Abstract: This study investigates the mechanisms behind fluidity loss in superplasticized limestone calcined clay cement (LC3), a sustainable alternative to ordinary Portland cement (OPC). Despite its environmental benefits, in presence of superplasticizers, LC3 experiences significant challenges in maintaining workability, an issue of which this paper examines the root cause. It focuses on the role that initial reactions play in creating additional surface area and the consequence thereof on the performance of polycarboxylate ether superplasticizers (PCE) in LC3. Experimental results reveal that while PCEs initially disperse the cement particles, fluidity decreases rapidly over time, primarily due to the continuous generation of those new surfaces that exceed the adsorption capacity of PCEs. The study also examines the potential intercalation of PCE side chains into calcined clays and shows that even in the worst‐case scenario with montmorillonite clays, intercalation is not a significant contributor to slump loss when the clays are calcined. These findings suggest that alternative strategies, such as slowing down the initial reactivity of the calcined clays, for example by combining PCEs with other additives like diphosphonates, may be necessary to improve flow retention in superplasticized LC3 systems.