Steel corrosion resistance in model solutions
and reinforced mortar containing wastes
D. A. Koleva and K. van Breugel
Delft University of Technology, Delft, The Netherlands, Faculty of Civil Engineering & Geosciences, Materials & Environment, Stevinweg 1, 2628 CN
Summary
This work reports on the corrosion resistance of steel in alkaline model solutions and in cement-based materials (mortar). The model solutions and the mortar specimens were Ordinary Portland Cement (OPC) based. Further, hereby discussed is the implementation of an eco-friendly approach of waste utilization for corrosion control and/or achieving superior properties and performance by modifying the model solutions and the mortar mixtures with waste materials i.e. “Red mud” (RM) and Blast Furnace Slag (BFS).
The objective was to define the steel electrochemical behaviour and the properties (morphology and composition) of the product layer that forms on the steel surface within treatment in chloride-free and chloride-containing model solutions. The motivation for this research is related to several aspects, mainly to exploring the possibility for achieving superior corrosion performance of steel in cement-based materials by using (combination of) wastes.
The study reveals that RM has a distinctive positive effect on the corrosion performance of steel, especially in chloride containing environment. The presence of RM favours steel passivation and results in an increased corrosion resistance. The positive effect of RM is especially pronounced for steel in BFS solutions and BFS mortar specimens.
1. Introduction
This paper presents a comparative investigation of the corrosion behaviour of low carbon steel in model alkaline solutions and reinforcing steel in mortar, using Ordinary Portland cement (OPC), Blast furnace slag cement (BFS) and Red Mud (RM) in chloride containing environment. The variables of main interest are: the steel corrosion parameters in both aqueous (model solutions) and solid (mortar) environment and the alterations in the bulk cement-based matrix (porosity, pore size distribution) fort the reinforced mortar cases. The investigation is performed as a comparative study of control (non-corroding) specimens from each group and corroding such (conditioned in 5% NaCl).
Steel corrosion, being a major problem in civil engineering, presents an enormous cost (e.g. 30 to 50% of annual costs for infrastructure maintenance in EU are spent on corrosion-related issues). Dealing with steel corrosion in an eco-friendly manner, using wastes for example, is a sustainable way of solving corrosion problems.
Additionally, the cement replacement by wastes in a reinforced concrete system would even further create sustainable solutions (less cement utilization, less CO2 produced). Recycling of waste materials in the construction industry is an eco-friendly and a technically successful option. However, major wastes (e.g. the bauxite residue – the highly alkaline red mud) are still not practically utilized and cause serious and alarming environmental issues: each tone of produced aluminium results in 0.5–2 tones waste; approximately 145 million tones of red mud are annually produced [1]. Red mud (RM) is reported to increase steel passivity in alkaline solutions [2]; RM additions to cement-based materials are also recently reported to result in mortars and concretes for shielding X-ray radiation [3], heavy metal (and other toxic substances) binding effects [4-6] while maintaining sufficient mechanical properties [7]. Practical applications are however extremely limited. Blast furnace slag (BFS) is another largely generated waste (from the steel production); in contrast to RM, it is widely used in European countries; e.g. in the Netherlands it has a market share of more than 50%. A practical inconvenience however is the impeded cement hydration of BFS, compared to Portland cement (OPC) [8, 9], resulting in a coarser structure at earlier hydration ages. Further, BFS concrete requires proper and careful water curing at early ages and even then, reinforcing steel in BFS concrete exhibits reduced properties of the passive layer, due to: limited oxygen availability and thus a shift of the steel corrosion potential to a more negative state [10,11]; lower calcium content and pH decrease [12]. Despite the higher chloride binding capacity of BFS,
the passive layer on the steel surface is weakened as the Fe3+ species are reduced
by the reducing agents from BFS cement itself; additionally BFS concrete is highly susceptible to carbonation and freeze-thaw durability issues [13].
In order to establish superior corrosion resistance and improved matrix properties of reinforced concrete, containing wastes, a possible solution is to use a combination of RM and BFS as a replacement of Portland cement. This paper presents investigation in model solutions, where the combination of BFS and RM is used (for the complete study please see [14]. Further, a mixture of RM and BFS as OPC cement replacement was also studied. A comparative investigation of control and corroding specimens is presented for the period of approximately 250 days of conditioning (for more details on this investigation, please see [15]).
2. Materials and methods
2.1. Tests in model solutions
Steel electrodes (low carbon steel St37, surface area of 4 cm2) were tested in model
alkaline solutions (in a common 3-electrode electrochemical cell, with a Saturated calomel electrode (SCE) as a reference and Pt as a counter electrode) for time intervals of 1h, 24h, 3 days and 5 days. The model solutions were prepared as follows: Two types of cement, CEM I 42.5N and CEM IIIB 42.5N (i.e. OPC and BFS, Table 1) and RM (as 20% replacement of the cement portion) were used. The red mud used in this study was supplied from Suriname (XRF analysis of the received supply gives major contributions of: Al2O3 29.1 wt.%, Fe2O3 24.9 wt.%, Na2O 14.3 wt.%, SiO2 20.2 wt.%, CaO 3.5 wt.%). Cement (or cement + RM respectively) and
water were mixed in a ratio of 1:1.The pH of these suspensions develops with time, the measured values until 7h are given in Fig.1.
Table 1 Chemical composition of OPC, BFS and RM
CEM I 42.5 N (OPC), ENCI, NL CEM III/B 42.5N (BFS), ENCI, NL RM (XRF) CaO % 63.91 44.60 3.54 SO3 % 2.68 3.55 0.53 SiO2 % 20.60 27.60 20.24 Al2O3 % 5.01 12.20 29.10 Fe2O3 % 3.25 1.19 24.85
Further, the containers containing the suspensions were rotated for 24h and subsequently filtrated, thus receiving the model solution as an environmental medium for testing electrochemical behaviour of steel electrodes. The pH of the as received solutions is 12.7 – 12.9. 12.6 12.7 12.8 12.9 13 13.1 13.2 13.3 13.4 13.5 13.6 0 1 2 3 4 5 6 7 8 Time (h) p H BFS CE OPC CE OPC+RM CE BFS+RM CE
Figure 1: Development of pH fort he as admixed suspensions before filtration
The as received solutions define thus the sample designation: Group CE (cement extract) stands for a solution from ordinary Portland cement (OPC only); Group BFS stands for a solution, prepared from BFS only; Groups CERM and BFSRM stand for solutions of OPC and BFS where 20% of the initial cement portion was replaced with RM. Sub-groups of each above defined group form solutions, where NaCl was added as a solid, achieving final concentration of 5% in the testing solution. Thus, the steel electrodes were monitored in 8 solution types and form 8 groups respectively: controls are designated as CE, BFS, CERM and BFSRM; corroding specimens are designated as CEn; BFSn; CERMn and BFSRMn. Three replicates per type and per time interval were measured. Characterization of the product layers (morphology and composition) was performed on replicate specimens of each type after 5 days treatment (i.e. corresponding to the latest hereby monitored time interval).
Experimental techniques: Electrochemical methods involved were electrochemical
impedance spectroscopy (EIS), potentio-dynamic polarization (PDP) and Cyclic voltammetry (CVA) (hereby, partly presented, for the full investigation, please see [14].The measurements were performed after open circuit potential (OCP)
stabilization for all cells. PDP was performed in the range of -0.15 V to +0.75 V vs OCP at scan rate 0.5 mV/s. The EIS measurements were carried out in the frequency range of 50 KHz to 10 mHz by superimposing an AC voltage of 10 mV. The CVA scans were in the potential region of -1.4V to 0.6V at a scan rate of 5 mV/s [16]. All potential values were hereby reported vs saturated calomel electrode (SCE). The used equipment was a High performance Autolab PGSTAT302N, combined with FRA2 module, using GPES and FRA interface. Morphological observation and semi-quantification of the product layers formed on the steel surface were performed by Scanning Electron Microscopy (using ESEM Philips XL), coupled with EDX. Additionally, XRD measurements were performed by scanning the whole surface (~ 2 cm2) of the test specimens, using D8 Advance Diffractometer, “Bruker AXS”. A VANTEC position sensitive detector (window 6 degr.) was used for detection. Energy
source was CoKα (1.789Å) and the tube settings were 45kV and 35mA.
2.2. Tests in reinforced mortar
Reinforced mortar cylinders (d = 3.5 cm; h = 20cm) were cast from OPC CEM I 42.5
and CEM IIIB 42.5, cement/sand ratio of 1:3 and water/cement ratio of 0.6. The waste Red mud (RM) was added as 20% OPC or BFS replacement (chemical composition in Table 1). Eight groups (5 replicates per group) were monitored: four control groups (non-corroding) with (designation RM) and without red mud, denoted as OPC, RMOPC, BFS and RMBFS respectively, and four corroding groups, with and without red mud, denoted OPCn, RMOPCn, BFSn and RMBFSn. All specimens were cured for 7 days in fog room (20 ºC and 98% RH) and maintained in lab air further on. An external solution of 10 % NaCl was used as a chloride-induced accelerator for the corroding groups (the specimens were 1/3rd of height immersed in the solution; the control specimens were immersed in tap water). Red mud was added (after drying at 105ºC and grinding to cement finesse) for the RMOPC, RMOPCn, RMBFS and RMBFSn specimens.
The steel re-bars (construction steel FeB500 HKN, d=0.8cm, h=10cm, composition
according NEN6008 (in wt.%): C < 0.12 wt.%, Si max 0.6, P max 0.05, S max 0.05, N max 0.012) were embedded “as received” i.e. there was no preliminary treatment of the bars before casting. Both ends of the steel bars were isolated (to avoid crevice corrosion) and the bar was positioned in the middle of the concrete specimens. Mixed Metal Oxide (MMO) titanium mesh served as a counter electrode; SCE electrode was used as a reference electrode. The experimental set-up and specimen’s geometry are as previously used and reported in [17,18].
Experimental techniques: surface morphology and electrochemical behavior were
investigated via the above mentioned techniques (as used for steel in model solutions). Microstructural analysis (porosity and pore size distribution) of the bulk cement-based matrix were obtained via ESEM imaging and image analysis (OPTIMAS software): a set of ESEM images of the cement matrix was obtained in backscattered electron (BSE) mode with the magnification of 500x. The results are an average of 35 locations per sample (details about the sample preparation and procedures as reported in [19-23]).
3. Results and discussion
3.1. Electrochemical behaviour and surface analysis of steel electrodes in model solutions.
Figures 2 and 3 present the potentiodynamic (PDP) curves for the time intervals of 1h and 5days for all investigated conditions (the curves are equal length i.e. PD polarization was in the range of -0.15 V to +0.75 V vs OCP).
Overlay control cells CE and CERM, 1h and 5d
1 2 3 4 1- CE1h 2 - CE5d 3 - CERM1h 4 - CERM5d -0.50 -0.25 0 0.25 0.50 0.75 1.00 1.25 -9 1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10 -4 1x10 -3 1x10 -2 1x10 -1 1x10 0 1x10 E / V (SCE) I / (A /c m 2 )
Overlay corroding cells CEn and CERMn, 1h and 5d
1 2 3 4 1- CEn1h 2 - CEn5d 3 - CERMn1h 4 - CERMn5d -0.75 -0.50 -0.25 0 0.25 0.50 0.75 1.00 -9 1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10 -4 1x10 -3 1x10 -2 1x10 -1 1x10 0 1x10 E / V (SCE) I / (A /c m 2 ) a) b) i / A .c m -2 i / A .c m -2 E vs. SCE / V E vs. SCE / V
Figure 2: PDP curves for all CE cases as an overlay of 1h and 5 days time intervals
Overlay control cells BFS and BFSRM, 1h and 5d
1 2 3 4 1- BFS1h 2- BFS5d 3-BFSRM1h 4-BFSRM5d -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 1.00 1.25 -9 1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10 -4 1x10 -3 1x10 -2 1x10 -1 1x10 0 1x10 E / V (SCE) I / (A /c m 2 )
Overlay corrding cells BFSn and BFSRMn, 1h and 5d
1 2 3 4 1- BFSn1h 2- BFSn5d 3-BFSRMn1h 4-BFSRMn5d -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 1.00 1.25 -9 1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10 -4 1x10 -3 1x10 -2 1x10 -1 1x10 0 1x10 E / V (SCE) I / (A /c m 2 ) a) b) E vs. SCE / V E vs. SCE / V i / A .c m -2 i / A .c m -2
Figure 3: PDP curves for all BFS cases as an overlay of 1h and 5 days time intervals
For CE, CERM and BFSRM control specimens the behavior with external polarization is similar and as expected – very low corrosion and anodic current densities are recorded. The BFS control cell, however, presents a significantly different behavior (Fig.3a). The higher (although in passive range) corrosion current and anodic currents for BFS are clearly observed after 1h treatment (curve 1, Fig.3a), resulting from possible inability of the steel electrode to form an initially adherent and stable passive layer in the model solution of BFS. Further, the positive effect of RM is well pronounced, since both time intervals (1h and 5d) depict behavior of the control BFSRM cell almost identical with the one in CE and CERM cells, whereas even more cathodic corrosion potentials are recorded for BFS cell at 5days (compare Fig.2a and 3a). The relatively “bare” or “clean” steel surface in BFS cells (as observed by ESEM and XRD) supports the hypothesis for an impeded formation of a stable and protective layer on the steel surface in BFS environment and supports the EIS
results, showing superior performance of steel in CE and/or when RM is added to both CE and BFS. In other words, the disadvantage of BFS cement (as used in reinforced concrete for practical applications) in terms of impeded steel passivation (when no external detrimental influences, as chloride, are involved) can be overcome by partial BFS replacement with the waste RM.
For corroding conditions (Figs. 2b and 3b), the hereby investigated cases CEn, CERMn, BFSn and BFSRMn similarly show the positive effect of RM. As seen from Fig.2b, the anodic current densities for CERMn are lower; a shift to more noble corrosion potentials, compared to CEn, is observed. Here again, as also recorded by EIS, RM apparently exerts stabilization of the product layer and thus higher corrosion resistance. Initially (1h response), the highest corrosion current densities, anodic currents and most cathodic potentials are recorded for the BFSn and CEn cells; further after 5d of treatment, corrosion activity remains the highest in BFSn, whereas both RM containing corroding cells BFSRMn and CERMn remain with lower anodic currents.
Corrosion current density, corroding cells
0 2 4 6 8 10 12 14 16 18 0.05 0.04 0.05 0.05 0.08 0.04 0.05 0.04 0.15 0.27 2.71 5.42 0.10 0.08 0.06 0.05 m ic ro A m p s /s q .c m 0.5 to 1.0 µA/cm2 - high corrosion rate
> 1 µA/cm2 - severe corrosion
CEn CERMn BFSn BFSRMn b) 1h 24 h 3 d 5d 1 h 24 h 3d 5d 1 h 24h 3 d 5d 1h 2 4h 3 d 5d i / µ A .c m -2 1h 24 h 3 d 5d 1 h 24 h 3d 5d 1 h 24 h 3d 5 d 1 h 24h 3d 5 d
Figure 4: Corrosion current densities for corroding specimens
Fig.4 depicts a summary of derived corrosion current densities fort he corroding cases (for the control cells the corrosion current densities, except the BFS specimens, are in the range of 0.04 to 0.08 µA/cm2, whereas the BFS specimens, although being control cases behave as corroding such, exhibiting current densities in the range of 0.15 to 5.4 µA/cm2). The corroding cells CEn and CERMn (Fig.4) present corrosion activity immediately after immersion: for time interval 1h, corrosion
current density for CEn cells being approximately 6 µA/cm2; for CERMn – 3 µA/cm2.
Further, corrosion current densities are slightly decreased (as a result from competing processes of activity and diffusion limitations i.e. attempt for re-passivation), reaching values in the range of 1.14 µA/cm2 for the CERMn cells. At the
end of the test (5 days) the derived currents are 8.4 µA/cm2 for CEn and 3.7 µA/cm2
for CERMn. For the corroding cells BFSn and BFSRMn, since significantly higher corrosion and anodic currents, as well as more cathodic corrosion potentials are observed, the corrosion current density for BFSn cells reaches 15.7 µA/cm2 at the latest interval of 5 days, whereas 5.2 µA/cm2 is the recorded current in the presence of RM (BFSRMn cells) after 5 days.
Overlay 1st scans BFS BFSRM CE CERM -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 -10 1x10 -9 1x10 -8 1x10 -7 1x10 -6 1x10 -5 1x10 -4 1x10 -3 1x10 -2 1x10 -1 1x10 0 1x10 E / V (SCE) I / (A /c m 2 ) Overlay 1st cycles CE BFSRM CERM BFS -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0 0.25 0.50 0.75 -0.00012 -0.00009 -0.00007 -0.00004 -0.00002 0.00001 0.00003 0.00006 0.00008 E / V (SCE) I / (A /c m 2 ) I II III IIIa IV Va V Vb VI R M c) Fe act ivity passivity O2 e v o lu ti o n a) b) i / A .c m -2 i / A .c m -2 E vs. SCE / V E vs. SCE / V
Figure 5: Overlay 1st cycles for control cells (a); linear plot depicting main peaks and regions (b)
Cyclic voltammetry supports the PDP results. It was performed on replicate electrodes at 5 mV/s scan rate in the potential region of -1.4 V SCE to 0.6 V SCE (scan rate and potential region considered representative for the investigated systems, also with respect to RM additions, as reported in [16]). Fig.5 depicts an overlay of the 1st (out of ten) cycles for the control specimens CE, CERM, BFS and BFSRM (including linear plots as zoomed regions of main features in Fig.5b). The main regions in the voltammograms (Fig.5b) are as recorded for steel in alkaline media (as reported for various types of solutions, including cement extract from OPC cement [24-32]). The voltammograms present a few typical features for all control conditions e.g. oxidation peaks in the forward scan between -870mV to -980mV (peak I, Fig.5b) and -650 to -750 mV (peak III, Fig.5b) and broad reduction peaks between -610 to -970 mV and around -1250mV and -1350 mV (peaks V, VI in Fig.5b). The oxidation peak I (Fig.5b), appearing at -950 mV for BFSRM and CERM cells is well pronounced for the former and very low in current density for the latter case. The oxidation peak I is related to the formation of Fe(OH)2 via: Fe0 +2OH− = Fe(OH)2+2e−. Region II refers to the onset of magnetite formation at peak III according the reaction: 3Fe(OH)2+2OH- = Fe3O4+4H2O+2e−. Peak III, Fig.5b (with a corresponding reduction peak V) is pronounced in the 1st cycles only for cells CERM and BFSRM (both at -740 mV). For the CE cell peaks I and III are not distinguished in the 1st cycle. For the BFS cell peaks I and region II are merged and anodically shifted, peak III is almost not visible (-710 mV) i.e. metal dissolution rather than magnetite formation is observed. In short, the CVA tests confirm the relatively clean steel surface for BFS specimens and the inability of stable passive layer formation, whereas a stable passive film form in the BFSRM specimens (for a complete discussion on CVA tests, please see [14]). In addition to the peculiarities in behaviour for the corroding cells, it can be firmly stated that the RM addition has a significant positive effect for both chloride containing and chloride free BFS solutions. The result is increased corrosion resistance for BFS cells in control conditions and improved corrosion performance for BFSRMn (as well as CERMn) as corroding conditions. Steel surface morphology and composition proves the above discussed electrochemical behaviour – Figs. 6 and 7.
Figure 6: Morphology product layer and EDX: corroding cells CEn (a) CERMn (b)
Figure 7: Morphology product layer and EDX, corroding cells BFSn(a), BFSRMn (b)
As can be observed in Figs. 6 and 7, a significant amount of Cl-containing corrosion products accumulated on the steel surface for CEn and BFSn specimens, wherease limited or none such were observed fort he RDm containing cases CERMn and BFSRMn, where calcium carbonates were mainly recorde on the steel surface. Finally, for steel treated in model solutions it can be stated that the presence of RM in both CE and BFS solutions (with and without NaCl) favors steel passivation and results in an increased corrosion resistance. The positive effect of RM is especially pronounced for steel treated in BFS solutions. The most plausible mechanisms and related phenomena are denoted to the presence of Fe2O3 and Al2O3 (from RM and BFS), mainly in terms of the effect of Fe3+ on redox activity and thus stabilization of
the passsive layer. Secondly, the adhered red mud particles act as adsorption sites for Ca2+, resulting in a product layer (adhered CaO/Ca(OH)2 and/or Ca-substituted iron oxides/hydroxides) with higher corrosion resistance.
3.2. Electrochemical behaviour of steel reinforcement and bulk matrix properties for reinforced mortar specimens.
Electrochemical impedance spectroscopy (EIS) is a useful technique for obtaining
knowledge of the steel/concrete system as it provides information for both the steel surface (electrochemical parameters) and the concrete bulk matrix (concrete bulk and pore network resistance) [33,34]. The elements of the equivalent circuit used in this study (Fig. 8 right) present the following physical meaning: The first part of the circuit i.e. the high frequency domain (C1,R1 and C2, R2) is attributed to the properties of the concrete matrix in terms of pore network: Ro, R1 and R2 denote for the resistance of the environmental medium (water for control cells and 10% NaCl for corroding ones), the bulk matrix resistance in terms of solid phase, including disconnected pore pathways and the resistance of the pore network in terms of continuous connected pores; The second part of the circuit i.e. middle and low frequency domains (time constants R3C3 and R4C4) deals with the electrochemical reaction on the steel surface including the contribution of redox processes, taking place in the product layers on the steel surface. An overlay of the experimental EIS response (in order to account for bulk matrix characteristics, EIS was performed in the frequency range of 1MHz to 10 mHz) for all groups is presented in Fig.8(left), relevant to the final so far recorded time interval of 250 days, which corresponds to the time interval of microstructural analysis (Fig.10 further below). Summarized data for the best fit parameters (EIS responce) are presented in Table 2.
0 4 8 1 2 1 6 2 0 0 4 8 1 2 1 6 2 0 Z ' / K o h m -Z '' / K o h m 0 2 4 5 7 9 0 2 4 5 7 9 Z ' / K o h m -Z '' / K o h m 0 . 5 1 . 5 2 . 5 0 0 . 5 1 . 5 2 . 5 Z ' / K o h m -Z '' / K o h m 10mHz 10MHz 50kHz
Control cells (250 days)
2 3 4 1 1 2 3 4 0 0. 5 1 . 0 1. 5 2. 0 2 . 5 0 0. 5 1. 0 1. 5 2. 0 2. 5 Z ' / K o h m -Z '' / K o h m 1 0 M H z 5 0K H z 10 m H z 50 K 3 4 2 1 3 4 1- OPCn 2- RMOPCn 3- RMBFSn 4- BFSn
Corroding cells (250 days)
10mHz
Figure 8: (left) EIS response for control (top) and corroding (bottom) cells after 250 days of
conditioning (zoomed areas in the Nyquist plots give the high frequency response); (right) equivalent circuit, employed for EIS data interpratetion
Ro C1 R1 C2 R2 C3 R5 C4 R6
The high frequency response corresponds to the concrete bulk resistance, including the contribution of electrolyte resistance. The obtained (Rel+b+p.netw) values (Table 2) correspond to the overall concrete resistance. For deriving polarization resistance (Rp) from EIS measurements in reinforced concrete (Rct and Rred respectively), the medium-low and low frequency limits of the impedance spectra are generally considered, as reported in [35,36], previously discussed and reported for reinforced mortar and concrete in [17,22] and used in the present study as well. At the stage of 250 days similar EIS response in terms of electrochemical behavior was recorded for all control cells, evidenced by the close to capacitive behavior at low frequencies (indicating situation of passivity). The corrosion resistance for BFS cells is lower, compared to OPC cells (Table 2, Rp values). The influence of RM as BFS replacement is well evident (Table 2, Fig.8, curves 2 and 3, top left). The response for all OPC and BFS specimens differs in the high frequency domain, denoted to the bulk properties of the cement-based matrix in BFS, compared to OPC. The former matrix is generally reported to be denser compared to the latter one [37-41], which gives the difference in bulk matrix resistance, Table 2. Figure 8 (left, bottom) presents the EIS response for the corroding cells at 250 days, reflecting still passive state for the corroding RMBFSn group and enhanced corrosion activity for the BFSn, OPCn and RMOPCn groups (reduced magnitude of |Z| and phase angle drop to below 40º). The bulk matrix resistance (Rb+p.netw) for all specimens (corroding and control groups) increases with time as a result of cement hydration (NaCl slightly influencing the electrical resistivity values), the major difference being denoted to the type of cement i.e. OPC cells (corroding and control) presenting bulk matrix resistance in the range of 89 – 116 kOhm.cm2, while for BFS cells, this range is 490 - 800 kOhm.cm2 at 250 days.
Table 2: Best fit parameters EIS response
Control cells
Cell type Rel+b+p.netw
(R0+R1+R2) Rp.netw. (R2) Cs (C1) Cp.netw. (C2) Rct. (R3) Cf (C3) Rred. (R4) Cred (C4) Rp (R3+R4) E mV kOhm.cm2 nF/cm2 µF/cm2 Ohm.cm2 µF/cm2 Ohm.cm2 µF/cm2 kOhm.cm2 SCE
OPC 93 39 9.90e-03 1.51 127 26 2192 20 2.3 -183
RMOPC 89 45 10.10e-03 1.22 165 23 2846 17 3.1 -170
BFS 812 209 1.21e-03 0.31 312 15 923 23 1.2 -65
RMBFS 496 244 2.41e-03 0.41 335 14 1719 15 1.7 -120
Corroding cells
Cell type Rel+b+p.netw
(R0+R1+R2) Rp.netw. (R2) Cs (C1) Cp.netw. (C2) Rct. (R3) Cf (C3) Rred. (R4) Cred (C4) Qred,Y0×10-3 Rp (R3+R4) E mV
kOhm.cm2 nF/cm2 µF/cm2 Ohm.cm2 µF/cm2 Ohm.cm2 µF/cm2 Ω-1sn kOhm.cm2 SCE
OPCn 114 48 3.40e-03 1.81 37 628 123 0.15 -470 RMOPCn 116 57 3.60e-03 0.61 41 1125 115 0.16 -540 BFSn 493 83 1.10e-03 8.6e-06 22 88 69 1.67, n=0.65 1.69, n=0.62 0.73, n=0.61 0.09 -480 RMBFSn 710 100 0.81e-03 5.8e-05 79 30 1615 15 1.7 -130
The charge transfer resistance (Rct) decreased for all corroding cells, compared to the control ones, the specimens RMBFSn, however, behave as control ones (Eocp=-130 mV), exhibiting Rp values in the range of the control cells (1.7 kOhm.cm2), capacitive behavior and higher phase angle (Table 2, Fig.8, specimen RMBFSn). As evident from the electrochemical measurements, 20% red mud as cement replacement, particularly for the BFS cells, results in a corrosion delay in the very
aggressive environment of 10% NaCl. The presence of RM in reinforced mortar (and concrete respectively) will account for an increased corrosion resistance in the wastes-modified systems. In order to clarify the responsible mechanisms and related phenomena, microstructural analysis of the bulk cementitious matrix is performed and discussed in what follows.
Bulk matrix properties (microstructural parameters): The electrolytic path in reinforced
cementitious systems is dependent on the kinetics of ion transport mechanisms. These mechanisms, in addition to the cement hydration and the morphological alterations, are affected by the pore size distribution and the pore connectivity of the bulk concrete material.
Porosity - Control cells (250 days)
15.57 13.94 10.86 9.35 0 5 10 15 20
0.1 1 pore size (um) 10
p o ro s it y % OPC RMOPC BFS RMBFS
Porosity - Corroding cells (250 days)
11.74 9.77 6.78 5.97 0 5 10 15 20
0.1 1 pore size (um) 10
p o ro s it y % OPCn RMOPCn BFSn RMBFSn
Figure 9: Porosity and pore size distribution (bulk matrix) for control cells and corroding cells
A set of SEM images on polished sections were made and were further subject to image analysis. The hereby discussed porosity refers to the bulk matrix only. The structural parameters were averaged from at least 35 locations in the bulk matrix (sample of 2x2 cm). The final data were considered after performing a statistical evaluation in terms of frequency of occurrence (% distribution) vs class (porosity in %). Lower porosity was observed for the corroding cells, compared to the control cells, Fig.9, (as a result from the influence of NaCl as accelerator of cement hydration at initial stages). Further, the influence of BFS and RM is well visible, the lowest recorded porosity being in the RM containing BFS cells (the critical pore size for all investigated groups was similar (approximately 0.634 µm), except for the control BFS cells (0.951 µm), for morf details please see [15]). For the BFS control cells, carbonation of the mortar cover was specifically relevant (well known is the high susceptibility of BFS to carbonation), resulting in coarser structure of the mortar cover, higher pore network connectivity and therefore influence on critical pore size. Additionally, the difference in mortar cover porosity (not hereby presented) and internal bulk matrix porosity and connectivity respectively, for the BFS specimens, resulted in some discrepancies in the correlation of EIS and structural parameters. The microstructural investigation reveals that the RMBFSn corroding specimens, which electrochemically behave as control ones (i.e. higher corrosion resistance compared to OPCn, BFSn and RMOPCn) exhibit the lowest porosity. In addition to the specific properties of RM in terms of inducing steel passivity, the combination of RM and BFS apparently exerts a significant positive effect (reduced permeability,
increased chloride binding mechanisms) and thus is responsible for the observed higher corrosion resistance in these specimens. Porosity alone is, however, not a factor solely determining the significantly different electrochemical behavior. What has to be considered is the pore interconnectivity in the cement-based matrix and thus the matrix permeability. A correlation can be made between pore network parameters and electrochemical (EIS) parameters, mainly pore network capacitance (Cp.netw) and pore network resistance, Rp.netw (Table 2). The higher porosity (P%) in OPC control and corroding cells corresponds to the highest Cp.netw and lowest Rp.netw, the values being lower for OPC corroding cells, compared to OPC control cells. The lowest Cp.netw corresponds to the lowest permeability/porosity values, derived for specimens BFS and BFSRM (both corroding and control groups). The result is very well in line with the derived global bulk electrical resistivity of the matrix in BFS as well (Table 2). Consequently, the BFS matrix (and especially in the presence of RM) in the corroding specimens would be characterized with a larger pore surface area but also increased portion of disconnected and isolated conductive pore pathways (increased pore network resistance). The capacitance values for BFS corroding cells are significantly lower than those for the OPC corroding cells, therefore a comparison of pore network permeability and interconnectivity between OPC and BFS specimens can be reliably derived on the basis of pore network capacitance and resistance. Consequently (and moreover, after verification with microstructural analysis), it can be stated that EIS is a powerful, non-destructive technique for evaluation of pore network parameters. Combined with the electrochemical parameters, derived for the embedded steel, EIS allows a thorough evaluation of a reinforced concrete system.
Clearly, the addition of RM has a significant positive effect on the corrosion resistance of both OPC and BFS reinforced mortar specimens. However, further investigation is necessary in order to claim feasibility in practical applications, since the 20% RM replacement in both OPC and BFS cases results in reduced compressive strength of the mortar matrix – Fig.10.
0 10 20 30 40 50 60 70 80 90
1day 7day 14day 28day 56day
Curing ages (day)
C o m p re ss iv e s tr e n g th ( M p a ) OPC OPC+RM BFS BFS+RM
Figure 10: Compressive strength of mortar with and without addition of RM
The recorded values of approximately 30 - 40 MPa at 28 and increasing and 56 days are sufficient according standards for some applications, but are significantly lower than the non-modified matrix of OPC and BFS. It can be observed, however, that the
RM addition to BFS leads to higher values than those for OPC cases, which was actually one of the objectives of this work – to study a simultaneous application of BFS and RM as OPC replacement. Considering the mixing proportions for this study i.e. the very high water-to-cement ratio of 0.6 (generally used for acceleration of corrosion initiation) the mechanical properties are logically affected. Therefore, a further systematic study on the mixture proportions is necessary in order to achieve optimum corrosion control and mechanical performance in reinforced cement-based systems.
4. Conclusions
Summarizing, this study discussed the corrosion behaviour of steel electrodes in model alkaline solutions and the performance of reinforcing steel in OPC and BFS concrete with and without Red Mud replacement, subjected to chloride-induced corrosion (5% NaCl) and in comparison with control cases. Additionally, for reinforced mortar, the electrochemical parameters for the embedded steel were correlated with the microstructural parameters and properties of the bulk concrete matrix.
For steel in model solutions: the presence of RM in both CE and BFS solutions (with
and without NaCl) favours steel passivation and results in an increased corrosion resistance. The positive effect of RM is especially pronounced for steel treated in BFS solutions. The mechanisms and related phenomena are denoted to the presence of Fe2O3 and Al2O3 (from RM and BFS), mainly in terms of the effect of Fe3+ on redox activity and thus stabilization of the passsive layer. Secondly, the adhered red mud particles act as adsorption sites for Ca2+, resulting in a product layer (adhered CaO/Ca(OH)2 and/or Ca-substituted iron oxides/hydroxides) with higher corrosion resistance.
For reinforcing steel in mortar: After 250 days of conditioning, corrosion resistance is
higher in the RM containing BFS corroding cells, whereas active behaviour was observed for the RM free corroding cells and the RM containing OPC cells. The higher corrosion resistance in RMBFS reinforced mortar is denoted to decreased pore-network interconnectivity, evident from the significantly lower pore network capacitance and higher pore network resistance, as derived from EIS and verified on the basis of microstructural analysis. Further investigation for optimization of the mixing proportions sin the presence of RM is necessary in order to claim simultaneously improved corrosion resistance and mechanical properties for reinforced cement-based systems.
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