Replacing fly ash with a natural pozzolan in normal strength concrete and ultra-high performance concrete

Abstract

Fly ash is a widely-used supplementary cementitious material (SCM) in concrete. Common reasons for using fly ash in normal strength concrete (NSC) are to reduce cement consumption and mitigate alkali-silica reaction. Additionally, fly ash is being used in ultra-high performance concrete (UHPC) to reduce use of silica fume which is more expensive than fly ash. Fly ash and silica fume are important SCMs in UHPC that provide a dense microstructure, improving UHPC strength and durability. However, fly ash is becoming less available because the energy industry has been investing in renewable energy production and removing coal burning power plants from operation. Therefore, there is a need to identify sustainable SCMs to replace fly ash in NSC and UHPC mixtures. Natural pozzolans are a diverse class of siliceous or siliceous and aluminous materials that can form compounds possessing cementitious properties in the presence of water. Since natural pozzolans are easily mined, they are usually considered to be more sustainable than Portland cement and nearly as sustainable as other SCMs such as fly ash and silica fume. This experimental study evaluated the effects of a natural pozzolan (pumicite) as a potential replacement for fly ash that can produce comparable rheological and mechanical properties in NSC and UHPC mixtures. Results indicated that workability of both NSC and UHPC mixtures containing pumicite was consistent enough to achieve the targeted workability in just a few trials. Additionally, NSC mixtures with 30% pumicite had comparable compressive strength compared to NSC mixtures with 30% fly ash. Modulus of rupture (MOR) for NSC mixtures containing 30% pumicite (greater than 4.10 MPa) exceeded the MOR for specimens containing 30% fly ash. For UHPC mixtures, results showed that up to 75% of the fly ash in the control mixture could be replaced with pumicite while still producing acceptable compressive and flexural strengths. Based on these results, it appears that pumicite can be a reliable alternative for fly ash in NSC and UHPC mixtures in terms of rheological and mechanical properties.