I found this online and it worked well and was cheap. It's strong smelling so be careful. A pressure washer would work best, but I got by with just a hose.
A Heavy-Duty Floor Cleaner
Here's a recipe for a solution that will remove extra-tough dirt and grime from asphalt, glazed or unglazed ceramic tile, concrete, flagstone, and slate floors:
* Mix:
1/4 cup low-sudsing, all-purpose cleaner
1 cup ammonia
1/2 gallon cool or cold water.
Caution: Wear rubber gloves, and work in a well-ventilated area when using this powerful solution.
* Apply the solution to the floor with a sponge mop, using pressure for heavily soiled areas; rinse with cool, clear water for spotless results.
* Dry with a soft cloth.