ORCID Profile
0000-0001-6911-0994
Current Organisation
University of California, Irvine
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Publisher: Springer Science and Business Media LLC
Date: 03-05-2018
DOI: 10.1038/S41467-018-04174-Z
Abstract: The time-dependent response of structural materials dominates our aging infrastructure’s life expectancy and has important resilience implications. For calcium-silicate-hydrates, the glue of cement, nanoscale mechanisms underlying time-dependent phenomena are complex and remain poorly understood. This complexity originates in part from the inherent difficulty in studying nanoscale longtime phenomena in atomistic simulations. Herein, we propose a three-staged incremental stress-marching technique to overcome such limitations. The first stage unravels a stretched exponential relaxation, which is ubiquitous in glassy systems. When fully relaxed, the material behaves viscoelastically upon further loading, which is described by the standard solid model. By progressively increasing the interlayer water, the time-dependent response of calcium-silicate-hydrates exhibits a transition from viscoelastic to logarithmic creep. These findings bridge the gap between atomistic simulations and nanomechanical experimental measurements and pave the way for the design of reduced aging construction materials and other disordered systems such as metallic and oxide glasses.
Publisher: Wiley
Date: 05-09-2019
DOI: 10.1111/JACE.16750
Abstract: Calcium‐silicate‐hydrates (C–S–H) gel, the main binding phase in cementitious materials, has a complex multiscale texture. Despite decades of intensive research, the relation between C–S–H's chemical composition and mesoscale texture remains experimentally limited to probe and theoretically elusive to comprehend. While the nanogranular texture explains a wide range of experimental observations, understanding the fundamental processes that control particles’ size and shape are still obscure. This paper strives to establish a link between the chemistry of C–S–H nanolayers at the molecular level and formation of C–S–H globules at the mesoscale via the potential‐of‐mean‐force (PMF) coarse‐graining approach. We propose a new thermomechanical load‐cycling scheme that effectively packs polydisperse coarse‐grained nanolayers and creates representative C–S–H gel structures at various packing densities. We find that the C–S–H nanolayers percolate at ~10% packing fraction, significantly below the percolation of ideal hard contact oblate particles and rather close to that of overlapping ellipsoids. The agglomeration of C–S–H nanolayers leads to the formation of globular clusters with the effective thickness of ~5 nm, in striking agreement with small angle neutron and X‐ray scattering measurements as well as nanoscale imaging observations. The study of pore structure and local packing distribution in the course of densification shows a transition from a connected pore network to isolated nanoporosity. Furthermore, the calculated mechanical properties are in excellent agreement with statistical nanoindentation experiments, positioning nanolayered morphology as a finer description of C–S–H globule models. Such high‐resolution description becomes indispensable when investigating phenomena that involve internal building blocks of globules such as shrinkage and creep.
Publisher: American Chemical Society (ACS)
Date: 02-03-2017
Publisher: Springer Science and Business Media LLC
Date: 08-09-2017
DOI: 10.1038/S41598-017-11146-8
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.
Publisher: American Chemical Society (ACS)
Date: 03-08-2017
Abstract: Intermolecular interactions control the collective behavior of colloidal clusters. The overwhelming majority of literature focuses on cohesive attributes of intermolecular forces as they govern the jamming process. However, the overlooked sliding component plays a critical role in the slow relaxation dynamics of colloidal aggregates and the emergence of discontinuous shear thickening in dense suspensions. Here, we use crystalline calcium-silicate-hydrate (C-S-H) as a model system to explore synergistic cohesive and sliding interactions. We use the free energy perturbation approach to reconstruct potential of mean force profile between two finite-sized nanolayers in an aqueous environment. We show that sliding free energy barriers decay exponentially as the separation distance increases. The characteristic length scale of the decay is related to the interface corrugation. We introduce a simple yet effective mechanism to capture the sliding behavior of colloids with surface roughness. Moreover, we develop a global free energy landscape model by juxtaposing cohesive and sliding interactions. This model enables us to measure the height of energy barriers, which is essential to elucidate deformation mechanism and dynamics of colloidal aggregates. For cohesive colloids, our approach predicts a sublinear relation between applied normal and shear stress at the onset of sliding that is in contrast to Amontons' laws of friction. We demonstrate that the sublinear trend is due to the adhesion and nature of soft contact at the nanoscale. The proposed framework provides a new route to draw a more realistic picture of intermolecular interactions in nanoparticulate systems such as geomaterials, cementitious systems, complex colloidal assemblies, and dense suspensions.
No related grants have been discovered for Mohammad Javad Abdolhosseini Qomi.