Why toothpaste and cement harden over time

(University of Delaware) Cements, clays, soils, inks, paints, and even toothpaste. Many paste materials, also known as dense colloidal suspensions, stiffen as they age. Structural dynamics, or changes in the loads the materials undergo over time, are partly responsible for this change, but for decades, experts have suspected that there's more going on inside these materials. Now, a University of Delaware professor and an international team of researchers have discovered a process called contact-...