How come my lime mortar comes out gray/brown?

[MdB]

There are some products with lime in the title that also have lots of other things in them (often cement) so they aren't the best. However, the hydralime data sheet http://www.lafarge.co.uk/CementDatasheet/Hydralime.pdf gives the following:

Hydralime is manufactured by the following method:
• Calcining limestone or chalk in a kiln at high temperature to produce quicklime.
• Hydrating (slaking) the quicklime under carefully controlled conditions to produce a fine, dry, white,
calcium hydroxide powder.

This exactly what you want.

Also:

Available lime, as Ca(OH)2 (%) 95.0 to 97.0

So of the bag, 95% is usable lime. Again this is exactly what you want.

Now lime doesn't give you anything like the strength of cement mortars so there is a danger of people using it for unsuitable building jobs where mortar strength is important. The manufacturer therefore cover their arses with statements that it isn't suitable for mortar without cement. Some do, some don't but it all the same stuff. If it is the wrong stuff then various others here and myself are using the wrong stuff too.

The only thing I think you are doing wrong is the mix. 3:1 sand:hydrate is about the same as 4.5:1 sandutty which is rather a weak mix. I use 2:1 sand:hydrate. I am confidant that this is OK as if I mix 2 buckets of sand and 1 of hydrate, I only end up with 2 buckets of mortar. i.e. the lime has only filled the gaps between the sand with nothing spare. With 3:1 there will still be gaps between the sand grains. I wouldn't worry too much though. Its certainly not something to redo.

 

[Zebra]

Thank you so much. I feel much better. I will alter the mix strength as you say.
However, I phoned Old House Store to ask the same question and they did say that Hydrated Lime is inferior to Hydraulic, and what they use in their ready mixed mortar is Hydraulic...
Just one other thing:

Now lime doesn't give you anything like the strength of cement mortars so there is a danger of people using it for unsuitable building jobs where mortar strength is important.

Where I have had to repoint the south face of my chimney, what I am pulling out is half a centimetre of cement, then nothing but powder all the way through the wall. To avoid the chimney falling down I am doing one course per day!! (The north and west faces are OK, existing mortar is firm and solid.) Clearly here mortar strength is important. Is my existing mix still OK?
Thanks so much, glad I don't have to do it all again.

 

[Zebra]

I use 2:1 sand:hydrate.

What type of sand do you use? I've tried all sharp sand. I've even tried kiln dried. What is the important factor, is it the amount of grit, or the ratio of big grains to small ones (using the technical terminology)?

 

[MdB]

Hydrated lime is weaker than hydraulic (I believe it it would count as NHL 1 if it was hydraulic) but it is more flexible so deals with movement better. It also only sets via carbonation so the surface gets stronger before the lower layers. For these reasons it is ideal for pointing and plastering but not the best for building a wall for instance as the centre won't carbonate for years.

I started out using ready mixed mortar (3:1) before mixing my own from putty (3:1). I then moved on to making my own putty from hydrate before turning it into mortar (3:1). Next was mortar made directly from hydrate (3:1) again until I realised that the hydrate shrinks by around 30% when water is added so I now make mortar from hydrate in a 2:1 ratio. Of all the different ways I've made it, I can't say I've noticed any significant difference in the resultant mortar nor any differences in what has been applied. The more expert lime people here claim that the putty based stuff is better though and they should know. I just haven't seen it myself.

All ready mixed lime is non hydraulic lime as wet hydraulic lime would just set in the bag. I guess there was a bit of miss communication with the Old House Store. What they probably said was lime putty based mortar was better than hydrated lime based mortar (Mike Wye states this also). This is possibly the case but as above I haven't noticed it myself - it could also be a marketing exercise... There is a danger of hydrate carbonating in the bag which is why I always turn a complete bag into either mortar or putty when I open it, so this is one reason against hydrate.

Sand wise others have pointed out that the sand that would have been used historically would have been the sand nearest to hand - most likely from the hole in the ground a couple of hundred yards away. They might have graded it if feeling keen but probably just used it as is so don't get too hung up on the type of sand. I use completely the wrong stuff (normal builders sand) as I'm not looking for a great deal of strength for the pointing and have had no problems so far with what I've done. A more graded sand of different size grains is normally what is recommended.

 

[Zebra]

Wow! Thanks so much. There's some experience talking! I haven't done any more replacing mortar this week, too busy digging French drain, but very glad of the reassurance. Thanks.