How it Works: Cement!

By Justine Nicole Dator

Cement is a major part of the industrial world. It is the binder of concrete, and concrete makes up the structure of many buildings. Without concrete, many infrastructure would not be around today, or would be made using other materials like wood, steel, and the like.

Exactly how does cement work? In this article, we’ll focus on one of the most common types of cement – Portland cement. Cement is made first by quarrying and crushing different materials and combining them in exact proportions. These materials include calcium oxide (CaO), silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3). The resulting combination is put in a cylindrical kiln which heats up the mixture. The kiln is on an incline, and the raw materials enter through the higher end, and slowly move the length of the kiln as it rotates. At the end of the kiln, fuel is used to heat up the materials, causing them to react.

As it moves down the cylinder, the mixture changes. First any excess water is lost by evaporation. Due to the loss of water and carbon dioxide, decomposition happens. The process is called calcination. Calcination is subjecting a substance to heat without the intent of fusing the mixture together, but with the intent of causing a chemical change, such as decomposition of compounds. Next is clinkering, where calcium silicates form. The final stage is the cooling stage.

The resulting compound is called a clinker. The clinker contains certain compounds which affect the overall properties of cement. Mixing these compounds in different proportions can make different kinds of cement.

COMPOUND PROPERTY IT AFFECTS
Tricalcium aluminate Frees plenty of heat during hydration;
Tricalcium silicate Hardens quickly; responsible for early cement strength gain
Dicalcium silicate Hardens slowly; responsible for later strength gain of cement
Ferrite Reduces melting temperature of raw materials; does not contribute to strength

 

Once water is mixed, the cement forms a paste with the aggregates available to make the concrete. The water caused the hardening of the concrete through hydration. In hydration, the compounds in cement form chemical bonds with the molecules of water to become hydrates. The water is very important because the ratio of water to cement is what determines how strong the concrete is.

The expound more on how cement works when mixed with water. We’ll discuss the interaction of water with tricalcium silicate. Once pure water is added (the water must be pure in order to avoid any unnecessary reactions from occurring), the tricalcium silicate reacts to release calcium ions, hydroxide ions, and heat. Once the system becomes saturated, the calcium hydroxide begins to crystalized, while calcium silicate hydrates form. From this, more calcium silicate hydrate forms, and the crystals grow thicker, making it harder for water to reach the tricalcium silicate. Finally, the calcium silicate hydrate hardens and the cement paste is now solid.

The other components of the cement interact in the same way with water. Their reactions may be a bit more complicated since they involve the compound gypsum. Nevertheless, hydration is a very important process that is undergone by cement. With all the complicated terminology over, I hope that you have properly understood how cement binds with aggregate to create concrete – which is a vital material in the modern-day world.

 

References:

[1] http://matse1.matse.illinois.edu/concrete/prin.html

[2] http://www.engr.psu.edu/ce/courses/ce584/concrete/library/construction/curing/Composition%20of%20cement.htm

[3] http://www.lafargeholcim.com/cement-solutions (Picture)

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