Calcium aluminate cement (CAC) is an alternative to Portland cement, valued for its superior early strength and thermal resistance. Partially replacing CAC with Fly ash (FA) can reduce carbon footprint and production costs of CAC, producing sustainable cementitious binders. This research investigates on various properties (i.e., hydration kinetics; phase assemblage evolution; compressive strength) of [CAC + FA] binders. Using 13 distinct FAs, up to 50% of CAC was substituted. The study measures hydration kinetics, compressive strength, and employs the number of constraints to estimate FA reactivity. Advanced quantitative analysis draws links between hydration kinetics and compressive strength and elucidate the role of FA (including composition and reactivity) on binder performance. Techniques such as X-ray diffraction and thermodynamic modeling illuminate phase compositions in [CAC + FA]. This study also develops a novel method that evaluates the degree of reaction of FA in non-Portland cement binders. Then the correlation between degree of reaction of FAs at 3 days number of constraints is revealed.