We are trying to make white pavers for crosswalks versus painting. In 2013 we made some that are still in fair shape, did not write down what we used as a formula. I think it was 3 to 1, one part Portland to 3 sand. When we broke one open it looked like porcelain. The ones we have made since are not holding up to freeze thaw and heavy equipment doing snow removal. We made some the other day using I part Portland 2 parts sand, we mixed acryl 60 1 part to 2 parts water. We mixed Portland and sand first then added water mixture mixing only long enough to hydrate. When we removed from the forms you could see they were full of holes. What's up ???
For maximum strength your first recipe, 3 to 1, is ideal. A better material that will increase the bonding strength of the concrete molecules is shredded fiberglass. Added in proportion to your mix it can substantially increase the concrete. I would also not use any admixture which is normally used to increase the bond when pouring fresh concrete to cured.
I agree with your mixing as little water as needed. That step alone will yield the strongest concrete. Conversely, it will also make it the most difficult to work. So finding the median can be difficult.
If the pavers are plain and with out an intricate pattern adding some sort of framing or reinforcing bar will allow the subsequent poured concrete additional rigidity able to withstand more compressive forces. Depending on the thickness of the pavers a 3/8 inch re-bar square an inch shorter from the edges of the paver will help a lot. Or a steel mesh or grid is also sufficient.
You should also allow the concrete to set as slowly as possible. Increasing the cure time by slowing the water that evaporates will produce a stronger concrete. Once the pour has become firm wet it down gently with cool water and cover it with plastic.
The air pockets found in the cured paver may have occurred do to the way it was mixed or how it was poured. If you are using a drill paddle switch to either a barrel mixer or a shovel. Either will decrease the amount of air that becomes entrained into the concrete.
I would use a little extra cement and add more water. This will help get rid of the bubbles. A handheld vibrator on the edge of the form would also shake the bubbles out.
For more strength add small aggregate in place of some of the sand. I usually use 1,2,3 ratio for hand mixing. 1 scoop cement, 2 scoops sand & 3 scoops gravel.