Elsevier

Construction and Building Materials

Volume 76, 1 February 2015, Pages 158-167
Construction and Building Materials

Innovative application of silicon nanoparticles (SN): Improvement of the barrier effect in hardened Portland cement-based materials

https://doi.org/10.1016/j.conbuildmat.2014.11.054Get rights and content

Highlights

  • Amorphous silicon nanoparticles synthesized by sol–gel promotes the ionic transport of Si4+.

  • The electric field forced the SN to move toward the cement matrix.

  • Pores blocking, caused by the ingress of SN, was supported by N2–Physisorption results.

  • The resistance to carbonation was improved by the block pore effect.

  • SN treatment is a promising method since improves the durability on concrete’s hardened state.

Abstract

The present study investigates the introduction SN prepared using the sol–gel method into a hardened Portland cement matrix by means of an electric field. The SNs were prepared from Si(OC2H5)4 and C2H6O with a mole fraction of 0.1051 in an alkaline medium. The XRD and TEM analyses confirmed that an irregular, nanometric (20–30 nm) amorphous material was obtained. FTIR analysis showed the characteristic bands of amorphous silicon. For SN migration, a suspension of 0.1% weight was prepared and placed in a cell in contact with a mortar specimen (50 mm diameter, 50 mm length). A voltage was applied (10–20 V, DC) for 4 h followed by a 54 days period of immersion in tap water. During this period, resistivity measurements were carried out until a significant change was observed in the specimens. Subsequently, the specimens were exposed to an atmosphere with CO2 for 7 days. The tests performed showed both an increase in the electrical resistivity and a significant decrease in the carbonation depth for all mortars treated with SN. The latter conclusion is confirmed by SEM images, which show the evolution of the microstructure of the matrix, due to the presence of the silicon ions.

Introduction

Reinforced concrete (RC) structures deteriorate in the short to long term due to its interaction with the environment, thus causing loss of durability [1], [2], [3], [4]. Discussing the durability of RC structures is a complex matter due to the existence of external agents responsible for propagating different types of deterioration in both concrete and steel [5]. Concrete permeability is a property directly involved in all deterioration mechanisms affecting RC structures. This permeability derives from the porous nature of the concrete, and it has an effect on the entrance of aggressive agents (such as CO2, Cl and SO4) that may lead to the accelerated degradation of RC structures. Worldwide, it is estimated that 40% of all RC structures require some type of maintenance and/or repair, whereas the remaining structures must be replaced altogether [6]. Steel reinforcement corrosion is one of the main causes of maintenance and repairs worldwide [1].

Concrete permeability is adversely affected by a thin concrete cover, poor compacting, high water-to-cement or water-to-cementitious-material ratios, low cement content, and poor curing conditions. Therefore, the durability of RC structures is subject to considerable changes and, they often start showing signs of decay much earlier than expected for the structure’s service life. This behavior has led to interest in the development of innovative technologies to reduce the permeability of hardened concrete and to delay or inhibit the interaction of environmental agents with the cement matrix and steel reinforcements.

Regardless of the quality of the design or installation of a concrete structure, once it has been built and put in service, few procedures are available to reduce the high permeability of poor quality concrete. The use of coatings and waterproofing on the concrete surface has proven over time to be an inefficient option. Alternatives using electric fields to transport passive pore-blocking agents have been developed and patented [7], [8], [9]. In addition, the other uses of electric field for concrete rehabilitation have been studied, i.e., electrochemical chloride removal and electrochemical injection of corrosion inhibitors [10], [11], [12], [13]. Recently, the transport of SN toward the interior of the cement matrix by electrophoresis through the application of an electric field has been proposed [14], [15], [16], [17], [18].

Electrophoresis makes it possible to employ complex compounds or phases of functional materials (SiO2, ZnO, Fe2O3) and to effect the migration of these nanoparticles into porous materials. A two-stage reaction mechanism has been proposed to describe this process when silica-based particles are used: the first stage is flocculation, followed by a reaction in the second stage to produce a dense gel phase that blocks the pores; this resulting gel phase has been shown to contain calcium silicate hydrates (Csingle bondSsingle bondH) [16].

The literature includes a number of studies in which a mixture of nano-silica and cement was successfully utilized in the elaboration of the cementitious mixture. Flores and Hui [19], [20], for example, utilized nano-silica synthesized by the sol–gel method in the fabrication of Portland cement mortars and observed improved mechanical properties relative to reference mortars. Analysis of the plasters with nano-silica additives demonstrated that this improvement was due mainly to the behavior of the nanoparticles, which accelerated hydration reactions by generating nucleation sites for the formation of Csingle bondSsingle bondH. A very compact microstructure was generated by increasing the Sisingle bondOsingle bondSi bonds between layers of the Csingle bondSsingle bondH structure in a process similar to a pozzolanic reaction with the hydrating products of Portland cement. Recently investigations [21], [22], [23] have reported the use of nano-silica for improved concrete properties (especially compressive strength and durability) and drawn some discussion on the possible reaction mechanisms during hydration of cement in presence of this nanoparticles.

Currently, silica-based nanoparticles are among the most widely employed charge carrier materials because, at the nanoscale, they are capable of significantly modifying chemical properties [15]. Cárdenas [16] investigated the application of the electro-kinetic treatment of silica nanoparticles and silica doped with aluminum oxide; the results indicate that the treatment was effective for maintaining the pH and chloride content below threshold levels. Indirectly, it was also demonstrated that this method reduces permeability and, consequently, the ionic transport of chlorides within the concrete.

Additionally, among the mechanisms of deterioration, corrosion due to carbonation usually occurs in reinforced concrete structures, especially in urban areas, which are often characterized by high emissions of carbon dioxide into the environment from vehicles or industrial plants. Carbonation (the process by which CO2 reacts with the alkalis present in the pore solution of the concrete matrix) reduces the alkalinity of the concrete to pH values of 9.0 or less. When the carbonation reaches the steel reinforcement region, the steel’s passivity is broken, thereby initiating generalized steel corrosion. Thus, it remains to be determined whether the proposed technique is capable of reducing the diffusion coefficients of CO2 and chlorides. In the present work, the effect of introducing silica-based nanoparticles obtained through sol–gel synthesis in the carbonation of hardened cement-based materials under the effect of an electric field was analyzed. This study focuses on the analysis of the physicochemical properties of the obtained nanoparticles, as well as the electrical resistivity, porosimetry and carbonation of the hardened cement matrix.

Section snippets

Synthesis and physicochemical characterization of silicon nanoparticles (SN)

Silicon nanoparticles (SN) were obtained through sol–gel synthesis at 70 °C, utilizing the following reagents: tetraethyl orthosilicate (TEOS: Si(OC2H5)4, Sigma Aldrich, 99.9%) as a precursor to silica; ethyl alcohol (C2H6O, Sigma Aldrich, 95%); and ammonium hydroxide (NH4OH). The following mole fractions were used: Si(OC2H5)4/C2H6O = 0.1051, NH4OH/C2H6O = 0.0301, H2O/C2H6O = 0.7816). The synthesis method consisted of placing ethyl alcohol in a three-necked Schlenk flask, which was then vigorously

Characterization of the silicon-based particles

Fig. 3 shows the diffractogram of an SN sample. A very wide reflection can be observed within the interval 2θ = 22–23°, confirming that the SN-based material has the characteristics of an amorphous material (in this case, silica) and indicating high atomic disorder. This result coincides with the reported spectra of fresh silicon gels [3], [19].

As mentioned above, the morphological properties of the fresh material prepared by this soft chemical method were determined using TEM. The images shown

Conclusions

The application of SN on hardened cement-based materials, such as reinforced concrete structures, under the experimental conditions presented here, allows us to conclude the following:

Through characterization, it was confirmed that amorphous silicon was obtained, with predominantly nanometric, heterogeneous irregular particles of sizes ranging from 10 to 50 nm, and mostly between 20 and 30 nm.

Sonication promoted the separation of OH radicals from the silicon precursor, resulting in ionic

Acknowledgements

The authors express their gratitude to PROVERICYT, PAICYT and SEP-CONACYT for the financial support of projects IT573-10, IT720-11 and CB-2012-01/177839. We also thank the Laboratorio de Catálisis Heterogénea of the UJAT for the characterization of the materials. D.M.A. Cruz-Moreno would like to acknowledge CONACYT for the Graduate Studies scholarship granted for the realization of his studies.

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