Special-Uses-Alkali - Expanded Shale & Clay

1. Lowering the pH of the concrete’s pore solution to suppress the initial silicasolubility;2. Reducing the free alkali metal ion (sodium and potassium) concentrations present torestrict gel formation; and3. Reducing the permeability of the concrete to restrict water ingress, hence preventingthe gel from expanding.(1)Generally, New Zealand has had an informal agreement with the local cement manufacturers tokeep the equivalent alkali level in cement below 0.60%. By maintaining a maximum alkali limitof 0.16 pound per cubic foot (lb/ft), a normal concrete can last up to 50 years and experienceonly cosmetic effects. If the alkali levels cannot be limited, or the concrete is specified as highperformance, SCMs can be used to control ASR. BRANZ Limited uses a minimum of 15%replacement of cement with metakaolin to mitigate ASR expansion.SCMs reduce the permeability of the concrete by filling the voids and limit the reaction withavailable water to expand the ASR gel. Also, by replacing cement with a SCM, the total alkalicontent is reduced, lowering the number of free alkali ions in the concrete. It is also believed thatby using a mineral admixture with pozzolanic properties, the pozzolans react with calcium tofurther lower the pH, reducing the potential for the formation of the ASR gel.2.1.2. Cement and Concrete Association of New ZealandThe Cement and Concrete Association of New Zealand (CCANZ), recommends mineraladmixtures to reduce ASR expansion, and reduce the cost of the concrete. Reactive aggregatescan then be used satisfactorily with the addition of a SCM. With the addition of SCMs thestrength of the concrete is generally increased as the permeability is decreased.Metakaolin, a highly reactive pozzolan, has been found to resist ASR expansion when the SiO 2content is above 45% by mass and used at a 15% replacement of cement. In addition, curingtimes are generally shorter with the use of metakaolin. However, because of the fast reactingnature of the pozzolan, temperatures may get much higher and additional preventive measures4