Leaching of calcium sulfoaluminate cement pastes by water at regulated pH and temperature: Experimental investigation and modeling

Two cement pastes, prepared with a ye'elimite-rich calcium sulfoaluminate binder containing 0% or 20% gypsum, were leached by water at regulated pH (7) and temperature (20 degrees C) for three months with the aim of understanding and modeling the degradation processes. The cumulative quantities of Ca2+, OH- and SO42- ions in the leachates increased linearly versus the square root of time, showing that leaching was controlled by diffusion. The gypsum-free material exhibited the highest decalcification rate. Examination of the solid samples after three months revealed three zones: - the surface layer, highly porous and composed of AH(3), perovskite and possibly C-(A)-S-H, - an intermediate zone, less porous, in which several precipitation and dissolution fronts occurred, and - the sound core. The hydrated phase evolution along depth was accurately determined and qualitatively reproduced with a model coupling transport by diffusion and chemical reactions. (C) 2013 Elsevier Ltd. All rights reserved.