Graphene oxide-regulated performance and microstructural evolution of lithium slag–GBFS geopolymers

This study investigated the effects of graphene oxide (GO) on the macroscopic performance and microstructural evolution of lithium slag–ground granulated blast-furnace slag (LS–GBFS) geopolymers. A face-centered central composite design was employed to evaluate the linear, quadratic, and interaction effects of the LS replacement level, GO content, and alkali equivalent within predefined ranges of 15–25 wt%, 0.01–0.03 wt%, and 5–7%, respectively. Twenty mixtures were examined in terms of flowability, initial and final setting times, and 28 d compressive strength, while selected specimens were characterized by SEM–EDS, XRD, and FTIR. The fitted quadratic models yielded coefficients of determination ranging from 0.9515 to 0.9708, with nonsignificant lack-of-fit terms, indicating satisfactory agreement with the experimental observations within the investigated factor window. Increasing the replacement of GBFS by LS prolonged the setting time and reduced both flowability and 28 d compressive strength. At a GO content of 0.02 wt% and an alkali equivalent of 6%, increasing the LS replacement level from 15 to 25 wt% reduced the 28 d compressive strength from 62.1 to 47.7 MPa. Internal-standard Rietveld refinement quantified amorphous fractions of 34.6 wt% in LS and 97.5 wt% in GBFS. Multi-region EDS indicated a lower mean Ca/Si ratio in L16 than in L5 (p = 0.007), whereas the difference in Al/Si was not statistically significant (p = 0.997). Increasing GO content reduced flowability and produced a nonlinear strength response. An intermediate GO level within the tested range was associated with improved local matrix continuity, whereas the highest GO level was accompanied by greater local heterogeneity, agglomerated products, and cracking. A higher alkali equivalent enhanced precursor dissolution but was also associated with more pronounced calcite reflections, heterogeneous reaction-product distribution, and reduced 28 d strength. Microstructural observations suggest that GO may provide local adsorption and nucleation sites and influence the deposition and spatial distribution of reaction products. These results establish the factor–response relationships within the investigated compositional window and provide a basis for the controlled use of LS in GO-modified LS–GBFS geopolymers.