Size diversity in cement nanoparticles optimizes packing density to give concrete its strength
Researchers in MIT’s Concrete Sustainability Hub are making steady progress toward learning exactly how the cement paste that works as glue in concrete hardens during the first hours after water and cement powder are mixed. First they determined that cement paste can be described as a granular aggregate, where the particles or basic nanoscale units pack together most densely when arranged orderly. A few years later they discovered that the calcium-silicate-hydrate (C-S-H) molecules that make up the basic nanoscale unit of cement have a disorderly geometric arrangement, rather than the orderly crystalline structure scientists had long assumed. In new work, they found that the size of C-S-H particles themselves is also somewhat disorderly. The particles form at random sizes, not in homogenous spheres, and the diversity in particle size leads to a denser, disorderly packing of the particles, which corresponds to stronger cement paste. A paper by CEE postdoctoral associate Enrico Masoero, Professor Franz-Josef Ulm, senior research scientist Roland Pellenq and co-authors appeared in the Oct. 12 issue of Physical Review Letters. Read the news story.