Abstract
Calcium silicate hydrate (C-S-H) is the main binding phase in Portland cement. The addition of C-S-H nanoparticles as nucleation seeds has successfully been used to accelerate the hydration process and the precipitation of binding phases either in conventional Portland cement or in alternative binders. Indeed, the modulation of the hydration kinetics during the early-stage dissolution-precipitation reactions, by acting on the nucleation and growth of binding phases, improves the early strength development. The fine-tuning of concrete properties in terms of compressive strength and durability by designed structural modifications can be achieved through the detailed description of the reaction products at the atomic scale. The nano-sized, chemically complex and structurally disordered nature of these phases hamper their thorough structural characterization. To this aim, we implement a novel multi-scale approach by combining forefront small-angle X-ray scattering (SAXS) and synchrotron wide-angle X-ray total scattering (WAXTS) analyses for the characterization of Cu-doped C-S-H nanoparticles dispersed in a colloidal suspension, used as hardening accelerator. SAXS and WAXTS data were analyzed under a unified modeling approach by developing suitable atomistic models for C-S-H nanoparticles to be used to simulate the experimental X-ray scattering pattern through the Debye scattering equation. The optimization of atomistic models against the experimental pattern, together with complementary information on the structural local order from 29Si solid-state nuclear magnetic resonance and X-ray absorption spectroscopy, provided a comprehensive description of the structure, size and morphology of C-S-H nanoparticles from the atomic to the nanometer scale. C-S-H nanoparticles were modeled as an assembly of layers composed of 7-fold coordinated Ca atoms and decorated by silicate dimers and chains. The structural layers are a few tens of nanometers in length and width, with a crystal structure resembling that of a defective tobermorite, but lacking any ordering between stacking layers.
Highlights
Cementitious binders based on ordinary Portland cement (OPC) are complex, multiphase and nanostructured systems resulting from the reaction of a mixture of finely ground calcium silicates, aluminates and alumino-ferrites with water
The system complexity further increases when OPC is partially replaced by supplementary cementitious materials (SCMs), both as naturally occurring materials or as industrial by-products that take part in the pozzolanic reactions, or by formulating alternative OPC-free binders as the alkaliactivated materials (AAMs) [5,6,7]
The Me-S-H/PCE wide-angle X-ray total scattering (WAXTS) pattern showed few and broad peaks compatible with those produced by a nanocrystalline tobermorite structure (Figure S1)
Summary
Cementitious binders based on ordinary Portland cement (OPC) are complex, multiphase and nanostructured systems resulting from the reaction of a mixture of finely ground calcium silicates, aluminates and alumino-ferrites with water. The final product is a hardened paste constituted by several hydration products, the most abundant of which, and responsible for the chemical and mechanical properties of the binder, is a poorly crystalline calcium-silicate-hydrate commonly known as C-S-H [1]. The complexity of this system arises from the hydration of multiple phases, differing in chemical and mineralogical composition. The particle size, the modulation of water-to-cement ratio and the use of chemical admixtures to modify the particle dispersion and the kinetics of the reaction during hydration directly influence the workability of the binder as well as the strength development and the microstructure of the hardened material [1,2,3,4]. In blended cements and alternative binders, the chemical-mineralogical and microstructural features of nanocrystalline/amorphous hydration products (namely C-A-S-H, C-N-A-S-H, N-A-S-H, . . . ), which incorporate
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