Sodium sulfate heptahydrate in weathering phenomena of porous materials

Salt weathering is widely recognized as one of the most common mechanisms for deterioration of porous materials: monuments, sculptures and civil structures. One of the most damaging salts is sodium sulfate, which can have different crystalline modifications: thenardite (anhydrous), mirabilite (Na2SO4.10H2O), and heptahydrate (Na2SO4.7H2O), which is thermodynamically metastable. Na2SO4.7H2O has a well-defined supersolubility region limited by the so-called heptahydrate supersolubility line. To predict and prevent crystallization damage of porous materials it is necessary to know the salt phase that is responsible for damage as well as its nucleation and growth behavior. The crystallization of sodium sulfate can be induced by increasing the supersaturation either by drying or by cooling of a sample. In this study the supersaturation was measured non-destructively by Nuclear Magnetic Resonance (NMR). First, the crystallization was studied in bulk solutions. For this purpose an NMR setup was combined with time lapse digital microscopy, allowing simultaneous measurement of supersaturation in a droplet and visualization of the crystal growth. Two crystallization mechanisms were tested: diffusion controlled and adsorption controlled. The crystallization of heptahydrate was found to have so-called adsorption-controlled behavior. As a second step towards understanding sodium sulfate crystallization in porous materials, mineral powders were added to sodium sulfate solutions. This allowed studying the transition from in-bulk to in-pore crystallization of sodium sulfate. It was found that mineral powders act as additional nucleation centers, which accelerate the precipitation of crystalline phases from a solution, but do not have an effect on the crystalline phase that is growing. Next, the crystallization of sodium sulfate in porous materials was studied. The internal properties of the materials influence the dynamics of crystallization by providing a surface for nucleation. This is in correspondence with grain-boundary crystallization theory. It was found that the internal properties of porous materials do not influence the crystalline phase that is formed. In all measurements that were performed, the formation of sodium sulfate heptahydrate was observed with a reproducibility of 95%. No spontaneous crystallization of mirabilite directly from a solution was observed. Finally, the crystallization pressure was studied. To this end NMR measurements and optical length measuring techniques were combined. This allowed studying the crystalline phase being formed and the crystallization pressure caused by crystal formation during cooling and drying of the samples. It was found that a crystallization pressure capable to damage common porous materials can be expected from mirabilite. Series of weathering tests showed two ways for mirabilite formation: cooling of sodium sulfate solution to cryohydrates and rewetting of previously formed thenardite