Some advances in understanding the pozzolanic activity of zeolites: The effect of zeolite structure

To investigate the pozzolanic activity of zeolites, experimental blended cements prepared by mixing together ordinary Portland clinker, isochemical sodium zeolites type A [LTA] or X [FAU], and gypsum, were subjected to the Fratini’s test, i.e., the official test recognized in the European Standards. The same mixtures without gypsum were used to prepare cement pastes with deionized water, which, after curing for 3–28 days, were subjected to mechanical strength measurements. Both zeolites proved to be effective as pozzolanic materials. Their action involved various steps, including (1) cation exchange, (2) dissolution and/or breakdown of the zeolitic structure, (3) possible formation of transient gel phases, and (4) precipitation of hydrated calcium silicates and aluminates. Pozzolanic activity was demonstrated to depend on zeolite structure. Zeolite A reacted, in fact, more readily than zeolite X. Zeolite X, on its turn, being slightly more siliceous than zeolite A, contributed to a greater extent to the development of the mechanical resistances of the cement pastes at short curing times.