Electrochemical and physico-mechanical characterizations of fly ash-composite cement

Construction and Building Materials 243 (2020) 118309

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Electrochemical and physico-mechanical characterizations of fly ash-composite cement Mohamed Heikal a,b,⇑, A.I. Ali a, B. Ibrahim a, Arafat Toghan b,c,⇑ a

Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Chemistry Department, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia c Chemistry Department, Faculty of Science, South Valley University, Qena 83523, Egypt b

h i g h l i g h t s
Herein, characteristics of corrosion events in fixed FA-CCP-matrix have been explored.
Rebar immersed in pure cement extract showed higher corrosion resistance.
The results confirmed that spinach extract is a typical eco-friendly corrosion inhibitor.
At first 6 h, the increased of passive-film thickness followed by decrease corrosion rate.
Significant increase in CS and X was obtained up to 90 days in the 10% FA-CCP mix.
With prolonged hydration, dense crystalline-CSH products were formed in CCP-matrix.

a r t i c l e

i n f o

Article history: Received 13 November 2019 Received in revised form 23 January 2020 Accepted 29 January 2020

Keywords: Fly ash Compressive strength Gel/space ratio Spinach extract Eco-friendly corrosion inhibitor

a b s t r a c t In this paper, the corrosion of C-steel in OPC-fly ash-composite cement pastes (OPC-FA-CCP) was examined with different fly ash (FA) contents (0, 10, 25 and 50 mass%) and immersion times in the absence and presence of spinach extract as a green-corrosion inhibitor. The effect of replacing FA in cement composition on its physical, chemical, mechanical and microstructure properties using a variety of techniques has been also studied. The results indicated a significant increase in compression strength (CS) and gel/ space ratio (X) upto 90 days in the case of OPC-FA-CCP containing 10 mass% FA. At higher FA replacement levels (25–50 mass%), the CS and X values decreased. SEM confirmed the presence of a surface layer of amorphous-ill-crystalline C-S-H as a gel product on the FA grains, with prolonged time showing dense crystalline hydrated fibril-C-S-H products. The C-steel immersed in the pure OPC cement (FA0S) has the highest corrosion potential (Ecorr = 461 mV) vs. SCE. With the increase of the FA content the Ecorr shifts to a more negative value with an indication of the decrease in the corrosion resistance of the rebar. During the first 6 h of immersion, the increased thickness of the passive film leads to a remarkable decrease in the corrosion rate. As the dipping time increases, the current corrosion density in all tested OPC-FA-CCP mixtures is unexpectedly reduced. A significant improvement in corrosion resistance of rebar was achieved in the presence of spinach extract. All the corrosion parameters were also calculated. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Corrosion of steel is one of the most important factors that damage structures of reinforced concrete, mainly due to carbonation and chloride-induced corrosion [1,2]. Carbonation corrosion is that induced by entering atmospheric CO2 into the reinforced concrete structures. Although in an extremely alkaline environment, a ⇑ Corresponding authors at: Chemistry Department, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia. E-mail addresses: [email protected] (M. Heikal), arafat.toghan@yahoo. com (A. Toghan). https://doi.org/10.1016/j.conbuildmat.2020.118309 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

protective oxide layer is formed predominantly on the steel surface, but chloride ions can also attack and destroy this passive layer. The chloride that attacks concrete structures comes from spraying of deicing salts, salty and marine environments. In light of this, scientists are always looking for solutions to avoid the serious problem of rebar corrosion. One possibility is to cover the surface of the rebar with a thin layer of other materials using different deposition methods such as chemical vapor deposition, hydrothermal treatment, electrodeposition, and magnetron sputtering [3–7]. An alternative method is to use organic corrosion inhibitors that can absorb strongly and favorably on the steel surface and thereby

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M. Heikal et al. / Construction and Building Materials 243 (2020) 118309

enhance the passivity and anti-chloride-corrosion ability of rebar [8–10]. But in most cases, the classical synthesis of these organic substances is associated with environmental damage. Recently, the environmentally friendly electrochemical method has been successfully used to synthesize new catechol derivatives [11]. Also, the extraction of corrosion inhibitors from natural materials is currently one of the most important methods used to protect reinforcing steel from corrosion [12–14]. Spinach extract (SE) contains carbohydrates, quinine, phytosterol, glycosides, terpenoids, proteins, vitamin C, vitamin A, iron, potassium and flavonoids [15]. Therefore, it was used as a green corrosion inhibitor of steel in an acid solution at different exposure temperatures [16]. Also, it is considered a simple, practical as well as cost-effective method. Adsorption of the SE extract on the surface of steel in a 0.5 M HCl solution obeys Langmuir adsorption isotherm. Considering the fact that industrial products produced annually by the chemical and agricultural industries have recently led to environmental problems associated with the disposal of these substances. Nevertheless, the use of one or more of these materials in the cement composition is growing rapidly in the construction industry recently. This is due to a range of considerations, the most important of which are cost savings, resource conservation and energy savings [17]. In particular, FA is an alumino-silicate material obtained from various thermal power plants and its properties depend on many factors such as coal type, carbon content, coal pulverization degree, coal origin, storage, method of collection, oxidation state, flame temperature, and combustion for removal of SOx (x close to 2), thus it is now considered a typical component of concrete structure [18,19]. With the replacement of cement with a certain percentage of FA, it performs many functions in a concrete structure like assembly, plasticizer, active metal mixture, and micro-filler. In view of the above points, the present work aims to explore the effect of FA replacement in cement composition on its physical, chemical, mechanical, and microstructure characters as well as the anti-corrosion of reinforced in activated FA-CCP using a variety of techniques. Accordingly, the impact of using spinach extract (SE) as eco-friendly corrosion-inhibitor has also been investigated.

2. Experimental section Ordinary Portland cement of ASTM Type (I) (OPC, Lafarge Cement Company, Egypt), and fly ash (FA, Sika Chemical Company, Egypt) were used. OPC and FA have Blaine surface areas of 3150 and 3774 cm2/g, respectively. X-ray fluorescence (XRF) was used to identify the chemical composition of raw materials. The data of the chemical analysis is given in Table 1. The mineralogical, chemical composition and microstructure of FA has been studied by XRD and SEM equipped with EDX as shown in Fig. 1. The SEM image indicates that the FA consists of vitreous particles of different sizes in the range of < 20 mm, hollow and spheres as shown in Fig. 1A. Fig. 1B represents the XRD patterns of the FA and the data reveals that it consists mainly of quartz and mullite structure. Superplasticizer (SP, Sika Company, El-Abor City, Egypt), its physicochemical parameters were: light brownish liquid, 36% solid residue, 5.2–5.3 pH range, q = 1.06 kg/l, and 52.25% carbon.

Altogether eight samples, varying in mix chemical composition, were studied as shown in Table 2. Each mix is first intermixed in a ball mill of ceramic precursor using five balls for 1 h to ensure 100% homogeneity, and then kept in a sealed container. Consistency and setting times (STs) of fresh-mixing blends were monitoring according to ASTM: C191 [20]. The mixing blends were inserted into 2. 54  2.54  2.54 cm3 moulds, then compressed the paste into two layers manually, then was stored in a cabinet of humidity (100% RH) at 20 ± 1 °C for 24 h, and then stored underwater until the suitable time (up to 90 days) was reached after each interval. The hydration reaction of specimens was stopped by grinding 10 g of the representative sample in a 1:1 methanol/acetone mixture, and then stirred for 1 h. The mixture was filtered through sintered glass G4, twice washed with a stop solution and diethyl ether, then dried at 70 °C for an hour, then collected in polyethylene bags; sealed and stored in desiccators for analysis [21,22]. 600kN capacity SEIDNER Riedinger, compression-machine was used to determine the compressive strength [23,24]. For XRD determination, PW 1730 with X-ray source a Philips diffractometer of Cu Ka radiation (k = 1.5418 Å) was used. The speed of the scan of 2h/min between 5° and 65°. The X-ray tube voltage at 40 kV and the current were 25 mA. The analysis of XRD was performed with computer software search of the PDF diffraction data (JCPDAICDD), 2001. TGA/DTA was carried by using Shimadzu DSC-50 thermal analyzer at a heating rate of 20 °C/min. the rate flow of nitrogen was 30 cm3/min. The microstructure was investigated by ESEM ‘‘Inspect S”, FEI Holland. to study the specimen’s morphology without any coating. Freshly plant materials spinach was used as a green inhibitor (anti-corrosive) for this study as shown in Fig. 2A. After drying the spinach leaves it was ground into a fine powder, then inserted into a vial containing double distilled water. The flask was then refluxed in water-bath for 10 h. Afterward, the mother liquor was filtered and then evaporated to complete dryness. Thus the residue was aqueous plant extract. Interestingly, spinach-leaves contain many active ingredients, including flavonoids, antioxidants, antiproliferators, and anti-inflammatory in biological systems [15]. FT-IR (Fourier transform infrared) spectroscopy is used to analyze the plant extracts using a Nicolet 10 spectrophotometer within a range of 4000–400 cm1 (see Fig. 2B). Filtrates of a soluble component of OPC-FA-CCP slurries were obtained in three different ways. First, by mixing the OPC-FA-CCP with bi-distilled (1:10 ratio) and stirring with a magnetic stir bar for 4 h. Other solutions were obtained by mixing-up OPC-FA-CCP with different percentages of FA (0, 10, 25 and 50 mass%), and then stirred also for about 4 h. A third sample was prepared by adding a 5 mass% NaCl solution to the OPC-FA-CCP as a corrosive agent. Carbon-steel rods were used as working electrodes (WE) for the electrochemical studies. It is manufactured to have a fixed exposed surface area of 1 cm2 with a chemical composition (mass%): 0.250C, 0.350 Mn, 0.024P, 0.003Si, and 99.373 Fe. WE electrodes with epoxy resins were installed at one end in Pyrex glass tubes with the appropriate diameter leaving an exposed length of 1 cm to connect the solution. All the electrochemical experiments (EC) were performed on a PGP 201 potentiostat-Galvanostat electrochemical workstation. Thus, the corrosion potential (Ecorr) and the instantaneous corrosion current (icorr) can be precisely monitored. The EC was

Table 1 Chemical composition of the raw materials, percentage, mass%. Material

OPC FA

Composition, % CaO

SiO2

Fe2O3

Al2O3

SO3

MgO

Na2O

K2O

L.O.I

63.00 2.33

20.41 63.10

3.40 5.40

4.94 26.54

3.22 0.09

1.90 0.02

0.40 0. 52

0.22 0.85

3.04 2.40

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Fig. 1. Morphological characters and chemical structure of FA; (A) SEM image; (A) XRD diffraction patterns.

Table 2 Mix composition of the investigated cement blends, (mass%). Material (%)

Mix composition

OPC FA SP

FA0

FA10

FA25

FA50

FA0S

FA10S

FA25S

FA50S

100 0 0

90 10 0

75 25 0

50 50 0

100 0 1

90 10 1

75 25 1

50 50 1

Fig. 2. (A) Optical photograph for spinacia oleracea, and (B) FT-IR spectrum for the spinach extracts.

carried out in a three-electrode configuration: a platinum wire, steel bars, a SCE (saturated calomel) used as a counter, working and reference electrode; respectively. Regarding the WE, as described above a cylindrical of reinforced steel with a 0.38 cm2 bottom surface area was used. The WE electrode was mechanically polished with different emery grade papers and then rinsed with distilled water. Subsequently, it was washed with acetone and eventually with bi-distilled water before being introduced in the test solution. Potentiostatic polarization measurements were carried out by scanning the potential from 800 to 800 mV at a scan rate of 5 mV s1. Thus, the corrosion parameters of the samples to be examined were estimated by extrapolation of the cathode and anodic Tafel lines to the corrosion potential (Ecorr). Each experiment was repeated twice and the average value was chosen. The inhibition efficiency (IE) can be calculated from the polarization measurements according to the following equation:

 IE% ¼ ½1 

icorrðiÞ icorrðoÞ

  100

where icorr(0) and icorr(i) are the corrosion current densities obtained at C-steel WE (mA/cm2) in the absence and presence of eco-friendly corrosion inhibitor, respectively.

Polarization resistance (Rp (can also be calculated from the current–potential curves using Stren-Geary equation [25]:

Rp ¼ 1=2:303  ðba xbc Þ=ðba þ bc Þ  1=icorr where ba and bc are Tafel slopes and icorr is the corrosion current density. 3. Results and discussion 3.1. Consistency and setting times Fig. 3A,B represents the variation of consistency and STs of pastes containing different percentages of FA from 0, 10, 25 and 50mass% in the presence of 0, and 1 mass% SP. Consistency shows a remarkable increase with FA percentages (Fig. 3A,B). The increase of consistency reflecting the high surface area FA-grains [23]. Fig. 3B represents a remarkable decrease in the consistency with all mixes reflecting the effect of SP, highlighting that the SP increases of fluidity and workability of pastes. The STs are shortening with FA replacement upto 10 mass%, after then the increase in the replacement of FA from 25 mass% to 50 mass%, the STs are extended as shown in Fig. 3A. Fig. 3B shows a remarkable elongation in the final setting time. The elon-

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Fig. 3. Variation of water of consistency and STs of cement pastes with different percentages of FA in the presence of (A) 0, and (B) 1 mass% SP.

gation in STs is assigned to the decrease of OPC-portion as well as to the increase of FA replacement, which affected the hydration rate of FA-composite cement pastes (CCP) in comparison with pure OPC [26,27]. FA-CCP having 10 mass% acts as the nucleating and filling effect, which accelerates STs, hence the rate of pozzolanic reaction enhances to form extra hydration products of CSH, CAH, and CASH. 3.2. Physico-mechanical properties 3.2.1. Compressive-strength (CS) Fig. 4 illustrates compressive-strength (CS) of FA-CCP with age till 90 days. The values of the CS increase with age, reflecting the continued of hydration to produce of CSH, CAH, and CASH strength-giving-phases. These strength-giving-phases precipitated

1400

Compressive strength, kg/cm²

1200

1000

800

600 FA0 FA10

400

FA25 FA50 FA0S

200

FA10S FA25S FA50S

0 1

10

Curing time, days Fig. 4. Compressive strength of CCP cured up to 90 days.

100

in empty-pores to form compacted-matrix, then CS increases with the formation of extra-amounts of binding-strength-phases, due to the pozzolanic-behavior of FA and/or act as nucleating-sites leading to accelerating the hydration of FA in the FA-CCP mixes. Mix FA10 acts as both nucleating-agents and filling action facilitating the reaction with released CH from the cement hydration. Increase of the replacement level of FA, the CS decreases till 50 mass% FA. Increase the substitution of FA level, the CS diminishes, which influences the mechanical-properties of the solidified cementgiving pastes [27–31]. In the presence of SP, CS was enhanced and porosity decreased. SP improves the dispersion of OPC-FA-CCP cement grains, giving superior-compaction in the solidification process. The increase of the fluidity and/or workability decreases of initial porosity expedites the narrowing of the FA grains from released lime forming more excessive-amounts of binding-strength-phases (C-S-H, C-A-H, and C-A-S-H) to precipitated in empty pores-structure giving homogeneous close-matrix structure with higher CS. Mix FA10S mix shows the higher the mechanical-properties values than all OPC-FA-CCP pastes. 3.2.2. Correlation between the gel/space ratio and compressivestrength: The correlation between the gel/space ratio (X) and CS of FA-CCP with age till 90-days 0 mass% and 1 mass% SP are shown in Fig. 5. The values of X (ratio of cement hydrate volume to total quantities of cement to hydrate and capillary pores), calculated using degrees of cement hydration and water of consistency. The data X increases with age till 90-days. On increasing the values of CS, the amounts of gel/binder products increases; so, the products-formed precipitated in the pores of FA-CCP binder increase, so the gel/space ratio increases. Also, the data in Fig. 5B show that the values of CS of superplasticized OPC-FA-CCP are higher than those of pure-OPC-FA-CCP. On increasing the values of X , CS enhances. The volume of the cement gel formed after the hydration is ffi 2.2 times of the anhydrous cement [27]. This increase in the volume of gel cement formed implies the decrease in the porosity of the paste, this enhances the formed of hydrationgel, to increase the X of value. The increase of X values in the case of SP, this is due to the formation of dense-textured matrix. The gel/ space ratio of mixes FA10 and FA10S shows an increase than the other OPC-FA-CCP pastes (Fig. 5A,B). FA10 and FA10S mixes show a remarkable increase in the values of the X values after 7-days to 90-days. At higher replacement level FA upto 25-50mass%, the gel/space ratio decreases.

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1400

1400 (B)

(A)

1200

FA0

Compressive strength, kg/cm²

Compressive strength, kg/cm²

1200

FA10

1000

FA25 FA50

800

600

400

FA10S

1000

FA50S

600

400

200

0

0 1.99

2.00

2.01

FA25S

800

200

1.98

FA0S

2.02

1.98

1.99

Gel/space ratios, %

2.00

2.01

2.02

Gel/space ratios, %

Fig. 5. Relation between CS and gel/space of FA-CCP cured up to 90 days.

3.3. Phase composition: 100

3.3.1. XRD diffraction patterns Fig. 6 shows X-ray spectrometry of mix FA50S treated for 90-days. Fig. 6 illustrates the presence of diffraction lines of CH, C-S-H, CaCO3, b-C2S, C3S and quartz (Q). The peak represented to C-S-H shows an increase, wheat the peak discriminatory to CH shows an increase to 7 days, then decrease to 90 days. XRD patterns show the area under peaks corresponding to CaCO3 increases with age, due to the increase in the area under the peak of CH peak, which is available for atmospheric CO2 carbonation. The behavior of CH peaks is in accordance with the results of DTA/TGA data.

Weight loss, %

96

(A) 92

TGA

88

1 day 7 days 90 days

84 0

200

400

600

o

800

1000

Temperature, C 0.00

Temperature difference, C/mg

3.3.2. Differential thermal analysis Fig. 7A,B illustrate DTA/TGA thermograms of hydrated mix FA50S. The mass loss at 1000 °C calculated from TGA was 9.71%, 12.00%, and 15.23% for FA50S mix treated at 1, 7 and 90-days. The losses in the endothermic region < 200 °C were 3.09, 4.89 and 8.45% for 1, 7, and 90 days conforming mass losses of C-S-H, C-A-H and C-A-S-H products. The decomposition of calcium-sul phoaluminate-hydrates (ettringite (AFt) and mono-sulfate hydrates (AFm)) and C-A-S-H peaks are overlapped with the

o

_ CC

-0.10

(B)

CH DTA 1 day

CSH CAH CASH

7 days 90 days

-0.20 0

200

400

600

o

800

Temperature, C Fig. 6. XRD patterns of mix FA50S treated at 1, 7 and 90-days.

Fig. 7. DTA/TGA of mix FA50S treated at 1, 7 and 90-days.

1000

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decomposition of C-S-H peak [27,32,33]. The mass loss at 450– 500 °C attributed to the decomposition of Ca(OH)2 [34]. The peak at 750–785 °C is attributed to the decarbonation of calcium carbonate. The exothermic peak at 850–890 °C, is due to the formation of monocalcium silicate, which confirmed the pozzolanic action of CH with FA forming CSH [35–37]. The major characteristic of the thermograms is the increase of area under peak of C-S-H, C-A-H, and C-A-S-H with time, while the area under the peak between 450 and 500 °C, show an increase from 1 to 7 days, then decrease in 90 days, as the effect of pozzolanic reaction of FA with the CH liberated. 3.3.3. Microstructure Fig. 8 shows SEM photos of mix FA50S cured at 1 day and 90 days. SEM photos depocits the presence of a surface layer of amorphous ill-C-S-H crystals as a gel product on the FA grains at 1-day (Fig. 8) in addition to the formation of inner hydration within the FA grains [38]. With the prolong the time of hydration (90 days), the SEM photo shows a dense crystalline hydrated fibril C–S–H products. 3.4. Electrochemical monitoring of corrosion rate of rebar in OPC-FAcomposite 3.4.1. Effect of fly ash contents Typical potentiodynamic polarization curves (Tafel plots) of the C-steel working electrode immersed in OPC cement with various contents of FA is presented in Fig. 9. It can be seen that the steel rebar in the pure OPC cement (FA0S) has the highest corrosion potential Ecorr value (461 mV) vs. SCE as a reference electrode. With the increase of the FA content the Ecorr shifts to a more negative value. To obtain more information about what is going on the surface, all corrosion parameters were calculated as a result of fitting the anodic curves from Fig. 9 are represented in Table 3. When examining the data in Table 3, it can be seen that the steel reinforcing rod in the neat OPC cement (FA0S) has the lowest value of icorr of 0.06 mA/cm2 and the highest polarization resistance (Rp) of 118 X, indicating that this composition is superior in corrosion resistance. This is mainly due to the formation of the ferric oxide layer that is leading to a permanent state of passivity in high alkalinity environment [39]. However, the precise nature of the oxide layer in the alkaline solution is still under investigation; several views have been proposed to explain that [40,41]. One possibility suggested is that the formation of iron oxide solid solutions with spinel structures; such as magnetite (Fe3O4) and/or maghemite (c-Fe2O3), leads to fabricate a more efficient passive film on

Fig. 9. Log i-E curves for a steel rod immersed in OPC-FA-CCP containing different contents of FA. The experiment is carried out at scan rate of 5mVs1 and immersion time of 6 h at room temperature (25 °C).

the surface of the steel rod. An alternative possibility is that proposed the formation of nonporous 3D-dense layer of iron (III) (FeOOH, Fe2O3, and Fe3O4) on the steel surface. An interesting question here is what is the effect of replacing OPC cement with a certain percentage of FA on the corrosion rate of rebar? The answer to this question is given in Fig. 10. It can be seen that the replacement of OPC cement by a small percentage of FA (FA10S) leads to a sharp drop in the Rp value from 118 to 50.87 X (i.e. increase in the corrosion rate of steel bar). Surprisingly, any further increase of the FA content (above 10%) in OPCFA-CCP would change all corrosion parameters until it approaches the values of the OPC-FA-CCP in the case of a 50% replacement of FA(FA50S). Interestingly, the follow-up of the pH of the filtrate shows a slight decrease from 13.1 to 12.6 and finally to 11.4. Subsequently, detailed polarization experiments were performed for a C-steel rod in an OPC paste containing 0, 10, 25 and 50% FA, with a different immersion time and their data were presented in Table 4. It shows that the values of the anodic and cathodic Tafel slopes are changed together in all mixtures, indicating that the anodic and cathodic sites on the working electrode share the electrode interaction. This result can also be explained based on that FA is containing a negligible amount of SO3 (0.09%) and a high concentration of Al2O3 (26.54%) as shown in Table 1. Also, the OPC-FA-CCP suspension contains Na and K ions (Table 1), hence they can form aluminate soluble complex, such as AlO 2, M2O.Al2O3 and/or AlO 4 [42,43]. The presence of adsorbed aluminate ion (AlO2) in the corrosion medium rivals the passive formation reaction that occurs on the

Fig. 8. SEM micrographs of mix FA50S treated at (A) one, and (B) 90 days.

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Table 3 Electrochemical corrosion parameters of steel electrode in OPC and OPC-FA-CCP filtrate devoid of and containing 0, 10, 25, 50 mass% FA at 298 K. Data is taken and calculated from the Tafel plot in Fig. 9. Mix

Ecorr (mV)

ba (mV decade1)

-bc (mV decade1)

icorr (mA cm2)

Rp (X)

FA0S FA10S FA25S FA50S

   

125 95 162 56

98 60 88 68

0.06 0.38 0.25 0.10

118.0 50.87 92.43 98.56

461 530 435 420

electrode surface. This situation leads to the weakness of the passive layer formed, and thus the rate of corrosion is increased. However, there is a competition between the adsorption of the hydroxide ion and aluminate ion on the anodic sites, but only the later adsorption has occurred, this is attributed to the AlO 2 ions exist with large volume comparing to those of the hydroxide groups, and therefore they can adsorb on the anodic sites by electrostatic interaction forming loosely adherent and more porous layer on the steel surface. Thus, a corroding to the following consecutive reaction, the protective passive iron oxide film is formed [44]: At the anode (oxidation), the dissolution of iron to its ions that pass into the solution is taken place:

Fe ! Fe2 + 2e

Fig. 10. Influence of FA contents on the corrosion rate of C-steel rod electrode. Data is taken and calculated from the Tafel plot in Fig. 9A, corrosion potential, corrosion current density, and polarization resistance.

ð1Þ

At the cathode (reduction), the spare electrons in the Fe will combine with H2O and O2 to form hydroxyl ions:

2e + ‘ O2 + H2 O ! 2OH

ð2Þ

Table 4 Electrochemical parameters obtained from galvanostatic polarization curves of C-steel in OPC-FA-CCP containing 0, 10, 25 and 50% FA with different immersing time and at room temperature. Mix

Time (h)

Ecorr (mV)

ba (mV decade1)

bc (mV decade1)

icorr (mA cm2)

RP (X)

FA0S

0.5 1 2 3 4 6 24 48

657 555 519 515 507 461 665 678

96 101 141 186 122 125 201 29

66 70 43 96 55 98 108 42

0.29 0.25 0.20 0.15 0.12 0.06 0.19 0.34

32.99 45.33 61.28 68.39 95.33 118.0 85.02 28.37

FA10S

0.5 1 2 3 4 6 24 48

660 652 542 530 506 503 674 704

94 112 102 89 98 95 127 159

71 68 72 74 75 60 81 57

0.98 0.85 0.68 0.50 0.41 0.30 0.42 0.55

10.56 14.35 20.90 28.97 42.12 50.87 38.91 20.18

FA25S

0.5 1 2 3 4 6 24 48

533 510 481 464 439 435 648 662

94 95 108 135 174 162 181 151

66 74 59 75 95 88 63 83

0.75 0.63 0.40 0.35 0.30 0.25 0.32 0.42

26.17 35.75 41.62 48.92 85.77 92.43 42.92 25.55

FA50S

0.5 1 2 3 4 6 24 48

709 652 667 630 668 657 715 743

83 108 122 130 85 56 87 97

59 67 75 85 101 68 84 120

0.60 0.53 0.37 0.32 0.15 0.10 0.18 0.33

28.97 39.71 45.12 51.87 90.65 98.50 65.14 25.45

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After some intermediate stages, rust products (red and black) will be formed:

Fe2þ + 2OH ! Fe(OH)2

ð3Þ

4Fe(OH)2 + O2 + 2H2 O ! 4Fe(OH)3

ð4Þ

2Fe(OH)3 ! Fe2 O3 H2 O + 2H2 O

ð5Þ

3Fe + 8OH ! Fe3 O4 + 4H2 O + 8e

ð6Þ 2

As shown above, although the adsorption of the AlO ion has led to the formation of a layer on the surface of the metal, it turns out that this layer is porous and therefore insufficient to strongly protect the metal surface from corrosion. Consequently, a small replacement of the OPC cement composition by FA (FA10S) leads to an increase in the corrosion rate due to increasing the aluminate ion AlO 2 in the corrosive medium (see Fig. 10). What we expect now is that this problem should increase when the FA content in the chemical composition of cement increases. Surprisingly, although increment of the FA percentage to 50mass% (FA50S) has to increase the aluminate content in the corrosive medium, but the corrosion rate is going down comparing to mixes FA10S and FA25S. The reason for this unpredictable behavior is that at 50mass% of FA, the concentration of aluminate ion increases in the corrosion medium, followed by an increase in the thickness of the aluminate adsorbing layer on the C-steel surface, which leads to the isolation of the metal surface from attacking the corrosive agent, thus improving corrosion resistance. Although the corrosion rate at FA50S mix is still higher than in the elegant OPC (FA0S) which is probably due to the formation of a compact oxide layer on the surface of C-steel in the neat assembly. 3.5. Effect of different immersion times in the presence of FA Returning to the data presented in Table 4 that were drawn from the polarization curves measured at different immersion time of the C-steel bar in different corrosion media (i.e. different mixes). It can be observed that corrosion current densities, up to the only 6 h, reduce by increasing the dipping time in all tested media (cement suspension mixtures). This can be attributed to the fact that in the early dipping times C-steel can be passivated almost similarly in all media examined, and the oxide passive layer act as an effective barrier between the metal surface and the electrolytes. During the first 6 h of immersion time, the increased thickness of

the passive film leads to a remarkable decrease in the corrosion rate. Fig. 11A illustrates the SEM micrograph for corrosion of the C-steel bar surface after immersion time for 6 h in the elegant corrosion medium of FA10S suspension. This figure indicates the formation of a compact passive layer on the WE electrode surface. Surprisingly, although the thickness of the passive film increased by increasing the immersion time; the rate of corrosion began to gradually increase again (i.e. shift the Ecorr to more negative values) after further aging (i.e. exposure period up to 24 h). This result is mainly attributed to the cracking in the passive layer, which comes from increased internal pressure as the film thickness grows [45]. At this point, a portion of the C-steel surface will be stripped (i.e. uncovered area), thus directly exposed to the corrosion medium leading to further corrosion rate. Fig. 11B shows the SEM micrograph of C-steel specimen after 24 h exposure in the neat corrosion medium of FA10S suspension. It is now clear that the passive layer cracking is responsible for increasing the corrosion rate again. In practice, it is known that the cement setting will be completed within the first 6 h, so the laboratory studies have a benefit during this period. Increased corrosion rate after a period of 6 h does not affect the real application because at this time the entire structure becomes solid.

3.6. Effect of 5% NaCl solution Fig. 12 shows the data extracted from the potentiodynamic polarization curves of C-steel (vs. SCE) in OPC-FA mixes containing 5% sodium chloride solution at a scan rate of 5 mVs1 (see also Table 5). In high alkalinity environments, C-steel bars are expected to acquire a permanent state of passivity due to the increased oxygen that stabilizes the passive film on the metal surface. However, the results in Fig. 12 (Table 5) indicate that the addition of 5% NaCl increases the corrosion rate in the rebar electrode in all tested electrolytes compared to the measured data in the absence of chloride ion (see Table 4). The corrosion potentials were thus shifted to more negative values with the addition of chloride to the corrosive media. One might ask here, why the addition of chloride increases the corrosion rate? This is not surprising, as this effect may be explained by the breakdown or even rupture or of the passive oxide film/C-steel in the presence of chloride ions in the corrosive media. In particular, this passive layer was broken down as a result of the interaction of Cl that can induce pitting corrosion on the steel surface [46]. What is expected is that the chloride ions may propagate through the passive film and act as a catalyst for pitting

Fig. 11. SEM images of the surfaces of C-steel bar obtained at the same magnifications after two different immersion times in the corrosion medium of a FA0S at room temperature: (A) 6, and (B) 24 h.

M. Heikal et al. / Construction and Building Materials 243 (2020) 118309

9

Fig. 12. Effect of immersion time in presence of 600 ppm of spinach extract on C-steel electrode in cement of mixes FA10S and FA50S on: (A) corrosion potential, (B) inhibition efficiency, and (C) corrosion current.

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M. Heikal et al. / Construction and Building Materials 243 (2020) 118309

Table 5 Potentiodynamic polarization parameters for the corrosion of C-steel obtained on OPC-FA-composite containing different percentage of fly ash (0, 10, 25, and 50%) in the presence of 5% NaCl at room temperature. Mix

Time (h)

Ecorr (mV)

ba (mV decade1)

bc (mV decade1)

icorr (mA cm2)

RP (X)

FA0S

0.5 1 2 3 4 6 24 48

691 645 616 610 590 517 698 710

46 57 75 90 97 112 39 95

41 81 71 60 74 127 30 89

0.99 0.74 0.58 0.45 0.40 0.31 0.53 0.70

8.63 20.93 25.35 38.64 42.18 56.39 20.18 11.32

FA10S

0.5 1 2 3 4 6 24 48

695 665 662 660 655 590 675 701

85 91 102 127 150 74 100 97

55 68 65 78 89 102 55 66

3.35 2.64 1.97 1.28 0.97 0.77 1.23 1.60

2.36 3.81 5.12 6.62 12.57 19.67 8.76 4.42

FA25S

0.5 1 2 3 4 6 24 48

645 575 564 525 518 461 675 701

67 81 69 99 100 103 77 118

59 73 82 71 100 108 78 71

1.76 1.43 1.06 0.89 0.75 0.51 0.75 1.37

6.52 8.47 12.10 18.88 22.34 30.42 12.43 6.73

FA50S

0.5 1 2 3 4 6 24 48

689 663 665 589 640 673 701 720

79 81 115 78 77 70 90 104

63 59 68 77 62 51 60 57

0.90 0.80 0.72 0.60 0.52 0.42 0.40 0.52

17.21 20.60 257.85 30.41 38.54 54.44 29.64 19.27

corrosion, and thus can destroy the passive film. Because a critical concentration of the chloride ions is necessary to initiate the pitting on the metal surface [47]. This also depends on several other factors such as the nature of the metal or alloy, thermal treatment, and the surface condition. The initiation of pitting can be explained by competition in adsorption between additive (chloride anions) and OPC-FA-CCP (OH/H2O dipoles) species on the oxide film. Firstly, the chloride anions can displace the adsorbed passivating species at a certain point, and create a hole in the passive layer, and hence accelerate of the local anodic dissolution. In a second step, the pitting propagation involves the metal matrix dissolution followed by hydrolysis, resulting in a high degree of acidity at the bottom of the corrosion cavity. Due to the increase of the Fe2+ concentrations in the corrosion cavity as written above in Eq. (1), the chloride ions will migrate to maintain neutrality. Hence, the formed metal chloride (FeCl2) is hydrolyzed by H2O to produce

insoluble ferrous hydroxide and hydrochloric acid, and therefore the alkalinity of the electrolyte will be decreased according to the following reaction [48,49]:

FeCl2 + 2H2 O ! Fe(OH)2 + 2HCl

ð7Þ

Accordingly, the pH value inside the pit will be diminished, thus causes, of course, a further acceleration of the corrosion process, while the pH of the bulk solution remains unchanged. Consequently, the corrosion rate inside the cavity is higher than that of bulk areas, and so the bulk areas which act as cathode electrodes are partly or fully protected cathodes. Thus, the polarization resistance increased in the first 6 h of cement setting, followed by a decrease with increased immersion time in the absence and presence of 5% sodium chloride in all tested concrete mixtures as shown in Fig. 12A. As described above, after 6 h of immersion time

Table 6 Electrochemical corrosion parameters of C-steel in OPC-FA-CCP containing different percentage of FA in the absence and the presence of different concentration of spinach extracts after immersion time of one hour at 298 K. Mix

Conc. (mg l1)

Ecorr (mV)

b a

icorr (mA cm2)

IE%

FA10S

0 50 100 200 400 600

652 616 613 595 590 584

112 41 46 45 41 63

(mV decade1)

b c

68 45 39 37 45 62

(mV decade1)

0.85 0.30 0.27 0.23 0.20 0.15

... 65 68 73 76 82

FA50S

0 50 100 200 400 600

657 656 650 639 600. 580

108 56 54 59 65 34

67 58 68 46 76 29

0.53 0.41 0.31 0.28 0.26 0.18

... 23 42 47 51 66

M. Heikal et al. / Construction and Building Materials 243 (2020) 118309

the passive layer is thickened and the film cracking depth increases, thus the steel surface will be exposed directly to the corrosion medium which reduces its corrosion resistance (see SEM images in Fig. 11). 3.7. Effect of spinach extract The experimental results shown in Table 5 indicate that the corrosion parameters for C-steel in mix FA30S are close to those in the pure OPC sample (FA10S). Whereas, the mix FA10S has the highest corrosion rate compared to neat cement OPC. Thus, only these two mixes (FA10S and FA30S) were selected to explore the effect of a spinach extract, as a green corrosion inhibitor, on the corrosion of rebar. 3.7.1. Effect of spinach extract concentrations Table 6 shows the data obtained from the Tafel plots for steel after 1hr immersion in FA10S and FA30S mixes in the absence of different concentrations of spinach extract. Of course with the addition of spinach extract to the concrete composition, there will be competition between the adsorption of the plant constituents and ingredients of the corrosive media on the active sites of the metal surface. However, the results in Table 6 strongly support the adsorption of plant components on the active sites of the metal surface and are preferred over those in the corrosion medium. It can be seen that the corrosion current density decreases by increasing the concentration of spinach extract. In particular, a sharp decrease in the corrosion current density from 0.85 to 0.30 mA cm2 was achieved in mix FA10S by changing the concentration of the extract from 0 to 50 ppm, respectively. As a result, the inhibition efficiency ratio was increased by increasing the concentrations of plant extract to a maximum of % IE up to 82 and 66% at 600 ppm for the FA10S and FA30S mix, respectively. In general, an increase in plant concentrations from 50 to 600 ppm leads to a gradual increase in IE percentages. From the above, it can be said that the adsorption of plant molecules acts as a barrier between the steel surface and the aggressive corrosion medium leading to the improved corrosion resistance of the rebar. The question now is, why adsorption of plant ingredients is more favorable? The phytochemical analysis of spinacia oleracea extract proved that it contains carbohydrates, quinine, phytosterol, glycosides, terpenoids, proteins, and flavonoids [15]. Almost all of these organic materials contain different heteroatoms such as O

11

and N in their chemical composition. These are typical properties to be used as a corrosion inhibitor. As we know, heteroatoms are characterized by a lone pair of electrons. So these compounds can be easily/quickly attached to the metal surface by forming coordination bonds, i.e. chemically adsorbed. Thus, the adsorption of plant molecules on the active sites of the electrode surface was easier and more effective than anything else. 3.7.2. Effect of different immersion time in presence of FA and spinach extract Fig. 13 shows a comparison between the corrosion potential and the efficiency of inhibition of reinforced iron in mixtures FA10S and FA50S at different immersion times in the presence of a constant plant extract concentration = 600 ppm at room temperature. It is evident that the FA10S mix has less negative potential than FA50S (Fig. 13A), and this, of course, is accompanied by an increase in the inhibition efficiency of the FA10S mixture over the FA50S (Fig. 13B). This can be explained by the fact that since FA10S mixture has a lower amount of FA content (10mass% FA) compared to mixing FA50S (50mass% FA), the active sites available

Fig. 14. A SEM micrograph of a C-steel surface was obtained after submerging it for up to 6 h in FA10S mix and in the presence of 600 ppm of spinach.

Fig. 13. Variation of the corrosion potential (A); and the inhibition efficiencies (B) for the steel rebar in mixes FA10S and FA50S at different immersing time. Experiment conditions; spinach extract concentration = 600 ppm at 25 °C.

12

M. Heikal et al. / Construction and Building Materials 243 (2020) 118309

on the C-steel surface will be larger in the case of FA10S mixture which gives the opportunity to adsorb the extracted Spinach molecules easily and quickly on the metal surface. Conversely, in the case of the FA50S mix, the free active sites on the steel surface are limited, since the surface of the electrode is blocked with a high amount of aluminate complex, thus only a small amount of plant components will be adsorbed on the surface of the electrode. This argument was supported by the SEM micrograph of a C-steel surface after submerging it for up to 6 h in FA10S mix and the presence of 600 ppm of spinach extract (see Fig. 14). Interestingly, efficiency has the highest values in the initial period of immersion time, and then gradually decreases when immersion time increases as shown in Fig. 13B. Early, spinach extract molecules can easily and quickly adsorb on the surface of the C steel, with increased exposure time, the repulsion between adsorbed molecules may occur; therefore, this provides an opportunity to introduce corrosion solution into the bare metal again, thus increasing the corrosion rate. 4. Conclusions In conclusion, setting times (STs) are shortening with FA replacement upto 10mass%, after then the increase in the replacement of FA to 50mass% the STs of OPC-FA-CCP are extended. The obtained data indicated a significant increase in CS and X upto 90 days in the case of OPC-FA-CCP containing 10% FA. At a higher replacement level of FA up to 25–50 mass%, the CS and X values decrease, which influences the mechanical-properties of the solidified cement-giving pastes. SEM confirmed the presence of a surface layer of amorphous-CSH gel product, with prolonged hydration time dense crystalline CSH products formed. In the current work, the general characteristics of corrosion events in fixed OPC-FA-CCP structures have been explored with a variety of fly ash (FA) contents (0, 10, 25 and 50%) and immersion time in the absence and presence of spinach extract as a green corrosion inhibitor. Effect of replacing FA in OPC composition on its physical, chemical, mechanical and microstructure properties has been also studied. The results indicated that the rebar immersed in the pure OPC cement (FA0S) has the highest Ecorr value. It has been shifted to a more negative value with the increase of the FA content indicating a decrease of the corrosion resistance of the rebar. During the first 6 h of immersion time, the increased thickness of the passive film leads to a remarkable decrease in the corrosion rate. As the dipping time increases, the current corrosion density in all tested cement mixtures is unexpectedly reduced. The results also confirmed that spinach extract is a typical green corrosion inhibitor. CRediT authorship contribution statement Mohamed Heikal: Supervision, Data curation, Visualization, Validation, Investigation, Writing - review & editing. A.I. Ali: Conceptualization, Visualization, Methodology, Investigation, Software, Writing - first draft, Validation. B. Ibrahim: Methodology, Investigation, Software. Arafat Toghan: Conceptualization, Writing - original draft, Investigation, Software, Writing - review & editing, Data curation, Visualization. Acknowledgment The authors wish to express the deepest sense of gratitude to Dr. Arafat Toghan; Chemistry Department, Faculty of Science, South Valley University, Qena for his valued help during the production of this work.

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