Hi
I need to patch an area of render that I believe is lime based. What's the best way to go about it?
My local builders merchant has both hydraulic and hydrated lime.
How much sand will I need to add?
Will I need to add hair or is that only for internal plasters?
TIA
Nick Brooks
If the good stuff is quite hard then it will likely be a mix of lime cement and sand probably 1:1:6. If it is pale and soft (what colour sand is it?) it will be sand and lime, something like 4 or 5 to 1.
No hair is use these days. I'm not sure what hydraulic lime is. There is quick lime = calcium oxide (not used in building as it is dangerous. I have an idea it is used on playing fields to mark out pitches) and slaked lime = calcium hydroxide -which is a lot safer.
There's a kind of calcium cycle (he says, trying to remember 'O' level Chemistry)...
You start by mining the chalk/limestone, Calcium Carbonate.
Heat Calcium Carbonate to make Calcium Oxide and drive off Carbon dioxide. This is Quicklime (or old name, Burnt Lime), a rather nasty substance.
Add Calcium Oxide to water to form Calcium Hydroxide (and enough heat to boil the water instantly if you're not careful). This is Hydrated Lime or Slaked Lime. This is what you use in Lime mortars, plasters, and renders. (I'm not sure what the difference is between Hydrated Lime and Hydraulic lime -- I suspect they are chemically the same, but one is dryed out and the other is in liquid/paste form due to excess water.)
Calcium Hydroxide absorbs carbon dioxide from the air and turns back into chalk -- this is the lime mortar setting process, and that's why it doesn't set in a couple of hours;-).
This brings you back to what you started with, chalk, except it's now carefully shaped to fit round all your bricks ;-)
A couple of comments on your message Andrew:
It's called the carbon cycle.
Slaked lime kept covered in water, preventing contact with CO2, is the basic ingredient of traditional lime putty sold in tubs and used for mortars, plastering and rendering etc and has a very slow setting rate. Hydrated lime is a fine powder sold in bags like cement. You're right, it is chemically the same as slaked lime, but it has been heated again to dry it out and crushed and sieved to make a fine dry powder. This is used mainly in proportion with cement and sand to make gauged mortar for bricklaying and rendering, and gives greater workability, plasticity than plain cement mortar, and lightens its colour. Hydrated lime can be soaked, stirred and filtered again to make lime putty, but it's a long process and the result is less pure, so that it sets more quickly.
Hydraulic Lime is also sold in powder form. But it has substances called pozzolans added to the lime, which cause it to set quickly, sometimes very quickly, like Plaster of Paris. Pozzolans include things like brick dust or fuel ash. Hydraulic lime gets its name because, unlike lime putty, it will set under water. The Romans discovered this this and used hydraulic lime to build aquaducts.
Anna will be along soon and will probably find something wrong with what I've said! I'm sure she'll have something to say to Mike about hair not being used in plaster any more. :o)
Peter
I found
MBQ
Andrew wrote
LOL. And I haven't visited that little village Upper Ladder for a long time now!
That's a good mechant. A few have one and most have neither. If the render is painted, use the hydraulic outside as it sets quicker. Otherwise you will need to slak the lime, then mix about 1:4 or 5 with the correct colour sharp sand.
No
No complaints what you said looks sound to me :-) I will just add that what you use depends on what surface you are working onto. If its rigid like brick or block then hydraulic is OK and is especially good outdoors cos it is more weather resistant. If its a flexible surface like lath then don't use hydraulic its too hard and brittle & will crack so if you can't get readymade lime putty then make your own with hydrated lime.
3 sharpish sand : 1 lime putty or if you use hydraulic then read the bag.Quite right too! Hair is still used to give tensile strength. That is not necessary for rigid surfaces like brick & block but it is important to add plenty of hair for work onto lath & suchlike. Any strong fibre will give the tensile strength eg hemp, straw but hair is preferred if you want a good finish and hair from a white horse is the most invisible.
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Plaster conservation and lime plaster repair / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Anna Kettle wrote
Out of interest Anne, would you be happy to use synthetic reinforcing fibres (as used in concrete mixes and screeds) instead of hair? This is the sort of thing I mean:
advantage that it doesn't rot so the mortar can be mixed up way in advance. I'm not planning to start using synthetic fibre myself cos lots of my work is on old buildings so I try to keep as close to the original materials as possible
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Plaster conservation and lime plaster repair / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Or go modern and buy some glass cloth or chopped strand mat :D
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