I need to fill some small gaps around an old wrought iron balcony where it enters the brickwork of the building. Is lime mortar the best choice for this (1 part natural hydraulic lime/4 parts building sand)? I'm being cautious in case there's any reaction between the lime and iron which might weaken the balcony.
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Cubist
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It's understood that lime, because it's an alkali, will not induce corrosion, so your plan is safe. A couple of good coats of red oxide should be enough to protect the iron-work but if you are very concerned, and the metal is fairly thin, you could wrap the sections that will be buried with a heavy insulation tape as one might do with copper pipes.
Feltwell
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You could put denso tape around the buried sections first. Wear rubber gloves if you do, it's sticky old stuff, but it does work well on stopping corrosion on anything that's buried.
Craig89
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My house (circa 1915) was built with cavity walls and lime mortar. When I demolished the gable end for an extension I found a number of wrought iron wall ties, there was very little left of quite a number of them, but they had been in the wall 100 years. But I would be more inclined to believe this is more to do with them getting wet from driving rain onto the bricks rather than purely being in contact with lime
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Seems I've wandered into a mine-field. According to my references the high ph content of lime mortars, and cement for that matter, can/does actually protect embedded steel/iron and inhibits corrosion - but I hasten to add, it cannot absolutely stop it. The corrosion problem typically arises at the point at which the steelwork is exposed to atmosphere/weather.
OK thanks everyone. Really appreciate all your replies. I've already treated the external parts of the balcony with rust converter followed by a couple of coats of paint. I used Zinsser All Coat - not sure if this offers the best protection but I'll keep an eye on it. So taking all your comments under consideration, I think I'll apply another coat of paint or two to the bits of iron that will be buried, then some tape (I'll have to Google 'denso tape' 
) and then use a weak mix of perhaps 1 part NHL and 5 parts sand. I don't think it needs to be super strong, just needs to fill that hole to stop the rain getting in. Based on a previous comment in a separate thread I also thought of packing it with oakum and topping up with lime mortar, but that oakum is expensive stuff ! Would this be a better option? If anyone can recommend a reasonably priced supplier that might be another way to go.
) and then use a weak mix of perhaps 1 part NHL and 5 parts sand. I don't think it needs to be super strong, just needs to fill that hole to stop the rain getting in. Based on a previous comment in a separate thread I also thought of packing it with oakum and topping up with lime mortar, but that oakum is expensive stuff ! Would this be a better option? If anyone can recommend a reasonably priced supplier that might be another way to go.
Cubist
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I don't think so. Admitted the oil/stockholm tar may help to inhibit corrosion, oakum will only weaken the connection to the masonry. And yes, oakum is ludicrously expensive - and I still have about half of the 5 kilo bail I bought 8 years ago.
Feltwell
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Denso tape - it's funny stuff, it's a thick tape impregnated with a sort of greasy tarry substance - like industrial vaseline! - used to wrap around things like steel pipes that are going underground, or around threads for hold down studs for street lighting - basically anything with iron content that is being buried or exposed that you don't want to corrode. Ever.
OK got it. Thanks
I thought someone had suggested oakum in a previous thread but looking back that was for around an oak beam, not the wrought iron - my mistake ! Thanks again for the advice.I just Googled denso tape and this came up. Is this the stuff? https://www.toolstation.com/petro-t...NLs1f2qESoxSrfiwmksaAhaGEALw_wcB&gclsrc=aw.ds
Feltwell
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Yep, that's the stuff. Denso tape is a brand name that they all get referred to by, rather like Hoover to vacuum cleaners. The Toolstation stuff will just be a generic equivalent.
Note I've never tried to use it for what you're doing, but it certainly works underground. It is very sticky, disposable gloves are a must.
OK that's great, thanks. I'm sure it'll provide considerable protection for the iron balcony, but even if it doesn't, it can't do any harm. Might as well throw the kitchen sink at it ! Many thanks again
malcolm
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That will be a very weak mix. You can vary the strength of cement that way but lime will fall apart when mixed weaker than 1:3. For a weaker lime mix use NHL 2.0, putty, or just hydrated lime and keep the quantities the same.