The natural pozzolana ‘Rhenish trass’ and its effect on ASR in concrete
ABSTRACT: Industrial byproducts and natural pozzolanas are increasingly being used as supplementary cementitious materials, the latter one with volcanic ashes as the most prominent group. Despite the fact that volcanic pozzolanas were used since Roman times, not much is known about their role in influencing the long term durability of concrete. The paper presents results of the volcanic pozzolana ‘Rhenish trass’, which is quarried in western Germany and its role during an alkali silica reaction (ASR). Field data and laboratory experiments indicate that Rhenish trass releases alkalis into the pore solution but also changes cement paste chemistry and alkali binding behavior. There seems to be a sensitive balance between alkali release and increase of alkali binding capacity, both mechanisms crucial for the course of an ASR in concrete.