Lava rock brand

[Watermelon]

Hello - My 4 15 gallon pots call for 5 gallons of lava rock in each pot.

I did get 6 bags of the quickrete brand below, but im wondering are these even ment to mix in with soil?

The rocks are really big and have been smashing them, but a lot of the time they just turn to dust. So I'm not sure if I should mix them in my soil?

Any help with this would be great. I already mixed the rocks in with the soil, so I'm hoping this will work out.

 

[Gardenseed]

Very good question my friend. I'm still growing in soil but looking to go Hydro. I'm looking into it and I can't wait to see what replies you'll get.