The paper discusses the background of cementitious materials, and generally compares natural (Pozzolanic) cement to manufactured (Portland) cement. Fly ash is discussed as a common Pozzolan, and in particular, the low-energy requirement for fly ash as compared to Portland cement. Also, photomicrographs of fly ash particles and of chemically bonded fly ash are compared to a photomicrograph of Portland cement particles and a photograph of Portland cement concrete to dramatically illustrate the differences in the matrix formation of these cementitious materials. Laboratory data is used to show that simply adding some fly ash to a Portland cement mix can greatly reduce the permeability of the concrete under certain curing conditions. In addition, laboratory data is used to show that chemically-bonded fly ash can be engineered so that its structural properties (i.e., compressive strength, flexural strength, modulus of elasticity, etc.) mimic those of Portland cement concrete. However, bonded fly ash has a far denser matrix than Portland cement, and that dense matrix, in turn, gives the bonded fly ash a relatively low permeability without' significant curing. While not completely impermeable, as a structural material bonded fly ash is much more resistant to a chemical or biological intrusion than is Portland cement, when used as an expedient repair material, as demonstrated by laboratory comparison tests. Finally, although bonded fly ash is denser than Portland cement mortar, but since it does not require rock aggregate, it actually has lower density that Portland cement concrete. This means that bonded fly ash can be used to decrease the total weight of a structure, while also reducing the energy requirements of the materials, and, at the same time, increasing the resistance to chemical or biological intrusion.