:

 

 

 

   

 

 

 

 

 

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Cement-based vs. Lime-Based Mortar

For the end-user, each type of mortar has their own advantages and disadvantages. 

Cement-based mortars offer higher strength, quicker set-times, and can be applied year round in cold climates as long as precautions are taken during set-up to protect the masonry joints from freezing temperatures.  However, they are also highly corrosive to skin and flora in its uncured “wet” state, destroy soft stone and brick masonry, offer a short working-window, and require a lot of energy to manufacture.

Lime-based mortars, on the other hand, must be protected from direct sunlight, drying winds, and rain for at least a week after application, and are really only practical for use in warmer climates or during the summer season in northern climates (the uncured mix does not fair well in cool to cold temperatures).  They do, however, allow 24-hour workability (as long as the mix is sealed in a fairly airtight container), produce incredibly long-lasting joints (since it ‘moves’ like the soft masonry units it joins and is ‘self-healing’), offer easy cleanup (since the wet mix is not nearly as damaging to skin or flora), and have a substantially smaller carbon footprint than their cement-based relatives.  Plus, lime mortars remove CO2 from the air far more effectively then cement-based mortars since the process most accurately mimics how shells are made (i.e. carbonation).  You can read more on the various processes that entail the manufacture of each material by viewing the above referenced Wikipedia links.

Saved Just in Time

Luckily, I got to our foundation just in time.  The cement-based mortar patching had almost completely caused the disintegration of the underlying lime mortar by allowing water in behind it, soaking the hidden lime mortar but not allowing it to readily dry.  This caused it to simply fall apart.  Many of the joints were vacant any mortar at all.  The seal between the cement and the stone failed since although the stone moved around, the cement mortar stayed in one place, cracking the seal.  This interaction also caused many of the stone faces and edges to disintegrate.  In effect, something had to give in this tug-of-war and the softer stone ended up suffering.  And of course this effectively widened the joints even further.  If you’ve ever seen a brick wall where the faces of many of the bricks are blown clear-off, you’ll understand the effect.  To reiterate, cement and soft stone/brick do not mix!

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Before, In Progress, and After South Wall:

The “Lost” World of Lime Mortar

When I first looked into the world of lime mortar I was somewhat put off.  This was due to several realities.

1.  The material itself is presently hard to find, which is particularly tragic given the plethora of “Lime Kiln” roads that exist in our neck of the woods.  When I called my local masonry supply house and they had no idea what I was asking for, they wrongly tried to sell me masonry cement with lime in it (lime is sometimes added as an ingredient to increase the workability of certain cement-based mortars).

2.  Most of the information provided, by what centralized suppliers do exist (most are located in the mid-Atlantic states or on the west coast) or by the few trade organizations who have websites, has a very proprietary feel to it.  In other worlds, they’ll offer it for sale or discuss its benefits, but if you’re not “in the club” they’re not going to fill you in on the particulars of how to work with it, or will do their best to mystify its use.

3.  General lack of historical knowledge, by anyone I knew or had any contact with, about lime-based mortars.  This user-knowledge has not weathered the test of recent time well.

In the interest of overcoming these stumbling blocks for someone else, I am happy to not only explain where I ended up finding the stuff locally but also to pass along my experience in working with it.

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Hydraulic Lime

The Lime Mortar

The particular lime mortar I ended up choosing is denoted as “NHL 3.5”, which stands for “Naturally Hydraulic Lime Mortar” with moderate hydraulic characteristics. 

For more info on what this means, and how it differs slightly from non-hydraulic lime mortar, I’ll once again point you in the direction of the Wikipedia Lime Mortar page.

At first glance, it appeared that my only option to get hold of any was to have it shipped by UPS from Virginia to our location in Vermont.  Since NHL 3.5 comes in 55lbs bags, I was not looking forward to the added freight cost.  After digging a bit more, however, I found out that the North American importer of the all-natural St. Austier lime mortar (a product of France) had an office in Montreal, Quebec.  It’s really too bad it was neither easy nor intuitive to find this out.  However, after some telephone calls and emailing, I ended-up driving the four hour round-trip with the trailer attached to our car and picked up all I needed for less then it would have cost to have it shipped.

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Clean, Sharp, Well-Graded Sand?  What’s that?

Next, I had to find a local source of what all the retailers, and trade organizations, and product instructions I found online referred to as “clean, sharp, well-graded sand.”

I called the local sand pits, quarries and the masonry supply house, but no one had any idea if the various sands they offered met this definition.  I called the mid-Atlantic retailers (one of which actually manufactures their own line of domestic lime putty – most is imported) but they only deepened my confusion, echoing as they did the “clean, sharp, well-graded” mantra while also supplying to me the largely useless ASTM specification.  Sandpit operators in my area apparently have no idea what “ASTM C144-04” refers to, nor do I.

I finally secured the foolishly simple answer to this bizarrely mysterious question by locating and then getting in touch with a renowned mason in our area with experience in historic preservation.  He was generous enough to return my cold call and during our conversation effectively ‘spilled the beans’.  His advice arrived, “…we always use masonry sand and we just add some larger aggregate for filling larger joints.”  I thanked him, and also quietly thanked my lucky stars for my propensity and tenacity to get at the truth.  Otherwise, I’d likely still be searching for the ridiculously elusive “clean, sharp, well-graded sand.”  Why no one else just called it what it is I’ll never know.  I paid my local concrete yard a visit and took away all the masonry sand I would need for a small fee.

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Preparing the Foundation Walls

Then, it came time to actually prepare the foundation walls for the new mortar.

I got a kick out of the many references I came across online that practically forbade the use, by any self-respecting mason, of pneumatic chisels for removing old mortar.  Now, had I heeded that advice, I’d still be at work on my first section of wall.  Of course, I’ve since come to realize, through experience and by reading between the lines, that the widespread concern is wholly unfounded for stone walls.  Although the focus for all of these warnings was originally brick walls (where it is completely valid), somehow, over time, the original intent was lost in translation and the moratorium was extended to stone walls.

I personally found using a pneumatic chisel to be the best, most effective way to remove the hard cement mortar and the failed lime mortar in preparation for the new joint.  To then remove the left-over residue, I used a spray of high pressure air provided by attaching a spray nozzle to my compressor hose.  For interior work, I supplemented a shop-vac for this portion to keep from blowing dust all over the place.

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Prepared West Wall and Finished West Wall:

 

 

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Preparing the Mortar

Lastly, I prepared the new mortar using a ratio of 2.5:1, sand to lime.

I mixed batches in a steel wheelbarrow with a hoe and either transferred the mortar to a smaller container which I fed in through a basement window (for interior work) or simply troweled the mix right out of the barrow.  I made sure to thoroughly soak the portion of the wall that I was working on with a garden hose (outdoors) or a spray bottle (indoors) shortly before packing in the new mortar, and to then keep the wall moist as I worked.

Per the suggested practice I worked in “lifts”, filling the joints approximately half or two-thirds full on my first pass, then returning to where I’d started to go over the joints again with the finish layer.  There are many cautions about mixing the mortar too wet, but in my experience the only negatives a too-wet mortar would suffer from was that it made it harder to pack into the joints (too soupy) and that it would crack a little as it dried.  A couple of times I used a wetter mix for an initial lift and then simply smoothed over any cracks while using a dryer mix for the second lift.  No big deal.

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Prepared North Wall & Finished North Wall

 

 

 

 

 

 

 

 

 

 

 

Final Steps

When the joints were full to my satisfaction, I covered the portion of the wall I had finished with plastic tarps touching or almost touching the wall.

Approximately 24-hours later I returned to remove the tarps and using a stiff brush, brushed the joints, cleaning them up while removing any errant mortar from the face of the stone.  A couple of times I waited for ~35 hours before I completed this step since the walls on the north side of the house took longer to cure to the point where brushing could occur.  I realized the necessity of waiting longer when I began to brush the wall and too much mortar was removed.  So, in these instances, I simply waited another night and the next morning or mid-morning finished the process.

After the brushing, I reinstalled the tarps and left the wall(s) covered for seven days.  On the south side of the house, which is in direct sunlight, I used drop cloths soaked in water in between the tarps and the wall and then soaked them daily for the first few days to keep the walls from drying too quickly.  Elsewhere, I just used the plastic tarps and occasionally (once a day for the first few days) sprayed the walls lightly with a garden hose.  Indoors, I left the walls uncovered, but made sure to keep the basement windows closed which seemed to afford a humid enough environment for proper curing.

A week later, I removed the coverings and, in our case, added a band of ¾” crushed stone up against the foundation and extending outwards approximately two and a half feet, bordering the entire foundation.  Since I’d excavated approximately a foot to a foot and a half below grade to get at more of the foundation for re-pointing, I simply took the excavated soil off-site and filled the space back in with crushed stone after grading the soil base away from the house and tamping it down.  I figured that it would be good for the foundation joints I’d re-pointed to be able to breath (for drying) and the addition of the stone perimeter really cleaned up the look around the foundation.

All-in-all, this was a fairly simple and straight forward job but one that did take some time and required a little patience.  I highly suggest lime mortar for anyone working with stone and look forward to using it in future projects.

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Tree root growing along Foundation, In Progress, and Finished North Wall

 

  

110 comments to Reviving Lime Mortar: A Stone Foundation’s Best Friend

  • Marcia Prager

    Thanks to everyone on this thread. I have an 1895 Queen Anne Victorian in Philadelphia and want to parget the interior basement mostly below-grade walls that have the typical degradation and crumbling sand mortar sifting to the floor. I was advised to use a mortar mix called “irish Plaster” for parging basement fieldstone walls 1800’s to early 1900’s. This is the formula. Can you comment?

    Irish Plaster

    For basement fieldstone walls, 1800’s to early 1900’s

    mix one part Portland cement,
    three-quarters part limestone,
    and an amount of sand no less than 2.5 times and no more than four times greater than the amount of cement (for 10 pounds of cement, that works out to 7.5 pounds of limestone and 25 to 40 pounds of sand), and add water until the mixture is the consistency of pancake batter.
    Wet the surface of the wall, and spread the mixture on – not too thick – with a trowel. If it doesn’t bond to the surface, there could be too much water.

    Never mix more than you can use in 30 minutes.

    Can add bonding mix

    • Kai

      Marcia – My feeling is that this amounts to an attempt to bandaid an underlying problem. Instead I suggest that you repoint the failing mortar rather that try to cover over it with something that relies on adhering to that which it is applied to. If the mortar is failing, what’s going to stop the parget from failing as the mortar behind it continues its inevitable decline? I don’t think even a cement based product could hold back this tide. Perhaps someone else has an opinion?

  • JudithW

    I am so glad I found this page! I am in the process of purchasing a 1787 home in Massachusetts that needs some work done. It’s been a work in progress to obtain the full 203K loan plus a lot of research and reaching out to experts in historic renovation. Getting them to talk with a single woman looking to restore a historic home has come with its challenges. I want to be able to seal the foundation myself and possibly with the help of an expert with more knowledge than myself about old foundations. Your detailed information above will be of so much help for me and the many projects ahead. Thank you.

    • Kai

      Judith – We’re so glad you find it helpful! 🙂

    • carole roseman

      Judith W
      I can so totally commiserate. I bought a 1790 Brick Federal House in 1995. I GC’d the complete restoration. Presumably, the brick had all been repointed and the foundation redone. It’s now 2019 and there are problems…. so here we go again….
      Luckliy there are blogs like this now to share and ask on.
      Carole R.

  • Henry

    It seems that it’s hard to do lime mortar in the US/Canada. Well done on your repair. I thought I could helpfully bring a few points together as here in the UK lime is common and there are quite a few local suppliers remaining.

    I imagine that’s where you got your ‘sharp sand’ confusion from. In the UK, ‘sharp sand’ is the normal term for just that – sand that isn’t rounded. It’s purchased from all DIY shops or builders’ merchants, and is probably exactly what you discovered it’s called over the water: ‘masonry sand’. Using round pebbles or smooth sand as an aggregate would result in a weak mix because there aren’t any edges to lock together and create friction between the different particles.

    Graded simply means that the sand grains are of a consistent size. In reality, one will find that historic builders used what was there in the local area as it was difficult to transport large amounts of heavy material before the railways. Earth/lime mixes, stone, sharp gravels… The colours of mortars across the UK vary because of the local sands. If you read builders’ manuals from the early 1900s (Portland Cement mortars came in majorly from the 1930s) then you will find lots of discussion on lime and aggregates from different parts of the country and how it’s to be treated. Often with an injunction to consult a local mason for their knowledge!

    The advantages of lime are myriad. It is less environmentally damaging than cement in that it takes less energy to produce (embodied energy) and it is easy to remove from bricks and stones. The curing process turns lime mortars slowly back into the calcium carbonate they were made from – limestone. Lime mortars and plasters can be powdered and burnt again to create quicklime, which is slaked, aged, mixed with sand and becomes mortar again.

    Cement is too strong. It sticks to bricks, stones and blocks so strongly that it often breaks them when one tries to get it off to re-use the masonry. Lime is slightly alkali, so when in contact with wood it discourages insect and fungal attack – witness 500 year old buildings with their timber walls built into a lime layer on top of a small footing wall. It is flexible, so moves with the building which prevents cracks. It is breathable, allowing water vapour to pass through and evaporate, which balances internal humidity in a breathable construction and prevents water being retained in a wall and causing mould or rot. At present I am building a timber framed house adjoining a 19th century barn (rubble and lime walls with no foundations) using no plastic membranes, and as little concrete as my nervous engineer will let me get away with. This is possible using breathable materials (wood fibre insulation, timber, wood-wool board etc.) and considering the humidity gradient. It also works with the old rubble and lime wall which has been damaged by cement render retaining water in the walls.

    The only reason cement supplanted lime is because of the faster curing times and less requirement for craftsmanship to get it to work properly. Blame pernicious rhetorics of productivity and efficiency over building physics and health!

    Hydraulic lime cures in the presence of water, while a slaked lime putty (NOT hydrated lime from the builders’ merchants! That’s a lime-based additive for increasing the plasticity of cement) cures by carbonation from the air. The higher the NHL (Natural Hydraulic Lime) number, the more ‘cement-like’ the final product. i.e more rigid, impermeable, strong and brittle.

    These used to be called ‘feebly, normally and eminently hydraulic’ (NHL 2, 3.5, 5). No portland cement is used, with the mortar consisting only of aged lime putty and sand. The same is used as plaster, though the finish coats will have finer sand and occasionally ground marble for a remarkably smooth finish.

    For external renders, an NHL lime is recommended for its superior weather resistance, although ‘pozzolans’ or ash additives are available for lime putty mixes that require some hydraulic qualities. NHL limes are simply limestones that have inherent impurities which create the hydraulic properties without pozzolans.

    Some useful websites from the UK which might help further anyone in North America with lime questions are the Welsh Ty-Mawr natural building company (www.lime.org.uk) and the Scottish ‘Building Limes Forum’ (www.buildinglimesforum.org.uk).

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