Laboratory Performance of Heat-Assisted Fly Ash-Based Geopolymer Concrete as a Potential Thin Protective Layer for Asphalt Pavements

This study presents a laboratory-scale assessment of heat-assisted fly ash-based geopolymer concrete as a potential thin protective layer in one asphalt–geopolymer pavement configuration. The programme comprised water penetration under pressure, abrasion, initial skid resistance, wheel tracking, four-point-bending fatigue, mechanical strength, freeze–thaw response, de-icing-salt scaling, thermal characterization, and qualitative scanning electron microscopy. The selected geopolymer reached mean flexural, compressive, and splitting tensile strengths of 10.756, 61.058, and 3.797 MPa, respectively. Final rut depths were 1.57 mm after 7 days and 0.72 mm after 28 days, with corresponding WTSAIR values of 0.020296 and 0.014238 mm per 103 cycles. After 106 cycles at 10 Hz, 72–85% of the initial stiffness modulus remained across the tested strain levels. Mean de-icing-salt scaling was 0.500 kg/m2 after 28 days and 0.995 kg/m2 after 56 days. Standalone geopolymer specimens underwent full-depth water penetration, whereas no leakage through the asphalt layer was observed in the intact layered specimen during the 5 bar, 72 h test. This system-level observation supports functional tightness only under the tested intact condition and does not establish intrinsic material impermeability or long-term interface durability. Interpretation is limited to this laboratory-scale configuration: the 90 °C heat-assisted curing protocol limits transfer to conventional in situ paving; no ambient- or standard-cured control and no same-condition conventional overlay control were included; and only one composite geometry was evaluated. The findings therefore define application boundaries for further validation rather than a field-ready specification or proof of comparative superiority.