I'm making concrete countertops, so sand-only is the way to go to reduce voids in the forms. (I've done a half dozen samples). I've looked everywhere for mix ratios, but it's a weird request, and I can only find either concrete with aggregate ratios, mortar ratios, or someone wants me to give them a bunch of money to tell me 3 numbers. My best guess is 0.5 water/1 cement/2 sand/ 0.0035 superplasticizer. The sample looks fine, and doesn't break when I hit it with a hammer... but I'm just worried that I'm doing it wrong, and it's going to crumble in 3 months or something.
First I want to point out that the hardness has a lot to do with cure time and temperature. Longer cures (6-8 weeks) at cool temperature (say 50 °F) are harder. Short, hot cures are harder initially, and crumble later.
Generally, the 1pt cement : 2pt sand ratio is best and will have a 3500 psi compression stress. A 1:3 ratio will have less than 3000 psi. Gravel can actually strengthen the mix... up to 3 parts. The mix of choice (concrete) is 1pt cement :2pt sand : 3pt gravel, because it's the best/hardest (for the money).