MgO as an incredient for PC was recently brought up in the papercreters yahoo forum.
Here's Ernie Phelps' take on it:
I'll be honest and say I had not heard of MgO before you mentioned it
here. I like the idea and here are the pros and cons as I see it:
1. Mixes well with Cellulose based materials.
2. Very mold, fire and water resistant.
3. Somewhat better for the environment than portland.
4. Available both as a mix and as pre-made panels.
1. Very expensive compared to portland ($2 per lb vs $.075 per lb).
2. Untested within the papercrete arena (not a big problem as it has
been thoroughly tested with a large variety of aggregates).