Effect of alkalinity on early-age hydration in calcium sulfoaluminate clinker

Early-age hydration with neutral or alkaline (0 M to 8 M) solutions was studied in calcium sulfoaluminate clinker (CSA) using isothermal conduction calorimetry, X-ray diffraction (both continuous and discontinuous, i.e., after detaining hydration with isopropanol) and infrared spectroscopy. Hydration in 0.1 M NaOH was observed to be similar to hydration in water. At the next highest concentration studied (1 M), however, the reaction rate was slower, due primarily to lower AFt stability in that medium. Faster hydration but less total heat was observed in the pastes hydrated with 2 M NaOH, a finding associated with the presence of a metastable compound, U-phase, that subsequently transitioned to AFt, C3AH6 and thenardite. Hydration with highly alkaline (4 M and 8 M) solutions inhibited AFt formation completely, although ye'elimite reacted more vigorously (particularly with 8 M NaOH) to form katoite, AH3 and thenardite.