Auckland Basalts as a Source of Alkali in Concrete

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Title: Auckland Basalts as a Source of Alkali in Concrete

Author(s): Reiner Goguel and Neil B. Milestone

Publication: Special Publication

Volume: 171

Issue:

Appears on pages(s): 429-444

Keywords: Alkali-aggregate reactions; basalt; leaching; lithium hydroxide; rocks.

Date: 8/1/1997

Abstract:
The basalts of the central Auckland area, classified as nepheline basanites, exhibit a high potential for alkali release. When they are combined with Waikato River sand in concrete, an expansive reaction could occur with the alkali release. Methods developed at the New Zealand Institute for Industrial Research and Development (IRL) were applied to the few failed public structures at Auckland. These showed levels of contribution by the basalt of sodium and potassium in the pore solutions of the concrete exceeding 5kg Na20eq/m3. Bulk chemical and x-ray diffractometric phase analysis of the Auckland nepheline basanites reveal only minor variations. Feldspathoids (nepheline and in the case of Mt. Wellington basalt also leucite) are singled out as the main alkali-releasing phases. Their content can be estimated from the composition of a simple acid leach. This determines only the full alkali-release potential which is probably not frequently developed in Auckland concrete structures. Experimental mortars attain these high alkali-release levels if small aggregate grain size and high W/C are applied. At moderate W/C (eg 0.47) and curing time (eg one year) the rate of alkali release in concrete develops greater differences between and within quarries. These differences are controlled by the interstitial surface area and the phases that can be accessed by the pore solutions. Their alkali release decreases in the following order: feldspathoids (nepheline and leucite) > interstitial glass > feldspar (labradorite). Slow cooling of the lava, and incipient alteration have a delaying effect on alkali release. Screening tests that are faster than mortar curing and address these variables are: a ) analysis of alkaline leachates using 0.3 M LiOH and 0.3M tetramethylammoniumhydroxide (TMA-OH). b ) the measurement of internal surface area and micropore volume of the basalt by absorption-desorption porosimetry or sorption characteristics for heavy alkalies (Rb, Cs). C> electron microprobe investigation of alkali-bearing phases. Although none of these tests on its own provides all the desired information, the analysis of the alkaline leachates is likely to provide a practical measure for the eve1 of alkali that will be released.