A Novel K+ Slow-Release Cementitious Material Developed from Subway Tunnel Muck for Ecological Concrete Applications

This study explored a novel cementitious material developed from subway tunnel muck (STM) intended for ecological concrete (EC) preparation. The effects of three alkaline activators (NaOH, KOH, and CaO) on the properties of the cementitious materials were systematically examined. The results indicated that NaOH exhibited the most effective activation performance, followed by KOH, with CaO being the least effective. The NaOH-activated materials exhibited the highest compressive strength (reaching up to 12.15 MPa), the densest microstructure (characterized by the lowest porosity and smallest average pore size), the most substantial gel formation (evidenced by the highest mass loss in thermogravimetric analysis), and the optimal gel structure (indicated by the pronounced peak sharpening in Fourier transform infrared spectroscopy) after a 28-day curing period. Moreover, the crystallization of potassium salts under KOH activation detrimentally impacted the microstructure of KOH-activated materials. To balance the need for structural strength and nutrient provision, NaOH + KOH-activated materials were selected for the preparation of EC. Notably, the application of NaOH + KOH-activated materials resulted in a significant increase in K+ concentration in the soil layer, compared to common soil. Furthermore, NaOH + KOH-activated materials exhibited a slow-release effect, thereby offering sustained nutrient support conducive to plant development.