What is the best mix for cement render?

Question

I have some bricked exterior areas that had some cement render on them, and then paint. The render has fallen off the bricks in patches.

I think the problem is that large chunks of the brick work was done using cement blocks rather than clay bricks as the areas that have come away tend to be more over these bricks than the clay bricks.

The cement bricks though are not the ordinary breeze block type porous heavy versions, but are more aerated and lighter than normal. They also appear to soak up moisture a lot easier (what I suspect is more the problem).

So given the problems with the bricks, what would be the best mix for a render coating that means I do not need to actually replace the bricks themselves?

Answer 1

I'd add a mortar plasticiser to a cement to building sand render mix made at a ratio of about 1:4 - I believe your namesake Travis Perkins offer a suitable product ;)

Damp the wall thoroughly before applying the render to slow down the transfer of water from the mortar to the wall.

I guess you could also use a layer of PVA ontop of the wall before rendering to provide some kind of barrier but that's not something I've done personally.