Optimization of Mix Proportion and Performance Study of Metakaolin-Slag Geopolymer Mortar Based on Orthogonal Experiment

To promote the practical application of metakaolin-slag geopolymer materials in engineering repair, it is essential to clarify the influence of mix proportion parameters on macroscopic properties, given their inherent deficiencies of inferior toughness and volume stability. In this study, a five-factor and four-level orthogonal experimental design was adopted to systematically investigate the effects of slag content, water glass modulus, alkali equivalent, water–binder ratio, and sand–binder ratio on the fluidity, compressive strength, flexural strength, compressive-to-flexural strength ratio (toughness indicator), and drying shrinkage rate (volume stability indicator) of geopolymer mortar. Range analysis and variance analysis were conducted to clarify the primary and secondary order of influencing factors for each performance index, and the optimal mix proportion balancing multiple performance demands was determined. The results indicate that alkali equivalent is the core factor governing compressive and flexural strength, whereas slag content dominates the compressive-to-flexural ratio, fluidity and drying shrinkage. The geopolymer mortar achieves relatively optimal comprehensive performance when the slag content is 20%, the sodium silicate modulus is 1.6, the alkali equivalent is 12%, the water-to-binder ratio is 0.49, and the sand-to-binder ratio is 2:1, and all indicators meet the specification requirements for rigid repair mortar. Combined with SEM-EDS and XRD microstructural analysis, the main products of the metakaolin-slag system are amorphous N-A-S-H gel and C-(A)-S-H gel. Appropriate alkali equivalent and slag content can promote the dissolution of aluminosilicate raw materials and facilitate the formation of both gel products, providing microstructural support for the improvement of macroscopic performance.