## 1 Introduction  
Over the last decades, the geotechnical research has significantly contributed to the preservation of historical sites. It is more recent the awareness that the impact of climate changes is causing an increasingly rapid degradation of the landscape and monumental heritage, requiring new actions towards its conservation, within the integral respect of its material and formal consistency. The geotechnical aspects of heritage conservation require the most advanced methodologies and techniques to be implemented for sustainable mitigation of new challenges introduced by climate changes. In Italy, the case of Orvieto cliff [1] is paradigmatic in this sense. The stability of the cliff is linked to morphological features of the rock mass, but degradation of materials has been increased by the increasing intensity of environmental loads. Wetting and drying cycles of the rock mass surface are mainly responsible for its destructuration [2], causing a progressive degradation of the original cementing bonds. For a sustainable and durable conservation, the countermeasures should respect the dimeric feature of the rock mass and improve its mechanical resistance to face erosion and progressive degradation of the layers most exposed to environmental loads. In this experimental study, the hydro-mechanical improvement of the slightly coherent facies (pozzolana) of Orvieto Ignimbrite induced by addition of hydrated lime has been studied. The subsequent chemo-physical evolution has been investigated by means of a multi-scale analysis. Recent studies focused on the appropriateness and usefulness of lime treatment of pyroclastic soils [3; 4; 5; 6], investigating the function played on the engineering features by different parameters, such as lime per cent by weight, type of binder (CaO or Ca(OH)\({}_{2}\)), water content and curing time. The mechanical improvement is found to depend on different processes, i.e., short-time cation exchange between dissolved calcium ions and the clay minerals surface and hence, the development of pozzolanic reactions depending on the availability of calcium, aluminium and silicon in the system; it follows a microstructural reorganization derived from a redistribution of charges on clay particles in highly alkaline environment [7; 8]. [PERSON] et al. (2016) [9] evidenced that in the case of volcanic soils with low amount of exchanging minerals, cationic exchange only plays a secondary role. Nevertheless, the contribution of each of the mentioned processes should be evaluated along with the mineralogical composition of the natural soil. [PERSON] et al. (2017) [10] developed an experimental study aimed at investigating the microstructural modifications due to lime addition on a zeolitic pyroclastic soil (rich in Chabazite) belonging to a deposit of the Orvieto cliff in the Umbrian Region (Central Italy). The study showed the high reactivity of the system exhibited by the quick precipitation of hydrates as tangible products of the pozzolanic reactions and clearly responsible for the improvement of the mechanical response of the material upon shearing, which has been found to depend on confining stress, quantity of lime - percentage as well as curing time. In particular, upon shearing, the behaviour of such treated soil is fragile and dilatant; the soil brittleness increases with larger lime contents and longer curing times. For the investigated curing time intervals, the frictional aliquot of shear strength of the lime treated soil appears to be not influenced by the quantity of added lime and the amount of secondary phases, because of their low degree of crystallization. Starting from these considerations, the main objective of the present paper is to further investigate the link between the chemo-physical evolution detected at particle level and the macroscopic behaviour of the treated pyroclastic soil through the multi-scale analysis approach.