Alkali-Activated Slag Concrete with Recycled Aggregate: Long-Term Performance

This paper evaluates the long-term mechanical properties (up to 1 year) of alkali-activated slag concrete produced with recycled coarse aggregate (AAS-RA) and natural coarse aggregate (AAS-NA). The AAS-RA achieved a compressive strength of 35.20 MPa at 28 days and improved to 37.52 MPa at 365 days. This corresponded to a reduction in strength of 12.2% at 28 days and 7.7% at 365 days when compared with AAS-NA concrete. The flexural strength of both AAS concretes decreased with age, displaying a 10% and 11.8% drop after 1 year for AAS-NA and AAS-RA concretes, respectively, compared with that achieved at 28 days. However, the splitting tensile strength remained constant for the entire period. Furthermore, the elastic modulus of both AAS concretes decreased between 28 and 90 days; however, beyond 90 days, AAS-RA concrete maintained a constant elastic modulus while AAS-NA showed further decreased with time, such that by 365 days the AAS-NA had a 10% lower value than the AAS-RA concrete. This is attributed to alkali activation continuing beyond 90 days, producing additional C-A-S-H/C-S-H gel. This resulted in the combined effect of disjoining pressure and self-desiccation, which increased propagation of cracks and crack widths at later ages. However, in the AAS-RA there was also an increase in bond strength between the C-A-S-H gel and old cement paste (C-S-H gel matrix) on the recycled coarse aggregate, which resulted in a greater elastic modulus for the AAS-RA concrete. Experimentally observed long-term mechanical properties were compared with the predicted values in Australian and American Concrete Institute (ACI) codes. However, further studies are required to identify specific amendments required for these codes for the design of structural members manufactured from AAS concrete with recycled aggregate.