*3.1. Geopolymer Spheres Characterization*

Figure 2a shows the optical micrographs of RM-containing geopolymer spheres. The spheres have a spheroidal shape and narrow size distribution, with an average diameter of 2.9 ± 0.8 mm. The SEM micrographs of the geopolymer spheres' surface and interior microstructure are shown in Figure 2b. The spheres reveal a homogeneous smooth surface with small-sized pores, while their interior shows the presence of a large-sized and closed pore surrounded by mostly small-sized closed pores and a few open pores. The geometric density of geopolymer spheres was 0.57 g/cm3, with this value endowing their use as a lightweight aggregate.

**Figure 2.** (**a**) Optical microscopy and (**b**) SEM micrographs of the red mud-based geopolymer spheres.
