Effect of recycled autoclaved aerated concrete aggregate particle size and content on the microstructural and engineering properties of fly ash and silica fume based geopolymer composites

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.