I am thinking of using wood chips and shavings to make a lightweight mix. The obvious danger here is that it can burn; is there an easy way of overcoming this?
Yes. Immerse the wood first in a strong solution of calcium chloride, or silicate of soda, or in a lime slurry. You will need to experiment to get the proper degree of resistance, and also to determine the effect on the mix properties. Don't forget another danger--the concrete must be kept weather-tight, or it will still be subject to decay.