Water can make good mortar go bad
I have a very old solid-brick house, which I've been working on for many years. On the two corners on the north side of the house, the mortar and even some of the bricks are falling out. Why is this happening?
— Robert, New Carlisle, OH
General Contractor Tom Silva replies: Brick and mortar are porous materials, and when they are in constant contact with the ground, as yours are, they suck moisture up into the wall. That can cause big problems, particularly as the weather cycles back and forth between freezing and thawing. (It's also possible that the same freeze/thaw conditions are causing the soil to push the brick up and down.) The problem is worse on the north side because brick and soil don't dry out well.
The solution is to dig down at least 3 feet below grade close to the side of the foundation. When the wall is dry, apply a foundation sealer to everything you've exposed. Cover the soil with landscape fabric and backfill the trench with ¾-inch stone, which promotes drainage and allows movement of the soil during freezing weather.
From the way it's crumbling, I'd guess that the brick was originally pointed with lime mortar. You shouldn't repoint it with standard mortar, which is so hard that it can break off the faces of the old bricks. Instead, scrape out some of the existing mortar and have it analyzed so you can mix the sand, lime, and cement in proportions that won't harm your brick.