Effects of autoclave curing and fly ash on mechanical properties of ultra-high performance concrete

This study investigated compressive strength, flexural strength and fracture toughness of ultra-high performance concretes (UHPC) containing silica fume and different dosage of fly ash (0%, 10%, 20% and 30%) after exposure to different autoclave curing conditions with pressure of 0.5 MPa, 1.0 MPa and 1.5 MPa and duration time of 6 h, 8 h, 10 h and 12 h. The microstructure of UHPC samples were also measured by using MIP, XRD and SEM. The results show that the mechanical strength of UHPC specimen presents an increasing at first stage and a decreasing or stable trend later with the higher pressure or the prolonged duration time of autoclaving. The increasing autoclave pressure is needed for UHPC containing more fly ash to obtain the best strength. Incorporation of steel fibers could improve flexural strength while has little impact on compressive strength and the reinforcement effect of steel fibers gets more significant as the increasing autoclave pressure and duration. Autoclave curing degrades the fraction toughness for pure cement UHPC and the incorporation of FA can counteract or even eliminate this detrimental effect. The incorporation of fly ash and the increasing autoclave duration reduce the porosity of UHPC samples. The autoclave curing leads to the formation of xonotlite (Ca6[Si6O17](OH)2) and decreases the content of portlandite in UHPC samples.