Using a plasticiser admixture with lime render

by Keith Weller

Dear Paul,

I knocked up a 2:1 sharp sand and NHL3.5 mix with a whisk successfully, although I found it rather less 'buttery' than I was hoping for and it needed a fair amount of pressure to get it to stick to the (well wetted) internal wall I'm working on.
It seems to have bonded OK though now that it has set, but I was wondering if on the remaining walls a finer sand, like plastering sand, might be more workable (albeit slightly weaker)?

Also, I wondered if using some plasticiser admixture might in general improve the workablility of lime mortar as it does for OPC mortar? From what I've read hydraulic lime mixes should be naturally workable anyway once they've 'fattend up', and I also noted that the boggo liquid plasticiser I have says it's not to be used with lime - although maybe they just mean not to add hydrated lime to OPC as a plasticiser with the admixture. Anyway, I'm worried that adding it might weaken the mortar in some way if I do try it out.

Many thanks for the advice so far!

Hi Keith,

As you say you shouldn't need to use a plasticiser with lime - it is more to do with the sand. Sharp sand does provide extra strength among many other benefits but the big drawback is less workability.
If for whatever reason I'm using sharp sand - i tend to cheat a little and add some soft sand to the mix. The amount varies from 50:50 (sharp:soft), to 10:1. The amount is often trial and error and depends upon the sharp sand - you could try adding a very small amount of soft (10:1) and gradually increase until you're happy.


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