Recycled concrete fine powder (RFP) as cement partial replacement: Influences on the physical properties, hydration characteristics, and microstructure of blended cement

Abstract

Construction and demolition (C&D) waste has caused various economic, environmental, and social problems. This paper investigates the potential of recycled concrete fine powder (RFP) as a supplementary cementitious material. The effect of RFP with different replacement levels on the properties of cement and the mechanism of action were explored from the physical characteristics, mechanical properties, hydration kinetics, hydration products, and pore structure. In this study, scanning electron microscopy, Energy Dispersive X-Ray Spectroscopy, X-ray diffraction, X-ray fluorescence, Thermogravimetric analysis, and Isothermal calorimeter were applied. The results showed that RFP (less than 30%) shortened the setting time and improved the fluidity of cement. The early compressive strength increased by 17.62% of the cement mortar with 10% RFP. When the RFP content was 10%, the flowability of the blended slurry (175.5 mm) increased by 8.0% compared to the pure cement (162.5 mm). With the RFP content increase of the blended cement, the hydration induction period was prolonged, and the peak hydration exothermic rate and the accumulated heat release showed an apparent decreasing trend. The reaction rate of the nucleation and crystal growth (NG) process was much higher than that of the interface (I) and diffusion (D) processes. The low field nuclear magnetic resonance (NMR) technique revealed that RFP filled the large and medium pores of the mortar. The non-destructive layered scanning found that the pores were distributed mainly in the central zone and had a gradient distribution law of gradually decreasing from the middle area to the edge area. Visual analysis of the dynamic distribution of water molecules in the pores and the non-homogeneous characterization also demonstrated the good filling effect of RFP on the mortar. Therefore, RFP can be used as an economical and environmentally friendly cement mixing material to replace partial cement and contribute to reducing carbon emissions in the construction industry.