Synthesis of geopolymer using alkaline activation of building-related construction and demolition wastes

Abstract

There are over 2 billion tons of building-related construction and demolition waste (brCDW) produced annually in China, but less than 30% of the waste is presently recycled. The majority of the brCDW is being disposed of in landfills in suburban or rural areas, resulting in a serious waste of resources and environmental pollution issues. This study proposed a high-value utilization approach for the brCDW, which was used as a precursor material to synthesize the high strength and environmentally friendly geopolymer. The brCDW was classified into three main components, namely, brick, ceramic, and concrete. The X-ray fluorescence (XRF), X-ray diffraction (XRD), particle size analyzer, and unconfined compressive strength tests were conducted to evaluate the influences of chemical composition and particle size of precursor materials on the strength of geopolymers derived from single and mixed brCDW components. The results demonstrated that the brick geopolymer exhibited a low strength in the early stage but a significantly high strength in the late stage. The ceramic geopolymer showed a low strength in both the early and late stages. The strength of concrete geopolymer was high in the early stage, but increased slowly later on. The mixed brCDW geopolymer had the highest early strength and the second-highest final strength compared to the single-component derived geopolymers. Furthermore, the XRD, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) tests were conducted to explore their microscopic mechanisms. The precursor materials dissolved in the geopolymerization process, thereby generating gels to coat and bind the unreacted particles. The formed gel type varied due to the different chemical composition ratios of precursor materials. The morphology and structure of brick geopolymer gels differed from other geopolymers, with the main gel products being C-A-S-H and N-A-S-H. The primary gel of ceramic, concrete, and mixed precursor geopolymers was the C–S–H gel, possibly with small amounts of C-A-S-H and N-A-S-H gel. Both the formed gel type and the compactness of the microstructure had significant influences on the geopolymer strength.