**1. Introduction**

The construction industry, with its almost 30% share of the total industrial activity within the European Union (EU), is one of its largest industrial sectors. Besides the construction of new buildings, it also involves renovation, demolition, and the disposing of waste building materials. In 2020, more than 800 million tons of the produced construction and demolition waste in the EU had to be recycled or landfilled [1]. Due to the negative environmental impact of landfilled construction materials polluted by hazardous agents, the design of sustainable materials leading to a reduction in landfilling gained higher importance in recent years [1–3].

With over 25 gigatons produced per year, concrete is the most widely used building material worldwide and undoubtedly also a major component of construction waste [4]. Therefore, its reuse in the construction industry, e.g., as a recycled waste aggregate for new composites, is beneficial for the reduction in landfilled material as well as the protection of natural resources [5,6]. It should be noted that the use of recycled concrete aggregate has its limits and rules. Nováková and Mikulica [6] emphasized that in the case of landfilled concrete, special attention needs to be paid to the removal of impurities coming from other materials, such as asphalts, bricks or plastics. They also noted that the European standard EN 12 620+A1 requirements for the use of aggregate in concretes slightly differs from country to country [7].

**Citation:** Hotˇek, P.; Fiala, L.; Lin, W.-T.; Chang, Y.-H.; Cerný, R. ˇ Alkali-Activated Metashale Mortar with Waste Cementitious Aggregate: Material Characterization. *Mater. Proc.* **2023**, *13*, 41. https://doi.org/ 10.3390/materproc2023013041

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 1 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The degree of the environmental impact of the construction composite production significantly depends on the used binder and filler. Although alternative binders can positively influence the total environmental impact of the composite, the choice of aggregate plays the decisive role since it is present in a significantly higher amount than the binder. Alkali-activated composites (geopolymers) based on aluminosilicate waste/industrial product precursors (slag, brick dust, fly ash, metakaolin, metashale, etc.) offer a good environmental performance as far as the binder is concerned [8,9]. Despite the use of alkali activators (potassium/sodium hydroxides and water glasses), which are characterized by a higher negative environmental impact, the amount necessary for activation is low in comparison with the amount of binder and filler [10,11]. Geopolymers have a huge potential in civil engineering due to their various beneficial properties, e.g., high initial mechanical properties and chemical and fire resistance, ensuring durability and a low environmental impact, which can be further improved by replacing natural products with recycled aggregate [12–14].

This paper is focused on the design of a geopolymer mortar based on the mix of the newly produced and waste metashale binder, potassium hydroxide/silicate activator and recycled concrete aggregate filler. The composite was supplemented with a small amount of carbon fibers to examine their influence on electrical properties, which would be crucial for the new functional properties, such as self-sensing, self-heating or energy harvesting. The composite was characterized in terms of the basic physical, thermal and electrical properties.

### **2. Experimental Stage**
