Lime mortar workability/mix

[Molesy]

Hi Folks

I am having a great deal of difficulty with the cement my local supplier is now providing which I am using to make a lime mortar. I am using this as I have a Victorian terraced house which largely seems to have stayed standing largely due to the 'flexibility'/accomodation of such a mix.

Originally, they were supplying me with Lafarge Mastercrete, which I was mixing 6:1:1 sand, cement, hydraulic lime. Now they have switched to Rugby cement (not the Premium), which seems to have very poor cohesion and workability. I am really struggling to work the stuff into a 'sausage' to make a 'fat' mortar, get it to stay on the trowel or stick to anything much. I expect it should have a butter-like consistency, which I could get with the previous mix. I had previously added some Adabond in error which makes it even better, though I understand you should not add this if you are making a lime mortar.

There does seem to be some confusion between hydraulic and hydrated lime. I have tried the Rugby stuff with hydrated lime as well after their helpline said I should use this and it would work 'no problem', but I got the same problems. The mortar tries to 'fall apart' before I even get it off the mixing board. It may be necessary to make a wetter mix with this material, but I think that is a bad idea. I am going to have 35m2 of re-pointing to do at some stage, and it is looking to be a nightmare unless I can find a solution (I understand some experts can do 70m2 a day!!).

Should I only be using Rugby Premium, or is there something else I am doing wrong? One fella at the merchants said I should use more cement, but it is already coming out with a greenish tinge, which I think is a sign that I am using too much. I am being very careful about measuring the correct proportions.

I am keen to avoid all this standing overnight with lime mixes, although ultimately I'd like to use a solution that works and is practical. Just to mention, for the front elevation I am planning a tinted mix with some ash and lime colourant (for accurate colour consistency) to produce a dark grey colour to set off the facing bricks. I don't know if you think that issue should affect the mix (or procedures) I should be using.

Thanks! -Molesy.

 

[mointainwalker]

I don't know how much mixtures vary between different brands, however when I was doing lime-plastering on my barn here in France, instructions were to use 4 1/2 sand to 1 of lime ( hydraulic I think ), no cement.

I found it easy to use , with no previous experience and it was ready to use straight after mixing.

If mixtures for pointing and other uses are normally very different anyway, then please disregard my comments.

 

[marshman]

If you are using hydraulic lime then you don't usually add cement, and your mix should be about 3&1 with hydraulic lime being the 1
and a semi-sharp to sharp sand. Have a search on hydraulic lime mortar and you'll find some good info on what mix you'll need.

 

[JohnCo]

Agree, hydraulic lime won`t move if you add cement to the mix. Regards JohnC

 

[amwc]

Agree with the above hydraulic lime should only be mixed with sand not cement, 4:1 and should be very paste like. It takes ages to go off but is the best solution with old properties that need a lime mortar. If you are mixing cement mortar you need hydrated lime in a 1:1:6 mix. I have used loads of cements and never had any problems with them unless they are out of date etc.

 

[stuart45]

Although the lime/sand mix for brickwork was thought to be 3/1, recent analysis of the old lime mortars has shown that was more like 1.5/1 or 2/1. The 3/1 is now thought to have been done by weight and not volume. The old mixes also contained pozzolans like brick dust which made the mix slightly hydraulic.
Weaker lime mixes for pointing have often failed. A moderately hydraulic lime is probably you best bet.