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.
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