Q: What are the chemical reactions between portland cement and aluminum that cause: (1) Hydrogen gas (as in pumping through aluminum pipes)? (2) Expansion of the aluminum (as in buried conduit)? Does the latter occur when calcium chloride is not present? What are the actual chemical equations that represent these actions? Precisely how does hydrogen gas affect the strength of the concrete? By loss of cementing action on the aggregate particles? By creating planes of weakness? What are the corresponding sets of chemical equations when pozzolan/lime or fly ash/lime concretes are used? Are the reactions with aluminum any less severe in such cases?