Glass Pozzolan Powders

Glass Powder Pozzolans now have their own separate ASTM Standard passed early in 2020. The new standard, ASTM C 1866/C1866M-20, describes two types of glass pozzolans; Type GE, which is a low alkali glass extremely effective for ASR mitigation, and type GS glass pozzolans, which work well in concrete mix designs without reactive aggregates. Table 1 shows the chemistry of this new category of pozzolans compared to fly ash, GGBF slag.

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 has three types of Glass pozzolans, two of which fall under the ASTM C 1866 standard. VCAS is a low alkali type GE glass powder extremely effective at mitigating ASR at cement replacement levels as low as 12-15%. ACAS is a recycled plate glass powder with excellent pozzolanic properties for mix designs with non reactive aggregates or indoor applications. Both VCAS and ACAS are whiter than white cement and very suitable for decorative concretes. The third glass pozzolan is an extremely white nanosilica that is similar to silica fume and meets all the ASTM C1240 standards for silica fume.