Glass Pozzolan Powders

Glass powders are widely used as pozzolans in concrete. A pozzolan is an amorphous mineral that is not cementitious by itself, but when added to a concrete mix it reacts like the cement powder to form the cement binder called calcium silicate hydrate, CSH. The glass pozzolan therefore is used to replace 10-30 percent of the cement powder.

Chemically, the reason this works so well is that when you add water to cement, about 80 percent of the hydrated cement turns into the strength building CSH, while 20 percent turns into useless lime (CaOH). The lime causes efflorescence, which is an annoying whitish buildup on the surface of concrete that obscures color and takes away from the aesthetics of concretes and mortars.

It all starts when you add water to the mix of cement powder, pozzolans, and aggregates. The glass pozzolan starts to dissolve in the high pH environment of the concrete and begins reacting with the lime to form additional CSH while serving to diminish the efflorescence problem. So if 20 percent of the cement powder is replaced with a glass powder, less lime is produced and the lime that is produced is mostly converted to CSH by the reaction with the pozzolan.

The net effect is stronger, more durable concrete with less efflorescence issues. Other widely known pozzolans are fly ash, silica fume, and metakaolin.

Vitro Minerals three types of glass pozzolans includes VCAS, which is a very white low alkali glass used for decorative concrete; ACAS, which is a pozzolan derived from post consumer bottle glass; and HR 50 Fumed Nanosilica, which is an extremely reactive, very white pozzolan that can replace silica fume or metakaolin.