# Pore Structure and Water Transfer in _Pietra d'Aspra_ Limestone: A Neutronographic Study
## 3 Results and Discussion
### Neutron Radiography Analysis  
Time-lapsed radiographs of water capillary suction collected for PA1 sample at nine different selected time-steps are displayed in Figure 6, as an example. Figure 5: Weight variation (\(\%\)) (**a**) after salt crystallization test and (**b**) after temperature/relative humidity jumps. Figure 6: False-colored transmission images of water imbibition in PA1 sample at (**a**) 40 s, (**b**) 2560 s, (**c**) 3840 s, (**d**) 5120 s, (**e**) 6400 s, (**f**) 7680 s, (**g**) 8960 s, (**h**) 10,240 s, and (i) 11,560 s. In green, the \(5\times 5\) cm\({}^{2}\) surface area of the limestone is displayed, whereas the blue/purple/red regions account for the liquid phase characterized by an ever-increasing amount of water. The quantitative evaluation of the water content is addressed from the analysis of the wetting profiles (see text for details). A first inspection of Figure 6 reveals how the water rising in PA1, as for the majority of the investigated samples, is characterized by an extremely fast \"rate\" of spontaneous imbibition during the first stage of the absorption test, followed by a slow dynamic. This evidence, together with a nonuniform distribution of water during the uptake process, supports the well-known heterogeneity and poor interpore connectivity of the analyzed stone, as also revealed by the pore size distribution analysis (see Figure 4). This result can be considered, in a way, experimental evidence of the effect that the bimodal distribution of porosity in _Pietra d'Aspra_ limestone exerts on the water transfer behavior. In fact, as the stone is not uniform whether in terms of porosity or in terms of the size of the individual natural components (i.e., calcareous bioclasts and lithic fragments), a regular rise, as observed in a large variety of similar systems [54], was not expected. Furthermore, as the water kinetics shown in Figure 6 is descriptive of the PA1 sample, consolidated and subjected to a climatic chamber, a difficulty in the migration of saline solutions through the capillary porous lattice can be hypothesized. As a result, salts may shrink and crystallize, leading to a partial granular stone disaggregation. However, as we see later in the text, the presence of nano-SiO\({}_{2}\) brushed onto the limestone surface strongly reduces the amount of water (wt.%) within the stone network with respect to that observed for the reference sample (PA-TQ), proving a remarkable effectiveness of such a coating as a consolidating agent against accelerated weathering tests. Moreover, after a time interval of \(\sim\)2600 s, a slight formation of salt efflorescence can be detected as a blue spot that stretches out over the lateral surface of the stone in the bottom-right area of each image (except for Figure 6a). This evidence reveals the development of a preferential pathway for the liquid water during the imbibition toward the right-lateral surface of the stone, which spilled out after about 2600 s.  
In order to provide a quantitative picture of the effects that consolidants exert on the behavior of the water suction through capillarity in our investigated materials, in Figure 7, typical plots of water content (WC) as a function of the sample height are displayed. A first inspection of Figure 7 reveals that, for all the investigated samples, enhanced water penetration during the first stage of water contact is achieved, as suggested by the diminishing of the distances between two consecutive curves as time increases. This phenomenon can be associated with the presence of diagenetic cement, which slows the absorption process, in conjunction with a progressively diminishing of the \"free\" space available for water upon approaching the top of the material. Concerning the reference sample (PA-TQ), the obtained values of water content and height were found to be equal to \(\sim\)3 wt.% and \(\sim\)50 mm, respectively.