Most mastics turn brown over time when exposed to air.
Marble tile getting darker.
Cement board still allows water to pass through.
The problem is most likely a bad installation and or cracks or voids in the grout allowing water to get trapped behind the tiles where it cannot evaporate.
Unfortunately rust stains are the most difficult of stains to remove from marble.
A simple test of this is to let the lid off the mastic container for a few days or look at the mastic trowel the next day.
Please please do consider using kerdi everywhere from and for the floor pan and up onto the walls as far as the walls could possibly get wet.
Marble a crystallized form of limestone is a popular choice for worktops flooring and wall tiles in both kitchens and bathrooms.
Marble shower tile getting darker.
The top picture was taken right after the install the bottom is after about two.
Wet stone is always darker in color so the white marble tiles become gray and stay gray since they are saturated with water.
It is not a problem with the tile but the installation or perhaps a serious water leak.
If it was a tub with shower is one thing but a full shower requires the extra moisture protection especially at the base.
Because marble is porous you should clean spills immediately or you run the risk of the stone developing stains.
Available in a variety of colors and shades marble can be.
What type of thinset to use when installing marble tile.
For instance different kinds of.
It is an indication of a problem that can cause long term structural damage if left unattended.
Rust stains can usually be removed with a poultice or a non acidic rust remover.
If anything the marble will get darker as time goes on and the mastic continues to cure.
If you try these methods and still do not have.
Instead it just absorbs into the marble.
Be careful if you are removing a stain from darker colored marble as hydrogen peroxide can cause the marble to lighten.
When installing tile in your home you can use a variety of products to adhere the tile to the substrate.
I had tile installed i my bathroom.
Marble tile is just as susceptible to staining as solid marble but you can use the same method.
Since i know very little about tile and plumbing i had everything professionally installed.