Abstract
The incorporation of Al and increased curing temperature promotes the crystallization and cross-linking of calcium (alumino)silicate hydrate (C-(A-)S-H), which is the primary binding phase in most contemporary concrete materials. However, the influence of Al-induced structural changes on the mechanical properties at atomistic scale is not well understood. Herein, synchrotron radiation-based high-pressure X-ray diffraction is used to quantify the influence of dreierketten chain cross-linking on the anisotropic mechanical behavior of C-(A-)S-H. We show that the ab -planar stiffness is independent of dreierketten chain defects, e.g. vacancies in bridging tetrahedra sites and Al for Si substitution. The c -axis of non-cross-linked C-(A-)S-H is more deformable due to the softer interlayer opening but stiffens with decreased spacing and/or increased zeolitic water and Ca2+ of the interlayer. Dreierketten chain cross-links act as ‘columns’ to resist compression, thus increasing the bulk modulus of C-(A-)S-H. We provide the first experimental evidence on the influence of the Al-induced atomistic configurational change on the mechanical properties of C-(A-)S-H. Our work advances the fundamental knowledge of C-(A-)S-H on the lowest level of its hierarchical structure, and thus can impact the way that innovative C-(A-)S-H-based cementitious materials are developed using a ‘bottom-up’ approach.
Highlights
It was previously reported that Al-incorporation does not change the mechanical properties of nano-crystalline C-A-S-H in alkali-activated slag (AAS) cement (38 ± 3 GPa) relative to C-S-H(I) (38 ± 7 GPa)[25]; the differences in the sample preparation conditions employed for each material and the lack of chemical crystallographic characterization shed doubt on the reliability of this conclusion
Starting from defective tobermorite structures[26,27,28,29,30,31], simulations of C-(A-)S-H have been reported, there exist clear missing gaps that restrict its improvement: (a) a lack of direct experimental tests to validate the simulated mechanical properties at the atomistic scale; (b) to date, focus has been directed on the structural change but limited work has been performed on its influence to the mechanical properties[30,31]; and (c) the published large volume of classic molecular dynamics study may not provide information as accurate as ab initio calculation does, in terms of investigating the interatomic bonding, especially in the interlayer space[32,33]
C-(A-)S-H samples studied here are of the same Ca to
Summary
Reported for the first time, we used samples synthesized at 80 °C to experimentally assess the influence of Al-induced crystallographic changes (e.g. dreierketten chain cross-linking) on the mechanical properties of synthetic nano-crystalline C-(A-)S-H. Their lattice-axial and volumetric responses to hydrostatic loading (up to ~10 GPa) were determined by synchrotron radiation-based HP-XRD, which were used to calculate the bulk moduli values. The results of this work provides fundamental information for understanding the structure and mechanical behavior of C-(A-)S-H at the lowest level of its hierarchical structure, which is essential to establish a predictive multi-scale model of modern cementitious materials
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