THE USE OF A CHELATING AGENT TO DISTINGUISH BETWEEN CRYSTALLINE AND POORLY CRYSTALLINE SILICA: APPLICATIONS TO ALKALI - SILICA REACTIVITY

Alkali Silica Reaction (ASR) is a deleterious, expansive reaction in cement and concrete that can compromise the integrity of roads, buildings, and other structures. Several ASTM methods have been developed to test potential aggregates for possible ASR risk, including chemical methods and physical methods, such as the mortar bar test. A common chemical method uses NaOH digestion, followed by spectroscopic methods, to estimate the amount of reactive silica present in an aggregate. NaOH digestion, however, is destructive to both crystalline (i.e. quartz) and non-crystalline (e.g. opal), or poorly crystalline, silica, potentially yielding falsely high estimates of reactive silica.

Many studies in soil and plant sciences have used Tiron, an alkaline chelating agent (C₆H₄S₂O₈Na₂), as a method of digesting opaline silica for spectroscopic analysis. Here we suggest that Tiron may be used to estimate the amount of reactive silica present in road aggregate for ASR studies. We do this by applying both the Tiron method and a modified ASTM method to various aggregate sources with ASR tendencies known to the Georgia Department of Transportation. Preliminary data show that Tiron extraction of reactive silica is more selective, in some cases extracting only ~10% of the silica compared to the harsher NaOH extraction. Variation is due to differences in aggregate mineralogy. The ease of sample preparation and spectroscopic analyses, along with the less toxic chemicals, make this a promising alternative or complementary analysis in ASR studies of potential road aggregate.