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Ultra-high performance concrete (UHPC) is an emerging material with remarkable mechanical and durability properties that contains large amounts of cementitious materials. Silica fume is a main supplementary cementitious material (SCM) in UHPC, however, it is more expensive than cement and other SCMs, so it is often substituted with inexpensive class F fly ash. Unfortunately, future availability of fly ash is uncertain as the energy industry moves toward renewable energy. Fly ash shortages create an urgent need to find cost-effective and environmentally-friendly alternatives for fly ash. This study investigated replacing cement, fly ash, and silica fume in UHPC mixtures with ground granulated blast-furnace slag (GGBFS), metakaolin, and a natural pozzolan (pumicite). To identify acceptable UHPC mixtures (28-day compressive strength greater than 17,000 psi [120 MPa]), workability, compression, and flexural tests were conducted on all mixtures. Then, durability properties including shrinkage, frost resistance, and chloride ion permeability (rapid chloride permeability and surface resistivity tests) were evaluated for the acceptable UHPC mixtures. Results showed that 75, 100, and 40% of the fly ash in the control mixture could be replaced with pumicite, metakaolin, and GGBFS, respectively, while still producing acceptable UHPC mixtures. Flexural strengths were greater than 2000 psi (13.80 MPa) for all mixtures, which should be considered acceptable for many UHPC applications. For durability, the UHPC mixtures had shrinkage strains no greater than 406 mstrain, durability factors of at least 105, and “very low” susceptibility to chloride ion penetration. These results indicate that the durability for all of the UHPC mixtures appears to be excellent. In general, the results indicate that pumicite, metakaolin, and GGBFS are all suitable candidates to replace most or all fly ash and potentially replace some silica fume in UHPC.


Tran-SET Project: 21CNMSU60