Is NHL mortar too strong?

[plasticpigeon]

Today I have been grinding/chiseling mortar out of a section of wall in order to repoint. I am hoping my pointing is up to scratch now. Some of the wall had rock hard cement in it, and other bits had some NHL 3.5 pointing that I did last year. This pointing wasn't the best as I was (and still am) learning. I am taking it all out to do it again. Unsurprisingly the cement was hard to get out. The best method I found was to grind a channel down the middle and try and lightly tap the edges where it adhered to the brick. The method works ok for the most part. More surprising to me was how hard the NHL mortar had got. When grinding there was almost no resistance, but when chiseling I found that it was about as easy to gouge out the mortar as to gouge a brick, meaning that the NHL mortar was about as hard as my bricks. Although not as brittle as cement, in places it did crack rather than crumble. The original lime is very much weaker. I hate to admit this, but Andrew Bodenham did warn me of this in a post a few months back!!!!

I think I want to modify my mortar mix, but what to? I was using standard 3 sand 1 NHL, but I was thinking that 2 possibilities are 4 sand to 1NHL, or even 1 NHL, 1 hydrated lime and 6 sand a bit like 1:1:6 mortar except using NHL instead of cement. I'd be grateful if anyone has tried this or has any ideas.

P.S. I know I could use putty but it is not available near me, and I appreciate the faster set of NHL.

 

[Nigel Watts]

NHL comes in different strengths. Which one were you using?

 

[skier-hughes]

Try using NHL 2.

 

[LadyArowana]

Tru using marzipan or thick chocolate sauce ..... you will find out very quickly that it won't work and can then justify a teabreak to use up the extra materials.

 

[Zebra]

Plasticpigeon, I've found the same thing, that pointing I did in NHL 3.5 on my chimney 18 months ago, or on my DPC last summer, has set harder than I expected. Also I did an experimental patch of NHL3.5 render on my new extension, and had to chip it off yesterday ready for the proper render, and it was stuck really hard and took a bit of the blockwork away with it, I was quite surprised. I used either a ready mixed mortar from the Old House Store or a mix of 1.5 parts soft sand; 1.5 parts sharp sand; 1 part NHL 3.5.
However, I don't think the mortar is trapping water in the brick in the same way that cement would, so I am leaving it where it is. I chose 3.5 on the advice of the Old House Store because they said that a chimney requires a stronger mix because of wind loading - to use it for the dpc was my own choice. Also where I've dismantled my repointed brickwork again the mortar did give way before the brick, so hopefully any movement in the building would make the mortar shift and not crack the brick.
Elsewhere on the old parts of the house I have used a putty based mortar and render (either ready mixed or 2 soft; 1 sharp; 1 putty) and this has dried very soft - perhaps too soft really although the (17th/18th centrury?) bricks are very soft so I'm sure it's the right thing where its not quite so structurally important. The render is very easy to damage and the mortar very easy to crumble away. So perhaps a putty based mortar would be too soft.

 

[plasticpigeon]

Hi Zebra, that's interesting, I don't think that water was being trapped in my bricks either. In fact I can sometimes see at the base of the wall below dpc that the bricks pointed in lime and the lime mortar look dry, while the cement pointed bricks look a bit wet (darker colour). I was thinking that if there was any small movement that it would be better if the mortar was definitely weaker than the brick.

Nigel, I was using NHL 3.5 from Roundtower.

 

[DJH]

Have you tried using a 10 mm wide disc in your grinder? They have them on sale in Jewsons etc to fit the 4" or 110 mm grinder and I used them for cutting chases in joints to fix flashings in. They remove a lot of mortar and make it easier to chip out the remaining stuff.

Doug

 

[tobydog]

PP - it's interesting you mention Andrew's earlier advice because that's been at the back of my mind for some time now. I have some pointing to do shortly (if the weather ever warms up!) and I was worried about how hard NHL 3.5 was even in a 1:3 mix. I used it inside to rebuild parts of the plinth because it's likely to stay damp for some time. Having said this, the mix is still drawing out the moisture and doing it's job.

Andrew recommended lime putty which is what I planned to use going forward on the drier parts. I will probably get it pre-mixed in the same ratio but I've heard that in it's normal state it can be too wet and that it's best to let it dry a bit before using it for pointing. Does this work okay?.

NHL 2 has also been suggested. Would that actually be just as good a using putty for pointing?.

 

[MdB]

Andrew recommended lime putty which is what I planned to use going forward on the drier parts. I will probably get it pre-mixed in the same ratio but I've heard that in it's normal state it can be too wet and that it's best to let it dry a bit before using it for pointing. Does this work okay?.

Let it stand before use in a sealed tub. This allows the excess water to pool at the top where it can easily be poured off. Remix and potentially add some liquid back if too dry. I normally had to do this when mixing up from putty when using wet sand as the resultant mix was too wet. A day or two of standing is normally enough.

I wouldn't let it air dry as this will allow carbonation to potentially start.

 

[plasticpigeon]

I have tried an experiment by using 1 NHL:1 hyrdrated lime:6 sand on some non structural brickwork inside. The mix was totally different to use than 1:3 with NHL. Even when the mix was very dry it was still very plastic and sticky. With straight NHL it was not putty like or malleable at all, and tended to be either wet or very solid and not very workable. I don't know if this new mix is stromg enough, but I messed about and can make my pointing look really nice with the hydrated lime added as it is so much nicer to use. I think I will carry on with this, especially as it is so much easier to do nice pointing with it and I would put money on it being as strong as the original lime, which has lumps of white lime dotted through it!!!

 

[andrew gayton]

In a word 'yes' - someone once said 'but all historic limes were hydraulic' ............ maybe in certain instances they were made slightly hydraulic via contamination from aggregates and the slaking process, but thats about it (aside from intentionaly adding brick and stone dust) and no where near as strong as todays limes which are hydraulic because of the limestone used for the quicklime.

1 part non hydraulic and 2 parts sand has always served me very well for pointing.

 

[Roger H]

Where would hydrated (bagged) lime sit on the scale of hardness?

 

[andrew gayton]

just as 'hard' as putty lime.

I could show you hydrate lime mortar that is spot on. The outside of my flint house being one. Sand is an equally important consideration - a rough guide being that the sharpest particle size needs to be at least a third the size of the joint that is being pointed ........................... graded 'sharp' sand is typically 3mm down to 150 microns.

 

[plasticpigeon]

Hi Andrew, was it you that flagged up NHL being too strong before, if so I'm sorry I got your name wrong.

 

[andrew gayton]

Hello P.P - it might well have been me ............ I have said this sort of thing more than once, although hopefully there is someone else saying similar! Another thing worth remembering regarding historic quicklimes - quite often they were from burnt chalk (as opposed to a 'harder' form of limestone) and therefore weaker still. It could be argued therefore that in certain instances even using a modern non hydraulic based lime is going to be stronger than the primary/early build. Each case on its merits, but great care is needed in selecting the right mortar. Saying that, if required - say for chimneys and the like - I would far rather specify/use a composite mortar than an NHL one.

 

[worms]

P.S. I know I could use putty but it is not available near me, and I appreciate the faster set of NHL.

I take the point about faster set... But why people pay to have water shipped about the country by buying pre-made putty defeats me! But then, nowadays people also buy ready-diluted screen-wash and pancake batter, so I guess anything is possible!

 

[andrew gayton]

P.S. I know I could use putty but it is not available near me, and I appreciate the faster set of NHL.

I take the point about faster set... But why people pay to have water shipped about the country by buying pre-made putty defeats me! But then, nowadays people also buy ready-diluted screen-wash and pancake batter, so I guess anything is possible!

if this forum had a 'like this' button, I would have used it for this.

 

[tobydog]

P.S. I know I could use putty but it is not available near me, and I appreciate the faster set of NHL.

I take the point about faster set... But why people pay to have water shipped about the country by buying pre-made putty defeats me! But then, nowadays people also buy ready-diluted screen-wash and pancake batter, so I guess anything is possible!

if this forum had a 'like this' button, I would have used it for this.

I think the pre-mix tub world, apart from the convenience for use and storage, is good for those still lacking in confidence in what they are doing, and what they are doing it with. I have mixed NHL 3.5 with sharp sand in a 1:3 ratio to date and had mixed results ( :? ) but I know from this forum and other threads that putty is probably better going forward for the repair areas I will be working on. I plan to pick up a few tubs of pre-mix putty from Anglia Lime next time I'm in Sudbury but I certainly wouldn't pay to have it delivered. I suppose I would rather wait to get the putty than continue now with the NHL.

BTW - I buy concentrated screen wash but I'm not too good at making pancakes. Apparently the batter comes out like excellent well matured putty .