Blown render on freshly rendered block work building

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Phoebe

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Done a job rendering onto block work on seafront outbuilding and all render has blown. Thermalite type blocks, galvanized mesh, 3:1 scratch coat ( I know that's strong) with wterproofer. Left overnight then following day topped with 3:1 again and floated up as normal. come 16:00 all looked well. Following day I spot some cracking, have a little tap and it's all blown. Rip some off, it comes off in massive plates and it looks like the scratch coat has pretty much all dried up and turned to almost powder. One theory is that the suction just drew too much moisture out of the scratchcoat and therefore didn't properly key. Another theory is that we used building sand for scratch coat and a too strong mix of cement. And our last thory is that it's got something to do with the sea air. Apparently last week there was spray coming off the sea and onto building. Any help would be fantastic as am a bit stumped and now have to re-render so need to resolve. Thankyou
 
the block manufacturers reccomend a mix of

6:1:1

sand/cement/lime

so your mix far too hard..

and leave longer before second coat..
 
The lime will butter the mix up but it helps to retain moisture in the mix and reduce cracking, so I believe.
 
You didn't control the suction from the thermalites. All the mosisture was sucked out before the chemical reaction could cure the the mix. This means your topcoat is sitting on dry sand and powdered cement causing it to blow. Its as simple as that.

Thermalites + render - any form of pre rend = Bad times
 
I've done plenty of renders on Thermalites/Celcons without any pre rend without any problems.
It's all about the ratios and curing of scratch coats.
 
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