Zen and the art of mortar mixing

[Steve]

Hello.

I am totally confused! I need to do some repointing on my Edwardian semi. From investigation I'm led to believe that lime based mortar would have originally been used. However, there seems to be lots of, often contradictory, information about the type of mortar I should use.

1. Use 3:1 lime mix. Some say hydrated lime mixed dry is OK, others say the hydrated lime needs to be soaked in water for as long as possible, others say hydrated lime is unsuitable and you should use commercial lime putty.

1. Use 1:1:6 cement:lime mix. Or use 1:2:9 cement mix.

2. Sand. Some say use sharp sand, others builders sand, others a mix of the 2.

....and so it goes on.

Simply, the more I search, the more differing opionions I seem to find.

I've taken a picture of the existing mortar -

'd be grateful for some conclusive guidance as how to go about producing a mortar mix to as closely replicate the existing mortar.

Cheers,

Steve.

[andrewpreece]

First off, I am not an expert: with that disclaimer, I shall comment on your questions. I have heard many opinions on the amount of time needed to soak lime prior to use. The longest I have heard to date is 60 days, plus another 28 after you add the sand to make lime! All I can say is it still sets if you mix it with water

24 hours ahead of use, which is what I have done. The hydrated lime mixed with water is called 'lime putty' by the way, whilst I believe that the lime putty with sand added to it is called 'rough stuff' or something daft. Lime putty and rough stuff will keep indefinitely if kept wet and away from the air. It takes about six weeks to set, so be patient. To my knowledge, what goes into commercial lime putty is hydrated lime, that is they take quicklime, CaO, add water to slake it ( standing well back ) then you get hydrated lime, which is probably CaOH2O or similar, which is what you get in a bag of hydated lime. AFAICS, the only difference is the length of time it's been soaking in water, and I'm not entirely sure what reaction is supposed to be taking place when lime putty is just standing around out of contact with the air ( lime mortar sets by reation with carbon dioxide, making calcium carbonate ).

I have used the first of these mixes as well, using white portland cement as opposed to the grey stuff. It works too, and goes off quicker, though whether it is as flexible and permeable as a straight lime mortar I don't know.

I used sharp sand ( silver sand ) as I wanted a white mortar. I have to say that the lime mortar ( I assume it is simple lime mortar and not lime-cement mortar ) used on my house originally looks to have been made with 'lime grit' instead of sharp sand. As I understand it, lime grit is a wide range of sizes of rounded river particles, some of which are quite large ( several mm ). I could not source any so used sharp sand ( silver sand ) as it gave me the colour I wanted. It is not as nice to work/adjust/handle as builders' sand, but I have managed to lay bricks with it.

Looks like your mortar uses lime grit: I have no idea if it is still available. I couldn't find any. How much fidelity do you actually want? If you are going for a perfect match I think you will need to spend some time searching about for grits/sands etc and try a few mixes out. It won't look the same even if you match the original mix, as it has weathered. Brand new lime mortar made with silver sand is going to be a lot whiter than what you have, though it will settle down a bit over a year or two, especially if the rain can get to it. You can get cement colourants to tweak the colour of the mortar if need be, but you'd have to be careful about matching batches.By the way, keep the lime mortat covered for a week whilst setting, as it needs moisture to set.

Andy.

[Andrew Gabriel]

In Victorian/Edwardian times, sand was not so accurately graded as it is today. I suspect the sand in the lime plaster was probably the finest available at the time, and even that has large lumps in it which I've found when taking bits off. That must have made it a real bugger to get flat and polished, but they seem to have managed even so.

A brickie (actually he teaches building, but loves doing brickwork too) at the other end of my terrace said if they're trying to get an authentic sand to match ~1900 properties, they usually do it by mixing various current sand grades together, to get the large variation in particle size which sand used at the time had.

[Stuart Noble]

Chalk being well known for its water resistance anf longevity

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[The Natural Philosopher]

The truth of the matter is that any combination of lime, cement, and sand will more or less set hard and not crumble unless there is a huge amount of sand and not that much of anything else.

A bag of cement will set all by itself.

Sharp sand is griity, and daoesn;t flow as well as bulders, and has bigger spaces that tend to make it more porous if you don't use a lot of cement. Sometimes this is what you want.

Adding lime makes the mixture take l;onger to set, and in a way a full lime mortar never dies set - it remaions slightly crumbly.

Using a lot of cement fills up the holes between the sand completely, making the mortar impervious, but according to another poster, it may crack on setting, nullifying the benefits. Never seen it myself even in lumps of 100% cement.

For brickwork, you want a mortar that is flwable enough to tamp bricks down ion accurately, and stays soft long enough to get a decent line laid, and yet is solid enough not to splump when you lay the next course over it. and is relatively tough and water resistant.

A lot depends on the bricks too - porous commons will suck the mortar dy unless soaked first, and cause it to go fairly solid fairly quickly. Thats someimes useful, as it stabilises the course ready for the next one.

I'd say about 4:1 to 6:1 builders sand to cement makes a decent mix, and you can add lime and subtract cement.

I never know what I mix to. Its pretty much a half bag of cement a half bag of lime and about 20 shovelfuls of sand in the mixer..maybe a squirt of plasticiser or winter mix.

I was laying flagstones on half a bag of cement and 20 shovels of sharp sand. That came out porous, but hard...at 125kg of cement to 850kg of sand..thats about 7:1 I make it, weight wise.

So probably my mortars are 3:1 or 4:1 sand to cement. or 6:1:1 with lime?

Something around that area anyway.

,

.

[The Natural Philosopher]

;)

[wig]

I'm no expert either and did not read Andy's impressive attempt. But check out these guys, if anyone knows lime, they do!

[Andrew Gabriel]

Some previous occupant of my house repointed a couple of areas, very small fortunately, with 100% cement. It resulted in a neat crack line all the way along, and was impossible to remove without damaging the brick faces. You probably wouldn't notice shrinkage in a lump, but force it into a gap and let it set, and you'll notice it shrinks lots.

Some cement:lime ratios are particularly bad though, with the cement failing to add strength and preventing the lime's healing properties from working. 1:1 is safe from this though.

You also need to know if the wall is intended to be one solid item, or if it's designed to move around. Cement mortar is fine for a solid wall, but lime mortar walls are not intended to be solid, and mixing areas of 'solid' cement mortar in to one can cause bad damage to the wall as a whole.

[biff]

There are all sorts of reasons why you should not use any Portland cement on an old house. The arguements between using bagged hydrated lime and wet lime putty are analogous to the difference between a Rolls Royce and a mini. They will both get you there but... When the Victorian and Edwardian houses were built, the quicklime (limestone that has been burnt in a kiln to change the calcium carbonate to calcium oxide) was slaked on the building site and used straight away. It was recognised that leaving the lime putty in the slaking pit for weeks or months improved it but this was not a practical proposition in the housing boom around the turn of the 19th century. The lime was often not very well sieved, hence the large particles of lime found in old mortars. These days we are just too careful with the ingredients and find the rough and ready methods of old difficult to reproduce.

[Mike]

No - doesn't produce a nice 'fat' mix

Not nice to do if the house still has lime mortar intact but does work

No - cracks

Hard one. Best thing is to take a small sample from the wall and have it analysed for a match.

If you google for your nearest "lime putty" centre (MikeWye, Oldhousestore, Bleaklow, etc) they will offer lots of advice.

[bigcat]

There are 2 basic types of mortar: lime, and cement. (Lime & cement mixes are all of the cement type.)

Cement mortars on soft brick cause 3 problems:

1. Bricks breaking due to normal movement. Lime mortars are softer and will crack instead of breaking the brick, which is very much preferable. Also lime mortars self heal by recrystallising across the cracks.
2. deterioration of bricks due to freeze damage. Lime evaporates damp from the wall, cement doesnt, so water at the brick surface freezing causes bricks to slowly disintegrate in some cases.
3. Damp: lime allows the evaporation of damp, cement does to a much lesser extent.

So lime mortar is much preferred, although cement will work.

Cement mortar: there is only one reasonable choice, 1:1:6. 1:2:9 is now known fail prematurely.

Lime mortar:

You can mix hydrated bagged lime into a putty, store it in a closed container for a few weeks and use it 3:1. This is easy and cheap. Other options are buying lime putty or using bagged lime and water without storage. Cover the job for a while so hard rain doesnt wash it out before its hard.

Sand: use whatever the heck you want. From your pic, use the gritty sand, and add some lil bits of chalk if you can find it and want it to look the same. What you have is lime mortar, cment will never match it.

NT

[Elessar]

If you're really stuck

will analyse your existing mortar for £85 (and presumably advise further).

-- LSR

[Anna Kettle]

Time doesn't affect the setting but affects the workability which is how easy it is to make it do what you want

Coarse stuff

Not and not

Have a look round to see what aggregate is available and then if necessary mix various things together to get as good a match to the old as you can. For pointing whether you use sharp or builders sand isn't nearly so important as getting a good colour and texture match using a mixture of large, medium and small grains. The original aggregate probably came from somewhere really local

pointing for as long as you can cos it looks better old

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

01359 230642

[bigcat]

that brings up an important point. Where there is cement, do not remove any sound pointing, only repoint the patches that need it. Removing cment pointing, unless its falling apart, and unless great care is taken, damages bricks and leads t their gradual disintegration.

NT