Matching Mortar to Bricks

Urbandad posted “A soft mortar should be used with the older bricks which are softer. We mixed some type N mortar ourselves down to Type O since type O is not commonly stocked, although it can be ordered.” We recently repointed the 1895 brick on the front and rear of our house. Both times we implored the masons to use soft mortar and they said they would but I am highly doubtful they did. I don’t think they even knew what I was talking about. How do we check now that the work is already done? If it’s not the right mortar, is it possible to remove the new mortar and repoint? How would we get the correct mortar? (I’m an ordinary homeowner, I can’t order or mix mortar.) Thank you.

cate

in General Discussion 4 years and 6 months ago

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cate | 4 years and 6 months ago

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Thank you very much for this detailed info, Urbandad and Arkady.

hkapstein | 4 years and 6 months ago

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If the mortar contains a lot of uncured lime it will fizz if you spray vinegar on it, and that would be the case for an air lime mortar, but it will not tell you if you have a Type O mortar. My guess is you would have to have it tested. You could possibly try to repoint the bricks with softer mortar, I don’t know if the removal of the hard mortar would damage the bricks. With the work already done, I might be tempted to cross my fingers and hope that they used type N rather than type S, and that the bricks can withstand the type N mortar. I’d guess the historical brick is around 500-600 psi, but it’s possible it could be harder and there’s definitely variation. Type O mortar is about 350 psi, and type N is about 750. But on a new project I would definitely try to use type O. Also note that certain lime based mortars and hydrated Lime also come in type N and Type S but that refers to something completely different with different compressive strengths, so here we are talking about Type N, S, and O portland lime cement mortars.

Modern mortar is basically a mix of lime, sa nd, and portland cement. The higher the percentage of portland, the harder the mortar will be. There are simple recipes, and the ratios are here in this link and many other places, I believe there is an engineering spec published for the mortar types and tolerances out there, but that’s not really needed because extreme precision is not necessary for a small project in my view.

https://stlbrickrepair.com/2014/03/10/mortar-types-m-s-o-n-k-tuckpointing-and-laying-masonry/

As you can see, type O has more lime and sand and less portland compared to type N. If you have bags of type N available, you could add additional lime and sand when you are mixing the mortar with water for use to bring it down to Type O, and you can see the math is fairly simple. Type N is 1:1:6 portland:lime:sand.
Type O is 1:2:9. So for 1 bag of type N, about 1/8th of the bag is portland, so you need to add about 1/8 bag of lime, and 3/8th bag of sand to achieve the type O ratios. I think that is right, it’s been a while since we did the calculation.

There is also a type K specification which is even softer. So do you need to type K or type O? How strong are the bricks? You could research the specs for your type of bricks or actually have the compressive strength of a brick tested in a lab. This may sound fancy and expensive, but there are a lot of materials testing labs and I doubt it would cost much to do it, but I did not do this at the time. Perhaps it will come back to bite me, but I think I got it right.

So why not just use the original type of mortar? The original mortar was most likely made from a non-hydraulic lime/sand mix with no portland. It was hot mixed on site from quicklime, and this product is no longer available to the public in the US, although this is still available in parts of Europe. Pre-slaked lime putty is available in the US, but expensive as the water weight must be transported, and it isn’t commonly sold. It would also require training the mason to use it properly.

Another option is to use Hydraulic Lime mortars which are portland free, such as an NHL type mortar. These come in premixed bags, and have different strengths, and are available from specialty suppliers. You won’t find them at home depot, but you can get them in bulk fairly easily, and they probably have qualities more similar to an air lime than would a type o mortar which is mainly held together by portland cement.

Arkady | 4 years and 6 months ago

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I’m not sure the link is there. Basically, it says if you have a soft mortar it will “fizz” if you spray vinegar on it whereas a regular mortar will not.

Arkady | 4 years and 6 months ago

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Try this: https://www.google.com/search?rlz=1C1CHBD_enUS831US831&sxsrf=ALeKk022kFUsCSsloUFA8YsAhmzMcvd2fQ%3A1608647801219&ei=eQTiX5noDIKq1QGFtZKYDQ&q=how+do+you+identify+soft+mortar&oq=how+do+you+identify+soft+mortar&gs_lcp=CgZwc3ktYWIQAzIICCEQFhAdEB46BAgAEEc6BwgjELACECc6BAgAEA06BggAEAcQHjoGCAAQDRAeOgoIABANEAUQChAeOgQIIRAKUITeA1ihgwRggqwEaABwAngAgAGbAYgBzA2SAQQxOC4zmAEAoAEBqgEHZ3dzLXdpesgBCMABAQ&sclient=psy-ab&ved=0ahUKEwjZiIqG6OHtAhUCVTUKHYWaBNMQ4dUDCA4&uact=5#kpvalbx=_xATiX-_KK7uk5NoPwPG2gA017

Guest User | 4 years and 6 months ago

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+1 to this question. I know I’m going to need to replace some degrading bricks in the front of my building as well as tackle pointing elsewhere and am trying to amass the info needed to get it to match the mortar (and the bricks) to the existing.