Not one person asked how it got there. Throw the tank away. Every little spec of sand, grit, gravel will find its way to the fuel filter. You will be battling a plugged filter for months to come. Start with a new tank and be done with it.

you need a new tank. and if the engine was run while the quikcrete was in the tank, you need a new fuel system and possibly a complete engine cleaning. particles can destroy an engine if they get past the fuel filter.

Buy a new gas tank. Or take it to a radiator shop and pay them to clean it.

Take the tank apart and chisel out what you can, sandblast the rest.

Assuming it's dry.

Still wet, I disconnect the tank, and start flushing it with a high power hose; then open and clean it.