Learned that old brown mortar joints can cause wall failure in just 8 years

I was at a supply house in Cleveland last Tuesday picking up lime for a repointing job, and I got chatting with the old timer behind the counter. He asked what I was working on and I mentioned a 1920s warehouse wall that had some crumbling joints. That's when he dropped a fact I'd never heard before. He said those brown mortar joints from the 1920s and 30s were often made with slag cement which has almost no bonding strength compared to modern stuff. He showed me a binder from 1927 that had test results and apparently those joints start losing integrity after about 8 years of freeze thaw cycles. I've been laying brick for 12 years and nobody ever taught me that in any training. Has anyone else run into old slag mortar and had to deal with it differently?

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thea1432mo ago

Oh man that old timer was absolutely right. I learned that one the hard way on a church job back in 2017. We started repointing this 1920s chapel and the mortar just crumbled in our hands like sand. Had to completely rip out all the joints and switch to a Type N mix because nothing would stick to that old slag stuff. Even then we had to wet the bricks extra and wait longer between lifts. Total nightmare job that took twice as long as we bid it for.

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