## 2 Material and Methods  
The investigated material belongs to the Northern part of the Roman Magmatic Province in the Vulsini district (Central Italy), locally known as the Orvieto-Bagnoregio Ignimbrite [12]. A polyphasic eruptive pattern characterized the volcanic activity of the Vulsini District: a first strombolian and effusive activity, followed by caldera-forming eruptions associated with ignimbrite deposits and a final eruptive phase characterized by strombolian magmatic and hydromagnatic events [13]. The average thickness of the ignimbrite deposits in the studied area is about 10 meters, but it reaches a maximum value of 60 meters in the town of Orvieto, sited on the top of a small plateaux resulting from intense erosion processes. These deposits exhibit two facies, a weak rock material (tuff facies) and a slightly coherent pozzolana facies. The investigated soil, here denoted as OR soil (identified as PWD4 by [PERSON] et al. (2014) [11]), belongs to the pozzolana facies (Figure 1) and it is largely enriched in zeolites. The raw OR soil was crushed and the fraction passing to the sieve 450 \(\upmu\)m selected and oven-dried for 24 h at 60 \({}^{\circ}\)C. Mineralogical and chemo-physical properties of OR natural soil (raw material) are described in [PERSON] et al. (2017) [10]. The OR-soil is characterized by a relevant presence of Ca-chabazite and iron-enriched altered amorphous phase, both clearly identified by SEM observations (Figure 2a,b). The prominent presence of chabazite is furtherly confirmed by all the used investigation techniques. X-Ray powder diffraction also allowed the identification of minor phases such as: sanidine, augite, K-feldspar, analcime and biotite (see Section 3.2, raw material). Samples preparation of lime-treated samples was carried out identically for mechanical, microstructural and chemo-mineralogical tests: the 5% of lime (by dry weight of soil) was firstly mixed to the soil; then, once obtained a homogenous powder, the 20% of distilled water (by dry weight of soil) was added to the soil-line mixture. The hydrated lime used for treatment is composed of more  
Figure 1: The Orvieto southern cliff. The samples were retrieved at the base of the outcrop. Figure 2: Micrographs at different magnification factors: (**a**) Chabazite crystals surrounded by altered amorphous phase, (**b**) glassy amorphous phase, altered in its external surface. than 95% by Portlandite. Reconstituted samples were compacted in three layers. After compaction, the specimens were stored before testing in order to avoid environmental contamination and drying. Microstructural investigation was carried out for a wide range of curing times, from 24 h to 90 days, while mechanical testing was performed until 28 days of curing. Several investigation techniques and analyses were performed to gain an insight of the microstructural features of raw and lime treated OR soil. At the micro-scale of investigation, the experimental programme consisted overall of mineralogical and thermogravimetric analyses (XRD, DTG), as well as scanning electron microscopy analyses (SEM). XRD analyses were carried out using a Brucker AXS D8 Advance Diffractometer with CuK\(\alpha\) (\(\lambda\) = 0.154 nm) radiation and a step size of 0.021\({}^{\circ}\) on crushed and randomly oriented powders. A quantity of 50-100 mg of finely ground material was used for thermogravimetric analyses and heated at a rate of 10 \({}^{\circ}\)C min\({}^{-1}\), from lab-room temperature up to 1000 \({}^{\circ}\)C. A Netzsch STA 449F3 Jupiter instrument was used, equipped with a mass spectrometer; Netzsch Proteus software was used for results interpretation. Further details are given in [PERSON] et al. (2017) [10]. Mercury Intrusion Porosimetry (MIP) on the raw and lime treated samples was carried out by means of the Micromeritics AutoPore III, which allows to detect pore diameters from 0.07 \(\upmu\)m to 300 \(\upmu\)m (max Hg pressure respectively 0.20685 MPa and 206.85 MPa).