Fresh properties of fly ash-based geopolymer: the role of the testing conditions on the rheological measurements

Abstract

Rheology is a powerful tool to assess the fresh properties of cementitious systems accurately. However, uncertainties and systematic errors in measurements can impact the obtained values, even though the rheological properties should not depend on the measurement procedure. Contrary to Portland-based systems, few reports systematically assessed the role of the testing conditions on the rheological properties of geopolymers. This study evaluates the effect of testing conditions on the measurement of the rheological properties of fly ash-based geopolymer paste. Different testing geometries, including two sets of parallel plates, two concentric cylinders, and a four-blade vane spindle, were used to obtain flow curves by continuous and stepwise shear rate increments. The results show that reducing the ratio between the spindle and container radii for coaxial geometries can induce wrong shear stress/shear rate calculations and local instability. Additionally, the differences in shear stress measured with ramp and steps protocols increase as the Ri/Ro ratio (i.e., the ratio between the spindle and the container radii) increases. Parallel plates require an extra ring to keep the paste between the plates during shearing. Increasing the maximum shear rate applied reduces yield stress and viscosity values, indicating improved particle dispersion. Comparative tests with a Portland paste demostrated that the mini-slump test was suitable for assessing the yield stress of the geopolymeric system. However, its limitation regarding viscosity assessment was evidenced. Therefore, the measurement setup must be carefully chosen for the geopolymer under evaluation, and preliminary tests are highly recommended to support this selection.