restoring sandstone wall

[Anonymous]

This was covered in cement render now removed and sanded back to something of a flat surface, albeit still with holes. Joins are now gouged out ready for pointing.
Does anyone advise using lime? If so any tips would be good. If not, what alternatives do you recommend? How about filling the inevitable pits and holes in 150 year old stone? I have saved the stone dust.
Help please!
celia

 

[Anonymous]

Lime is the only sensible choice. Mike.

 

[Anonymous]

Yup - it has to be lime. A coat at a time - eventually the pits, holes and general unevenness will be smoothed out. It takes time, and it's not possibly as cheap as unsuitable modern stuff, but you will be doing a job which will be best for the building.

Ready mixed - we are currently using Rydale Conservation lime (Chalk Hill). There's the Period Property Shop, and Mike Wye... many others produce it.

I know some people have used hydrated but it's not something I've ever tried.

 

[Anonymous]

I took advice from a plasterer, who showed me the french way. One bucket of red sand, seven eights of lime 'hydraulique' and half a bucket of lime 'aerienne'. Precise amounts as you can see!
It's just starting to dry out at present and it looks really good but I feel I may have to go back and start again with a few seams that I did first - its the same as everything, you learn from trial and error.
A friend is trying to persuade me to do the rest his way, which is a mixture of white sand and some other substance, which I think but am not sure must be some sort of lime; I continued to use the lime on a wall of 'moellons' a kind of hotchpotch of large and small stones stuck together with clay, which is also looking good. This took far longer than the sandstone wall!
celia

 

[Anonymous]

Last line in particular:

"We have a brick-built, Victorian terrace house with sandstone surrounds around the windows. They are in need of attention because, being on the seafront, they have endured a lot of weathering over the years. Can you advise us of the correct product we need to make repairs to the sandstone, where we can buy it and also how much it costs?

"David Snell writes: If you try to repair them with sand and cement, the tri-calcium aluminate and sulphate will react with the chloride in the sea air to break down the sandstone. Similarly, a lime mortar, in any situation will dissolve the sandstone. "

Will it????

 

[Anonymous]

"...a lime mortar, in any situation will dissolve the sandstone. "

Will it????

No

Sandstone is, essentially, grains of sand cemented together with very small calcite crystals. The sand itself, being quartz, is insoluble in pretty much anything short of concentrated hot hydroflouric acid. The calcite cement is chemically the same as lime. Adding more lime to it will not dissolve it.

I'd go with the Frenchman.

 

[Anonymous]

I'm not an expert but the chaux we have just gauged out of a sandstone wall seems to have done no damage at all to the stone in the last 150 years.
celia

 

[Anonymous]

I thought not either, but I wonder what the Telegraph is doing giving such inaccurate advice?

They ended up recommending a resin-based product. Tut tut.

 

[Anonymous]

I've just emailed Angela Pertusini, property section editor on the Telegraph, to ask her to invite David Snell to set up a demonstration of lime mortar dissolving sandstone.
I've offered to sell tickets and eat several hats if he can make it happen.

 

[Anonymous]

He's phrased it badly maybe? I think what he was getting at, maybe, was that calcium hydroxide (lime putty) can react with sulphates, to form gypsum. This can then form a black crust, which is very damaging to sandstone - it blocks the pores. You can see this in some industrial northern cities wich had very high atmospheric pollution providing sulphates.

(PS. Get stocked up on hats Gervase - very caustic liquids (pH13) can disolve *some* types of reactive silica to make a gel - this happens in ASR in concrete.)

 

[Anonymous]

"very caustic liquids (pH13) can disolve *some* types of reactive silica to make a gel - this happens in ASR in concrete"

Yes, but sandstone does not have 'reactive silica'. The quartz crystals that make up the sand grains in sandstone are just about the most resistant substance found on earth.

 

[Anonymous]

but ive always believed everything ive read in the torygraph

 

[Anonymous]

I think I'll stick to beans on toast for now; David Snell very kindly replied to my email this morning:

"Bob Bennett of The Lime Centre advises me that lime
mortar can be injurious to sandstone by the action of
water running over it and leaching out the
constituents. He states that in the main, damage is
caused where the mortars are derived from oolitic
limestone and that there are other limestones which
might not be as harmful. But as much of the lime
mortar available in this country is made from oolitic
limestone, his general advise is not to use them in
combination. As, perhaps, this country’s leading
advocate and exponent of lime mortar, his opinion
carries great weight with me.
He gives, as an example, the experiences they are
having in Edinburgh where the sandstone of many
buildings is being attacked by the action of the rain
on the lime mortar used to construct them. This has
resulted in grooves beneath perps and a widening of
the gap between the stone of the walling and proud
lintels.
I could have used the words ‘damage’ or ‘etching’
instead of ‘dissolve’ but they are only words to
describe an injurious process.
As to my suggested solution. If you read my answer
carefully you will see that I favoured replacement or
turning. But if patching was chosen I suggested a
resin mixed with powder from the stone to match. I
stand by that even though I accept that it might not
be a permanent solution. 23 years ago I used a similar
method to patch and repair broken brickwork and, to my
knowledge, those repairs have held and are invisible
to the naked eye.
I don’t believe for one moment that, in the situation
of extreme exposure described by the reader, any lime
or cement mortar would have stayed for very long.
This is perhaps an irritable answer to your
‘intrusion’. I have 44 years experience in property
and building, much of it, before I took up the pen,
hands on. I don’t know everything but I know a lot on
quite a broad spectrum. For the bits I am not so sure
of, I rely on a number of contacts and experts whose
opinions I value. I cannot, however, hope to compete
in an argument between two scientists with different
opinions and do not therefore wish to get into a
protracted exchange on this issue."

 

[Anonymous]

ahh, but they're not always. Reactive silica can occur in any type of rock made from silica, though I'm not saying its common in sandstones. Thats why 'concrete cancer' came as such a suprise when it was discovered (volcanic rock being eaten by cement? sounds unlikely)

 

[Anonymous]

"Reactive silica can occur in any type of rock made from silica, though I'm not saying its common in sandstones. Thats why 'concrete cancer' came as such a suprise when it was discovered (volcanic rock being eaten by cement? sounds unlikely)"

No, no, no! Sand grains in sandstone are made of the mineral quartz. They will not react. You really do have to boil them in hydrofluric acic in a gold/platinum alloy crucible before they will disolve. I've done it! Sandstone is not a volcanic rock. Many volcanic rocks weather quite easily because they have unstable Mg.Fe.Al.silicates. Sandstone is made of the bits that did not dissolve after the original rocks weathered, were transported to the sea and redeposited. There are essentially two ways the sandgrains are stuck together in a sandstone; either by calcite which could dissolve in acid conditions from, say, polluted rainwater, or by more quartz, in which case it produces a stone so strong and durable it is usually not used because it is too hard to work.

As for the sea air damaging lime mortar, I notice Telford's harbour works around the northern coasts of Scotland are coming up for their bicentenary soon.

 

[Anonymous]

I'm not suggesting lime mortars are not an excellent idea in general am I? Some types of lime, used in certain situations near the sea, might not be suitable, which is a different thing to say.

Sandstone is not volcanic, I know that, but it is often made up of weathered igneous rocks - eg. Millstone Grit

With respect, I don't think you understand the chemistry of ASR. It would be a miracle if you did - its still little understood by research scientists.

Quartz is a mineral silica, & there are different types of quartz even. You cannot possibly say it is non-reactive with out testing each individual type from each quarry. ASR does not attack the bonds in sandstone - it attacks the crystaline structure of the silica itself. There are even identified cases of ASR in quartz impurities found in crushed limestone

http://trb.org/publications/trnews/rpo/rpo.trn217.pdf

 

[Anonymous]

ASR won't happen if sandstone is mortared with lime. ASR is a problem that affects concrete made from some types of Ordinary Portland Cement. It occurs when there is an excess of sodium or potassium ions resulting in a pH approacing 14.
Yet another reason never to risk using cement on a period property.

 

[Anonymous]

Again, I think you are grossly over-simplyfying. In spite of what you're trying to insinuate, I think, I'm not advocating the use of Portland Cement.

ASR is actually 2 reations, the first which turns silica into a gel requires only hydroxide ions & reactive gel. (The second is a reaction with potassium & sodium to turn the gel hydrophilic & expansive). Actually the reaction can occur below pH of 13, but more slowly. So, my initial claim, that some sandstone can be 'dissolved' by highly alkaline solutions is correct.

ASR does not need portland cement to occur, nor concrete. Research indicate that some types of natural stone undergo a naturlal weathering similar to asr, just very, very slowly. Increasing any of the variables (moisture availability, hydroxide ions, potassium & sodium availability, pH) increases the rate of reaction.

 

[Anonymous]

Yes, we are simple folk on this forum. As I understand it, the pH in concrete is usually about 12 to 13 but can approach 14, which is when ASR really becomes a problem. Lime mortar will have a pH about 8.5, which is well below any ASR danger.

I would be interested to hear of an example of sandstone) made of quartz grains cemented with calcite) that has suffered any chemical weathering because lime mortar was used in the construction of the wall or because it was sprayed with sea water.

So when Snell said "...a lime mortar, in any situation will dissolve the sandstone" I think he, and his adviser, are wrong and the best advice to the original poster who asked about repairing a sandstone wall is still to use lime mortar.

 

[Cloudscape]

Not wishing to rekindle any controversy (this is quite an old thread), I need to repair some sandstone in a way that is cosmetically OK (will have to be repainted anyway) with something that won't damage the sandstone.

I've read what I can find on the internet. Suggestions included c*m*nt mortar (non-breathable and I would have thought this should be the last thing you consider doing!) and resin-based stuff (non-breathable and unsympathetic to the sandstone!).

Some sort of lime-based mortar/plaster is would have imagined was the way to go, but I also found the piece quoted further up this thread where someone says that lime will dissolve sandstone. If I hadn't found this thread, I would have assumed they were right.

But I found this product:

http://www.lime-mortars.co.uk/stone-repair/stone-repair-mortar?gclid=CJmEuKiX4pgCFQ48QwodfGoDeQ

Basically, I have a large painted sandstone arch. I've no idea what it's been painted with over the years, but I think I've spotted limewash, oil-based paint, and I know the upper layers are Sandtex (I found the cans in the shed). Quite large areas have blown and the surface you see isn't in contact with the stone beneath. I haven't investigated fully, but some areas are salting (probably caused by earlier inappropriate repairs). The sandstone had surface striations, possibly mason's saw marks? Where the paint has fallen away, I can see that the tooled surface has crumbled away in places.

Has anyone any thoughts on surface repairs to sandstone, or any experience of using a product like the one above?

If I can effect the repairs, would the sanstone be better limewashed (it must remain white).

Before it is painted, is there anything that should be used on the stone to 'help' it in any way?