The chemistry and structure of calcium (alumino) silicate hydrate: A study by XANES, ptychographic imaging, and wide- and small-angle scattering

Abstract Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to optimizing concrete properties such as compressive strength and durability; yet questions remain around the coordination environments of Ca and Al in its structure with various chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6–1.6, Al/Si = 0–0.1, and equilibrated at 7–80 °C is studied by nanoscale soft X-ray spectroscopy at the Ca L2,3- and Si K-edges. Highly distorted CaO7 complexes occur in the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in the interlayer of C-(A-)S-H is highly distorted from an ideal octahedral coordination. Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-(A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake in the bridging and/or cross-linked sites, as well as a contraction of Si O bond lengths. C-(A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized platelets with comparable thickness regardless of Ca/Si or Al/Si at 7–50 °C. Coarser C-(A-)S-H foils occur at 80 °C and higher Al/Si ratios relative to lower temperatures and Al content.

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