New render not stuck when dry

[Matt]

I have rendered a foamed concrete block wall using sand cement (5:1 + rendermix additive). The problem is that (for two attempts) when the render has gone off, the bottom half is not stuck to the wall and is free to swing in and out.

I had pva-d the wall with a 5-1 water/pva mix and then a 3-1 pva mix before applying the muck.

Please help! What's going wrong, and what should I do to fix it?

I have decided against sand cement render and i'm going to use aquapanel for what will be the tiled bit inside the shower enclosure (it's a bathroom). Should I browning/bonding the rest of the wall or use plasterboard?

[Stuart Noble]

Sounds like you may have over sealed the blocks. Whatever you use, get a thin coat on first and give it a chance to bond. A thicker coat will slump under its own weight. You should get a good result in the end but the initial coat can be difficult on this type of surface. I'd use bonding plaster, which is lighter in weight and warmer to the touch.

[marvelus]

I thought you were supposed to do an initial scratch coat of 3.1 or stronger with a key scratched into it then do the top float coat of

5.1 with waterproofer. This obviates all this PVA mumbo jumbo.

[Andy]

Did you use a choppy motion when rendering the wall? It's better to use a sort of slightly up-and-down ramming action when applying to graunch the render into the crevices in the wall surface. Did you use sharp sand? I found the best consistency to use was a mix that would be too wet for bricklaying, i.e. it slumped a fair bit if you tried to heap it up on a horizontal surface. Having said that I did not PVA my breeze blocks before rendering.

Just some thoughts,

Andy.

[John Rumm]

Staple expanded metal lath to the blocks and try again ;-)

[Matt]

Cheers for the replies!

I used a firm upwards sweep to press it on...

I had problems with the first attempt slumping due to the mix being fairly wet - this didnt happen on the second attempt as I used a firmer mix and a sceed batton across the bottom of the wall.

I would probably say the wall was over sealed - the blocks looked shiny to begin with to be honest - like a grey chocolate mousse or something.

Needless to say, i've screwed cement board to the wall now!

John Rumm wrote:

[The Natural Philosopher]

seconded..