Good question. You have a few options for window and door bucks in an ICF (insulated concrete frame) structure.
The 2006 IRC suggests you should use "naturally durable" wood or wood that is preservative-treated. Many people believe cedar to be a naturally durable wood, but you'll want to check with your local building official or inspector to ensure it is allowed in your area or if you are required to use something treated.
If you decide to go with cedar, you'll want to look for western red cedar ? it is much more abundant than the northern white cedar typically found on the East Coast. If possible, you should also attempt to find cedar that has been certified by a third party as sustainably harvested. Certified sustainable wood is becoming much easier to find through traditional lumber supply companies, home improvement centers or local green building material stores.
If the inspector disapproves of the natural cedar, you'll want to find preservative-treated wood that doesn?t pose potential detrimental impacts to the environment or to your family. Look for lumber that has been treated with borate- or silica-based treatments. They are less harmful to the environment than the typical copper (ACQ)-based treatments and not known to be harmful to your family like chromated copper arsenate (CCA) (banned from new playground wood in 2003) and ammonium copper arsenate (ACA) treatments. Copper treatments, though not harmful to humans, are highly toxic to marine organisms and corrode the steel fasteners attaching the wood to the slab and other framing members. Like cedar, it would be nice to get pine that has been certified as sustainably harvested.
There are other alternatives on the market, including a PVC-based product and proprietary systems devloped by certain ICF manufacturers. PVC is very durable but comes with negative beginning-of-life and end-of-life environmental and health impacts that are too big a topic to get into here. It would be nice if an ICF manufacturer came up with a recycled plastic buck (you could ask your building official about using recycled plastic or composite lumber), but I haven?t seen one. Good luck with your project and enjoy the energy savings, sound qualities and thermal comfort of your ICF home.