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

Abstract

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.

A review of analogues of alkaline alteration with regard to long-term barrier performance

“…Quartz also decreased significantly in the first millimetres of mudstone in contact with the concrete. The zone of pronounced mineral growth at Tournemire is 1.5 cm thick, with ettringite and calcite forming, whereas further away (5À10 cm), a texturally unperturbed zone shows K-feldspar overgrowths, (Peterson et al, 2006) Dam wall 65 Greywacke Alkali-silica gel in reaction rims on aggregate grains, occasionally crystallized into rosette/bladed crystals (Blackwell and Pettifer, 1992) Dam wall 60 Dacite Alkali-silica gel and/or K-, Na-substituted C-S-H mineral (okenite?) in reaction rims on aggregate grains (Shayan, 1988) Dam wall 50 Granite Alkali-silica gel, turning to zeolite-like needles on exposure to air (Fernandes et al, 2004) Power station 40 Siliceous limestone Alkali-silica gel lining voids and cracks (Grattan-Bellew, 1995) Dam wall 30 Siltstone, sandstone Alkali-silica gel lining pores, with K-, Na-substituted okenite in reaction rims on aggregate grains (Cole et al, 1981); (Cole and Lancucki, 1983) Dam walls, bridge 15À50 Granites; alluvial sediment Alkali-silica gel in reaction rims on aggregate grains (Fernandes, 2009) (Marfil and Maiza, 2001) Dam wall, bridges, road pavement, quays, median road barriers 5À70 Siliceous limestone, rhyolitic tuff, quartzite Alkali-silica gel in reaction rims on aggregate grains, occasionally with rosettes of unidentified crystalline material (Bérubé and Fournier, 1986) Bridges ?…”

Reactividad y reciclado de pizarras para su empleo como áridos en construcción

“…44,45 Una tercera corriente aúna las dos anteriores al observar que los áridos que liberan mayor cantidad de sílice son aquellos con alto contenido en microcristales de cuarzo que presentan gran densidad de dislocaciones. [46][47][48][49] Recientes estudios sobre el tema revelan que las distintas formas mineralógicas del cuarzo en los áridos son susceptibles de liberar sílice en disolución y dar lugar a reacciones álcali-sílice, aunque con distintas velocidades de reacción en cada caso. 50 La presencia de otros elementos químicos afecta también a la concentración de sílice disponible en las disoluciones de los poros de hormigón.…”