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

  • Hamilton A
  • Hall C
  • Pel L
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It is well known that sodium sulfate causes salt crystn. 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 geochem. We use hard synchrotron x-rays to detect the formation of this metastable heptahydrate on cooling a porous calcium silicate material satd. with sodium sulfate soln. The heptahydrate persists indefinitely and transforms to mirabilite only below 0 DegC. At the transformation, which is rapid, the soln. is highly supersatd. with respect to mirabilite. We est. that crystn. of the heptahydrate and of mirabilite have assocd. Correns pressures of about 9 and 19 MPa, resp., exceeding the tensile strength of building stones. We detect lattice strains in the salts from x-ray measurements consistent with these values. [on SciFinder (R)]

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