**2. Experiments**

*2.1. Materials*

The precursor used to produce the alkali-activated material was a combination of two aluminosilicates materials: finely ground granulated blast furnace slag (produced by Kotouˇc Štramberk, Czech Republic) and metashale (produced by Cesk ˇ é Lupkové Závody, A.S., Czech Republic). Finely ground granulated blast furnace slag (GGBFS) is a secondary raw material from iron production. Metashale (RON D460 HR) is produced by heat treatment of the waste material from metashale Mefisto L05 production. Both precursors are shown in Figure 1 and their chemical composition is presented in Table 1.

**Figure 1.** (**a**) Finely ground granulated blast furnace slag (GGBFS); (**b**) metashale (RON D460 HR).



Waste alkalis (WAA) based on sodium hydroxide from the industrial processes of glass production (AGS Glass, Teplice, Czech Republic) were used as an alkaline activator. These alkalis are in solid form in the form of pellets (see Figure 2), from which an activating solution of the appropriate molarity is produced. For the purpose of this research, an 8 M solution of WAA was used.

**Figure 2.** (**a**) Solidified waste alkalis based on sodium hydroxide (WAA); (**b**) fibers from the waste fiberglass reinforcement fabric (WFF).

Fibers from waste fiberglass reinforcement fabric (WFF) were used to reinforce the alkali-activated composite. This fabric is very important for ensuring high quality and a strong final layer in the interior plaster. The fabric was cut into individual fibers 25 mm in length (see Figure 2). The producer reported that the minimal tensile strength of the fiber is from 2000 to 2200 N/50 mm, while the elongation is 3.5%.

A mixture of fine standard aggregate in three fractions (0.08–0.5, 0.5–1.0 and 1.0– 2.0 mm) was used for the mortar samples.

The experimental program was divided into two sets of sample mixtures. One set of samples was without WFF; the other had WFF fibers. The procedure for mixing the WFF was as follows. The mixture was mixed one step at a time. First, the dry components (a mixture of the respective precursors and fine aggregates) were mixed and then an 8 M solution of WAA was added. At the very end, WFF fibers were inserted in batches to avoid the fibers clumping in the matrix.

The individual mixtures differed in the percentage of the respective precursors, i.e., GGBFS and RON, which changed after 20%. The composition of the studied mixtures is shown in Table 2.


**Table 2.** Composition of 1 m3 of the studied mixtures.
