ECO-UHPC with High-Volume Class-F Fly Ash: New Insight into Mechanical and Durability Properties

Abstract

Ultra-high-performance concrete (UHPC), despite its superior mechanical and durability properties, has a high CO2 footprint owing to its high portland cement content. This drawback can be offset to a notable extent if a high volume of supplementary cementitious materials can be utilized to produce UHPC while maintaining mechanical and durability properties that are comparable to those of conventional UHPC. In this study the effects of high-volume cement replacement by Class-F fly ash (up to 70% by mass) on the mechanical, durability, and microstructure properties of UHPC are investigated, with the aim of encouraging moderate- to low-volume cement use in UHPC (from 600 down to 300 kg/m3). Environmentally friendly-UHPC (ECO-UHPC) mixes, which have CO2 footprint intensities of less than 5 kg/m3/MPa, have been synthesized with different replacements of cement by Class-F fly ash. Results suggest that concretes with ultrahigh strength (>150 MPa) and a strength greater than 100 MPa can be prepared with up to 40% and 70% replacements, respectively, without employing any special curing or fibers. A model has been developed to predict the compressive strength of fly ash–based UHPC from the contents and chemical compositions of its binders. UHPCs made with up to 60% cement replacement by Class-F fly ash exhibit durability properties comparable to that of UHPC without fly ash, in terms of water absorption, initial rate of water absorption, and corrosion risk. The depth of carbonation remains below the detection limit of 0.5 mm, up to 70% replacement.