Here is a good formula for a beautiful strong concrete countertop:
4 parts white cement
1/4 fly ash
4 white sand
4 marble dust
1/3 fiber
It creates a nice white counter top that is also very strong. We do custom concrete counters and find that this formula really works. We sometimes add decorative aggregate. One that really works is broken up salvaged glass from discarded tempered windows. The tempered glass breaks up into nice little pellets. You can really pile it in there to the point where there is almost more glass than anything else. It gives a nice effect.
Another similar formula we like:
1 part cement (30% fly ash, 70% white cement)
2 part sand/dust (50% white sand, 50% marble dust)
2 parts glass pellets
1/4 water (50% concrete glue, 50% H2O)
This has a slightly coarser look, although it feels the same since you sand it down with super fine marble sanding.