Investigating the alkali-silica reaction of recycled glass aggregates in concrete materials

Application of crushed recycled glass in concrete materials can offer significant economical and environmental benefits provided that the alkali-silica reaction (ASR) of glass in concrete is properly controlled. Previous work on the use of glass sand in mortars shows that the reactivity of glass is influenced by its particle size as mortars containing finer glass sand show reduced ASR expansions. This may be counterintuitive since ASR is considered to be a surface reaction and should accelerate by increasing the surface area (i.e., reducing the size) of reactive aggregates. This paper presents a more in-depth investigation of the size-effect phenomena using scanning electron microscopy (SEM)/energy dispersive spectroscopy imaging of mortars containing different size glass particles. The SEM micrographs reveal that ASR does not occur at the glass-paste interface; rather, it occurs inside microcracks that exist inside glass particles which were generated during the glass bottle crushing operations. Larger size glass particles show larger and more active microcracks which render their high alkali-silica reactivity. At its interface with cement paste, glass shows evidence of pozzolanic reaction which leads to the formation of nonexpansive CSH. For particles smaller than #30 sieve (0.6 mm), the intraparticle ASR is minimal and only the interfacial pozzolanic reaction proceeds. This agrees well with the results of ASTM C1260 tests showing that mixed color glass aggregate smaller than #30 sieve does not produce deleterious ASR expansions in mortars even when no ASR suppressant (e.g., fly ash) is used.