A Discussion on Causes and Mechanism of Paste Flash Setting

【Author】

Yaozhong Xi* China Building Materials Academy,No.1,Guanzhuang Dongli, Chaoyang Disrct, Beijing, China, 100024

【Abstract】

Intense hydration reactions of active cement minerals (C3A, C12A7, CA, C4A3?S, C11A7·CaF2) under activation of accelerating compounds (NaAlO2, Na2CO3, Al2(SO4)3·K2SO4, Na2O·nSiO2, NaOH) are the most important cause for paste flash setting. The setting can be regarded as a result of the free water in cement paste consumed to such an extent that solid particles coalesce, and physical water-consuming is as important as chemical one, therefore determination of water /binder ratio is a main issue for mix design of shotcrete. The flash setting of shotcrete is closely related to rapid evolution of heat including heat of wetting, heat of dissolution and heat of hydration. AFt is a major hydrate responsible for rapid setting and early strength in ordinary shotcrete. Enough CaSO4 is necessary to stabilize AFt phase in shotcrete. The high performance accelerators for shotcrete can be produced only if rapid setting cement minerals, accelerating compounds, thickening agents, wetting agents and dispersants are properly selected and proportionated.

【Keywords】

Flash setting, accelerators, shotcrete, mechanism, concrete admixtures, AFt phase.

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