Material-Level Fuzzy Logic Assessment of CEM III Mortars Under Raw-Material Variability: Effects of Fly Ash and Internal Sulfate Sources

Rapid construction development increases the demand for raw materials and makes variations in their quality, including mixing-water quality, increasingly relevant to cementitious-material performance. This creates a need for assessment tools capable of integrating several material properties under variable raw-material conditions. Fuzzy Logic can support such assessment by combining mechanical and physical responses into a single comparative index. This study evaluated the effects of gypsum and sodium sulfate on CEM III mortars with and without 10% fly ash. Compressive strength, flexural strength, and mass water absorption were measured after 28 days and integrated using a Mamdani Fuzzy Logic model into a Fuzzy Performance Index (FPI). The sulfate-free mortar containing fly ash showed mean compressive and flexural strength values 3.73% and 1.47% higher than the reference mortar. Gypsum produced the strongest adverse changes, with the higher dosage reducing compressive strength by 26.87% and increasing water absorption to 11.67%. The response to Na2SO4 was non-monotonic and depended on binder composition. Sensitivity analysis showed stable separation of the mixtures into performance groups, with Spearman coefficients of 0.9286–1.000, although closely ranked mixtures could exchange positions. The results support the use of Fuzzy Logic as a local screening tool for integrating conflicting material responses within the investigated experimental domain.