A comprehensive investigation on sulphate resistance of geopolymer recycled concrete: Macro and micro properties

Abstract

As a potential alternative to ordinary Portland cement concrete (OPC), geopolymer recycled concrete (GRC) is provided with great research value as it not only significantly reduces carbon dioxide (CO2) emissions but also promotes the resourceful use of construction and demolition waste (CDW). However, concrete is frequently subjected to extreme circumstances such as sulphate attack during service. To comprehensively explore the structural and performance damage mechanisms of GRC caused by sulphate assault, this study is conducted to erode GRC specimens with recycled aggregate (RA) replacement rate of 0%, 30%, 50%, 70% and 100% by using magnesium sulphate solution with 10% mass fraction. The macroscopic properties of GRC subjected to sulphate attack are examined, including mass loss rate, relative modulus of elasticity and compressive strength. The microstructure and phase evolution of GRC subjected to sulphate attack is analyzed by SEM, EDS and XRD. The micro-mechanical properties of interfacial transition zones (ITZs) in GRC are measured by nanoindentation. The results show that there is no loss of mass in all GRC specimens throughout the erosion age, and the relative dynamic elastic modulus of GRC first increases and then decreases. GRC with 30% RA substitution rate has the best resistance to sulphate attack. The degradation of GRC by sulphate attack is closely related to its internal structural changes. The deterioration of the micromechanical properties of three ITZs by sulphate erosion is not significant, which indicates the RA has favorable performances with geopolymer binder contributing to excellent resistance of GRC to sulfate attack. In addition, a prediction model for the evolution of GRC resistance to sulfate erosion with erosion age for different RA substitution rates is developed, which provides a theoretical guidance for the application of GRC.