Effect of ductile deformation of quartz-bearing rocks on the alkali-silica reaction

It has been known for a long time that strained, microcrystalline or cryptocrystalline quartz-bearing aggregates may cause alkali-silica reactions in concrete. Studying the relationship between deformation processes and microstructural characteristics of rocks, the reason for this behaviour can be better understood. Orthogneisses from the metamorphic basement of the Sierra Chica, Cَrdoba (Argentina), which were locally and differentially deformed in ductile shear zones, were used to analyse such behaviour. Petrographic analyses, accelerated mortar bar tests (ASTM C 1260, 2005) and chemical test (ASTM C 289, 1994) were conducted. Furthermore, corrosion tests were performed on polished rock surfaces using 1 N NaOH solution. It was seen that the reactivity of the quartz-bearing mylonites increased by ~ 30% with respect to the non-mylonitised sample due to the increment in the strained quartz content and specially with the extended subgrain development. The mylonitised rock affected by superimposed cataclasis and the development of pseudotachylytic veins incremented its reactivity by ~ 97% with respect to the non-mylonitised sample due to the combined effects of subgrain formation, grain size reduction and the formation of glassy material. It was also the only sample that showed significant differences in surface corrosion confirming the high reactivity of the rock. These results agree with expansion values measured on the accelerated mortar bar test and with silica leached in the chemical test. We believe that the simultaneous use of different tools to evaluate the potential alkali reactivity of the rocks in concrete is a good strategy rather than the use of isolated tools, which could lead to confusing interpretations of the process and therefore result in erroneous decisions.