Characterisation of rendering mortars by squeeze-flow and rotational rheometry

• Phase segregation plays an important role on the squeeze-flow of rendering mortars.
• Maximum particle size limits the minimum gap that can be achieved during squeezing.
• Good correlation was found between squeeze (5 × >) and rotational yield stresses.
• Extensional viscosities were ≈ 30 times higher than rotational shear viscosities.
• Very stiff mortars could not be properly evaluated by the rotational setup used.

This paper reports the first experimental comparison between squeeze-flow at controlled displacement rate and rotational shear at controlled speed for evaluating the rheological behaviour of mortars. Several Brazilian and European rendering products showed a wide range of workability behaviour in both testing modes. The flow curves and hysteresis effects during the shear cycle varied significantly, since the mortar's composition affects structural breakdown, interfacial slip and phase segregation. The latter plays a critical role during squeeze-flow and is the main reason for the higher loads required to deform the samples at lower rates. A gravimetric-based methodology was developed to assess phase segregation induced by rotational tests. Encouraging agreement between the methods was observed, with yield stress in the structured (unsheared) state showing a good linear correlation. For some mortars showing low segregation it was possible to compare shear viscosity and extensional viscosity and Trouton ratios between 20 and 40 were obtained.