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The preparation and characterization of innovative organic-inorganic hybrid geopolymers, obtained by valorizing coal fly ash generated from thermoelectric power plants, is reported for the first time. These hybrid materials are prepared by simultaneously reacting fly ash and dimethylsiloxane oligomers at 25 °C in a strongly alkaline environment. Despite their lower density, the obtained materials are characterized by highly improved mechanical properties when compared to the unmodified geopolymer obtained without the use of polysiloxanes, hence confirming the effectiveness of the applied synthetic strategy which specifically aims at obtaining hybrid materials with better mechanical properties in respect to conventional ones. This study is an example of the production of new materials by reusing and valorizing waste raw resources and by-products, thus representing a possible contribution towards the circular economy.

The paper compares and evaluates the influence of the presence of perforations on the sound absorption coefficient (SAC) of a negative stiffness metamaterial based on a foamed ceramic geopolymer. Chemical–physical, microstructural, dynamic–mechanical, and sound characterisations are presented. A rigid, lightweight geopolymeric porous material has been prepared using an inorganic/organic monomeric mixture containing oligomeric sialates and siloxanes foamed with aluminium powder. This process results in an amorphous rigid light foam with an apparent 180 Kg/m3 density and a 78% open-pore. The viscoelastic characterisation by dynamic mechanical analysis (DMA) carried out from 10−3 to 103 Hz indicates the behaviour of a mechanical metamaterial with negative stiffness enabling ultrahigh energy absorption at straining frequencies from 300 to 1000 Hz. The material loss factor (the ratio of dissipative/elastic shear moduli) is about 0.03 (essentially elastic behaviour) for frequencies up to 200 Hz to suddenly increase up to a value of six at 1000 Hz (highly dissipative behaviour). The corresponding storage and loss moduli were 8.2 MPa and 20 MPa, respectively. Impedance tube acoustic absorption measurements on perforated and unperforated specimens highlighted the role of perforation-resonant cavities in enhancing sound absorption efficiency, particularly within the specified frequency band where the mass of the negative stiffness foamed geopolymer matrix magnifies the dissipation effect. In the limits of a still exploratory and comparative study, we aimed to verify the technological transfer potentiality of using architected metamaterials in sustainable building practices.

This study presents an experimental overview for the development of photocatalytic materials based on geopolymer binders as catalyst support matrices. Particularly, geopolymer matrices obtained from different solid precursors (fly ash and metakaolin), composite systems (siloxane-hybrid, foamed hybrid), and curing temperatures (room temperature and 60 °C) were investigated for the same photocatalyst content (i.e., 3% TiO2 by weight of paste). The geopolymer matrices were previously designed for different applications, ranging from insulating (foam) to structural materials. The photocatalytic activity was evaluated as NO degradation in air, and the results were compared with an ordinary Portland cement reference. The studied matrices demonstrated highly variable photocatalytic performance depending on both matrix constituents and the curing temperature, with promising activity revealed by the geopolymers based on fly ash and metakaolin. Furthermore, microstructural features and titania dispersion in the matrices were assessed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) analyses. Particularly, EDS analyses of sample sections indicated segregation effects of titania in the surface layer, with consequent enhancement or depletion of the catalyst concentration in the active sample region, suggesting non-negligible transport phenomena during the curing process. The described results demonstrated that geopolymer binders can be interesting catalyst support matrices for the development of photocatalytic materials and indicated a large potential for the exploitation of their peculiar features.

Using innovative and sustainable materials has become crucial for developed countries. Reusing waste as a secondary raw material in industrial processes central to the circular economy could enhance environmental sustainability and support local economies. Building materials such as Portland cement have a significant environmental impact due to greenhouse gas emissions and construction and demolition waste (CDW), which is challenging to recycle. Research into sustainable alternatives is, therefore, essential. The European Union has set ambitious targets to reduce greenhouse gas emissions by 55% by 2030 and achieve climate neutrality by 2050. The National Recovery and Resilience Plan (PNRR) supports the green transition in Italy by promoting sustainable materials like geopolymers. These ceramic-like materials are based on aluminosilicates obtained through the chemical activation of waste rich in silica and aluminosilicate compounds. Though promising, these materials require further research to address challenges like long-term durability and chemical variability. Collaboration between scientific research and industry is essential to develop specific protocols and suitable infrastructures. This article provides a critical review of the advancements and challenges in using alkali-activated waste as construction binders, focusing on Italy, and encourages the exploration of alternative sustainable materials beyond conventional Portland cement.

A new, easy and cost-effective synthetic procedure for the preparation of thermosetting melamine-based epoxy resins is reported. By this innovative synthetic method, different kinds of resins can be obtained just by mixing the reagents in the presence of a catalyst without solvent and with mild curing conditions. Two types of resins were synthesized using melamine and a glycidyl derivative (resins I) or by adding a silane derivative (resin II). The resins were characterized by means of chemical-physical and thermal techniques. Experimental results show that all the prepared resins have a good thermal stability, but differ for their mechanical properties: resin I exhibits remarkable stiffness with a storage modulus value up to 830 MPa at room temperature, while lower storage moduli were found for resin II, indicating that the presence of silane groups could enhance the flexibility of these materials. The resins show a pot life higher than 30 min, which makes these resins good candidates for practical applications. The functionalization with silane terminations can be exploited in the formulation of hybrid organic-inorganic composite materials.

In this paper, geopolymer composite bricks were prepared using polyethylene terephthalate (PET) waste and the influence of PET amount, curing conditions, and durability were investigated, providing valuable insights for developing environmentally friendly building materials. Notably, there is a significant increase of 56% in compressive strength as the temperature rises from 30 °C to 70 °C. The presence of PET waste increases water absorption, which is positively correlated with thermal conductivity by 39%. Additionally, there is a negative correlation of 42% between water absorption and average compressive strength. The use of scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) reveals the presence of voids, fractures, and changes in the composition of the material at a microscopic level. Significantly, substituting PET waste ranging from 0 to 100% enhances thermal conductivity by up to 25%. The use of a clustering algorithm-based analysis highlights the relationship between microstructure and mechanical properties, of fundamental importance for improving geopolymer composite formulations. The results provide crucial information to predict and study the properties of geopolymer composite bricks, highlighting their ability to solve environmental issues in the building materials. Graphical abstract

Several researchers have recently worked to create sustainable building materials. One of the fundamental prerequisites for sustainable construction methods and environmental impact assessments is the use of green building materials and manufacturing processes. In this research study, geopolymer bricks were developed using polyethylene terephthalate waste and different industrial by-products (rice husk ash, ground granulated blast furnace slag, red mud, construction, and demolition waste) and investigated their performances. The polyethylene terephthalate waste was used as a replacement for sand filler in the geopolymer brick up to 100%. Key findings include a workability decrease of 14.75% and a compressive strength reduction of up to 75% with 100% plastic waste replacement, attributed to increased voids and weak geopolymer matrix interaction. Dry density consistently decreases, and water absorption rises to 13.73% with full sand replacement, indicating a porous structure. Impact resistance improves with plastic waste inclusion, enhancing ductility and thermal conductivity by 57% at full replacement. Microstructural analyses reveal correlations between physical–mechanical properties and changes in porosity, microcracks, and bond strength. Machine learning, especially linear regression, proves effective for strength parameter prediction (up to 100% efficacy, R-square of 0.998). The promising results obtained could offer a substantial environmentally friendly solution to the building and construction industry in line with Circular Economy principles.

With progress in the bone tissue engineering (BTE) field, there is an important need to develop innovative biomaterials to improve the bone healing process using reproducible, affordable, and low-environmental-impact alternative synthetic strategies. This review thoroughly examines geopolymers’ state-of-the-art and current applications and their future perspectives for bone tissue applications. This paper aims to analyse the potential of geopolymer materials in biomedical applications by reviewing the recent literature. Moreover, the characteristics of materials traditionally used as bioscaffolds are also compared, critically analysing the strengths and weaknesses of their use. The concerns that prevented the widespread use of alkali-activated materials as biomaterials (such as their toxicity and limited osteoconductivity) and the potentialities of geopolymers as ceramic biomaterials have also been considered. In particular, the possibility of targeting their mechanical properties and morphologies through their chemical compositions to meet specific and relevant requirements, such as biocompatibility and controlled porosity, is described. A statistical analysis of the published scientific literature is presented. Data on “geopolymers for biomedical applications” were extracted from the Scopus database. This paper focuses on possible strategies necessary to overcome the barriers that have limited their application in biomedicine. Specifically, innovative hybrid geopolymer-based formulations (alkali-activated mixtures for additive manufacturing) and their composites that optimise the porous morphology of bioscaffolds while minimising their toxicity for BTE are discussed.

A comparative “cradle to grave” Life Cycle Analysis between the production processes of ceramic stoneware products and geopolymeric materials obtained by valorizing ceramic wastes is reported. This study presents an effective eco-design approach to obtain sustainable materials through a low energy consumption manufacturing process, a feature that is essential in a historical period of high geopolitical instability which makes the supplying of fossil fuels difficult and particularly expensive. In particular, the possibility of lowering production costs (saving on the cost of waste disposal, using a raw-second material, and a low-temperature production process) could represent a strong contribution to the environmental and economic sustainability of the Italian ceramic industry, which is going through a time of severe financial crisis which due to the unprecedent high cost of raw materials and energy. Finally, the new geopolymeric systems proposed in this paper could be profitably used in the field of green building, art, eco-design, and technical-artistic value-added applications, such as restoration, conservation, and/or rehabilitation of historic monuments, or simply as materials for building revetments.

This paper examines how extrusion-based 3D-printing technology is evolving, utilising geopolymers (GPs) as sustainable inorganic aluminosilicate materials. Particularly, the current state of 3D-printing geopolymers is critically examined in this study from the perspectives of the production process, printability need, mix design, early-age material features, and sustainability, with an emphasis on the effects of various elements including the examination of the fresh and hardened properties of 3D-printed geopolymers, depending on the matrix composition, reinforcement type, curing process, and printing configuration. The differences and potential of two-part and one-part geopolymers are also analysed. The applications of advanced printable geopolymer materials and products are highlighted, along with some specific examples. The primary issues, outlooks, and paths for future efforts necessary to advance this technology are identified.