Macro and micro characteristics of eco-friendly fly ash-based geopolymer composites made of different types of recycled sand

The main objective of the present study is to explore alternative recycled sand materials to river sand. Within this context, six different types of recycled sand materials obtained from the recycling of construction materials were considered to be alternatives to the river sand. In producing fly ash-based geopolymer composites, the river sand was substituted with recycled sands. Then, the properties of geopolymer composites in the fresh and hardened states were studied comparatively. In this regard, flow table test, rheology, and fresh unit weight were determined as fresh state properties whereas dry unit weights, flexural and compressive strengths, and ultrasonic pulse velocity performance were determined as hardened state characteristics. In addition, the interfacial transition zone occurring between these sands and geopolymer paste was displayed using the scanning electron microscope technique. In a general perspective, all the investigated characteristics of the geopolymer composites were influenced by substituting the river sand with recycled sands. The flowability of the geopolymer mixtures decreased whereas they performed more viscous behavior when the river sand was replaced with recycled sands. Among the recycled sands, only the sand recycled from basalt tile increased the fresh and dry unit weights of geopolymer composite while the others resulted in decreases. Replacing the river sand with the sands recycled from tiles did not significantly influence the flexural strength, however, the sands recycled from basalt, granite, and marble tiles led to increasing the compressive strength. Regarding the results achieved in the current study, it could be stated that these recycled sands could be utilized in the production of the geopolymer composites in a controlled manner.