Study of the structure of alkali–silica reaction gel by high-resolution NMR spectroscopy

The alkali–silica reaction is a deleterious chemical process that can occur in concrete. The product of the reaction is an amorphous silicate material with gel characteristics, whose high expansion properties may cause cracking in the matrix and in the discrete aggregate of particles, leading to severe deterioration of the concrete structure. Structural information of this gel at the atomic scale can provide critical information on how to develop appropriate repair of the affected structure. Samples of this gel, produced under in-service conditions in a large concrete dam, were studied by 29Si and 23Na high-resolution nuclear magnetic resonance spectroscopy, triple quantum magic angle spinning 23Na nuclear magnetic resonance, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The short-range atomic structure of the compound was determined as an amorphous potassium–hydroxide–silicate glass, with a Q3-like dominant silicate connectivity, having a silicate speciation highly disproportioned when compared with potassium–silicate glasses with the same K2O content. Sodium ions are mostly segregated from the bulk amorphous silicate network, forming crystal domains attributed to the trona compound (sodium sesquicarbonate, Na2CO3 · NaHCO3 · 2H2O). The structural picture at atomic scale obtained in this study gives support for double-layer models of the expansive properties of the alkali–silica reaction gel.