I have noticed that most castable refractories effervesce if mixed with acid. In context, this is orthophosphoric acid added to promote the formation of a calcium-aluminium phosphate bonding phase. In one case, I estimated that 40% of the calcium aluminate cement had become carbonated by reaction with the atmosphere. Suppliers don't see it as a problem It is impractical for me to roast large quantities of refractory to drive off the carbon dioxide, so I considered chemical ways of decarbonation that would not ultimately weaken the refractory and found the problem much more challenging than expected. Treatment with formic acid vapour decarbonates OK but the formate ions don't contribute to strength. The same applies to all the obvious anions. Chemically, chromic acid might work but the environmental hazards don't bear thinking about.