Microsilica, also known as Silica Fume, is an ultrafine, amorphous (non-crystalline) polymorph of silicon dioxide. Collected as a by-product of silicon and ferrosilicon alloy production, its particles are roughly 100 times smaller than average cement grains.

In concrete applications, it performs two vital functions. First, as a micro-filler, it slips between cement grains, physically reducing the size and volume of microscopic voids. Second, as a highly reactive pozzolan, it reacts chemically with calcium hydroxide (a weak by-product of cement hydration) to form strong, durable calcium silicate hydrate (CSH) gel.

The result is a denser, vastly stronger matrix that is virtually impenetrable to water, chlorides, and aggressive chemicals, making it indispensable for marine structures, bridges, and high-rise buildings.