Densification of the interlayer spacing governs the nanomechanical properties of calcium-silicate-hydrate

Abstract

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.

Nanolayered attributes of calcium‐silicate‐hydrate gels

“…Therefore, the possibility of strong uncertainties arising from the first two cases is negligible, unless all proposed models and potentials are erroneous. This is very unlikely, as we have shown the nanoscale models match a wide range of experimental observations . Therefore, our assumptions regarding the structure and force fields are not going to significantly affect the mesoscale models considerably.…”

“…43 The nanoscale structure of the C-S-H is yet the subject of intensive research. 6,10,11,44,45 Although far from perfect, the proposed molecular models based on the so-called defective Tobermorite description can adequately reproduce a wide range of chemo-physical properties and therefore will be the focus of this work. Herein, we apply the combinatorial molecular optimization method developed by Abdolhosseini Qomi et al 10 to adjust Ca/Si ratio.…”

Effects of CO2 and temperature on the structure and chemistry of C–(A–)S–H investigated by Raman spectroscopy

“…Understanding the structural properties of C-(A-)S-H provides insight into the physical and mechanic behavior of the cement based materials. 15,16 The synthesis temperature is an important parameter affecting key properties such as, density, chemical composition, microstructure and compressive strength of cement based materials. 12,17 Moreover, due to the exothermical hydration reactions, concrete structures may experience signicant temperature rise when used in large volume, e.g.…”

Pressure‐induced anomalous behavior of thaumasite crystal

“…The high‐pressure XRD (HPXRD) technique can provide valuable experimental data to compute intrinsic mechanical properties of crystalline minerals. For example, the structural responses of calcium aluminate hydrates and calcium silicate hydrates have been investigated using synchrotron X‐ray radiation . Additionally, HPXRD results can be directly combined with molecular modeling, which is also an effective tool for simulating a high‐pressure state and for studying the structure of crystals at an atomic level under various conditions.…”

“…For example, the structural responses of calcium aluminate hydrates and calcium silicate hydrates have been investigated using synchrotron X-ray radiation. [15][16][17] Additionally, HPXRD results can be directly combined with molecular modeling, 18,19 which is also an effective tool for simulating a high-pressure state and for studying the structure of crystals at an atomic level under various conditions. Obviously, molecular modeling is more valuable when used in conjunction with available experimental data, such as those from HPXRD.…”