Sodium sulfate heptahydrate : direct observation of crystallization in a porous material

Hamilton, Andrea and Hall, Christopher and Pel, Leo (2008) Sodium sulfate heptahydrate : direct observation of crystallization in a porous material. Journal of Physics D: Applied Physics, 41 (21). 212002. ISSN 0022-3727

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Abstract

It is well known that sodium sulfate causes salt crystallization damage in building materials and rocks. However since the early 1900s the existence of the metastable heptahydrate has been largely forgotten and almost entirely overlooked in scientific publications on salt damage mechanics and on terrestrial and planetary geochemistry. We use hard synchrotron x-rays to detect the formation of this metastable heptahydrate on cooling a porous calcium silicate material saturated with sodium sulfate solution. The heptahydrate persists indefinitely and transforms to mirabilite only below 0 ◦C. At the transformation, which is rapid, the solution is highly supersaturated with respect to mirabilite. We estimate that crystallization of the heptahydrate and of mirabilite have associated Correns pressures of about 9 and 19MPa, respectively, exceeding the tensile strength of building stones. We detect lattice strains in the salts from x-ray measurements consistent with these values.