Densified silica fume: particle sizes and dispersion in concrete

Abstract

Dry densified silica fume is by far the most common form of silica fume used in current concrete practice; the alternative, slurried silica fume has become unavailable in many places. Densified silica fume as commonly supplied consists of particles of sizes up to several millimeters, which are generally not dispersable into individual silica fume spheres. Densified silica fumes from some sources can be dispersed by moderate ultrasonic treatment into small clusters or chains of spheres; others resist such treatment and mostly remain as large agglomerates. Under conventional concrete mixing, substantial contents of agglomerates almost always remain in the concrete. Thus the assumption that the densification process is somehow ‘reversible’ is not generally warranted. The sizes of undispersed agglomerates remaining in concrete after mixing often exceed the sizes of Portland cement particles, thus limiting any potential benefits attributed to the fine particle filler effect. Large undispersed grains appear to always undergo chemical reaction in concrete, but such reactions may induce ASR damage only under especially unfavorable circumstances.