Mesoscale modeling of cement matrix using the concept of building block
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Mesoscale modeling of cement matrix using the concept of building block Denvid Lau1,2, Zechuan Yu1, Oral Buyukozturk2 1
Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. Email:[email protected], web site: http://bccw.cityu.edu.hk/denvid.lau/ 2
ABSTRACT Calcium silicate hydrate (C-S-H) gel is the cohesive phase in cement paste and critically controls the cement hydration. Atomistic models can reproduce reasonable structural and mechanical properties of C-S-H gel at the nano scale. However, the length and time scale of such all-atom modeling technique are restrained by limited computing power. Under this context, coarsegrained modeling technique emerges as a useful alternative for investigating cement paste at extended length and time scale. Here, we regard the building block of cement as ellipsoid and develop a coarse-grained model of cement matrix based on the Gay-Berne (GB) potential. Emphasis of the present paper is on the parameterization and interpretation of the GB potential formula. 1. INTRODUCTION Calcium silicate hydrate (C-S-H) gel is a porous and amorphous material that binds various components in cement paste. The mechanics of concrete is largely related to C-S-H gel, which possesses elusive structures.1 _ENREF_2At the sub-micro scale, the C-S-H gel can be described as a highly heterogeneous aggregation of disordered hydrated globules of diverse sizes.2 Down to the nano scale, the globules contain sandwich-like structures made of calcium silicate clay-like layers together with confined water molecules in between.3 Numerical models can provide insightful understanding of the structural and mechanical complexity of C-S-H gel.4 Among a series of modelling techniques, the molecular dynamics (MD) simulation is a useful tool to investigating the C-S-H gel at the nano scale. The all-atom (AA) MD simulation is a bottom-up approach that simulates the behavior of atoms and atomistic simulations of C-S-H gel can reproduce reasonable results corresponding to the experimental results.5 Such all-atom (AA) simulations are accurate, but very expensive in terms of the computing power required. The unaffordable computing power burden normally precludes the atomistic simulations at large length scale and long time scale. Under this context, an alternative simulation technique is needed in extending the length to hundreds of nanometers and the time to hundreds of nanoseconds. Coarse-grained (CG) simulations are an emerging technique for simulating large scale systems with high computational efficiency. Unlike the AA model which describes the system atom by atom, the CG model represents a group of atoms by one interactive site, which constitutes as the building block of the large system. When setting up CG models, there exist different grouping strategies. One strategy assumes that the building blocks are spherical rigid beads so that C-S-H gel can be mapped into
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