Is the clay "exchange capacity" concept wholly applicable to pozzolans?

One of the most characteristic properties of clay is their "cation exchange capacity", otherwise known as their "exchange capacity". The research reported in this article shows that for various reasons some natural and artificial pozzolans exhibit this property . Ion exchange is only partial in the latter, however, because contrary to clays, on hydration with Portland cement, pozzolans may be nearly entirely converted from their mainly vitreous and/or amorphous original state into a variety of new more or less crystalline compounds. For this purpose a total of 142 types of cement were tested in this study: 8 industrial and 134 laboratory cements, the latter prepared by mixing Portland cements with different proportions, by weight, of three natural and three artificial pozzolans. All these cements were analysed by the Frattini test, 7 and/or 28 days after mixing. Both [OH-] and [CaO] and the percentage content of Na+ and K+, expressed as Na2O and K2O, respectively, were determined in the liquid phase. The cation concentration was only determined for the 45 blended cements most relevent to the objective of the study. Finally, the three natural pozzolans were analyzed by XRD to identify their mineral composition. The results obtained confirmed the working hypothesis. They also showed that the higher the content of Na+ and/or K+, the higher the likelihood of exchange capacity in the pozzolans, in this case primarily with Ca2+. Nonetheless, the very special mineral composition of certain pozzolans, with minerals such as zeolite and phillipsite, may likewise contribute to this capacity. In any event, due to the cation exchange between Na+/K+ and Ca+, the blended cements involved met the Frattini test requirements more readily and had shorter setting times, especially: where the Portland cement with which they were mixed was not highly sulfate-resistant, and where the Portland cement/pozzolan ratio was 60/40. Under these circumstances the behaviour of one of the pozzolans studied may, in one sense, be described to be "more apparent than real".