" What can I tell the contractor to use to give a concrete finish to our outdoor decks? "

Do you mean a grey color like concrete? If so get to a BM or SW store and pick out a solid color deck stain.

I want somethingI that looks like Concrete popossibly leveling compound or tile. Weight is the concern

Wood deck boards typically are never butted tight and a wood deck is in not in any way, shape or form designed to carry the load of either leveling compound or tile. That's a lot of weight that will be reduced to rubble from the decks natural movement on short order if you were to even find a way to install the leveling compound or tile.

I want to remove all the cedar decks boards I saw this product online

The existing framing system needs to be upgraded immensely. I was not only referring to the deck boards when I spoke loading and deflection. Tile and leveling compound all weigh a lots and the deck as a whole is not designed for the weight. It will fail in short order.