I need help with mud texture: part 2

So thanks for all your comments on my post yesterday, today I tried something. I went to a nearby store and asked for grout. The problem is that under the English world "grout" there are a lot of different products I could buy here. In the end I chose... Concrete. Thinking it would dry rock solid and be less of a headache after settling down I mixed it with coconut fibers, PVA, wood pieces of various forms and shapes, sand and some synthetic gravel. In the end the look is PERFECT. What is bugging me is that it won't stay still, in fact it crumbles as if it is just sprinkled above. Any suggestion for sealing it once and for all? I thought about mixing PVA and acrylic with some chalk and just spread the mix all around. What do you think? Thanks in advance for your suggestions.

So thanks for all your comments on my post yesterday, today I tried something. I went to a nearby store and asked for grout. The problem is that under the English world "grout" there are a lot of different products I could buy here. In the end I chose... Concrete. Thinking it would dry rock solid and be less of a headache after settling down I mixed it with coconut fibers, PVA, wood pieces of various forms and shapes, sand and some synthetic gravel. In the end the look is PERFECT. What is bugging me is that it won't stay still, in fact it crumbles as if it is just sprinkled above. Any suggestion for sealing it once and for all? I thought about mixing PVA and acrylic with some chalk and just spread the mix all around. What do you think? Thanks in advance for your suggestions.