This section will define metakaolin as a pozzolanic material derived from the calcination of kaolin clay. It will discuss the chemical and physical properties that make metakaolin an effective supplementary cementitious material (SCM). The mechanisms by which metakaolin interacts with calcium hydroxide released during cement hydration to form additional calcium silicate hydrate (CSH) will be detailed, enhancing the microstructure and decreasing the pore size distribution within the concrete.