The colloidal nature and osmotic potential of alkali-silica reaction products and their role for the ASR expansion mechanism

The chemical basics of the ASR are largely revealed and widely accepted, but the nature of the expansion mechanism is still not yet sufficiently well understood. Recent observations showed that ASR products could be considered as colloidal systems. In order to clarify if and to what extent this is the case and whether it could help to better understand the nature of the ASR products and the mechanism of ASR expansion in concrete, 10 ASR products of different composition, water content and synthesised at two temperatures (40 and 60 °C) were investigated over a period of 1.5 years. The ASR products were studied by means of NTA, SEM, ²⁹Si NMR, XRD and an osmotic cell test. The results show that ASR products contain particles of colloidal size, mainly between 50–600 nm and of different shape. The particles are unable to pass pores with a size smaller than themselves what represents a mechanism of semi-permeability in all concrete constituents with respective pore sizes, resulting in the Donnan effect and osmosis. The particles are irreversibly linked by the addition of Ca, which leads to a decrease in the particle concentration, the formation of crystalline phases and thus to a decrease in the osmotic potential of the ASR products. Based on the colloidal nature of the ASR products, expansion caused by ASR in concrete can be explained osmotically.