patchy sandstone slabs

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Hi
any idea whats happened here and if its likely to go away?
Slabs laid on solid base wettish 1:6 sharp sand / cement On top of existing concrete.
not sure but hoping it's not similar to what you get when you spot bed and it shows through?
Any ideas welcome!
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how long have they been down? the problem with laying over a solid concrete base is hat there is nowhere for water to go but up through the flags leaving efflorsence trails and patchs.

If they have been down a long time it looks like something was spilt on them.

a picture from directly above would be useful as that one is not super clear.
 
Cheers for the reply, I laid them about 4 weeks ago. Looked fine when they went down as you can see in this pic
Not sure if anythings got on them. I'll try and get a pic from above of later
There was a strip which I wet concreted on the day before laying which I reckon might line up with most of the patches, was that a big mistake, reckon it'll eventually dry out?
Thanks again
 
if they are only down that long it could still be efflouresence coming out of the mortar. this is especially common on a solid concrete base as the moisture has to come up.

If it still looks funny after 2 months and drier summer weather it may be something else.

Did you use sbr in your bedding by any chance?
 
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Thanks mate. Yes I think and hope you're right. It's changed again now and gone white and strange blotches so that's definitely efflouresence.
no didn't use sbr, used a bit of normal plasticiser, ?
 

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