4. Grey grout, black silicone and the mould that keeps coming back
Grout and silicone problems are moisture problems with a mineral complication. Hard water leaves a porous mineral film on grout that holds soap and water against the surface, and mould colonises that film.
Start by distinguishing the two. Grey, chalky grout that scrubs back toward white is dirt and mineral deposit. Black spots or a black line along the silicone that will not scrub out is mould, and its roots are inside the sealant.
For dirty grout, use a grout brush with a paste of bicarbonate of soda and water, working along the lines rather than across. Follow with a diluted limescale remover if the grout is cement-based and the tiles are ceramic or porcelain. Never use acid on natural stone tiles or on grout between them.
For mould in silicone, scrub with a diluted bleach solution or a mould spray, applied on a strip of kitchen paper pressed along the joint and left for two hours. If the black returns within weeks, the sealant is colonised through its thickness and no product will fix it. Cut it out with a sharp blade and a silicone remover tool, clean and dry the channel completely, and re-seal with a bathroom-grade sanitary silicone containing a fungicide. Materials cost about $12, CA$18 or £10 and the job takes an hour.
Then attack the cause. Run the extractor fan during every shower and for twenty minutes after — most fans need far longer than people leave them. If the room has no fan and only a window, open it. A £10 hygrometer showing above 60 percent relative humidity for long stretches tells you the ventilation is losing.