An Atomistic Model Describing the Structure and Morphology of Cu-Doped C-S-H Hardening Accelerator Nanoparticles

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.