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Geopolymers: Promising Materials For Underground Applications
Another concern among construction specialists is whether it is possible to utilize common mix design guidelines such as ACI 211.1 in the design and proportioning of geopolymer concrete. Some professionals recommended abandoning such guidelines because they were designed for Portland cement and do not address some of the aspects of geopolymer technology such as the alkali activation system or curing mechanism. However, the TTC chose to take a semi-empirical approach to study all these aspects and incorporate them into a computerized version of the guideline, a custom developed software package that enables its users to select the most suitable geopolymer mix for their specific application using a modified version of the ACI guideline to obtain geopolymer proportioning (Figure 2).
Geopolymer’s outstanding properties include high early strength, high corrosion resistance to acids and sulfates, thermal stability up to 2,300 degrees F, and resistance to fire without exploding, unlike hydration based concretes. Its unique polycondensation mechanism, which uses water only as a conveyor and not as a reactive, produces a stable silicoaluminate network which is responsible for these properties. In theory, any material with sufficient amounts of silicon and aluminum oxides can be used as a raw material, and can potentially produce geopolymer concrete, with quality level dependent upon on parameters such as particle distribution, amount of bound water and impurities, among others.
The TTC has conducted extensive studies on fly ash-based geopolymer for a number of properties including compressive and flexural strength, pot life, acid and sulfate resistance, carbonation, chloride penetration, curing methods and workability. This research demonstrates that depending on the activation system and curing method, geopolymer concrete with early compressive strength of up to 16,000 psi can be achieved. Moderate temperature curing (140 degrees F) is still the preferred curing method of geopolymer concretes, although research at the TTC has also demonstrated that ambient cured geopolymer can gain strength in a similar timeframe to Portland cement systems.