Hydrated Lime for render

16 Jul 2006
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United Kingdom
Hi, looking to render a small wall over the weekend.

I put a scratch coat on 6-8mm with ordinary cement 4-1.

Put another coat on {10mm} and floated that off, 5 sand, 1 cement, 1 hydrated lime 5-1-1.

Now I want to put on final coat, perhaps a pattern/effect.

Reading the sites on the net, someone said I can use hydrated lime with just sand, no cement and it will set in a few days, but on the rugby website it states:

Hydrated lime being non-hydraulic is not suitable for use in
mortars without cement being present.

What are your thoughts on this, as im getting conflicting information.

I am using both rugby's cement and lime. Many thanks
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If the answer is yes you must use cement with hydrated lime, what is the minimum amount of cement to lime, sand ratio

ie would 5 - 1 - 0.5 be ok, sand, lime, cement

i imagine it would look quite light in colour with that mix, but I dont know if it will be good enough

many thanks
I'm no expert, but from my limited understanding I'd ask why use a lime render as a top coat when you've already used cement in the first coats.

The benefit of lime render (no cement) is to allow the wall to breathe, flex and cracks to self-heal.
You've already ruled out those qualities so why not go down the normal sand/cement/lime render for top coat.
I've always used 1:1:4 or 5 or 6, cement/lime/sand.
Hi mate, thanks for the post. The only reason why I wanted to use just lime is because I am aiming for a light colored render.

The rugby cement is lighter than others, but its still grey and looks like cement :D

Anyways, did a test yesterday...
Mixed up 10 Sand, 5 Hydrated Lime and 1 Cement. Going outside to check it now.
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Sorry, it was 10 - 4 - 1 ~ s/l/c

Just checked it out now, quite surprised, its rock hard and looks much lighter than the previous coat, set overnight, as its only 2-3mm thick I think I'll carry on using the same mix.

I think what has helped too, is by using a paddle mixer. I am putting the water in first, adding the cement and lime, mixing this runny stuff for a few minutes then adding the kiln dried sand.

I don't think you would get such a consistant mix with a shovel. Slowly mixing it to avoid too much air entering.
i believe you're right morg, hydrated lime needs cement and as redherring says, you've already used cement in your scratch and float so lime only finish coat is not necessary, you could have used white portland cement in the mix.
I'm not sure that I'd agree with you 100% TrowelMonkey.
But your nom de plume suggests that you know what you're talking about so I'll tread very carefully as I'm a layman.

Hydrated lime doesn't need cement to cure. It cures in a reaction with carbon dioxide, not water.
But I'd certainly agree hydrated lime needs cement with it in an exernal use. Otherwise it should be at least a feebly hydraulic lime, which wouldn't necessarily need cement with it.
Hi RH2

i'm no expert either mate but just going on what i've learnt from various sources and trying to dispel the mysteries of lime in it's various forms as an education for myself more than anything else.

It seems you could be right from what i can find out, hydrated lime is very similar to lime putty or slaked lime from what i've recently read.

It's basically quicklime (limestone fired in a kiln to drive off all CO2) with just the right amount of water added to it so as not to leave a water residue like in putty, the adding of the water turns it to calcium hydroxide from calcium oxide, when it's mixed with water and sand to make the render and applied to the wall it then takes on CO2 as you say in a process called carbonation, essentially turning it back to limestone.

I've read that hydrated lime is an inferior form to lime putty, but why i can't say yet, still searching ;) having said all this RH2, if you've got it on your wall and it's looking good then where's the problem? and sounds like you know how to treat it when using it so hats off to you there!

absolutely agree that for anything exterior you need a hydraulic lime

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