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GSBN:YakWoman asks about Lime (FWD)

The following is a response from Harry Francis, probably the most knowledgable "Limey" in North America. You may recall that 'Arry contributed to the "Lime Plaster" issue
of The Last Straw last year, which Athena and Beel Steen guest-edited.

--------- Forwarded message ----------
From: CALXA@...
To: archilogic@...
Cc: klerner@...
Date: Fri, 7 Dec 2001 11:39:50 EST
Subject: Re: YakWoman asks about Lime


I think Tom is corrrect...either his mix or your propposed mix will work fine. I think the contractor is used to using cement for stucco work, and just a little lime to make it more plastic ( workable). (This is typical of today's need to get it finished quickly mentality).

The 1:1:6 will work fine, I think, but will be slower setting, than his suggested mix...

The main idea here is to use as little cemnetiscous materials as possible (lime and/or cement) and as much sand as the cementiscous materiaqls will hole. The reason is that the heat expansion of sunlight striking the walls can result in cracks if the materials do not expand uniformally. Literature says that lime/sand mixes crack less because the expansion of the lime and the sand are about equal....whereas the cement expands at a different rate...causing cracks.

Cement is often caustic, causing burns of workers hands, etc. Quicklime is also caustic...not due to the chemistry, but due to the heat generated when hydrated ( mixed with water). The resulting hydrated lime is not caustic unless additives have been added making it so...

His concerns about your mix being too hot - caustic - may result from experience in slaking quicklime. If the hydrated lime is thoroughly soaked and aged 24/48 hours before using in the mix, any un-hydrated quicklime
will be hydrated and no heat generated.

However, the portland cement will produce sufficient heat on hydration to cause burns, and many Portlands have sufficient sodium hydroxide contained to make it chemically caustic..and result in chemical burns....to the skin and eyes...

The old literature suggests 1:3 ratios cementiscous material to sand.....with comments to sue as much sand as the stucco will carry....to prevent thermal expansion cracking.

The contractor may dislike the slow setting of pure lime stuccos, and the 1:1:6 does help the mix harden quicker...and the specific sand used makes the difference in ratios needed to make a satisfactory project.... the old NLA handbook suggests no more than 20 % Portland cement be added

"Cement, however,  should not be added in excess of 20% of the lime used."

"Then, too, after the lime has hardened it has practically the same heat and moisture co-efficients of expansion as does the sand it bind in place."

Kelly, I hope these comments help in your negotioations with the contractor...Admititily, many projects today, are simply Portland cement and sand, with just a little lime as a plasticizing agent...resulting is short life, cracks, debonding, etc... and a bad name for stucco....What A shame....

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Robert W. Tom
Kanata, Ontario, Canada

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