Coupled polymer gelation and cement hydration in the structural build-up of xanthan gum-modified cement pastes

Although microbial polysaccharides (MPs) are widely employed to enhance the initial rheological properties of self-consolidating concrete and 3D printed concrete, their early-age structural evolution, particularly the coupling between polymer networks and cement hydration, remains insufficiently understood. In this study, the structural build-up of cement pastes containing different dosages of xanthan gum (XG) was systematically evaluated over 240 min by monitoring the evolution of static yield stress. Rheological and calorimetric tests demonstrated that at low XG dosages (< 0.2%), the structural build-up followed a phased evolution. During both the initial and induction periods of hydration, it increased linearly over time, whereas it grew exponentially upon entering the acceleration period. In contrast, when the XG dosage exceeded 0.2%, the behavior exhibited consistent linear growth over time. Pore solution and polymer entanglement tests revealed that this transition above 0.2% XG was governed by XG entanglement, where an in-situ XG-Ca²⁺ gel network formed. In the first two hydration stages, the polymer network dominated the weak hydration network, acting as the primary driver for the structural build-up. In the acceleration stage, microstructural analysis suggested that the polymer network provided a bio-template for hydration, synergistically strengthening interparticle interactions and resulting in a uniform structural evolution. This study provides a deep understanding of the structural build-up process of cement pastes containing MPs, enabling improved control of buildability and stability in applications.

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