Effect of recycled concrete powder on the rheological properties of cement paste: New findings

Recycled concrete powder (RCP) influences cement paste's rheology, but the mechanisms remain unclear. This paper intends to fill this gap by employing a new method for measuring water absorption, the minimum water requirement method, an organic carbon analyser, and a laser particle size analyser. The cementitious material's water absorption ($W$), packing density (${\phi }_m$), water reducer adsorption ($Q_{ad}$), and particle size distribution are determined. Results show that, as the RCP's content increases from 0 % to 25 %, the cementitious material's $W$, ${\phi }_m$, $Q_{ad}$, volume fraction, and average particle size increase by 21.7 %, 0.9 %, 26.2 %, 1.4 %, and 30.6 %, respectively. Consequently, the particle's surface covered by the water reducer ($\theta$) and distance ($H$) decrease by 26.5 % and 32.6 %, respectively, resulting in an increase in the paste's yield stress (${\tau }_0$) and plastic viscosity (${\eta }_{pl}$) by 1946.6 % and 45.3 %, respectively. Based on an existing yield stress model, RCP affecting ${\tau }_0$ can be attributed to changes in the particle system's colloidal and contact interactions. A decrease in $H$ increases colloidal interactions. Conversely, an increase in ${\phi }_m$ and a decrease in fine particle content reduce contact interactions. Colloidal interactions are more significant, thus ${\tau }_0$ increases. Based on the functional expression for the ${\eta }_{pl}$ developed here, RCP affecting ${\eta }_{pl}$ can be attributed to changes in hydrodynamic interactions and contact interactions. A decrease in $H$ increases hydrodynamic interactions. An increase in ${\phi }_m$ combined with a decrease in fine particle content decrease contact interactions. Additionally, an increase in $Q_{ad}$ reduces pore solution's viscosity. Hydrodynamic interactions are more significant, thus increasing ${\eta }_{pl}$.