Mechanical Properties of Ultra-High-Performance Concrete Containing Natural Pozzolan and Metakaolin

Ultra-high-performance concrete (UHPC) is an emerging construction material with exceptional mechanical and durability properties. Silica fume has been used in UHPC as the main supplementary cementitious material (SCM) to improve mechanical and durability properties. However, silica fume is more expensive than cement and other SCMs, so it is often replaced with class F fly ash. Unfortunately, future availability of fly ash is uncertain because the energy industry has been removing coal burning generating stations from operation creating a need to find new, cost-effective, and environmentally friendly alternatives to fly ash. This study assessed use of metakaolin and a natural pozzolan (pumicite) as potential replacements for fly ash and silica fume in UHPC mixtures. Results from workability, compressive strength, and flexural strength tests indicated that replacing up to 75% of the fly ash in a control mixture with pumicite as well as replacing 100% of the fly ash and 25% of the silica fume in the control mixture with metakaolin led to acceptable 28-day compressive strengths [greater than 17,000 psi (120 MPa)]. Compressive and flexural strengths of UHPC mixtures decreased with increasing pumicite and metakaolin content, although strengths remained above 17,000 psi (120 MPa).