Abstract: In the making of a mortar or concrete rich in aluminate cement wherein the cement is mixed with water to hydrate and is thereafter permitted to harden, the improvement which comprises adding to the mixture of cement and water an ionic compound containing anions X.sup.n- whereby there are formed mixed crystals of the formula[Ca.sub.2 Al(OH).sub.6 ].sup.- [( m/n)X.sup.n- (1-m) Al(OH).sub.4.sup.- mAl(OH).sub.3 ; aq].sup.-Preferably X is NO.sub.2.sup.-, SO.sub.3.sup.2-, S.sub.2 O.sub.3.sup.2-, Fe(CN).sub.6.sup.3-, PO.sub.4.sup.3-, FE(CN).sub.6.sup.4-, MnO.sub.4 --, ClO.sub.3.sup.- or CrO.sub.4.sup.2-. The mixed crystals are new.