Microstructure of thermoactivated recycled cement pastes

Abstract

This paper focuses on the microstructural characterisation of pastes produced with thermoactivated recycled cement (RC). To this end, a comprehensive experimental program combining thermogravimetry (TG), X-ray diffraction (XRD), isothermal calorimetry, scanning electron microscopy (SEM), quantitative backscattering electron (BSE) imaging, mercury intrusion porosimetry (MIP), nitrogen adsorption and mechanical tests, was performed. The hydration and microstructure evolution of RC was monitored over time, between 8 h and 28 days, and compared against ordinary Portland cement (OPC) pastes of equal w/b or workability. RC showed effective rehydration with formation of C-S-H, AFm, AFt and carbonation phases since very early ages, developing high initial reactivity, with lower coarse porosity and higher volume of hydration products than OPC pastes of equal w/b up to 3 days. The RC paste is defined by two sub-structures composed by the outer hydrated phases of bulk paste and the inner RC hydration products, which affect its fresh and hardened behaviour.