Concrete Countertop Mixtures

There are several options for actually making the concrete for your concrete countertops.
1) Prepackage countertop mix - Buy all of the ingredients preweighed out, all you do is mix. Buddy Rhodes, Encounter, quikcrete, and regional suppliers all provide these mixtures.
2) Modified packaged mix - Buy regular outside sackcrete dry concrete and add colors and water reducers to make it work for countertops. Cheng Pro-formula is a prepackaged mixture you mix with regular 5000psi quikcrete to make a countertop. His mixture contains water reducing admixture, accelerating admixture, fibers, and color.
3) Your own recipe - With a little knowledge you can make your own.

The important aspects are workability (to get it in the forms), strength (to get it out of the forms and installed), low shrinkage (to prevent unwanted cracks), and low air content (to give a nice surface).

For the sink I developed my own mixture. Since it had very intricate shapes I replaced the coarse aggregate (gravel) with the same volume of sand. So the sink is actually mortar and not concrete, but for most people it is a fine line. Here is the testing summary for about 15 mixes I did. To speed up the process I was placing most of the iterations based on 1-day strength. *Remember concrete gets hard from a chemical reaction process called hydration. This process goes on forever. However, the majority of strength is gained in the first 28-days. Since the goal was to remove forms after 24 hrs, that's when I tested strength.

Testing Summary of 15 mixes:
Mixture 1 was standard Quikcrete proportions with sand instead of gravel, which contains about 540lbs of cement/cubic yard of concrete (pcy). The water-to-cement ratio (w/c) was 0.45. Lower water-to-cement generally means higher strength. I also included a deair-entraining admixture to make sure the surface didn't get holes.
1-day strength 2361 psi (pounds per square inch)
4-day strength 3393 psi
7-day strength 4570 psi (typical concrete is 4000 psi at 28-days so this is pretty good)

Mixture 2 took mixture 1 and increased the cement to 900 pcy, while lowering to w/c 0.30 with a high-range water reducing admixture (HRWR). It takes some special chemicals to lower the w/c while still maintaining workability.
1-day strength 7484 psi
4-day 8457 psi
7-day 10,582 psi (high strength concrete)

Mixture 3 maintained 900 lbs total cementitous, but substituted 15% metakaolin clay for cement. MKC is a fine white processed clay that lowers the permeability and lightens the color. This mixture also included a latex polymer to lower shrinkage and increase tensile strength. The flexural strength was improved by adding polypropylene fibers (think Easter grass) and cellulose fibers (think paper mache).
1-day strength 6391 psi
1-day tensile strength 793 psi

The ultimate mixture had good workability and strength with low permeability and shrinkage. Platinum color was added to give the sink a brighter concrete color.