I know the recipe for both concrete and refined concrete in assembly machines do not allow for productivity modules, but did you know that the foundry recipe that makes concrete from molten iron does allow for productivity modules?

Even if it just means the regular concrete from lava recipe allow for productivity, where the refined concrete from lava does not.

Just something to consider.

I think I remember something like that. I could allow the prod modules for these specific recipes. But I recommend the mod here:
https://mods.factorio.com/mod/concrete-productivity
which is fully compatible with my mod.
Also, I currently work on a big update and I can include your suggestion with that.
EDIT: As I rework a big chunk on the backend, I don't think this idea will be in the next version, unless I find a good way for this exception.