Atomic structure of nanocrystalline and amorphous ASR products

Limited information about the atomic structure of amorphous and nanocrystalline alkali–silica reaction (ASR) products is available. Here, reactive molecular simulations were employed to construct a model of these products based on defective shlykovite. To introduce disorder into the models, various annealing temperatures (ranging from ambient to temperatures above silicate dissolution) were tested. Structures obtained from annealing at 700 K reproduced the main peaks observed experimentally in X-ray diffraction patterns of "ASR-P1" phase, a nanocrystalline or amorphous alkali–silica reaction product recently identified in synthetic and field concrete. Pair distribution functions and structure factors computed for this structure also compared well with previous data reported for "amorphous" ASR products. These results demonstrate that what was previously considered "amorphous" ASR products might be related to nanocrystalline defective shlykovite structures.