The course of alkali-aggregate reactions in concrete

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Title: The course of alkali-aggregate reactions in concrete

Author(s): Z. Owsiak

Publication: KILW

Volume: 54

Issue: 3

Appears on pages(s): 593-608

Keywords: alkali,reactivity,cement

Date: 7/1/2008

Abstract:
The alkali-aggregate reaction in concrete is associated both with the presence of reactive aggregates and with alkali content in concrete, which is most significantly influenced by sodium and potassium content in the cement. The alkali-aggregate reaction occurs between the pore solution in the mortar or concrete and certain aggregates, causing concrete expansion which may lead to its cracking and destruction. The problem of the alkali-aggregate reaction is still topical and, despite the numerous investigations and publications, many issues have not been explained yet. This paper presents the results of the Author's years-long research into alkali-aggregate reactions. The research objective has been to determine the influence of alkali content in the cement on the scope of the expansion, the significance of changes in sodium and potassium hydroxide concentration in the liquid phase, Changes in the composition of sodium-potassium-calcium silicate gel in the function of time as well as the significance of adventitious phases accompanying the alkali-silica reaction. The research has covered the changes in linear dimensions of mortar bars, changes in the concentration of sodium and potassium ions in the liquid phase, as well as the mortar microstructure and X-ray analysis of post-expansion mortar microsurfaces. For the purposes of investigating the microstructure, scanning microscopy was used, together with simultaneous X-ray analysis of the microsurface. The results demonstrate that expansion value depends on potassium and sodium content in the cement, and the decrease rate of sodium and potassium ion concentration in mortar pores points to an ongoing silica reaction and the accompanying mortar expansion. An indicator enabling prediction of further course of the aggregate-alkali reaction may be alkali content in the liquid phase. Experimental results support the hypothesis that calcium ions replace sodium and potassium ions in the gel, releasing these components into the solution and contributing to further expansion increase. Hence the influence of calcium hydroxide presence in the mortar is deleterious. Secondary ettringite which accompanies concrete deterioration processes is likely to result from carbonatisation of hydrated calcium aluminates; yet the role of ettringite in the expansion requires further study.


Polish Academy of Sciences, International Partner Access.

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