Shape effect of cement particles on the ionic diffusivity of hardened cement paste—a three-dimensional numerical investigation

https://doi.org/10.1016/j.conbuildmat.2020.118736Get rights and content

Highlights

  • Shape effect of cement particles on the ionic diffusivity of pastes is presented.

  • Sphericity is an important factor affecting the diffusivity of HCP.

  • Hydration age and w/c ratio can affect shape effects on diffusivity.

  • The intrinsic reason is microstructure, especially pore structure.

Abstract

A three-dimensional numerical investigation is presented to study the shape effect of cement particles on the ionic diffusivity of hardened cement paste (HCP), whose microstructure is generated based on the random packing models of spherical and polyhedral particles. The microstructure of HCP composed of four phases is first generated and then converted into a voxel-presented one. Based on the digitized microstructure, the diffusivity of HCP can be obtained by a lattice modelling. The reliability of simulations is validated by comparing to experimental data and the shape effect of cement particles on the diffusivity of HCP is discussed. The results indicate that the shape of cement particles significantly affects the ionic diffusivity of HCP due to its effect on the hydration degree: the cement particles with smaller sphericity contribute to the hydration of HCP, which reduces the ionic diffusivity of HCP. This effect decreases as the hydration age elapses and increases with the increasing water-cement ratio. The intrinsic reason causing this effect is illustrated from the microstructural view of HCP: the shape effect has an influence on the volume fractions of three diffusive phases and pore structures.

Introduction

Diffusion coefficient, one of significant parameters to evaluate the deterioration mechanisms of coastal or cold-regions concrete infrastructures, is closely associated with the morphological characters of concrete which possesses a complicated multi-scale structural property [1], [2], [3], [4], [5], [6], [7], [8]. The morphological characters of cement-based materials at different scale are closely related to the packing particles which are commonly assumed as spheres in most of early hydration models including the HYMOSTRUC3D model [9], the DuCOM model [10] and the μic model [11]. However, the fact that the shape of particles are non-spherical in reality results in a deviation of morphological characters based on the above hydration models. The random packing models of non-spherical particles (including ellipsoid [12], polyhedron [13], [14], superball [15], [16], [17], [18], and arbitrary-shaped particles [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]) have been developed to simulate the structures of cement-based materials, which is relatively closer to the actual situation.

The models mentioned above, coupling with specific algorithms (i.e., 3D lattice modelling, lattice Boltzmann method, and finite element method, etc), can provide us a more comprehensive understanding about the transport properties of multi-scale materials with and without the shape effect of particles. The transport properties of mortar and concrete at meso-scale without the shape effect of aggregates have been investigated by Kamali-Bernard et al. [29], [30] and Bentz et al. [31], respectively. The shape effect of aggregates on the transport properties of mortar and concrete at meso-scale have been studied in many literature [32], [33], [34], [35], [36]. For mortar, the shapes of aggregates discussed by Liu et al. [32] and Abyaneh et al. [33] including sphere, ellipsoid and polyhedron. For concrete, the shapes of aggregates involved in literature [34], [35], [36] including ellipse, triangle, square and the above mixed aggregates in 2D, as well as spheroid, ellipsoid and polyhedron in 3D. The above researches for mortar and concrete at meso-scale was merely conducted based on the packing structures of aggregates, and do not involve the cement hydration due to the limit of scale. The transport properties of HCP at micro-scale without the shape effect of cement particles have been studied by Liu et al. [37] and Zhang et al. [38]. Few studies are related to the shape effect of cement particles on the diffusivity of HCP considering the reason that the process of cement hydration make it more difficult to study the shape effect on transport property.

So far, Zhu [39] has revealed that the shape effect of cement particles has a significant impact on the cement hydration and microstructures of HCP based on the HYMOSTRUC model and random packing models of convex polyhedral particles. Liu et al. [27], [28] have investigated the shape effect of cement particles on the cement hydration, the capillary pore structures, and the microstructural characteristics of HCP. It is worth noting that the 3D lattice modelling, a useful numerical method, can be employed to assess the diffusion coefficients [32], [34], shrinkage [40], [41], and mechanical properties [42] of materials. Following the current researches, this paper is going to study the shape effect of cement particles on the diffusivity of HCP by the 3D lattice modelling.

In order to study the shape effect of cement particles on the diffusivity, the HCP is simulated based on the random packing model of spherical or polyhedral particles. The 3D microstructure of HCP composed of unhydrated cement cores (UCC), high-density calcium silicate hydrate (HD C-S-H), low-density calcium silicate hydrate (LD C-S-H) and large capillary pores (LCP) is generated by a hydration model, and then converted into a voxel-presented one. The diffusivity of HCP is predicted by the 3D lattice modelling and its reliability is validated by comparing to experimental diffusivities of HCP from literature. The shape effect of cement particles on the diffusivity of HCP is discussed, and the reason causing this effect is illustrated from the microstructural view of HCP.

Section snippets

Methodology

Three main parts are included in the process of simulation for the diffusivities of HCP: generation of microstructure, digitalization of microstructure and lattice modeling for diffusivity. First, a 3D microstructure for HCP, composed of UCC, HD C-S-H, LD C-S-H and LCP, is generated by a hydration model based on the random packing models of spherical or polyhedral particles. Subsequently, the microstructure is converted into a corresponding mesh represented by voxels, which are identified as

Validation

A number of experiments have been carried out about the diffusivity of HCP, which could be utilized to validate the reliability of simulation results. Patel et al. [57] have provided a comprehensive overview of existing experimental approaches (i.e., through-diffusion, in-diffusion, electro-migration, and electrical resistivity) to determine the effective diffusion coefficients of water saturated ordinary Portland cement-based materials. Because the 3D lattice modelling for diffusivity

Results and discussion

The shape effect of cement particles on the diffusivity of HCP will be discussed based on the above numerical modelling. Two main kinds of particles including sphere and polyhedrons were involved in the modelling. Polyhedrons discussed in this paper including tetrahedral, cubic, octahedral, dodecahedral and icosahedral particles. The above particles were characterized by sphericity. The w/c ratios in the modelling were set as 0.4, 0.5 and 0.6. For a given w/c ratio, the 3D microstructures of

Conclusions

In this paper, the shape effect of cement particles on the diffusivity of HCP is investigated. Spherical particles and polyhedral particles are used as the packing particles and characterized by the shape factor (i.e., sphericity). Based on the packing models of above particles, the 3D microstructures of HCPs, which are composed of UCC, HD C-S-H, LD C-S-H, and LCP, are first generated and then converted into voxel-presented ones. The diffusivities of HCPs are obtained from the digitized

Funding

This study was funded by the National Natural Science Foundation of China (Grant No. 51,978,241 and No. 51678219). Zhu gratefully acknowledge the project funded by China Postdoctoral Science Foundation (Great No. 2019 M651668). Chen gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51878152).

CRediT authorship contribution statement

Lin Liu: Conceptualization, Methodology, Software, Writing - review & editing, Supervision, Funding acquisition. Guanghui Tao: Software, Formal analysis, Investigation, Writing - original draft. Huisu Chen: Validation, Resources, Writing - review & editing, Visualization. Zhigang Zhu: Methodology, Resources, Data curation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the funding support by the National Natural Science Foundation of China (Grant No. 51978241 and No. 51678219). Zhu gratefully acknowledge the project funded by China Postdoctoral Science Foundation (Great No. 2019 M651668). Chen gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51878152).

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