Eco-friendly mortar for a wall

Hi everyone,

I'm thinking of building a low wall in my garden from old bricks which I've found in various places in the garden (including some that were buried). It's not a supporting wall and will only be about knee-high, so safety concerns aren't a major issue.

I was wondering if anyone can suggest eco-friendly mortars that I could use instead of cement (materials that I can source easily and in small quantities)? If not, I'll use cement, but I just thought I'd ask in case there's an easy substitute.

Thanks

If it's only knee high, do you need mortar at all? Have you considered dry-stacking? I did a dry stack 4 ft x 8 ft bed about 3 years ago - the lack of mortar was an asset when Hubby bumped one corner with the tractor. I just dug out behind it and reset the blocks - it was mostly out of breeze-blocks.

Jay's suggestion for dry stacking is great for low walls. Cracking isn't an issue and bricks and/or rocks can be moved around as garden needs change. For added support, I backfill / parge the less exposed side using a sand + clay + water mortar mix at a ~1:3 slope. The walls vary in height from 1 - 5 feet. This method works best when walls gently curve around the garden.

Amy Gardener wrote:Jay's suggestion for dry stacking is great for low walls. Cracking isn't an issue and bricks and/or rocks can be moved around as garden needs change. For added support, I backfill / parge the less exposed side using a sand + clay + water mortar mix at a ~1:3 slope. The walls vary in height from 1 - 5 feet. This method works best when walls gently curve around the garden.

Thanks for the suggestions. I was considering doing some sort of dry stone wall or not using mortar for the bricks but I don't think it'll be the best option for what I'm doing.

Amy, could you explain more about your backfill suggestion, please, I don't understand how it works.

I learned this method from an amazing adobe builder here in New Mexico. Here we have a lot of clay and sand and he could put it to good use for all sorts of projects. Rocks and bricks are used for their beautiful facings, since they are more rare than clay and sand. He used the word “parge” but when I looked it up for this post, it didn’t seem to apply since the definition was more about decorative stucco work for unattractive cinderblock. In this particular application of the mud is thickly troweled as I’ll try to describe in more detail.
What this builder did was lay out the brick or stone to be beautiful on one side then trowel up the back at an angle for support. He worked one row at a time building up the parged material in a thin layer then doubling up the layer as the wall grew higher. The bottom of the wall had much more material on the back than the top: a steep slope.
If you imagine a right triangle where the tall vertical side is the height of the low wall and the short bottom of the triangle is level with the ground, that right triangle is the back fill or parge (there must be another word). As the mortar is troweled into the back, the hypotenuse is compressed or pushed into the brick or rock.
In the case of the brick, they were recycled so most only had one good face. The attractive side faced out and the broken or jagged side held the clay "mortar" mix. In the case of rock, the largest stones were at the bottom and the thinner, but equally large facings were at top.
After the builder did his work, I copied the method around the yard and using stone to “face” berms and low garden walls. I’ve never had a collapse of a wall built like this.
If you need more detail, feel free to ask, or make a little prototype. Mix a mud and sand mortar in a wheel barrow using local material and mixing with a masonry hoe. Scoop the material behind your wall and trowel it up the back then go higher. It takes a little practice but no one sees the back and it’s only mud so you can try again. Good luck G!

Amy Gardener wrote:I learned this method from an amazing adobe builder here in New Mexico. Here we have a lot of clay and sand and he could put it to good use for all sorts of projects. Rocks and bricks are used for their beautiful facings, since they are more rare than clay and sand. He used the word “parge” but when I looked it up for this post, it didn’t seem to apply since the definition was more about decorative stucco work for unattractive cinderblock. In this particular application of the mud is thickly troweled as I’ll try to describe in more detail.
What this builder did was lay out the brick or stone to be beautiful on one side then trowel up the back at an angle for support. He worked one row at a time building up the parged material in a thin layer then doubling up the layer as the wall grew higher. The bottom of the wall had much more material on the back than the top: a steep slope.
If you imagine a right triangle where the tall vertical side is the height of the low wall and the short bottom of the triangle is level with the ground, that right triangle is the back fill or parge (there must be another word). As the mortar is troweled into the back, the hypotenuse is compressed or pushed into the brick or rock.
In the case of the brick, they were recycled so most only had one good face. The attractive side faced out and the broken or jagged side held the clay "mortar" mix. In the case of rock, the largest stones were at the bottom and the thinner, but equally large facings were at top.
After the builder did his work, I copied the method around the yard and using stone to “face” berms and low garden walls. I’ve never had a collapse of a wall built like this.
If you need more detail, feel free to ask, or make a little prototype. Mix a mud and sand mortar in a wheel barrow using local material and mixing with a masonry hoe. Scoop the material behind your wall and trowel it up the back then go higher. It takes a little practice but no one sees the back and it’s only mud so you can try again. Good luck G!

Thanks! I like the sound of this technique. If I use old bricks for the wall, can I put any mud/mortar in between the bricks or does it only go behind the bricks? I was wondering if the wall would be a bit more solid if I also put some mortar between the bricks.

Can you also clarify the contents of the mortar and ratios of each material, please? Thanks

This is a very important fine point that you are bringing up now, G. Prentice! As you can see from beautifully mortared brick/stone work and the remains of crumbled brick/stone buildings, cement mortar sticks to brick like adhesive. Clay, sand, unfired adobe, cob, straw, natural fibers, and dried dung all mix and stick to each other and become solid together. If you use unfired bricks and stack them with mud mortar, all will compress together and become a solid wall mass. Brick, on the other hand, is gripped or held by mud but mud doesn’t stick so well to brick (or lumber). Mud mixes need rough surfaces to fill; little pockets where the clay will enter and dry. If you create a nice mud layer between the bricks then expose the brick to rain, the water will run off the mostly impervious brick, flow into mud mortar where the clay will suck up the water and erode away rather quickly. However you can use clay mortar AND the back “parge.” That mass of material behind the wall sticks to itself AND sticks to itself as mortar. The  combination of steep sloping mud at the back of the wall, dry stacking the impervious faces of brick AND troweling mud into large rear gaps where bricks curve work better than mud mortar for brick.
Imagine the letter ‘E’with the slope of mud that I mentioned propping up that long vertical E in a right triangle. Now you get adhesion for the mortar at the horizontal E lines and the backfill or parge for unifying the mass of mud mortar. Bump this wall and it won’t budge. Bump a freestanding brick wall mortared with mud and it will tumble easily.
As far as the mix, sand provides compressive strength and clay provides adhesion. Since you want more adhesion, go with a mix with high clay content: about 30% clay, 70% sand. Mix with water to form a wet mortar. Sand and fibers will prevent shrinkage. Fibers will also weave everything together. Chop straw, woody plant trimmings, and other fiber to make your mortar drier and workable: test and feel your way to a mix that works for you. Exact measurements are not necessary for this kind of work. Use the material at hand and add sand to reduce cracking, clay to increase sticking.

Cob has been used for garden walls in Devon and I've seen examples of it used as a mortar before, the problem is that it can pull away from the stone in very hot and cold weather (I believe due to the stone having different thermal expansion to the cob). These cob walls usually have pretty large overhanging eaves or capstones to keep much of the English rain away from the clay.

Creating two parallel walls of dry-stacked stone and brick, then infilling with subsoil or hardcore is another way to go. This method was used to build the hedge banks across much of England and Wales. The wall should have larger "through stones" that tie them together. You could use topsoil for the final few inches and grow turves or other plants to support the structure (in Pembrokeshire and Devon they grow hedges on top).

A final option is to purchase lime putty or lime mortar and use that in place of cement. The lime will re-absorb the carbon that was driven off during its creation (it begins life as limestone, or similar, has the carbon removed through heat and then re-absorbs the carbon from the atmosphere to turn back into limestone when fully cured!) so the footprint of the mortar is significantly lower than cement (it is the energy required to heat the lime, plus transportation). A well-build lime wall will last centuries as many Roman buildings can attest to.

Thanks!

I may look into both methods, so Amy, when you say 'clay' for the mix, what product am I buying - is it some sort of powder? I know what clay is, but presumably you're not talking about standard clay in this context!

And, Luke - are there any pros/cons of using lime putty over lime mortar, and am I likely to find either of them in a hardware shop or would I need to buy it online?

If you have some land, you probably have clay right at your feet. To find it, look for an open place to dig a hole to examine the layers of soil. Low spots are usually best. Once you pick an open area, follow these steps:
1) Gently move the topsoil (soil with earthworms and composted debris) to the point where you don’t see humus or living organisms. You’re looking for your subsoil
2) Now take a 1 pint sample of that lifeless subsoil and put it in a quart jar
3) Add water to the top of the jar and shake up your subsoil and water
4) Set this down on a counter and do not disturb for at least a day
5) Look through your jar for layers. The sand will sink to the bottom. Clay will float in solution and eventually settle on top. The middle layer is probably slightly larger particles than clay but workable as part of your clay percentage if it has a greasy or slippery feel
6) Now look at the sand percentage of the total relative to the clay. Ideally you’ll have about a 70% sand at the bottom and 30% clay at the top of the sample distribution. If it’s 50/50, fine, work with that and add lots of fiber to prevent clay cracking due to shrinkage. If you have more clay, it probably would be fine since the cracks won't show. If you do not find a slippery top layer that is at least 10%, take other subsoil samples in other places on your property until you find a workable deposit for your project

If you live on a complete sand dune, you can buy ball clay (or the cheapest junk clay) at a ceramics shop or school as a last resort. In such a case, use 10% by volume of pure clay with your sand.

Lime putty is a form of slaked lime (lime mixed with water) that has usually "matured" for a period of time. I've heard that the longer the putty can mature the better and I know that many people prepare large batches and leave them lying around. I found this supplier who has 4-month matured lime putty (which doesn't sound like a lot to me but it is better than nothing!).

Lime mortar (my bad in using such a generic term) is any lime-based mortar. This can be powdered lime ("quicklime") that is hydrated and mixed, much like cement. Some mixes include sand, some do not. This is fundamentally the same thing as lime putty but the mix will take longer to cure ("go off") than a well-mature quicklime.

You can buy pre-mixed lime mortar which, I suspect, will sit somewhere between the two in terms of curing time.

If you wanted to make your own lime putty, you could buy quicklime, mix it with water and leave it in a waterproof vessel (the blue plastic barrels with the tops cut off are ideal), covered with water, in the open for a few months . This would be cheaper than buying premade and greener too, given the lower weight of the product being transported (plus lime, like cement, comes in paper sacks rather than the plastic buckets the wet putty is supplied in). It is obviously much slower though.

For load-bearing applications, buying pre-mixed lime mortar or putty might provide some additional reassurance as there is likely to be a strength rating associated. A home-made mixture may not be optimal in that respect. For your application, however, I suspect it wouldn't matter.

Hey Luke! Thanks for the lesson on lime putty. For this low wall application and other light projects, would it be okay to mix lime putty with sandy subsoil? Does the sand have to be washed of minimal clay or other particulate? Have you ever heard of builders combining lime and subsoil for a mix that is stronger than cob?

Hi Amy! I'm by far the most experienced person when it comes to building with lime (but I do know someone with far more experience, someone teaching me woodturning at the moment - I'll give him your questions and see what he says).

I know of at least one project that has added lime to their cob mixture, Sacred Groves in Auroville, India. They added lime to help bind it together when wet/waterproof it, for use in a toilet and shower block. I spent a few days mixing and using the mixture (which wasn't terribly pleasant, it is quite caustic before it has cured and I was using my bare hands/feet).

I would imagine that using a sandy subsoil for non-critical applications (such as a low wall or a floor) would be absolutely fine. I know that people use concrete like this to create earthcrete and I don't believe lime would behave differently - it would just take longer to cure.

It's worth mentioning that, despite the longer curing time, lime is eventually more durable than modern cement. It is softer, and not as "strong", but it is breathable and flexible and continues to harden indefinitely (as more and more of the CaOH, slaked lime or lime putty, is converted to CaCO, limestone or calcium carbonate).

Amy - Excellent replies - thank you.
I think I am understanding you correctly, butif it is possible, maybe you could post some photos of your construction process, or maybe some drawings?

Here in New Zealand you can buy hydrated lime from building supply, or agricultural supply stores. While it is still caustic, it is much safer to use than quicklime, as it doesn't give off heat when mixed with water. The bagged lime can be mixed with sharp sand to make a traditional mortar like those used  before Portland cement was developed.

A ratio of 1 part lime to 3 parts sand would be a good starting point. If the sand used is dirty (mixed with clay or silt) it will reduce the strength of the mortar, so you may want to increase the lime quantity. This may not be a problem for a garden wall. To increase strength and to speed up set time, some wood ash could be added to the mortar.

It should be noted while lime is better then portland cement from an environmental viewpoint, it does still require quite a lot of energy in its production.

Matt, Builder, NZ

Thanks for all the info’. My wall is a summer task, so I’ll be working on it in the next few months - -and may come back with some more questions! I’ve been in an essay writing hole recently, hence my very slow response!

We made the raised beds in my greenhouse out of roundish stone from on site, with mortar made of mud (mix of onsite sandy soil and clayey stuff that was brought in). It's 3 years old now, holding in watered soil on one side. I sit on it and occasionally step on it or even walk on it. A good 3 months of winter the greenhouse is below freezing at night and above during the day. I expect some of the stones to roll out at some point and I'll have to put them back, but it hasn't happened yet.

It was made more or less as Amy described the triangle cross-section earlier.

I'm sorry the poor photo quality is from cropping the background of some other photo to focus on this wall.

It's a very nice wall - well done!

Luke Mitchell wrote:Hi Amy! I'm by far the most experienced person when it comes to building with lime (but I do know someone with far more experience, someone teaching me woodturning at the moment - I'll give him your questions and see what he says).

Do'h! I just re-read this. It was meant to say:

I'm far from the most experienced person

What was supposed to be a caveat/warning about my advice was turned into an accidental, immodest (and untrue!) brag!

Luke Mitchell wrote:

Luke Mitchell wrote:Hi Amy! I'm by far the most experienced person when it comes to building with lime (but I do know someone with far more experience, someone teaching me woodturning at the moment - I'll give him your questions and see what he says).

What was supposed to be a caveat/warning about my advice was turned into an accidental, immodest (and untrue!) brag!

Did your wood turning friend have anything to add on the subject?