Effects of Fly Ash–Metakaolin Composition and Sodium Silicate-to-Sodium Hydroxide (SS/SH) Ratio on Hybrid Geopolymer Concrete After Elevated-Temperature Exposure: Mechanical, Microstructural, and Machine-Learning Assessment

This study investigates the coupled effects of metakaolin (MK) replacement and the sodium silicate-to-sodium hydroxide (SS/SH) ratio on the fresh-state, mechanical, thermal, and microstructural properties of ambient-cured hybrid geopolymer concrete. Fly ash was replaced by MK at levels of 0%, 25%, 50%, 75%, and 100%, and SS/SH ratios of 2.0, 2.5, and 3.0 were adopted, producing 15 mixtures activated with a 12 M NaOH solution. Fresh-state slump was measured, and compressive and splitting tensile strengths were evaluated at ambient temperature and after exposure to 300 °C. XRD, FTIR, TGA/DTG, FE-SEM, and EDX analyses were also conducted to characterize the reaction matrix of a representative mixture (FA50–MK50–SS/SH2.5) after exposure to 300 °C. Increasing both MK content and SS/SH ratio reduced workability but substantially enhanced mechanical performance. At an SS/SH ratio of 3.0, replacing fly ash entirely with MK increased the ambient compressive strength from 20.17 to 39.59 MPa and the splitting tensile strength from 1.79 to 3.41 MPa, corresponding to improvements of 96.27% and 90.25%, respectively. Following exposure to 300 °C, the corresponding residual strengths reached 13.81 MPa and 1.60 MPa, representing increases of 79.31% and 77.76%, respectively, over the MK-free mixture. Nevertheless, heating caused considerable absolute losses in strength across all mixtures. At an SS/SH ratio of 2.0, increasing MK content reduced thermal compressive strength loss from approximately 60% to 44.77%, indicating greater retention of compressive strength after exposure to 300 °C. Microstructural observations revealed a predominantly amorphous reaction matrix containing residual precursor particles, local voids, and microcracks. The representative FE-SEM images also showed locally continuous matrix regions and apparently well-bonded aggregate–binder interfaces. These observations are consistent with the development of alkali-activated reaction products and locally homogeneous matrix regions. Experimental evaluation on three newly tested and two independently reproduced compositions showed close agreement in the limited evaluation set (descriptive R2: 0.906–0.999; MAPE: 0.47–1.40%). The original 95% prediction intervals had a strict coverage of 90%, whereas a supplementary resolution-adjusted assessment gave a coverage of 95%. These results support the local predictive usefulness of the framework within the investigated design domain, and uncertainty-aware Pareto optimization identified MK content as the dominant design variable. Overall, MK content and activator composition jointly govern the balance among workability, strength development, and residual mechanical performance of hybrid geopolymer concrete after exposure to 300 °C.