Investigation of Sodium Sulfate Phase Transitions in a Porous Material Using Humidity- and Temperature-Controlled X-ray Diffraction

Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. To investigate such deleterious processes, appropriate in situ techniques are required. This paper describes the use of X-ray diffractometry under controlled conditions of temperature and relative humidity (RH-XRD) for the direct observation of phase transition reactions in a porous substrate. An improved environmental chamber without temperature gradients is presented and applied to the investigation of phase transformations in the system Na2SO4 + H2O. This salt is generally considered as particularly damaging and frequently used in accelerated weathering tests. It is demonstrated that RH-XRD can be successfully applied for the direct observation of several relevant phase transitions in glass frits used as porous substrates. The conversion of Na2SO4(III) to Na2SO4(V) and the hydration of Na2SO4(V) both proceed fairly rapidly as true solid-state reactions without deliquescence of the educt phases. In contrast, crystallization from solution is kinetically hindered as there is a strong tendency of aqueous Na2SO4 to form supersaturated solutions also in narrow pores. The important implications of this behavior of the salt are also briefly discussed in the paper.