I plan to repoint my stone house, and need a fail safe historically appropriate lime mortar recipe. Any advice will be most appreciated, thanks.
Contact me if you are still looking for a source of natural lime in Canada. There are only a few but more are popping up as the restoration industry in Canada develops.
contact me here if you have any questions
For a foundation like yours you should probably use a NHL 3.5 or even NHL 5.0 lime. Your mortar mix should be 1 parts lime and 3 parts sand. NHL (natural hydraulic lime) 3.5 is usually used for repointing exterior walls on historical buildings. The NHL 5.0 is used for high exposure areas and is much stronger. I would use the 3.5 for the majority of your wall and then switch to the 5.0 in areas close to your soil line if you are concerned about damage from freeze/thaw cycles.
The sand that you use should consist of 2 parts brick sand (fine grains) and 1 part concrete sand (course/sharp grains). This mixture allows for maximum strength while at the same time allowing for some flexibility in the mortar to increase durability.
Avoid the use of lime from portland cement. Because of the manufacturing process and additives, Portland cement lime usually attracts water and is not breathable, which traps the water and causes ice crystals to form in the mortar in our Canadian climate. Use a natural hydraulic lime that allows your mortar to wick the moisture out of the bricks and evaporate into the outside environment.
So your finally mix will be 1 part lime and 3 parts sand or 1 part lime, 1 part sharp sand (concrete sand) and 2 parts brick sand (fine sand). This is the most traditional lime mortar I have come across and used in the restoration of historical houses and churches in Toronto. The architectural specs for these jobs specify the same mix. If you have any other questions just email me. www.oliverestoration.ca
Many thanks for your informative answer. I will take you up on your offer and email you in the spring, as I have questions regarding curing periods, keeping areas damp with wet burlap shrouding etc. I am really looking forward to starting this project.---Mal
Here is another company down in Texas. I did not use them as the fellow redoing our bricks was a restorer on the NM State historical list and knew what he was doing. But I did have several discussions with these folks before finding the guy I used. I found them very helpful. You can buy in bulk. They will match the original if you want.
This is the recipe we used in the East Row Historic District in Newport, KY. However, in our area most homes have 19th century brick. I don't know if you must use the same care with slate. Nevertheless, here is our recipe for your use.
4 C white, non-staining Portland Cement
1X5 Gal bucket hydrated lime
2x5 Gal buckets sand
A proportion of 1 part lime to 2 parts sand is useful as a starting point. No more than 20% of the total volume of the lime and cement combined should be Portland cement. Any greater amount of cement increases the hardness of the repointing mortar to a potentially damaging level.
Color matching should be accomplished by using buff colored sand and tinting powder.
See, this is my quandary; the addition of ANY portland to a lime repointing mix. I get info from all over the net, all of which is conflicting. A well-thought-of local heritage mason here uses some white portland in his mix, but it's the devil to many others. I understand this in context with soft old brick, but I'm referring to fieldstone here. The more I research, the less I know.
All the old mortars had cement in them, its the proportion of lime that made it compatible with old brick that wasn't fired to the high temperatures of todays brick. As a result, the old stuff will spall crack and loose its face if too much cement is in the mix, ergo the lime. As I said, I don't know if the same problems exist with slate - I think not, but you would be safer using the mix I gave you than anything store bought. By the way, if you don't use cement as a hardening agent with the lime what are you going to use that will stand up to the elements and changes in temperature and moisture?
If you research portland cement (what is commonly called cement today) you will find that it was not invented until the late 1800's and not widely used within Canada until the 1920's. Pozzolan mortars have been used for thousands of years and Lime mortars falls into this category. Pozzolan is a siliceous and not to bore you with the chemical details but these products react with water and calcium hyrdoxides to form compounds possessing the properties of common day cement except when cured are breathable and do not attract water. Lime mortars with no portland cement were the only mortars used in most of Canada's historical structures prior to the 1920's. Hope this information helps. The confusion between portland cement and natural limes in mortars are one of the most misunderstood materials in modern day masonry restoration.
Just as a foot note, a master mason in the UK once told me that if someone used mortars with any portland based limes on a heritage building in England there are severe penalties including possible imprisonment.
If you have any questions please feel free to contact me (Brick and Stone repair on heritage buildings in Canada)