Dielectric properties of the pore solution in cement-based materials

https://doi.org/10.1016/j.molliq.2020.112548Get rights and content

Highlights

  • Dielectric response of multi-component aqueous solutions is quantified.

  • Age-dependent composition leads to an age-dependent dielectric response.

  • We quantify the repercussions of the effects of the age-dependence of dielectric response on the macroscale response.

Abstract

Composition changes in the pore solution affect significantly the dielectric response of porous materials? The presence of ions is recognized to affect the dielectric response of electrolytes in a complex way that depends on the ion-water and ion-ion interactions and respective concentrations. Pore solutions in cement-based materials are complex and age-dependent exhibiting different ion-pair states, which make them an interesting candidate for the study of the complex pore solution found in geomaterials in general. In cement-based materials, the measurement of dielectric properties is used for non-destructive monitoring and assessment of concrete structures conditions and to unravel details of the hierarchical pore structure of the material, notably at early-age when the microstructure of the material changes significantly. Here, we study the dielectric properties of bulk aqueous solutions representative of the pore solution found in cement-based materials using molecular dynamics simulations. Broadband dielectric spectra and frequency-dependent conductivities are provided. We show that the changes in the ionic composition of the pore solution due to the aging of the material translate in significant changes in the dielectric response: the static dielectric constant varies from 55.2 to 73.9. A mean-field upscaling strategy is deployed to compute the dielectric response at the cement-paste scale. Our results can be used in the interpretation of high frequency electromagnetic methods, such as ground-penetrating radar, enhancing the quality of interpretation and improving the confidence in the corresponding results.

Introduction

High Frequency Electromagnetic (HF-EM) method [[1], [2], [3]], as well as man-made geomaterials such as cementitious materials (e.g. [[4], [5], [6], [7], [8]]) have been deployed in the last decades to assess water content of soil and rocks. Among these methods, one can quote L-band remote sensing [9], Ground Penetrating Radar (GPR) [10], capacitive probe [11], etc. The success of such methods is caused by the dipolar character of the water molecules resulting in a high permittivity in comparison to other phases such as solid particles or gas voids. However, evidence suggests that the frequency-dependent dielectric permittivity contains far more information than water content only: structure [12], density [13], mineralogy, cohesion forces on the pore solution [14] and even material strength may leave distinct signatures that could potentially be quantified. Nevertheless, despite its great success, HF-EM methods are still confronted with theoretical challenges. Interaction of the pore solutions and solid phases leads to strong contributions to the EM properties. These interactions are not well understood at the moment: theoretical studies are urgently needed.

As for other physical properties [[15], [16], [17], [18]], micromechanics can be applied to upscale the dielectric permittivity of heterogeneous materials [19,20]. However, there is a lack of important information concerning the behavior of the constituent phases, and it is this information that is required to provide reliable predictions of the effective dielectric permittivity of nanoporous materials. Dielectric properties are recognized to be dependent on pore size [21,22] and pore solution composition [[23], [24], [25]]. The relative static dielectric permittivity ϵ(0) of aqueous solutions are in general one order of magnitude larger (e.g. for bulk water is [26]ϵ(0)=80.103 at 20°C ) than the dielectric permittivity of solid phases. Therefore, the precise quantification of the dielectric response of the pore solution is crucial to upscale the dielectric properties and to interpret the experimental spectra of porous materials.

In cement-based materials, the composition of the pore solution is crucial to the stability and assemblage of phases in these materials [27]. Therefore, cement hydration processes and chemical alterations of cement-based materials (due, for instance, to durability issues) are closely related to the pore solution composition. The composition of the pore solution depends on the composition of the binder (e.g. ordinary Portland cement, alternative cements, and Supplementary Cementitious Materials (SCM)), age and temperature [[27], [28], [29]]. Atomistic simulations have been successfully used to unveil the details of water interaction in cement-based materials, specially in calcium silicate hydrates (e.g.[[30], [31], [32], [33], [34], [35]]). Recently, the authors have studied for the first time the dynamics and structure of bulk aqueous solutions representative of the pore solution found in cement-based materials through molecular dynamics simulations [36]. The repercussions of the dynamics and structure differences observed in these pore solution on the dielectric response are still to be quantified.

Here, the dielectric properties of the pore solution of cement-based materials are computed taking into account the variability of ionic concentrations in aqueous solutions. To do so, we perform MD simulations to compute the dielectric response of the pore solutions. The composition of the pore solutions, atomic configurations and force fields, which are recalled in this article, are taken from Honorio et al. [36]. For the first time, the dielectric spectra of cement-based materials pore solutions are computed accounting for the age-dependent composition variations. We observe that the Cole-Cole equation fits well the dielectric spectra obtained for all pore solutions. Finally, we quantify the effect of the variability of the dielectric response of the pore solution on the effective response of cement-based materials using Monte Carlo Micromechanics [37]. Our results contribute to a better understanding of the electromagnetic behavior of cement-based materials and can be readily used in the interpretation of dielectric probing of cement-based materials, enhancing the performance of HF-EM testing and improving the confidence in the corresponding results.

Section snippets

Dielectric response and conductivity from molecular simulations

For a system with n particles i with charge qi, the total system polarization P is defined as the sum of the polarization (or dipole moment) μi(t) of each particle i at time t [38]: P(t)=i=1nμi(t)=i=1nqiri(t)where ri(t) is the position of the (center of) particle i.

The total system polarization P is related to the electric field E via the complex frequency-dependent dielectric susceptibility χ(f) = χ′(f) − ″(f) by P(f)=χ(f)ϵ0E(f)where ϵ0 is the vacuum permittivity. The dielectric

Results and discussion

Fig. 2 (a)–(e) shows the water solvation shells of the ions present in the pore solutions PC1–PC7. As discussed previously [36], cation-water and hydroxide-water structuration is similar in all pore solutions studied; whereas sulfates-water pair exhibits structure variations according to the pore solution. We compute the ion pair states following contact ion pairs (CIP), single solvent-separated ion pairs (SIP) and doubly solvent-separated ion pairs (DSIP) configurations depicted in Fig. 2 (f).

Conclusions

The dielectric spectra and frequency-dependent conductivities of bulk aqueous solutions representative of the pore solution found in cement-based materials were computed for the first time using classical molecular dynamics simulations. Our main conclusions are as follows:

  • Classical molecular dynamics simulations provide a powerful tool to compute the dielectric response of pore solutions with complex composition. Pore solutions in cement-based materials are cement type, age, and

CRediT authorship contribution statement

Tulio Honorio: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Thierry Bore: Conceptualization, Methodology, Writing - original draft, Validation. Farid Benboudjema: Conceptualization, Methodology, Supervision. Eric Vourc’h: Conceptualization, Methodology, Writing - original draft, Supervision. Mehdi Ferhat: Conceptualization, Methodology.

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.

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