Modified Hydrotalcites as
Smart Additives for Improved
Corrosion Protection of
Reinforced Concrete
Modified Hydrotalcites as
Smart Additives for Improved
Corrosion Protection of
Reinforced Concrete
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 23 juni om 10.00 uur
door
Zhengxian YANG
Master of Science in Physical Chemistry,
South-Central University for Nationalities, China
Prof. dr. R.B. Polder
Samenstelling promotiecommissie:
Rector Magnificus, Technische Universiteit Delft, voorzitter Prof. dr. R.B. Polder, Technische Universiteit Delft, promotor Prof. dr. ir. K. van Breugel, Technische Universiteit Delft
Dr. J.M.C. Mol, Technische Universiteit Delft
Prof. dr. B. Elsener, Swiss Federal Institute of Technology in Zürich, Switzerland Prof. dr. C. Andrade, Institute ‘Eduardo Torroja’ of Construction Science, Spain Prof. dr. G.F. Peng, Beijing Jiaotong University, China
Dr. H.R. Fischer, TNO, The Netherlands
Prof. dr. ir. L.J. Sluys, Technische Universiteit Delft, Reservelid
ISBN: 978-94-91909-25-2
Keywords: Modified hydrotalcites; Layered double hydroxides; Smart additives; Reinforced concrete; Durability; Service life; Corrosion; Chloride; Corrosion inhibitors; Amino acids
Copyright © 2015 by Zhengxian Yang Email: zhengxian.yang@gmail.com
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Table of Contents
Chapter 1 General Introduction ... 1
1.1 Background of this research project ...2
1.2 Objective and scope of this research ...4
1.3 Outline of this thesis ...4
References ...7
Chapter 2 Modified Hydrotalcites as Smart Additives for Improved Corrosion Protection of Reinforced Concrete: A Literature Review ... 9
2.1 Introduction ... 10
2.2 Corrosion of the steel in concrete ... 10
2.2.1 Carbonation ... 12
2.2.2 Chloride induced corrosion ... 13
2.3 Factors that affect corrosion of reinforcement ... 15
2.3.1 Permeability of concrete ... 15
2.3.2 Chloride binding ... 15
2.3.3 Concrete cover ... 16
2.3.4 Environmental conditions ... 17
2.4 Corrosion preventive measures ... 18
2.5 MHTs and their application in cementitious materials ... 18
2.5.1 General aspect ... 18
2.5.2 Synthesis and characterization ... 20
2.5.3 Ion exchange of MHTs and its role in capturing chloride ... 21
2.5.4 Application in cementitious materials ... 22
2.6 Concluding remarks ... 25
References ... 25
Chapter 3 Inhibition Performance Evaluation of Some Amino Acids against Steel Corrosion in Simulated Concrete Pore Solution ... 33
3.2 Experimental... 36
3.2.1 Materials ... 36
3.2.2 Testing methods ... 36
3.3 Results and discussion ... 38
3.3.1 The effect of inhibitor addition on pH of the alkaline solution ... 38
3.3.2 Open circuit potential (OCP) measurements ... 39
3.3.3 Linear polarization resistance (LPR) measurements ... 43
3.4 Conclusions ... 47
References ... 48
Chapter 4 Synthesis and Characterization of Modified Hydrotalcites Using Selected Inhibitors as Modifiers ... 51 4.1 Introduction ... 52 4.2 Experimental... 53 4.2.1 Materials ... 53 4.2.2 Synthesis ... 53 4.2.3 Characterization ... 54
4.3 Results and discussion ... 54
4.3.1 X-ray diffraction analysis ... 54
4.3.2 Infrared analysis ... 57
4.3.3 Thermal analysis ... 60
4.3.4 Intercalation efficiency under the calcination-rehydration condition ... 63
4.4 Conclusions ... 65
References ... 65
Chapter 5 Anti-corrosion Performance Evaluation of Synthesized Modified Hydrotalcites in Simulated Concrete Pore Solution ... 69
5.1 Introduction ... 70
5.2 Experimental... 70
5.2.1 Materials ... 70
5.2.2 Ion exchange of MHT with chlorides in simulated concrete pore solution ... 70
5.2.3 Anti-corrosion performance evaluation in simulated concrete pore solution ... 71
5.3 Results and discussion ... 72
5.3.1 The role of MHT in capturing chlorides ... 72
5.3.3 Anti-corrosion performance of the selected MHT ... 78
5.4 Conclusion ... 85
References ... 86
Chapter 6 The Influence of Two Types of Modified Hydrotalcites on Chloride Ingress in Cement Mortar ... 89 6.1 Introduction ... 90 6.2 Experimental... 91 6.2.1 Materials ... 91 6.2.2 Sample preparation ... 92 6.2.3 Testing methods ... 92
6.3 Results and discussion ... 95
6.3.1 The effect of MHTs on workability of fresh mortar ... 95
6.3.2 The effect of MHTs on mechanical properties ... 95
6.3.3 The effect of MHTs on porosity ... 98
6.3.4 The effect of MHTs on chloride penetration ... 99
6.4 Conclusion ... 105
References ... 106
Chapter 7 The Anti-corrosion Performance of Two Types of Modified Hydrotalcites in Cement Mortar with Embedded Steel ... 109
7.1 Introduction ... 110
7.2 Experimental... 110
7.2.1 Materials ... 110
7.2.2 Sample preparation ... 110
7.2.3 Anti-corrosion performance evaluation ... 114
7.2.4 Chloride analysis ... 121
7.3 Results and discussion ... 122
7.3.1 Accelerated chloride migration test ... 122
7.3.2 Cyclic wetting-drying test ... 130
7.3.3 Natural diffusion test ... 136
7.3.4 The applied test methodology and the effect on chloride diffusion/migration and chloride threshold ... 139
7.3.5 Effect of MHT on time to corrosion initiation ... 141
References ... 146
Chapter 8 Conclusions and Recommendations for Future Research ... 149
8.1 General conclusions... 150
8.2 Industrial application potentials and valorization ... 154
8.3 Recommendations for future research ... 154
Summary ... 157
Samenvatting ... 160
Acknowledgements ... 163
Curriculum Vitae ... 165
Chapter 1
1.1 Background of this research project
Reinforced concrete is the most widely used construction material across the world, due to its relatively low cost and the excellent marriage between reinforcing steel and bulk concrete. Its technical success has resulted from the complementary mechanical properties between reinforcing steel (source of good tensile strength) and concrete (source of good compressive strength) and their excellent physical and chemical compatibility. The similarity of concrete and steel in thermal expansion contributes to eliminate large internal stresses due to differences in thermal expansion or contraction [1]. In addition, the high alkalinity of the concrete pore solution promotes corrosion protection by passivation of mild (reinforcing) steel [2]. All of these factors have led to a large stock of structures which are expected to last very long; nowadays a service life of 100 years is required for major structures, e.g. tunnels and bridges [3, 4]. However, corrosion protection can be lost due to ingress of chloride ions, typically present in de-icing salts and marine environment [5, 6]. Chloride ion transport by diffusion or capillary absorption in the concrete pore system is relatively fast and chloride-induced corrosion has been recognized as a main culprit to the durability of reinforced concrete [1]. A secondary issue is the carbonation of concrete, which reduces the pH of the concrete pore solution to values where passivation disappears [7]. Combined chloride ingress and carbonation increases the corrosion risk even more. Both degradation processes threaten the initial passivation and thus concrete durability on the time scale of 10 to 50 years. In all above-ground outdoor structures, plenty of oxygen and water are available to promote relatively rapid corrosion. Corrosion products are much more voluminous than the parent steel, causing tensile stresses in the concrete cover and subsequently, its cracking and spalling on a relatively short term (< 10 years after corrosion initiation) [8, 9]. In the midterm (10-20 years), steel cross section loss may cause insufficient tensile capacity and thus may threat structural integrity and safety [10]. Consequently, signs of corrosion (rust staining, concrete cracking) signal the need to repair and to reinstate corrosion protection, or to even replace complete elements or structures. Such measures are at least laborious and time consuming. Results are high unplanned maintenance costs (potentially up to the initial construction costs), significant unavailability (out of service time) and waste of materials and energy with associated emission of carbon dioxide. This problem is highly relevant for civil engineering structures in the transport sector, such as bridges, tunnels, harbour quays and parking structures [11]. Consequently, the construction industry is in need of improving the corrosion protection of reinforced concrete structures, preferably by low-cost measures.
Modern service life design approach aims at providing sufficient concrete cover depth to the reinforcing steel, while taking into account the resistance against chloride transport and the critical chloride level at the steel for corrosion initiation. Traditional Standards oversimplify the complexity of the mechanisms involved and provide insufficient performance in aggressive environment. More advanced regulations [3, 4, 12] are based on modelling of chloride transport
up to the critical corrosion initiating level and testing of materials under laboratory conditions, e.g. by Rapid Chloride Migration testing [13, 14]. In particular the critical chloride content [15-18] and the time-dependency of chloride diffusion [19] are not well understood. Consequently, high uncertainties are still associated with long-term material behaviour and the influence of execution variables that dominate concrete properties as produced on site. Presently available options for improved corrosion protection are either too costly or too complicated; or insufficiently effective. Stainless steel reinforcement is 5 or 10 times more expensive than reinforcing (carbon) steel [20, 21]. Cathodic prevention and protection may be effective but both are a special niche expertise and are thus not applied on a wide scale [22, 23]. Coatings on the concrete surface do not last long enough (10-20 years), which causes a maintenance cycle of its own [20]. Corrosion inhibitors have been proposed but are generally not reliable in terms of long-term efficiency [24]; some are toxic, such as nitrites [25]. 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 their underlying working mechanism. Among them, modified hydrotalcites (MHTs) may represent a promising option for use in concrete as new type of smart functional additives [26].
Recently, a study on the application of amino acid modified hydrotalcites in cementitious materials has formed the basis of a joined TNO-AIDICO patent “corrosion inhibition of reinforced concrete” (WO 2011/065825 A1) [27], that consists of hydrotalcites intercalated with organic species, in particular eco-friendly amino acids, which can be directly applied (without paint or polymeric carrier) as aqueous emulsion on a metallic substrate, or it can be mixed into fresh concrete with the various components. However, its scale was relatively small and further work was considered necessary by the applicants and their organisations.
Our preliminary work has shown that ion exchange occurs between free chloride ions in the simulated concrete pore solution and anions intercalated in MHT reducing the free chloride concentration which is equivalent to increased binding of chloride [28, 29]. Indications exist for the natural occurrence of hydrotalcite in hydrated blast furnace slag cements [30-32], which are known to bind more chloride ions than Portland cements. Increased binding would slow down chloride transport. The preliminary work has also shown that certain organic anions with known inhibitive properties could be intercalated, which then can be slowly released, possibly 'automatic' upon arrival of chloride ions. Such inhibition increases the chloride threshold level for corrosion initiation and/or reduces the subsequent corrosion rate. Less aggressive electrochemical potentials have been observed in simulated concrete pore solution with MHT as compared to solutions without MHT [29]. These results suggest that MHT has a high potential as active component in concrete with corrosion protection properties that can be tailor made.
Hydrotalcite belongs to a large mineral group of naturally occurring Layered Double Hydroxides (LDHs), in general formula [MII1-x MIIIx (OH)2]x+ [(An-x/n)]x-·mH2O, where MII and
MIII are di- and trivalent metal cations (MII: Mg2+, Ca2+, Zn2+, Ni2+, etc; MIII: Al3+, Fe3+, Ga3+, Co3+, etc), and An- is an exchangeable interlayer anion with valence n. In this thesis, the hydrotalcites particularly refer to Mg-Al hydrotalcites.
1.2 Objective and scope of this research
The research project aims at developing a new promising additive as an alternative approach against chloride ingress into mortar (or concrete) and/or chloride induced corrosion based on modified hydrotalcites (in particular those modified by eco-friendly amino acids) and understanding and quantifying their effects in mortar (or concrete): (1) their interaction with chloride ions and (2) their influence on steel corrosion. These effects should be measurable in terms of reduced chloride diffusion rates and increased chloride threshold level for corrosion initiation. The overall objective is to increase the tolerance of mortar/concrete structures with regard to chloride induced corrosion and to increase their maintenance-free service life by utilization of modified hydrotalcites. Potential applications are as (1) admixtures to fresh mortar/concrete, (2) pre-casting treatment of reinforcing steel in new building and repair situations and (3) as addition to repair mortars. This new additive when applied appropriately, either incorporation of a small amount in bulk mortar/concrete or as a surface coating of the reinforcing steel could prevent/delay the corrosion initiation. In addition, it is expected to have no adverse side-effects on the properties of fresh and hardened mortar/concrete.
1.3 Outline of this thesis
As shown in Figure 1.1, this thesis is organized in 8 chapters. Chapter 1 gives the background, motivation and the objective and scope of this research.
Chapter 2 presents a literature review of the existing knowledge with regard to synthesis and characterisation methods of MHTs, ion exchange within the MHT structure as well as the application of MHTs in cementitious materials. On top of that the mechanism of corrosion in reinforced concrete, factors that affect corrosion of reinforcement and relevant corrosion preventive measures are also briefly reviewed. This part is intended to be brief, with which many textbooks [1, 10, 33-35] have explicitly dealt.
Chapter 3 evaluates the inhibition performance of some amino acids (in particular, glycine, 6-aminocaproic acid, 11-aminoundecanoic acid and p-aminobenzoic acid) against steel corrosion in simulated concrete pore solution. The objective of this chapter is to select the most promising amino acids with good inhibition performance as candidate modifiers for synthesis of MHTs.
Based on the results obtained from Chapter 3, six MHTs (with Mg/Al atomic ratios of 2.2 and 2.7) intercalated with nitrites and the selected amino acids inhibitors were synthesized in Chapter 4 using the reconstruction method. They were characterized by means of X-ray powder
diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetry (TG), Differential scanning calorimetry (DSC) and relevant elemental analysis.
Chapter 5 investigates the ion exchange characteristics of the six synthesized MHTs and their anti-corrosion performance in chloride-rich simulated concrete pore solution based on electrochemical methods such as open circuit potential (OCP) and linear polarization resistance (LPR). The objective of this chapter is to select the MHTs with best anti-corrosion performance for use in both plain and reinforced mortar tests. Two MHTs, i.e., Mg(2)Al-NO2 and
Mg(2)Al-pAB were finally selected as the more promising MHT candidates for mortar test, which is the main topic of the following two chapters.
Chapter 6 explores the influence of the two selected modified hydrotalcites on chloride ingress into plain mortar, while Chapter 7 focuses on their anti-corrosion performance in mortar with embedded steel. In Chapter 6, the effect of the two MHTs in plain mortar is studied by workability test, strength test, porosity test, and rapid chloride migration and natural diffusion test. In Chapter 7, the effect of the two MHTs on reinforcement corrosion is investigated by three designated testing methods based on chloride exposure and OCP/LPR measurements with custom designed reinforced mortar specimens:
a) An accelerated chloride migration test b) A wetting-drying cyclic test
c) A natural diffusion test
Chapter 8 summarizes the results obtained in this research and gives the conclusions. Some recommendations are given for future research.
Figure 1.1 Outline of the thesis. Chapter 8
Conclusions and recommendations
Chapter 6
The influence of two types of MHTs on chloride ingress in
cement mortar
Chapter 7
Anti-corrosion performance of two types of MHTs in cement mortar
with embedded steel
Chapter 1 General introduction Chapter 2 Literature review Chapter 3 Inhibition performance evaluation of some amino acids against
steel corrosion in simulated concrete pore solution Chapter 4 Synthesis and characterization of MHTs using selected inhibitors as modifiers Chapter 5 Anti-corrosion performance evaluation of synthesized MHTs in simulated concrete pore solution
References
[1] Bertolini L, Elsener B, Pedeferri P, Redaelli E, Polder RB. Corrosion of steel in concrete: prevention, diagnosis, repair. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2013.
[2] Gouda V. Corrosion and corrosion inhibition of reinforcing steel: I. Immersed in alkaline solutions. British Corrosion Journal. 1970;5(5):198-203.
[3] Fédération Internationale du Béton, Model code for service life design. Lausanne, Switzerland: fib Bull. 34; 2006.
[4] The European Union—Brite EuRam III, DuraCrete-Probabilistic Performance Based Durability Design of Concrete Structures. Final Technical Report, Document BE95-1347/R17. CUR, Gouda, 2000.
[5] Polder R, Larbi J. Investigation of concrete exposed to North Sea water submersion for 16 years. Heron. 1995;40(1):31-56.
[6] Polder RB, Hug A. Penetration of chloride from de-icing salt into concrete from a 30 year old bridge. Heron. 2000;45(2):109-24.
[7] Neville AM. Properties of concrete. Fourth edition. Harlow: Prentice Hall/Pearson; 2006.
[8] Shah V, Hookham C. Long-term aging of light water reactor concrete containments. Nuclear engineering and design. 1998;185(1):51-81.
[9] Smith J, Virmani YP. Materials and methods for corrosion control of reinforced and prestressed concrete structures in new construction (No. FHWA-RD-00-081). 2000.
[10] Bentur A, Berke N, Diamond S. Steel corrosion in concrete: fundamentals and civil engineering practice: CRC Press; 1997.
[11] Gaal G. Prediction of deterioration of concrete bridges. PhD Thesis. Delft, Delft University of Technology; 2004.
[12] Wegen G, Polder RB, Breugel KV. Guideline for service life design of structural concrete: A performance based approach with regard to chloride induced corrosion. Heron. 2012;57(3):153-68. [13] Luping T, Nilsson L-O. Rapid determination of the chloride diffusivity in concrete by applying an
electric field. ACI materials journal. 1993;89(1).
[14] NTBuild492. Concrete, mortar and cement-based repair materials: Chloride migration coefficient from non-steady-state migration experiments. NordTest, Espoo. 1999.
[15] Polder RB. Critical chloride content for reinforced concrete and its relationship to concrete resistivity. Materials and Corrosion. 2009;60(8):623-30.
[16] Alonso C, Andrade C, Castellote M, Castro P. Chloride threshold values to depassivate reinforcing bars embedded in a standardized OPC mortar. Cement and Concrete Research. 2000;30(7):1047-55.
[17] Glass GK, Buenfeld NR. Chloride threshold levels for corrosion induced deterioration of steel. In: Nilsson L, Ollivier J, editors. 1st RILEM International Workshop on Chloride Penetration into Concrete: Rilem Publications SARL; 1995. p. 429-40.
[18] Angst U, Elsener B, Larsen CK, Vennesland Ø. Critical chloride content in reinforced concrete—a review. Cement and Concrete Research. 2009;39(12):1122-38.
[19] Visser J, Polder R. Concrete binder performance evaluation in service life design. In: Kovler K, editor. ConcreteLife'06-International RILEM-JCI Seminar on Concrete Durability and Service Life Planning: Curing, Crack Control, Performance in Harsh Environments. Dead Sea, Israel: RILEM Publications SARL; 2006. p. 330-40.
[20] Cigna R, Andrade C, Nürnberger U, Polder R, Weydert R, Seitz E. COST 521: Corrosion of steel in reinforced concrete structures-final report. Luxembourg: European communities EUR20599;2002.
[21] Elsener B, Addari D, Coray S, Rossi A. Stainless steel reinforcing bars–reason for their high pitting corrosion resistance. Materials and Corrosion. 2011;62(2):111-9.
[22] Pedeferri P. Cathodic protection and cathodic prevention. Construction and Building Materials. 1996;10(5):391-402.
[23] Polder R, Peelen W, Lollini F, Redaelli E, Bertolini L. Numerical design for cathodic protection systems for concrete. Materials and Corrosion. 2009;60(2):130-6.
[24] Elsener B. Corrosion inhibitors for steel in concrete: state of the art report: Woodhead Pub Limited; 2001.
[25] Rosenberg A, Gaidis J. The mechanism of nitrite inhibition of chloride attack on reinforcing steel in alkaline aqueous environments. Materials performance. 1979;18(11).
[26] Yang Z, Fischer H, Polder R. Modified hydrotalcites as a new emerging class of smart additive of reinforced concrete for anticorrosion applications: A literature review. Materials and Corrosion. 2013;64(12):1066-74.
[27] Fischer HR, Adan O, Lloris Cormano JM, Lopez Tendero MJ. Corrosion inhibition of reinforced concrete. WIPO Patent WO 2011/065825A1, 3 Jun 2011.
[28] Yang Z, Fischer H, Polder R. Synthesis and characterization of modified hydrotalcites and their ion exchange characteristics in chloride-rich simulated concrete pore solution. Cement and Concrete Composites. 2014;47:87-93.
[29] Yang Z, Fischer H, Cerezo J, Mol J, Polder R. Aminobenzoate modified MgAl hydrotalcites as a novel smart additive of reinforced concrete for anticorrosion applications. Construction and Building Materials. 2013;47:1436-43.
[30] Roy DM, Sonnenthal E, Prave R. Hydrotalcite observed in mortars exposed to sulfate solutions. Cement and Concrete Research. 1985;15(5):914-6.
[31] Wang S-D, Scrivener KL. Hydration products of alkali activated slag cement. Cement and Concrete Research. 1995;25(3):561-71.
[32] Kayali O, Khan MSH, Sharfuddin Ahmed M. The role of hydrotalcite in chloride binding and corrosion protection in concretes with ground granulated blast furnace slag. Cement and Concrete Composites. 2012;34(8):936-45.
[33] Poulsen E, Mejlbro L. Diffusion of chloride in concrete: theory and application: CRC Press; 2010. [34] Tang L, Nilsson L-O, Basheer M. Resistance of concrete to chloride ingress. Testing and modelling:
Spon Press; 2012.
Chapter 2
Modified Hydrotalcites as Smart
Additives for Improved Corrosion
Protection of Reinforced Concrete: A
Literature Review
Part of the work described in this chapter has been published as: Yang, Z., Fischer, H., Polder, R. Modified Hydrotalcites as A New Emerging Class of Smart Additive of Reinforced Concrete for Anti-corrosion Applications: A Literature Review. Materials and Corrosion, 2013, 64(12):1066-1074.
2.1 Introduction
Concrete is a porous and highly heterogeneous composite with features of various dimensions ranging from nanometer-sized pores and calcium-silicate-hydrate (C-S-H) gel to micrometer-sized air voids, millimeter-micrometer-sized aggregate particles and to steel reinforcement that can be meters in length. Concrete can consequently be penetrated by corrosive agents (e.g. certain chemical and microbiological substances), liquids (e.g. water, in which various ions are dissolved) or gases (e.g. oxygen and carbon dioxide present in the atmosphere) through capillary absorption, hydrostatic pressure, or diffusion. Some of them promote corrosion of reinforcing steel. In addition, disintegration of concrete exposed to freeze-thaw cycles in cold climates may also compromise the protection of reinforcement. All of these factors potentially impose a serious threat on 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 [1-4]. Among those above-mentioned factors, the dominant aggressive external influence for civil infrastructure, e.g. bridges and harbour structures, is the ingress of chloride ions, typically present in de-icing salts and sea water [5-7].
Modified hydrotalcites (MTHs) represent a group of technologically promising materials for addition to concrete to improve its durability in aggressive environment, owing to their low cost, relative simplicity of preparation, and plenty of unique composition variables that may be adopted [8, 9]. Up to date, a lot of academic work and commercial interest on MHTs have been invested, but relatively few studies focus on cementitious materials, particularly in exploiting their potential applications in corrosion protection of reinforced concrete structures. In this chapter, the mechanism of corrosion in reinforced concrete, factors that affect corrosion of reinforcement and relevant corrosion preventive measures are briefly introduced. In addition, the existing knowledge with regard to synthesis and characterization methods of MHTs, ion exchange within the MHT structure as well as the application of MHTs in cementitious materials were reviewed. As a new emerging class of smart additive of reinforced concrete, MHTs are expected to contribute to the effort of searching for effective measures to improve the durability of reinforced concrete.
2.2 Corrosion of the steel in concrete
Steel in concrete is normally in a non-corroding and passive condition. During hydration of cement, a highly alkaline pore solution (pH>12.5) develops, which facilitates the formation of a passive oxide/hydroxide film on the surface of the steel. This protective film is only a few nanometers thick and can effectively insulate the steel from the pore electrolyte so that the onset of corrosion is delayed, allowing decades of relatively low maintenance [10]. However, this protective film can be disrupted (i.e., depassivation) by the ingress of chlorides and carbon
dioxide from the atmosphere (i.e., carbonation) [5, 11]. Once corrosion has started, three main consequences occur [12]: 1) corrosion of the reinforcement (either local pitting in cases of chloride attack or uniform corrosion in the case of carbonation); 2) decrease of ductility due to reduction of cross section of the reinforcing steel; 3) cracking and spalling of the concrete cover due to build-up of voluminous corrosion products, which in turn foster the ingress of moisture, oxygen and other aggressive agents into the concrete. Figure 2.1 schematically shows typical corrosion damage as occurring to reinforcing steel. In increasing order, these consequences may significantly compromise the structural integrity and safety. The development of corrosion in reinforced concrete structures can be divided in two main stages according to Tuutti's corrosion model as shown in Figure 2.2 [13, 14]. The first stage is the initiation of corrosion, in which the reinforcement is passive but phenomena that can lead to loss of passivity, e. g. chloride penetration into or carbonation of the concrete cover take place. The second stage is corrosion propagation which is dependent on the availability of water and oxygen in the vicinity of the steel. It starts when the steel is depassivated and may proceed until some form of failure associated with internal and/or surface cracking and spalling of the concrete cover. The time before such failure is often referred to the service life of the reinforced concrete element, which is determined by the total duration of these two stages. It is worth pointing out that modern service life design philosophy somehow only considers the initiation stage and neglects the propagation stage.
2.2.1 Carbonation
The gradual ingress of carbon dioxide causes its reaction with the alkaline constituents of concrete present in the pore solution (mainly as sodium and potassium hydroxides) and in the solid hydration phases, reducing the pH of the concrete pore solution to a value (pH≈9.0) where passivity of reinforcing steel is destroyed [15]. Calcium hydroxide, Ca(OH)2, is the main alkali in
cement hydration products that reacts most readily with CO2 to produce calcite (CaCO3). The pH
of the pore solution can be reduced to 8.3 [16], if all Ca(OH)2 has been depleted. The reaction
that takes place in aqueous solution is:
Ca(OH)2 + CO2 CaCO3 + H2O (2.1)
In normal practice, the corrosive impact of carbonation is limited and relatively easy to avoid [5]. For concrete with high cementitious material content and low w/c (<0.4), carbonation rates are typically on the order of 1 mm per decade or less [17]; loss of passivity due to this cause within a normal design life is generally not a concern. However, carbonation must be anticipated at concrete cracks, where air essentially has direct access to the reinforcement, irrespective of concrete cover and quality. In general, corrosion occurs more rapidly under conditions of
Figure 2.2 Schematic illustration of various stages involved in the development of reinforcement
exposure to chlorides and chloride induced corrosion is of a more serious concern for reinforced concrete in particular in a salt-laden environment.
2.2.2 Chloride induced corrosion
Soluble chlorides present in seawater, ground water or de-icing salts may enter concrete through capillary absorption or diffusion. Chlorides may also be introduced into concrete by using contaminated aggregates or mixing water in the production of concrete. Sometimes excessive chlorides can be present in chemical admixtures e.g., when calcium chloride is added in fresh concrete as a set accelerator, which was reported to cause premature structural problems related to steel corrosion [18]. While the chloride ion (Cl-) has only a small influence on the pH of pore solution, concentrations as low as 0.6 kg/m3 by weight of concrete have been projected to compromise steel passivity [17]. According to the European standard EN 206, the maximum allowed chloride contents are 0.2-0.4% chloride ions by mass of binder for reinforced and 0.1-0.2% for prestressed concrete [19]. Chlorides can be chemically or physically bound, being adsorbed to the hydrated cement paste. Numerous researchers have related corrosion risk to chloride content. The chloride content in concrete is usually expressed in terms of the amount of total chloride by mass of cement or cementitious binder. It should be realized that the total chloride content is not really responsible for corrosion. The free chlorides instead of bound chlorides are the only ones that can possibly destroy the passive film on the surface of the
reinforcing steel and therefore initiate corrosion. In addition, plenty of oxygen and water are available in all above-ground outdoor structures to facilitate significant corrosion. Corrosion is assumed to start when the concentration of chlorides at the embedded steel surface has reached a certain so-called “chloride threshold” value or critical chloride content. Once the chloride threshold has been exceeded, the local disruption of the passive film initiates corrosion cells between the active corrosion zones (anode) and the surrounding areas that are still passive (cathode). Figure 2.3 [17, 20] schematically shows the electro-chemical process and the main reactions involved in chloride contaminated reinforced concrete. As can be seen, the chlorides actually act as a catalyst in this process accelerating the corrosion. The chloride ions release from the soluble complex of ferrous chloride by hydroxyl ions produced in cathodic reaction, so the rust itself contains no chlorides. Depending on the condition of the steel surface and corrosion degree, the composition of the rust can be varied. As shown in Figure 2.4, the volume of produced rust can be six times larger than that of the original steel [21]. The resulting expansive stress due to the increased volume is restricted inside the concrete. As corrosion propagates, it could eventually cause cracking, spalling, or delamination of the concrete cover, which further the accessibility of airborne carbon dioxide. It may need to be noted that although corrosion due to carbonation proceeds at a much lower rate than that due to chloride ingress [22], the combination of chloride ingress and carbonation will make the corrosion process more complicated and the corrosion risk due to the combined effect is sometime higher than that of either cause separately [2, 23].
2.3 Factors that affect corrosion of reinforcement
2.3.1 Permeability of concrete
Concrete, as a porous and highly heterogeneous composite, is subject to the ingress of various ionic and molecular species from the environment. Excessive accumulation of certain species can be a vitally deleterious factor affecting the service life of concrete structures. In general, permeability indicates the property of concrete to allow substances to intrude the concrete and attack the reinforcing steel resulting in corrosion. The penetration of gases, liquids or ions into concrete takes place through pore spaces in the cement paste and paste-aggregate interfaces or microcracks according to four basic mechanisms, namely: capillary suction, permeation (due to pressure gradients), diffusion (due to concentration gradients), and migration (due to electrical potential gradients) [5, 24]. Permeability is believed to be a decisive characteristic of concrete durability, which is related to its microstructural properties, such as the size, amount, distribution, tortuosity and connectivity of pores and microcracks [25-27]. The microstructure of concrete is influenced by the water to cement ratio (w/c) of the concrete, the degree of cement hydration and the inclusion of supplementary cementitious materials (SCMs) which serve to refine the pore structure [28]. Although the bulk hardened cement paste influences the permeability significantly, the influence of the paste-aggregate interface is also not negligible [29, 30]. Microcracking in the paste-aggregate interface (also known as ITZ) can alter the connectivity of pores making disconnected pores become connected and creating pathways for the flow of water and dissolved ions. Substantial hydration, a relatively low w/c ratio and a well-developed ITZ, as well as inclusion of SCMs (e.g., blast furnace slag, fly ash, silica fume) [31, 32], all contribute to reduce the permeability of concrete and thus enhance its resistance to corrosion damage or other relevant degradation issues.
2.3.2 Chloride binding
Chloride binding in concrete may be defined as the interaction between the porous matrix of concrete and chloride ions which results in effective removal of chlorides from the pore solution [33]. It is known that hardened cement paste has the ability to bind chlorides which makes concrete itself the first natural barrier against chloride ingress, although this binding capacity to some degree is limited [34, 35]. Effective binding will remove a part of chloride from the transport process as well as alter the pore solution concentration and therefore the concentration gradient driving chloride diffusion. The critical chloride content expressed by total mass able to initiate corrosion of reinforcing steel will be increased accordingly for concrete with a high chloride binding capacity. Binding of chlorides can take place through both chemical combination and physical adsorption. Chlorides may interact with hydrated cement forming different chloride bearing AFm-like (tetracalcium aluminate monosulfate) phases, such as
Friedel’s salt (3CaO•Al2O3•CaCl2•10H2O) or its iron analogue (3CaO•Fe2O3•CaCl2•10H2O)
and/or Kuzel’s salt (3CaO•Al2O3•½CaSO4•½CaCl2•12H2O) and solid solutions with other
AFm-like phases [36-39]. Chloride-ettringite has however been reported to occur only below 0°C and usually at very high chloride concentrations [40, 41]. Chloride can also physically be adsorbed on the surface of the C-S-H as well as in interlayer spaces and possibly be chemisorbed by C-S-H as an oxychloride complex (i.e., 3CaO•CaCl2•xH2O) or substituted in the structure [39, 42-45].
Compared to the main binding product (i.e., Friedel’s salt), oxychlorides are highly soluble and only stable at very high chloride concentrations [38]. From samples exposed to long term submersion in sea water, it appeared that most of the chloride was bound either chemically or physically to C-S-H, rather than by chemical binding in detectable crystalline compounds [6].
There are many factors associated with the constituents of the concrete affecting the chloride binding capacity. These factors include cement type, curing temperature and age, pH of pore solution, w/c ratio, and chloride concentration and so on. Based on previous work, Glass and his co-workers found that the C3A (tricalcium aluminate) content in cement and the type and
proportion of cement replacement materials are the most important factors influencing the binding capacity of concrete [46]. Cements containing high content of C3A and C4AF
(tetracalcium aluminoferrite) can make hardened cement pastes rich in AFm, which has the ability to accommodate chlorides and could be able to bind relatively large amounts of chlorides [47-49]. An increase of sulfate content in cement reduces the chloride binding capacity since Friedel’s salt is not stable when excess sulfate ions are present, and tends to be converted to ettringite [50]. However, it is worthy to be pointed out that the effect of external sulfates on binding may constitute a difference between exposure to sea water (with significant sulfate) and de-icing salts (usually pure chloride-bearing compounds). Chloride binding is significantly influenced by the incorporation of SCMs, such as blast furnace slag, fly ash, silica fume and natural pozzolans. Generally, blast furnace slag and fly ash increase chloride binding but silica fume can decrease the chloride binding capacity [51-53]. It is also reported that a part of the bound chloride could be released when the pH drops to values below 12 (e.g., carbonation or sulfate attack) [54]. Although the protective passive film at the steel surface is still thermodynamically stable at this pH, it might be assumed that all the chloride released will be available to sustain local passive film breakdown.
2.3.3 Concrete cover
Corrosion of steel in reinforced concrete structures is considerably different from corrosion of steel exposed to the atmosphere, as in the former case the steel is protected by the concrete cover, which provides a good physical barrier protecting the steel from corrosion [55, 56]. This barrier also limits the diffusion of oxygen that is necessary for sustaining corrosion. The thickness and quality of concrete cover are very important factors in controlling the level of protection provided to embedded steel. A test conducted on reinforced concrete showed that when the cover thickness
is increased from 30 to 40 mm, the rate of corrosion of the embedded steel could be reduced about 91% after six cycles of wetting and drying [57]. Eurocode 2 [58] fixes minimum values of the concrete cover ranging from 10 mm for a dry environment up to 55 mm for prestressing steel in chloride-laden environments. Considering the construction variability met in practice, Eurocode 2 suggests that these minimum values should be increased to obtain nominal values by 10 mm. However, the cover thickness cannot exceed certain limits for mechanical and practical reasons. In particular a very thick cover may not work as well in defending against corrosion as expected. In extreme cases, a thick unreinforced layer of concrete cover may facilitate the formation of microcracks or even macrocracks due to tensile forces exerted by drying shrinkage of the outer layer. In normal practice, the cover thickness shouldn’t be above 70 to 90 mm [5].
2.3.4 Environmental conditions
Corrosion itself is the consequence of environmental impacts. The environmental conditions relate to numerous factors that are not always independent of each other. From a corrosion point of view, these factors have simultaneous and complex synergistic effects connected to both the macroclimate and to local microclimatic conditions acting on the concrete structure such as: the source (internal or external) and type of chloride contamination, humidity, the temperature and the frequency of wetting-drying or freezing-thawing cycles. The source of chloride can influence both chloride binding and pH of the concrete pore solution [51, 59]. It was reported that a higher chloride content can be tolerated when it is added as an additive to the concrete mix [60, 61]. In terms of the type of the chloride salts, previous studies showed that the associated cation has a strong influence on the diffusion rates and the chemical binding of the chloride [62]. It was found that the diffusion coefficient of chloride combined with divalent cations is greater than that combined with monovalent cations. In addition, Tuutti reported that the ratio of chemically bound chloride to freely dissolved chloride for CaCl2 is approximately three times that for KCl [14].
Compared to arid or temperate climates, tropical or equatorial environment generally increases the corrosion risk in concrete [63]. Reinforced concrete under chloride exposure experiencing wetting-drying cycles could concentrate the chlorides at locations which have the highest average moisture content [64]. Consequently, a significant increase in the local chloride concentration results, which in turn leads to the occurrence of pitting. Freezing-thawing cycles can also impose serious corrosion risk to the reinforced concrete in particular in cold regions where de-icing salts are often used [65]. The European standard EN 206-1 [19] explicitly defines exposure classifications based on environmental conditions to which the concrete is exposed and that result in effects on concrete or the reinforcement or embedded metal. It should be noted that this classification limits to average regional exposure conditions; local microclimatic conditions however have not been considered.
2.4 Corrosion preventive measures
Prevention of reinforcement corrosion can be primarily achieved in the design stage by using high quality concrete, sufficient cover, and optimizing concreting materials and mix design as indicated in the European standard EN 206-1 and Eurocode 2 [19, 58]. Additional prevention options are generally adopted when severe environmental conditions occur (e.g. marine environment or the application of deicing salts), or on structures requiring very long service life, as well as in rehabilitation. A number of corrosion preventive measures with their own advantages and limitations have been applied separately or in a synergic manner either to existing chloride contaminated structures or to new structures [5, 17, 66-71]:
1) Physical barrier systems such as applying waterproofing membranes, sealers and overlays on concrete surfaces to retain and/or restrict subsequent chloride penetration; surface coating of the reinforcing steel such as using epoxy-based coating to restrict oxygen’s access to the surface of rebar.
2) Electrochemical techniques such as cathodic protection for polarizing the steel to a potential at which new pitting is suppressed and existing pit growth is at least substantially retarded, electrochemical chloride extraction (ECE) which aims at removing chloride ions from the cover zone of chloride contaminated concrete and electrochemical realkalisation (ER) which aims at restoring the alkalinity of carbonated concrete cover. 3) Use of corrosion resistant reinforcement such as galvanized steel, alloy claddings,
stainless steel and polymer-based reinforcing materials.
4) Use of corrosion inhibitors added to fresh concrete as an admixture, or applied to the hardened concrete surface or used as a surface treatment on rebars before concreting. 5) Use of new methods and technologies such as nano-materials, self-healing, mineral
admixtures and multifunctional additives, etc.
2.5 MHTs and their application in cementitious materials
2.5.1 General aspect
Hydrotalcite is a natural mineral that was discovered in 1842 in Sweden and the first exact formula, [Mg6Al2(OH)16]CO3•4H2O, was published in 1915 by Manasse [72]. Subsequently
hydrotalcite gave its name to a large mineral group of naturally occurring Layered Double Hydroxides (LDHs), which are also known as hydrotalcite-like materials or are commonly referred to as hydrotalcites. Although LDHs are available as naturally occurring minerals, nowadays a great number of pure LDH compounds has been prepared and characterized in the laboratory [73, 74]. Since all of these synthetic LDHs have the same structure as their parent material hydrotalcite, they can also be termed modified hydrotalcites (MHTs), a term which is used throughout this thesis. MHTs are structurally similar to the minerals Brucite (Mg(OH)2), and
Portlandite (Ca(OH)2), in which a central divalent metal cation is surrounded by six hydroxyl
groups in an octahedral configuration. These octahedral units form infinite, charge-neutral layers by edge-sharing, with the hydroxyl ions sitting perpendicular to the plane of the layers. The layers then stack on top of one another to form the three-dimensional structure. In MHTs, a fraction of divalent cations is isomorphously substituted by trivalent cations, which generates a net positive charge on the layers that necessitates the incorporation of charge-balancing anions in the interlayer galleries. The remaining free space of the interlayer is occupied by water molecules via hydrogen bonding. The most common anion found in naturally occurring hydrotalcites is carbonate. In practice, there is no significant restriction to the nature of the interlayer charge-balancing anions. The MHT structure can accommodate various cations in the hydroxide layers with varying MII/MIII ratios as well as a wide range of anionic species in the interlayer regions. This high variation allows the large variety of materials to be represented by a general formula [MII1-x MIIIx (OH)2]x+ [(An-x/n)]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 (CO32-, SO42-, Cl-, NO3-, NO2- and carboxylates, amino or
polyamino carboxylates, etc.) with valence n. The x value is in the range 0.20-0.33. A typical structure of MHTs can be schematically shown as in Figure 2.5.
In cement chemistry, LDHs or hydrotalcite-like materials represent a group of phases that form during cement hydration. The hydration products of tricalcium aluminate (C3A) and
tetracalcium aluminoferrite (C4AF) are hexagonal-layered materials denoted C2(A,F)H8,
Figure 2.5 Schematic representation of a typical carbonate hydrotalcite crystal structure (d-spacing
C4(A,F)H13, and C4(A,F)H19. These hydrates along with the AFm phases are recognized as
hydrotalcite-like compounds [75]. As mentioned above, it is well known that the stability of AFm phases play a very important role in controlling the performance of concrete since chloride ions could interact with hydrated cement forming chloroaluminate phases, such as Friedel’s (a chloride-bearing AFm phase) salt and/or Kuzel’s (a chloride- and sulfate-bearing AFm phase) salt and solid solutions with other AFm phases [39, 47]. Formation of these specific chloride-containing salts is potentially a mechanism for retarding chloride diffusion and thus mitigating chloride-induced corrosion.
2.5.2 Synthesis and characterization
A wide variety of cation combinations (e.g., MII, MIII) and different anions in the interlayer are found in the structures of synthetic MHTs. It is suggested that only MII and MIII ions having an ionic radius not too different from that of Mg2+ may be accommodated in the octahedral sites of the brucite-like layers to form LDH compounds [74]. Cations that are too small, such as Be2+ or too large such as Cd2+ will not be able to fit into the lattice, hence, other types of structures will be formed. The most widely occurring compositional range corresponds approximately to an MIII/(MII+MIII) ratio of x between 0.20 and 0.33 as mentioned above. There are a number of techniques that have been successfully applied to synthesize MTHs, among which three main methods are frequently used. The most commonly method used is a simple coprecipitation of two metal salts in alkaline solution at a constant pH value of about 10. The second one is based on the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed LDHs to produce specific anion intercalated MTHs. The third method is a lattice reconstruction after heating (calcination), which is based on the hydrotalcite-like materials’ unique feature of “structural memory effect”, due to which the original structure is reproduced after re-hydration. Therefore, this method is called the “calcination-rehydration”. In addition, some other methods, such as sol-gel synthesis using ethanol and acetone solutions, and a fast nucleation process followed by a separate ageing step at elevated temperatures have also been reported. A common concern with all the methods is that in preparations of MHTs with anions other than carbonate, it is always important to avoid contamination from ambient CO2
since the carbonate anion is readily intercalated and firmly held in the interlayer space. Consequently, decarbonated and deionized water has been often used and exposure of the reacting materials to the atmosphere needs to be carefully avoided during the preparation. More details about the synthetic techniques for the preparation of MHTs can be found in the literature [76, 77]. It should be pointed out that once formed, MHTs are stable in alkaline and neutral solution although they are soluble at pH below 4.0 [78], which means that in alkaline and carbonated concrete (pH around 9.0), MHT is a stable solid compound.
A variety of modern techniques has been employed to characterize synthesized MHTs. Powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) are
principally performed to check the structural characteristics of MHTs whilst other techniques such as X-ray fluorescence (XRF) [79], X-ray absorption spectroscopy (XRS) [80] and electron spin resonance (ESR) [81] as well as Raman spectroscopy [82] are less extensively employed although reported. Energy-dispersive X-ray spectroscopy (EDX) [75], nuclear magnetic resonance (NMR) [83, 84], atomic absorption spectroscopy [85], ion coupled plasma optical emission spectrometer (ICP-OES) [86], ion chromatography (IC) [82] and Mössbauer spectroscopy [79] as well as elemental analysis of carbon/sulfur, oxygen/nitrogen & hydrogen [82, 86] are also performed to determine the compositional formulae of MHTs. The thermal behavior of MHTs are commonly studied using thermogravimetry (TG), differential scanning calorimetry (DSC) and differential thermal analysis (DTA) [87, 88]. In certain cases, these techniques are used in combination with a mass spectral analyzer to study the nature of the evolved gases during the thermal treatment. Morphology and particle size are often checked by electron microscope (e.g., SEM, TEM, FESEM) and particle size analyzer.
2.5.3 Ion exchange of MHTs and its role in capturing chloride
The key feature of the application of MHTs is their high anionic exchange capacity (2 to 4.5 millequivalents/g) which makes exchange of the interlayer ion by a wide range of organic or inorganic anions versatile and easily achieved [89-91]. The anion-exchange capacity of MHTs is inversely dependent on the layer charge density (i.e. the MII/MIII molar ratios in the brucite-like sheets). Additionally, it is affected by the nature of the interlayer anions initially present. Generally, the naturally occurring hydrotalcite, which has the chemical formula [Mg6Al2(OH)16]CO3•4H2O, is regarded as the parent material of MHTs. The carbonate anion is
tenaciously held in the interlayer space, so ion-exchange reaction hardly occurs in carbonate form hydrotalcite [92]. However, such materials are believed to have a great potential to be modified or tailor-made. MHTs have greater affinities for multivalent anions relative to monovalent anions [74, 76]. It was reported that the ion-exchange equilibrium constant tends to increase as the diameter of the anion decreases [93]. Previous research [94] has also found that the interlayer interactions can be direct or mediated through other species present in the interlayer region, both by coulomb forces between the positively charged layers and the anions in the interlayer and by hydrogen bonding between the hydroxyl groups of the layer with the anions and the water molecules in the interlayer. In summary, MHTs have a very rich interlayer chemistry and can participate in anion exchange reactions with great facility. For the promising use as a smart functional additive for concrete against chloride attack, certain inorganic or organic anions with known inhibitive properties may be intercalated in the structure of hydrotalcite. Then, an ion exchange reaction in MHT can be triggered possibly 'automatic' upon arrival of external chloride ions. The present chloride ions are captured and the intercalated inhibitive anions are simultaneously released, which provide further inhibitory protection to the reinforcing steel. The
anion exchange process can be described in the following Eq. 2.2 [95] and also shown schematically in Figure 2.6:
HT-Inh + Cl-(aq) HT-Cl + Inh-(aq) (2.2) where Inh- represents the intercalated inhibitive anions,; Cl- is the free chloride ions present in concrete pore solution. As such, MHT plays a dual-role against chloride-induced corrosion in reinforced concrete acting as a chloride scavenger and providing corrosion inhibitors in parallel as an internal inhibitor reservoir and protecting reinforcing steel from corrosion continuously.
2.5.4 Application in cementitious materials
Previous studies have demonstrated that several ion exchangers based on hydrotalcite dispersed in polymeric coatings are strong corrosion inhibitors for the metallic substrates [95-106]. The corrosion is hindered due to the release of an inhibitive anion that diffuses through the pore space of the coating, and it is exchanged for chloride ions in the environment, which are “entrapped” into the interlayer space of the unique layered structure of hydrotalcite [95-97, 107-109]. These
results suggest that MHT has a high potential as active component in concrete with corrosion protection properties that can be tailor-made.
Sustainable development of concrete infrastructure continues to be of importance to the construction industry. The use of supplementary materials or functional additives in concrete is an integral component of these initiatives. Hydrotalcites or hydrotalcite-like phases have been found in hydrated slag cements, which are known to be able to bind more chloride ions than pure Portland cements [51, 52, 110]. Nonaka and Sato [111] patented a cement modifier that contains hydrotalcite or hydrotalcite and blast furnace slag and provided a method capable of blending this modifier into concrete/mortar without inhibiting the hydration reaction of cement and the method for imparting required corrosion resistance and durability into the cement/mortar. The existence of hydrotalcite-like phases such as Friedel’s salt or its iron analogue has been believed to be a main contributor for chloride binding in cementitious materials and thus has enhanced the corrosion resistance of reinforced concrete. The beneficial effects of Friedel’s salt on binding chloride have indicated the possibilities for using other hydrotalcite-like compounds in concrete as an effective chloride capturer or scavenger. Increased binding capacity would consequently slow down chloride transport in concrete. Furthermore, MHTs are expected to be able to retain bound chloride even in a neutral and high temperature environment, for example if the pH of the pore solution drops due to carbonation. Raki et al. [75] demonstrated the promise of MHTs as suitable hosts for intercalation of organic admixtures with the long-term view of controlling their release rate in concrete by blending inorganic-organic nanocomposites (in small amounts) with the cement. In their study, nitrobenzoic acid (NBA), naphthalene-2,6-disulfonic acid (26NS), and naphthalene-2 sulfonic acid (2NS) salts, commonly used as admixtures in concrete production, were intercalated through anion-exchange of nitrate in the host material, [Ca2Al(OH)6]NO3•nH2O.
It was suggested that potential future applications of these composite materials could be to control the effect of admixtures on the kinetics of cement hydration by programming their temporal release. In addition, in a recent patent [112], Raki and Beaudoin disclose that a controlled release formulation for a cement-based composition can be produced by means of hydrotalcites modified with different anions, which confer functions such as an accelerator, a set retarder, and a superplasticizer when they are released. The patent also mentions the potential corrosion inhibition function of this formulation but unfortunately no examples are supplied. Ashida et al. [113] and Mihara et al. [114] disclose a cement admixture that comprises hydrotalcite and other functional additives in which it is claimed that the hydrotalcite-based admixture has excellent ability to capture chloride or carbonate ions, thus can simultaneously prevent concrete deterioration caused by chloride and carbonation. Tatematsu et al. [115] synthesized a hydrocalumite-like material (calcium-aluminum based MHTs) and added it into cement mixtures as a salt adsorbent. Their results showed that once the admixed salt adsorbent contacts with chloride ions it could adsorb them and release the intercalated nitrite anions in the meantime. The released nitrite ions on the other hand could work as an efficient inhibitor for induced corrosion. The corrosion inhibiting effect of the salt adsorbent on
chloride-induced corrosion was further confirmed by the experiments performed with a large-size specimen. In another patent, Tatematsu et al. [116] also disclosed a hydrocalumite that contains nitrite or nitrate ions that can be added directly to the concrete as a chloride ion scavenger to prevent choride-induced corrosion. Kang et al. [117] patented a corrosion inhibitive repair method for reinforced concrete structures using repair mortar containing nitrite-based hydrocalumite and confirmed its effectiveness in inhibiting corrosion to some degrees. A cement additive for inhibiting concrete deterioration was developed by Feng [118] and Tatematsu et al. [119] with a mixture of an inorganic cationic exchanger: a calcium-substituted zeolite capable of absorbing alkali ions (e.g., sodium, potassium) and an inorganic anionic exchanger: hydrocalumite capable of exchanging anions (e.g., chlorides, nitrates, sulfates, etc.) that is directly incorporated into mortar or concrete mixtures. The testing results from this specific mixture indicated the potential of protecting concrete from deterioration by exchange of alkali and chloride ions to mitigate alkali-aggregate reaction and corrosion of reinforcing steel under the control of ion exchange mechanism. It is noted that corrosion inhibition systems based on this type of cement additive refer to hydrotalcites intercalated with inorganic anions. However, the possibilities of these kinds of corrosion inhibition systems using organic anions as the interlayer inhibitive species are not discussed anyway. In another relevant patent, Kashima [120] describes on one hand, the use of a carbonate hydrotalcite as a system able to incorporate chloride ions from the environment in the hydrotalcite structure and on the other hand, polycarboxylic acid and organoamine as corrosion inhibitors are also added to the cement-based materials to prevent carbonation and corrosion of reinforcing steel due to the penetration of water, oxygen, etc., as well as to adsorb and remove salt contained in the concrete. However, this work also does not deal with the integral use of organic anions intercalated in the hydrotalcite structure although those organic species are used as components of the concrete mixture. In view of the ease of hosting of molecular anions in the interlayer space of MHTs, certain organic inhibitors which are suitable for use in concrete could also be intercalated into the structures of MHTs. For the envisaged use as a new additive to concrete, Fischer et al [121] recently patented a corrosion inhibition system for reinforced concrete against chloride induced corrosion that consists of MHTs intercalated with organic species, in particular eco-friendly amino acids, which can be directly applied (without paint or polymeric carrier) as aqueous emulsion on a metallic substrate, or it can be mixed into fresh concrete with the various components. The corrosion inhibition system is believed to be able to work as a smart storage of active corrosion protective agent and therefore can keep the chloride concentration below the threshold that would initiate corrosion of reinforcing steel during the concrete service life. The experimental results from their work demonstrate that the corrosion inhibition system based on some amino acids modified hydrotalcites is at least as effective as those based on MHTs intercalated with inorganic inhibitors in particular such as nitrites which are well known for their excellent corrosion inhibiting performance in reinforced concrete system.
2.6 Concluding remarks
Corrosion-related durability issues are nowadays associated with high maintenance costs and safety concerns, so any improvement in the design, production, construction, maintenance, and materials performance of concrete could be crucial in improving the service life of concrete structures, which in turn may have enormous social, economic and environmental benefits. Modified hydrotalcites (MTHs) designed for chloride scavenging and release of inhibiting agents represent a class of technologically promising materials owing to their low cost, relative simplicity of preparation, and plenty of composition variables that may be adopted. Up to date, a lot of academic work and commercial interest in MHTs has been invested, among others for corrosion protection of metals, but not much of it has been directed towards cementitious materials. In particular, research is needed for exploiting their potential applications in corrosion protection of reinforced concrete. We are confident that future work on applications of new smart functional concrete additives based on MHTs will expand rapidly and contribute greatly to the effort of searching for effective measures to improve the durability of reinforced concrete.
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