Rheological Properties of Cement Mortar with Fly Ash and Silica Fume for 3D Printing

This study examines the rheological behavior of cement-based mortar mixes designed for 3D printing by varying fly ash (FA) and silica fume (SF) contents and testing at different resting times. Six mixes were prepared: three with 0%, 15%, and 30% FA at a 0.30 water-to-binder (w/b) ratio, and three with 0%, 10%, and 20% SF at a 0.45 w/b ratio. Anton Paar MCR 302 rheometer was used to perform a standard hysteresis loop test to measure static yield stress, dynamic yield stress, plastic viscosity, and thixotropy at 0, 15, and 30 minutes after mixing. Results showed that the 15% fly ash mix developed notably high static and dynamic yield stresses along with the greatest thixotropy at later resting times, implying a beneficial balance of flowability and structural build-up. In contrast, the 30% fly ash mix showed a significant drop in both yield stress and thixotropy after 30 minutes. This suggests it required excessive water and that the pozzolanic reaction was slower. For silica fume mixes, the 20% SF mix had the highest yield stresses overall. However, the 10% SF mix achieved better thixotropy at 30 minutes. These findings support ongoing efforts to create mix designs that improve pumpability, extrudability, and layer stability for additive manufacturing in construction.