Cement-based materials are in wide use - partly in the belief that their reactivity is due to their glassy nature. The question we wish to investigate is: how specific is the link between the pozzolanic reaction and glassy materials? In order to answer it, we take the case of quartz, a material often used in cement-based matrices, reputed to be chemically inert due to its crystalline structure. Our tests have shown the pozzolanic reaction of quartz to be a complex dissolution-precipitation reaction involving multiple parameters. One of these, we find, has particular importance: above a certain critical particle diameter, the reaction becomes barely perceptible.