*2.1. Materials*

The designed composite was based on the mix of two aluminosilicate precursors, Mefisto L05 and RON D460 HR (Figure 1). Mefisto L05 is a very fine metashale powder (*d50* = 3 μm, *d90* = 10 μm) originating from the thermal and granulometric treatment of clays and float kaolins. RON D460 HR is an industrial waste from the production of Mefisto L05, consisting of some unburnt particles that are of slightly higher size than in Mefisto L05 (*d50* = 4.5 μm and *d90* = 23 μm). The chemical composition and particle size distributions of the precursors are summarized in Table 1 and Figure 2.

**Figure 1.** (**a**) Mefisto L05, (**b**) RON D460 HR.

**Table 1.** XRF analysis of Mefisto L05 and RON D460 HR precursors (%).


**Figure 2.** Particle size distribution of Mefisto L05 and RON D460 HR.

Metashale precursors were activated by a mix of potassium alkalis: potassium water glass with a K2O/SiO2 ratio = 1.7 and potassium hydroxide to avoid efflorescence, which occurs upon activation by sodium water glass/hydroxide [15]. The filler—a recycled cementitious aggregate—was of a grain size within the range of 0.063 to 2 mm, prepared by crushing damaged structural formworks. The H-shaped structural formwork (Figure 3a) was first crushed into smaller pieces using an EDB 400 mechanical press. Subsequently, smaller pieces were crushed using a crusher with 2.2 mm maximum jaw distance. The resulting aggregate was sieved and sorted into individual fractions of 0.063–0.5 mm, 0.5–1 mm and 1–2 mm, following the pattern of standardized quartz sand fractions (Figure 3b). The fresh mixture was supplemented with a small amount of carbon fibers to examine their influence on the electrical properties of the designed composite.

**Figure 3.** (**a**) H-shape structural formwork, (**b**) fractions of waste aggregates.
