Expanding mortar additive

RobinClay · 7 Oct 2010

We have an ancient stone wall, from which lumps have fallen out. If we stuff 'em back in with OPC mortar, the mortar will shrink and they'll fall out again.

Does anyone know of an expanding additive ?
Name, supplier, cost ?

stanna · 7 Oct 2010

Pre mixed (easier) lime mortar.

Blagard · 8 Oct 2010

RobinClay said:
We have an ancient stone wall, from which lumps have fallen out. If we stuff 'em back in with OPC mortar, the mortar will shrink and they'll fall out again.

As Stanna has suggested a lime mortar would be the better solution. For your infomation, when using an OPC mix for filling a gap, use as little water as possible so the mix is like the damp sand you would use to make a sand castle! Shinkage is then very minimal. Simply pack it in hard and if you need to point it up, just spray a little water on the surface.

Expanding mortars I am not familiar with. The only time I have used some thing like that is when grouting up steelwork base plates with a non-shrink grout.

RobinClay · 8 Oct 2010

Thank you for your response.

[quote="Blagard";p="1760626] The only time I have used some thing like that is when grouting up steelwork base plates with a non-shrink grout.[/quote]
Expanding / non-shrink - what's in a name ?

Do you have a brand name for it, or where I can get it ? Please ?

[Harrumph] 30 years ago, in Hong Kong..... I was working on the Mass Transit Railway. The Spec required expanding mortar to fill the pockets in the cast-iron lining. The Contractor proposed a product, and gave us a sample, which we sent to the lab. The result came back, "Yes, provided it's not used for grouting pre-stressing ducts". I asked the Contractor why he proposed this particular stuff. "Oh, we had some left over from a pre-stressing job." Ooopppsss !

andyb30 · 8 Oct 2010

lime putty is your best bet,mixed with a bit of aggregate.
wet the joint,bob some in,push some wet stones into it to pack it out then
cover with more putty mix and trowel off.
seal the edges and keep fine misting it for a couple of days.
you can wash it with a wet paint brush,this will take the top skin off the putty and show the aggregate.clean the stones with wet sponge and
sit back and admire.

Do Not Use OPC as it will kill your stone.

Blagard · 8 Oct 2010

RobinClay said:
Do you have a brand name for it, or where I can get it ? Please ?

Conbextra is one name that comes to mind with non-shrink grouts. It usually has to be ordered through the builders merchant as few will keep a stock of it. But please don't use it on the stone!

Lime based mortars are really what you need. One particular feature of lime mortar is the way minor cracks heal themselves.

noseall · 8 Oct 2010

RobinClay said:
We have an ancient stone wall, from which lumps have fallen out. If we stuff 'em back in with OPC mortar, the mortar will shrink and they'll fall out again.

No it won't if done properly.

There is no amount of shrinkage, amongst regular opc mortars, that allows masonry to simply fall out if packed and pointed correctly. Wetting the area helps too.

Softer 'fattier' building sands do shrink a very small amount but rinsed sand such as plastering sand shrinks very little.

It will either be down to the construction of the existing wall or the way in which the lumps are being re-fixed that is the problem.

stuart45 · 8 Oct 2010

Sulphoaluminate cement expands when used in damp conditions. It's used a lot in the US to plug leaking basements as it goes off really fast as well as expanding.
However as Noseall stated the problem lies with the workmanship.

RobinClay · 28 Mar 2012

Well, we've bought the lime and the sand, and now the weather's a bit warmer we hope to get around to doing it "real soon"

Thanks for your help.

^woody^ · 28 Mar 2012

FM, post back in 2014 and let us know how you got on

RobinClay · 28 Mar 2012

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Expanding mortar additive

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