Early age hydration of calcium sulfoaluminate (synthetic ye'elimite, C sub(4)A sub(3)S) in the presence of gypsum and varying amounts of calcium hydroxide

Suspensions of synthetic ye'elimite (C sub(4)A sub(3)S) in a saturated gypsum (CSH sub(2)) and calcium hydroxide (CH) solution were examined in-situ in a wet cell by soft X-ray transmission microscopy and ex-situ by scanning electron microscopy. The most voluminous hydration product observed was ettringite. Ettringite commonly displayed acicular, filiform, reticulated, and stellate crystal habits. Additionally, pastes with C sub(4)A sub(3)S2, 15% CSH sub(2), and varying amounts of CH were prepared and examined with X-ray diffraction (XRD) and isothermal calorimetry. The XRD experiments showed that increasing CH content caused more solid solution (SO super(2) sub(4) super(-)/OH super(-)) AFm phases to form at early ages (< 1 d) and more monosulfate to form at later ages (> 1 d). Calorimetry indicated that the increased production of solid solution AFm was accompanied with an increase in the initial (