True about delayed expansion:

Another important additive to amalgam alloy is zinc. Originally, zinc was added to conventional amalgam as a processing aid to suppress oxidation of the key elements in the alloy.  Zinc tends to oxidize preferentially forming a zinc oxide film that covers the surface of liquid alloy during manufacture and suppresses oxidation of other elements.  Generally, 1% or more is added to accomplish this end. However, some (0.2% to 1%) is left in the amalgam alloy at the end.  A detrimental side effect of this residual zinc was that moisture contamination before setting converted the zinc to zinc oxide and produced hydrogen gas that could expand the amalgam excessively, resulting in patient pain, a phenomenon called as delayed expansion.