Hydration and Pozzolanic Behavior of Ultra‐High‐Performance Concrete Incorporating Coal Bottom Ash

ABSTRACT This study investigates the feasibility of using ground coal bottom ash (CBA) as a partial replacement for Portland cement (PC) in ultra‐high‐performance concrete (UHPC). PC was replaced with CBA at levels of 10, 20, 30, and 50 wt.%, and the resulting effects on hydration behavior and mechanical properties were systematically evaluated. A 10 wt.% replacement of PC with CBA resulted in improved compressive strength, whereas higher replacement levels led to strength reduction mostly due to a dilution effect. Nevertheless, increasing CBA content enhanced overall pozzolanic activity, accompanied by improved reactivity of both CBA and silica fume. It revealed that Al‐ and Fe‐bearing sources released from CBA were preferentially incorporated into the calcium silicate hydrate phase rather than forming distinct AFt or AFm phases. Despite the partial replacement of PC with CBA, the sample containing 10 wt.% CBA exhibited excellent mechanical performance, indicating an optimized balance between PC hydration and pozzolanic reactions. Overall, the results demonstrate that CBA can be effectively utilized as a binder in UHPC, contributing to reduced cement consumption and improved sustainability.