Globally, over 350 million tonnes of thermoplastic waste are generated annually, with more than 60% either landfilled or mismanaged. This attracts innovative pathways to increase their recyclability, among which particulate-filled recycled thermoplastic composites (RTCs) are emerging as a potential waste reuse strategy for diverse civil and industrial applications. This review systematically analyses the current understanding of the physical, mechanical, and durability performance of RTCs, focusing on how various particulate filler types, content, and interfacial compatibility influence key properties. Reported studies show that incorporating particulate organic or inorganic fillers such as waste glass, sand, wood flour, etc., can increase density by 10–45%, tensile and flexural moduli by 30–120%, and thermal stability by up to 40%, though strength and ductility often decrease by 15–50% due to poor filler–matrix adhesion. This review further evaluates durability enhancements under prolonged exposure to water, thermal, and UV radiation, where filler addition reduces water absorption and UV degradation by 20–60%. Despite these advancements, challenges remain in optimising interfacial bonding, long-term performance modelling, and scalability for civil infrastructure. This review also outlines research directions to advance high-performance, sustainable RTCs through a structured review approach using defined keywords on recycled thermoplastics, fillers, and durability.

This study investigates hygrothermal durability of bio-epoxy composites reinforced with carbon, E-glass, basalt, and flax fibres. Fibre yarns and bio-composites were exposed for 3000 h at 60 °C and 98% relative humidity. The tensile strength reduction in the fibres and the interfacial shear strength (IFSS) reduction in the composites were assessed after ageing. Chemical deterioration was evaluated using energy-dispersive X-ray spectroscopy (EDS); morphological changes in fibres and composites fracture surfaces were examined using a scanning electron microscope (SEM). Results indicated that the durability was significantly influenced by fibre types. Tensile strength reduction was higher in carbon, glass and basalt compared to flax yarns because of chemical degradation of the sizing layer in synthetic fibres, while only physical damage was observed in flax. The IFSS reduction was highest in flax composites (10%), and lowest in carbon (4%). EDS indicated the hydrolysis and erosion of fibre sizing, with reduced silica content in glass and basalt fibres. SEM revealed matrix-dominated failure in carbon/bio-epoxy, interfacial debonding in glass and basalt composites, fibre slip and pull-out in flax/bio-epoxy. Overall, the results highlighted damage propagation pathways and demonstrated that bio-epoxy composites exhibited reasonable performance under hygrothermal ageing, supporting their potential as a sustainable alternative in durability-critical applications.

This study investigates the degradation mechanisms of glass-fibre- and carbon-fibre-reinforced polymer (GFRP and CFRP, respectively) composites fabricated either with epoxy, vinyl-ester, or bio-epoxy resins under a hygrothermal environment. Composite laminates were manufactured using the vacuum-assisted resin infusion technique and exposed to high moisture and elevated in-service temperatures of 23 °C (room temperature), 40 °C and 60 °C for up to 125 days. Changes in the physical, microstructural, chemical and mechanical properties were then assessed. CFRP and GFRP composites showed distinct differences in their hygrothermal ageing depending on the resin system used in the manufacturing. CFRP composites consistently demonstrated higher stability than GFRP composites. Epoxy resin exhibited high resistance to water absorption and hydrolysis under hygrothermal exposure. After 125 days at 60 °C, glass/epoxy (GE) and carbon/epoxy (CE) composites retained 79.0% and 72.1% of their tensile strength and 46.9% and 72.6% of their interlaminar shear strength (ILSS), respectively. Vinyl-ester composites showed high mechanical retention, with glass/vinyl-ester (GV) and carbon/vinyl-ester (CV) retaining 70.8% and 83.1% of tensile strength and 67.5% and 80.3% of ILSS, respectively. Despite this mechanical stability, evidence of hydrolysis indicated ongoing chemical degradation of the vinyl-ester resin under prolonged hygrothermal exposure. In contrast, bio-epoxy composites exhibited relatively low overall durability. Glass/bio-epoxy (GB) retained 126.5% tensile strength and 68.8% ILSS, whereas carbon/bio-epoxy retained 61.0% tensile strength and 44.3% ILSS after 125 days at 60 °C. Overall, fibre and resin types were found to have a significant effect on the hygrothermal ageing of polymer composites.

Geopolymer concrete (GPC) has emerged as a promising low-carbon alternative to ordinary Portland cement (OPC), yet wider adoption is limited by the lack of standardised mix-design procedures. Precursor, activator, curing, and aggregates strongly interact to affect properties, but findings are scattered and hard to generalise. This review consolidates and normalises published findings to clarify how key parameters-precursor type, activator dosage and concentration, activator-to-binder ratio, curing temperature, and aggregate gradation-control fresh and hardened performance. Overall trends indicate that calcium-rich systems enhance early strength by 80–100% at typical replacement levels; while optimum activator conditions of 12 M NaOH and sodium silicate/sodium hydroxide ratio = 2.5 commonly improve strength by 40–60% relative to sub-optimal ratios; alkaline activator-to-binder ratios of 0.4–0.7 provide the most practical strength-workability balance. Heat curing at 80–100 °C significantly accelerates early-age property development by 50–200% compared to ambient curing, depending on duration and activator chemistry. A target-strength mix design is demonstrated through a 40 MPa case study. Using a compiled dataset for fly ash (FA)-based GPC, a modulus–strength framework is proposed; common OPC code equations over-predict elastic modulus for 15–50 MPa, and calibration yields a conservative, code-compatible relation: E = $2.75\sqrt{f{c}_{MPa}}$. Key limitations are highlighted, including variability in raw materials and durability uncertainties, and future directions are proposed toward performance-based design and standardisation to support structural use of GPC in sustainable infrastructure.

This paper presents an experimental investigation of six different types of composite sandwich panels manufactured from waste-based materials, which are comprised of two different types of skins (made from hemp and recycled PET (Polyethylene terephthalate) fabrics with bio-epoxy resin) and three different cores (composite wood, recycled plastic, and styrofoam) materials. The skins of these sandwich panels were investigated under five different environmental conditions (normal air, water, hygrothermal, saline solution, and 80 °C elevated temperature) over seven months to evaluate their durability performance. In addition, the tensile and dynamic mechanical properties of those sandwich panels were studied. The bending behavior of cores and sandwich panels was also investigated and compared. The results indicated that elevated temperatures are 30% more detrimental to fiber composite laminates than normal water. Composite laminates made of hemp are more sensitive to environmental conditions than composite laminates made of recycled PET. A higher-density core makes panels more rigid and less susceptible to indentation failure. The flexible plastic cores are found to be up to 25% more effective at increasing the strength of sandwich panels than brittle wood cores.

Fiber-reinforced polymers (FRPs) are effective for strengthening masonry walls. Debonding at the polymer–masonry interface is a major concern, requiring further investigation into interface behavior. This study utilizes detailed micro-modeling finite element (FE) analysis to predict failure mechanisms and analyze the behavior of brick masonry walls strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) under in-plane loading. The research investigates three CFRP strengthening configurations (X, I, and H). The FE model incorporates the nonlinear behavior of brick masonry components using the Concrete Damage Plasticity (CDP) model and uses a cohesive interface approach to model unit–mortar interfaces and the bond joints between masonry and CFRPs. The results demonstrate that diagonal CFRP reinforcement enhances the ductility and capacity of masonry wall systems. The FE model accurately captures the crack propagation, fracture mechanisms, and shear strength of both unreinforced and reinforced walls. The study confirms that the model can reliably predict the structural behavior of these composite systems. Furthermore, the study compares predicted shear strengths with established design equations, highlighting the ACI 440.7R-10 and CNR-DT 200/2013 models as providing the most accurate predictions when compared to experimental results.