Initial sequence for alkali-silica reaction: Transport barrier and spatial distribution of reaction products

Alkali-silica reaction (ASR) is the result of complex chemical reactions. The exact sequence of ASR gel formation has yet to be fully understood. One promising hypothesis is that ASR gel starts to form only in localized regions where the availability of calcium is restricted. A transport barrier (C-S-H) around the aggregate has been hypothetically suggested. However, the existence of this physical barrier and the formation of ASR gel inside this barrier has been questioned because it has never been observed experimentally. This paper firstly presents a direct observation of the physical barrier, spatial distribution of reaction products, and crack formations in a reactive aggregate exposed to a model reactant system. Combined analyses using X-ray micro tomography and other chemical characterization techniques shows that ASR gel preferentially starts to form at localized areas covered by C-S-H, loosely packed reaction products with pores, and pre-existing defects.