Water effect on internal friction of set plaster

Set plaster is brittle when it is dry and becomes plastic with a drastic drop of mechanical properties in the presence of water. In this work, mechanical spectroscopy has been used to investigate this water effect. Internal friction was measured as a function of strain amplitude at different humidity conditions. A weak dependence is observed at low strain amplitude, followed by a steep linear rising above a threshold strain amplitude. Increasing the relative humidity increases the internal friction and lowers the threshold of the rising part. The results provide an experimental evidence of a relative sliding of gypsum crystals enhanced in the presence of water. A simple viscoelastoplastic rheological model is proposed to describe this effect.