P - Technische Universiteit Eindhoven

8. NMR combined with accelerated weathering 100
o
thernardite c = 3.6 M
decahydrate c = 1.6 M
c) Cordova cream limestone
Fig. 8.5: Efflorescence pathway diagram for the drying part of the weathering cycles (see also
figure 8.4). The total amount of salt in the solution, cθ (in moles per kg) is plotted
against the moisture content θ. The two possible salt phases which can be formed at
22 ◦ C are indicated by c = 3.6 M for thenardite and c = 1.6 M for mirabilite.
For the fixed amount of 0.2 ml solution used in these experiments, in the case of the
Indiana limestone all the water would have been taken up in the crystals if there would
have been a complete transformation (see table 8.1). The only partial transformation
is most probably caused by a competition between the thenardite dissolution rate and
the transformation rate. As a result in the pores a mixture of thenardite and decahydrate
will be present, as was also observed by Rodriguez-Navarro et al. [89]. From the
initial amount of thenardite formed during drying and the measured concentration immediately
after rewetting we can estimate the transformation ratio (for a calculation see
the appendix). The resulting values are included in table 8.1. As can be seen from this
table our measurements indicate that only up to about 30 % of the thenardite is directly
transformed into mirabilite.
Because of the competition between dissolution of thenardite and formation of decahydrate
the solution remains highly supersaturated with respect to decahydrate. Therefore
a high crystallization pressure is induced as soon as the sample is re-wetted and a
rapid expansion of the sample is observed. Based on the measured concentration we can
estimate the crystallization pressure using equation 8.1 or 8.2. The results are plotted in
figure 8.6, which shows that upon re-wetting the crystallization pressure increases by a
factor of 2 to 3.
By inserting the measured expansion in equation 8.3 we can obtain an indication of
the poromechanical pressure. In figure 8.6, the poromechanical pressures calculated before
and after re-wetting are presented as horizontal lines. The point after re-wetting represents