plaster mix.

[formerly known as]

experimenting at the weekend .................. I needed to plaster a wall in one hit as the electricity board etc has to be mounted fairly shortly afterwards. 2 buckets of sharp sand to 2 buckets of builders lime to 1 bucket of approx. 50mm chopped barley straw made a render/plaster that could be put on in one coat over flint and brickwork with very open/recessed joints. Made up quite stiff, it went on with ease and finished with a gross grain. No cracking - I know its only 4 days in - and its all good. I think that this will become the norm for all applications.

I just thought that some might like to try.

 

[malcolm]

Are you using hydrated lime from builders merchants (rather than the confusingly similar sounding hydraulic lime)? Are there major advantages or disadvantages in using that over lime putty other than the advantages of price and easy availability of hydrated lime?

I've not got around to doing anything much inside yet and have been using NHL 3.5 outside, but your system sounds extremely low cost for a good breathable and crack resistant conservation method so is very interesting.

 

[formerly known as]

It is hydrated NON HYDRAULIC lime.

The only difference between hydrate and putty, aside from cost, is the amount of water added to quicklime in the production process - less for hydrate, more for putty.

I have used hydrate for years for mortars, renders,plasters, grouts and limewash.

Most people seem to be scared of using it because they read the blurb saying that it should be used as a plasticiser only.

Personally, I would only use hydraulic where I would be concerned about obtaining a quicker set or for the core of thick rubble walling.

I was speaking to an architect last week, he said that he would NEVER specify hydrate, because it was 'inferior' - I asked him why and he couldn't answer - another case of being blindly led without questioning.

Remember also that the age old 1:3 mix is based on quicklime and sand - which can double in volume once slaked - so you need 1:2 minimum.

The gauged mix in post 1 is drying well, still no cracking - bearing in mind that this is over flint/soft red/patch of dense modern brick - I am very pleased.

 

[formerly known as]

I forgot to add - hydrate is easier to gauge in a bucket ..... less messy! Also, with a sealed bag hydrate, you know what you are getting - the quality of putty can be very unpredictable. Don't forget that aggregate choice is important too - try and use the sharpest that you can get your hands on; my local pit does brilliant sharp that is 5mm down and well graded (4 leaf aggregates, carbrooke, Norfolk)

 

[livias]

Hydrated is cheap and is fine for general work (though not for my pargeting) but it has he one defect that like gypsum but unlike putty, it must not be allowed to get damp, so buy from somewhere wih a good turnover, check the manufacturing date and discard half used bags

 

[formerly known as]

That was certainly the case when hydrate was sold in paper bags, but now it is sold in paper bags with a plastic liner. Anything which isn't used is either sealed in a 40 gallon sealed container, made to putty, limewash or for limewater.

 

[88v8]

Yes, I'd viewed hydrated as 'inferior' and have always (always, huh, I've only been into lime since 2011, previously it was a cocktail ingredient) always used hydraulic.
There's an internal wall I'd like to plaster, but I want the finish to be quite swoopy with hills and vales following the dip and peak of the underlying stone.
I envisage achieving this by bagging, but how to put on a second coat I've no idea. So your one coat mix is attractive.
Did you wood-float it, or bag it?

Ivor

 

[formerly known as]

applied with normal float, then scrubbed with a plastic cross grain float ................. final light sweep with a sponge - a nice uniform open texture, ideal for limewash. You can only see the odd bit of flattened straw.

 

[malcolm]

Super, thanks. I shall purchase a bag of this new fangled hydrated lime and have a play. It sounds ideal for internal plastering. I like the straw idea for the base coat too - no problem of rot on an internal wall and much more convenient than shaving horses in the dead of night.

 

[formerly known as]

and for external - wattle and daub / clay lump survives and it is full of barley straw.

you will not be dissapointed!

 

[xanthe]

Where do you get chopped straw? And what do you mix it in/with?

Could you use chopped hemp instead? Would it make any difference to insulation properties using one or the other do you think?

 

[Roger H]

Where do you get chopped straw?

Maybe run over some with a lawnmover? (on a hard surface, not lawn).

 

[Zebra]

Whenever I buy a bag of hydrated lime, I make it into putty straight away. A bag makes about three paint tubs-full. Covered with water and with lids on, it keeps indefinately and improves with age. If you want to use it straight away then it needs to be made of a stiff consistency, I find. If you are going to store it a while, it can be made runnier and allowed to settle.
Whether or not it makes a suitable mortar or plaster, compared to hydraulic lime, depends on the softness and porosity of the brick/stone, surely?

 

[Flyfisher]

I've never quite understood the 'improves with age' thing when it comes to lime putty. What's the mechanism for any improvement? As I understand things, the only difference between hydrated lime and putty is the amount of water. Chemically, both are calcium hydroxide waiting to be carbonated to calcium carbonate, as per the lime cycle. I suspect that hydrated lime was not used historically because the precise manufacturing process required to add just the right amount of water to slake quicklime was not easy or possible back then and it was far simpler just to add an excess of water and end up with lime putty - which is perfectly useable anyway.

These days, the manufacturing processes are more precise and controllable and it's more convenient to ship pure hydrate around the country rather than waste energy transporting an excess of water with it. But, hydrate or putty, the end result is surely the same?

Personally, I buy hydrate and mix it 2:1 (on advice from our CO) with sharp sand for mortar work and it seems to work well as long as the bricks are well damped before laying and the wall is kept damp by spraying for a few days. If I have any hydrate left over then I'll make it into putty in the usual way so it will keep until needed. I can't honestly say I've noticed any difference in use or performance between hydrate or putty based mortar mixes, but then I'm probably not expert enough to notice the difference. My experience has been that spraying the work to keep it damp for a few days has been the biggest factor in preventing failure.

 

[livias]

Chemically, the 'improves with age" feature is quicklime converting to calcium hydroxide a bit more. Most of the conversion happens quickly but there is a tail off effect and lime putty which is old has had time for more of the reaction to occur. Hydrated lime doesn't get a chance to continue the process. For general purpose plastering this is no great problem cos enough of the conversion has happened for it to be perfectly useable

I have heard of but never seen ... 'popping' of the plaster surface on the wall when lime has been used too young

Some people mix up hydrated lime mortar a day or two in advance to give it time for a bit more converting. I'm not sure if this is a useful thing to do or not. Perhaps someone like formerlyknownas can comment?

My pargeting work and anyone who is doing detailed or fine work needs lime mortar which is very plastic and very even in consistency, so ageing is vital. I tend to use mortar which is about a year old but mostly because each spring I say "it is lime buying season again"

 

[malcolm]

How do you mix lime putty with sand? Do you just mix for a very long time?

For the powdered stuff I tend to pre-mix the lime with the sand to make sure everything is evenly distributed and then add the water. I prefer a fairly stiff mix for pointing and plastering and wouldn't rate my chances of evenly mixing in any further sand to that mix.

 

[xanthe]

Can you buy thing pre-mixed and just use them straight out of the bucket/tin? Just wondering if I need to find space in the shed/cellar for sacks of sand and lime...

Also what do you mix things in? A cement mixer or just a big bucket?

 

[Flyfisher]

Chemically, the 'improves with age" feature is quicklime converting to calcium hydroxide a bit more. Most of the conversion happens quickly but there is a tail off effect and lime putty which is old has had time for more of the reaction to occur. Hydrated lime doesn't get a chance to continue the process. For general purpose plastering this is no great problem cos enough of the conversion has happened for it to be perfectly useable

Hmm, though doesn't that imply that hydrated lime is somewhat impure if it contains appreciable quantities of quicklime? Given that it's a very large-scale industrial process these days it seems a bit surprising it would be so inefficient.

[does a bit of googling . . . . ]


Double hmm. This datasheet for 'Hydralime' (which is what I usually buy from my local Jewson) states 'Available lime, as Ca(OH)2, 95-97%' though it doesn't give an actual figure for quicklime (CaO) content. http://assets.jewson.co.uk/product-docs/8390/8390.pdf

Even assuming up to 5% quicklime in hydrated lime, how long would that take to be fully slaked when mixing from dry in a mixer? Having to leave it for a year seems a bit extreme - and would the workers of old have done this?

Interesting!

 

[Flyfisher]

How do you mix lime putty with sand? Do you just mix for a very long time?

For the powdered stuff I tend to pre-mix the lime with the sand to make sure everything is evenly distributed and then add the water. I prefer a fairly stiff mix for pointing and plastering and wouldn't rate my chances of evenly mixing in any further sand to that mix.

That's what I do with hydrate, but even after thorough dry mixing (at least 30 mins) I still give it about another hour after adding the water. It means it has to be (or should be!) the first job of the day and gives me plenty of time to get everything else together.

I rarely use previously stored putty for mixing mortar as I usually use it for limewash, but when I have I just leave the mixer running as long as it needs, as judged by my inexpert eye. My feeling is that this is likely to be at least as good, and probably far better, than a hand-shovelled mix from 'the good old days'.

 

[88v8]

If plastering with hydraulic lime, wet wall until very wet, and keep damp while proceeding.
Is this how it is with hydrate?

I too would never add sand to a wet mix. Was alarmed to see my builder doing that with cement mortar and even worse, adding cement to wet mix. Some of his mortar has proved very weak.

As regards ageing of putty, I have read that for quality work it was usual to make it up months in advance.

Ivor