Solidifying friable sandstone

[Itchinfilth]

Now removed cement render from the 6th window. The sandstone underneath is crumbly to say the least, this is by far the best condition one we've uncovered. Any advice on anything that can be added to the stone to try 'solidify it'? All have / will be allowed to dry out and lime pointed however the earlier completed ones are still very fragile.

All are double chamfered with cavetto moulding to the mullion for the geeks like me Although the cavetto is more straight shaped after years of determination under the 'c' stuff.

 

[a twig]

Probably not what you want to hear but replacement of the damaged sections of stonework by a stonemason is really the only proper solution if the damage is that bad that the picture is the best one.

You could attempt repairs on some of the least worst bits using a colour matched NHL based stone repair mortar but given that amount that would be needed for the worst bits wouldn’t be a long term solution particularly as it will be harder than the sandstone

 

[aockenden]

Not sure how to do it without altering the appearance of the stone relative to the other un-stabilized surfaces of stone. But if you didn’t care about that you could try a limewash or some linseed / tung oil painted onto the surface with a paintbrush? There are likely some more modern products that wouldn’t alter the color / lustre of the stone surface.

 

[Itchinfilth]

Not sure how to do it without altering the appearance of the stone relative to the other un-stabilized surfaces of stone. But if you didn’t care about that you could try a limewash or some linseed / tung oil painted onto the surface with a paintbrush? There are likely some more modern products that wouldn’t alter the color / lustre of the stone surface.

Thanks for the advice. I'm a big fan of linseed oil will try a patch. I've also heard lime water can solidify with enough spray applications.

 

[aockenden]

Thanks for the advice. I'm a big fan of linseed oil will try a patch. I've also heard lime water can solidify with enough spray applications.

Yeah well lime water is essentially just a highly diluted lime wash so that makes sense.

 

[Stevers]

No personal experience of crumbly Sandstone to draw on as I'm in limestone country, and the old red sandstone cap up on the Mendip Ridge is a) not red, and b) much harder than nails!

Sandstones are quite variable in that the 'cement' that binds the grains can be silicate, or non-silicate such as calcite - worth a mention as it might dictate the correct solution. The Wiki article on Sandstone (https://en.wikipedia.org/wiki/Sandstone) makes specific reference to an expensive and hazardous chemical that might help stabilise sandstones - presumably the silicate ones:

"Sandstone that becomes depleted of its cement binder through weathering gradually becomes friable and unstable. This process can be somewhat reversed by the application of tetraethyl orthosilicate (Si(OC2H5)4) which will deposit amorphous silicon dioxide between the sand grains. The reaction is as follows. Si(OC2H5)4 (l) + 2 H2O (l) → SiO2 (s) + 4 C2H5OH (g)"

Presumably if it's a calcite sandstone then multiple applications of a very dilute limewash (coloured to match the stone) might be worth a try.

An option that is unlikely to be approved of here, but I would have thought preferable to linseed oil, is very dilute PVA as synthetic replacement for what the stone has lost. As Toby Newell of this parish has pointed out, PVA is of itself breathable. I've used dilute PVA Bond to stabilise lime plaster that had been weakened through water ingress, and the first one of those walls has been good for twenty years now. Do a test patch first as dilute PVA has huge potential to change the appearance of the stone in a way that you won't like. There may be other synthetic binders less likely to change the appearance of the stone that you could try.

 

[Itchinfilth]

No personal experience of crumbly Sandstone to draw on as I'm in limestone country, and the old red sandstone cap up on the Mendip Ridge is a) not red, and b) much harder than nails!

Sandstones are quite variable in that the 'cement' that binds the grains can be silicate, or non-silicate such as calcite - worth a mention as it might dictate the correct solution. The Wiki article on Sandstone (https://en.wikipedia.org/wiki/Sandstone) makes specific reference to an expensive and hazardous chemical that might help stabilise sandstones - presumably the silicate ones:

"Sandstone that becomes depleted of its cement binder through weathering gradually becomes friable and unstable. This process can be somewhat reversed by the application of tetraethyl orthosilicate (Si(OC2H5)4) which will deposit amorphous silicon dioxide between the sand grains. The reaction is as follows. Si(OC2H5)4 (l) + 2 H2O (l) → SiO2 (s) + 4 C2H5OH (g)"

Presumably if it's a calcite sandstone then multiple applications of a very dilute limewash (coloured to match the stone) might be worth a try.

An option that is unlikely to be approved of here, but I would have thought preferable to linseed oil, is very dilute PVA as synthetic replacement for what the stone has lost. As Toby Newell of this parish has pointed out, PVA is of itself breathable. I've used dilute PVA Bond to stabilise lime plaster that had been weakened through water ingress, and the first one of those walls has been good for twenty years now. Do a test patch first as dilute PVA has huge potential to change the appearance of the stone in a way that you won't like. There may be other synthetic binders less likely to change the appearance of the stone that you could try.

Hi Stevers,
Incredibly helpful information. I'll take a bit of discoloration to save what we have. Never knew red sandstone was harder than our stuff!