The pozzolanic reaction of fly ash in mortar was primarily examined using a K-value from Feret's law. As a result, the pozzolanic reaction would be mainly controlled by the diffusion kinetic, and the diffusion constant would increase with water content in the mortar. The K-value is useful to estimate the degree of the pozzolanic reaction of mortar. Secondly, the relation between K-value and assessed pozzolanic activity index derived from accelerated chemical test (API method) was inspected. It is concluded that the API is one of the useful acceleration methods to evaluate the degree of the pozzolanic activity, because API has a good relation with K-value. (c) 2006 Elsevier Ltd. All rights reserved.