Hardeners are substances that are added to materials to increase their strength and durability. However, there are several synonyms for the word "hardeners" that can be used to describe these types of substances. For instance, one could use the word "strengtheners" to describe materials that increase the strength of a substance. Alternatively, the term "fortifiers" could be used to describe materials that make a substance more durable. Additionally, "consolidators" can be used to describe substances that make materials more stable and solid. Whatever term is used, hardeners, strengtheners, fortifiers, and consolidators all refer to substances that make materials stronger and more durable.