There you have it. That's what happens.
If you take a closer look at the photo though, you can see clearly what the issue is with Portland cement. It holds onto water like a sponge; an unbreathable sponge at that. Notice how the concrete render is cracking in a grid. Those cracks are occurring at the joins between bricks which are filled with concrete mortar. The mortar has retained the moisture, which has then seeped into the concrete render, and ultimately destroyed it.
This knowledge is crucial if you're thinking of repairing an old stone wall, rendering a traditional building, or maintaining a natural home. As Period Living says, "Sadly, the wrong techniques and materials are all too frequently used for repairs so moisture is sealed in, resulting in damage and decay, often with disastrous consequences to walls and floors."
So what should you use instead? Lime plaster and cretes, clay plasters and mud mortars are the way to go. Here's a useful article on the subject.
Have you restored an old natural home? Do share your tales and your tips.
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