The mortar is crumbling and falling out from between the stonework (or brickwork) of my old house. Is it OK to patch it up with cement?
No! If the mortar's crumbling it's because it's doing its job. It sounds crazy but that's what it's there for! Old houses were constructed with soft lime mortars which tend to crumble in time. Mortar should always be softer than the brick or stone of a wall in order to allow the building to absorb movement, and to carry away any moisture that finds its way into the walls. Damp will always follow the path of least resistance as it evaporates. Damp from the ground carries harmful salts which crystalise as they reach the air. These crystals expand as they form and can flake away the surface of brick or stonework. But they do very much less harm if they cause mortar to crumble. Replace it with soft lime mortar, using a traditional recipe, and your old house will be much better off than if you were to use a modern, hard cement mortar.
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