Internal corrosion characteristics of endogenous sulfate introduced by recycled aggregates in recycled aggregates concrete: Insights into the macro-mechanical and meso-mechanical properties

Abstract

The old cement paste adhering to recycled aggregates (RA) carries some sulfate corrosion media because of the external sulfate attack occurred in parent concrete. These sulfate corrosion media would be introduced into recycled aggregates concrete (RAC) that is prepared using RA. For studying the effects of the internal sulfate corrosion media on RAC, different Na2SO4 solutions (0%, 1%, 5% and 8%) were used to soak RA for 120 days. The effects of the soaked RA on macro-mechanical properties and meso-mechanical properties of RAC were explored. It was found that the endogenous sulfate ions diffused from RA into the new cement paste to react with cement hydration products (such as Ca(OH)2) to generate corrosion products, ettringite and gypsum. The moderate corrosion products could fill pores and cracks at interfacial transition zone (ITZ) to enhance the early compressive strength and flexural strength of RAC. However, as the erosion level of RA and curing time of concrete increased, much corrosion products caused the obvious deterioration in strength of RAC and microhardness of ITZ, as well as the increase of thickness of ITZ. For easing the comprehensive evaluation of microhardness and thickness of ITZ, a new parameter (Δζ) was proposed. The smaller the Δζ, the higher the quality of ITZ.