Urgent answer needed for render mix

[michael]

I need to mix up a hydraulic lime render today, for exterior use and to match in with existing. I’m proposing to use three parts washed plastering sand to one part (3.5) NHL with a blob of lime putty thrown into the mix to act as a plastisizer. I also want to add some coal dust as a colorant to try and match the existing and prevent the repair looking to obvious. Should I be removing a measured amount of sand from the mix in proportion to the amount of coal dust being added or just add the coal dust into the 3 to 1?

Many thanks and sorry for the urgency

 

[GRB]

Hi Michael

I would reduce the amount of sand proportionate to the amount of coal, assuming the 'dust' actually has bits equal to the size of the sand and would be substantial enough to form part of the aggregate. If it is just dust then probably not - although dust may speed up the initial set as it did for me (although I was using putty). Is the sand well-graded sharp sand? If you do put the putty into the mix I would be interested to know how it turns out... Good luck

 

[MdB]

NHL + water makes putty (that will set) so adding normal putty isn't going to make a great deal of difference. Won't harm though.

 

[michael]

I’ll be using Westerham washed plastering sand which is a local product in the Kent /East Sussex area and seems to be used buy all the lime people around here and supplied by a local slaked lime producer/ hydraulic lime supplier. It’s not particularly gravely though. I’ve used it for my scratch coat (without coal dust colorant) and it seems to have adhered and set quite well. I also put a dollop of lime putty into the mix.

As far as the coal dust is concerned, I’ve been sifting this through a sieve ready to add to my top coat mix as I didn’t want small black crumbs in the mix but perhaps I should throw a few of these crumbs back in. what do you think?

I understand that by putting a bit of lime putty into the mix, it keeps the mortar more moist and workable even though I am spraying down the surface (brickwork/scratch coat) before application. As mentioned above, the scratch coat has set quite well over the weekend even with the dollop of putty in the mix.

I have noticed that hydraulic lime from different producers also affects the colour of the mortar. The lime suppliers I use have two makes of hydraulic lime: Singleton Birch and Hanson castle. Used with the Westerham sand which is quite orange, the setting colour of the Singleton Birch mix remains quite orange whilst the Hanson Castle mix sets to a more creamy bough colour. I’ll be using the latter but I want to try and get some sooty blacks and greys in there as well to match existing old render
I wonder if I could also rub some coal dust into the freshly rendered surface with a paint brush a bit like fresh plaster frescoing or would this just weather away?

Anyway if I understand correctly, no reduction in sand if only dust is used and a proportional reduction in sand if gritty coal dust is used?

 

[MdB]

I understand that by putting a bit of lime putty into the mix, it keeps the mortar more moist and workable

I'm not sure how that would work. It is the same stuff. If you put a lot in then you would be weakening the mix a bit i.e. NHL 3.5 would turn into NHL 2 but a small amount wouldn't make any difference. Moisture retention, workability will be unaffected. Could be wrong though.

 

[GRB]

If you are adding the coal dust as a colourant then I would add it into the mix, to make a tinted render, as this will be consistent throughout if the surface does weather. I personally wouldn't reduce the sand content either or put non-hydraulic putty in with NHL, as this would alter the lime to sand ratio.

 

[michael]

Is it the same stuff? I thought that hydraulic, the strength of which is determined by its pozzolins (feebly, moderately or eminently) sets in water and hydrate or lime putty sets in air purely by carbonation.
Or maybe not. I’m still confused after all these years.

Interestingly I notice that in answer to KeithJ’s question “lime to sand mix ratio” Penners reference to:-

http://www.periodpropertyshop.co.uk/aca ... ering.html

And the guidance chart to choosing the right lime within:-

http://www.periodpropertyshop.co.uk/acatalog/LimeGuide.pdf

All references within the chart are to hydraulic lime with the exception of the blue category which they call coarse stuff. Dose coarse stuff mean lime hydrate /lime putty and, as they suggest, can it only be used on old soft brick and stone on sheltered sites?

I have an old soft brick property in an exposed location and the chart suggests that I should be using either feebly or moderately hydraulic lime which I understand to mean NHL2 or NHL 3.5

 

[MdB]

Hydrated lime / lime putty will only set through carbination. NHL is mainly the same stuff but also has pozzolins in it which cause it to set chemically as well as through carbination. The chemical set is stronger than the carbination so if you add hydrate / putty to NHL then you are just diluting the pozzolins a bit so hence a bit less strong a set.

It all really doesn't matter that much. You may be interested to know that the error margins on NHL specifications are so large it is entirely possible to buy a bag labelled NHL 2 (feeble), NHL 3.5 (moderate) and NHL 5 (strong) and the three bags actually have the same stuff in all of them...

 

[GRB]

Hydraulic lime sets with water and non-hydraulic lime does indeed carbonate. Lime putty usually refers to non-hydraulic lime which is quicklime slaked with enough water to form the putty (like cream cheese) - it is now ready to use but apparently best matured. I think hydraulic lime is slaked only enough to form the powder and then starts to set when water is added to make the mix - NHL or natural hydraulic lime does come in the 3 strengths, and this depends on the content of clay and other impurities within it. Pozzolan is added to non-hydraulic lime putty mixes to give it an initial set similar to NHL.

The coarse stuff mentioned on the link is for Limetec's (presumably a brand name for the limes products by Lime Technology?) and will be a non-hydraulic lime putty to sharp sand pre-mix (usually 1:3). I rendered the gable of our house in non-hydraulic lime (1:3) and this is on an exposed face in the Durham dales and it seems to have done the job. The front has old render (poss. original 1750's) which was presumably made with a much poorer quality non-hydraulic lime putty (or even hot lime mix judging by all of the bits of un-burnt limestone in it) and this is still doing the job.

 

[michael]

Just doing some experimental mixes to try and achieve the right colour. These comprise of:-

(a) 3 parts sand, 1 part coal dust, 1 part NHL 3.5
(b) 3 parts sand, 2 parts coal dust, 1 part NHL 3.5
(c) 3 parts sand, 3 parts coal dust, 1 part NHL 3.5
(d) 3 parts sand, 4 parts coal dust, 1 part NHL 3.5

Obviously I need to let these samples dry out overnight to get some idea of what the resulting colour might be and allow for it to get even lighter when completely dry but nevertheless you can see from the above that a considerable amount of coal dust is likely to be needed in the mix to achieve the desired shade which has to be like an old dirty cement even though it is hydraulic lime.

Considering the amount of coal dust needed do you still think that I should be maintaining the 3 parts sand or should I be reducing this.
Ok the colourant is only dust but it still seems to amount to a lot of aggregate when used in the above quantities

 

[MdB]

That looks like an awful lot of coal dust to me. Is the current stuff really that colour or does it just need a good clean?

With that amount of additional material you really need to increase the amount of lime and I would also confirm with the supplier that it was a suitable additive. Coal dust may do something unexpected to the set - I don't know either way but it is something I would want to check.

 

[michael]

The area that needs patch repairing is not a flat rendered wall, its rendered detail on the tympanum of a gable end together with two rendered areas at each end of a brick wall. The existing render colour is a mixture of grey with patches of sooty black and white mottled areas together with kaki green and patches of sandy camel colour It’s generally mottled and old and has no doubt undergone repairs during its 190 year life. Uncoloured white or light sand lime render repairs would stand out like a sore thumb so I need to shade them down considerably.
I’ll see how my test mixes look tomorrow. Do you think 3 sand 1 lime to ½ coal dust would be an acceptable mix?

 

[michael]

I think that what I probably need is a Roman cement similar to that developed by James Parker in the 1780s,

http://en.wikipedia.org/wiki/Roman_cement

Does anybody know if it’s still possible to get something like this?

 

[Keithj]

I would be very wary of adding coal dust to lime. Coal dust is largely carbon, which will presumably do interesting things to the carbonation process of the lime. At least, speak to one of the expert lime suppliers first.

My inclination would be to use unadulterated lime render, and then to apply several coats of limewash in the colour you want for the final appearance. You would expect to be limewashing the exterior every few years anyway, I would expect.

 

[michael]

Assuming that I can no longer obtain James Parkers ‘roman cement’ i.e. natural cement produced from ‘marl’ otherwise known as cement stone , see:-

http://en.wikipedia.org/wiki/Roman_cement

Then it seems I have two options and I would appreciate a vote on these options from you learned people.

(a) Following my test mixes, the best coloured mortar for my purposes would be
3 sand, 2 coal dust, 1 NHL (3.5) however from the comments above, some doubt has been cast on the strength and stability of the mix.

(b) Render with a normal mix of 3 sand to 1 NHL (3.5) and then paint to disguise the repairs. The area is inaccessible and requires scaffolding so limewash would be impractical from a maintenance point of view. I would therefore propose using an Earthborn silicate masonary paint Ref:-

http://www.earthbornpaints.co.uk/silicate-masonry-paint.php

Votes or other alternatives appreciated.

 

[Schoolmarm]

Use pigments in powder form to dye the mix. A little goes a long way and won't spoil the strength. There is a black one.

 

[michael]

Thanks schoolmarm. Nice idea
Unfortunately I’ve got quite a way to travel to get the pigments and not much time. Scaffold has to come down.
Do you know if adding a trowel full of top soil (earth) into the mix would darken the colour and affect the strength/ stability of a hydraulic lime mix?

I seem to remember earth being mentioned on this site as a component of some old lime mixes

If I end up having to use just a straight forward 3/1 sand lime mix, I will also be considering dabbing some coal dust onto the wet render surface and painting just the repaired parts with a watery mud and yogurt mix to encourage algae growth and an aged effect.(good idea or not?) All I really need to do is dull down the stark brightness of new lime plaster but dulling the finish without affecting the strength is proving difficult. If only I could walk round the corner to look at the pigments.

 

[GRB]

It's a shame about the time constraints - scaffolding is far too expensive and many a job has had to be rushed to save on the hire! The pigment sounds like the best solution as the colour will remain constant. Off the top of my head I know Rose of Jericho used to do some mid and darkish greys, and they can deliver fairly quickly.

 

[Schoolmarm]

Have you got a cast iron fire that you burn coal on? Or has a neighbour? There is usually lots of soot inside at the top, If you can try and capture some in a container that would be good.

 

[michael]

I’ve got a wood burner with a layer of wood ash sitting in the bottom. Don’t know if that would work?
Anyway, I’m off to 'Chalk Down Lime' early tomorrow morning

http://www.chalkdownlime.co.uk/

our ‘local ,,ish’ lime suppliers/slaked lime producers to look at pigments and to take along some samples of the render I’ve cut away. Hopefully they will be able to suggest a good mix to match the existing.
Speaking to them on the phone today they also advised that they have a couple of bags of roman cement in stock so fingers crossed ,I hope to come up with the right solution.