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 [Na.sub.2]S[O.sub.4] + [H.sub.2]O. 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 [Na.sub.2]S[O.sub.4](III) to [Na.sub.2]S[O.sub.4](V) and the hydration of [Na.sub.2]S[O.sub.4](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 [Na.sub.2]S[O.sub.4] to form supersaturated solutions also in narrow pores. The important implications of this behavior of the salt are also briefly discussed in the paper.