Experimental and molecular dynamics modeling studies of interlayer swelling: water incorporation in kanemite and ASR gel

Abstract

Molecular dynamics (MD) modeling is an effective tool for studying the structure, dynamics and energetics of cement materials at the molecular level, and is especially useful when used in conjunction with experimental data that allows detailed definition of the systems to be modeled. Here we present a combined MD and experimental study of the swelling behavior of kanemites (hydrous alkali layered silicates) and alkali silicate hydrate (A-S-H) gels that leads to testable hypotheses concerning the mechanisms of water incorporation into the gels produced during the alkali silica reaction (ASR) in concrete. The MD computational results show that entry of water into the interlayer spaces of kanemite [Na,K)HSi2O5·nH2O] is structurally and energetically limited and, thus, suggest that expansion of A-S-H gels produced during the alkali silica reaction (ASR) is due principally to incorporation of water molecules between nano-particles, rather than within kanemite-like interlayer galleries. Kanemite-like volumes appear to be a significant component of A-S-H gels with compositions in the range observed for in-service concrete. XRD data suggest that these nano-particles have coherent diffraction lengths of the order of 10 nm perpendicular to the silicate layers. The MD-computed structures of Na- and K-kanemite are in excellent agreement with those determined from X-ray diffraction data. Likewise, the computed energetics of water sorption are in excellent agreement with the observed X-ray diffraction, water sorption, TGA/DTA, and29Si NMR data for kanemites and A-S-H. The water sorption and TGA/DTA behaviors of the kanemites and A-S-H gels show many similarities but also significant differences.