Effect of the Pretreatment on the Properties of Cement-Based Recycled Powder

Abstract

Three pretreatment methods including calcination, carbonization, and a carbonization-calcination combined pretreatment were studied to understand the pretreatment mechanisms for cement-based recycled powder (CRP). The mineral and microstructure of the CRP sample were investigated through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermal gravity (TG) analysis, and scanning electron microscopy (SEM) after exposure to different thermal temperatures (400 °C, 600 °C, and 800 °C), carbonization times (6 h, 1 d, and 3 d), and pre-carbonization for 1 d followed by heating at 800 °C. The results showed that the optimal thermal pretreatment temperature was approximately 720–800 °C. Through the process of calcination, the C-S-H, Ca(OH)2, and CaCO3 minerals in the CRP sample underwent decomposition to produce CaO or C2S. During carbonation, the pretreatment not only results in the increased production of CaCO3 owing to the reaction of the C-S-H gel and Ca(OH)2 with CO2, but also enhances its properties and the strength of chemical bond between CaCO3 and the post-hydration products. Both CaCO3 and CaO were present after the combined pretreatment, which indicates that the CaCO3 mineral formed superior stability after it had been pre-carbonated. Due to fewer impurities in CRP, the positive effect of the pretreatment on CRP was significantly better than that on recycled powder derived from construction and demolition waste.