.rising stains through stone slabs

[markactive]

I am about to lay 20mm light granite slabs onto a full mortar bed and I will brush a SBR slurry coat on the underside of the slab. I hear tales of rising cement stains through these stone slabs especially thin light coloured ones.
Has anyone encountered this and can I do anything to counteract it ?
Thanks

[deegan1979]

From what I gather the sbr slurry should prevent it

[markactive]

Thanks for that. I have spoken to the SBR manufacturers and the say that it will not stop the rising stains in theory because it is semi-permeable. I have found a topic on this website that can help the problem by using silver sand and white cement as a mortar bed. The white cement is readily available but I cannot buy silver sand anywhere in the midlands.

[lutonlagerlout]

soaking the flags just before you lay them should help
if they are dry then you can get a wick effect where water/cement travels up microscopic capillaries in the stone and gives the surface a picture frame effect
cheers LLL

[markactive]

Thanks LLL, all I need now is a silver sand supplier.

[deegan1979]

I was advised that in order to stop efflourescence rising up through my sandstone that using a cement / sbr slurry on the back of the slabs
stop this with the added benefit of making sure the slabs will really bond to the wet mix.

[markactive]

Thanks for that deegan. Apparently efflourescence(salts) is not what the stains are caused by, after reading the article on this site it is the dark impurities within the cement and sand that contaminates the underside of the slab and travels upwards. However perhaps the SBR slurry will slow it down.
I am perhaps worrying about something that will not happen but with about 100 square metres of paving I want to make sure it is right first time.

[deegan1979]

No probs mate.
I was advised through the ask an expert link on this website that using ordinary grey cement and sbr on the back of the slab would negate the need to use white cement.
Hopefully it's correct and I have misunderstood something important. Hopefully someone else will advise us on the matter

[PavingSuperstore]

I think silver sand is the same as play sand - it has a very fine grain, so may not create the best bond.

George

[lutonlagerlout]

I have laid 100's of m3 of flags and never used a bond bridge
but I always use a strong wet mix and add SBR to the mix
efflorescence can occur from time to time but nothing can really stop it,it either happens or its doesnt
LLL

[markactive]

Thanks for your input guys. Efflorescence, if it occurs will be easier to deal with than this "5 0'clock shadow" that quite a few people have mentioned away from this forum including the supplier of the granite slabs.

[lemoncurd1702]

I've used this stuff slurry primer recently on ceramic paving and it's the dogs proverbials. Why bother with an sbr slurry when this is available.

[markactive]

Thanks for that Lemoncurd, it looks like good stuff especially the working time which is about one hour, more than an SBR slurry. The only thing is if I make my own SBR slurry up I have control over the colour by using a white cement which may prevent the stains that I have been warned about.The colour of Pro-prime is grey.
I will possibly use this in the future with thicker slabs that are darker in colour.
Thanks.

[DempseyLiverpool]

I've just laid 25mm paving and 50mm granite setts, wet down prior to laying on moist 75mm 6:1 grit/general purpose cement + sbr, backs coated with cement/sbr slurry and pointed with 4:1 grit/cem1 + sbr slurry with no staining.

Some pics on Twitter ashleydempsey@dempseypaving

[markactive]

Thanks for that Dempsey. I understand the thinner the granite the more prone it is to rising staining, the ones I am laying are 20mm nominal so could be as thin as 18mm. Apparently it can take quite a while to migrate upwards as well.
Cheers.