ABSTRACT
There are several materials that can be, and are, used as partial replacements for cement in a concrete mix and since they are produced as by-products in other industrial processes the cynic may think that this is just a convenient way of getting rid of unwanted waste products that would otherwise have no commercial value. In fact there are considerable benefits to be obtained because of the effects they may have on the properties of both the fresh and the hardened concrete. They fall very roughly into two categories, those that consist of more or less pure silica and depend for their effectiveness on their ability to react with the free lime in the mix, and those that contain compounds somewhat similar chemically to those in Portland cement and under the influence of an initiator will hydrate to produce hydration products similar to those produced in the hydration of cement itself. In the first category are pulverized fuel ash (pfa) which is also sometimes referred to as fly ash, microsilica which is also referred to as silica fume, and, in appropriate parts of the world, material such as rice-husk residues. In the second category are ground granulated blast-furnace slag (ggbs) and, possibly in the future, gasiher slag which is obtained in the process of gasification of coal. The ones important at present are pfa and slag, which are in fairly common use, and micro-silica which is relatively new but is likely to be used in increasing quantities.
