A disadvantage to utilizing silica fume in cement is the cost. Condensed silica fume costs $400-$1,000/ton, significantly higher than Portland cement ($90/ton). Some materials, such as silica dust, will pose no threat of explosion, while metal dust from magnesium or aluminum, for example, is very explosive. In general, about 70% of dust is explosive. In addition, non-explosive materials (sand or Silica, for example) could become explosive when mixed with other explosive materials (such as organic or metal dust) in sufficient concentration. Materials such as Silica, limestone, sand, cement, fly ash, etc., are inert in their pure form (i.e., these materials will neither burn nor support combustion nor pose a risk of fire or dust explosion). The zeta potential showed that the isoelectric points of nano-fumed Silica and modified nano-fumed Silica are pH=4. Lime softening is the most common and effective technique for removing granular Silica from boiler water. This process involves adding lime, also known as calcium hydroxide, to the water. Lime acts like a water softener for Silica, causing calcium and magnesium — the particles that cause hardness — to precipitate out.
Shelf Life: MasterLife SF 100 silica fume admixture has a minimum shelf life of 24 months. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual particles are extremely small, approximately 1/100th the size of an average cement particle. At about 870 °C, the coating acquires a smooth surface, and heating above 1170 °C destroys it by twenty percent. It is insoluble in water. The solubility of Silica in water is influenced by the presence of acid or alkali in the water. The predominant form of dissolved Silica is mono silicic acid, Si(OH)4. The polymorph quartz usually controls silica solubility at reservoir temperatures roughly above about 185°C; solubility at temperatures less than about 185°C may be controlled by the polymorph chalcedony, which is more soluble than quartz.