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.

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (CA) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in CA dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of CA occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto CA, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO on the partially dissolved CA solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for CA dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon CA dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated CA and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

PJM 2018, 'The chemistry and structure of calcium (alumina) silicate hydrate: A study by XANES, ptychographic imaging, and wide-and small-angle scattering', Cement and Concrete Research. https://doi.Abstract 27 28 Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended 29 cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to 30 optimizing concrete properties such as compressive strength and durability; yet questions 31 remain around the coordination environments of Ca and Al in its structure with various 32 chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6-1.6, 33 Al/Si = 0-0.1, and equilibrated at 7-80°C are studied by nanoscale soft X-ray 2 35 the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in 36 the interlayer of C-(A-)S-H are highly distorted from an ideal octahedral coordination.37 Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-38 (A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain 39 polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake 40 in the bridging and/or cross-linked sites, as well as a contraction of Si-O bond lengths. C-41 (A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized 42 platelets with comparable thickness regardless of Ca/Si or Al/Si at 7-50°C. Coarser C-43 (A-)S-H foils occur at 80°C and higher Al/Si ratios relative to lower temperatures and Al 44 content. 45 46 47 Keywords 48 Temperature, calcium-silicate-hydrate, third aluminate hydrate, blended cement, C-A-S-49 H 50 51 1.Introduction 52 53 Calcium (alumino)silicate hydrate (C-(A-)S-H) † is the main binding phase in blended 54 cement concrete[1]. The Ca/Si ratio in calcium silicate hydrate (C-S-H) in hydrated 55 Portland cement (PC) is ~1.7 [2]. The addition of supplementary cementitious materials 56 (SCMs), e.g., fly ash and blast furnace slag, leads to the uptake of Al and a decrease in 57 Ca/(Si+Al) ratio in this phase [3, 4]. C-(A-)S-H equilibrated at room temperature is † Cement chemistry shorthand notation is used throughout the text: A, Al 2 O 3 ; C, CaO; H, H 2 O; S, SiO 2 ; and , SO 3 . S 3 58 structurally analogous to defective tobermorite, which contains calcium oxide polyhedra 59 sheets flanked with "dreierketten" -tetrahedral (alumino)silicate chains -on one side and 60 counter-ions (e.g., Ca and alkalis) and water in an interlayer on the other [5-9]. At 61 Ca/Si<0.6-0.8, long silicate tetrahedral chains occur, which predominantly consist of 62 repeating units of one bridging site (Q 2 B ) ‡ connected to two paired silicate tetrahedral 63 sites (Q 2 P ) on either side. At higher Ca/Si ratios >~1.0 these chains are significantly 64 shorter albeit structurally similar, and have varying degrees of vacant tetrahedra in 65 bridging sites [10]. 66 67 Aluminum incorporated into these chains occur in tetrahedral AlO 4 sites. Five-fold 68 coordinated aluminum (Al V...

Calcium (alumino)silicate hydrate (C-(A-)S-H) is the critical binding phase in modern Portland cementbased concrete, yet the relationship between its structure and stoichiometry is not completely understood. In this study, Raman spectroscopy is used to analyze the effects of varying Ca/Si molar ratio (0.6-1.6), Al/Si molar ratio (0.0-0.1), and synthesis temperature (7-80 C) on the chemical composition and atomic configuration of C-(A-)S-H. The experimental results indicate that increasing Ca/Si molar ratio produces less cross-linked C-(A-)S-H structures, while the addition of Al into the system increases the long-range order of its chain-like structure. Furthermore, increasing the synthesis temperature leads to the formation of more polymerized structures, especially in the Al-containing samples. The Raman spectra also suggest the formation of vaterite in C-S-H samples synthesized at low temperatures.Finally, this study reveals that uptake of atmospheric CO 2 in C-S-H and C-A-S-H favors the formation of long-range ordered chain-like structures.