Alkali-reactivity of greywacke aggregates in Maentwrog Dam (North Wales)

Greywacke as a rock group has been implicated in promoting alkali–aggregate reaction (AAR) in a number of structures throughout the world. The Maentwrog Dam is believed to be the first major case recorded in the U K. Petrographic examination of concrete cores taken from the dam showed that the structure is affected by AAR, notwithstanding any other contributory causes of distress. SEM examination revealed three main AAR gel types having morphological, situational and chemical characteristics consistent with those in flint aggregate concrete promoting the alkali–silica reaction. X-ray diffraction studies showed that matrix phyllosilicates did not exhibit exfoliations diagnostic of the controversial alkali–silicate mechanism, corroborating the petrographic examination evidence that an alkali–silica mechanism is the most likely explanation for distress. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed that the microcrystalline quartz, acting as a matrix cement in the rock, has local dislocation densities of 10¹⁴ m⁻², and this is believed to be the reactive constituent. Similarities between the Maentwrog greywacke and a Canadian greywacke, which has promoted AAR in the Malay Falls Dam (Nova Scotia), are shown. This confirms that greywackes are vulnerable to the alkali–silica reaction rather than alkali–silicate reactions.