Air Slaking

Sodium geopolymers will “bloom” after a couple weeks while drying. I understand that this is unreacted sodium that migrates to the surface, probably as sodium carbonate. The bloom can be washed off easily, and may come back once but is not very persistant in my experience. Perhaps a perfect specimin with no unreacted alkali will not do this. Potassium geopolymers possibly do not bloom because they are not molecular sieves (the sodium has no channles to migrate through). (I have only losely gotten this impression from reading and have not compared sodium and potassium in this respect).

In his groundbreaking book on hydraulic mortors, Le Châtelier discusses substances that are slaked by humid air but not by water. Over-burned lime will air slake over time and destroy what looked like a good concrete. Le Châtelier sites sodium sulfate as an excellent example of a substance which seems to set fine with water but then will blossom in humid air. I have seen this impressively in sodium chloride and in sodium carbonate mud mixtures. In geopolymers I believe the free alkali reactes with carbon dioxide from the air, and then expands and grows as humidity allows it to migrate. The change in volume causes internal pressure which weakens or destroys the piece. This is just my thinking.