The hydrolysis of pure and sodium-substituted calcium aluminates and cement clinker phases was investigated in situ in the temperature range 25-170 degreesC, using the angle dispersive powder synchrotron powder X-ray diffraction technique. The final hydrolysis product in all cases was Ca3Al2(OH)(12). The intermediate phase Ca4Al2O7.19H(2)O was formed from the pure calcium aluminates, and the intermediate phases Ca4Al2O7.xH(2)O, x = 11, 13, or 19, were formed from the cement clinker phases.