Résumé
The use of viscosity-modifying admixtures (VMAs) is essential to ensuring cohesion and stability incementitious systems, yet most synthetic options remain costly and unsustainable. This study investigatesAscophyllum nodosum (AN), a brown seaweed rich in alginates (ALG), as a natural, low-cost alternative. The seaweedpowder was pre-dispersed in heated water (40 °C) to promote alginate release and tested both alone and in combinationwith two high-range water-reducers (HRWRs), polycarboxylate ether (PCE) and polynaphthalene sulfonate (PNS),using purified alginate as a reference. Cement hydration, rheology, mini-slump flow, and compressive strength wereassessed. Results showed that AN (1%) and ALG (0.125%) produced comparable effects on plastic viscosity and yieldstress, despite the much lower dosage of ALG. ALG exhibited greater compatibility with PNS in terms of viscosity,yield stress, and workability retention, whereas AN exhibited strong compatibility with both HRWRs. Regardingstructural build-up, ALG improved rigidity more effectively with PCE, while AN enhanced rigidity with bothHRWRs. Hydration kinetics and strength development revealed antagonistic interaction with PCE, particularly forAN, but beneficial synergy with PNS, which mitigated retardation and enhanced compressive strength. Overall,combined with PNS, AN offers reliable compatibility, rheological stability, controlled hydration, and superiorperformance in eco-efficient cementitious systems.