Cement Americas provides comprehensive coverage of the North and South American cement markets from raw material extraction to delivery and tranportation to end user.
Issue link: https://cement.epubxp.com/i/119488
CEMENTSCOPE Cement Particle Size Diversity Optimizes Packing Density, Concrete Strength By Denise Brehm Concrete may be one of the most familiar building materials on Earth, but its underlying structure remains a bit of a mystery. Materials scientists and concrete engineers still don't fully understand exactly how the cement paste that works as glue in concrete hardens during the first hours after water and powder are mixed. PHOTO: Enrico Masoero New technologies are making it possible for researchers at the Massachusetts Institute of Technology Concrete Sustainability Hub to make steady progress toward solving this mystery. First they determined that cement paste is a granular material, where the particles or basic nanoscale units pack together most densely when arranged orderly. They discovered that the calcium-silicate- Size diversity allows cement nanoparticles to pack together tightly, increasing the strength of the cement or concrete matrix. 4 hydrate (C-S-H) molecules comprising the basic nanoscale unit of cement have a disorderly geometric arrangement, rather than the orderly crystalline structure scientists had long assumed. PARTICLE SIZE IMPLICATIONS In new work, Hub researchers find that the size of C-S-H particles themselves is also somewhat disorderly: They form at very diverse sizes and this diversity in the size of the nanoscale units leads to a denser, disorderly particle packing, which corresponds to stronger cement paste. Researchers hope this understanding will allow materials scientists and concrete engineers to alter the C-S-H particles at the molecular level to develop stronger, more durable concrete that will have a reduced environmental footprint. If concrete is stronger, less of it is needed. And if it's more durable, structures made from it will last longer. "While previously our models showed that the particles – think of them as homogenous sized oranges – pack together most densely when arranged orderly in a grocer's pyramid, our new work shows that when C-S-H units form in a variety of sizes, they can pack more densely when in disarray," said Concrete Sustainability Hub lead researcher Franz-Josef Ulm, the George Macomber Professor in the MIT Department of Civil and Environmental Engineering. "If you imagine a box randomly filled with many types of fruit, you can see that the berries will naturally fill the space between apples and oranges, and the apples and oranges will do the same between larger fruits." Ulm's earlier mathematical models assumed that the nanoscale units were identical in size and shape. Results from them showed that those units packed naturally at the two densest CEMENT AMERICAS • Spring 2013 • www.cementamericas.com