『Abstract
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℃). 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℃ under equilibrium conditions. Non-equilibrium crystallization
of thenardite at temperatures below 32.4℃ 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.
Keywords: Sodium sulfates; Humidity; SEM; X-ray diffraction; Degradation』
1. Introduction
2. Materials and methods
3. Results
4. Discussion
5. Conclusions
Acknowledgments
References