Granite

Pyllonite

Ultramylonite

From P.J.M. Monteiro, K. Shomglin, H.R. Wenk, and N. P. Hasparyk Effect of Aggregate Deformation on the Alkali-Silica Reaction ACI MATERIALS JOURNAL, MAR-APR, 2001, V98(N2):179-183.

Quartz-bearing rocks such as granites, quartzites, and gneisses may be potentially reactive with the cement paste matrix. Unfortunately, many large concrete dams were built using such rocks as aggregates and now are showing manifestations of distress.

Expansion of mortar specimens prepared with granodiorite, mylonite, phyllonite, and ultramylonite from the Santa Rosa mylonite zone in southern California were studied to investigate the effect of aggregate deformation on the alkali-silica reaction in concrete. While these rocks have basically the same chemical and mineralogical composition, they differ in the grain size distribution, the deformation state, and the structural organization of polycrystals. The ASTM C 1260 expansion tests indicate that mortar expands increasingly when made with granite, mylonite, phyllonite, and ultramylonite respectively.

There is a significant correlation between expansion and the development of foliation, and accompanying reduction in grain size. The results suggest that the alkali-silica reaction depends on more factors than simply the crystallinity of quartz. Deformed granitic rocks provide a good system to quantify these parameters. Texture analysis of these rocks indicated that there is a quantitative relationship between the degree of deformation and reactivity.