Effects of Various Concrete Waste Micropowders on Hydration Gel Products and Mechanical Performance of Ultra-Low w/b Ratio Cementitious Composites

Partial replacement of cement by concrete waste micropowder in ultra-low-w/b cementitious composites is an effective strategy to reduce carbon emissions and promote the high-value utilization of concrete waste. While existing studies have predominantly focused on the effect of concrete waste micropowder on high-w/b cementitious composites systems, investigations on ultra-low-w/b cementitious composites remained limited. Therefore, this study examined the influence of concrete waste micropowder type and replacement ratio on the performance of ultra-low-w/b cementitious composites. The results indicated that moderate cement substitution by concrete waste exerted a minor impact on hydration and hydration products and enhanced multi-scale mechanical properties. In contrast, high-volume replacement (above 30%) weakened the hydration process, reduced both the quantity and quality of hydration products, and degraded the micro-to-macro performance. Nevertheless, even at 70% replacement, the ultra-low-w/b cementitious composites retained exceptional mechanical performance and a high strength retention rate, with a 28-day compressive strength still exceeding 120 MPa. Based on the hydration data of ultra-low-w/b cementitious composites incorporating concrete waste micropowder, a kinetic model was established and the corresponding kinetic parameters were determined. In addition, the ultra-low-w/b cementitious composites with ground concrete micropowders and those with ground mortar micropowders exhibited comparable microstructure and mechanical performance, both of which were superior to those with ground paste micropowder. Co-incorporation of granulated blast furnace slag or fly ash with concrete waste micropowder further improved the microstructure and multi-scale mechanical performance.