Hydration of polyphase Ca3SiO5-Ca3Al2O6in the presence of gypsum and Na2SO4

Alite (Ca3SiO5: C3S*) and calcium aluminate (Ca3Al2O6: 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) Ca3SiO5-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 C3S. The C3S 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 Na2SO4. 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 Na2SO4, recrystallization of ettringite from the AFm phases is postponed from 1 day to 7 days. *Cement chemistry notation: C-CaO, S-SiO2, A-Al2O3,S over bar -SO3, N-Na2O.