# Pore Structure and Water Transfer in _Pietra d'Aspra_ Limestone: A Neutronographic Study
## 3 Results and Discussion
### Neutron Radiography Analysis
It is worth noting that the changes in pore dimensions as a consequence of the aforementioned phase transition are relatively small. However, as can be seen in Figure 8, where the comparison between the WC profiles in PA-TQ and PA4 is reported, the high spatial accuracy achieved by neutron radiography, in terms of liquid distribution within the stone network, allowed us to hypothesize such structural variations otherwise impossible to be highlighted through laboratory-based methodologies. Finally, the relatively high WC value (\(\sim\)4-5%) between \(\sim\)25 and \(\sim\)35 mm in PA4 was probably due to an extended fracture already present inside the stone or caused by the development of high crystallization pressures. As far as PA2 and PA5 are concerned, accounting for samples exposed to 15 cycles of salt crystallization, a substantial reduction in the amount of absorbed water can also be detected. In PA2, WC was found to be equal to 1 wt.% after 3.7 h, reaching approximately \(\sim\)18-20 mm. On the contrary, a remarkable increase in WC can be observed in PA5, with a mean value around 3-3.5%. This occurrence, in agreement with what was already observed in the case of PA1 and PA4 samples, supports the hypothesis that NS, more than NL, clearly enhances the structural properties of the analyzed stone by improving its overall hydrophobicity. Salt crystallization cycles may lead to the formation of fractures and plane distortions to a major extent with respect to those induced by thermal/RH stresses. Accordingly, liquid solutions penetrate more efficiently and faster thanks to the presence of new \"channels\", making the structure more susceptible to the presence of water even for short exposure times. In fact, by comparing the WC profiles of PA5 and PA-TQ, at equal contact times, we found that the mean water content value (wt.%) is always greater in PA5 than in PA-TQ. This occurrence suggests a scenario according to which the new artificially induced cracks in PA5, after 15 cycles of salt crystallization, are now filled with water through spontaneous imbibition, resulting in a higher WC value (\(\sim\)3.5%) with respect to that of PA-TQ (\(\sim\)2.5%). This hypothesis is also supported by the fact that, in PA6 (sample treated with nanoline  
Figure 8: Water content profiles for samples PA4 (NL consolidated) and PA-TQ (reference) between 20 and 50 mm. Dotted lines mark peaks which reveal, better than others, the enlargement induced by the thenardite–mirabilite phase transition activated by microclimatic stimuli (T/RH jumps). and not subjected to any accelerated weathering tests), the water content is slightly lower (\(\sim\)2.5%) with respect to PA5, suggesting that the main reason for the observed increase in WC in PA5 must be ascribed to the salt crystallization test. NL, in this case, does not affect the limestone almost at all. In the case of PA1, the NS consolidant reduces the access of water through the occlusion of the outer shell pores, reducing the amount of included water and slowing the water dynamics with respect to both PA5 and reference (PA-TQ). To further investigate the effects of the application of consolidants onto the limestone surface, the mean penetration depth of water (\(h\)) was evaluated as the sample height associated with the inflexion point (IP) of the decreasing WC profiles depicted in Figure 7.  
IP can be considered as the position from the bottom of the sample at which a water-rich/dry zone transition occurs, and it is, hence, equivalent to the \"front\" of the wetted region during the capillary suction process. It is worth noting that an exact determination of IPs starting from neutron radiographs can be carried out easily only in those cases in which a uniform capillary absorption occurs, as observed in a large variety of porous materials characterized by a homogeneously distributed porosity [56; 57].