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

• Calcium sulfoaluminate cement pastes are leached by water at pH 7 and 20 °C.
• Leaching is controlled by diffusion.
• The gypsum-free material exhibits the highest decalcification rate.
• Mineralogy of the leached samples shows a zonation process.
• This process can be modelled by coupling diffusion and chemical reactions.

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 °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 AH3, 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.