I need a recipe for render

[Keef]

Hello again, I am building a straw bale house, walls are all now finished with a a 1 inch thick cement render, rather rough finish but good and solid. I am about to start on the final render coat. A few years ago I did a workshop on rendering in which we used a mix of white cement, sand and some red clay(for colour) mixed with lime and left overnight. This lot was then combined with some sawdust (for texture and flexibility) and then mixed up in a cement mixture. I think we might have added a dash of bondcrete also.
This mix covered well and gave a great finish, problem is that it was a few years ago and I can't remember the exact detail of the mix. Has anybody out there heard of this method or something similar. Idea is to end up with an earthy organic look, weatherproof and not dusty or flaky, able to be applied by a not very expert renderer.
If you happen to be in Northern NSW you could even come and help me slap it on the walls (I wish!)
Keef

[jackiew]

Suggest you ring up the people who ran the course and ask them. They should be quite happy to tell you as you went on their course.

[ozwinner]

A few years ago I did a workshop on rendering in which we used a mix of white cement, sand and some red clay(for colour) mixed with lime and left overnight.

Hi
Just curious, why did you leave the mix overnight, it wasnt wet obviously?

This lot was then combined with some sawdust (for texture and flexibility) and then mixed up in a cement mixture. I think we might have added a dash of bondcrete also.

Is the original stuff still stuck on the wall? I would have thought that with sawdust added it, it would be full of holes, weaker and liable crack to more easily.

Just curious, Al

[wombat47]

Sounds a bit like the old fashioned sand and hydrated lime mixture - you mix that up with the minimum of water and leave at least overnight. It becomes more plastic as it sits. It doesn't contain cement and therefore doesn't react with old mortars. Takes longer to set than cement based mixes but is just as hard and water resistant. The Romans used lime mortar and plasters.

I'm not 100% sure but I think that old plaster walls (like mine) were applied with a thick base coat of sand and lime, and then a much thinner top coat of lime soaked in water to give a fine eggshell finish.

[bitingmidge]

A few years ago I did a workshop on rendering in which we used a mix of white cement, sand and some red clay(for colour) mixed with lime and left overnight.

I'm pretty dubious about the left overnight bit as well... Cement goes "off" as part of a chemical reaction. No doubt someone reading this will understand the process and elaborate, but it is not reversable.

By leaving the brew overnight, you will have a claggy mix that will be relying on drying only to adhere, rather than the chemical properties of the brew.

Curiously, my knowledge of this was gained first hand after a 35 story building job I was running had to have its paint removed by hand scraping, and re-applied correctly. The paint was a cement-based primer and the subcontractor mixed half a day's brew at a time, then proceeded up the building to work for half a day.

Because the initial chemical reaction was complete by the time the paint was applied, when it dried it was effectively a coat of hard talc.

Let's just say the rectification costs were significant!

Can't help with the question though sorry,

P

[ozwinner]

Initial set for cement is 45 minutes, final set is 10 hours.
But cement will continue to harden for the next 10 years, after that it starts a gradual decaying process.

I got learned that doing my bricklaying apprenticeship.

When ever I mix mud, I only ever mix what I can use straight away, I dont leave the perpetual mix in the mixer going around all day.

Al

[bitingmidge]

Initial set for cement is 45 minutes

Yep, thats the time the chemical reaction takes place. After that, no hope of getting the design strength.

Oz you are now the fourth Bricky I have ever come across who has actually done an apprenticeship....one more and I will have met them all!

P

[ozwinner]

Funny F#cker.



No wonder I love comeing here, good one


Al

[ozwinner]

Your right though, most brickies just seem to pick it up.

But just a couple of minute talking with them and you can tell.
Sorry if I seem tradest, ( as in sexest ) :eek:

Cheers, Al

[bitingmidge]

Your right though, most brickies just seem to pick it up

Tradest is OK I think, although I would have used another word before "it up" with regard to many!

P

[ozwinner]

I used to have this uncle, and when my dad wasnt busy I used to get lent to him for the day.

Every day without fail at knock off time, he would slop up the unused mud for the next day.
He was a real tight bastard, next day, knock up the mud and use it.
I couldnt/cant beleive it.
This is a bloke who would drive 20 miles ( it was in England ), to save a couple of bob on a carton of smokes.
The petrol alone would have cost more than the saveing on the smokes.

Al

[Keef]

I'm back...thanks for all the responses. Unfortortunately I have not had any luck in tracking down the bloke wot ran the course. It was only the lime and clay which was left overnight to soften. An old bathtub was filled with wet clay and a lot of lime was added. In the morning this mix was worked with a hoe thingy with holes in it (wots the proper name for that anybody?)
Then this was added to some sand and white cement, mixed for a bit and then enough coarse sawdust was added to make the mix a bit fluffy. ( see it's all starting to come back to me now) The resultant mix was lovely to apply. The chemistry might be a bit dodgy but the builder/workshop person had been using this for many years and it was claimed to be crack resistant and weatherproof. I might have to go back to the hose I worked on and ask the present owner wot he knows.
Keef

[sailingamerican]

This sounds like Adoby mix. Straw does not help make Adoby stronger. when you mix Adoby you make bricks and stand them up on end to dry.
Lime in your mix is good for freezing weather. It helps morter from expanding.
Why the saw dust? This adds nothing to the mix. The basic mix you have detailed is your basic stucco mix which we use in the Southern US and South America. You can make walls out of compressed mud. Many Indian tribes and cultures around the world have used this method and after thousands of years still remains. Try going on line. Here in California USA they have built many straw bail homes. I have seen it on the TV. You can find it on line. I do not think they leave the mix over night. Mix what you can use for a few hours. You can put just about anything in the mix for texture. Saw dust does not add strength. I think I have seen it used but why? They also dig into the earth and use tires and rebar packed with dirt. Then they build floors, roofs, and put windows and doors on the front. To get a building permit they drove a 5 yard cement truck over the tires. It held.
Have a great day.
Richard in CA USA

[journeyman Mick]

Keef,
hoe thingy with holes in it is called a larry and is made for mixing mortar. Now largely superceded by the cement mixer, but still useful for mixing small batches.

Mick

[Keef]

Richard...I am constantly amazed by how many helpful replies I get on this forum. The mix I described was not actually designed for strawbale but I think it would work well. The builder who came up with the method used it to transform boring looking houses. First day he would rock up with a truckload of rusty old roofing iron and nailgun this horizontally onto the walls of the house, (caused instant phone calls of complaint from the neighbors). Second day he would apply the mix as a scratch coat ( coarse sawdust in the mix). Final day the mix had some finer sawdust, it went on easily and was finished with a sponge after going off for about a half hour. The builder claims to have done hundreds of these renders with zero failures.......Keef