*2.4. Materials Characterization*

The chemical composition of the solid precursors was determined via X-ray fluorescence (XRF- Philips X'Pert PRO MPD spectrometer, Malvern Panalytical, Malvern, UK). The morphology and porosity of the spheres were examined using an optical microscope (Nikon, H550S, Tokyo, Japan). The particle size distribution of expanded perlite was conducted using laser diffraction in the dry mode (HORIBA scientific LA-960V2, Kyoto, Japan). Scanning electron microscopy (SEM—Hitachi SU 70; energy dispersion spectroscopy—EDS Bruker) was also used to characterize the microstructure of the spheres. The geometric density was calculated via the ratio of dry mass to volume using three cubic specimens (4 × <sup>4</sup> × 4 cm3) of each formulation cured for 28 days. The compressive strength of composite specimens (4 × <sup>4</sup> × 4 cm3), measured 28 days after their synthesis, was performed using a Universal Testing Machine (Shimadzu AG-25 TA, Kyoto, Japan) at a 0.5 mm/min load rate in triplicate per batch, following the standard EN 1015-11:1999 [10]. The thermal conductivity was measured on three cubic specimens (4 × <sup>4</sup> × 4 cm3) of each composition by using a heat flow meter apparatus following the ASTM C518-04 standard [11], where the sample is placed between two parallel plates, with a constant temperature gradient of 15 ◦C.
