Rheological Control of 3D Printable Cement Paste and Mortars

Recent advances in concrete construction such as three-dimensional concrete printing (3DCP) have given rise to new requirements on the control of both the hydration and rheology of cementitious materials. To meet these new demands, and to move toward adoption of 3DCP on a commercial scale, in-operando control of hydration and rheology will be required. In this study, two cement paste mixtures containing limestone powder of two different median particle sizes are used to create 3D printed structures with a cement paste printer. Hydration control in the form of acceleration is achieved with the addition of the limestone powder to the cement and rheology control is achieved by using limestone with different median particle sizes. Rheology measurements conducted concurrently with printed structures indicate that yield stress and a measure of thixotropy of the cement paste provide an indicator as to whether a material will produce a multi-filament free-standing structure for a given 3DCP system. Simulations of particles flowing in a pipe are used to study the rheological behavior of paste and mortar. For the case of a mortar, the flow rate of suspended particles (sand) follows the same functional form with driving force as the matrix fluid (cement paste). Shear-induced particle migration increases the density of particles toward the center of the pipe, a result that implies that the aggregates may not be uniformly distributed.