Neat cement slurry

[handyattimes]

I put a patio down a few years ago. It was directly onto concrete. I used a 3:1 wet mix. Once it had dried some of the slabs started to come up. Then some more. I reckon about half in the end. Due to other projects and the fact it annoyed me only now am i thinking of giving it another go. I hear a lot about neat cement slurry to coat the slab/concrete so slabs will stay down. Any suggestions to get the best slurry? Also, any improvements on my 3:1 mix as it's going directly onto concrete?

Cheers