Influence of organic thickening admixtures on the rheological properties of mortars: Relationship with water-retention

In this study, we consider the influence of different types of polymer thickening admixtures on the rheological behavior of render mortars. The admixtures considered include cellulose–ethers with different molecular weights, a guar ether, and a starch ether. A thorough rheological investigation is performed, including the characterization of the steady state flow behavior and the microstructure at rest. The latter is examined through measuring linear viscoelastic properties using oscillatory rheology. First, it is shown that the fresh mortars behave generally as Hershel–Bulkley fluids with a non-trivial evolution of the rheological parameters with admixture content. The evolution of the viscoelastic properties with admixture content shows that the cellulose ethers and guar ethers decrease the gel-like response of the mortars, while starch ether increases their gel feature. This indicates that there should be a profound difference between the two types of admixtures regarding their organization within the pore solution and their interactions with the mineral particles. The rheological properties are discussed in relation to water retention. It is shown that the ratio between dissipation and elasticity (or cohesion) correlates well with water retention. The more the admixture introduces viscous dissipation (as compared to cohesion) into the system, the more it increases its water retention capacity. Water retention is also correlated with air-entrainment, indicating that the surfactant property of the admixture is a contributing factor.