|WHAT'S THAT STUFF?
Volume 76, Number 35
CENEAR 76 35 1-72
The Joys of Cement
Who has not experienced the thrill of traveling along a road, turning around a bend, and suddenly seeing a spectacular view of a city, coastline, or mountain range?
And how many chemists have not experienced the thrill of a successful experiment in a laboratory or an unexpected revelation or sudden enlightenment when turning the page of a chemistry book, journal, or magazine?
Chemistry never ceases to amaze me. Just when I think I have sampled most of the delights the subject can offer, I realize yet again that there are vast uncharted territories out there waiting to be explored.
It happened in January when I visited Queen's University of Belfast in Northern Ireland to research a story on room-temperature ionic liquids (C&EN, March 30, page 32). Before I embarked on this story, I had not once heard of or read about these materials--and surely, I thought, I would have done so if they were important. After all, my career as a chemist in industry, academia, and publishing stretches back to the 1960s, and I like to think I'm at least aware of the broad fronts of current research. How could I have missed this large area of chemistry that has so much potential?
And it happened again in July. The subject on this occasion was much more mundane, and I have been aware of it for most of my life. The subject was cement. Fascinating? Well, yes.
This time, as I was turning the pages of the latest issue of Chemical Communications, I came across a paper titled "Molecular Design and Testing of Organophosphonates for Inhibition of Crystallization of Ettringite and Cement Hydration" by Peter V. Coveney, senior research scientist at Schlumberger Cambridge Research in Cambridge, England, and his coworkers at the University of Manchester Institute of Science & Technology [Chem. Commun., 1998, 1467].
The title is unlikely to inspire even the most ardent chemist. Nevertheless, I started to read the paper. This is the first sentence: "Cementitious materials are among the most widely used by mankind while being among the least well understood." This is the next: "The detailed physicochemical processes involved in the hydration and setting of cement slurries are very complex, and a clearly defined quantitative account is still lacking; indeed, even the precise composition of the cement powder is unknown." And the next: "Although numerous additives are known and used to retard the cement setting process, little is understood of the mechanism by which they act."
So here is a material that is used in virtually every village, town, and city in the world, and I now read that little is known about its chemistry. This was good enough bait for me to make further inquiries.
First of all, I checked several general chemistry textbooks that I have in myoffice. Two do not list cement in the index. The others devote two or three paragraphs to Portland cement. They define this as a complex mixture of calcium silicates and aluminates that is made by heating a mixture of clay and limestone to about 1,500 oC in a kiln. The mixture is then cooled, pulverized, and gypsum (CaSO42H2O) is added. When the powder is mixed with water, complex reactions take place and the cement sets to a solid mass.
I next contacted Coveney in Cambridge to inquire about his research. "We have, for the first time ever, put cement chemistry on a rational basis," he explained. "Until now, it has been simply a jungle of disconnected facts and fairy tales."
His reply fueled my curiosity. I got back to Coveney again, requesting further information about the chemistry of cement. "Chemical compositions vary from cement to cement according to the kiln conditions and the detailed composition of the local ingredients with which the kiln is fed," he replied.
The physical state of the powder, such as the size distribution of all the particles, also affects reactivity. The main reaction in setting is hydration by water, and this is largely surface controlled, Coveney explained.
The properties of cement can vary "in an alarming manner" from manufacturer to manufacturer and even from batch to batch from any single producer, he pointed out. He added that even a small variation in the chemical composition or physical state of cement can cause substantial variations in performance, usually in ways that cannot be foreseen at all.
"You can find endless facts about cementitious materials, but not one of them brings one any nearer an ability to control the process of hydration and set," he said.
I asked Coveney about the fairy tales. "These abound," he said. "They are passed from hand to mouth by old cement hands."
He gave two examples. Some old hands say that the role of magnesium oxide is critical in the setting of cement. And others say that the setting time of cement can be controlled by stirring the cement in a certain way.
"In reality, cements, like the human body, are highly multivariate nonlinear systems whose properties are greater than the sum of the parts," according to Coveney.
This was all news to me. I had never realized that so little is known and so much is to be learned about the chemistry of cement. And I haven't even mentioned the chemistry of concrete--a cement-sand-rock composite.
What were the outcomes of my pleasurable trek through this unexplored territory? First, I wrote a science/technology concentrate titled "Novel macrocyclic organophosphonates retard cement setting" (C&EN, July 20, page 47). Second, my knowledge of chemistry has been broadened yet again in an unexpected way. And finally, the trek confirmed my view that we are still just scratching the surface in some very important areas of the world of chemistry.
Chemical & Engineering News