Pointing Mortar Mix

First a bit of history.
My Chimney has been pointed twice in th e10 years I've owned my house. The first time about 9 years ago, the second about 3 years ago.
The first time, it looks like the guy just scraped out about 1/4" of mortar and filled it back in. Now that thin layer of new mortar is falling out. However, its still hard as a rock.
The second time, just a few spots were patched but the mortar used looks like it has turned to sand and is disintegrating. I've been able to remove a few bricks where this is happening.
I figure I can do just as good a job as the "professionals" who screwed it up before so I'm going to give it a try.
The question I have is what type of mortar is used? I see the home centers sell a type N mortar mix and a type O mortar mix. Type N being stronger and is used for exterior walls and type O being weaker and is sometimes called pointing mortar.
So far I have all the affected joints cleaned out to about 3/4 inch depth, and all the old mortar has been cleaned from the few bricks that I was able to remove. I ready to put the new mortar and bricks back in.
-dickm
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Besides the joint has to be clean, acid and a powerwash work
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The correct way to do pointing is to have the existing mortar tested for its exact ratio of sand, lime, and cement. You need to take samples from several sections as the mix may have changed with each batch during the original construction. Once you know the correct ratio, you mix your own to the same specifications.
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I don't know anything about types of mortar mix but I do know that any mortar or cement repair job will not last long if the mortar or cement dries out too quickly. You will need to dampen the repair area before the mortar is put in, protect it from sun and otherwise keep it damp for as long as possible. Misting the area afterward can help, once the mortar has set up enough.
Don Young
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You don't indicate the age of the structure or type of brick used. IF you put high compressive strength mortar with very old bricks, you will destroy the bricks. The strength of the mortar is subject to the ratio of Portland, lime, and sand. The color is most affected by the color of the sand.
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