It is possible to embed 12×12 inch ceramic tiles into a freshly poured concrete slab, but there are a lot factors to consider. It is recommended to let the concrete cure for 28 days before adhering the tile to it.

Tile Installation Guidelines and Instructions for Installing Ceramic Tile, Glass Tile, and Stone Tile for Residential Installations. Hiring the right installer is the difference between a delightful result and a nightmare.

Yes, movement joints (expansion joints) should be at all plane transitions and where there are transitions from different substrates and materials. Door thresholds, wall penetrations around escutcheons, and light fixtures also should have resilient joints.

Full adhesive contact is defined as continuous contact between the back of the tile and its substrate. Spot bonding tiles is not acceptable unless an epoxy is being used for interior wall applications only. Spot bonding is where only dabs of thin-set are applied in the four corners and in the center of the tile.

I assume that the concrete slab is an interior application. Generally speaking you don’t what to install tile over a concrete slab that has a hydrostatic water problem unless it is remediated first.

There should be a water proof membrane on the shower floor and walls, and it needs to be installed correctly with the appropriate transitions and slopes to drain.

Mechanical anchoring of the stone is always considered the safest method of installation.

As long as the existing marble floor is structurally sound and well bonded to its substrate without excessive deflection, you can bond over the marble.