Crumbling new Lime Mortar Pointing

[ihatetripe]

Hi Everyone

I am renovating brickwork on my chimney breast. I have raked out the old mortar, and am repointing with some ready mixed Rough Aggregate 1:3 mix lime mortar.

Now, I left some lines too long before buffing the lime to get the nice colour and show the aggregate in the mix. Because of this I had to really rub hard with a brush to get the top layer off. To my dismay, after getting through a hardened top crust, the underlying new pointing just turned to loose sand.

OK I thought, maybe I got my pointing wrong, wet the joint too much, maybe not enough, so I left a bit of the mortar out to dry on a board. A lump of the lime mortar (after about 1 week drying) about the size of a two pence piece just crumbled again to sand when I crushed it between my fingers.

Am I doing something completely wrong here. I know its weaker than cement mortar, I didn't expect it to be this weak! Have I got a duff batch maybe?

Thanks in advance for any replies.

James

[biff]

It is important to understand how lime mortar works. The lime is calcium hydroxide. It reacts chemically with carbon dioxide in the air to form calcium carbonate. This is the hard, strong mineral that you want. It is what limestone is made of. The reaction proceeds slowly in the presence of air and moisture. Your hardened top crust shows that it is happening but it takes weeks, even months, to complete the process. Your mortar will be much stronger next year if you leave it alone. You probably have done nothing wrong except shown a bit of impatience.

One of the many advantages of lime mortar is that it absorbs CO2 from the atmosphere and requires lower temperature in kilning so contributes less to global warming than Ordinary Portland cement.

[Rick]

We intend to use a similar techniqe to point our hosue, so have been expirimenting to see what happens.

It takes a good while for the mortar to set all the way through, after a couple of weeks we can see its on its way, but we expect it to take somewhat longer to completly set.

Rick

[Andrew Gabriel]

Well, it absorbs exactly the same amount it gave off whilst being calcined (burned) during production (ignoring any produced by the energy required to heat it). It starts off as calcium carbonate in the ground, and it ends up as calcium carbonate in your walls, but reshaped to fit precisely around your bricks.

[Stuart Noble]

Perhaps you could dig some out with your little finger to test the quality :-)

[Stuart Noble]

I think you have discovered the hard way that lime mortar is totally useless for re-pointing. I would guess that where it is effective the bulk of the mortar never dries fully due to the skin formed by the initial set. In thin layers the whole lot reverts to chalk and sand, and my hunch is it won't improve, however long you leave it.

[<me9]

But it can take hundreds of years to do so. It will also dissolve in acid rain.

[biff]

Please Stuart, don't give such misinformation. Lime mortar is definately the most suitable material for repointing brick and stone buildings that were built before the introduction of Ordinary Portland cement. That means almost all building more than 100 years old and a good many younger ones. Lime mortar does not 'dry', rather it sets by carbonation of the calcium hydroxide to calcium carbonate, a process that takes weeks or months. If you don't believe me because you don't trust me as an expert in this area then please consult English Heritage, The Society for Protection of Ancient Buildings, The National Trust, any local authority Conservation Officer or any of the other organizations whose business is the protection of our built heritage. Any of these will tell you that using Portland cement to repoint old buildings presents many problems including irreversible damage. There would, potentially, be cases where using Portland cement, instead of lime, to repoint a listed building could constitute a criminal offence.

It is hard to stress strongly enough that Stuart's advice should not be heeded.

[John Rumm]

Leave it long enough and it will work just fine...

[BigWallop]

Having never used lime mortar myself, but hearing the different site guys talking about it on renovations and things, is it true that it's supposed to mix more to the consistency of putty rather than just normal mortar?

[Stuart Noble]

That was a party political broadcast on behalf of the Taliban. I hope James keeps us informed of the progress of his lime mortar as it carbonates over the years. I suspect it will continue to crumble between his fingers as it does now.

[bigcat]

Is this IMM? If not, theyll probably gang up.

For the OP, lime mortar should idally be kept damp for a few weeks to hasten its set. It can be covered with sheeting, or sprinkled.

NT

[andrewpreece]

No, when you mix up lime with water you get lime putty, which does rather have the consistency of putty, but when that has soaked for a while, you then mix the lime putty with sand to get 'coarse stuff', which behaves exactly like mortar.

Andy.

[andrewpreece]

Having used lime mortar, I can testify to its reluctance to set, but give it about six weeks and the outer surface will have set sufficiently that you cannot scratch it away with a fingernail. For comparison cement mortar reaches 50% strength in about 3 days.

Andy.

[Stuart Noble]

How would keeping it damp hasten its cure? I think you cover it until the surface has cured/carbonated/dried sufficiently to withstand rain. Not exactly a practical proposition, which is presumably why hydraulic lime was used. Maybe the sand also has a pozzolan effect. How would you ever get a house built otherwise?

[BigWallop]

So a mixture with too much sand is not a good thing either then? I thought it had to stay as a putty consistency to be any good for pointing, which means that the sand and lime mixture had to be equal in the mix. I like to learn these things. :-)

[bigcat]

At least youre now accepting you dont know the basics of lime mortar. Thats a good start. Now you can find out. Google is your friend.

NT

[Stuart Noble]

I'll take that as a don't know then?

[biff]

Stuart, your comment about Taliban is in extremely bad taste. Please try to learn about lime mortars instead of attempting to be insulting.

BigWallap, you wouldn't want an equal lime sand mix. There should be just enough, but no more, lime to fill the spaces between the sand grains. This is a bit tricky to work out because it depends on the size of sand grains and, especially, whether the sand grains are all the same size or have a range of grain sizes. In practice you won't be far from the ideal with a 3:1 sand:lime mix for mortar. For the top coat of lime plaster a mix with more lime, less sand can be used as it's then easier to get a smooth finish and strength is not so important.

[Mike]

It behaves similarly but mixed correctly lime mortar for pointing feels different to Portland mortar - more 'fatty' for want of a better word.