Investigation on the Early Performance and Hydration Control Mechanism of Polyacrylamide in Mortar with Accelerator

To address the shortage of natural sand resources and environmental issues, the industry has shifted to using manufactured sand as a substitute. Flocculants are often used to treat sand washing wastewater in wet production processes, which can lead to excessive flocculant residues in mechanized sand, negatively affecting the performance of shotcrete. In this study, the influences of polyacrylamide (PAM) on the workability, mechanical properties, and microstructural development of shotcrete were investigated by simulating different residual PAM levels in manufactured sand. The addition of PAM resulted in a significant loss of mortar workability and a shortening of the net mortar setting time. PAM adsorbs Ca 2+ to occupy adsorption sites, thereby inhibiting the formation of [ Al ( OH ) 4 ] − generated in an alkaline environment from participating in AFt formation, thus slowing down the hydration of C 3 A . By weakening the adsorption of AFt, it promotes the hydration of C 3 S to form more C─S─H gel and CH structures. As the reaction progresses, the AFt structure formed by the hydration of C 3 A also exerts an adsorption effect on C 3 S , causing the hydration rate of C 3 S to slow down again. As a result, the maximum decrease in mortar strength after 6 h was 19.4%, and the maximum increase in mortar strength after 24 h was 8.7%. Through this study, we hope to provide guidance on the control of PAM residue in mechanized sand used in shotcrete and provide a theoretical basis for the preparation of high-quality mechanized sand shotcrete.