Synergistically enhanced alkali-activated recycled mortar with kappa-carrageenan and aluminum silicate fiber: Mechanical properties and microstructural evolution

To address the issue of insufficient strength in alkali-activated recycled concrete powder (RCP) mortar at high replacement rates, this study proposes the use of K-carrageenan (K-C) and aluminum silicate fibers (ASF) to construct a polymer-fiber composite reinforcement system. Through mechanical property testing and microscopic characterization methods such as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and nitrogen adsorption, the study investigated the synergistic effects of these two materials on the mechanical properties and microstructure of alkali-activated recycled concrete powder (AAM) mortar. The results indicate that K-C significantly enhances mortar strength, with an optimal dosage of 0.6%; ASF suppresses crack propagation and maintains toughness through physical bridging effects, with an optimal dosage of 2%. A significant interaction effect was observed between the two in the analysis of variance, confirming their synergistic enhancement; The 28-day flexural strength of the A2C6 group exceeded the optimal values of the respective single-additive mixtures while maintaining high toughness, achieving a balanced optimization of strength and toughness. Microstructural analysis revealed that K-C improved matrix density and promoted the formation of cement paste with a high Ca/Si ratio; at a dosage of 0.3%, it reduced porosity, but further addition caused porosity to rise again due to viscosity effects; ASF releases Si/Al through alkali leaching to participate in matrix reactions, while simultaneously causing porosity to increase monotonically. This synergistic system provides a new approach for the efficient utilization and performance regulation of high-replacement-rate RCP, offering valuable insights for improving the utilization rate of construction and demolition waste.