Yes, fly ash concrete seems more sustainable than concrete without it (provided that the fly ash already exists) in certain applications where its particular properties are beneficial. I seem to recall it having greater durability under compressive loads, so roads can be built that require less frequent repair. In the US, we have enough roads falling apart I think we could use all the fly ash concrete we could possibly make, and we wouldn't have any extra to build substandard, energy-inefficient housing.