On Brazil we hadn't much thermo-electric (coal power plant) but we have a very huge (giant) production of sugar cage (for suger and ethanol for cars) and there is enormous amounts of cane bagasse after the squeeze the sugar cane juice. This bagasse it's most times burned to generate energy (electric and thermal) to power the sugar refinary usine.
Well, they found the ashes of sugarcane bagasse can be used as partial replacement to portland cement or to make the portland get sme good properties. Not sure if works for aircrete, but I found it's good for cements for ecologic bricks (earth compressed bricks with portland).
https://www.researchgate.net/publica...nt_replacement
https://www.researchgate.net/publica...ies_of_mortars
With few like replace 5% of portland cement with ashes make it even better.
https://www.researchgate.net/publica...ment_of_Cement
About romans, their concrete formula last mouch more than our modern portland cement. That's why many roman builds are still stand.