I have one tile that came off. I got a replacement tile, and some grout. But the online guides say you need mortar to glue the tile to the ground. Can I use the grout as mortar? I want to save money so I only want to buy one thing.
Yes you can do it, is simple answer, if what you have is sanded grout.
If you can add a little acrylic glue available in 4oz tubes at building material centers for a couple of dollars it will be even better.
Alternatively you may be able to ask to take a torn bag of ThinSet for free, such as I have done. They actually encourage this to keep the shelves clean.
The mortar or what is known by many in trade as Readymix, is for applications where you may need to level more than one inch thickness. or your tiles or marble is bigger than 12x12 inches for it offers structural volum integrity and easier trawling.