Hydration of polyphase Ca 3 SiO 5 ‐Ca 3 Al 2 O 6 in the presence of gypsum and Na 2 SO 4

Alite (Ca 3 SiO 5 : C 3 S * ) and calcium aluminate (Ca 3 Al 2 O 6 : C 3 A) are the major phases in Portland cement, which have an essential role in the development of early age properties. The effects of gypsum content, fineness, and Na 2 SO 4 addition on the early‐stage hydration kinetics are investigated for polyphase (co‐sintered) Ca 3 SiO 5 ‐Ca 3 Al 2 O 6 model systems using calorimetry, X‐ ray diffraction, thermal analysis, and solid‐state 27 Al and 29 Si nuclear magnetic resonance (NMR) spectroscopy. The results demonstrate that the hydration of C 3 A significantly affects the hydration of C 3 S. The C 3 S and C 3 A hydration is hindered considerably in severely over‐sulfated systems (where C 3 A hydration is suppressed due to a very high gypsum content) and systems with additional Na 2 SO 4 . Although there is a considerable amount of Al incorporation in the C‐S‐H phase, no clear trends with respect to gypsum content, hydration age or Na 2 SO 4 addition are observed for the Al IV /Si ratios of the C‐S‐H phase determined from 29 Si NMR. With the addition of Na 2 SO 4 , recrystallization of ettringite from the AFm phases is postponed from 1 day to 7 days. *Cement chemistry notation: C‐CaO, S‐SiO 2 , A‐Al 2 O 3 , S ¯ ‐SO 3 , N‐Na 2 O.