<p>This paper presents phase diagrams for Portland cement-slag-fly ash ternary cements. The study investigates various binder combinations of Portland cement, slag and fly ash, ranging from 30–100, 0–50 and 0–50, respectively. The thermodynamic equilibrium of given ternary cement systems was simulated using thermodynamic modeling coupled with simulated degree of reaction of slag and fly ash that vary with the binder compositions. The obtained results suggest that a 56% substitution factor for supplementary cementitious materials is feasible without significantly altering the formation of C–(A)–S–H. Additionally, it is found that ettringite and straetlingite are stable in the same cementitious material combinations, but ettringite becomes unstable in mixtures that favor monosulfate formation. Overall, this study provides a catalogue for selecting an appropriate proportion of binders to create mixes that complement one another, providing characteristics that are specifically tailored to the application. The results may have important implications for designing and optimizing ternary cement compositions.</p>