Igneous rock silicate minerals and their effect on durability:
Quartz: normally, quartz is hard, relatively inert and insoluble. However, quartz in rocks subjected to tectonic activity may become fractured, shattered, and strained. Fracturing increases the surface area accessible to fluids; strained quartz surfaces have high surface charges. The strained active surfaces attract cations of solutions, initially by strong adsorption, which may eventually be involved in chemisorption and chemical reaction. In an alkali environment of cement paste (strong concentrations of alkali elements such as Na+, K+, and Ca+), the quartz surfaces dissolve to produce silica gel. The net result is expansion and cracking, the well know Alkali Silica Reaction.
Feldspar: Granites have K and Na -rich feldspars; gabbro, basalt, and diabase have Ca rich feldspar. The K,Na feldspars may participate in alkali reactivity similar to that of quartz, but to a much lesser degree. The Ca-rich feldspars are prone to a more rapid chemical alteration that may produce expansive clays and intermediate minerals. The alteration may already be present before the rocks are quarried, the result of exposure of the rocks to hot, saline circulating fluids within the earth's crust. In some instances, the relatively unstable Ca-feldspar may undergo slow alteration after crushing and exposure to surface fluids. The decay of these minerals produces clays, some of which may be expansive. The expansive clays produced are sensitive to wetting and drying cycles, and expand, eventually destroying the aggregate piece or the material encasing it.
Ferromagnesian Minerals: Olivine, pyroxene, amphibole, and mica are the principal ferromagnesian minerals. The first two are high temperature minerals, and relatively unstable in surface environment, especially when exposed to moisture and solutions. Their decay produces clays, as explained above.
