SILICA FUME Among the various types of cementitious ...

SILICA FUME
Among the various types of cementitious admixtures, the finest of all is silica fume. Silica fume has other names which include: microsilica, ferrosilicon dust, arc furnace silica, silica flue dust, amorphous silica, and volatized silica. Silica fume is defined as: “…a waste product recovered from the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys…” (LEED Credit Guidelines - Precast Sustainability). When used as a mineral admixture in concrete, silica fume is most important and advantageous. It is similar to fly ash and natural pozzolans in its high reactivity content from the fume’s chemical and physical properties. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual constituents of silica fume are about 1/100th the size of a cement particle. The comparison of the cementitious admixture sizes with a typical cement particle can be seen in Figure 1. The extremely fine quality of the silica fume helps to fill the voids between the cement particles, which in turn decreases permeability. The high reactivity of the fume when used in cement mixtures comes from the large overall surface area of the product, the fineness of the particles, and the high silicon dioxide content. If silica fume has low water content, this increases the resistance of penetration of corrosive chloride ions. The standards ASTM C 1240 and AASHTO M 307, both entail the quality and specification of silica fume used in cement mixtures. AVR, Inc. states that the contributions of silica fume to concrete besides lower permeability and corrosion resistance is: increased concrete strength (12-15,000 psi), increased modulus of elasticity, and improved durability. Silica fume contributes to the early strength of concrete in relation to set time. Three to five times the compressive strength is yielded from the use of silica fume in concrete as compared to portland cement when similar amounts of portland cement and silica fume are used.

Figure 1: The four types of cementitious admixtures (Meininger)
The USGBC created the Leadership in Energy and Environmental Design (LEED) certification for construction and engineering so that buildings are to use recycled content in their building materials in order to have a more environmentally stable construction method and outcome. Precast concrete is mixture that uses the recycled cementitious aggregates (such as fly ash, slag, natural pozzolans, and silica fume) to divert construction waste from landfills. Since the manufacturing of cement has such a large environmental impact, the original cement was substituted with pozzolanic materials such as fly ash or silica fume to reduce the overall consequences of material production. The final reduction is cement use from original concrete can be up to 60%, but this can vary through mixture design requirements. The fume used ...