Properties of Concrete Incorporating Ultrafine Fly Ash and Silica Fume

A laboratory study on the influence of the combination of ultrafine fly ash (UFFA) and silica fume (SF) on the properties of fresh and hardened concrete is described. Also compared are the performance of concrete incorporating UFFA and SF (ternary blend of cement), concrete incorporating UFFA or SF (binary blend of cement), and control portland cement concrete. The test results show that the incorporation of SF or UFFA in concrete resulted in higher strength and improved durability (resistance to chloride penetration). These benefits were found to be more pronounced in the SF concrete. However, the SF concrete demonstrated several limitations such as low slump and high early-age shrinkage. These limitations were not observed in the UFFA concrete; addition of UFFA increased the slump and decreased the early-age shrinkage. To minimize the shortcomings of SF without losing its strength and durability benefits, a concrete mixture incorporating both SF and UFFA was prepared. The test results show that the incorporation of both SF and UFFA produces a concrete mixture that demonstrates high early-age strength and improved durability similar to those properties in SF concrete. In addition, unlike SF concrete, the new concrete mixture demonstrates a higher level of slump and a lower level of free shrinkage.