This paper reviews about geopolymers fundamental focusing in clay materials. The review includes the chemistry and structure of geopolymers, reaction mechanism and prime materials involved in geopolymer formation. The characterization of geopolymers in term of microstructural, crystallographic and functional groups is elucidated. Besides, the important factors (such as alkali concentration, mixing parameters, curing conditions, and water contents) affecting properties (such as setting time, bulk density, strength, thermal properties, and stability) of clay-based geopolymers are critically reviewed. Finally, the paper also includes the previous geopolymer applications that have been successfully marketed and the potential current and future areas or focuses of study and application. (C) 2016 Elsevier Ltd. All rights reserved.