MaterialsMaking better, cleaner cement

It is no surprise that humans the world over use more water, by volume, than any other material. In second place, at more than seventeen billion tons consumed each year, comes concrete made with Portland cement. Portland cement provides the essential binder for strong, versatile concrete; its basic materials are found in many places around the globe; and, at about $100 a ton, it is relatively cheap. Making it, however, releases massive amounts of carbon dioxide, accounting for more than five percent of the total CO2 emissions from human activity.

“Portland cement is the most important building material in the world,” says Paulo Monteiro, a professor of civil and environmental engineering at the University of California at Berkeley, “but if we are going to find ways to use it more efficiently — or just as important, search for practical alternatives — we need a full understanding of its structure on the nanoscale.” To this end Monteiro has teamed with researchers at the U.S. Department of Energy’s Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory.

A Berkeley Lab release reports that most recently, at ALS beamline 12.2.2, Monteiro and his colleagues gradually squeezed specks of fine dust of the mineral tobermorite between faces of two diamonds in a diamond anvil cell, until they achieved pressures like those 100 miles below the surface of Earth. This was the first experiment to determine tobermorite’s bulk modulus — its “stiffness” — from diffraction patterns obtained by sending a bright beam of X‑rays through the sample, revealing how its structure changed as the pressure increased.

The results, which will appear in  Cement and Concrete Research and are now available online to subscribers, led to new insights into calcium-silicate-hydrate (C‑S‑H), the material primarily responsible for the strength and durability of concrete made with Portland cement.

Cement on the nanoscale
Portland cement is made by baking limestone (calcium carbonate) and clay (silicates) in a kiln at over 1,400 degrees Celsius to make “clinker,” which is then ground to a powder. When the powder is mixed with water, calcium-silicate-hydrate (C-S-H) is formed, which, although poorly crystallized, is a binder critical to the strength and durability of the cement paste.

“We and many other groups have developed sophisticated computer models to understand the crystal structure and mechanical behavior of C‑S‑H, based on observations of how it performs,” says Monteiro. “But we’re the only group that uses minerals to validate the results of