**1. Introduction**

The production of ordinary Portland cement (OPC)-based concrete is responsible for a large amount of energy consumption, in addition to a very high share of the anthropogenic CO2 emission (~8%), which is one of the major causes of climate change. Besides this, the energy inefficiency stemming from the heating and cooling needs of the building also raises concerns regarding sustainability in the building sector [1]. In this context, there is an urgent need for alternative and highly efficient new materials with a lower carbon footprint. Geopolymers emerge as one of the most promising alternatives due to their lower embodied CO2, but also due to the possibility of using industrial wastes as precursors instead of non-renewable raw materials [1]. The production of lightweight and thermal insulating geopolymer concrete is a particularly interesting approach to increase the energy efficiency of buildings, as these materials can mitigate the energy losses [2,3]. One of the common routes involves incorporation of natural (e.g., cork [4]) or synthetic (e.g., expanded polystyrene, crumb rubber [5,6]) lightweight aggregates that ensure weight reduction. As the construction industry is striving to minimize consumption of virgin resources and precursors derived from fossil fuels, the production of aggregates from wastes or industrial by-products is given priority. In line with the above, highly porous red mud-based geopolymer spheres were synthesized in this work and used as lightweight aggregates to produce low density composite materials. Red mud is a by-product produced in large amounts during alumina production (0.8–1.5 tons of red mud per ton of alumina), which is considered an environmental hazard [7]. Then, the mechanical, thermal, and physical properties were determined to infer the influence of geopolymer spheres' incorporation content on the produced composites using slag as a binder. A comparison with a commercial and

**Citation:** Alves, Z.; Labrincha, J.A.; Novais, R.M. Lightweight Geopolymer Composites: The Impact of the Aggregate. *Mater. Proc.* **2023**, *13*, 30. https://doi.org/10.3390/ materproc2023013030

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

Published: 21 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/).

non-renewable lightweight aggregate was also performed, in this case by using expanded perlite. Geopolymer mortars containing expanded perlite have been proven to produce materials with better fire resistance and higher thermal insulation than siliceous sand [8].
