SciTech

The process of concrete: cement, sand, water mix together

Those of you with too much time on your hands may have found yourself wondering from time to time: how does concrete work? We walk on it almost every day. It’s used for roads, walls, columns, and housing foundations, to name a few applications. But what is it made of?

A fellow by the name of Joseph Aspdin, an English cement manufacturer, patented the method for producing Portland cement, a major ingredient of concrete, in the year 1824.
But the history of concrete isn’t as concrete as you might imagine, since its origins date as far back as ancient Egypt and Rome and materials similar to concrete can be found in the pyramids and the Pantheon.

Aspdin’s method has changed little since the 1820s and continues to be used regularly throughout the world. Concrete is, both metaphorically and literally, a key ingredient in the foundation of modern day building infrastructure, valued for its high strength and ability to be formed into almost any shape. In fact, over two billion tons of cement are produced each year!

Concrete is generated via a chemical hydration reaction between cement and water. It is important to note here that cement and concrete are not the same thing. Cement is composed of limestone and clay, which are heated together at 1450 degrees Celsius in order to induce a reaction generating calcium silicates:

CaCo3 + SiO2 → CaSiO3 + CO2

In this newly-reacted powder form, cement readily undergoes the previously mentioned hydration reaction that enables the creation of concrete. Concrete is a mixture of cement, water, sand, and small rock particles.

It works as follows:

Cement grains are dissolved in a mixture of water sand, and small rock particles, releasing calcium and silicon ions in the process.

Once the ions spread throughout the mixture and reach a critical concentration in solution, they begin to precipitate out of solution and form a cement film around the small rock particles. A general ratio used by many companies is one part cement to three parts sand to three parts small rock particles.

This cycle of dissolution, diffusion, and precipitation continues for about an hour, causing the film to grow and the concrete to strengthen until the precipitated grains begin to impinge on one another and the entire mixture solidifies.

It is at this point that the concrete begins to “set” and lose its fluid properties. Hydration of concrete is visually complete in only a few hours; however, on the microscopic level, hydration can continue to occur for months or even years.

As you can imagine, concrete is an extremely complicated material with a fairly disorganized atomic structure. Interestingly, although concrete is used nearly twice as much in construction than any other building material, the crystalline structure of concrete was only just revealed by scientists at the Massachusetts Institute of Technology in 2009.

As we begin to understand concrete’s molecular make-up, we can make inroads into making concrete crack-resistant and more environmentally friendly. With these benefits, concrete will continue to be used to build roads, houses, and foundations. The future of concrete is bright!