Disposal of fly ash for sustainable applications is the need of hour. A lot of research works are available and being carried out on use of fly ash in concrete or soil for modifying properties and economy in construction. Use of fly ash for the manufacture of coarse aggregates is one option to replace conventional aggregates particularly for light weight concrete or low cost concrete or low strength concrete. There is need to develop appropriate technology in this area.
There are a number of lightweight aggregates derived from fly ash on the market, at least in the UK. Lytag is perhaps one of the best known.
But, there are better uses for fly ash than for manufacturing aggregates. Aggregates are cheap and low-carbon when compared to cement. It makes more sense to focus on making maximum use of fly ash as an SCM.
I agree with professor Leon Black that now there are better uses for fly ash than for manufacturing lightweight aggregate.
I would say, it's not as simple as it seems at first. The aggregates either coarse or fine are inert materials which should contribute to the better physico-mechanical properties of the concretes. It must not react to the concrete mixtures. If it reacts then the changes in volume, density and mineralogical composition are expected. The fly ash is used as SCM and a reaction will occur within the concrete. Unless you have sintered the fly ash to cause the surface crystallization, I would never recommend using fly ash as the aggregates.
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