*Proceeding Paper* **Lightweight Geopolymer Composites: The Impact of the Aggregate †**

**Zélia Alves \*, João A. Labrincha and Rui M. Novais**

Department of Materials and Ceramic Engineering, CICECO—Aveiro Instituto of Materials, University of Aveiro, Campus Universitário de Aveiro, 3810-193 Aveiro, Portugal

**\*** Correspondence: zeliaralves@ua.pt

† Presented at the 10th MATBUD'2023 Scientific-Technical Conference "Building Materials Engineering and Innovative Sustainable Materials", Cracow, Poland, 19–21 April 2023.

**Abstract:** In this work, lightweight geopolymer composites were produced by varying the content of expanded perlite and geopolymer spheres, used as aggregates. Firstly, highly porous geopolymer spheres containing mostly red mud as the solid precursor were produced, which leads to the valorization of significant amounts of this metallurgical waste. Then, the incorporation of each aggregate in the slag matrix demonstrated that the geometric density, compressive strength, and thermal conductivity of the geopolymer composites decrease when their content increases. Composites with low density (0.8 g/cm3) and low thermal conductivity (0.130 W/m K) were produced by using the expanded perlite. However, the sphere-containing composites showed higher specific strength and good thermal insulation properties. The results show an interesting potential of using a waste-derived aggregate, synthesized at much lower temperatures compared to expanded perlite, addressing the demand for developing novel energy efficient materials.

**Keywords:** slag; expanded perlite; geopolymer spheres; lightweight material; thermal conductivity
