Corrosion behavior of steel re-bars in mortar, containing waste products.
D.A. Koleva1, K.van Breugel1, J.H. W.de Wit2
1,2
Delft U-ty of Technology, The Netherlands, 1
Faculty Civil Eng. & Geosciences, Dep. Mater. Sci., Stevinweg 1, 2628 CN Delft; 2
Faculty 3mE, Dep. Mater. Sci.&Eng., Mekelweg 2, 2628 CD Delft
This work presents the first results from initial tests in the frame of a large research project on chloride-induced corrosion and corrosion prevention in reinforced concrete, containing the waste “red mud” as a partial replacement of the cement portion in the mixture. The waste “red mud” is generated during aluminum production from bauxite. The annual production of 1 tone of metallic aluminum generates about 2 tones of red mud. The disposal of this alkaline waste sludge is expensive (up to 1–2% of the alumina price); the enormous quantity of red mud generated every year (about 66 million tones) posses very serious and alarming environmental problems. Consequently, research on ways of making use of this residue is of significant importance. Moreover, in addition to the economic and ecological advantages, the use of “red mud” as cement replacement will not create new residues.
Red mud is recently reported to increase steel passivity in alkaline solutions (pre-treatment procedure); “red mud” additions to cement-based materials are reported to result in mixtures, shielding X-ray radiation having or heavy metal (and other toxic substances) binding effects, while maintaining sufficient mechanical properties. To this end, the purpose of our ongoing 3-year project is to investigate the application of “red mud” as additive for reinforced concrete for achieving corrosion protection on one hand, improving the concrete bulk microstructure on the other and thus finally contributing to cement cost reduction and waste utilization in an ecologically friendly manner.
Materials and experimental methods: The hereby
presented results cover the first (initial) period of ongoing electrochemical monitoring of steel embedded in mortar, with and without red mud additions for a period of 36 days. The steel reinforcement (construction steel FeB500 HWL, d=0.8cm, h=10cm) was cast “as received” in mortar cylinders (d=4cm, h=15cm), made from OPC CEM32.5, water-to-cement ratio 0.5, cement-to-sand ratio 1:3. The specimens free of red mud are denoted as OPC, the specimens containing red mud (20% replacement of cement by weight) are denoted as RM. All specimens were cured for 7 days in fog room (20ºC and 98% humidity) and then
placed in lab conditions: lab air, 22ºC and 1/3rd of height
immersed in water or 10% NaCl. The specimens, immersed in water (from both OPC and RM) served as “reference” specimens for a comparison of electrochemical parameters with the “corroding” specimens (both OPC and RM immersed in 10 % NaCl). There were 5 replicates for each type and condition; the hereby presented results are an average of the recorded responses and parameters.
Results: The red mud composition, determined by
XRF and the cement composition are given in Table 1.
Table 1 Red mud and OPC composition Red mud (XRF) OPC CEMI 32.5) Z wt.% StdErr StdComp. Wt.% Na2O 14.25 0.39 Na2O 0.24 MgO 0.124 0.014 MgO 2.00 Al2O3 29.10 0.30 Al2O3 5.03 SiO2 20.24 0.45 SiO2 21.00 P2O5 0.155 0.003 P2O5 0.16 SO3 0.525 0.058 SO3 3.00 Cl 0.142 0.016 Mn2O5 0.06 K2O 0.118 0.013 K2O 0.65 CaO 3.54 0.21 CaO 63.90 TiO2 6.22 0.27 TiO2 0.30 Fe2O3 24.85 0.48 Fe2O3 2.83
Figure 1 (top) presents potential mapping for the “reference”
specimens (OPC and RM) immersed in water from 7 to 30 days; Fig.1 (bottom) refers to the “corroding” specimens OPC and RM for the same period. As seen from the plots, a stable passive condition establishes for the “reference” specimens in approximately 1 week, both OPC and RM exhibiting similar behavior. For the “corroding” groups, a significant difference is recorded between the specimens OPC and those, containing red mud (RM) i.e. the transition from active to passive behavior between 7 and 12 days hold for both groups, after 15 days however, the OPC specimens exhibit a cathodic shift of the corrosion potential, denoted to localized corrosion on the steel surface, while the corroding RM specimens behave as “reference” specimens.
Figures 2 to 5 depict the impedance response as a comparison of different ages. The results are in line with the potential mapping: the “reference” RM specimens (Fig.2) and “reference” OPC specimens (Fig.3) behave similarly with time. With conditioning, the “corroding” RM specimens (Fig. 4) behave like “reference”, despite the very aggressive corrosion medium (10% NaCl); the magnitude of impedance is lower compared to the “reference” RM, but the trend is towards increasing (passivation of the steel surface), rather than the expected active behavior, as observed for the “corroding” OPC specimens (Fig.5). The EIS response is supported by potentio-dynamic polarization results, summarized Rp values for the presented intervals of 7, 23 and 36 days are given in Table 2.
Table 2: Rp in kOhm.cm2
7 days 23 days 36 days OPC “reference” 126.77 317.25 247.50 RM “reference” 109.20 271.75 258.00 OPC “corroding” 147.50 109.75 105.25 RM “corroding” 116.75 175.12 172.65
"Red mud" specimens (water) at 7, 23 and 36 days
7d 23d 36d 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Z' / ohm -Z'' / ohm -2 -1 0 1 2 3 4 5 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 3.8 0 10 20 30 40 50 60 log(f) log( Z ) -p has e / de g
"OPC specimens" (water) at 7, 23 and 36 days
7d 23d 36d 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Z' / ohm -Z'' / ohm -2 -2 -1 -1 0 1 1 2 2 3 3 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 3.8 0 10 20 30 40 50 60 70 log(f) log( Z )( o) -p has e / de g( +)
"Red mud" specimens (10% NaCl) at 7. 23 and 36 days
7d 23d 36d 0 700 1400 2100 2800 3500 0 700 1400 2100 2800 3500 Z' / ohm -Z '' / ohm -2 -1 0 1 2 3 4 5 1.5 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 0 10 20 30 40 50 60 70 log(f) lo g (Z )( o ) -p has e / deg( +)
"OPC" specimens (10 % NaCl) at 7, 23 and 36 days
7d 23d 36d 0 700 1400 2100 2800 3500 0 700 1400 2100 2800 3500 Z' / ohm -Z '' / o h m -2 -1 0 1 2 3 4 5 1.5 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 0 10 20 30 40 50 60 log(f) log( Z )( o) -p h a se / deg (+)
In conclusion, from these preliminary investigation it is clearly seen that the addition of 20% red mud (as cement replacement) in reinforced mortar significantly improves the corrosion resistance of the embedded steel, especially in chloride containing environments.
-600 -500 -400 -300 -200 -100 0 7 10 11 15 19 22 23 27 30 OPC water,A RM water,A -700 -600 -500 -400 -300 -200 -100 0 7 10 11 15 19 22 23 27 30 OPC NaCl,A RM NaCl,A Fig.1 Fig.2 Fig.3 Fig.4 Fig.5
