Abstract The reaction of synthetic Ca 2AlFeO 5 at 20 °C with (a) H 2O; (b) H 2O, CaSO 4·2H 2O; (c) H 2O, CaSO 4· 1 2H 2O ; (d) H 2O, CaSO 4·2H 2O, Na 2SO 4; (e) H 2O, CaSO 4·2H 2O, K 2SO 4; (f) H 2O, CaSO 4·2H 2O, Ca(OH) 2; (g) H 2O, CaSO 4·2H 2O, Ca 3SiO 5; (h) H 2O, CaSO 4 ·2H 2O, KOH; was studied by differential scanning calorimetry and X-ray powder diffraction. Acetone dried samples were examined at hydration times between 2 min and 28 days, and 4 months in the instance of experiments (f) and (g). In (a) AFm forms immediately followed by hydrogarnet, while in (b-e) the formation of AFm is delayed, since the initial product is AFt, which then transforms into AFm. In the more basic samples (f-g), AFm formation is further delayed due to the introduction of a pronounced dormant period during AFt formation. The strongly basic sample (h) also exhibits this dormant period, but less AFt is formed and AFm appears as in (b-e).