*3.2. Geopolymer Composites Characterization*

The geometric density of the geopolymer composites is reduced with the addition of both lightweight aggregates, reaching values below 1.40 g/cm<sup>3</sup> in all studied composites, which represent a reduction of at least 39% in comparison with the reference specimen prepared without the lightweight aggregate (Figure 3). A small density decrease is observed between the first and the twenty-eight day of cure as a result of the dehydration process during the polycondensation reaction. Noteworthy, a major reduction in the geometric density of composites is seen when increasing the amount of the aggregates, the lowest geometric density (0.75 g/cm3) reached by the higher volume (80 vol.%) of expanded perlite, followed by the composite with 85 vol.% of spheres (0.84 g/cm3). At 80 vol.% of aggregate, the geometric density of composite containing expanded perlite is roughly 1.3 times lower than the density with the same volume of spheres (1.00 g/cm3). However, no significant difference is observed with 75 vol.% of each aggregate, even though the expanded perlite's geometric density is roughly five times lower than the density of the geopolymer spheres. This feature is attributed to the fragile nature of expanded perlite, which can be partially destroyed during the mixing step, leading to higher density values than those expected when considering the apparent density of this aggregate. In fact, this phenomenon led to specimens having heterogeneous mechanical properties, as can be observed in Figure 4.

**Figure 3.** Geometric density of geopolymer composites varying the amount of expanded perlite and geopolymer spheres (measured on the 1st and 28th day). Reference means the control specimen without addition of lightweight aggregate.

**Figure 4.** Compressive strength (MPa) of geopolymer composites varying the amount of expanded perlite and geopolymer spheres.

The mechanical performance under compression of all composites is shown in Figure 4. The compressive strength decline is more pronounced at the highest incorporation volume of the aggregates, with the lowest value for specimens composed with 80 vol.% of expanded perlite (0.5 MPa) followed by the one with 85 vol.% of spheres (1.0 MPa). Despite the small difference in their geometric densities, the composite with 85 vol.% of spheres has a compressive strength two times higher than composite containing 80 vol.% of expanded perlite. With 75 vol.% of expanded perlite, the compressive strength is approximately twofold higher than the composite using the same amount of spheres. However, the result seen for the perlite-containing composite is abnormal and should be considered with caution. These samples were heterogenous, possibly due to a partial destruction of perlite during the mixing step, and this explains the very high standard deviation of composites with 75 vol.% of expanded perlite. Nevertheless, it should be noted this feature was not observed when using higher volumes of this aggregate. Indeed, with the incorporation of 80 vol.% of aggregate, an opposite tendency occurs, where the composite with spheres has a compressive strength fourfold higher than the expanded perlite, which can be explained by its higher composite's geometric density. The specific strength of the specimens with lower densities was determined and showed the following: 80 vol.% of spheres (2.1 MPa cm3/g) > 85 vol.% of spheres (1.8 MPa cm3/g) > 80 vol.% of expanded perlite (0.7 MPa cm3/g). The high specific strength of composites containing the spheres demonstrates the interesting properties of this material, enabling their use as a lightweight aggregate in the production of low density geopolymers.

The thermal conductivity of composites drops when the content of each lightweight aggregate rises, as shown in Table 2. The lowest value of thermal conductivity (0.130 W/m K) was observed in the composite with the highest volume of expanded perlite. For the same concentration, the addition of expanded perlite generates materials with lower thermal conductivity than the use of geopolymer spheres, but it is only significantly different at 80 vol.%. These results show that thermal conductivity can be controlled by the type of lightweight aggregate and by its content. The thermal conductivity value (0.175 W/m K) of composite containing the utmost volume of geopolymer spheres validates the strategy of using them as a lightweight aggregate. Moreover, their use has sustainable advantages over expanded perlite, since spheres are synthesized at 80 ◦C, this being much lower than the common temperatures involved in the expansion of perlite (850–1100 ◦C) [12,13]. Additionally, they are mostly produced using hazardous and abundant (150 Mt/year) industrial waste, contributing not only to reduce its stockpile but also to avoid the use of virgin raw materials. The lowest value of thermal conductivity obtained here (0.130 W/m K with expanded perlite) was compared with the literature and demonstrated to be smaller than geopolymer mortars incorporating expanded perlite (0.370 W/m K) [8] and crumb rubber (0.279 W/m K) [6], similar with the addition of waste-expanded polystyrene (0.121 W/m K) [5], but higher than those seen when using cork (0.072 W/m K) [4].

**Table 2.** Thermal conductivity (W/m K) of geopolymer composites produced with distinct expanded perlite and geopolymer spheres content.


#### **4. Conclusions**

Geopolymer composites were produced here using expanded perlite or geopolymer red mud-based spheres as a lightweight aggregate. The effect of their content in the composite was investigated via the mechanical, thermal, and physical properties. Results show that by increasing the aggregate amount, the compressive strength, geometric density, and thermal conductivity tend to decrease, demonstrating that these properties can be tuned considering the application envisioned. The composition containing the highest expanded perlite volume achieved the lowest value of geometric density (0.8 g/cm3) and of thermal conductivity (0.130 W/m K). Nevertheless, the use of red mud-based spheres resulted in a higher specific strength. These results suggest that this waste-derived aggregate can be a promising alternative in the development of sustainable and energy efficient geopolymer materials, also contributing to reduce the environmental impact associated with waste landfilling.

**Author Contributions:** Conceptualization, R.M.N. and J.A.L.; methodology, Z.A., R.M.N. and J.A.L.; software, Z.A.; validation, Z.A., R.M.N. and J.A.L.; formal analysis, Z.A.; investigation, Z.A.; resources, R.M.N. and J.A.L.; writing—original draft preparation, Z.A.; writing—review and editing, R.M.N. and J.A.L.; visualization, Z.A.; supervision, R.M.N. and J.A.L.; funding acquisition, R.M.N. and J.A.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The authors would like to thank the project SMART-G (ERA-MIN/0001/2019), sponsored by FCT.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Publication cost of this paper was covered with founds of the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023–Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
