Abstract
Maarten Broekmans has studied the alkali-silica reaction in two Dutch concretes by means of optical microscopy, geochemical analyses on bulk material and after selective digestion in acid, and with element mapping in polished thin sections. He furthermore characterized the nature of the silica/quartz in the Norwegian mylonites studied previously by
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Wigum in his PhD-thesis (1995), in the Ohio cherts previously studied by Kneller (1967), and some Dutch cherts by assessing their crystallinity indices following the XRD-method of Murata&Norman (1976).
The thesis suggests a simple and straight-forward system to rate concrete damage by assessing and classifying the crack fabric in impregnated full cores under fluorescent illumination. This method does not distinguish between different causes for the observed cracking, but it is quick and does not require expensive instrumentation nor special training. However, identifying the true cause of the cracking is of course essential and can be done as a second stage by thin section petrography.
Typically, chert is the main alkali-reactive constituent in Dutch concrete. However, petrographic observations on sand- and siltstone grains in the aggregate indicate that these generate ASR-gel as well, which was not generally accepted in the Netherlands until before recently. The necessary alkali may very well be provided by interstitial clay minerals, and eventually by detrital muscovite on a very local scale with very limited reach. In geological-sedimentary literature, the catalytic effect of sheet silicate minerals (clays, micas) upon the dissolution of quartz is well documented. There is a strong resemblance of the observations made in the ASR-concrete to the features described from sedimentary-diagenetic sandstone compaction, and a similar process is suggested to occur in ASR. This is partly supported by element maps on Na, K, Ca, Si, Fe and S on intact and ASR-cracked chert, and intact and ASR-cracked sandstone. There are marked differences in their alkali-household.
The silica involved in an alkali-silica reaction is hardly characterized, often only identified as silica or quartz. However, there are many qualities and/or properties of silica that may or may not affect its solubility, including polymorphism, lattice irregularities (vacancies, dislocations, twinning planes, domain structures), foreign ions and water in the quartz structure, etc, etc. Which of these qualities and/or properties do apply depends on conditions. Thus, merely identifying the alkali-reactive silica as 'quartz' is not good enough, but rises questions about which methods to use for that, and with which criteria? There are few procedures available to determine the quality of the quartz lattice; among those most frequently used is the XRD-method of Murata&Norman (1976), who use relative peak-heights in the quintuplet at 67.74 degrees two-theta in a diffractogram. However, all qualities that do have an effect on the quartz lattice are lumped together, and no further distinction can be made which of these are relevant for that particular sample. Thus, increased (alkali-) solubility cannot be attributed to one single quartz property. This is confirmed in literature, by the great discrepancies reported when the same samples are analyzed by XRD and IR or DTA/TGA. Quartz content of analyzed silica may vary at random from 0% with one method to 100% with another, and may very well be the opposite for another sample. Broekmans found that the samples analyzed (Norwegian mylonites, Ohio cherts, Dutch cherts) span the entire range of crystallinity indices from <1 to 10, but there seems no correlation with available expansion data. Therefore, the applicability of the crystallinity index in its present form to determine the alkali-reactivity potential of quartz is only limited.
The thesis contains sixteen full-color plates of graphs, photographic images of impregnated cores, thin section details and element maps, and a numbered reference list with 212 entries.
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