A hierarchical study of the mechanical properties of gypsum

The flexural strength of gypsum is reported for freestanding single crystals in three-point bending carried with a nanoindenter. The elastic modulus, splitting tensile strength, and fracture toughness of monolithic gypsum consisting of interlocking needle-like microcrystals are also reported as functions of porosity and accelerator addition. This study shows that geometric configurations, in addition to porosity, affect the mechanical properties of gypsum. The properties are improved by 50–100% when the crystal network changes from needle aggregates to one made up of homogeneous randomly oriented single crystals. An Ashby geometric model for open-cell foams is adopted to link the properties of the individual crystals and the bulk properties. The lower and upper bounds of the measured elastic modulus are in accordance with bending-dominated behavior and stretch-dominated behavior predicted by the model, respectively. However, the strength of gypsum is much lower than values predicted by the model, which is based failure on fracture of individual crystals, suggesting that the strength of monolithic gypsum may be instead controlled by the failure of weak intercrystalline contacts.