Mitigating the impact of incompatibility between PCE superplasticizer and recycled C&D waste powder on fresh properties by optimizing cementitious compositions

Recycled construction and demolition (C&D) waste powder had been used as a supplementary cementitious material for low-carbon cement-based systems, while its irregular surface morphology and high water absorption caused poor compatibility with polycarboxylate ether (PCE) superplasticizers. To mitigate the impact of this, fly ash was introduced via a simplex centroid design to optimize the cementitious composition and mitigate adverse interactions. The mechanisms were clarified through particle packing, water film thickness, zeta potential, PCE adsorption, and ionic concentration. Results showed that recycled powder improved initial flowability due to better particle packing, lower surface area, and higher negative charge, but significantly compromised flowability retention due to increased PCE adsorption and water absorption. Partially substituting recycled powder with fly ash remarkedly improved flowability retention and setting behaviors compared to using recycled powder alone, but close to that of pure cement paste. This enhancement was attributed to the dilution of recycled powder, the ball bearing effect of fly ash, and more PCE molecules remained in the interstitial solution. Finally, synergistically incorporating 20 % recycled powder and 20 % fly ash could achieve the optimal performance. This study provides a theoretical and practical guidance for recycled powder applications in cement-based materials under diverse conditions. © 2025 Elsevier Ltd