Educate me on pozzolans

[jayc89]

I'm restoring our understair cupboard. It drops to approx 500mm below ground level, and the floor is slabs laid direct to soil so the floor and bottom of the walls are damp.

 

I'm going to laid a limecrete floor to sort the floor out and it's been recommended to render the walls with a lime + pozzolan mix. Apparently the addition of pozzolan kicks off a chemical reaction that allows the lime to set in damper conditions.

 

This all sounds great, but from reading up on pozzolan, my understanding is that it also reduces capillary and vapour permeability characteristics too. If that's the case, what's the difference between using lime + pozzolan and a cement render?

[jfb]

Hydraulic lime (which is what you want to be using, not hydrated) already sets in damp conditions. In fact it is best if it does not dry out too quickly which is why when you are working with hydraulic lime you should always damp down the render/mortar on days after applying it. It is the chemical reaction with the water that makes it set hard.

 

So I wouldn't see any need to add pozzolans to the lime.

 

Rendering with lime is not really any different to rendering with cement (except plasterers who aren't used to it might say otherwise) - you just have to make sure the walls are well damped down before application (especially in summer now) and are kept damp for a while after (you can spray it once a day).

[jayc89]

Hydraulic lime is what I've been recommended to use, this stuff - https://www.mikewye.co.uk/product/secil-reabilita-cal-cs/ - mixed with pozzolan though https://www.mikewye.co.uk/product/argical-pozzolan/ as the harling coat.

 

Which is what confused me. My understanding is pozzolans reduce capillary action and vapour permeability, I'm not sure by how much, but if that's the case its seems as beneficial to use cement render, which in itself seems counter intuitive. 

 

 

[jfb]

I don't see the point of adding anything to a lime render - it is going to set fine.

You could also consider just getting some hydraulic lime and sand separately as it tends to be a lot cheaper. But if it is just a small section may be easier to go for the premixed bags.

[SteamyTea]

As usual, when comparing lime and cement based render, finding real numbers for porosity, water retention, and vapour permeability is just about impossible.

No wonder there is so much confusion.