This experimental study investigates the feasibility of recycling the autoclaved aerated concrete (AAC) waste as a fine aggregate in fly ash/silica fume-based geopolymer composites to address the environmental challenges of AAC disposal. In this regard, the microstructural and engineering characteristics were evaluated in detail. To produce geopolymer composites, natural sand was partially replaced with recycled AAC (r-AAC) sand in 10% increments, up to 60% by volume. Three different particle size distributions (0.125–4 mm, 0.125–2 mm, and 0.125–1 mm) were examined to assess the influence of aggregate size on composite performance. To mitigate potential degradation due to the incorporation of r-AAC sand, 10% silica fume (SF) was incorporated as a precursor by mass. A total of 38 geopolymer mixtures were produced using a mixture of sodium hydroxide and sodium silicate as an alkaline activator. The engineering properties, including workability, unit weight, and compressive strength, were evaluated alongside comprehensive microstructural analyses using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), mapping, and X-Ray Diffraction (XRD). Results indicated that increasing r-AAC content and decreasing the particle size led to a decrease in 28-day compressive strength; however, the addition of SF enhanced the mechanical performance. Microstructural findings confirmed a reduction in matrix compactness with higher r-AAC replacement, directly impacting the material performance. The study demonstrates the feasibility of utilizing r-AAC in geopolymers while highlighting the critical role of particle size and SF addition.

Fiber metal laminates (FMLs) sandwich panels are typically installed using mechanical joints. This investigation aims to enhance the bearing behavior of pin joints made of aluminum 1050–E glass FMLs. A novel core modification was achieved using a functionally graded material (FGM) technique. The impact of inserting aluminum layers at the mid-thickness of the specimens and stacking sequences of the composite core ([Al/(0o/90o/0o)s/Al] and [Al/(90o/0o/90o)s/Al]) on joint performance was also examined across different ratios of joint geometries. Furthermore, composite-like and sandwich-like layups were also compared. All tests were conducted in accordance with ASTM D5931/5961 M-17. The results indicated that the sandwich-like layup slightly enhanced the bearing strength of these joints, ranging from 1.37% to 6.16% depending on the joint geometry. Notably, inserting the mid-aluminum layer increased joint bearing strength by 11.18% by constraining delamination within the composite core. Using an FGM technique can increase the bearing strength of pin joints in FMLs by up to 27%. Furthermore, visual examination indicated that the [0°] layers exhibited shear-out, while the [90°] layers failed in net tension. The SEM images show fiber breakage, fiber-matrix debonding, matrix crack, delamination between the [0°] and [90°] composite layers, and delamination between the composite and metal layers.