Microstructural evolution of amorphous silica following alkali–silica reaction

Combining X-ray diffraction, NMR (CP-MAS and 29Si NMR-MAS) and diffuse reflectance infra-red Fourier transformed spectroscopy (DRIFTS), the local structural changes induced during the reactivity of amorphous (a-SiO2) under alkali–silica reaction (ASR) have been investigated. XRD patterns provide useful and new qualitative information concerning the reactivity of a-SiO2. A shift of the maximum of the halo towards the high values of 2θ is observed when the reaction progresses. NMR shows that ASR is accompanied in a-SiO2 by the diminution of Q4 species and by the formation of Q3 (and to a less extend to Q2) species. This importance played by Q3 species confirms the results obtained with natural alkali–silica gels. The formation of OH bonds observed by DRIFTS can be followed at ca. 950 cm−1 and confirms NMR results. The microstructural state of silica (here represented by the medium- and short-range order (MRO) of amorphous silica) plays a crucial role in governing the kinetics of the ASR.