P - Technische Universiteit Eindhoven

5. SODIUM SULFATE HEPTAHYDRATE FORMATION IN POROUS
BUILDING MATERIALS
Sodium sulfate is one of the most damaging salts for porous building materials. Three crystalline
phases of sodium sulfate are known: thenardite (Na 2 SO 4 , anhydrous), mirabilite
(Na 2 SO 4·10H 2 O, decahydrate), and the thermodynamically metastable heptahydrate
(Na 2 SO 4·7H 2 O). In this work, the in-pore crystallization of sodium sulfate heptahydrate
is studied in four different porous building materials with pore sizes from 0.5 to 100 µm.
Nuclear Magnetic Resonance (NMR) is used to measure the hydrogen and sodium content,
which enables us to obtain the sodium concentration of the pore solution non-destructively.
The NMR experiments show that the first phase to precipitate in the porous materials
is heptahydrate, which only transforms to mirabilite by further cooling to very low temperatures
(around −8 ◦ C). The time evolution of the volume fraction of heptahydrate is
determined from the measured sodium concentration. It appears that the nucleation rate
is influenced by the mineralogy of the porous material.
5.1 Introduction
Salt weathering is widely recognized as one of the most common mechanisms of deterioration
of monuments, sculptures and civil structures, and one of the most damaging salts
is sodium sulfate. As shown in figure 5.1, three main crystalline forms of this salt can be
distinguished [22]: thenardite (Na 2 SO 4 ), mirabilite (Na 2 SO 4·10H 2 O) and the thermodynamically
metastable heptahydrate (Na 2 SO 4·7H 2 O). To predict and prevent crystallization
damage, it is necessary to know the salt phase that is responsible for damage, as well
as its nucleation and growth behavior in a porous material.
Sodium sulfate heptahydrate (hepta) has a well-defined nucleation threshold at the
so-called heptahydrate supersolubility line [28,59]. In recent work, hepta was shown to be
the phase formed by cooling either droplets of sodium sulfate solution [22] or suspensions
of quartz and calcite powder in the solution [60]. The growth rate of one hepta crystal
was determined by optical microscopy, while the sodium concentration was measured
simultaneously by Nuclear Magnetic Resonance (NMR). Analysis of the crystal growth
kinetics in droplets indicated that the growth is interface-controlled and that the growth
rate is proportional to the supersaturation of the solution [22]. The higher tendency
for hepta to nucleate compared to mirabilite has also been observed in the presence of
mineral powders (quartz and calcite), which promote heterogeneous nucleation [60]. In
that study the influence of the substrate surface on the nucleation of hepta was described
by a modified Avrami analysis that considers the change of both the solution volume and
the supersaturation during crystallization with an interface-controlled growth rate. Nonisothermal
crystallization was investigated by Differential Scanning Calorimetry (DSC)