What Are the Benefits of Silica in Cement?

Silica

• Silica is crystalline silicon dioxide. For each atom of silicon it contains, it has two atoms of oxygen. It's highly abundant because quartz and sand are made from silica. Heating a mixture of sand, crushed limestone and clay in a kiln produces hard pellets called clinkers. The high heat induces formation of calcium oxide from the crushed limestone, which is originally calcium carbonate. The calcium oxide in turn reacts with the silica to make tricalcium silicates and dicalcium silicates, which have the chemical formulas Ca2SiO4 and Ca3SiO5.

Alumina

• Clay is made primarily of aluminosilicates, which contribute alumina to the reaction in the kiln. This ingredient reduces the temperature needed to form high quantities of tricalcium silicate from the silica and calcium carbonate. Both tricalcium and dicalcium silicate are vital to make high-quality clinkers. Once they are cool, the clinkers are ground into a fine powder with calcium sulfate and sold in bags to the end user. Adding water causes another series of chemical reactions that are still incompletely understood. Ultimately, these reactions form a calcium silicate hydrate gel, a sort of mineral glue with calcium hydroxide.

Reactions

• Silica is essential to the chemistry of cement at each step of its formation. Without silica, tricalcium silicates and dicalcium silicates wouldn't form in the clinkers; without those compounds, the process wouldn't produce the gluelike network that forms during hydration. Together with alumina from clay and calcium carbonate from limestone, silica is one of the three essential ingredients to make clinkers, although manufacturers often include other compounds to impart various desirable properties.

Strength

• The most remarkable feature of cement is its strength as a binding agent. Combining Portland cement with sand and gravel produces concrete, a tough mixture important for construction. Cement owes its strength and rigidity to calcium silicates, which like silica are hard and durable minerals; just think about how tough it is to grind sand. Inside the cement, the calcium silicates form a 3-D network that provides cement and concrete with their strength.