Grey sandstone slabs have changed colour

[McQLon]

Hi,

I recently had grey sandstone paving slabs laid in my back garden and some of the slabs (around 12 out of a 60sq metre area) have changed colour over the course of a few weeks. Basically the slab has gone more or less dark brown.

Does anyone have any idea as to the cause of this? The supplier has said that due to some of the slabs being hollow, the water has got in underneath the slabs and drawn colour from the sand/cement mix through when it's subsequently dried out.

The landscaper has said that it's a manufacturing fault and the supplier should supply more slabs as a result.

Can anyone shed any light on this for me? I have pictures of the slabs but can't see any upload facility on this forum.


Thanks
McQLon

[London Stone Paving]

Hello

email the pictures to me and I will upload them for you

steven@londonstone.co.uk

Cheers

Steve

[Tony McC]

Sandstone is not "manufactured" - it's quarried.

The flags themselves can't be hollow, but the bedding might be, espcially if it's the unacceptable spot-bedding, and that could help explain what you are seeing to some extent.

However, for grey sandstone (which is probably Kandla Grey) to turn brown is exceptionally rare. It's generally such a reliable stone that it makes me think there's summat else going on here.



And there's a guide to uploading images in the Notifications section

[PavingSuperstore]

Do you know if an acid cleaner has been used on the stone to remove excess mortar?

[McQLon]

I have sent the pictures to Steven at London Stone to upload. Thanks Steven.

Yes it's Kandla grey and no it's not the stone that sounds hollow there sounds like gaps in some places between the stone and the bedding underneath. It wasn't dot and dabbed as I saw it being laid but there does sound like voids under the slabs for whatever reason. Maybe the mix was too dry? I'm no expert and just wanted advice on next steps as I have a second patio in my garden that needs to be laid using the same slabs.

Either that or it's the stones themselves.

No cleaner has been used. Simply sharp sand and cement for laying the slabs and a mix of building sand and cement for the pointing.

[sy76uk]

My first thoughts were has a cleaner been used. Second thoughts are could be iron oxide in the stone turning rusty.
I have no experience with kandla grey so have no idea if it's a common occurance with that type of stone.

[McQLon]

Thanks for your reply. Hopefully Steve can upload the pictures in the morning but no cleaner has been used at all.

If it's an issue with the slabs then how do I prove that with the supplier?

[London Stone Paving]

Photos uploaded on behalf of Chris









This is quite an unusual one and unfortunately there are no 100% answers as to the cause of the discolouration.

I can say with certainty that its got nothing to do with voids under the slabs.

Although grey sandstone is a reliable stone its worth mentioning that its incredibly ferrous. Its the iron content in the stone that gives it its grey colour and another reason why Kandla Grey is so hard. In the past I have seen occasional stones of this colour come straight from the crate. So its worth noting that this discolouration can happen regardless of any external influences.

the most common reasons for this discolouration to be "activated" is down to the use of incorrect cleaning products. Cleaning Products which can cause this are anything containing hydrochloric acid or concentrated Chlorine based products. Another common cause is the sand in the bedding layer having a high iron content (often the case with strongly coloured yellow or red sand)

What's odd in this case is the high concentration in the two areas. This to me suggests something localised has caused it. If you say the paving hasn't been cleaned with anything then it could only be the mortar bed or something else underneath

The good news is that in most cases this can be fixed with an application of Lithofin Bero followed by a diluted solution of Lithofin Builders clean.

The worst case scenario is if the stone has been laid on a very ferrous sand in which case the iron will continue to bloom as it leaches through the stone. If this is the cause then there is quite possibly no solution as each time you apply the treatment the discolouration will return.

hope this helps

Steve

[higgness]

Hi Chris
I am curious, how long did it take to dis-colour?
Were the slabs in question already brown/brownish when they were laid?

I sell lots of this grey. I have seen some in our the crates that are already brown, we try very hard not to send them out.

Once ever did I get a major complaint about the grey, all of it turned brownish after an acid wash was used to clean cement stains.

[PavingSuperstore]

From the initial description it sounded like iron oxide from an acid based cleaner. The photos show a very clear perimeter around the outside of the slabs so either something has been applied topically that's avoided the edges - again still thinking acid cleaner or the mortar is pulling the colour through, but the edges are fine. Can you be 100% sure that not acid based brick cleaner was used to remove surplus mortar? Acid causes iron oxide within the stone to rust which would accept for brown staining.

[RAPressureWashing]

Is the paving supposed to look that "washed" out, as something doesn't look right here to me.
There does look a possibility of an acid wash but the brown ones are to neat if you see what I mean no over spill onto the others? unless the whole area has been washed and the brown ones are now damaged.

[PavingSuperstore]

I was wondering if an acid wash has almost been 'painted' over a couple of badly marked slabs as the markings are so precise.

[Tony McC]

You occasionally get rogue flags, pieces of crap or stone from a bad seam, slipped into a few crates by unscrupulous exporters, and that's what I thought these were at first glance, but when I look closer, I can see the unaffected edges, and that sort of look can only be achieved by treating the stone with summat.

When Kandla Grey gets the acid treatment, it tends to flare up in distinct blotches rather than affecting the whole flag evenly. Now, this assumes we are talking the usual 5% (ish) dilution of hydrochloric as found in so-called Patio Cleaners. If a much stronger dilution was used, such as 30-50% acid etching fluid, this *might* affect a whole flag and the presence of cementitious material around the edges *could* neutralise the acid before it had the time to eat into the mineralised iron - just a theory.

Or maybe a more aggressive acid has been used. Oxalic can be pretty nasty, and then if some eejit who has heard that acid cleans stone and consequently decided to use nitric or sulphuric....well, who knows what might happen!

I don't buy the contaminated bedding theory - how can the bedding be so precisely limited to individual flags, and in two discrete areas? No: if it was iffy bedding, it would be a whole patch of adjacent flags, not just a couple of isolated spots.

It would be interesting to find out just what has caused this, but, for me, it's definitely something that's happened post-installation, otherwise how can the unaffected edges be explained?

[London Stone Paving]

Its certainly a weird one. The OP called me earlier though and cast iron assured me that the stone hasn't been cleaned with anything. He has purchased all the materials for the job and employed a contractor to do the install only. So unless the builders are completely pulling the wool over his eyes you have to assume it hasn't been treated

He has also sent me some more photos of fresh staining emerging on new slabs so looks like the issues are not as isolated as they appear to be on the original photos.

[RAPressureWashing]

Oxalic Acid wouldn't really do that sort of damage, if anything it would have bleached the paving not turn it brown so for me not that, if anything has been used at all. For me it's the neatness of the staining that don't seem right plus the whole paving as I said before looks washed out/pale, insipid etc, and that is what I'd expect to see if hydrochloric acid has/had been used.