Pozzolanic activity is a measure of the reaction rate between a pozzolan and Ca²⁺ or calcium hydroxide (Ca(OH)₂) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition, and the active phase content.

Physical surface adsorption is not considered as being part of the pozzolanic activity, because no irreversible molecular bonds are formed in the process.

The pozzolanic reaction is the chemical reaction that occurs in Portland cement upon the addition of pozzolans. It is the main reaction involved in the Roman concrete invented in Ancient Rome and used to build, for example, the Pantheon. The pozzolanic reaction converts a silica-rich precursor with no cementing properties, to a calcium silicate, with good cementing properties.

In chemical terms, the pozzolanic reaction occurs between calcium hydroxide, also known as portlandite (Ca(OH)₂), and silicic acid (written as H₄SiO₄, or Si(OH)₄, in the geochemical notation):

or summarized in abbreviated cement chemist notation:

The pozzolanic reaction can also be written in an ancient industrial silicate notations as:

or even directly:

Both notations still coexist in the literature, depending on the research field considered. However, the more recent geochemical notation in which the silicon atom is tetracoordinated by four hydroxyl groups (Si(OH)
₄, also commonly noted H
₄SiO
₄) is more correct than the ancient industrial silicate notation, in which silicic acid (H
₂SiO
₃) was represented in the same way as carbonic acid (H
₂CO
₃) whose geometrical configuration is trigonal planar. When only considering mass balance, they are equivalent, and both are used.