*2.3. Synthesis of the Geopolymer Composites*

For the lightweight geopolymer composites preparation, firstly, the slag was activated with the potassium hydroxide solution with a liquid-to-solid ratio of 0.684. After mixing for 1 min, the lightweight aggregate was incorporated into the activated slag for 2 min. The volume ratios for each lightweight aggregate and the binder were firstly evaluated using preliminary tests. These tests showed that 80 vol.% and 85 vol.% were the maximum amount of expanded perlite and geopolymer spheres that could be added to the matrix, respectively, without disintegration of the composites. Three different compositions varying the amount of expanded perlite (70, 75, 80 vol.%) and other three varying the amount of geopolymer spheres (75, 80, 85 vol.%) were produced. Reference specimen was also produced without the addition of aggregates. The pastes were transferred to steel molds (4 × <sup>4</sup> × 4 cm3), sealed to maintain the humidity, and cured for 24 h at room temperature. Then, the cubic specimens were removed from the molds and cured at room temperature until the 28th day.
