How does sodium sulfate crystallize? Implications for the decay and testing of building materials

The fundamental behavior of sodium sulfate crystallization and induced decay in concrete and other building materials is still poorly understood, resulting in some misinterpretation and controversy. We experimentally show that under real world conditions, both thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O) precipitate directly from a saturated sodium sulfate solution at room temperature (20°C). With decreasing relative humidity (RH) and increasing evaporation rate, the relative proportion of thenardite increases, with thenardite being the most abundant phase when precipitation occurs at low RH in a porous material. However, thenardite is not expected to crystallize from a solution at T<32.4°C under equilibrium conditions. Non-equilibrium crystallization of thenardite at temperatures below 32.4°C occurs due to heterogeneous nucleation on a defect-rich support (i.e., most porous materials). Anhydrous sodium sulfate precipitation is promoted in micropores due to water activity reduction. Fast evaporation (due to low RH conditions) and the high degree of solution supersaturation reached in micropores before thenardite precipitation result in high crystallization pressure generation and greater damage to porous materials than mirabilite, which crystallizes at lower supersaturation ratios and generally as efflorescence. Data from the environmental scanning electron microscope (ESEM) show no hydration phenomena following wetting of thenardite; instead, thenardite dissolution occurs, followed by thenardite plus mirabilite crystallization upon drying. These results offer new insight into how damage is caused by sodium sulfate in natural geological, archaeological, construction and engineering contexts. They also help explain some of the controversial results of various commonly used sodium sulfate crystallization tests.