Limescale or mildew?


Can someone help me out with the difference between limescale and mildew?
I have horrible brown bits on the plaster between the wall tiles and my shower tray in my en suite. The build-up is underneath the plaster and therefore is impossible to remove without pulling off the plaster and risk water penetration into my downstairs room.
I can't remember how many times I've had to remove the plaster, clean off the limescale / mildew, apply new plaster and wait for it to dry. There must be an easier solution. We live in a hard water area and the en suite has an extractor fan.
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"Serious Man" wrote:

Limescale is very hard calcium salts, normally off-white, whereas mildew is live fungus and is normally black, brown or grey in colour. Brown bits are more likely to be fungus rather than limescale. Plaster is normally between the wall surface (brick or breeze block) and the tiled surface, not between tiles and the shower tray. There is normally grout or silicone sealant between the bottom row of tiles and the shower tray, and fungus will grow in that area. Are you confusing grout with plaster? If so, the grout needs raking out, the gap treated with a fungicide, and then filled with a fungus-resisting silicone sealant. If the edge of the shower tray is subject to flexing when in use then anything used to fill the gap between the tray and tiles will tend to crack and let water penetrate, and fungus growth will follow.
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