I have used a lot of the foam for posts and love it. You can buy 4 gallons off Amazon for $150, kind of expensive but I am barb wire fencing my 26 acres, so getting concrete to all holes is very difficult, my skid steer can not even get to all the places needed. The foam is said to also seal the post from below ground rot. Getting the right amount mixed is a learning curve, it also depends on the air temperature for the rise of the mix. But it cures quickly and is rock solid.
I don't know why you would need fast setting mix? Do not add as much water and it will set very rapidly.