Clay minerals are largely available in nature and due to special characteristics of clay, have successfully been used for dam core as a water-tightening material. Taking into consideration the execution necessities and so as to reduce the associated costs, it is required to reassess fundamental of the conventional plastic concrete mix design used for constructing water tightening element of the large dams. In this study the effects of two different types of clay minerals with the intention of evaluating the possibility of changing portion of the clay materials on compressive strength of plastic concrete are investigated. The required samples have been prepared and tested according to a special laboratory program. The results indicate that the type of clay mineral has meaningful effects on the strength of the plastic concrete. The results also have shown that the slump of the new mixture of plastic concrete is dependent on the amount of clay in the mix design. Consequently an empirical equation is derived for estimation of compressive strength based on two parameters of clay content and sample age.