Hello, last summer i laid a patio in sawn sandstone, jointed with GFTK VDW 815. It is the same job that i posted about last year, It has since turned black and the client, understandably is not very happy.
Im not involved with rectifying it and am just providing a little guidance to the architect that is.
The stone was literally pulled out of the ground, sawn and immediately laid. The flags have been recently taken up and fresh stone laid from the same quarry, it is to be jointed this week with the 815. The client is insisting that it is sealed, as the quarry reckon that'll stop it going black, im not so sure. Anyway, i am wondering can you apply a sealant on top of the 'sheen' left by the 2 part resin, and if so what sealant?
I have little experience of sealants and have always advised the client against them, but this one is adamant.
Architect is sending pictures over shortly.
Regards, Dan.
Sawn sandstone gone black
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Tony McC
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Is this lichen, Dan? I'm not sure how else a sandstone can turn black.
The vexed question of whether to seal then joint, or joint then seal keeps cropping up. There are pros and cons to both approaches, as you'd probably guess!
Pre-sealing helps minimise any risk of the selected jointing product (whichever it is) stainiong the pavement surface BUT....the open joints are a nuisance and can swallow expensive sealant if it's applied with, say, a roller.
Jointing before sealing helps minimise loss of sealant and allows the whole of the pavement to be treated in one fell swoop, but because most jointing compounds require plenty of water during application, there will often be a minimum 3-7 day delay between jointing and then sealing, which means the poor contractor might not get paid for that bit longer.
I can't say one approach is better than the other. With highly sensitive stones, I tend to support pre-sealing, preferably a protective seal pre-installation. For the more robust stones and for more-or-less all concretes, I'd rather get the jointing done, leave the paving for a few months to give time for any issues, such as efflorescence to sort themselves out, and then seal once I'm sure the paving is stable and bone dry.
The vexed question of whether to seal then joint, or joint then seal keeps cropping up. There are pros and cons to both approaches, as you'd probably guess!
Pre-sealing helps minimise any risk of the selected jointing product (whichever it is) stainiong the pavement surface BUT....the open joints are a nuisance and can swallow expensive sealant if it's applied with, say, a roller.
Jointing before sealing helps minimise loss of sealant and allows the whole of the pavement to be treated in one fell swoop, but because most jointing compounds require plenty of water during application, there will often be a minimum 3-7 day delay between jointing and then sealing, which means the poor contractor might not get paid for that bit longer.
I can't say one approach is better than the other. With highly sensitive stones, I tend to support pre-sealing, preferably a protective seal pre-installation. For the more robust stones and for more-or-less all concretes, I'd rather get the jointing done, leave the paving for a few months to give time for any issues, such as efflorescence to sort themselves out, and then seal once I'm sure the paving is stable and bone dry.
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DNgroundworks
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I was hoping you would reply Tony. I have no idea. The steps up to the patio are fine apparently and made out of the same stone, but the flags up top are completely black aside from 10mm around the edge of each flag.
It strikes me as some sort of mineral reaction or oxidation issue maybe? All i know is that it has been relaid and that they are at the jointing stage....ive sent them to Gary at NCC, he's the man for the question i reckon.
Also, for reasons unknown and something that stressed me at the time is that i was instructed to fall the paving backwards, rather than falling the patio down the steps. Falling a patio in this way that is otherwise completely enclosed with dwarf walls didn't sit well but i digress. This resulted in very minimal fall towards a woeful drain in the back corner owing to concrete heights underneath etc. It may have had standing water over it which may have exacerbated the issue.
It strikes me as some sort of mineral reaction or oxidation issue maybe? All i know is that it has been relaid and that they are at the jointing stage....ive sent them to Gary at NCC, he's the man for the question i reckon.
Also, for reasons unknown and something that stressed me at the time is that i was instructed to fall the paving backwards, rather than falling the patio down the steps. Falling a patio in this way that is otherwise completely enclosed with dwarf walls didn't sit well but i digress. This resulted in very minimal fall towards a woeful drain in the back corner owing to concrete heights underneath etc. It may have had standing water over it which may have exacerbated the issue.
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Tony McC
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MartinL
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I've got an old concrete patio that also tends to go black. A jetwash cleans it to a degree (though previous use of jetwash has probably damaged the slabs surface making them more porous). Jetwashing results are far from perfect. I think the issue is caused by a combination of moss/lichen/atmospheric pollution IMO. I also had a brand new tiled clay tiled roof put on about 18 years ago as that is now much much darker than it started out too hence my theory about atmospheric conditions. My property is surrounded by heavily farmed fields and also around 400m from a major A road.
Solution I have come up with on the patio is spraying it with a 3-4% Sodium Hypoclorate solution (aka bleach but 14-15% Sodium Hypochlorate patio cleaner is stronger) to clean it up. This has cleanned up my patio a treat and my black patio is now literally back to an attractive and spotless light sandstone colour again though I guess it will turn black again over time unless I seal it.
Does the sandstone clean up with Sodium Hypochlorate? If it does it seems likely to be a similar problem to what my patio has even though its a diferent material.
I'll probably get a response now saying never use sodium hypoclorate on slabs/flags - all I can say is that it has made my patio look a million percent better (from a neglected very low point admittedly)
Solution I have come up with on the patio is spraying it with a 3-4% Sodium Hypoclorate solution (aka bleach but 14-15% Sodium Hypochlorate patio cleaner is stronger) to clean it up. This has cleanned up my patio a treat and my black patio is now literally back to an attractive and spotless light sandstone colour again though I guess it will turn black again over time unless I seal it.
Does the sandstone clean up with Sodium Hypochlorate? If it does it seems likely to be a similar problem to what my patio has even though its a diferent material.
I'll probably get a response now saying never use sodium hypoclorate on slabs/flags - all I can say is that it has made my patio look a million percent better (from a neglected very low point admittedly)
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Tony McC
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