Influence of slag and recycled concrete aggregates contents on the performance of fly ash based self-compacted geopolymer concrete

Geopolymers are considered a sustainable substitute for Ordinary Portland Cement, but they do have an impact on the early age and long-term qualities of concrete, specifically its workability. This study evaluates the feasibility of developing sustainable self-compacted geopolymer concrete, using recycled concrete aggregates. The geopolymer consisted of ground granulated blast furnace slag and fly ash activated with sodium alkalis. The fly ash was replaced with slag at contents of 50% and 70% while the natural coarse aggregates was replaced with recycled concrete aggregates at contents of 30%, 50%, and 100%. A series of five mixes were designated in this study with a total binder content of (450 kg/m3) and an alkaline/binder ratio of 0.5. There were three mixes of self-compacting geopolymer concrete that were made using different amounts of recycled concrete aggregates 0%, 50%, and 100%. Additionally, there were two more mixes that used 100% recycled concrete aggregates, but with different proportions of fly ash and slag binders: 70% fly ash and 30% slag, and 50% fly ash and 50% slag. The fresh properties were assessed through slump flow, J-ring flow, V-funnel, J-rig height, L-box, and sieve segregation tests. Compressive strength, flexural strength, and static modulus of elasticity in terms of mechanical characteristics and fracture parameters such as the load-displacement curve, fracture energy, and stress intensity factor were also measured. The sorptivity of the developed mixtures was also evaluated in this study. The findings confirm that recycled concrete aggregates and slag negatively affect the workability of self-compacted geopolymer concrete mixtures, except for segregation resistance. The mechanical properties, fracture parameters, and sorptivity of fly ash based self-compacted geopolymer concrete are reduced by recycled concrete aggregates and enhanced by slag. It was found that a high-strength (77 MPa) self-compacted geopolymer concrete can be made with a mix containing 50/50 fly ash/slag and 100% recycled concrete aggregates. The results were compared with prediction models of mechanical properties developed by standard codes of practice and previous studies. It was shown that these predictive models do not effectively capture the properties of the self-compacted geopolymer concrete.