Effects of Silica Fume Purity on Behavior of Ultra-High Performance Concrete

Abstract

The use of silica fume in high-strength and ultra-high performance concretes has been studied from many perspectives including optimum replacement rate, implications on late-age strengths, and rheological properties. However, the effects of silica fume purity (e.g., silicon dioxide [SiO2] content) are not nearly as well documented, especially when shorter duration curing regimes at higher temperatures are utilized. In this paper, four silica fumes with varying SiO2 contents were used to quantify the effects of SiO2 purity in silica fume on the behavior of ultra-high performance concrete. Several variables were evaluated to determine if changes in SiO2 content significantly influenced properties of interest. Overall, silica fume purity had the most impact on thermal set time, maximum temperature generated during hydration, and compressive strength. Thermal set time values differed by over 7 h, and maximum temperatures differed by 7°C. After one day of room-temperature curing, compressive strength varied by 50 MPa, and elastic modulus varied by 14 GPa. When specimens were cured at elevated temperatures for short durations of time (e.g., one to six days), fc values ranged from 35 to 47 MPa.