I've asked about https://www.workshop.bunnings.com.au/t5/Garden/Correct-concrete-mix-for-holding-landscaping-rocks-in-place/m-p/144031 how to repoint (add mortar mix between rock gaps) before back in Feb, finally I've got time to work on this problem 😉
Some new question arise before carrying out the job, I'd appreciate to get expert advice before rolling. TIA
Is it recommended to fill the big gaps between rocks before filling with concrete mix?
Reason why: when pulling out the weeds, sands were pulled out together, so I reckon sands (or sand motor / cement mix) were used to fill the gaps up till the point where it is applicable to use mortar mix.

Sands, lots of sands..
