Lime mix for repairing external mouldings

[Nigel Watts]

My builder hired a lime plasterer last year to do a number of repairs, including replacing a small length of external moulding on my stucco facade. Part of this has now come loose an I have had to remove it. The problem was not with the final layer which formed the moulding, which was hard enough, but with the layer below which was very soft and damp - hardly carbonated in parts - and which had probably expanded in the cold weather, causing the hard outer surface to crack ( a similar thing has happened to the render on my front garden wall, the outer surface of which is hard but the inner layer is still soft and has expanded, causing the whole thing to crack). I suspect the problem was either that the lime mix was faulty or that the lower layers were not given sufficient time to carbonate before the final layer was applied.

A few weeks before the cold snap I strated to repair the moulding myself and put up a first layer of lime which was about 3 parts sand to one of lime putty. When preparing to add a second layer yesterday I noticed that this was still very soft and weak so I removed it. It should have had time to carbonate before the very cold weather so I am wondering if the problem was that it got too wet.

Is the answer to use a hydraulic lime?

I have now put up a mix of about two parts sand, one part pozzolan, one part lime putty. This should in theory create a hydraulic lime which should be quicker setting, harder and capable of setting when wet. I am hoping it will set before the predicted frosts at the weekend.

 

[Nigel Watts]

I have just found this online, which leands me to think that I should up the proportion of lime putty:

"ONE PART SLAKED LIME OR ONE PART QUICKLIME?

It is vital to understand that, until the Second World War, a majority of limes were still prepared from freshly burnt quicklime delivered to site, as opposed to ready-to-use slaked putties, which would have been extremely heavy to transport, or bagged dry-hydrates. For general mortars the quicklime was then usually slaked to a crude powder (technically, a dry-hydrate) on site. One of the most popular methods to achieve this was to place a one-third measure of quicklime broken down to the size of nutmegs within a cubic yard of ringed sand, and then apply the minimum of water necessary to slake it, before quickly drawing the sand over it as it both heated and broke down in slaking. After slaking was completed the pile would be turned over dry to fully integrate the sand and lime. One option was then to add extra water to bring it to the working consistency of mortar ready for immediate use. Alternatively, the dry mix could then be thrown with the shovel through a large inclined 5mm (¼") meshed screen to remove large inclusions before mixing it with water, thus producing a top-quality 'front mortar' that was generally reserved for facade masonry.

The important thing to note here is that the lime used in the ratio of 1:3 was not prepared slaked lime (calcium hydroxide) but unslaked quicklime (calcium oxide), a fundamentally different substance in several respects, including volume. This vital point has frequently been overlooked and has led to misinterpretation of a great many historical mortar mixes based on original documents recording mortar ratios, or on those recorded within old craft books. A simple but very good example of this is to be found in an architect's private site book, for an entry dated 1927 on preparing lime mortar as follows: 'Mortar: Lime 1, Sand 3. Lime: slack [slake] with water and then cover with sand. After lime is thoroughly slack, screen through upright screen and then mix with water to desired consistency'.

The proportions used by this architect for mixing quicklime with sand would not apply to a mix made with hydrated lime (whether hydraulic or non-hydraulic) because all quicklimes increase in volume when they are slaked. The amount of increase varies according to the type and class of lime but typically this is between 60 and 100 per cent. Therefore the resultant lime:sand ratio for the finished mortar is always more lime-rich than the originally-stated ratio. That is why, under analysis, the majority of historic lime mortars are not commonly found to be 1:3 but typically vary between 1:1½ and 1:2, just as the original mortar makers and craftsmen intended. This is borne out by extensive analysis carried out over many years by The Scottish Lime Centre Trust. (At the last count the organisation has analysed around 4,500 historic mortar samples, approximately 80 per cent of which were from Scotland, 10 per cent from England with the remaining 10 per cent from various other countries.) The average lime:sand ratio on the organisation's entire database of historic mortar samples is around 1:1½.

The 1:3 quicklime:sand ratio suited most general building sands. However, sometimes builders had to use a naturally fine and more uniform local sand, not the ideal well-graded building sand, but one that demands an increased lime content to make good mortar. Then the craftsmen simply adjusted the quicklime content accordingly. A good example of this was discovered during archaeological works to the external brick fabric of Aspley House, Bedfordshire (late 17th-century and enlarged 1745). The Scottish Lime Centre Trust, on behalf of the writer in his role as historic brickwork consultant, undertook detailed analysis of several samples of original mortar, which is known to have been made using the fine sand obtained from within part of the curtilage of the property, and mixed with the local (Totternhoe) feebly hydraulic grey chalk lime. These mortars had been used for both the mansion brickwork and on a long and very high garden boundary wall to the rear of the property. The main house brickwork mortars, from both phases of construction, were to identical ratios of 1:1.4, but interestingly, the mortar for the garden wall brickwork was to a ratio of 1:0.7, revealing that the bricklayers had simply doubled the ratio of lime to sand as a logical and pragmatic way to gain additional strength and the weathering capabilities deemed necessary for this most exposed of elements."

 

[plasticpigeon]

I an tell you are a real perfectionist, and with such a lovely property I'm not surprised!!

In an old book I have called "builders work", from around 1880, stucco is just lime with 3 or 4 parts sand. However as you mentioned, that was quicklime and not putty or hydrated lime, so about twice as strong a mix is needed I think. Also I'm not sure that lime from way back was extremely pure, so may have had some hydraulic properties. I spoke to a guy at SPAB, and he said the reason lime based mortars are porous, is because the lime shrinks slightly on setting, but cement expands slightly on setting. this means there are tiny voids around each piece of aggregate making it porous. Cement mortars don't have this property. It means that even hydraulic limes will ends up a bit porous. I do not know if this is 100% true, but it is interesting to note that a polished lime surface is not very porous at all, but a rough textured one is, which would tend to back up his comments.

I am hoping to try and repair some frost damaged and cack handed builder damaged bath stone mouldings on my house this summer. I hope to use bath stone dust and hydraulic lime, and perhaps a bit of colour if need be, so I am very interested to see how you get on. Please keep us informed.

 

[DJH]

Nigel, in Millars Plastering Plain and Decorative, Batsford 1st Edition 1897 pages 101 to 104 inclusive, he writes about Stucco and gives some recipes. On page 117 he writes about Fixing Enrichments. I can scan these for you at some stage if you like.

Doug

 

[Nigel Watts]

Doug

Thanks for the offer. I think I have a copy of Millar's somewhere - not a first edition though. I will look it up.

 

[Keithj]

The builder doing the repairs on our church did some lime work in early December. I remarked at the time that I thought he was optimistic; he thought it would be fine. I was right. It turned to mush, and most of it washed out with the rain a couple of weeks later. Winter isn't the time for lime. He's planning to try again in April.

I'm not an expert, but the one who did our house (very well) insisted she would always use a 50:50 lime and chalk mix for pargetting and the like.

 

[DJH]

Doug

Thanks for the offer. I think I have a copy of Millar's somewhere - not a first edition though. I will look it up.

I mentioned the first edition Nigel because the subsequent editions were published after Millar's death and edited by George Bankart who left out quite a lot of the original, including some recipes, in favour of pictures of his own work.

Doug

 

[Nigel Watts]

Winter isn't the time for lime. He's planning to try again in April.

I was also coming to this conclusion.