Roman Cement Repairs

[Nigel Watts]

My 1988 English Heritage Handbook on Mortars, Plasters and Renders has just arrived.

My stucco seems likely to be Roman cement as it has "the distinctive pinky-brown colour". Posher London Houses, such as Nash's Regents Park terraces, seem to have been given a mastic finish which mine clearly wasn't, but a piece of detached moulding has a dark grey clinker-like quality, the composition of which which I have yet to identify.

The book states that as these Roman cements are no longer made the best material to use for repair is a cement, lime and sand mix and that the colour can be approximated through a careful choice of sand. The use of cement in the mix rather surprises me. I wonder if this is still considered best practice? I guess lime alone would be too weak.

Interestingly the author states that "much Roman cement stucco was coloured with with a fine wash of lime putty, tallow and copperas (iron sulphate which is brown-red in colour) to simulate Bath stone colour. This coating was usually applied while the Roman cement was damp and therefore stained the outer portion of the render."

It recommends that hollow sounding bits should be cut out and re-rendered and recommends, where the render has been scored to simulate ashlar blocks, that the old render in the defective areas is cut back to these simulated joint lines to give a neater finish, even if that means cutting out areas of sound original work - a good tip I think.

 

[jocelyn plummer]

They make it sound so easy!

 

[Nigel Watts]

Just found this after a Google search:
http://www.lbhf.gov.uk/Images/GuidelinesonrepairsStPetersSq_tcm21-112603.pdf - see Appendix A, which appears to be based on a report by one of the co-authors of the original English Heritage Handbook.

Hydraulic Limes are recommended here.

I am so delighted that planning applications by people wanting to paint their stucco white are being rejected and they are being told instead to paint in a stone colour.

 

[Schoolmarm]

Oh, glad to see you have found a recommendation for hydraulic lime mortar. Cement and sand guaged with lime has been mentioned a few times as being a mix likely to fail.

 

[MdB]

Oh, glad to see you have found a recommendation for hydraulic lime mortar. Cement and sand guaged with lime has been mentioned a few times as being a mix likely to fail.

Other way around I think. My understanding was mixes with less cement than lime were likely to fail as the cement clumped. More cement than lime wasn't a problem.

 

[Schoolmarm]

Oh! Lol. Well there is no way I can remember where I read an article that said both were compromised by the addition of the other.

 

[Penners]

I read an article that said both were compromised by the addition of the other.

Did that apply universally, 'Marm, or only in particular rendering applications? For as long as I can remember, the mantra has always been "add a proportion of lime to your cement mortar - this makes it more flexible and less likely to crack".

Has that been wrong all along?

 

[Schoolmarm]

Well, I thought I had read that is just weakened the cement but now I am getting worried.

 

[Flyfisher]

I've also been told that lime is frequently added to a mainly cement-based mortar for the reasons Penners' described (and to make it easier to work) but couldn't 'marm also be right? Wouldn't a 'weakened' mortar be more 'flexible and less likely to crack' ?

 

[MatthewC]

The church I used to look after was first rendered in 1828, and when it was re-done in 1993 they used a 1:1:5 mix of cement:lime:sand (although I forget what type of sand, but it was pretty fine). This is still well attached after 17 years, apart from a few places where the inside wall was mistreated at different times. One area has some pink gypsum plaster inside and both the plaster and the render are deteriorating fast; another place has a 3mm skim of cement over the plaster and the sheet of render has popped on the outside. Where there is lime plaster inside, the render seems OK, so I wouldn't say that the strength/durability has been affected (yet!).

 

[c r]

just to bump back to this thread...

Nigel, thats an interesting link you posted about St peters sq.

I just thought you might be interested in this, if you are not already aware of it of course-
http://www.masonsmortarshop.com/prompt-natural-cement.html

I think this is the only commercially produced product that bears similar resemblance to the characteristics of 'Roman'/'natural' cements used in 19th century. Despite the name it is not based on, nor is it a variant, of OPC (Portland cement)- I think its more accurate to describe its characteristics as being more akin to a strong hydraulic lime.

These two links from the manufacturer take some reading but there is some interesting (& occasionally comprehensible!) data/info in them both.

http://www.romanportland.net/files/doc/cahier-technique_chx-cnp_anglais.pdf

(particularly section 3+4 of this)
http://www.romanportland.net/files/doc/cahier_technique_cr_cnp_eng.pdf


As well as the supplier above, I think Ty Mawr also stock 'prompt' cement, but it does not seem to be at all widely available in general, which is a shame as so many of our towns/cities have buildings with external features that were likely originally formed using 'Roman' cement. It seems a versatile product, a bit like plaster of paris, but with external durability.

note it can be used with an additive which retards the set. One of the above documents described it being suitable for use in thin layers, with fine aggregate, so for example for touching up areas of your stucco. Certainly also for (re) running external mouldings etc, as would have been done originally.

 

[biffvernon]

I've also been told that lime is frequently added to a mainly cement-based mortar for the reasons Penners' described (and to make it easier to work) but couldn't 'marm also be right? Wouldn't a 'weakened' mortar be more 'flexible and less likely to crack' ?

I think Peter Ellis's article, Gauging Lime Mortar, in Building Conservation may be the one to start with:

http://www.buildingconservation.com/articles/limegauging/limegauging.htm

followed by Graham O'Hare, Lime Mortars and Renders - The Relative Merits of Adding Cement:

http://www.buildingconservation.com/articles/cement/cement.htm

They draw upon the work of the Smeaton Project. Here's the Ashurst et al. paper: http://www.scribd.com/doc/29910110/Smeaton-Project