Re: Footings for sandstone block wall.

[Stevers]

If I still lived in my old house I could nip out and provide numerous examples all within a few hundred yards where rubble stone boundary walls have been ribbon pointed in cement mortar and the stone is in horrendous shape. A few houses further on where the wall had been left to it’s own devices, no stone damage whatsoever

I currently walk an elderly dog round the block, giving me plenty of time to closely inspect the sort of damaged stonework you mention!

My garden walls had been re-pointed in a mix of cement and Mendip stone dust and this type of mortar rings like a bell when struck. The mortar was not usually attached to the stone, quite often just falling out. Some of the stone had broken up into cubes, and other bits were still OK, but it was a poor job overall. I repointed with NHL3.5 3:1 with half stone dust and half sand - and that has stayed put. For the very first test section I used 1:1:6 cement:lime:aggregate and that started to fail very quickly, and was repointed in NHL a couple of years ago and looks good so far.

 

[Cubist]

It’s now been some years since I reached the time of life in which it was advisable to perform regular shoulder checks to determine whether any of my mental faculties had diminished or been lost. I will freely admit that entropy has had some negative impacts and periodically I have been known to forget to press the ‘equals’ key after dividing by 3 to determine the weight of a 6 foot length of oak and I frequently come back indoors to find I did something other than that first planned when I went out.

This debate concerning the relative merits/de-merits of using cement or lime mortars to bond brick or stone in outdoor walls has been rather interesting and challenging insofar that many of the offered opinions seem to fly in the face of, or simply ignore, the rules of rational logic and, my now admittedly dimming knowledge, of the physical laws of the universe.

Sadly no-one has thus far offered a reasoned and evidence based argument to sustain the view that the use of lime based mortars prevents or inhibits spalling of bricks and stone and nor has any such been offered to support the contention that cement mortars actively or passively promote the conditions in which spalling more frequently or significantly occurs.

I am however aware of one theory that would, if it were true, explain why all these various contentions could be true and accurate. This concerns a suggestion that we each create our own unique universe simply by observing it. In such a universe I, or you, would create our own understanding of the rules of logic and the physical laws governing the properties and behaviours of our personal universe. Sadly, proving any contention I might make to someone occupying their own unique universe, is therefore an impossible mission.

I will therefore gracefully retire, with tongue in cheek, from further debate on this particular topic - unless and until someone asks that I explain my reasoning in support of the view that the use of either cement or lime mortar in a garden wall can only make a very small and marginal contribution to the occurrence and extent of spalling.

 

[a twig]

An article for you @Cubist - https://www.buildingconservation.com/articles/gardenwalls/gardenwalls.html

I've taken the liberty of quoting their reasoning, which makes the point I was trying to make but much more eloquently.

It was not realised until later in the 20th century that mortars rich in OPC were incompatible with traditional structures, and by then the art of using lime had all but died out. The cement made the mortars too hard, too inflexible and too impervious for traditional construction techniques and they were entirely unsuitable for repointing walls which had been originally constructed with a lime-rich mortar, causing considerable damage to softer stone in particular. The problem is partly caused by differential movement. As lime mortars do not set as rigidly as those rich in OPC any movement in the wall, for example due to thermal expansion, can be absorbed by joints filled with a soft lime mortar. However, if a dense mortar rich in OPC has been used in later repointing, the surface of the wall becomes rigid while the core remains flexible. The slightest movement leads to stress at the surface where the joints have been repointed, and structural failure commonly occurs across the arrisses (or corners) of the stones or bricks. In some instances, the entire surface of some stones may disintegrate.

As well as being relatively more flexible than a cement-rich mortar, lime mortars have the advantage of being significantly more porous, helping to ensure that the wall dries out quickly after a shower of rain.

I would be interested to understand your reasoning, as I always enjoy a healthy and civil debate as an opportunity to learn

 

[Stevers]

Outside are some Mendip rubble walls that I re-built with 1:1:6 or 1:2:9 cement:lime:sand/stone dust mixes over twenty years ago, that are as sound today as when I built them. Holding our garden up above the back lane is a sixty foot long, eleven foot high retaining wall that was refaced in the same 1:1:6 mix professionally twenty five years ago. It was more than a little 'bulgy', and the builders reckoned that putting them back like that for another hundred years was what they did before structural engineers were invented. I have to say that when that apparently lovely old wall is viewed at considerable leisure with the elderly dog, another hundred years looks quite possible!

When I started to repair the old garden walls I just used the same mixes, but it quickly became obvious that those mixes wouldn't stay put. All of the (DIY?) 60's cement smear pointing of my garden walls has failed by starting to separate and fall out, apart from a few sections that don't ring like a bell when struck and have visible lime particles in them, leading me to think that those sections were deeply (and professionally) re-pointed with a cement/lime mix.

My conclusion from all this would be that building a wall from scratch with cement/lime mixes can work - if mixed weaker than the stone used, but that pointing with a cement containing mortar over a wall built in lime mortar is a complete waste of time. I would be nervous about building a stone wall with just a cement:sand mix even as weak as 1:4 or 1:5, although it might work for a while. To add balance I have seen a professional do an extensive stone repoint/repair using such a mix that has held up OK so far, but the stone here (other than the oolites) is pretty much as sound as limestone gets.

The problem with the mixed weaker than clause is how exactly do you achieve that, with anything other than luck, or a lot of experience that may have been hard earned?

 

[Cubist]

An article for you @Cubist - https://www.buildingconservation.com/articles/gardenwalls/gardenwalls.html

I've taken the liberty of quoting their reasoning, which makes the point I was trying to make but much more eloquently.



I would be interested to understand your reasoning, as I always enjoy a healthy and civil debate as an opportunity to learn

Hi Twig,

Although I have not read that particular article before I was already aware and had agreed with the debate and conclusions regarding the use of OPC mortars and how their increased rigidity can be detrimental when used to re-point a wall in which a lime mortar had been previously used. Ostensibly the issues arising were rooted in differential warming and cooling of the brick or stone work and the different mortars producing movement that in turn resulted in cracks and breaks. As I understand it these movement stresses can sometimes be powerful enough to, in effect, pulverise the brick or stone and in a recent post I discussed how differential heating could cause some types of stone, if used in a wall behind a fireplace, to crack and even explode

Frankly I do not think that issue is directly relevant to the debate at hand. Some may challenge this but I believe that 'spalling' of brick or stone work is the result of repeated freezing and thawing of water trapped near its exposed to atmosphere surfaces and that, in effect, 'blows' the surface material off its faces. I have already admitted that I can see how the use of an OPC mortar 'may' cause water absorbed through the face of a brick to pool at its interface with the mortar and thus perhaps contribute to some damage but to my mind its moot whether a lime mortar would do much better to prevent this. The argument that bricks and stone are impermeable and therefore that any spalling damage must, as a consequence, be due to the choice of mortar type - forgive the pun - just doesn't hold water. All stone and brick will absorb some water.

Happy to hear contra views, I too have a lot to learn; that's why I participate here.

 

[malcolm]

I didn't say bricks were impermeable. Bricks are more permeable than cement mortar but less permeable than lime mortar.

 

[stuart45]

The problem with the mixed weaker than clause is how exactly do you achieve that, with anything other than luck, or a lot of experience that may have been hard earned?

Usually it's experience, although from a technical point of view, the compressive strengths of various mortars can be seen in a table, and the strengths of the masonry units can also be seen.
Limestone is like a sliced loaf, and should be laid so the slices are flat on the bed, and not so they can delaminate.

 

[a twig]

To my understanding there are two processes by which spalling can occur:

1) Differential movement between “soft” bricks/stone and a “hard” inflexible OPC mortar which can produce crushing forces. Dependent on the mortar the pressure inflicted can be greater than 1000 psi - many historic bricks or natural limestones struggle beyond 300 psi. That force then produces cracks and fractures

2) Moisture - this doesn’t just have to be freeze / thaw but can also be purely from differential pressure driving moisture in, if it can’t escape quickly enough to relieve the pressure it literally forces the face off the brick.

Granted you are unlikely to get such large differentials on a boundary wall as you would on a house so it will be a lesser (but I’d argue still potentially significant) action, but without that differential driving the moisture there could be a counter argument that the moisture is slower to leave the wal and so freeze/thaw would more likely.

In either case providing maximum breathability for water to get out again and appropriate features (e.g. coping) to limit water ingress would seem a sensible action.

Using an impermeable OPC mortar, moisture can only really escape through the face, increasing the likelihood of spalling, whereas using a lime based mortar that moisture can travel through the top, bottom and side faces as well, maximising the chance of keeping moisture at a less than damaging level.

Although having re-read your post, is part of your point that spalling is a specific thing that only applies to (2) and (1) isn’t considered spalling, so they shouldn’t be conflated?

 

[a twig]

Usually it's experience, although from a technical point of view, the compressive strengths of various mortars can be seen in a table, and the strengths of the masonry units can also be seen.
Limestone is like a sliced loaf, and should be laid so the slices are flat on the bed, and not so they can delaminate.

There was a “repair” done to some Bath stone ashlar in my house where a doorway had been removed, one of the blocks used had been laid on its side, so used to peel every winter. It irritated me every day but it was considered “part of the history of the house” so got left alone, even though it was “wrong”

 

[stuart45]

Salts cryststalzing just below the surface can also cause it.

 

[Cubist]

To my understanding there are two processes by which spalling can occur:

1) Differential movement between “soft” bricks/stone and a “hard” inflexible OPC mortar which can produce crushing forces. Dependent on the mortar the pressure inflicted can be greater than 1000 psi - many historic bricks or natural limestones struggle beyond 300 psi. That force then produces cracks and fractures

2) Moisture - this doesn’t just have to be freeze / thaw but can also be purely from differential pressure driving moisture in, if it can’t escape quickly enough to relieve the pressure it literally forces the face off the brick.

Granted you are unlikely to get such large differentials on a boundary wall as you would on a house so it will be a lesser (but I’d argue still potentially significant) action, but without that differential driving the moisture there could be a counter argument that the moisture is slower to leave the wal and so freeze/thaw would more likely.

In either case providing maximum breathability for water to get out again and appropriate features (e.g. coping) to limit water ingress would seem a sensible action.

Using an impermeable OPC mortar, moisture can only really escape through the face, increasing the likelihood of spalling, whereas using a lime based mortar that moisture can travel through the top, bottom and side faces as well, maximising the chance of keeping moisture at a less than damaging level.

Although having re-read your post, is part of your point that spalling is a specific thing that only applies to (2) and (1) isn’t considered spalling, so they shouldn’t be conflated?

When preparing my initial reply I was focused on the 'gross' effects of Differential Movement and failed to highlight that it can produce minor symptoms identical to spalling. My reference to the Freeze / Thaw cycle too was a little loose and, as you point out, it is indeed unnecessary for the trapped water to freeze in order to produce the spalling effect. As both phenomena share the same root cause - thermo cycling - DM and Spalling could, and to my mind should in that sense, be conflated. Not least because our failures to recognise the true root cause can lead us to make false conclusions regarding the guilty party and incorrectly point the finger of blame.

Which is why I took issue early in this thread insofar that many contributors seemed to be pointing the finger of blame at OPCs as a primary cause for the failures of walls they had observed where others constructed with lime mortars had survived undamaged. To cast OPC mortar as the single and absolute culprit simply because it is less 'breathable' is fundamentally flawed. I was also concerned that some of our more seasoned members appeared to support this erroneous implication/inference out of preferential bias rather than an acknowledgment of provable facts.

To my mind, and in the hope of finally closing the door on this debate: -

1. Thermal cycling is the ultimate and fundamental cause of the failure of, in this case, a wall, any wall, every wall and their constituent bricks/stones and mortar.
2. There is no way to stop it - it can however be delayed or accelerated by our choice of the constituent materials. But no one can say definitively by how much.
3. OPC mortars are not inherently impermeable, they are merely 'less' permeable than a lime mortar.

Hence my view that the beneficial contribution delivered by a lime mortar for a garden wall would be at best only marginally better than an OPC based mortar and therefore of little to no measurable and meaningful significance.

 

[noj]

To throw my musings into the mix...

Hard OPC pointing of otherwise soft (moveable) lime mortar could potentially add disproportionate pressure to the face of the stones/bricks as stuff shifts around a little behind it? The masonry really just being rigidly held by the depth of the pointing whatever other forces are involved.

Effects take a long time to manifest, we're only just learning about the perils of adding 'waterproofing' coatings to a previously breathable structure.

My personal inclination is to try to avoid mixing materials with very different properties.

It would be interesting to know if OPC mortar used for the full construction would behave significantly differently.

Final thought; all stone will erode a bit over time, lime will go at least as quickly (generally quicker) leaving no obvious mark as to where the stone originally started. Hard OPC mortar, particularly ribbon pointing is left sticking out like a sore thumb making a problem of its own.

 

[malcolm]

It's well worth anyone reading this thread also read some more traditional explanations for brick spalling. I don't think we should be recommending cement mortar. Some links below:

Heritage House | Brickwork spalling

The word ‘spall’ refers to the breaking of a material into pieces, particularly to cracking below the surface that may cause part of the surface to come off.In construction ‘spalling’ refers to the flaking, cracking, peeling, crumbling or chipping of stone or bricks, particularly where areas of...

www.heritage-house.org

A guide to the repair of old brick walls

The SPAB is often asked about historic brickwork over our Technical Advice Line. Brick can be one of the most durable building materials, as testified by the survival of notable Roman examples. Not infrequently, however, poor repairs, a lack of maintenance and inappropriate alterations lead to...

www.spab.org.uk

https://historicengland.org.uk/imag...lls/heag144-repointing-brick-and-stone-walls/ (pdf)