Getting the right mortar mix for repointing a 1950's house

[DaveyH]

Hi,

I've been reading a lot about using the correct mix of mortar for repointing. Apparently your mortar should always be softer than your bricks and I can believe this, because looking at some of the houses around the estate there are a lot of blown bricks around on repointed houses, a few houses have also been rendered or partially rendered, I'm guessing they might have done that because of blown bricks. I'm also guessing our bricks are probably classed as soft, even though they don't seem soft to me (they are brittle however and very easily chipped). I've put a few pictures of our bricks (I acquired a few spares from a fella up the road who's old gable end was getting partially demolished when he was having a side extension built).

The original pointing is quite yellow in colour (see pic) and has definitely got small bits in it, so I'm guessing some kind of sharp sand was involved. The original mortar seems to crumble off with minimal effort and is definitely way softer than the bricks.

The fizz test. I've read somewhere that if you take a small sample of your mortar then put white vinegar over it and it fizzes that's an indication that there could be lime in it. I tried it and it did fizz, but not violently.

Utterly confused on how to proceed. As there seems to be a several different types of lime. I've also read that some types of lime mortar over long a period of time can actually end up as hard as cement mixes. I've also read that they stopped using Lime on new builds in the early 1900's, so maybe the fizz test was a false positive for lime.

Any advice gratefully received.

Thank you.

 

[^woody^]

You may be reading too much.

Mortar should be weaker than the bricks not softer - soft mortar crumbles and soaks up water. The performance of the mortar is determined by how it's pointed or ironed, as that gives it it's tough weathering face.

A 1950s house is less likely to have lime mortar, more so if its cavity walls. Lime mortar is made with hydrated lime aka slaked or hydraulic lime

However, common builders powdered lime may have been used as a plasticiser in cement mortar. This can leave white lumps.

The fiz test is not a test of mortar type.

To get a buff mortar you use a yellow sand.

Mix 1:4 for cement mortar, 1:3 for lime mortar.

 

[DaveyH]

You may be reading too much.

I agree. I've been reading far too much and watching far too many videos on youtube. My head is completely mashed with it all.
There's Hydraulic Lime, Natural Hydraulic Lime , Non-Hydraulic Lime, Lime putty, Hot Lime, Cold Lime, I need a bloody lager and lime
. Then there's this mix ratio, that mix ratio, this plasticiser, that plasticiser, It's worse than mist coating new plaster (I never thought I would be saying that)

A 1950s house is less likely to have lime mortar, more so if its cavity walls.

Not sure what you're saying here. My walls do have a cavity in them. So are you saying the cavity suggests a greater or lesser chance that a lime based mortar was used for the original mortar?

Thanks

 

[^woody^]

It's unlikely that a cavity wall had been built with a lime mortar.

 

[Harry Bloomfield]

I've just had to patch point, around 3m of my 1955 house, where the 80's repointing hadn't been adequate. I used cement and a 1:3 mix.

 

[DaveyH]

I used cement and a 1:3 mix.

Hi, does this mean?

1 cement, 1 lime, 3 sand

or just 1 cement, 3 sand?

 

[Harry Bloomfield]

or just 1 cement, 3 sand?

*******Sorry - I have just reread what I wrote, and no I actually used a 1:4 mix. 1 cement, 4 sand. Pointing, doesn't need much mix, so proportions are tiny. I used an old, plastic 1/4 litre jug as the measure.

I suspected the old mortar hadn't been chopped out sufficiently deep (if indeed it had been cut back at all), when it was originally done in the 80's, so I cut it back 20mm deep, with a stone disk in an angle grinder. Not a nice job at all, but had to be done..

 

[stuart45]

You need to use building sand. If you need to match the colour, stick a piece of the existing mortar into brick acid and you'll be left with a sample of the sand.
I'd guess at something like a 6/1/1 sand/lime/cement mix. Rule of thumb for repointing is a similar mix to the original, just a touch weaker.

 

[DaveyH]

Thanks for all the replies so far.

You need to use building sand. If you need to match the colour, stick a piece of the existing mortar into brick acid and you'll be left with a sample of the sand.
I'd guess at something like a 6/1/1 sand/lime/cement mix. Rule of thumb for repointing is a similar mix to the original, just a touch weaker.

I'm not overly concerned about the colour of the pointing. Something yellowish to match the original mortar would be nice, but my main concern is that whatever I end up using doesn't blow the face off my bricks (which I've noticed has happened to several houses around our estate, where the cowboys have probably used the wrong product or a too strong mix).

If was to go for a 6/1/1 mix i.e. 6 parts building sand, 1 part lime, 1 part cement.

I'm guessing the cement bit would be just a simple OPC (Ordinary Portland Cement)?

I'm also guessing the building sand bit, is just any simple building sand?

However, the bit I'm really stuck on the is the lime bit. What type of lime should be used? To avoid any doubt whatsoever, it might best to just give me a link to an actual suitable product.

PS. Sorry for being so pedantic, but for the simple DIY'er like me it's a minefield, there's hundreds of products and many different types of lime, all with very different properties and uses.

Thanks

 

[stuart45]

You need a standard hydrated lime, as sold in all merchants. NOT hydraulic lime.

 

[DaveyH]

You need a standard hydrated lime, as sold in all merchants. NOT hydraulic lime.

Thanks,

Just to be 100% sure, is this an example of the stuff?

Blue Circle Hydrated lime, 25kg Bag | DIY at B&Q

Blue Circle Hydralime is a hydrated lime used for increased plasticity & cohesiveness in cement, mortars & renders

www.diy.com

 

[stuart45]

Yes, that's a good example.

 

[opps]

The colour of the sand and cement is critical. My mate, Colin the pointer, has the ability to look at existing pointing and intuitively knows how much white and grey cement to add. I look at it, and it doesn't register in my tiny brain.

 

[DaveyH]

Is a 6:1:1 (sand:cement:lime) mix the equivalent of a 3:1 (sand:cement) or is the lime part weaker than the cement part meaning that it would be more like a 4:1 (sand:cement) mix?

For the avoidance of any doubt.
The sand being simple building sand.
The cement being OPC (Ordinary Portland Cement).
The lime being hydrated lime (like the blue circle stuff I've linked on post number 11).

 

[DaveyH]

Is a 6:1:1 (sand:cement:lime) mix the equivalent of a 3:1 (sand:cement) or is the lime part weaker than the cement part meaning that it would be more like a 4:1 (sand:cement) mix?

I've managed to answer this question myself by reading the technical datasheet for blue circle’s Hydralime where I found this sentence

"A mortar of 1:1:5 to 6 (by vol) Portland cement:blue circle Hydralime:sand is equivalent to a 1:5-6 (by vol) Portland cement : sand + plasticiser in plasticity and strength."

For any mortar geeks reading, here is a link to this document.

https://uploadstarmaccom.blob.core.windows.net/bluecircle/sites/76/2024/03/BC_Hydralime_TechINFO_SVD1223.pdf

They've wrote the numbers in a different order to me, but it appears their Hydralime product adds a similar amount of strength as a plasticiser would, which I'm guessing won't be much?

So a 6:1:1 mix sounds just the ticket. thanks. That will surely be weaker than my bricks.