A compound that speeds up a chemical reaction. When added to paint, concrete, mortar, or grout mix, it speeds the rate of hydration and thereby causes it to set or harden sooner. For example, alum is used to decrease the setting time of plaster of Paris or Portland cement and metallic soaps are used to hasten the drying time for oil paints. Synonyms: promoter; activator; accelerant; drier. Museum of Fine Arts, Boston (2000); RS Means Building Glossary (undated). [J. Cassar, R. de Angelis]