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).
High-strength concrete has a compressive strength greater than 40 MPa (5800 psi). In the UK, BS EN 206-1 defines High strength concrete as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower.
Bob Vila is looking at 6000 PSI (40 MPa) concrete for countertops.
Three days after pouring a counter with Countertop Mix, the compressive strength of the slab will achieve resistance equal to 2,000 pounds per square inch (PSI). At seven days, it reaches 4,000 PSI, and at full cure (28 days), the countertop slab will reach 6,000 PSI. All-purpose concrete at full cure reaches only 4,000 PSI. Choosing the mix with the higher PSI will provide better defense against cracks (and limit crack repair) down the line.
Concrete Mix Design as per IS 10262-2009 – Procedure and Calculations. If that math isn't beyond you as it is me, then you could figure it out. The important part is a water-cement ratio of 0.35 or lower.