Performance of high strength geopolymer concrete containing recycled aggregates with different parent concrete strengths

Recycled aggregate (RA) in high strength geopolymer concrete (HSGPC) can provide a path toward sustainable development in construction. However, the impact of parent concrete (PAC) strength as a factor influencing the quality of RAs must be considered. The gap in understanding about the performance of RA with different PAC strengths in HSGPC and the critical role of RA quality in high-strength concrete distinguishes the need for substantial investigation in this field. Accordingly, this research evaluated the performance of slag-based HSGPC (activator solution = NaOH + Na2SiO3) that is made with three RAs originated from PACs with strengths of 20-60 MPa. Ten HSGPCs with 50-100% RAs were produced and assessed for compressive and tensile strengths, electrical resistivity, absorption and H2SO4 acid attack. Results show that 100% replacement of natural aggregate with RA originated from 60 MPa PAC strength and up to 50% replacement level for RA with 40 MPa PAC strength, provided a comparable or higher compressive strength than plain concrete. Although the porous nature of RA provided an easier condition to accelerate the migration of aggressive ions into the concrete structure which adversely affects the electrical resistivity and absorption of the HSGPCs, this is generally less pronounced for RA with a higher PAC strength. SEM observation revealed a significant deterioration with numerous cracks indicating a high crushing load loss of HSGPC fabricated with RA after being subjected to H2SO4 acid attack.