Why toothpaste and cement harden over time

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 researchers have discovered a process called contact-controlled aging that explains some age-related changes in paste materials.