Aminobenzoate modified MgAl hydrotalcites as a novel smart additive of reinforced concrete for anticorrosion applications
Graphical abstract
Introduction
Chloride-induced corrosion of the reinforcing steel is a major threat to the durability and serviceability of concrete structures, which accounts for large amounts of unplanned repairs, associated costs, out of service time and waste of materials and energy [1], [2], [3]. Once chloride initiates corrosion, three main consequences occur [4]: (1) local pitting corrosion of the reinforcement; (2) cracking and spalling of the concrete cover due to build-up of voluminous corrosion products; and (3) decrease of ductility and reduction of cross section of the reinforcing steel. In increasing order, these consequences may compromise the structural integrity. Therefore it is crucial to design concrete to sustain environmental aggressiveness; and after corrosion initiation, to prevent or slow down further deterioration. Traditionally available anti-corrosion options are too expensive or technically too complicated to be applied on a wide scale [5], [6], [7], [8]. Thus, continuing research in the domain of materials science is essentially needed in searching for more effective measures to improve the corrosion resistance of reinforced concrete.
In the last two decades, more research interest has been attracted in developing new or modified materials able to prevent corrosion initiation and slow down or even stop corrosion propagation, as well as in understanding the underlying working mechanism [9], [10], [11]. Among them, modified hydrotalcites (MHTs) may represent a promising option for use in concrete as a new type of functional additive [12], [13]. Deriving from their parent compound, i.e., the naturally occurring hydrotalcite, [Mg6Al2(OH)16]CO3·4H2O, MHTs are anion-exchangeable substances consisting of stacks of positively charged mixed-metal hydroxide layers between which negatively charged anionic species and water molecules are intercalated. The MHTs’ structure can be represented by a general formula [ (OH)2]x+[()]x−·mH2O, where MII and MIII are di- and trivalent metals respectively, (MII: Mg2+, Ca2+, Zn2+, Ni2+, etc.; MIII: Al3+, Fe3+, Ga3+, Co3+, etc.,) and An− is an exchangeable interlayer anion that could be inorganic or organic (, , Cl−, , and carboxylates, anions of amino acids or polyamino carboxylates, etc.) with valence n. The x value is in the range of 0.22–0.33. The key feature of MHTs is their high anionic exchange capacity (2–4.5 milliequivalents/g) which makes exchange of the interlayer ion by a wide range of organic or inorganic anions versatile and easily achieved [14], [15], [16]. A typical crystal structure of MHT is presented schematically in Fig. 1.
Hydrotalcite or hydrotalcite-like phases have been found in hydrated slag cements, which are known to bind more chloride ions than pure Portland cements [17], [18], [19], [20]. The existence of hydrotalcite-like phases such as Friedl’s salt (a chloride-bearing AFm phase) or its iron analogue and/or Kuzel’s salt (a chloride- and sulfate-bearing AFm phase) have been reported to contribute to chloride binding and thus enhance the corrosion resistance of reinforced concrete [21]. The beneficial effects of Friedl’s salt on binding chloride in cement support the idea of using MHTs in concrete as an effective chloride scavenger and the increased chloride-binding would definitely slow down chloride transport through concrete matrix. For the envisioned use as an additive to concrete against chloride attack, certain inorganic or organic anions with known inhibitive properties could be intercalated into the structures of MHTs, which then can be slowly released, possibly ‘automatic’ upon arrival of chloride ions. Such inhibition would also increase the chloride threshold level for corrosion initiation and/or reduce the subsequent corrosion rate of the reinforcing steel in concrete if corrosion has been initiated. Different from the other (typically single-function) protective materials, MHTs play a dual role against chloride-induced corrosion: capturing aggressive chlorides and simultaneously releasing inhibitive anions to protect the reinforcing steel from corrosion [22]. The mechanism of this function can be schematically shown as in Fig. 2. Within the MHT family, a class of materials with emerging importance is that constituted by MHTs intercalated with organic species [23], [24]. In this work, magnesium–aluminum-based hydrotalcite, Mg(2)Al-CO3 (Mg/Al atomic ratio 2:1) modified by p-aminobenzoate (pAB) was synthesized as a model material from the family of MHTs and experiments were designed to investigate the feasibility of MHTs with the selected intercalating organic anions to be able to act as a scavenger for chlorides. The primary objective of the paper is therefore to explore the promising use of novel MHT compositions with selected intercalating inhibitive anions as a new type of smart additive for reinforced concrete to reduce chloride-induced corrosion.
Section snippets
Materials
Mg(NO3)2·6H2O, Al(NO3)3·9H2O and NaNO3 were obtained from Merck KGaA. NaOH, Na2CO3 and p-aminobenzoic acid were obtained from Sigma–Aldrich. All reagents are ACS grade (>99% purity) and used as received without further purification. Boiled deionized water was used for the preparation of aqueous solutions and filtration. Steel electrodes prepared for anticorrosion evaluation were low-carbon steel (ASTM A36) coupons with an exposed surface area of 100 mm2. All electrodes were ground with Nos.
X-ray diffraction analysis
XRD patterns of the synthesized Mg(2)Al-CO3, its calcinated form Mg(2)Al-500C and modification derivative Mg(2)Al-pAB are shown in Fig. 4. As can be seen, before calcination, the Mg(2)Al-CO3 shows sharp basal reflections at lower 2θ angles which represent a characteristic layered structure of the hydrotalcite-like compound with good crystallinity [28]. After being calcinated at 500 °C for 3 h, all of the characteristic reflections before calcination disappeared, indicating that the layered
Conclusions
A carbonate form magnesium–aluminum hydrotalcite (i.e., Mg(2)Al-CO3) and its pAB modified derivative (i.e., Mg(2)Al-pAB) were synthesized and characterized by means of XRD, FT-IR and TGA/DSC. Total organic carbon content (TOC) analysis combined with TG/DSC results further confirmed that 35.5% pAB anions (by mass of Mg(2)Al-pAB) were successfully intercalated into the interlayer space of these hydrotalcites. The anticorrosion behavior of Mg(2)Al-pAB was evaluated on the basis of open circuit
Acknowledgements
The research was carried out under Project number M81.609337 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl). The first author greatly appreciates Dr. X. Shi of Western Transportation Institute at Montana State University for the gift of steel electrodes that was used in this work.
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