Sand and cement indoors - ratio?

[wayt43]

The property I am doing up has thick S&C render everywhere indoors, up to 20 mm in some places but usually around 10, topped by a plaster skim coat. The S&C seems to me at least partly structural - I say this because some of the walls are quite flimsy hollow clay blocks (only around 80 mm thick) and once you hack the render off you can almost push them over. The render is tough (compared to ordinary plaster) and adds a lot of strength.

What ratio of sand and cement should I use to maximise strength on an internal render? I normally use 4:1 for mortar, would this work?
Is there a disadvantage in using to much cement in a render (not enough flex, cracking etc)?
Do I need to use lime in the mix? I wouldn't unless its needed, as presumably this would reduce strength.

 

[DAZB]

A render mesh would add strength and below is a reply posted online to a similar query which may be useful....

"Make sure the wall is free of loose mortar, you may need to wet the wall before you apply the scratch coat as too much suction will cause the first coat to dry too quickly and crack.
The science behind cement render is that each coat should be weaker than the previous, I.e. 1 cement to 4 sand for scratch coat then 1 cement to 5 sand for render coat.
For your job I would say that a 1 cement to 5 sand is suitable for the scratch coat. And 1 cement to 6 for the render coat. You will not need lime as generally this was used as a plasterciser which has been replaced by Feb. Lime is sometimes used for the render coat in pebble dashing.
Put feb in both coats as per manufacturers instructions, this makes the cement more ‘workable’. You would be advised to put waterproof admixture in both coats also. Again as per the manufacturers instructions. Plasterers often put a third of the recommended waterproof dose in the scratch coat, this will reduce the suction for your top coat, too much will make the second coat slip off, then full recommended dose for the top coat."
And also this ....
"Make sure all surfaces are clean and dust free I would advise a slurry coat of sbr and cement painted on before the scratch coat mix, this will allow for better adhesion for the scratch coat. Scratch coat is 1 cement, 1 washed sharp sand and 4 sand either building sand or plastering sand second coat 1 cement to 4 building sand or plastering sand. Waterproofing liquid can also be added. The reason for the sharp sand in the first coat will make the mix extra strong hope this helps"

 

[wayt43]

May have been written for external renders (not sure why you would use waterproofer on the top coat indoors) but seems to confirm my suspicions about the bottom coat being the same as mortar. I will get someone in to do the skimming in any case. When I have been demolishing I notice that the mortar and render come off as 'H' shapes like they are the same substance, as the render is a continuation of the mortar, and they are visually identical in colour, consistency, strength etc.