Ion-dependent rheology and hydration of (κ)-carrageenan in cement suspensions

Using biopolymers to tune cement rheology is a sustainable strategy for lowering the carbon dioxide footprint of the construction sector. Carrageenan viscosity-modifying admixtures (VMAs) from red algae, Kappaphycus alvarezii and Chondrus crispus and pure (κ)-carrageenan were investigated in this work. Ionic environments were altered using dialysis and enrichment with monovalent and divalent cations. Modified biopolymers were dosed into cement pastes and evaluated for plastic viscosity, yield stress, viscoelasticity, structural build-up (SBU) kinetics and hydration behaviour. Dialysis of native seaweed extracts increased the suspension viscosity and rigidity, whereas ionic enrichment of pure (κ)-carrageenan reduced both parameters. Despite these contrasts, the SBU kinetics and hydration remained largely insensitive to ionic modification (dialysis and ion enrichment), indicating control by intrinsic molecular architecture (polymer structuration/junction-zone formation), particularly the presence and density of 3,6-anhydrogalactose bridges. The results revealed a dual control mechanism: ionic media governs flow resistance, while molecular structure sets time-dependent structuration and hydration responses. These insights provide practical levers for tailoring carrageenan-based VMA performance through processing, such as targeted dialysis or cation management, without compromising hydration. Overall, this work clarifies how ionic interactions and polymer structure jointly shape rheology in cement systems and informs the design of sustainable, bio-based admixtures for rheology control across mix designs and conditions.