*2.1. Materials*

The porous geopolymer spheres, to be used as synthetic lightweight aggregate, were produced using two solid precursors: red mud (RM), supplied by a bauxite mining company from Greece, and metakaolin (MK) (ArgicalTM M1200S from Univar®, Seattle, WA, USA). Prior to use, RM was milled and sieved to attain a particle size below 75 μm, while MK was sieved to reach a particle size below 63 μm. The chemical composition of RM (Table 1) determined via X-ray fluorescence (XRF) shows that this waste is an iron-rich material (41 wt.%) with a low silica and alumina content. For this reason, MK was employed to adjust the SiO2/Al2O3 ratio. To perform the chemical activation of these solid precursors, a mixture of 100 g of industrial grade sodium silicate solution (Chem-Lab, Belgium) and 13.22 g of sodium hydroxide (ACS reagent, 97%; Sigma Aldrich, Saint Louis, MO, USA) were used. Sodium dodecyl sulfate (SDS) and polyethylene glycol (PEG 600) were supplied by Sigma Aldrich to be used as a pore foaming agent and consolidation medium of the geopolymer spheres, respectively. For the geopolymer composites synthesis, the main solid precursor in the binder's production was a commercial slag (Koranel®, Metallo, Beerce, Belgium) activated via an alkaline solution consisting of a mixture of 100 g of potassium silicate (K40, Quimialmel, Albergaria-a-Velha, Portugal) and 15.25 g of potassium hydroxide (85%; PanReac AppliChem, Castellar del Vallès, Spain). Expanded perlite (Figure 1a), with a geometric density of 0.11 g/cm3 and particle diameter (D50) of 0.537 mm (Figure 1b), was used as a natural lightweight material and supplied by a Greek company.


**Table 1.** Chemical composition from XRF of metakaolin (MK), red mud (RM), and slag.

**Figure 1.** (**a**) Expanded perlite aggregate and its (**b**) particle size distribution.

### *2.2. Synthesis of the Geopolymer Spheres*

The geopolymer spheres were prepared following the experience from previous investigations [9], in which 31 g of solid precursors (55 wt% of RM and 45 wt% of MK) were chemically activated with an alkaline solution (30 g) and distilled water (8.3 g). A planetary mixer was used to mix the precursors and the activator for 5 min. Then, the foamed slurry was obtained by adding the SDS (0.9 g) to the slurry and mixing it for 2 min. Finally, the geopolymer spheres were synthetized through the suspension solidification method by injecting the foamed slurry in a polyethylene glycol medium, which was maintained under the bath temperature of 80 ± 5 ◦C. After collecting the spheres, they were washed with water and cured in an oven at 40 ◦C for 24 h. Afterwards, the geopolymer spheres were cured for 28 days at room temperature.
