Slate - White marks on slate

[lutonlagerlout]

it almost looks as if cement has wicked up through the stone===> highly unlikely with slate i know
but i do know from roofing work that welsh slate is prized more highly than Chinese slate due to superior quality
however as usual the cost factor comes into play
for a good durable driveway clay block pavers are unbeatable in my experience
20% dearer than concrete,but the colour lasts
tarmac to be done right is a an art form that sadly a lot of domestic installers do not have
personally of all the tarmaccers i know (not including the decent fellas on here) there are only 2 i would let near a roller
good luck michael
maybe leave the patio for a year and see how it looks,might be the path of least pain
LLL

[DNgroundworks]

Ditto with the tarmac LLL, it sure is a fine art. Also i agree with the clay pavers never done a decent sized job in them yet.

At first i thought they were foot prints

[pianello]

Thanks for the input. I'll check with our installer about the cement coming up through the slate. Welsh slate was our first choice but when we checked with the quarries they were mega expensive. I think most of their slate goes on public works like the pavements in Carnaevon where money is or rather was no object. I need to get something done sooner rather than later as I don't want the new drive being used for materials etc for rectifying the garden patio. I am going to see our slate supplier in the next few days so will post what happens.
I'll check out the clay pavers as well.
Thanks again this is a brilliant site.
Michael

[StevenHoward]

Hi Michael,
Knowing a wee bit about Geology from my Uni days, I am pretty sure these are Calcite mineral deposits on the surface of the slate. Calcite makes up part of the natural formation of slate ( the other minerals being mica, chlorite and quartz) and can make itself apparent in the white 'flaking' you are experiencing on your flags when exposed to oxygen and water.

It is very common and can happen to any slate formation; be it Chinese, Welsh or Brazilian. It can also manifest itself in-situ and may not have been apparent when first laid. It is pretty tough to tell which flags may show a physical deposit and it may show itself up to two years after being exposed to the elements.

A mild brick acid #be careful how you use it# will stop the deposit getting any worse and will take off the unsightly flaking. However, even then it will leave a faint shadowing on the surface where the deposit once was.

Other than that, you'll have to replace the flags and hope the deposit doesn't manifest itself. If you are going to do that, buy your replacement flags a couple of weeks before you get the job done and lay them out exposed to the elements and start looking for signs of natural deposits.

I hope this helps.

Steven

[pianello]

Hi Steven
Thanks for your comments about my patio. I understand what you are saying about the reaction and so if I replace the slabs the same thing could happen again so maybe I will have to live with it. We spent a lot of time researching the patio including time spent on this site and visiting lots of stone suppliers. This reaction didn't show up on any of the suppliers that we visited. If it had we wouldn't have gone ahead.
Could all the slabs react eventually to this deposit or perhaps it is only appearing on the slabs which are the thinnest cut?
Regards
Michael

[stapleme]

Hey London Stone ....If these slates were sealed or intensified when laid would that stop the minerals coming through ??
cheers

[London Stone Paving]

There is no real need to seal slate for exterior use. Its a dense material which doesn't absorb too much dirt or moisture. I cant really answer your question because I have never seen these marks develop before after the slate has been laid. As I have said on the post previously a percentage of our slate does have these marks on but its just a characteristic of Chinese slate.

[Jay9]

We have just layed our new patio with slate and have exactly the same markings. We addressed the issue with the suppliers before it was laid and they told us it was normal and it would just brush away (even to wire brush if some of the marking was stubborn). We gave the go ahead to the gardeners to lay the slate and the marks now won't come off. The suppliers are being evasive and unhelpful although I'm due another call with them on Tuesday. I have spent many hours trying to find a solution to the problem on the internet and the only thing that remotely worked was soaking the mark in vinegar for a few minutes and scrubbling like hell with a wire brush. Even then its labour intensive and doesn't remove all the mark. I'm guessing it's not great for the slate either). I haven't sealed my patio yet as I'm trying to resolve the matter first however I'm even more concerned that your marks have come out after a few years of being down! I'd be really interested if you find a solution to removing the marks.

[London Stone Paving]

Jay

I dont think you will get rid of the marks. They dont just sit on the surface, they are ingrained within the stone. You need to remember that stone is a natural material and it will contain these types of imperfections. I certainly wouldn't reccomend you putting vinegar on the stone, who knows what that could do.

Also, why are you sealing slate? It does not need to be sealed, unless its for a specific reason. I.e around a barbecue or something.

I understand why your upset, because you have been told by the suppliers that the marks will brush off. This is crap because it has given you false hope. The best thing to do is to get hold of the name of the MD of the company and contact him/her directly and tell him about whats happened. If heshe cares about their reputation then he will do something about it.

But going back to removing the marks, forget about it, its not happening