The alkali-silica reaction: mineralogical and geochemical aspects of some Dutch concretes and Norwegian mylonites

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 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

Microstructure of selected aggregate quartz by XRD, and a critical review of the crystallinity index

Reliable assessment of the potential of quartz in aggregate to develop deleterious alkali-silica reaction (ASR) is essential for the construction of durable concrete. The crystallinity index for quartz (QCI) introduced by Murata and Norman [15] has been applied to predict the ASR potential of quartz. Despite a number of technical shortcomings and omissions in the original paper, the method has arguably become the most popular alternative for the 'petrography + expansion testing' combo. This paper investigates the ASR potential of twelve Italian concrete aggregates, by petrography, mortar bar expansion testing, and test the quartz potential reactivity by calculating the QCI and by the line profile analysis of the XRD pattern. The results confirm that a relationship between QCI values and aggregate expansion behavior is absent. Contrary, the microstructural analysis is a powerful method for predicting the ASR-reactivity of quartz. Finally, the method introduced by Murata and Norman [15] is critically reviewed

Origin and mobility of alkalies in two Dutch ASR-concretes. II: Microscale element distribution around sandstone and chert. Implications for the mechanism of ASR

River gravel used as aggregate for concrete in the Netherlands contains several potentially deleterious components with respect to alkali-silica reaction (ASR), viz. porous chert, chalcedony, and impure sandstones (greywackes. mica- and sericite-rich sandstones, siltstones, arkoses etc.). Whereas cherts and chalcedonies are virtually free from alkalies prior to their incorporation in concrete, impure sandstones are not. Current regulations therefore impose a cumulative limit in terms of Na2O-equivalents on concrete. i.e. the sum of bulk Na20-equivalents of individual Components (cement, aggregate. Mier. additives, etc.). However, for understanding the fundamental mechanism of alkali-silica reaction itself, knowledge of the internal relationships within concrete, i.e. exchange between different types of aggregate (chert, impure sandstone), cement paste and fluid phase, is essential

Wetting-induced layer contraction in illite and mica-family relatives

When dry illite is wetted, its layer structure contracts along the c-axis by up to 0.2 Å. This behavior contrasts with that of smectite which shows interlayer swelling on wetting. Layer contraction has also been found to occur in glauconite and celadonite that are structurally more disordered than illite, as well as with phengite and artificially degraded hydromuscovite. In contrast, muscovite proper does not show contraction. The contraction is ascribed to the deprotonation of hydronium ions (H3O+ → H2O + H+) occupying interlayer K+ positions. The hydronium ion is approximately 5% larger than the neutral water molecule. This difference in size is proportional to the magnitude of contraction. The change in molecular volume and interlayer separation of illite particles may explain the anomalous decrease in density after dehydration under pressure. This paper reports on wetting-induced contraction in illite and related layer silicates, using humidity-controlled X-ray diffraction