How to make white portland cement stronger?
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.