Freeze–thaw performance of ambient-cured geopolymer concrete: An integrated mechanical, bond, and microstructural comparison with C50/60 normal concrete

This study evaluates the freeze–thaw performance of ambient-cured geopolymer concrete developed to achieve a practical balance of strength, workability, and setting time, in comparison with conventional C50/60 high-strength normal concrete. After 28 days of curing, normal concrete and geopolymer concrete specimens were exposed to up to 300 freeze–thaw cycles. Compressive, flexural, splitting tensile, and bond strengths, together with mass loss, ultrasonic pulse velocity, and XRD, FTIR, and SEM analyses, were used to assess mechanical, physical, and microstructural deterioration. The geopolymer concrete achieved an initial compressive strength of 60.97 MPa, close to 64.30 MPa for normal concrete. However, normal concrete retained 64.95% of its compressive strength after 300 cycles, whereas geopolymer concrete showed rapid deterioration after 200 cycles; its compressive strength decreased to 14.50 MPa at 250 cycles and became unmeasurable at 300 cycles. The combined results suggest that deterioration of the investigated geopolymer concrete was predominantly associated with progressive physical damage, including matrix discontinuity and microcrack development, while the principal chemical and mineralogical framework remained relatively stable within the analysed range. Overall, the ambient-cured geopolymer concrete achieved high initial strength but exhibited substantially lower long-term freeze–thaw resistance than normal concrete.