Stone Wall Restoration - Working With Lime Mortar

Introduction: Stone Wall Restoration - Working With Lime Mortar

About: I am an escapee from modern life, now living by the sea in a forest garden in France. After over 20 years industrial experience, I quit my managerial position to study for a degree in Engineering. That done I …

Our house is at least 300 years old and was constructed from local stone and a few choice pieces of granite 'borrowed' from the local monastery during the French Revolution. It has no foundations but is built directly onto the bedrock from a stone 'sandwich' laid in layers of local clay with a loose rubble and clay infill. This makes for both a solid and well-insulated wall, which starts at a base thickness of around 27", tapering up to 18" at the height of the roof.

Before we came out to live in France, I used to do a little moonlighting teaching maths and IT at my local college. Whilst there I found they had a great bricklaying class and decided to enroll and over the next eighteen months or so I became acquainted with some of the practical techniques of this fascinating trade. Our instructor was a professional builder and the tips and suggestions he made to me and my classmates regarding our own specific home projects were invaluable. Although Natural Building per se was still in its infancy or rather rebirth, we were very lucky in our instructor. One of the most important for me and avant garde for a mainstream building course, was his his stressing of the use, where possible, of lime mortar. We used this in class and became well acquainted with the medium. On finding out that I wanted to replace an ugly 1960’s fireplace with a cast iron Art Nouveau one he immediately cautioned me not to use the very expensive commercial fire cement but to opt for a ‘soft’ lime mortar mix to fit the new fire-back, Following his advice I was able to undertake the job at a fraction of the cost through using his recommendation and it worked perfectly. In fact the money I saved through not having to purchase the expensive refractory cement more than paid for the evening classes!

Now however, in our present house I'm in a whole new ball game with stone but the valuable lessons in using lime mortar are even more apposite for this ancient longhouse.

Step 1: Knowing and Respecting the Past

Over the centuries, including, as we live in Normandy, heavy shelling and bombing in WW2, the building has taken quite a battering and thus there are many different patches of restoration, these are in the main, of local stone and lime mortar or clay..

The object for us is not to destroy any of the charm of the original building (including all honest repairs), which through its 'stone book' tells the story of the people who lived here. For example, the third image above shows our fenêtre meurtrière - literally 'murder window' or rather arrow slit. This not only bears witness to the thickness of our walls but how much an 18th century French farmer's house was his castle. These windows allowed the occupants a wide field of view and thus fire, whilst being themselves protected by the walls with a formidable stone shield.

Whilst my knowledge of lime mortars is in no way as extensive as I would like, I have never found that the mortars I have mixed and used have done anything detrimental to the structure of the buildings. For further information on specific uses of lime mortars, I have included three of my favourite sites at the end of this instructable.

I’ve used lime mortar a lot in our present house firstly because it is obviously appropriate to a 300 year building, as it is sympathetic to the original build of stone laid on a bed of clay mortar, secondly because in the mortar mixes I have used it will never be stronger than the stone of the house and thirdly when used in wall construction, the wall is able to ‘breathe’, essential for the thick walls that we have here. Clay mortars were very popular in the 15th century and were still used in vernacular houses and buildings in the 18th century.

Step 2: Why Do We Need Mortar

Bear in mind that the mortar is there to keep the stones apart so it must have some ability to retain a shape but be sufficiently workable that when the stone is laid upon a bed of it, it will form a joint and support the stone.

If stones were to be laid directly in contact with each other, any high point on the stone face would be a pressure point and the concentrated load transferred through this point to the stone below would invariably cause that stone to break

Step 3: A Word About Natural Hydraulic Lime

Above: an area along our coast, famous for lime kilns, oysters and pirates.

Sea shells, chalk and limestone are all forms of calcium carbonate, when these substances are crushed and heated in a kiln between 800ºC –1 000ºC the carbon dioxide in the compound is driven off leaving solid calcium oxide(quicklime).

Adding water to the cooled medium results in an exothermic reaction and produces calcium hydroxide which subsequently, if left exposed to the air, will absorb carbon dioxide to form once again calcium carbonate. Adding sand to the calcium hydroxide produces a mortar known and used for thousands of years. If the limestone is not pure but contains impurities of clay then during the heating process the calcium will react with the clay to form compounds called silicates which will set with the addition of water. Not all of the calcium will be in the silicate form, there will still remain some calcium hydroxide, which will set by the carbonation process. Thus mortars of different strengths and setting times can be produced. depending on the level of the impurities.

There exists a classification for Natural Hydraulic Limes which describes their performance due to the level of impurities. Namely: feebly hydraulic NHL 2, moderately hydraulic NHL 3.5 and eminently hydraulic NHL 5. The higher the number, the faster the setting time and the harder the resulting mortar.

Step 4: Mixing Mortar by Feel

If I was to use the word Art in connection with lime, it would no doubt conjure up an image of High Renaissance fresco but in effect my meaning here is much more prosaic and refers to the mixing of mortar. Or rather it has become so, as over previous decades we have been taught to devalue what was an art or at least artisan, down to the level of manual labour. There is as much of science and art in getting mortar right as in any other endeavour, which is why no Mediaeval cathedral ever carried a ten year roofing guarantee.

Because my quantities of mortar were small I mixed it by hand in a mortar mixing tub which could then be carried into the house.

The first thing to do was to accurately measure the correct proportions of sand and lime. For pointing, this is 4 parts sand to one of lime.

That done, I dry mixed them to produce an evenly coloured, lump-free mix.

This stage is important because I want the lime to coat all the particles of sand.

Once satisfied with my dry mix, I add water a small amount at a time and with each addition, continued with the mixing.

The water needed to be evenly distributed throughout the mix so it was important to make sure that the lime/sand mixture in the corners of the mixing tub were moved to the centre.

As the quantity of water was slowly increased, the mortar becomes more homogeneous and the feel of it on the mixing spade changed. It was at this stage that I stopped and tested the mortar with a trowel.

The mortar should not be like a slurry, but one ought to be able to slice a sausage-shaped piece with two cuts of the trowel which can then be lifted onto the trowel, it should still retain its shape and yet have an elastic appearance when shaken.

Frankly, I find this to be a very subjective operation and this ’feel’ for a good mortar was one of the first things I learned in my evening class. Nevertheless, the too runny, slurry stage would be self evident.

Step 5: Pointing an Old Stone Wall

Our old walls over the centuries lost their clay mortar and now need that to be replaced. By using the mortar to 'point' the wall, this restabilises it and prevents the possibility of stones slipping out of place.

In some areas of the wall, so much clay has been lost that there are large voids behind the facing stone. These need to be repacked with stone and mortar before pointing.

The first stage in pointing is to rake back i.e. clear out the joints to a depth of about 1" (2.5cm). This is when you find which stones have become loose.

The wall is then brushed down to remove any remaining loose debris and dust.

Once the mortar is mixed, the wall is dampened down, I use a brush and bucket of water.

If there are any really deep holes that have become apparent it is at this stage that I would feed in mortar and then some clean wet stones to fill the void. The joint can then be filled.

I use a large trowel held upside down onto which I place a sausage-shaped piece of mortar. Using a smaller pointing trowel, I press the mortar down so it is thickness of the joint I want to fill. Now comes the tricky bit, using the pointing trowel, I cut a piece of this mortar and in the same motion as cutting, sweep the mortar onto the backface of the pointing trowel. If the mortar is too wet, it will slide off onto the floor! A correctly mixed mortar will sit on the pointing trowel without moving. This can then be transferred into the joint between the stones.

Obviously with natural stones, the gaps between them will vary.

Any mortar which has ended up on the face of the stone, should be removed with a stiff brush, I usually wait until the following day to do this. Stone is a lot more forgiving than brick!

Step 6: Conclusions & Links

Lime mortar is a wonderful medium to work with and a must if you have an old house or building to work on.

I would certainly recommend the following sites:-

http://www.buildingconservation.com/articles/articles.htm#limemortars for more information on the use of lime mortars in restoration work,

http://www.naturalhydrauliclime.net/FAQs.html for a contemporary review of the applications of the medium and

http://www.stastier.co.uk for comprehensive technical information on the performance of mortars made using lime-based products.

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14 Comments

0
tytower
tytower

1 year ago

Thoroughly enjoyed that thank you. The links are a wealth of information too .

The great wall of China used a mortar which contained starch powder in the form of the rice boiling water kept and used in the cement. I believe they did not know what they were creating at the time but time has tested it and found the rice flour or starch enhances and waterproofs the mix.

Has this been tried with lime mortars do you know? Do you think it would be beneficial. I have made concrete steps with 3 % or so starch powder and so far they seem to be totally waterproof after 10 years.
I think though that its the very fine particles in rice flour that do the actual waterproofing.

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi there and thank you so much for the comments - they are very much appreciated.
The waterproofing idea is fascinating - I never knew they used starch in mortar. I found this https://www.untdprimepub.com/journal-of-rice-scien... which has in depth information on the composition, chemistry and performance of 'sticky rice mortar' So this is a traditional lime mortar. My only caveat would be that this would not be used for houses or other constructions where you want the walls to breath.

I know in developing countries they make ferro-cement water tanks and this is not an ecological solution but it sounds to me like sticky rice mortar instead of conventional cement would be ideal. It's also a great way to get rid of over-cooked rice (or friendly rice as kind people call it!) I wonder too about potato starch which would be easier to get here, we do have rice but it's grown in the Camargue and is quite pricey.

You've sent me off down a whole new rabbit hole because we are looking to make a rain water harvesting system and this could be the non-plastic solution we've been looking for - thank you!

All the very best from Basse Normandie,

Andy

0
tytower
tytower

Reply 12 months ago

All good Andy ,i did really enjoy your writeup . Of ferro cement I happen to have a 50ft Ketch in my back yard but not ferro cement. Same armature layup almost except I used coils of high tensile stress relieved 7mm stringers and 5mm diagonals at maximum of 2" by 2" spacing. 7 layers of heavily galvanised 1/2 inch mesh 4 one side 3 the other. Tied at every stringer/diagonal crossing . I tell you this because it is the best and easiest material to work with if you can get a bit of experience first. Using normal cement and sand mix 2 to 1 ,coarse river sand (sharp) and cured for 28 days gives a very strong and waterproof layer 3/4 inch thick.

So for your water tank ,dont worry about starch . With water in it the tank leaches lime? (,white,) through it if at all and the finer particles in the suspension that might get through seal it up watertight very quickly.
Thats how I got into sticky rice additive.

I used a fine white beach sand /chopped glass strands / mix with polyester resin half way through from the outside and then puddled it in from the inside . a test panel stopped a 303 bullet from 20 foot. These days epoxy is almost the same cost here and it is definitely waterproof whereas polyester does allow water through especially osmosis where fresh in the bilge seeps through attracted to the salt water outside.

For your tank use standard reo sheet 4 inch squares ,put one layer of 1/2 inch square mesh each side ,tie in each center and put cement in in one shot. There is a method now of buying boxes of steel slivers about an inch long and a hairs width and mix it in with the cement and pour on a form or mould . Have not tried it but they do this with concrete pipes.

Anyway a bit of a ramble on.

0
tytower
tytower

Reply 12 months ago

Also I have not seen that link you supplied . Very interesting . I figured it might have accidentally discovered by using rice boiling water in the mix because all water would have had to be carried so waste not walk not!

0
haroun
haroun

12 months ago

These guys are a good source in the states- http://www.limeworks.us/ . I've used their NHL3 I think it was, & the marble dust to repair a pre-civil war house in Doylestown PA.

0
JohnC430
JohnC430

1 year ago

Thank you for the lesson and description of the materials and also for the web links.
I dont see any links to vote.

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi John,
I just got the answer back - apparently they stopped the voting option sometime ago but they did keep the button up for the fun of voting but apparently this caused a lot of confusion, so now they have taken it off altogether. All the contests now go straight to be judged by a panel.
All the very best and thanks for your support,
Andy

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi there John,

You are so welcome! No I don't see the links to vote either. I'll check out with admin and get back to you.

All the very best from France,
Andy

0
mikaelfernstrom
mikaelfernstrom

1 year ago

Great article. As I spent a lot of time last year learning - on my own - how to repoint old stone walls. I used NLH 2 and NHL 3.5 from https://www.roundtowerlime.com for my walls. I used the same mix ratio 1:4.

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi Mikael,

Glad you enjoyed the article and thanks for the link - it's always useful to get recommendations for raw materials. I've never used NHL 2. How did you get on with it?

Cheers from Normandy,
Andy

0
mikaelfernstrom
mikaelfernstrom

Reply 1 year ago

It turned out great! Also, Roundtower have great support via phone and email. They have tutorial videos online as well.
Today, I'm plastering.

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi Mikael,
Thanks - always good to know.
Have a great time!
Cheers,
Andy

0
waterwin2
waterwin2

1 year ago

Very interesting as we live in a not so old French house close to Verdun. Where do you buy your materials in France and what brands can you advise? Thanks, de Erwin

0
Organikmechanic
Organikmechanic

Reply 1 year ago

Hi there Erwin,

I'd either try a local builders merchant, he may give you a better price on quality limes such as Saint Astier and also be able to supply a local sand you can buy in bulk, so you just load up as little or much as you want. Tell them what you want to use it for and they will advise on the most appropriate and often the best priced.
I have also found that Brico Depot (you have several in your area) do good quality lime including brands such as Calcia but I see they have a 5 star review on one called Eqiom.

If you want to insulate with natural lime 'enduits', it is better to use materials like chopped linen for example, or hemp not from dedicated builders merchants as eco materials are pricey but you can get organic hemp (in bulk) from horticultural suppliers (used for mulch - paillage) for less than half the price of that designated 'building materials. Similarly with chopped linen - this can be bought from general feed/horticultural suppliers as Linabox (used for stables where the horse is allergic to straw) again this is a much cheaper way of buying it.

Good Luck with your projects and Best Wishes from Basse Normandie, Andy