Transformation of waste into resources is an important part of the circular economy. Nowadays, the recovery of materials in the most effective way is crucial for sustainable development. Composite materials offer great opportunities for product development and high performance in use, but their position in a circular economy system remains challenging, especially in terms of material recovery. Currently, the methods applied for recycling composites are not always effective. The aim of the article is to analyse the most important methods of material recovery from multilateral composites. The manuscript presents three case studies related to the recycling of products manufactured from composites: used tyres, wind turbine blades, and solar panels. It shows the advantages and disadvantages of currently applied methods for multilateral composite utilisation and presents further trends in composite recycling. The results show that increasing volumes of end-of-life composites have led to increased attention from government, industry, and academia.

Fly ash-based geopolymers can be considered as a greener alternative to ordinary Portland cement, featuring comparable properties and cost yet with lower CO₂ emissions. New wood fiber reinforced geopolymer composites with sand addition (WFSGC) have been synthesized at room temperature by mixing powder (fly ash, sand and randomly reinforced wood fiber) with sodium silicate and sodium hydroxide as alkaline activators. New WFSGC were cured at 90°C for 24 h. The design and investigation of WFSGC were based on a fix 5 wt.% percentage of sand, with variable wood fiber (5, 10, 15, 20, 25, 30 and 35 wt.%) and fly ash percentages. These WFSGC showed decreasing mechanical properties with increasing wood fiber addition as measured by the compressive strength at the cylindrical test (21.76–42.52 MPa), the compressive strength at the cubic test (31.79–39.17 MPa), the force load at upper yield at the cylindrical test (1.27–3.58KN), the flexural strength (7–10.76 MPa), compressive modulus at the cylindrical test (590,75–1021.17 MPa), the compressive modulus at cubic test (787.92–1059.79 MPa) and the flexural modulus (298.03–737.83 MPa). The density of WFRGC decreases with the addition of wood fiber (1.49–1.71 g/cm³). WFSGC with addition wood fibers up to 15 wt.% could be the limit for a promising green material for construction.

Geopolymers based on fly ash are an emerging alternative to ordinary Portland cement, with a low impact on the environment and a potential to increase the sustainability of the concrete-based construction. Reported here is the design and investigation of new wood fiber reinforced geopolymer composites (WFRGC) based on fly ash and wood fiber (5–35 wt%). These WFRGC showed reduced mechanical properties with rising wood fiber addition as calculated by compressive strength with the cylindrical test (7.83–59.82 MPa); compressive strength at cubic test (6.95–50.23 MPa); force load at upper yield (0.70–5.02 KN); flexural strength (1.71–11.05 MPa); compressive modulus at cubic test (0.13–054 GPa); and flexural modulus (3.91–1293.61 MPa). The adding of wood fiber showed decreases of the density WFRGC (1.58–1.87 g/cm³) and decreases the thermal conductivity of WFRGC. The addition 20 wt% wood fiber could be the limit for acceptable mechanical properties. WFRGC with 20–35 wt% wood fibers may be a promising green material for building.

The aim of this study was to evaluate the correlation between the percent of inorganic filler by weight (wt. %) and by volume (vol. %) of 11 flowable resin composites (FRCs) and their mechanical properties. To establish the correlation, the quantity of inorganic filler was determined by combustion and shape/size analyzed by SEM images. The compressive strength (CS), flexural strength (FS), and flexural modulus (FM) were determined. The CS values were between 182.87-310.38 MPa, the FS values ranged between 59.59 and 96.95 MPa, and the FM values were between 2.34 and 6.23 GPa. The percentage of inorganic filler registered values situated between 52.25 and 69.64 wt. % and 35.35 and 53.50 vol. %. There was a very good correlation between CS, FS, and FM vs. the inorganic filler by wt. % and vol. %. (R² = 0.8899–0.9483). The highest regression was obtained for the FM values vs. vol. %. SEM images of the tested FRCs showed hybrid inorganic filler for Filtek Supreme XT (A3) and StarFlow (A2) and a homogeneous type of inorganic filler for the other investigated materials. All of the FS values were above 50 MPa, the ISO 4049/2019 limit for FRCs.

Various powders of graphene oxide (GO), GO with silver (GO-Ag) and zinc oxide (GO-ZnO) were obtained. The powders were silanized with (3-aminopropyl) triethoxysilane (APTES) aiming to be used, in a future stage, as additives in the hydraulic lime mortars composition. The powders were characterized by Fourier Transform Infrared Spectrometry (FTIR) and Scanning Electron Microscopy (SEM) before and after the silanization process. GO, GO-Ag, GO-Ag-APTES, GO-ZnO and GO-ZnO-APTES powders were also investigated by Thermogravimetric Analysis (TG/DTA) and Ultraviolet–Visible Spectroscopy (UV-Vis). Likewise, the antibacterial effect of powders against five bacterial strains was evaluated. The peaks associated to the functional groups from GO, GO-APTES, GO-Ag, GO-Ag-APTES, GO-ZnO and GO-ZnO-APTES powders were identified by FTIR analysis. The mass losses of powders, analyzed by TG/DTA were lower than those recorded for GO. By UV-VIS analysis, maxima corresponding to the electronic π-π * and n-π * transitions were recorded. SEM images highlighted the lamellar and layered structure of GO, but also the presence of Ag and Zn nanoparticles on the surface of graphene sheets. All these results confirm the presence of Ag/ZnO/APTES on the GO. The antibacterial effect evaluated by recording the diameter of the inhibition zone ranged between 12–22 mm.

The demand for geopolymer materials is constantly growing. This, in turn, translates into an increasing number of studies aimed at developing new approaches to the methodology of geopolymer synthesis. The range of potential applications of geopolymers can be increased by improving the properties of the components. Future directions of studies on geopolymer materials aim at developing geopolymers showing excellent mechanical properties but also demonstrating significant improvement in thermal, magnetic, or sorption characteristics. Additionally, the current efforts focus not only on the materials’ properties but also on obtaining them as a result of environment-friendly approaches performed in line with circular economy assumptions. Scientists look for smart and economical solutions such that a small amount of the modifier will translate into a significant improvement in functional properties. Thus, special attention is paid to the application of nanomaterials. This article presents selected nanoparticles incorporated into geopolymer matrices, including carbon nanotubes, graphene, nanosilica, and titanium dioxide. The review was prepared employing scientific databases, with particular attention given to studies on geopolymer nanocomposites. The purpose of this review article is to discuss geopolymer nanocomposites in the context of a sustainable development approach. Importantly, the main focus is on the influence of these nanomaterials on the physicochemical properties of geopolymer nanocomposites. Such a combination of geopolymer technology and nanotechnology seems to be promising in terms of preparation of nanocomposites with a variety of potential uses.