I'm going to do my underfloor insulation with a Vermiculite concrete layer. Going online most of the sellers are either builder's suppliers doing a product called exfoliated Vermiculite (that looks like something, expanded you put in your loft to insulate it) or garden suppliers selling it as something more granular that you would mix in as compost additive.

Are they the same? What should I be buying to mix in with Portland cement to make Vermiculite concrete? Any good suppliers?