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Technical Tip formation of silicates, thereby getting rid of cal- mistakenly used in an attempt to decrease slump cium molecules and putting them into solution; loss. This was never the intended purpose of these and this, along with the coating on the cement admixtures. In hot temperatures, extending the set particle, keeps the nozzle accelerator from being time can in some applications give the mixture able to reactivate the mixture. more time to lose water due to evaporation, Retarders are normally added to the initial mix causing slump loss. water in the mixer. One must be careful when If there is a need to hold off the setting time of using retarders for applications in hot tempera- your shotcrete mixture, make sure that you do tures. Depending on the cement and the admixture some initial investigation or trials to make sure used, there have been instances of excessive that you are using the proper admixture or com- retardation or flash setting. bination of admixtures to attain the objective Unlike retarders, it is not known exactly how required for the project. set stabilizers work but there is a lot that is known about their application. Stabilizers can be used at References high dosages without the adverse effects such as Rixom, R., and Mailvaganam, N., 1999, Chemical Admix- flash set and poor strength development charac- tures for Concrete, third edition, Taylor and Francis Group teristics that can occur when using conventional Publishing, 456 pp. -Whiting, D., and Dziedzic, W., 1992, “Effects of Conven retarders. Stabilizers are commonly made from tional and High-Range Water Reducers on Concrete Proper- carboxylic acids and/or phosphorous-containing ties,” Research and Development Bulletin RD107, Portland organic acid salts, and as a result, often have a Cement Association, Skokie, IL. low pH in the 2 to 3 range. The way a stabilizer works is thought to be related to the inhibition of calcium silicate hydrate and calcium hydroxide nucleation. Many will Dan Millette, a Mining Engi- claim that the stabilizer will do a much better job neer, is the Director of the of controlling the nucleation process than re - Mining and Tunneling Division tarders. This claim has been supported in labora- of The Euclid Chemical Com- tory trials using calorimetric data and by viewing pany. Millette has over 30 years under a scanning electron microscope the surfaces of experience in the shotcrete of both stabilized and activated alites. As men- industry and is a certified tioned earlier, retarders will retard the formation EFNARC Nozzleman Examiner. of silicates, which is just one phase of cement He is a member of ACI Subcommittee 506-F, hydration, while a stabilizer will act on all phases Shotcreting-Underground, as well as a member of cement hydration, including the C3A fraction. of the Society for Mining, Metallurgy and Explo- Stabilizers also coat the cement particles but ration (SME); the Canadian Institute for Mining, it is thought that they can be penetrated by shot- Metallurgy and Petroleum; and the Tunneling crete accelerators to restart the hydration process. Association of Canada. The addition of a stabilizer is different than that of a retarder. To be completely effective, a stabi- lizer should be added to the mixture only after the cement has had a chance to completely wet out with the water in the mixture, thereby starting the hydration process. If a stabilizer is added to the STRUCTURAL mixture water up front, before the cement is SHOTCRETE added, it will only have approximately half the effectiveness of adding it after the cement has YSTEMS, INC.S wetted out. Another advantage to using a stabilizer LICENSE #579272 A www.structuralshotcrete.com is that it can be added again to the same mixture if the job is further delayed and a longer retarda- JASON E. WEINSTEIN, P.E. tion of the set time is required. VICE PRESIDENT When using either retarders or stabilizers, it must be realized that the purpose of these admix- tures is to hold off the setting time of cement, or 12645 CLARK STREET (562) 941-9916 more specifically to extend the dormant period of SANTA FE SPRINGS, CA 90670 FAX (562) 941-8098 the cement hydration process. They are also often Shotcrete • Summer 2014 53


2014SumShotcreteEMag
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