**1. Introduction**

Cement composites have become irreplaceable elements in the construction industry over the last few decades due to their properties and low production costs. However, their production involves the consumption of significant quantities of raw materials and primary energy for manufacturing. Moreover, the production and transport of cement place a high CO2 burden on the environment [1,2]. Despite all drawbacks, cement is the most widely used building material in the world due to its superior engineering properties. In terms of their environmental impact and material properties, alkali-activated materials (AAM) seem to be a suitable option and they also match the development direction of building materials due to the valorization of various by-products or industrial waste products [3].

AAMs are formed by the alkaline activation of aluminosilicates (precursors) using highly alkaline activators. The precursors are usually obtained as waste materials or secondary raw materials from industrial production (blast furnace slag, silica fume, metashale and metakaolin), but natural precursors (rice husk, palm oil ash) can be used as well. Through the utilization of various waste or natural precursors, the production of binders and the associated release of CO2 are noticeably reduced. As activators, water glass or hydroxide are the most commonly used [4–6]. Apart from being aggressive substances, their production, which is very energy-demanding, also represents a substantial burden on the natural environment and human health. For this reason, waste alkalis, which can also be obtained as waste raw materials from industrial production, should be used instead [7]. The selection and use of the precursors and activators significantly influence the final properties of the AAM, such as the mechanical, thermal or electrical properties, as well as their fire and frost resistance [8,9]. In regards to the properties of AAM, some of them can

**Citation:** Mildner, M.; Foˇrt, J.; Cerný, ˇ R. Fiber-Reinforced Alkali-Activated Materials Based on Waste Materials. *Mater. Proc.* **2023**, *13*, 1. https:// doi.org/10.3390/materproc2023013001

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

Published: 13 February 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/).

also be modified by adding fiber reinforcement to the material's matrix. The use of fibers can also lead to a reduction in the cracks arising from debonding, sliding and pull-out [10].

Glass fibers are synthetic amorphous silicates that are commonly available at low cost. Their main advantage is their high flexural strength [11]. Pernica et al. [12] studied a Na geopolymer E-glass composite and observed an increase in the flexural strength from 175.9 to 255.2 MPa. Sankar and Kriven [11] observed the behavior of geopolymer-reinforced E-glass lazy weaves and observed an increase in the flexural tensile strength as well.

This study aimed to assess the potential of waste or secondary raw materials as the main sources for the production of new alternative building materials. In addition to the waste precursor, the alkaline activator represents a waste material from industrial cleaning operations and can also be viewed as waste material. In other words, this research contemplated the production of alternative AAMs based on waste materials, with a significantly reduced environmental footprint. Thus, the utilization of selected waste products can be seen as beneficial in terms of the environment and cost savings achieved by avoiding costly disposal. To improve the mechanical properties and reduce the shrinkage of the material, waste glass fibers were utilized.
