ARCHITECTURAL AND ENGINEERING FEATS BEGIN WITH A BALANCED CEMENT EQUATION
The chemistry of cement, water, aggregate and additives as they combine to form concrete, has more happening beneath the surface than you may first think, as science activates to deliver building material properties of stability, consistency, flexibility, sustainability and workability.
As a building material used in Ancient Roman times, concrete or ‘liquid rock’ as it is sometimes called, has been transformed into some of the world’s most remarkable mega structures. The Pantheon, built in 125AD, is arguably the first building to exhibit the innovative versatility and strength of roman concrete – seen in its unreinforced concrete 142-foot dome.
Other landmark examples of engineering feats that were conquered by using concrete are the Panama Canal and Hoover Dam (for which more than five million barrels of cement were used in its construction). Today the phenomenal 163 floor Burj Khalifa (Dubai) definitively showcases the fortitude of the choice material of reinforced concrete.
Then there is the revolutionary Jubilee Church building in Rome that shows new potential for concrete as a self-cleaning, futuristic building material through the pioneering of what National Geographic and BBC refer to as “a new form of concrete[1]”. This landmark project is said to be “the crown jewel of the Vicariato di Roma’s Millennium project” and as such, the brief required the creators to deliver a material that was “both ancient and modern”.
According to the National Geographic documentary, what is progressive about the Jubilee Church is that the architectural team included “photocatalytic particles in the cement that absorb and neutralise the acids in the air”. This new approach effectively contributes to making the building an air pollution neutralising structure.
While the results of these buildings are clear to see on the surface, the science of concrete was silently at work during the construction phase to shape such remarkable structures. The basic equation of concrete isn’t hard to get your head around:
(cement + water) + aggregate = concrete
In essence, significant heat is produced when the above four elements are mixed. This is because of the exothermic process in the reaction between the cement and water, and is called hydration.
The more complex scientific equations come into effect within the concrete mix process, which Materials Science and Technology Teacher’s (MAST) Workshop hosted by the Department of Materials Science and Engineering at the University of Illinois explain comprehensively.[2]
The following excerpt from MAST provides sight of the complexity of the science underway in concretisation:
“When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. …
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate + Water—>Calcium silicate hydrate + Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O —> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ
Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH–) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.
The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier’s principle). The evolution of heat is then dramatically increased.
The formation of the calcium hydroxide and calcium silicate hydrate crystals provide “seeds” upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. …
The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage. The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. … It is at the end of this stage that initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours.”
Ultimately, the path to harnessing concrete, starts with getting the science of the cement equation to balance.
At Sephaku Cement we have a finely tuned scientific approach applied to the production of our cement products. We aim to consider the needs of the people who make the end product a reality by applying a hi-tech, progressive and passionate approach to the business of cement production.
We take our role seriously in the building value chain of kusasa (tomorrow) and test our cement rigorously in our concrete testing laboratory (see below).
Sephaku believes that just as the established role of cement in structural development has been concretised by history, the future of our planetary responsibility for using cement in new ways gives us the possibilities of new horizons. There is much that we can create and new thinking, rooted in consistent scientific application, will lead us on.
[1] Source: https://www.youtube.com/watch?v=NgCl1yf5oN8 [2] Source: http://matse1.matse.illinois.edu/concrete/prin.html
