Effect of aggregate contamination with pyrite on reinforcement corrosion in concrete
Introduction
According to Pradhan (2014) [1], concrete reinforcement corrosion is the most significant deteriorating mechanism for the reinforcement of structures. As it is the largest occurring phenomenon, it deserves studies on the favorable factors of the mechanism and preventive measures for it.
Corrosion is defined as the deterioration of a material, usually metallic, through chemical or electrochemical action of the environment, associated or not with mechanical stresses. The interaction between the material and the environment causes harmful and undesirable changes suffered by the material, making it unsuitable for its use in construction [2].
In general, studies related to corrosion in concrete structures analyze the phenomenon from the carbonation viewpoint (pH reduction) or chloride concentration (ion diffusion), since these are the main aggressive agents responsible for the deteriorating mechanism [3]. However, a study from the viewpoint of sulfate presence in concrete mixtures is interesting, since sulfide-contaminated aggregates are available and there are differences in contaminant attack mechanism and its content limit that, so its use may not be harmful to the concrete [4].
Concrete structures far from urban centers usually require the use of local aggregates that might present contaminations such as pyrite. Those contaminations can affect directly the corrosion mechanism of the concrete structures. Although expansion, microcracking and loss of stiffness are essential during the evaluation of structures affected by sulfate attack [5], [6], the sulfate contamination could influence the steel passivation and induce the corrosion initiation even with no significant mechanical changes in the concrete. Therefore, this paper contributes to the understanding of the corrosion phenomenon associated with sulfate attacks that can occur due to internal contaminations such as pyrite.
This study aims to analyze, through chemical and electrochemical tests, the influence that the use of aggregates contaminated by sulfides on the behavior of cementitious composites regarding the phenomenon of corrosion. The chemical analysis (SEM and XRD) allows discussions on deleterious products formed in cement paste from 28 to 720 days of age. The electrochemical analysis (corrosion potential, current density and electrical resistivity) supports the discussions of the susceptibility to the corrosion phenomenon of concretes contaminated by sulfides, from 28 to 1100 days of age.
The hypothesis raised in this study is that the contamination of the samples by sulfides would leave the steel bars more susceptible to the phenomenon of corrosion, since one of the consequences of the attack is the consumption of Portlandite, a material responsible for maintaining the pH of the matrix and protecting the reinforcement [7], and also allows the formation of ettringite in the concrete [8]. Thus, the combined analysis of the chemical and electrochemical properties of the contaminated samples makes it possible to study the products formed due to the attack, as well as the behavior of the reinforcement concerning the corrosion mechanism. And finally, a contamination limit from the electrochemical perspective can be proposed.
Section snippets
Literature review
The deleterious reactions of concrete structures that contain sulfide in their composition start with the oxidation of the sulfide mineral. Pyrite, for example, in the presence of water and oxygen, oxidizes, forming iron hydroxide and sulfuric acid, as shown in Eq. (1) [9].
The by-products generated can react with the compounds present in the cementitious matrix and initiate new chemical reactions and forming products with volumes greater than the initial volume of
Materials and characteristics of mixtures
The experimental program of this study consisted of the molding of samples of contaminated reinforcement concretes for electrochemical tests and cement pastes for chemical and microstructural tests. The materials used in both mixtures were: cement, fly ash and pyrite as a contaminant.
The binder blend used was obtained by a composition of 65% of commercial cement, CPV ARI – NBR 16697 [18] equivalent to Type III – ASTM C150 [19], and 35% of fly ash, equivalent to ASTM C618 [20] class F. The final
Electrochemical analysis
Fig. 3(A) and (B) show the corrosion potential in wetting and drying cycle and tank conditions, respectively. For the corrosion risk analysis, the limits presented in ASTM C876 [48] are shown in Fig. 3 (low, uncertain and high). In the analysis of the formation of the passivating film, it was taken into consideration the change of risk range, high risk to uncertain risk, as mentioned by Koga et al. [49]. However, it is important to note that the classification referred to in ASTM C876 [48] was
Conclusions
The electrochemical tests confirmed the higher susceptibility to the steel corrosion in reinforced concretes with contamination of 5.0% of SO3. In the two exposure conditions, cycle and tank, it was not possible to point out the formation of the passivating film for the series of higher contamination in any condition. Also, after 1100 days, the 5.0% of SO3 series presented higher corrosion current density, 0.7 µA/cm2, (moderate level) and lower potential of corrosion, −600 mV, (high-risk zone).
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.
Acknowledgments
The authors thank the support, as a scholarship, by Foundation Araucária, as well as the availability of the infrastructure and human resources made available by the Federal University of Paraná (PPGECC/UFPR-Brazil). Our gratitude also to the Center of Electron Microscopy of UFPR (CME/UFPR) by the support during the SEM tests.
Declaration of Competing Interest
The authors confirm that this work is unpublished and there is no declaration of interest. It strictly followed all ethical procedures and it has not been submitted to any other journal for publication.
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