Hi @hopeaotearoa
I am assuming your deck is timber.
To answer your question can you put breeze blocks onto any flat surface for low walls. The higher the wall the more they fast need more traditional concrete base and metal support rods.
You are better off putting a timber frame and screwing decorative panels too it for long term instead of railings
But lets explore
Yes you can actually glue breeze blocks together with outdoor liquid nails instead of mortar. This will glue to timber as well. I do it often for the odd loose pavers etc this defies traditional cementing more premanent builds but it works all the same. I just glued a row of glass bricks in a low non weight bearing garden wall it is solid looks very good in a decorative way only 600mm height including a besser capping glued on.
Often cemented capping all to often can be impacted and crumble off out door building glue and polymers hold better.
Why you probably shouldnt breeze block the deck directly
On a timber deck I probaly wouldnt go more than 2 breeze blocks high as they weigh in at approx 16kg each Warp your deck.
- Deck needs mantinace for longer life not possible with bricks on it.
- Deck might need replacement later on so your wall will need to come down best to build the breeze block wall on to the ground besides the deck/
