Hybrid Mortar Mixes

[Suffolk Lad]

Hi all, first post so please bear with me.

Would be very grateful if any one has any information on where the English Heritage stand on Hybrid Mortar Mixes.
On a Technical policy statement published by the English Heritage of which i have a copy they do state that at the moment the English Heritage "recommends that, for the time being, 'hybrid' mixes should not be used for conservation work. Until there scientific reshearch has been completed." Unfortunately this statement was published in 1997 and the research was due to be completed by 1998.

But here is where my problem lies, I cant find any page on there website or anyone at there offices who can confirm the results of the scientific research.

If any user knows of the results or could point me in a different direction on how to find the results these would be very much appreciated.

Thanks
Suffolk Lad

 

[biffvernon]

By 'hybrid' you presumably mean a mixture of lime and Ordinary Portland Cement. Opinion these days has hardened towards one that says there is pretty much no place for OPC within general repair and maintenance of old buildings. Some ratios of hybrids seem to risk particularly poor performance. See the Smeaton Project.

You haven't said what you are doing but it will probably be best to use hydrated lime. Hydraulic lime is appropriate in some situations. Tell us more about the proposed works.

 

[Suffolk Lad]

biffvernon

Actually what i meant by 'Hybrid' is a mixture of hydraulic lime and non-hydraulic lime. But i did check out the Smeaton Project which was very interesting so thanks for that.

There's actually no proposed work, the question came up during a discussion with a plasterer friend just wondered if anyone knew the answer or regulations of this 'hybrid' mix for future reference.

But thanks biff for your swift reply.
Thanks
Suffolk Lad

 

[biffvernon]

Ah

Hydraulic lime is decribed in terms of the proportion of reactive material, and given numbers, 2, 3.5 and 5, the bigger the number the more reactive so the faster and stronger the 'set'. If you mixed the ordinary hydrated lime with hydraulic you would, effectively, get a hydraulic with a lower number. It would be cheaper to start off with the lower number so there might not be much point. If you wanted a very weakly hydraulic lime, say a number 1, which is not generally available, then mixing a hybrid might get what you want.

 

[Gervase]

I've used hybrid mixes with great success to make feebly hydraulic lime mortar, mixing NHL2 with fat lime to get a mix to repoint the weatherward side of a building. It's cheaper and less faff than trying to guage each mix with the right amount of metastar or other pozzolan.
I think the Scottish Lime Centre is still experimenting with hybrid mixes, and that the jury is still out as far as any 'official' recommendation is concerned.

 

[biffvernon]

NHL 1 isn't really recognised by the EU yet, but there is a demand for it and Singleton Birch are working to get a standard approved. They intend to produce an NHL 1 in due course.

 

[NT]

NHL 1 isn't really recognised by the EU yet,

nor are pounds and ounces


NT

 

[Stephen]

Biff
The numbers after NHL indicate compressive strength (in Newtons I think).

 

[bournonite]

I think you'll find that NHL 3.5 meets the compressive strength requirements for a 2 storey building (I hope so, cos we just built one in Bridgnorth..) - Portland cement is a factor of ten harder with a compressive strength of more like 40. Why are we using it, when most european countries dont even sell the stuff for building.!

 

[biffvernon]

Biff
The numbers after NHL indicate compressive strength (in Newtons I think).

Ah, I'm sure you're right. It's not something I know much about. But I did go to a talk a couple of weeks ago by a bloke who made the stuff and he kind of explained it. It seemed a bit more complicated.

I think you'll find that NHL 3.5 meets the compressive strength requirements for a 2 storey building

Does that mean that all the three storey buildings in London that were built with NHL 1 should fall down? Don't tell them - they haven't realised yet.

 

[bournonite]

OK smartypants... maybe the regs in those days were a bit less demanding! I'm only quoting the 2006 regs remember....!!!!

 

[biffvernon]

BS EN 459-1, the standard that defines NHLs, gives the compressive strength stated in megaPascals (MPa) achieved at 28 days thus:
NHL 2 between 2 and 7 MPa
NHL 3.5 between 3.5 and 10 MPa
NHL 5 between 5 and 15 MPa

By comparison, hon-hydraulic hydrated limes typically achieve less than 2 MPa while Ordinary Portland Cement achieves ove 30 MPa. But note that limes can continue to increase in strength beond the 28 day standard date.

It is also worth noting the vagueness of the Standard BS EN 459-1. Some NHL 2 limes may be stronger than some NHL 5 limes. There is so much overlap in the standard.