Upcycling autoclaved aerated concrete into sustainable and energy-efficient geopolymers with improved structural attributes

This study aims to achieve the complete recycling of post-demolition autoclaved aerated concrete (AAC) and to develop sustainable, energy-efficient, and high-performance geopolymer construction materials. Recycled AAC powder with different particle sizes was used as the aluminosilicate precursor and mixed with alkaline activators consisting of water glass and sodium hydroxide at concentrations of 4, 8, 12, and 16 mol. The effects of alkaline concentration, activator ratio, curing temperature, and precursor particle size on the mechanical properties and bulk density of the synthesized geopolymer materials were systematically evaluated. Increasing sodium hydroxide concentration and the water glass-to-sodium hydroxide ratio enhanced mechanical strength, while higher curing temperatures decreased bulk density. Under optimal conditions (12 molar sodium hydroxide, activator ratio of 2, curing at 85°C, and a particle size of 150 µm), the compressive, tensile, and flexural strengths reached 27.03, 3.51, and 2.73 MPa, respectively. Water absorption after 24 hours was measured at 11.9 wt.%, and chemical/microstructural analyses revealed a dense and homogeneous matrix consistent with complete geopolymerization. A life cycle assessment demonstrated an 86% reduction in climate-change impact, along with lower resource consumption, water use, and toxicity compared to ordinary Portland cement, confirming the environmental advantages of the proposed approach. Overall, the findings demonstrate that post-demolition AAC can be fully upcycled into structurally efficient and environmentally responsible geopolymer materials, with strong potential for application in sustainable construction products including high-performance precast elements and masonry units.