Clay rendering on exterior walls, what mix is best?

I'm looking to render on old concrete blocks that I salvaged. I have a pile of clay, straw maure and sand but what is best mix. How best to make it waterproof on the exterior walls, is that possible?

Have you looked for external clay plastering?

Hey Midge.
The ratios depend on the type of clay and also sand.
And how thick you want and need to plaster.
If you need to cover uneven blocks and want to unify it all, I would suggest a first coat of almost 1“ using clay, sand and fiber.
Depending on the weather, you'll need to let it dry a bit.
And then add a second,finer coat. With sifted sand and no fiber. Maybe 1/4“ thickness.

You need to do some tests for finding the right ratios.
I would start with 1 part (volume) clay and 3 parts sand and/or fiber.

I don’t know if waterproofing is a good idea. Can you protect the plaster from direct rain?

Also don’t let the plaster touch the ground but build a little stone wall, like 3“ high, as a moisture barrier.

Hi Midge,

Sounds like a fun project! Do you mean clay plaster? I'll just get this out of the way first: you cannot waterproof clay with natural materials.

It's useability strongly depends on your climate. You have the perfect ingredients, but do you live in a really dry climate? In my area of the world houses are plastered in clay, some of it washes off in the seasonal rains, but during the annual dry season it is reapplied.

If you want something that is permanent, and that adheres to concrete blocks really well, try lime plaster. Add lime to your sand and when you've plastered it nice and smooth, add a soap wash for water repellence.

Otherwise, clay plaster is a gorgeous solution for interior walls.  

Midge,

Is the wall protected with some roof, cornice, eaves, cap?
What is the function of the wall?
Do you want to water proof the wall or the plaster itself?
Where are you located?

Many thanks for your comments

I'm in southern England, so yep rain is an issue, from your comments would it make sense to have a first layer of clay, sand and straw.
Then a finish layer of lime and filtered sand? Or just do a lime plaster, just I collected the clay already.

These concrete wall were going to be part of my geo thermal sunken greenhouse but I'm not going to cover the wall section and plant climbers to shade the greenhouse in summer
It's really making them look better

Clay could be used to plaster the interior walls.

This young artist and builder is delightful, expert and inspirational 👏 🙌 ❤️

The Nito Project.. natural building and plaster master

[youtube]https://youtube.com/@thenitoproject?si=j0owNjW6W_RB7Y5X[/youtube]

Midge,

Plaster layers should start with the strongest, going towards weakest. Applying one type of plaster already has its challenges and mixing two mediums needs testing (at least a full year cycle).
In my opinion there is no point to use clay plaster on concrete block on the outside. It would make perfect sense to use it on interior concrete walls in an inhabited building, so some wall breathability would be added to low porosity concrete and would improve air quality in such a building.
I would suggest to plaster it just with lime. Concrete plastering is pleasant, because it wicks plaster moisture slowly, giving enough time to finish it perfectly.