A FEASIBILITY STUDY OF ANTICORROSION APPLICATIONS OF
MODIFIED HYDROTALCITES IN REINFORCED CONCRETE
Zhengxian Yang (1, 2), Hartmut Fischer (3) and Rob Polder (2, 4)
(1) Materials innovation institute (M2i), Delft, The Netherlands
(2) Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands
(3) TNO Science and Industry, Eindhoven, The Netherlands
(4) TNO Built Environment and Geosciences, Delft, The Netherlands
Abstract
A carbonate form of Mg-Al-hydrotalcite with Mg/Al =2 and its p-aminobenzoate (pAB) modified derivative were synthesized and characterized by means of XRD, IR and TG/DSC. Mg(2)Al-CO3 was prepared by a coprecipitation method and was subsequently modified by pAB through the calcination-rehydration technique. The results from the relevant characterizations combined with total organic carbon (TOC) analysis further confirmed that pAB anions were successfully intercalated into the interlayer space of the hydrotalcite. The anticorrosion behavior of Mg(2)Al-pAB was evaluated on the basis of open circuit potential (OCP) monitoring of carbon steel in simulated concrete pore solution at pH 13 contaminated with chloride. The preliminary results from this study demonstrated that ion-exchange indeed occurred between free chloride ions in simulated concrete pore solution and the intercalated pAB anions in Mg(2)Al-pAB structure. The simultaneously released pAB anions were found to exhibit the envisaged inhibiting effect and cause a shift of corrosion initiation of the steel to higher chloride concentrations than without the modified hydrotalcite.
1. INTRODUCTION
Corrosion of the reinforcing steel is a major threat to the durability of concrete structures which accounts for large amounts of unplanned repairs, associated costs, out of service time and waste of materials and energy. Normally, steel in concrete is protected from corrosion by a passive film formed in the high alkalinity of the concrete internal environment. However, this protective film can be destroyed by the ingress of chlorides and carbonation of concrete. In normal practice, the corrosive impacts of carbonation are relatively mild and easily avoided compared to those due to the effects of chlorides [1]. Once corrosion initiates, three main consequences occur [2]: 1) local pitting corrosion of the reinforcement; 2) early 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. All of these consequences may compromise the structural integrity. Therefore it is crucial to design the concrete to sustain the environmental aggressiveness. Traditionally available anti-corrosion options such as coatings on the surface of concrete or reinforcement, stainless steel reinforcement, addition of inhibitors to the concrete and the application of cathodic protection are not able to guarantee a long enough protection or too expensive or technically too complicated to be applied on a wide scale [3-6].
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 the underlying working mechanism. Among them, modified hydrotalcites (MHTs) may represent a promising option for use in concrete as a new type of functional additive [7,8]. 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. A key feature 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 [9]. Hydrotalcite or hydrotalcite-like phases have been found in hydrated slag cements, which are known to bind more chloride ions than pure Portland cements [10-12]. The existence of hydrotalcite-like phases such as Friedel’s salt or its iron analogue and/or Kuzel’s salt have been believed to contribute to chloride binding and thus enhance the corrosion resistance of reinforced concrete [13]. For the envisaged use as an additive to concrete against chloride attack, certain inorganic or organic anions with known inhibitive properties could be intercalated in 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 other (typically single-function) protective approaches, MHTs play a double-role against chloride-induced corrosion: simultaneously capturing aggressive chlorides and releasing inhibitive anions to protect the reinforcing steel from corrosion. In this work, magnesium-aluminate-based hydrotalcite, Mg(2)Al-CO3 (Mg/Al atomic ratio 2:1) modified by p-aminobenzoate (pAB) was synthesized as a model material for a whole family of MHTs and experiments were designed to investigate the feasibility of MHTs with the selected intercalating organic species to be able to act as a chloride scavenger. The primary objective of the paper is therefore to provide preliminary information to explore the promising use of the other various MHTs compositions with selected intercalating anions as a new type of additive for concrete with a perspective view to reduce chloride-induced corrosion of reinforced concrete.
2. EXPERIMENTAL
2.1 Materials
Mg(NO3)2·6H2O, Al(NO3)3·9H2O and NaNO3 were obtained from Merck KGaA. NaOH, NaNO3 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 solution and filtration. Steel electrodes prepared for corrosion evaluation were low-carbon steel (ASTM A36) with a surface area of 100 mm2. All
electrodes were grinded with No. 320 to No.4000 emery papers in water and further cleaned with acetone under ultrasonication prior to immersion in the relevant testing solutions.
2.2 Synthesis
The Mg(2)Al-CO3 was synthesized by co-precipitation method. Typically, 128g Mg(NO3)2·6H2O and 93.6g of Al(NO3)3·9H2O were mixed together in 350ml deionized water and then the solution was added dropwise within 1.5-2.0 hours to a solution containing 140g, 50% aq. NaOH and 50g NaCO3 in 500ml deionized water under vigorous stirring at 40°C. Once the addition was completed, the resulting suspension was maintained at 65°C for 16hours under vigorous stirring, after which the precipitate was cooled down to room temperature (RT) and collected by filtration and washed thoroughly. The product was then dried for 16hours at 105°C under vacuum.
Modification of Mg(2)Al-CO3 was performed by calcination-rehydration method. Typically, 5g Mg(2)Al-CO3 powder was heated for several hours at 500°C and then was cooled down in N2 flow to RT. 350ml warm water (50°C) was subsequently added under vigorous stirring and then the mixture was cooled down again to the RT. Afterwards, 250ml solution containing 75mmol p-aminobenzoic acid and 75mmol NaOH was added. 6M NaOH or HNO3 was employed to adjust the pH of final solution to be around 10 if necessary. The resulted suspension was stirred vigorously for 10 hours before subjecting to filtration. The modified product, Mg(2)Al-pAB was then dried for 16hours at 105°C under vacuum.
2.3 Characterization
X-ray powder diffraction (XRD) was performed on a Bruker D5005 diffractometer equipped with Huber incident-beam monochromator and Braun PSD detector using Cu Kα radiation in the 2θ region between 5 and 90°. Thermal analyses on powder samples were carried out using a NETZSCH STA 449 F3 Jupiter® simultaneous thermal analyzer TG/DSC under flowing Argon (50ml/min) at a heating rate of 10 K/min from 40 to 1100°C. FT-IR spectra were recorded using a Perkin–Elmer Spectrum 100 Series spectrometer equipped with universal Attenuated Total Reflexion (ATR) unit over the wavenumber range of 4000 to 600 cm−1. Shimadzu TOC-VCPH total organic carbon analyzer was employed to analyze the intercalation amount of the corresponding organic anions (i.e., p-aminobenzoate anion ) after dissolution of a known amount of intercalation compound in dilute HCl solution. Duplicate tests were conducted with the specimen involved.
2.4 Anticorrosion evaluation of Mg(2)Al-pAB
The anticorrosion properties of Mg(2)Al-pAB with respect to steel specimens were evaluated by open circuit potential (OCP) in simulated concrete pore solution contaminated with chloride. At least two independent measurements per testing condition were performed to ensure better reliability of the results. The equipment used for measuring OCP is Eco-Chemie Autolab-Potentostat PGSTAT20 and all potentials are referred to the saturated calomel electrode (SCE). In these experiments, steel electrodes were immersed in relevant testing solutions and four types of testing solution were prepared: 0.1M NaOH (pH=13, as control case) simulating the alkaline pore liquid of concrete; 0.1M NaOH+Mg(2)Al-pAB; 0.1M NaOH+NaCl; 0.1M NaOH+Mg(2)Al-pAB+NaCl. Specifically, 0.5 g Mg(2)Al-pAB powders were mixed in 100 ml testing solution. NaCl was added as a source of Cl
-contamination starting from 0 M, and progressively increases with a step of 0.02M every 24 hours up to 0.08M.
3. RESULTS AND DISCUSSION
3.1 X-ray diffraction analysis
XRD patterns of the synthetic Mg(2)Al-CO3 precursor and its modification derivative Mg(2)Al-pAB are shown in Figure 1. As can be seen from Figure 1, both compounds, Mg(2)Al-CO3 and Mg(2)Al-pAB show sharp basal reflections at low 2θ angles indicating a typical layered hydrotalcite structure with good crystallinity. The basal spacing can be calculated from the (003) position using Bragg’s law, which gives 7.8Å for Mg(2)Al-CO3 and 15.4 Å for Mg(2)Al-pAB. The basal spacing values obtained are in good agreement with those previously reported in the literature [14,15] and carbonate and p-aminobenzoate anions are described to orientate perpendicularly between the host mixed-metal hydroxide layer with interacted water molecules. The expanded basal spacing d (003) from 7.8Å (in case of Mg(2)Al-CO3) to 15.4 Å (in case of Mg(2)Al-pAB) clearly indicates that the carbonate anions were replaced by 4-aminobenzonate anions following the calcination-rehydration process of Mg(2)Al-CO3. In addition, a slightly broader peak (006) in the case of Mg(2)Al-pAB, appeared in the same angle position (2θ=11.4°) when compared to peak (003) of Mg(2)Al-CO3. This finding may suggest the possibilities of overlapping of both peaks in diffractogram of Mg(2)Al-pAB and indicate certain amount of carbonate is present in the interlayer space of Mg(2)Al-pAB as well resulting from the unavoidable CO2 contamination during the preparation of Mg(2)Al-pAB. The content of intercalated p-aminobenzoate was thus further determined by total organic carbon (TOC) analysis and 35.5 % of p-aminobenzoate was detected out of the gross mass of intercalation compound, Mg(2)Al-pAB.
3.2 Infrared analysis
FT-IR Spectra of Mg(2)Al-CO3 and Mg(2)Al-pAB are shown in Figure 2. For both compounds, a broad band between 3700 and 3100 cm-1 is observed representing the stretching vibrations of the hydrogen-bonded hydroxyl group of both hydroxide layers and interlayer water. For Mg(2)Al-pAB, characteristic peaks of pAB were present in the spectrum at 1526
5 10 15 20 25 30 35 40 45 50 55 60 65 70 2Theta/ degrees (003), d=15.4Å (003), d=7.8Å (006) (006) Mg(2)Al-pAB Mg(2)Al-CO3 4000 3500 3000 2500 2000 1500 1000 500 Mg(2)Al-pAB Tr a ns m it/ % wavenummber/ cm-1 Mg(2)Al-CO3 1700 16501600 1550 1500 1450 1400 13501300 -COO-C=C -NH2 wavenummber/ cm-1 -CO3 1604 1589 1526 1428 1372
Figure 1: XRD patterns for
cm-1and 1428 cm-1corresponding respectively to the asymmetric and symmetric stretching vibrations associated with -COO-. Furthermore, a characteristic peak for the -NH2 bending mode is observed at 1604 cm-1 and the aromatic C=C stretching mode appears at 1589 cm-1. The presence of these peaks is comparable with previously reported studies [16] and indicates that p-aminobenzoate anions have been intercalated into the interlayer space of the hydrotalcite. The relatively less intensive peak at 1372 cm-1 when compared to the spectrum of Mg(2)Al-CO3 is probably owing to the co-intercalated carbonates in the interlayer space of Mg(2)Al-pAB which is also in agreement with the results of XRD analysis.
3.3 Thermal analysis
Thermogravimetric (TG) and differential scanning calorimetry (DSC) were employed to study thermal behaviors of intercalation compounds and the results are shown in Figure 3. As can be seen from the TG curves in Figure 3, both hydrotalcite and its modified derivative clearly show two major weight-loss processes. For Mg(2)Al-CO3, the first weight loss of 12.2% from 40 to 225°C corresponds to the elimination of interlayer water molecules, while the second one of 31.8% from 225 to 500°C is due to a concomitant dehydroxylation (i.e., loss of structural water) of the metal hydroxide layers and a decomposition of interlayer carbonates. It is often believed that there are two separate decomposition reactions proceeding in the second weight loss process [17,18] as evidenced by two endothermic peaks occurring in the associated DSC thermograms from 225 to 500°C. The first of these two peaks is attributed to the partial loss of hydroxyl group from the hydroxide layer and the second one to the complete loss of hydroxyl group and carbonate ions. In the case of the modification compound Mg(2)Al-pAB, the first weight loss of 6. 8% from 40 to 225°C corresponds to the elimination of interlayer water molecules, while the second one of 46.2% from 225 to 500°C is due to a concomitant dehydroxylation (i.e., structural water) and decomposition of interlayer pAB. Similarly, two notable endothermic peaks are observed in the associated DSC thermograms from 225 to 500°C. The first of these two peaks is attributed to the partial loss of hydroxyl group from the hydroxide layer and the second one to the complete loss of hydroxyl group and pAB. Moreover, the occurring of the small shoulder peak accompanied with the first one may suggest the decomposition of interlayer pAB has started as early as 270°C and this indicates the higher thermal stability of the intercalated pAB relative to its pure crystalline parent substance, i.e., p-amino benzoic acid whose melting point is 189°C.
Figure 3.TG/DSC curves for (A) Mg(2)Al-CO3 and (B) Mg(2)Al-pAB.
TG/DSC- Mg(2)Al-pAB 40 45 50 55 60 65 70 75 80 85 90 95 100 0 200 400 600 800 1000 1200 Temperature/ ℃ M ass ch an g e ( % ) -8.00E-01 -6.00E-01 -4.00E-01 -2.00E-01 0.00E+00 2.00E-01 4.00E-01 6.00E-01 H eat fl o w ( μ v/ m g ) Mass/% DSC/(μv/mg) exo B TG/DSC- Mg(2)Al-CO3 50 55 60 65 70 75 80 85 90 95 100 0 200 400 600 800 1000 1200 Temperature/ ℃ M ass ch an g e (% ) -2.00E+00 -1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 He at fl o w ( μ v/ m g ) Mass/% DSC/(μv/mg) exo A
3.4 Anticorrosion evaluation of Mg(2)Al-pAB on the basis of OCP evolution
In general, OCP evolution determines the occurrence of corrosion initiation. In an alkaline chloride-containing solution, corrosion, in particular localized corrosion could be initiated when the protective passive layer on the steel surface is disrupted. For the steel in the simulated concrete pore solution, corrosion will not be considered to be initiated if the OCP is equal to or more positive than -270 mV (SCE), a value which is widely accepted in the literature [19,20]. OCPs of all the freshly prepared steel electrodes were around -500±50 mV immediately after immersion and were increasing with time and up to around -270 mV after 24 hours. In these experiments, the OCP readings were recorded after 2 hours immersion and the results were reported as the average values of at least two samples per condition. Figure 4 shows the OCP evolution for the steel electrodes in the testing solutions. As can be observed, the Mg(2)Al-pAB did not significantly influence the OCP of the steel in the two control solutions (without Cl-) since both of the OCP values are more positive than -270 mv from about 24 hour immersion on, and went on to become more positive. This means a stable passive layer has formed on the steel surface in chloride-free alkaline solutions both with or without Mg(2)Al-pAB. For the chloride-containing solutions, various effects occurred. 1) the first addition of Cl- (at 0.02 M) made the OCP of steel in solution without Mg(2)Al-pAB drop by about 50 mV compared to the same solution with Mg(2)Al-pAB; 2) at 0.02M chloride, the OCP evolution of the solution with Mg(2)Al-pAB followed the control solutions without chloride; 3) upon the addition of more Cl-, up to 0.04M, the OCPs of the steel in the solution with Mg(2)Al-pAB stayed positive than -270mv until about 60 hours’ immersion and dropped markedly afterwards, but still was more positive than those without Mg(2)Al-pAB; 4) Upon another addition of chloride up to 0.06M, both electrodes showed more negative OCPs, definitely confirming that active corrosion was present in both cases. Nevertheless, the results herein clearly revealed that the chlorides have been exchanged with intercalated p-aminobenzoate anions which subsequently show some inhibiting effect and cause corrosion initiation shifting to a higher chloride concentration than without the Mg(2)Al-pAB. As expected from the proposed anion-exchange mechanism, the presence of chlorides leads to release of p-aminobenzoate ions. What is relevant from this study is that MHTs are sensitive to the concentration of chlorides. This statement is supported by the more positive OCPs of chloride-containing solution with Mg(2)Al-pAB than those without Mg(2)Al-pAB for a concentration of NaCl in solution of 0.02M and for 0.04M some time. However, when NaCl concentration is increased up to 0.06M after 84 hours immersion, the trend of OCP for both cases is quite similar. The observed effect is probably due to exhaustion of the anion-exchange capacity of Mg(2)Al-pAB, as most of pAB anions had already been anion-exchanged by chlorides. Taking account of 35.5% pAB intercalated in the hydrotalcite structure based on the TOC analysis, the maximum amount of released pAB is calculated to be 0.013M, corresponding to an anion-exchange capacity of 2.6 meqg-1, which is close to the theoretical exchange capacity of 3 meqg-1 for hydrotalcite-like compounds [21]. As mentioned previously, it is worthy to note that MHTs play a double-role against chloride-induced corrosion: simultaneously capturing chlorides and releasing inhibitive anions to protect the steel from corrosion. Therefore, the MHTs can be envisaged as both containers of corrosion inhibitors and traps of chlorides. Specifically, we can attribute the four abovementioned effects observed in chloride-containing solutions to the double-role function of MHTs (i.e., Mg(2)Al-pAB in this case). When the first 0.02M chloride was added, the ion exchange between chloride and pAB which caused the effective Cl- concentration to reduce from 0.02
M by 0.013 M if assuming all the intercalated pAB has been exchanged is believed to sustain the OCP of the steel in solution with Mg(2)Al-pAB about 50 mV higher than the same solution without Mg(2)Al-pAB. From 48 to 60 hours’ immersion after chloride concentration was increased to 0.04 M, likely due to the inhibiting effect of simultaneously released 0.013 M pAB, the OCPs stayed more positive than -270mV. Afterward, the OCPs dropped markedly although they were still more positive than those without Mg(2)Al-pAB, likely due to such inhibiting effect of pAB became dimmed. With further addition of chloride, no inhibiting effect can be observed, in particular after 84 hours’ immersion.
Figure 4. OCP evolution for the steel electrodes in simulated concrete pore solution with and without Mg(2)Al-pAB.
4. CONLUSIONS
A carbonate form hydrotalcite (i.e., Mg(2)Al-CO3) and its modified derivative (i.e., Mg(2)Al-pAB) were synthesized and characterized by means of XRD, IR and TG/DSC. Relative element analysis (TOC) and TG results further confirmed both carbonate and pAB anions were successfully intercalated into the interlayer space of these hydrotalcites. The anticorrosion behavior of pAB modified hydrotalcite i.e., Mg(2)Al-pAB was evaluated on the basis of open circuit potential monitoring of carbon steel in 0.1 M NaOH solution. The preliminary results from this study suggest that ion-exchange indeed occurred between free chloride ions in simulated pore solution and the intercalated p-aminobenzoate anions in Mg(2)Al-pAB structure, thereby reducing the free chloride concentration, which is believed to be equivalent to increased binding of chloride present in concrete. The simultaneously released p-aminobenzoate anions were found to exhibit some inhibiting effect and cause a shift of corrosion initiation on steel to higher chloride concentrations. Such results shed light on the promising use of modified hydrotalcites with intercalated inhibitive anions with different compositions as new types of additives for improved corrosion protection of reinforcement in concrete. Such additives are expected to contribute to the effort of searching for effective measures to improve the durability of reinforced concrete.
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).
-500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 0 12 24 36 48 60 72 84 96 108 120 Immersion time/ h E/ m v( vs S C E ) 0.1M NaOH 0.1M NaOH+ Mg(2)Al-pAB 0.1M NaOH+ NaCl
0.1M NaOH+ Mg(2)Al-pAB+ NaCl
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