If I poured concrete mix not concrete patching compound on cracks on my sidewalk will it stick? why not?

If you chisel out a "Vee"-section so a lot can adhere, and thoroughly Wet it, overnight, Maybe.

If it cracks again (probably tree root under it), I'd get root killer to stop problem at source, then use that Vinyl flex patch. '

In fact, I was hard-pressed to find a real cement mortar patch for my brickwork, short of buying 50# of it. I cant use flex in this application.

https://www.youtube.com/watch?v=1hqmO7OTmlg

A patch compound is better. Regular concrete will work, as long as the cracks are cleaned and there is enough surface to bond to. Concrete is intended to be poured in a larger mass like a slab or a wall, not intended to patch cracks. Concrete depends on the aggregate (sand and stone) to make a solid finished product. You can't have large aggregate in a mix to patch cracks.

no, regular concrete mix has too big of gravel in it to fill cracks. and you would need to make it too thick or it will just crack and come off. you can paint the edges of the crack with concrete glue so it will stick better.

You can get stuff to improve adhesion but, even if it sticks, it's going to crack again. Whatever the cause of the movement or shift to crack the sidewalk will almost certainly continue to move or shift.

It'll stick, but it won't help.

There is a cleaner that your suppose to use when putting concrete over older concrete, cleaned and then a bonding agent is used, your local hardware store has the crap, the names(like so many others) are escaping me at the moment. Directions are right on the bottles, easy enough to use.

Not enough adhesive properties to stick to dry concrete. Concrete will not take tension, so a thin sheet is brittle.

Water the racks first and cover the work with plastic sheeting for a week or two. That will assist bonding and curing.

I'd just put a dry silican sand cement mix into the cracks,and water it.

just mix some SBR resin into the mix and it will stick fine