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The reaction between certain reactive components of aggregates (like opal and metaestable silica) and concrete pore solution is well documented. Nevertheless, in this study it has been shown that some aggregates, like granite, could develop a rapid or slow alkali-silica reaction (ASR) depending on the deleterious component involved. Mortar bars were cast with two granitic aggregates extracted from concrete cores drilled in two Spanish Dams affected by ASR, being classified as granitic rocks. The main difference between them is the reactive component: microcrystalline quartz in one case and strained and microcracked quartz in the other case.  A petrographic examination was carried out in the mortar bars. Thin sections were cut and the alkalisilica gel was stained for an easier detection. Then, the thin sections were examinated with a stereomicroscope comparing the differences in the progress of the ASR for both aggregates.  It can be concluded that the main mechanism of formation and storage of gel is associated to the micro-cracks instead of the subgrain boundaries.<br><br>La reacción entre los componentes de la fase intersticial del hormigón y áridos con minerales como el ópalo o la sílice metaestable, se encuentra bien documentada. Sin embargo, en este estudio, se ha detectado que dentro de un mismo tipo de roca, como es el granito, puede haber diferencias en el tipo de reacción (lenta o rápida) dependiendo del componente reactivo que aparezca en la misma.  Se han fabricado barras de mortero con los áridos extraídos de dos presas españolas afectadas por la reacción álcali-sílice. Estos áridos son rocas graníticas y la diferencia entre ambos reside en que una de las muestras contiene cuarzo microcristalino como componente reactivo y, la otra, cuarzo deformado.  Mediante el estudio petrográfico de barras de mortero y el teñido del gel álcali-sílice se ha podido observar la evolución y progreso de la reacción para cada uno de los componentes reactivos.  La principal conclusión que se puede extraer de este estudio es que la formación y almacenamiento del gel se asocia a microfracturas en lugar de a límites de grano

Comparative study of the alkali-silica reaction (ASR) in granitic aggregates