HAVE YOU HEARD OF THE BENEFITS OF EPOXY GROUT???
This shower area had previously had a lot of movement due to the house needing to be re stumped, so the cracked grout had been replaced with silicone that had gone mouldy and began coming out in places. 
We removed existing caulk, thoroughly treated the area and replaced with ARDEX 2 part epoxy Grout. This means 100% waterproof no more mould and cleaning time cut in more than half. Stronger and better with a fantastic smooth finish that will last a life time!