This paper deals with the characterization of perlite-based geopolymer pastes, using fine perlite as raw material. The present study examined the effects of the main synthesis parameters such as perlite to activator ratio, NaOH concentration, the addition of soluble silica to the activator, and curing temperatureon the setting time, the stability in an aquatic environment, the viscosity of the paste, and the compressive strength of the solidified geopolymers. The results showed that these inorganic polymer pastes are non-Newtonian shear thinning fluids that achieve low viscosities at high shear stresses. The optimum synthesis conditions for the geopolymer pastes proved to be a) a low initial NaOH concentration in the alkaline phase (2–5 M) and b) a solid to liquid ratio of 1.2–1.4 g/mL. If very fast setting is necessary, the pastes should be prepared with a soluble silica-doped alkaline activating phase and cured at high temperatures around 90 °C.

The present research article investigates the extraction of aluminum (Al) from an engineered CaO-Al₂O₃-SiO₂ slag by leaching with aqueous HCl under atmospheric pressure conditions. The slag is the by-product of an aluminothermic reduction process to produce metallurgical-grade silicon (Si) in a more sustainable way compared to conventional carbothermic reduction. One major challenge in the acidic leaching of aluminosilicate materials, like the slag treated in this study, is the possibility of SiO₂ gel formation during the leaching process. Extensive SiO₂ gel formation can make the separation of pregnant leach solution (PLS) from the leached residue impossible. Consequently, any acid leaching of aluminosilicate materials must be optimized for avoidance of these gelation phenomena. The present study first explores the leaching behavior of the calcium aluminosilicate slag in relation to the S/L ratio, with all other hydrometallurgical factors remaining stable (20.2% wt. HCl solution, 80 °C, optimized stirring rate), to determine at which value of this parameter SiO₂ gelation occurs. After determining the optimum S/L ratio for this system, an analysis of the behavior of Si in the PLS is presented, and the mechanism of SiO₂ gelation is explained based on critical assessment of these results against data provided from the scientific literature. It was found that the value of the pH of the PLS and the concentration of dissolved CaCl₂ and AlCl₃ are critical factors for the formation of filterable amorphous SiO₂. Under the optimum leaching conditions, PLS containing 37 g/L Al was obtained with concurrent avoidance of gelation phenomena.

This paper investigates the development of geopolymer foam boards, using perlite wastes as raw material. This type of lightweight materials combines the geopolymerization technology with the foaming process. The mechanism of foaming is based on the generation of a gas that is retained by the geopolymer matrix in the form of individual or interconnected voids. In this study, the inorganic foaming agent is hydrogen peroxide (H₂O₂), which is added into the initial paste in different quantities by mechanical stirring. The produced porous materials have effective densities between 408–476.5 kg/m³, thermal conductivities between 0.076–0.095 W/m.K and different type of microstructure, depending on the concentration of the activator and the foaming agent content. To assess the porosity and the size distribution of the voids, image processing techniques were applied on digital images of the samples. According to these results, the synthesized lightweight materials exhibit similar or even better thermal properties than the current concrete porous materials.

This paper demonstrates the transformation of the industrial residue (copper slag) of a Swedish mining and smelting company “Boliden”, through geopolymerization, into advanced building materials. The main objective of this experimental study is the assessment of the appropriate conditions for the preparation of alkali-activated slag-based geopolymer pastes with further foaming production, by aluminum powder addition. The alkaline-activating solution used was KOH, at a constant concentration (8 M). The effect of crucial operating parameters, such as S/L ratio (3.5–4.5 g/mL) and aluminum powder addition (0.12%–0.22%), on the geopolymer paste were studied, in order to achieve the optimum rheological conditions of the slurry. The physical properties of the materials were examined after the appropriate curing process (24 h at 70 °C), with density values ranging between 805 and 1100 kg/m³. The mechanical performance of the materials ranged between 1.28 and 2 MPa (compressive strength), and from 0.25 to 0.85 MPa (flexural strength), indicating the strong correlation of physical and mechanical properties. To assess the porosity and the size distribution of the voids, image processing techniques were applied on digital images of selected samples. According to these results, the synthesized materials exhibit similar, or even better, properties than the current concrete porous materials.

Calcium aluminate slag produced by the aluminothermic reduction of silica is tested as a candidate raw material for the hydrometallurgical production of pure aluminium chloride hexahydrate (ACH) through leaching with hydrochloric acid. The crystallization of ACH follows by sparging the pregnant liquor with hydrochloric gas. Almost total extraction of Al is achieved with the use of azeotropic HCl acid solution (5.9 M) at 80 °C and 1 h retention time. A pregnant liquor with approximately 20 wt% AlCl₃ is produced as a base for ACH crystallization by sparging it with gaseous HCl. The ACH produced is re-dissolved and crystallized three to four times until high purity is achieved. High purity ACH acts as a precursor for producing High Purity Alumina (HPA), a high added value material used in LEDs and lithium-ion batteries and other niche applications.

This work deals with the development of lightweight geopolymeric boards for use in construction sector utilizing a solid perlitic waste as the main raw material. Hydrogen peroxide (H₂O₂) was used for the foaming of geopolymeric pastes and the production of porous and lightweight inorganic polymeric materials. The effect of geopolymeric synthesis parameters, such as the composition of activator and the curing conditions, on paste's properties that affect the foaming process, such as setting time and viscosity, were studied in detailed. Finally, the effects of H₂O₂ concentration on the properties (apparent density and % cell volume) and the microstructure of foamed boards were also studied. The produced porous boards have effective densities in-between 540 - 900 Kg/m³ and the thermal conductivity of the optimum product is 0.08 W/mK. Based on their properties, the developed lightweight geopolymeric boards have high potential to be used as building elements in construction industry.