AbstractThis paper presents the results of an experimental study investigating the recyclability of ground granulated blast-furnace slag (GGBFS) base geopolymer concrete as recycled aggregates (RAs) for both geopolymer concrete (GPC) and portland cement concrete (PCC). The effects of partial (0%, 25%, 50%, and 100%) replacement of recycled geopolymer concrete aggregates (RGAs) with natural aggregates (NAs), as fine and coarse aggregate, in either type of recycled geopolymer concrete (RGPC) and recycled portland cement concrete (RPCC) under ambient-cured conditions in similar laboratory circumstances was evaluated. The results were compared with those of recycled concrete counterparts made by recycled cement concrete aggregates (RCAs). A wide range of basic and mechanical properties [i.e., density, compressive strength, ultrasonic pulse velocity (UPV), tensile strength, flexural strength, and modulus of elasticity, water absorption, and shrinkage] along with scanning electron microscopy (SEM) analysis was investigated. The results highlight the high potential of RGAs in both RGPC and RPCC in terms of compressive and tensile strength with only an average reduction of 10% to 15% in 50% substitution of RGAs with NAs. However, there seems to be an incompatibility in using the heterogeneous type of recycled concrete aggregate and binder, which led to more dispersion of test result values. The results in the modulus of elasticity showed a negligible decrease of 3% to 6% by RAs substitution with NAs compared with the control samples. The reductions in flexural strength, water absorption, and shrinkage of RGPC were found to be slight, more than RPCC, which seems to be mainly attributed to the differences in properties and characteristics of geopolymer paste than the influence of RAs content. However, further incorporation of RAs content of more than 50% was found to lead to more significant negative effects in all recycled concrete groups, but the substitution of 50% RGAs to NAs as an optimum mixed proportion due to promising results can be considered for more investigation.Practical ApplicationsGPC is known as a sustainable alternative to PCC, admittedly with more compatibility with the environment. By expanding the use of GPC in the construction industry in the near future, the end-of-life strategies of this material should inevitably be considered as an upcoming concern according to sustainable development policies. Although recycling and reusing of PCC has been widely investigated in many studies for years, the applicability of using GPC itself as a recyclable material to be used for recycled concrete has been barely studied. Therefore, this study compares the properties of slag-based geopolymer recycled concrete with conventional cement-based recycled concrete. The results indicate the promising potential of GPC recyclability for partial replacement of recycled aggregate with natural aggregate and acceptable mechanical properties and durability as a sustainable material for the construction industry. The geopolymer recycled concretes recorded better compressive and tensile strengths and similar modulus of elasticity compared with cement-based recycled concrete. Additionally, the results in flexural strength, water absorption, and shrinkage of cement-based recycled concrete indicated a better behavior over time, which is more attributed to the characteristics of geopolymer binder rather than recycled aggregates. However, incorporation of RAs content of more than 50% was found to lead to more adverse effects in all recycled concrete groups.

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