Sulfate resistance of ultra-high performance concrete with recycled fine aggregate

The effect of sulfate attack (SA) on the strength of ultra-high performance concrete (UHPC) with recycled fine aggregate (RFA) is investigated. RFA is used to replace natural fine aggregate (NFA) at 0%, 50% and 100% by weight. UHPC samples are soaked in 10% sodium sulfate solution for 0–180 d (SA age). The sulfate ion concentration, flexural strength and compressive strength of UHPC are investigated. The microstructure of UHPC before and after SA is analysed through a multi-technique approach (ITZ, XRD, SEM-EDS and MIP). Results indicate that with the increase of SA age, the compressive strength of UHPC with RFA increases. The generated gypsum and ettringite refine the harmful pores (>20 nm), which is converted to harmless pores (<20 nm) in UHPC matrix. At the same SA age, the sulfate ion concentration of UHPC increases with the increase of RFA content due to more ITZ, pores and microcracks introduced by RFA. The strength development models of UHPC with different RFA content under SA are developed.