29Si NMR and small-angle X-ray scattering studies of the effect of alkaline ions (Li+, Na+, and K+) in silico-alkaline sols

Alkali-silica reactions (ASR) which occur in concrete can be simulated in laboratory by destabilization of silico-alkaline aqueous solutions by addition of calcium ions. The relevant features of the reaction depend on the nature of alkaline ions (Lif, Naf, or Kf) and on the silica/alkaline ratios which fix the distribution of the molecular species in the precursor solution. Si-29 NMR spectroscopy and small-angle X-ray scattering (SAXS) techniques were used to study the structure and size distribution of molecular and colloidal species in sols with different silica/alkaline molar ratio and several types of alkaline ions. Experimental SAXS curves were simulated using a simple structural model which assumes the sol to be composed of a multimodal set of weakly interacting hard spheres immersed in a liquid matrix. The results indicate the existence of strong interactions between alkaline ion and hydroxide and between alkaline ion and silicate species. The formation of large species (cyclic molecules and colloidal particles) is enhanced in solutions containing lithium. These results are consistent with those of previous investigations.