Collaborative Optimization of Mechanical and Alkali-Reduction Performance of Ecological Low-Alkali Porous Concrete Based on Alkalinity Gradient Regulation

Abstract Porous concrete (PC) is widely used in slope ecological restoration due to its good permeability and plantability. However, the synergistic evolution mechanism between its mechanical properties and pore alkalinity has not been systematically elucidated, which has become a key bottleneck restricting material synergistic optimization. Therefore, this paper takes PC as the research object and systematically explores the synergistic evolution mechanism of gradient alkali-reduction technology on its pore alkalinity and mechanical properties through a combination of internal and external methods, included unconfined compressive strength and pH value tests, combined with X-ray diffraction (XRD) and scanning electron microscope (SEM) microscopic analyzes. This technology is achieved through material composition design, gel system optimization, and various external alkali-reduction methods. The results show that the water-binder (w/b) ratios and aggregate sizes have a significant influence on the pore alkalinity and mechanical properties of the PC. Among them, the w/b ratio of 0.30 and aggregate size of 10–16 mm are the most superior in terms of balancing performance. Using low-alkalinity sulfoaluminate (LSAC) cement instead of ordinary Portland cement (OPC) can improve the PC’s mechanical strength and significantly reduce its pore alkalinity. Meanwhile, the synergistic incorporation of gypsum and silica fume further improves the PC’s mechanical properties and environmental adaptability, achieving dual performance enhancement. Among the various alkali-reduction approaches, the combination of the agronomic-physical method demonstrates the best performance regarding alkalinity control, long-term stability, and enhancement of mechanical properties. After optimizing the gradient alkali-reduction strategy, the mechanical strength of low-alkali porous concrete (LAPC) increased by 75.3%, and the pH inside the pores decreased by 45.9%, indicating good potential for engineering applications. In addition, outdoor vegetation experiments have also shown that the LAPC, achieved through alkaline gradient regulation, can facilitate rapid turf establishment and uniform coverage of plants, demonstrating excellent vegetation effects and confirming the effectiveness of the gradient alkali-reduction strategy.