Grout, cleaned back and sealed.
Grout is the porous part of a tiled floor. The tile can be spotless and the room still reads as dirty, because the lines between the tiles have taken years of mop water back into themselves and held it.
Cement-based grout is porous by design: it has to be, to bond and to move a little. That porosity is what takes on soil. Mopping pushes dirty water into the lines and it dries there, so a floor that is cleaned often can be dirtier in the grout than one that is not. Epoxy grout is a different material with effectively no porosity, and it does not discolour the same way, so the first thing worth knowing is which one you have.
In a wet area the problem is usually biological rather than dirt. Mould grows in the grout of a shower because the grout stays damp, and bleach takes the colour out of the mould without removing it, which is why it comes back in a fortnight looking the same. Efflorescence is the other common one: salts carried out of the bed by moisture, leaving a white bloom on the lines that will keep returning until the moisture source is dealt with.
The work is mechanical first. High-pressure hot water with a contained head lifts soil out of the pores rather than smearing it, and it does not eat the grout the way repeated acid washing does. Where cleaning cannot reach the original colour, a grout colourant is an option, and it is honest to call it what it is: a coating applied over the grout, which restores an even colour and will eventually wear in the traffic lane. Cement-based grout gets a penetrating sealer afterwards, which is the step that decides how long the result lasts.
Where the limit sits: grout that is crumbling, missing or cracked through has failed, and cleaning it is not the answer. It needs raking out and replacing, which is a tiling job. Cracks that follow the same line every time it is repaired are usually movement in the substrate, and grout is not the fix for movement.
Questions about grout.
My marble shower looks cloudy. Is it ruined?
Almost certainly not. The cloudy film on a marble shower is soap, shampoo and hard-water mineral build-up sitting on the surface, not damage to the stone. Domestic cleaners either do nothing to it or, if they are acidic, etch the marble underneath while leaving the film in place. A stone-safe strip removes it and the stone underneath is usually fine. The reason it built up is nearly always a failed or missing sealer, so the job finishes with a penetrating sealer that resists the build-up coming back as fast. You will also want a cleaner that is pH-neutral, which we leave with you. Glass doors and tiled walls collect the same film, so a shower that has been cleaned with a supermarket product for years often has both a film on the surface and etching under it. The order of work is the same either way: the film comes off first, then the stone is honed where the acid has dulled it, then it is sealed.
Other materials