Phase change in a lake sediment

I'm trying to simulate a freezing process at the bottom of a lake and I want to include phase change of the sediment. I have been trying to implement it and at the moment it takes ages to simulate a 24 h freezing with 1 hour time step. How do I shorten the simulation time and correctly use the phase change physics?

I have attached the file!

I'm new to Comsol as well!

/Jilip