Have I got the right cement?

[roofmog]

Preparing to render a small amount of exposed internal brickwork. I've got the sharp sand, cement and Sika Mix Plus Plasticiser . However the cement I've got is Blue Circle Mastercrete. This contains a small amount of lime, and the data sheet suggests that the use of the plasticiser is not necessary. Is this correct. Should I go ahead without the plasticiser, or would I be better off using a more basic cement and adding the plasticiser.

 

[tony1851]

Mastercrete has it's own plasticizer, so why bother buying a bag of OPC if you've already got the Mastercrete?

 

[^woody^]

You should not use plasticiser in render, only in mortar.

That cement is OK to use though.

 

[roofmog]

You should not use plasticiser in render, only in mortar.

That cement is OK to use though.

Thanks for the reply. Why do you not use plasticiser in render?

 

[^woody^]

Because plasticiser is designed to increase mortar workability by forming tiny air bubbles into the mix. This is desirable with mortar, but not desirable for render as it affects the waterproofing qualities.

 

[roofmog]

Because plasticiser is designed to increase mortar workability by forming tiny air bubbles into the mix. This is desirable with mortar, but not desirable for render as it affects the waterproofing qualities.

I see. The wall in question is an internal structure (fireplace surround) so waterproofing shouldn't be an issue. Out of interest, if the scratch coat was done with a basic cement and plasticiser, would there be any issue with doing the top coat with the mastercrete.

 

[stuart45]

You can get combined waterproofer/plasticiser. Using plaz does mean less water in the mix making it easier to work with, although lime in the mix and making sure it's well mixed also makes a workable mix.

 

[^woody^]

Mastercrete is just normal cement but supposedly more finely ground than OPC. You can use either.

TBH, with render sand, plasticiser makes no difference to workability. Lime does if that is what you are after, but for a fire place I can't see it making any difference for such a small area.

 

[roofmog]

Thanks for all the info. What are the minimum/maximum times I can leave between the scratch coat and the top coat?