Microscopic Characterization and Efficiency Coefficient Evaluation of Modified Recycled Concrete Micropowder in Cementitious Materials

To advance the development of sustainable buildings, this study investigates recycled cement-based materials. The core component of this material is concrete-based recycled micropowder (CRM), which is shaped and reinforced from recycled construction waste. It is then activated through high-temperature calcination to produce modified recycled micropowder (MRM), and the resulting changes in its properties are analyzed. X-ray diffraction, Brunauer–Emmett–Teller surface area, and hydration heat tests reveal that cementitious materials incorporating MRM800 contain more C-S-H and other hydration products, exhibit lower porosity, and demonstrate stronger hydration reactions. The results show that 800 °C is the optimal calcination temperature for CRM activation. For recycled silica-based mortar (RSM), the introduction of an efficiency coefficient (Kλ) allows for a quantitative, scientific, and intuitive evaluation of the contributions of three admixtures, aiding in the optimization of the mix ratio. RSM with MRM showed improved performance, with compressive strength ranging from 24.3 to 42.3 MPa. A 20% MRM addition effectively enhanced the mechanical properties of the mortar, while the mixture with 10% MRM and a 1:3 cement-to-sand ratio exhibited only 8.23% strength loss and 0.78% mass loss after 50 freeze–thaw cycles. MRM can improve the compactness of the cement matrix and thus optimize its freeze–thaw resistance, providing an eco-friendly technical solution for the engineering application of recycled mortar in cold regions.