sandstone patio slabs problem

18 May 2008
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United Kingdom
Can anyone help me please. I have started laying a patio using indian sandstone (22mm calibrated thickness purchased from Travis Perkins). Am bedding down on a sharp sand (not that sharp to be honest) and cement mix(6:1) at about 50mm thick over a compacted 20mm to dust sub base. My problem is that quite a few of the slabs are not bonding to this mortar mix and are just lifting. The mortar below has hardened perfectly, its just the slab is not bonding with it. Would it be a good idea to pre paste the back of each slab before laying with a neat cement / water mix to aid with the bonding process or is there a better way. Never had this problem with manufactured slabs before now. Is it something to watch out for with sandstone. Any advice please.
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Try soaking the slabs first as it could be the sandstone is absorbing the water from your mix to quickly.

I've layed various type slabs on a mix of 3 sharp:2 building:1 cement and they always bond to that.
This can be the problem with laying on a sharp sand only mix although it doesn't matter as your pointing will secure the flags and the sharp sand cement bed will keep them supported. You get a solid bed but don't get the grab you do with building sand.

Plastering sand is a good halfway house and can also be used for the pointing which speeds things up if you are buttering edges as you go.

You should use a building sand or plastering sand mix for any flags with free edges or steps etc to make sure it bonds.

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