[pinkpanda3310]

Just like you said ordinary portland cement is better than flyash. I think your on the right track with extra cement to up the mpa. I think the trick to get even more mpa will be to have lots of varying sized aggregate. Brickies sand is pretty much yellow sand of the same grain size and colour, on the other hand plastering sand has more clay content and varying size grains which lock in better.

How big is your large aggregate? You might consider crusher dust (fine blue metal) it's about 5mm IIRC. Having jaggered edges it will lock in good too but might be on the large side for your application.

Soaking it in water while it goes off (sets) is great for it. The more moisture you can retain the stronger it will be. For this reason try to get all faces as smooth as possible so it is less porourus when it dries out.