Calciuam-silicate-hydrate (C-S-H) is the principal binding phase in modern concrete. Molecular simulations imply that its nanoscale stiffness is ‘defect-driven’, i.e., dominated by crystallographic defects such as bridging site vacancies in its silicate chains. However, experimental validation of this result is difficult due to the hierarchically porous nature of C-S-H down to nanometers. Here, we integrate high pressure X-ray diffraction and atomistic simulations to correlate the anisotropic deformation of nanocrystalline C-S-H to its atomic-scale structure, which is changed by varying the Ca-to-Si molar ratio. Contrary to the ‘defect-driven’ hypothesis, we clearly observe stiffening of C-S-H with increasing Ca/Si in the range 0.8 ≤ Ca/Si ≤ 1.3, despite increasing numbers of vacancies in its silicate chains. The deformation of these chains along the b-axis occurs mainly through tilting of the Si-O-Si dihedral angle rather than shortening of the Si-O bond, and consequently there is no correlation between the incompressibilities of the a- and b-axes and the Ca/Si. On the contrary, the intrinsic stiffness of C-S-H solid is inversely correlated with the thickness of its interlayer space. This work provides direct experimental evidence to conduct more realistic modelling of C-S-H-based cementitious material.

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 reaction mechanism of alkali-silica reaction (ASR) is poorly understood due to the difficulties to directly characterize ASR products in concrete. In this study, ASR products with initial Ca/Si of 0.25 and (K+Na)/Si ratio 0.5 with different K/Na ratios are synthesized at 80 o C. The synthesized ASR products are characterized by different techniques, also the solution chemistry is analyzed and saturation indices are calculated. The results show that crystalline and nano-crystalline phases are formed in the presence of both alkalis and calcium. No ASR product is present in the absence of calcium. All synthesized crystalline ASR products highly resembles the crystal structure of shlykovite, indicating that a substitution of K in shlykovite by Na can occur. Its silicate sheet structure has strong similarity to the ASR products formed in concrete according to Raman spectra, while some differences are observed in their morphologies and basal peak of the XRD patterns.

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.

Properties of organic/inorganic composites can be highly dependent on the interfacial connections. In this work, molecular dynamics, using pair-potential-based force fields, was employed to investigate the structure, dynamics, and stability of interfacial connections between calcium-silicate-hydrates (C-S-H) and organic functional groups of three different polymer species. The calculation results suggest that the affinity between C-S-H and polymers is influenced by the polarity of the functional groups and the diffusivity and aggregation tendency of the polymers. In the interfaces, the calcium counterions from C-S-H act as the coordination atoms in bridging the double-bonded oxygen atoms in the carboxyl groups (-COOH), and the Ca-O connection plays a dominant role in binding poly(acrylic acid) (PAA) due to the high bond strength defined by time-correlated function. The defective calcium-silicate chains provide significant numbers of nonbridging oxygen sites to accept H-bonds from -COOH groups. As compared with PAA, the interfacial interactions are much weaker between C-S-H and poly(vinyl alcohol) (PVA) or poly(ethylene glycol) (PEG). Predominate percentage of the -OH groups in the PVA form H-bonds with inter- and intramolecule, which results in the polymer intertwining and reduces the probability of H-bond connections between PVA and C-S-H. On the other hand, the inert functional groups (C-O-C) in poly(ethylene glycol) (PEG) make this polymer exhibit unfolded configurations and move freely with little restrictions. The interaction mechanisms interpreted in this organic-inorganic interface can give fundamental insights into the polymer modification of C-S-H and further implications to improving cement-based materials from the genetic level.

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...

Tricalcium aluminate (cement clinker phase), gypsum, katoite, ettringite, and calcium monosulfoaluminate hydrate (abbreviated as kuzelite) are the major minerals in the hydration reaction of tricalcium aluminate in the presence of gypsum and have critical impacts on the kinetics and thermodynamics of early-age cement hydration mechanisms. Here, spectroscopic analysis of these minerals is conducted using scanning transmission X-ray microscopy (STXM). Their Ca L 2,3-edge near edge X-ray absorption fine structure (NEXAFS) spectra are measured and correlated to the known Ca coordination environments. The results indicate that these minerals have unique Ca environments that can be differentiated from one another based on the intensities and positions of the absorption peaks at 346.5-348.5 and 350.5-351.5 eV. It is concluded that Ca in tricalcium aluminate (cubic and orthorhombic polymorphs) and katoite is in cubic-like coordination with negative 10Dq, whereas Ca is in an octahedral-like coordination with positive 10Dq in ettringite, gypsum, and kuzelite. For tricalcium aluminate, the Ca atoms in both polymorphs are in similar chemical environments with slightly more distortion in the orthorhombic polymorph. As a common issue in STXM experiments, absorption saturation of NEXAFS spectra is also investigated. It is demonstrated that the optical density difference between pre-and post-edge absorption levels provides a reliable indication of the sample thickness in the systems studied. The present work provides a reference for the STXM study of the calcium (sulfo)aluminate reactions in cement hydration and natural aqueous environments, and in the former case, provides a more complete understanding of a system that may serve as a low-C alternative to Portland cement.

The rheology of modern Portland cement (PC) concrete critically depends on the correct dosage of gypsum (calcium sulfate hydrate) to control the hydration of the most reactive phase-tricalcium aluminate (C 3 A). The underlying physio-chemical mechanism, however, remains unsolved mainly due to the lack of high-spatialresolved and chemistry-sensitive characterization of the C 3 A dissolution frontier. Here, we fill this gap by integrating synchrotron-radiation based crystallographic, photon-energy-dependent spectroscopic and high-resolution morphological studies of the C 3 A hydration product layer. We propose that ettringite (6CaO•Al 2 O 3 •SO 3 •32H 2 O) is the only hydration product after the initial reaction period and before complete gypsum dissolution. We quantify the 2D and 3D morphology of the ettringite network, e.g. the packing density of ettringite at various surface locations and the surface dissolution heterogeneity. Our results show no trace of a rate-controlling diffusion barrier. We expect our work to have significant impact on modeling the kinetics and morphological evolution of PC hydration.