He says builders in Mexico level roofs repeatedly for as long as six months after the pour.

How do they do that? Puddle more concrete on top?

yup. They keep filling the depressions, which would take extra care in freeze-thaw climates. You can buy a two part cement-based sealer for aac, but I can tell you from experience that cracks will open as planks move under expansion and contraction.

I should explain that differential expansion is the problem with aac. Being half air, the planks are more stable than the mortar surrounding them (or so I surmise.) I suppose that mesh over the two-inch mortar joints could cure cracking. But I'd rather not rebuild the deck up there more often than once every 15 years.

I have Caribbean customers ordering the Kryton KIM (Krystol Internal Membrane) product from Kryton Int'l for waterproofing flat Quad-Deck roofs. Great product, added just prior to pour. On a Quad-Deck/Insul-Deck roof, it adds about $1.00 per square foot.....pretty cheap for that kind of insurance against water intrusion.

http://www.kryton.com/products/admixture