Factors affecting concrete pore solution: (A) Alkali fixation induced by drying and carbonation and (B) Alkali released from feldspars
Abstract
Two phenomena affecting the concentration of ions in the pore solution of cement paste, mortars, and concrete were studied: the removal or fixation of alkali ions as a result of simultaneous drying and carbonation, and the incorporation of alkalies released from feldspar aggregates. In addition, the effects of pressure of expression on pore solution ion concentrations were investigated. Drying without carbonation does not result in alkali fixation, but fixation of as much as 55% of the alkalies in the pore solution takes place when drying and carbonation occur simultaneously. Alkali fixation occurred with all the five cements studied and is expected to occur for all portland cement that is simultaneously dried and carbonated. No significant difference was observed among the different cements between the proportion of Na$\sp{+}$ and K$\sp{+}$ fixed. The alkali fixation is partially reversible if the previously dried specimens are saturated and stored sealed for a period of time. The amount of reversal is limited, and depends on the time allowed and the specific storage conditions. The results of this study suggest that it might be possible to fix a significant portion of the alkalies in field concrete affected by ASR if the concrete is dried and carbonated under controlled conditions. The possible formation of specific alkali-fixing compounds has been investigated, but none were found. It has been shown that in mortar specimens containing feldspar these minerals react and release alkalies into the pore solution, effectively increasing the pore solution alkali concentrations. In the presence of reactive aggregate the additional alkalies become active participants in the ASR process. Based on experiments conducted on feldspar fragments agitated in saturated CH solutions of high pH (controlled with KOH), it has been shown that: (a) the release of alkalies is strongly influenced by the temperature, (b) the amount of alkali released is a linear function of the surface area of the fragments; and (c) the rate of alkali release is not significantly affected by the KOH concentrations of the solution.
Degree
Ph.D.
Advisors
Diamond, Purdue University.
Subject Area
Civil engineering|Mechanical engineering|Chemistry
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