# Pore Structure and Water Transfer in _Pietra d'Aspra_ Limestone: A Neutronographic Study
## 1 Introduction  
The presence of water inside the porous matrix of building materials, whether artificial (concretes and mortars) or natural (stones), plays a main role in determining their mechanical performance and stability. As well known, salt crystallization and transportation of unwanted substances through water penetration can lead to the formation of cracks and plane distortions, strongly affecting the structural integrity of the construction itself. Restoration treatments aimed at \"conservation\" should preserve the original material while slowing down the decay process as much as possible. Stone durability is a term describing how well the material retains its original physical and mechanical properties. As a consequence, it concerns several aspects such as texture, structure, and mineralogical composition, as well as methods of quarrying and handling, applied loads, and environmental conditions to whichthe considered stone has been exposed [1; 2; 3; 4; 5; 6]. In particular, during the past few centuries, porous calcareous stones were extensively used as building material in several areas of the Mediterranean basin. In coastal urban environments, after a protracted period of exposure to the atmospheric agents, these lithotypes are very often subjected to extensive crumbling due to soluble salt crystallization in pore spaces [7; 8; 9; 10; 11; 12; 13]. Consequently, for this type of natural building material, the relationship among pore structure, water/moisture transport, and salt crystallization has been extensively investigated for understanding the mechanisms and kinetics of degradation processes [2; 9; 14; 15; 16; 17; 18; 19]. In this framework, different consolidant products have been commercialized and used in the last few years in the treatment of building materials of historical monuments, in order to preserve the whole structure and to guarantee a longer durability [20]. The high efficiency and minimum appearance alterations of such consolidating agents represent two highly demanded conditions in the field of conservation science. Both aspects depend on how these products interact with the relative substrates, affecting the penetration depth and the difference in porosity between the consolidant and the underlying bulk stone. In particular, the 3D pore distribution of both the stone and the coating layer represents a crucial parameter since it regulates the fluid mobility inside the material. Currently, liquid imbibition dynamics can be visualized through different approaches including gamma rays [21; 22], nuclear magnetic resonance (NMR) [23], and neutron radiography (NR) [24]. However, it must be pointed out that NR, among the other techniques, represents the best solution for the evaluation of the height of the water front and its 3D distribution, as all hydrogen-rich fluids are characterized by higher neutron attenuation coefficients with respect to elements typically present in limestone specimens. The study of water migration by capillarity in calcite-based materials (such as limestones) by means of NR has gained considerable attention by the scientific community in recent years [25; 26; 27; 28; 29; 30]. It has been successfully applied for the study of water movement by spontaneous imbibition in rocks [31; 32; 33], sandstones [34; 35], concrete [36; 37], and bricks [38]. Generally, a linear trend of the height of the water front (\(h(t)\)) with the square root of the exposure time is attained, according to the Lucas-Washburn (L-W) equation. It is worth nothing that the water rise by capillarity follows the aforementioned classical behavior only for structures characterized by a homogeneously distributed porosity. However, violations in the diffusion-like process can be often achieved [39; 40], at the early stage of water contact or after a certain period of time, revealing the occurrence of several processes during the water suction and/or the presence of nonuniform/disordered porosimetry within the inner structure of the investigated sample.