## 2 Materials and Methods
### Lightweight Aggregate (LWA) Preparation and Characterization
Equation (1) is used: \[\text{SF}\%=(\text{D}_{\text{d}}-\text{D}_{\text{f}})/\text{D}_{\text{d}}*100\] (1)
* **Water absorption after 24 h (WA%):** this test is governed by the UNI EN 772-21: 2011 [24] standard, and it involved the execution of a water absorption test at room temperature in which the aggregates (ten beads for each formulation) are immersed in distilled water, in such a quantity as to cover them entirely (about 200 mL of water in a beaker), and left in a static condition for 24 h. The following day, they were removed from the water, dried and weighed. The sample, after drying, was weighed before (W\({}_{\text{i}}\), initial weight) and after testing (W\({}_{\text{f}}\), final weight) with the immersion in water. The water absorption percentage WA (%) was quantified using Equation (2): \[\text{WA}\%=(\text{W}_{\text{f}}-\text{W}_{\text{i}})/\text{W}_{\text{i}}*100\] (2)
* **Apparent density:** for the calculation of the bulk density, a GeoPyc 1360 was used in which a first \"tare\" analysis was carried out with graphite powder (Dryflow). The test was carried out on both samples, for each of which two aggregates were sampled in order to obtain data redundancy and greater accuracy. Some preset values were kept constant in the instrument, such as the force = 28 N and the conversion factor (in our case for spherical samples) = 0.12840. The instrument carried out five measurement cycles for each sample so that the data obtained were for the mean values and gave a standard deviation. * **True Density:** the true density of the aggregates was measured with a He pycnometer, which calculates the volume of a porous solid (Micrometrics Accupyc 1340). Previously, ten LWA spheres were ground into a powder with a small agate mortar. The total porosity percentage (TP (%)) was obtained by processing the absolute (Myrometrics Accupyc 1340) and apparent (Enveloped Density Micrometrics Geopyc 1360) density data, indicated as \(\rho_{\text{abs}}\) and \(\rho_{\text{app}}\), using Equation (3): Total Porosity (%) = (True density \(-\) Bulk density)/(True density) \(*\) 100 (3)
* **pH and conductivity measurements:** pH and electrical conductivity measurements were carried out as reported in UNI EN 13,037:2012 (pH rule standard) [25] and UNI EN 13,038:2012 (conductivity rule standard) [26]. Bulk specimens (10 g) were placed in distilled water with a solid/liquid ratio of 1:5 under stirring conditions (360 rpm) for 1 h at room temperature. The liquid was filtered in order to obtain a transparent liquid fraction; with this eluate, the pH and electric conductivity were measured. * **Mineralogical analysis:** mineralogical analysis was carried out with an X-ray powder diffraction analyser (PW 3710, Philips Research Laboratories, Eindhoven, the Netherlands) with Ni-filtered CuKa radiation in the 5-70\({}^{\circ}\) 20 range and a speed of 1\({}^{\circ}\)/min, operating at 40 mA and 40 keV. Highscore Plus software version 3.0 coupled to the International Centre for Diffraction Data (ICDD) cards database was used to identify the crystalline phases with a qualitative method. * **Microstructural analysis (SEM):** microstructural analysis was performed using SEM (Model XL40, Philips Research Laboratories, the Netherlands) coupled with X-EDS equipment (Model QUANTAX-200, Bruker, MA, USA) and the following beam voltage operative conditions: 25 KW; spot size: 5.0; pressure: 0.60 Torr; working distance: 12-13 mm. Thanks to the scanning electron microscope, it was possible to carry out investigations relating to the morphology and microstructure of the materials in order to analyse the shape and size of the grains, the porosity and the defects and inclusions present. It was also possible to perform mineralogical characterizations to identify the phases within a material, determine their concentrations and search for the presence of heavy metals.