Macro–Micro Characteristics of Eco-Friendly Geopolymer Incorporating Cement Paste Recycled Powder

Reusing cement paste recycled powder (CRP) as a renewable binder in sustainable geopolymer offers a valuable approach for recycling construction and demolition waste streams. This study investigates the fresh, hardened properties and microcharacteristics of recycled powder geopolymer (RPG) incorporating CRP. The four experimental parameters include the replacement of CRP at 0% to 100%; the replacement method—with granulated blast furnace slag (GGBS), fly ash (FA), and both GGBS and FA; alkali dosage (M+); and alkali modulus (AM). The results show that the setting time significantly increased with the replacement of both GGBS and FA up to 75% but decreased with the single replacement of FA. Increasing M+ and AM caused the setting time to first increase and then decrease, owing to the content of Si4+ and Al3+. Additionally, the modified Bingham model was more accurate in fitting the rheological properties of RPG. The incorporation of CRP decreased the yield stress and plastic viscosity of RPG and elevated the energy threshold for destroying thixotropic structures. Furthermore, CRP had a negative impact on RPG’s compressive strength, which decreased by 90% when CRP replaced GGBS at a replacement rate of 100%. Based on the X-ray diffraction, Fourier transform infrared spectroscopy, thermal gravity, and scanning electron microscopy, the mineral composition mainly present in CRP was found in the RPG samples, and the microstructure of RPG loosened with an increase in CRP content. This behavior revealed that CRP is a powder with low activity and low effective calcium content and explained the significant decrease in compressive strength. Optimizing the replacement rate of CRP, M+, and AM can prepare sustainable geopolymer with good performance, and the upcycling of less than 50% CRP for RPG is feasible and sustainable.