Watching Cement Dry
When he first heard a colleague talking about cement research, Ronald Berliner, a nuclear physicist at the University of Missouri-Columbia, says, “I realized with horror that I had no idea what cement was.” Four years later, Berliner knows a lot about cement; in fact, he’s pushing out the frontiers of what we know about this ubiquitous but little-understood substance, which is crucial in making concrete-the world’s most common industrial material.
Cement is a complex mixture; Portland cement (the type commonly used for buildings) is made mainly of silicates and aluminates, with a sprinkling of sodium, potassium, sulfur, iron, and magnesia. Scientists have long known that the ingredients combine with water to form hydrates, which bind sand and stone into concrete. Yet a rigorous chemical understanding of this process has proved elusive, because most analytical methods don’t work on wet cement.
Particularly mysterious is the 6- to 8-hour “dormant” phase between the initial short period of rapid reaction, when water is first added to concrete mix, and a later period of rapid hardening. Berliner is using quasielastic neutron scattering to zero in on a key aspect of the dormant phase: the reaction of tricalcium silicate (the predominant compound in the cement mixture) with water. His work reveals that when water is added to cement, a crust forms around each cement grain. After that, it takes longer and longer for water molecules to diffuse through to the dry cement inside. That slow diffusion process accounts for the mysterious dormant phase.
Berliner is hoping that knowing precisely how the tricalcium silicate and water molecules combine will lead to ways to optimize the reaction, yielding stronger, more durable concrete. And even small improvements could yield tremendous savings, because so much concrete-350 million metric tons-is made every year around the world.
What’s more, improving the strength of concrete could reduce the volume required in structures, which would be good news for the environment. Cement manufacturing is a major culprit in global warming; an estimated 10 percent of the greenhouse gases generated by human activity are formed in cement production, which also uses an estimated 3 percent of the U.S. electrical generation capacity.
Berliner is now using neutron scattering techniques to attack the “concrete plague” that is causing the rapid deterioration of many roads and bridges. Finding a cure would make watching cement dry a lot more interesting than it might initially seem.