What do you mean they've never hardened and why are they still wrapped in plastic? Did you unmold these? Are they soft? If it hasn't hardened by now it's not going to. Cement does has a shelf life. If the cement gets little bits that have hardened up but can be turned back into a powder easily by hand I'll use it. If the lumps are rock hard but some of the cement is powdery don't use it, buy a fresh bag. I would think if you could put some kind of binder on the outside it would still be uncured on the inside and will be weak. Billie

I would try removing the plastic covering an see if that helps. I would lean towards thinking the cement has gone bad. Cement dries by chemical reaction along with evaporation. If the piece was sealed in plastic it should be hard but it could still be a little green. If you can cut a groove in it with your fingernail it is definately not drying. I had a failure last year from using old cement - if the cement asorbs two much moisture it will not set properly.
George