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Corrosion behaviour of steel in CAC-mixed concrete containing different concentrations of chloride
Construction and Building Materials 110 (2016) 227–234
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Corrosion behaviour of steel in CAC-mixed concrete containing different concentrations of chloride Sung Ho Jin, Hee Jun Yang, Jun Pil Hwang, Ki Yong Ann ⇑ Department of Civil and Environmental Engineering, Hanyang University, Ansan 426-791, Republic of Korea
h i g h l i g h t s Steel in CAC mixed concrete achieved 100%-inhibition against a chloride environment. CAC mixed concrete imposed higher chloride binding capacity compared to OPC. CAC mixed concrete had increased buffering against a pH fall in the cement matrix. A development of the strength of CAC mixed concrete was lowered, compared to OPC.
a r t i c l e
i n f o
Article history: Received 14 September 2015 Received in revised form 18 December 2015 Accepted 11 February 2016
Keywords: Calcium aluminate cement Corrosion Chloride binding Buffering
a b s t r a c t High aluminate in cement arising from Al2O3 in oxides is presumed to form the higher level of CA-type of hydration, which can subsequently bind chlorides then to reduce the corrosion risk. To maximise the chloride binding in the cement matrix, in this study, a mixture of calcium aluminate cement (CAC) with ordinary Portland cement (OPC) was used as binder. The ratio of CAC to total binder was 5%, 10% and 15%. The resistance of CAC mixture to chloride-induced corrosion was assessed by a monitoring of the corrosion rate, and its binding of chlorides and buffering against a pH fall of the cement matrix were simultaneously measured. As a result, the CAC mixture was very resistive to corrosion; there was no corrosion observed in CAC mixture at exceeding 3.0% of chlorides by weight of binder, whilst OPC produced about 0.5–1.0% of the critical chloride threshold for the onset of corrosion. The inhibitive measure of CAC mixture may arise from increased binding capacity of chlorides and buffering to acidification. In particular, the buffering zone for CAC mixture occurred at 10.5–11.5 and 11.8–12.6 in the pH, at which bound chlorides in the matrix would be kept immobile, unreactive in the corrosion process. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As aluminate-based hydration products such as C3A and C4AF in the cement matrix reacts with chlorides to remove from the pore solution, cement containing the high amount of Al2O3 in oxides may be intuitively thought as an inhibitive binder. Thus, calcium aluminate cement (CAC) containing around 40–50% of Al2O3 may be intuitively thought to enhance the inhibition effect of steel against chloride-induced corrosion. For example, it was reported that the CAC concrete could suppress the electrochemical reaction on the steel surface, then to mitigate the corrosion risk at a give chloride concentration at the depth of the steel [1]. In particular, CAC concrete cured at a lower temperature (i.e. <5 °C) had a low level of the porosity in the cement matrix then to lower percolation
⇑ Corresponding author. E-mail address: [email protected] (K.Y. Ann). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.032 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
of ionic transport and thus corrosion risk [2]. To enhance the effect of inhibition, a modified CAC mixture with zeolite was used for inhibiting corrosion of steel in concrete [3]. More currently, it was shown that a mixture of CAC with ordinary Portland cement (OPC) was very inhibitive to chloride-induced corrosion [4]; the chloride threshold level for CAC mixture ranged from 0.8% to 2.4% by weight of binder, whilst OPC produced 0.2–1.0% of the threshold level for the onset of corrosion. Notwithstanding, the inhibition mechanism of CAC concrete has not been clearly known except for electrochemical measurements, and moreover a hard evidence of the higher prevention of CAC, for example, chloride binding of CAC has not been proved to date. Prior to inhibitive measure of CAC, its concrete properties must be ensured. In fact, CAC concrete usually gains about 80–90% of the ultimate strength within 24 h together with a rapid setting and hardening, accounting for about 3 h [5]. The rapid strengthening of CAC concrete arises from a formation of the hexagonal CAH10
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S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
phase, which may be subsequently converted to the cubic of C3AH6 phase at higher and normal temperatures. In this process, porosities are further formed and in turn the density of the cubic phase is increased, leading to a subsequent reduction of concrete strength [6]. Moreover, when CAC is mixed with OPC in a mix, the concrete may be often be subjected to flash set [5]. Therefore, the use of CAC, in particular, a mixture of CAC with OPC is limited in application into in-situ, unless its problematic sources are removed or/and controlled. Simultaneously, the benefits of CAC mixture in inhibiting corrosion of steel must be promisingly guaranteed by any means, for example, increased chloride binding capacity. Moreover, CAC mixed with OPC may have economic benefit to compensate for the high price of CAC, of which the expense usually costs 5–7 times depending on its type, in terms of the amount of Al2O3 in oxides. In this study, the content of CAC in mix was limited to 5%, 10% and 15% to total weight of binder to produce CAC mixture with OPC to avoid or/and minimise the adverse effect to concrete properties. To ensure concrete properties, the setting time and strength development for the CAC mixture were preliminarily tested. The inhibition effect of CAC mixture was evaluated by electrochemical values in terms of the corrosion rate and potential, which were measured by the anodic polarisation technique. To investigate the inhibition mechanism, the chloride binding capacity of CAC mixture was measured by the water extraction method, and simultaneously its buffering capacity to a pH fall was determined by the resistance of CAC mixture against acidification of the cement matrix. In testing of corrosion, chlorides were mixed in cast to accelerate the corrosion process.
2. Experiments To investigate the inhibitive effect of calcium aluminate cement (CAC) mixture with ordinary Portland cement (OPC), the corrosion resistance was quantitatively measured by the electrochemical techniques and chemistry at chlorides in the cement matrix. Mixtures of CAC were fabricated, being in the range of 5%, 10% and 15% of CAC to total binder content. The oxide composition of CAC and OPC is given in Table 1. In this study, a CAC containing a very high level of Al2O3 was used, accounting for 71.0%, while normal CAC contains 40–50% of Al2O3. Mix proportion for the binder:mixing water:sand (Grade M):10.0 mm gravel was 1.00:0.40:2.45:3.17. The specific gravity of sand gravel was 2.58 and 2.65, respectively and their absorption was simultaneously 1.01%. Mortar specimens were used for testing of corrosion, while cement pastes were used to determine the chloride binding capacity and buffering to acidification in terms of acid neutralisation capacity. For a development of concrete strength, concrete was used at different ages. A free water/binder ratio was kept at 0.4 for all specimens.
2.2. Chloride binding and buffering Cement paste was cast to determine the chloride binding capacity of CAC mixtures rather than mortar and concrete, because aggregates do not affect the chemistry between cement hydration products and chloride ions. Six levels of chlorides were admixed in mixing water as NaCl: 0.0%, 0.5%, 1.0%, 1.5%, 2.0% and 3.0% by weight of binder. After casting the paste, the specimen was rotated at 6.0 rpm for 24 h to avid segregation of chloride ions, and then cured by wrapping in a polythene film at 20 ± 2 °C for 28, 56 and 91 days. Then, the specimen was dried in an oven at 104 °C for 24 h and then crushed/ground to obtain dust sample, which was subsequently sieved into a 300 lm sieve in the fineness. The dust sample was stirred for 5.0 min in 50 °C distilled water to extract the water-soluble chloride. After a further 30 min standing of the sample, the concentration of water soluble chloride ions in the cement paste was measured by the potentiometric titration against silver nitrate, which is taken as free chlorides [7]. Non-soluble chlorides in this procedure were, in turn, considered as bound ones. The buffering capacity of CAC mixture against acidification of the pore solution and cement matrix was determined by the pH measurement of the paste suspension against a given acid concentration. The dust sample was produced after 56 days of curing in the same way with the chloride binding capacity as above. Then, 3.0 g of the dust sample was dissolved in an acid solution consisting of 2.0 M nitric acid and distilled water to produce a 50.0 ml suspension sample. The range of acid concentration was restricted to 20.0 mol/kg binder. The details of acid concentration at a given volume of acid solution in the process of suspension production are given elsewhere [8]. After stirring the suspension for 10.0 min, the pH of the suspension was measured by a pH metre. Then, the acid neutralisation capacity (ANC) was determined at different pH values, as being defined as the acid concentration to a change in the pH in the process of acidifying the suspension. 2.3. Corrosion behaviour The corrosion of steel in concrete was measured by the anodic polarisation technique to quantify the corrosiveness of steel embedment. Mortar specimens were fabricated with a centrally located 10.0 mm diameter mild steel bar and equivalent levels of chloride to testing for chloride binding were simultaneously admixed in mixing water. The two ends of the steel bars were masked using rich cement paste, followed by a further covering with heat shrink insulation to avoid corrosion under the masking. The specimens were cured by wrapping in a polythene film at20 ± 2 °C for 56 days. To accelerate the corrosion process, the specimens were then subjected to a wet and dry cyclic condition consisting of 4 days wet and 3 days dry. During the wet condition, the relative humidity was kept 95% at 50 °C, while for the dry condition, it was 50% RH at 20 °C. The corrosion rate and potential were measured immediately at the completion of every wet cycle to minimise electrical resistance of mortar. In the measurement of the corrosion rate, the corrosion potential was measured by the standard calomel electrode prior to polarisation to determine the corrosion rate. Then, the potential was swept at a low scan rate of about 0.1 mV/s to a potential of 25.0 mV above the corrosion potential to achieve the polarisation resistance of the steel in mortar. The IR drop arising from electrical resistance of mortar was compensated by a current interruption technique. Then, the corrosion rate was calculated by the Ohm’s law as given in Eq. (1).
ICORR ¼
B RP
ð1Þ
where, ICORR indicates the corrosion rate, B for the constant potential (26 mV) and RP for the polarisation resistance, respectively.
2.1. Fundamental properties As concrete properties, a development of the compressive strength for CAC mixture was measured after fabrication of cylindrical specimens (Ø100 200 mm). Concrete specimens were demoulded 24 h after casting and then cured in a 95% humid chamber at 20 ± 2 °C. The setting time was determined by the penetration resistance of fresh concrete. Immediately after casting concrete, gravel in the fresh mix was removed by the 5.0 mm sieve. Then, mortar sample was poured in a cylindrical mould (Ø150 150 mm) to measure the penetration resistance, which was subsequently measured by a set of needles at 2.25, 3.19, 4.54, 7.15, 10.13 and 14.32 mm in the diameter at different time interval. The initial set was defined as the time for the penetration resistance to reach 3.43 MPa, and the final set to 27.46 MPa. The time of set was determined by interpolating the relation between the penetration resistance and time.
3. Results 3.1. Development of strength A development of the compressive strength for CAC mixture at 7, 28, 56 and 91 days is given in Fig. 1. For all concretes, the compressive strength was increased by curing age, irrespective of binder type. However, for CAC mixture, an increase in the CAC content in binder resulted in a decrease in the strength at all ages. For example, the strength for 5% CAC concrete was 25.3 MPa at
Table 1 Chemical physical characteristics of CAC and OPC. Oxides (%)
CAC OPC
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
SO3
28.5 63.8
0.2 22.1
71.0 5.0
0.1 3.0
0.4 1.6
0.2 0.35
– 0.64
– 2.0
Blaine (cm2/g)
Density
3750 3242
3.05 3.12
S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
OPC
5% CAC
10% CAC
15% CAC
Compressive strength (MPa)
40
effect to concrete properties, such as thermal cracking and thus lower strength. In the present study, treatments to secure/retard the setting of fresh concrete were not dealt with, for example, chemical retarder. 3.2. Removal of chlorides
30
20
10
0 0
20
40
60
80
100
Curing age (days) Fig. 1. Development of compressive strength for CAC mixture and OPC concrete with curing age.
28 days, and only 18.5 MPa of the strength was achieved for 15% CAC concrete at the corresponding days. The lower strength for CAC mixture is presumably associated with a very rapid setting of fresh concrete, leading to a high level of hydration heat and thus thermal cracking. At 91 days, however, 15% CAC concrete produced a rapid increase in the compressive strength, accounting for about 29.5 MPa, presumably due to a formation of cubic phase of CAH10, which is often precipitated in CAC paste in a long term then to stabilise the strength of CAC concrete, as the CAC used in this study contains a very high Al2O3 (i.e. 71.0%). The initial and final sets were determined by a monitoring of the penetration resistance of fresh concrete with time, as given in Fig. 2. The penetration resistance was best fitted with time to define the initial and final setting times. The CAC mixture dramatically reduced the setting time, depending on the CAC content in binder. The final set for 5%, 10% and 15% CAC was achieved at 201, 126 and 75 min respectively, while OPC achieved 377 min. A rapid setting for the CAC mixture, in fact, may impose adverse
OPC
CAC 5%
76 min.
126 min.
CAC 10%
CAC 15%
201 min.
377 min.
40
Penetration resistance (MPa)
229
30
Final set : 27.6 MPa
20
The presence of chlorides in the cement matrix was evaluated by chloride binding and buffering against a pH fall. The chloride binding capacity was expressed by the relation between free and bound chlorides at a given total chloride concentration, which was in fact rendered by the Langmuir isotherm, measured at 28, 56 and 91 days to identify the influence of curing age, as seen in Fig. 3. It is evident that an increase in the curing age resulted in an increase in the binding of chlorides, irrespective of binder type. In particular, the binding capacity of chlorides was dramatically increased after 28 days, and the binding capacity at 56 and 91 days was then mostly identical at a lower total chloride concentration. At a higher total chloride concentration exceeding 2.0% and 3.0%, the binding capacity at 91 days was slightly higher than at 56 days. It is notable that a CAC mixture always produced the higher level of chloride binding at a given chloride concentration at all ages. This may be attributed to a further formation of CA-type hydrations in the cement matrix. The high level of Al2O3 in CAC would enhance a formation of C3A or/and other CA-type hydrations, which could subsequently react with chloride ions to form chloro aluminate hydrates (i.e. Friedel’s salt). In fact, an increase in the portion of CAC in the mix resulted in an increase in the chloride binding capacity: for 15% CAC mix, the chloride binding was the highest ranked. The acid neutralisation capacity (ANC) of CAC mixtures was obtained as seen in Fig. 4, which depicts the resistance of hydration products against acidification at different pH values of the suspension. It is seen that OPC produced the higher buffering capacity at around 11.8–12.6 in the pH, at which Ca(OH)2 is presumed to be precipitated in the cement matrix [9]. At the lower pH values, there were only marginal peaks indicating the resistance to a pH fall. For CAC mixtures, the peaks at 11.8–12.6 in the pH were less present, seemingly still imposing the ANC of precipitated Ca(OH)2 to acidification. However, there were many, strong peaks even at lower pH values below 10.5–11.5 in the pH. It has an important implication in determining a chloride removal in the cement matrix. Chlorides bound by hydration products are often released into free at a lower pH, which would often take place at pit nucleation on the surface of steel in concrete. In this process, a number of electrons are released to acidify the cement matrix, thereby leading to a decomposition of chloro aluminate hydrates. Thus, the higher ANC may impose the increased buffering of a pH fall in the cement matrix and in turn bound chlorides could be kept immobile, unreactive in the vicinity of the steel, as being indicative of lower corrosion risk at a given chloride-bearing environment. The increased ANC of CAC mixtures was seemingly associated with CA-type hydration products. 3.3. Corrosion resistance
10
Initial set : 3.5 MPa 0 0
100
200
300
400
Time (min.) Fig. 2. Penetration resistance of fresh concrete with time to determine the setting time.
The corrosion behaviour of steel in CAC mixture was evaluated by the anodic polarisation technique to measure the corrosion rate and potential. As seen in Fig. 5, the corrosion potential was strongly influenced by binder type and chloride concentration in cast. For the lower chloride concentration at 0.0% and 0.5%, all specimens indicated mostly passive, exceeding the corrosion threshold voltage (i.e. 275 mV vs SCE). However, the corrosion potential for OPC specimens dramatically decreased up to about 400 mV, when the chloride concentration exceeded 1.0%. It may imply that OPC may have about 0.5–1.0% of the critical chloride concentration
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(a) OPC
(b) 5% CAC 56 days
91 days
28 days
2.5
2.5
2.0
2.0
Bound chloride (%, binder)
Bound chloride (%, binder)
28 days
1.5
1.0
0.5
28 days: Cb =
1.67 Cf 1 + 0.64 Cf
56 days: Cb =
2.54 Cf 1 + 0.50 Cf
91 days: Cb =
2.25 Cf 1 + 0.13 Cf
0.0 0.0
0.5
1.0
1.5
91 days
1.5
1.0
0.5
28 days: Cb =
3.52 Cf 1 + 1.88 Cf
56 days: Cb =
5.02 Cf 1 + 1.54 Cf
91 days: Cb =
5.83 Cf 1 + 1.76 Cf
0.0 0.0
2.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
Free chloride (%, binder)
(c) 10% CAC
(d) 15% CAC 28 days
56 days
91 days
28 days
2.5
2.5
2.0
2.0
Bound chloride (%, binder)
Bound chloride (%, binder)
56 days
1.5 28 days: Cb =
4.37 Cf 1 + 1.70 Cf
56 days: Cb =
4.73 Cf 1 + 1.17 Cf
91 days: Cb =
5.83 Cf 1 + 1.46 Cf
1.0
0.5
0.0
56 days
91 days
1.5 28 days: Cb =
5.90 Cf 1 + 1.95 Cf
56 days: Cb =
6.62 Cf 1 + 2.13 Cf
91 days: Cb =
4.08 Cf 1 + 0.71 Cf
1.0
0.5
0.0 0.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
0.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
Fig. 3. Relation between free and bound chlorides at a given total chloride concentration in CAC mixture and OPC paste at 28, 56, 91 days (Rendered by the Langmuir isotherm).
for the onset of corrosion, seemingly being in the range of previous studies [10,11]. For the CAC mixture, however, the corrosion potential kept always higher at all concentrations of chloride, irrespective of the CAC ratio in binder. It means that CAC mixture may keep the cathodic reactivity in mortar passive even at a very high concentration of chlorides. The corrosion rate of steel in CAC mixture was monitored, as seen in Fig. 6. It was observed that the corrosion rate was always in the range for passive zone (<1.0 mA/m2), when chlorides in cast ranged from 0.0% to 0.5%, except for OPC mortar after 10 cycles exposure. For OPC specimens, the corrosion rate at 1.0% of chlorides was highly ranged accounting for 3.1–15.6 mA/m2, at which range the depassivation of steel is known to start [12]. Then, the corrosion rate was rapidly increased with the concentration of chloride. However, the CAC mixture, in particular, 10% and 15% CAC, showed very low corrosion rates at all levels of chloride, indicating that no visible corrosion may takes place, accounting for below 1.0 mA/m2. Even for 5% CAC mixture, the corrosion rate indicated passive, except for at 3.0% of chlorides in cast, which was subsequently recovered to the passive state. The inhibitive nature of CAC mixture may be attributed to increased buffering capacity
of the cement matrix against acidification in the vicinity of the steel. Otherwise, the steel surface in the CAC mixture may achieve further passivity against corrosion arising from CA-type hydrations. After an exposure of the CAC mixed mortar containing different concentrations of chloride ranging 0.0–3.0% to 20 wet and dry cycles, steel embedded in the mortar was obtained by breaking and splitting the specimen. Representative steel rebars at each concentration of chloride in cast are given in Fig. 7. It is evident that all steel rebars were passive in chloride-free mortars, irrespective of binder type. However, for OPC, an increase in the chloride content in cast resulted in an increase in the rust amount on the steel surface, as expected. In particular, 2.0% and 3.0% of chlorides, all surfaces of the steel rebar were severely corroded. For CAC mixed mortar, all steel rebars were kept passive even at a very high chloride concentration, except for the 5% CAC specimen, for which the steel rebar was partially corroded at 3.0% of chloride, at which the corrosion rate exceeded 5.0 mA/m2 at early cycles and subsequently decreased to a small value. It is evident that steel rebars embedded in 10 and 15% CAC mortar were never corroded, implying an guaranteed inhibitive nature.
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(a) OPC
(b) 5% CAC 60
Resistance to a pH fall (mol/kg)
Resistance to a pH fall (mol/kg)
60 50 40 30 20 10 0
50 40 30 20 10 0
13
12
11
10
9
13
12
11
pH of suspension (pH)
(c) 10% CAC
9
10
9
(d) 15% CAC 60
Resistance to a pH fall (mol/kg)
60
Resistance to a pH fall (mol/kg)
10
pH of suspension (pH)
50 40 30 20 10 0
50 40 30 20 10 0
13
12
11
10
9
13
12
11
pH of suspension (pH)
pH of suspension (pH)
Fig. 4. Acid neutralisation capacity a pH fall for CAC mixture and OPC paste.
(a) 0.0% of chlorides
(b) 0.5% of chlorides 10% CAC
15% CAC
0
OPC
-100 -200 -300 -400 -500
10% CAC
15% CAC
0 -100 -200 -300 -400 -500
0
5
10
15
20
5
OPC
10
15
20
Number of cycle (weeks)
(d) 2.0% of chlorides
OPC
5% CAC
10% CAC
15% CAC
0 -100 -200 -300 -400 -500
0
Number of cycle (weeks)
0
5
10
15
Number of cycle (weeks)
(e) 3.0% of chlorides
5% CAC
10% CAC
15% CAC
0
OPC
Corrosion potential (SCE vs. mV)
Corrosion potential (SCE vs. mV)
(c) 1.0% of chlorides
5% CAC
Corrosion potential (SCE vs. mV)
5% CAC
Corrosion potential (SCE vs. mV)
Corrosion potential (SCE vs. mV)
OPC
-100 -200 -300 -400 -500
5% CAC
10% CAC
15% CAC
0 -100 -200
Corrosion potential: -275 mV vs. SCE
-300 -400 -500
0
5
10
15
Number of cycle (weeks)
20
0
5
10
15
20
Number of cycle (weeks)
Fig. 5. Corrosion potential of steel in CAC mixture and OPC mortar depending on chloride concentration in cast for 20 cycles (3 days wet + 4 days dry).
20
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S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
(a) 0.0% of chlorides OPC
(b) 0.5% of chlorides
CAC 5%
CAC 10%
CAC 15%
OPC
CAC 15%
OPC
1
0.1
10
1
0.1
0.01 0
5
10
15
20
OPC
CAC 10%
CAC 15%
10
1
0.1
0.01 0
5
Number of cycle (weeks)
10
15
20
Number of cycle (weeks)
(d) 2.0% of chlorides
CAC 5%
100
Corrosion rate (mA/m2)
10
0.01
0
5
10
15
20
Number of cycle (weeks)
(e) 3.0% of chlorides
CAC 5%
CAC 10%
CAC 15%
OPC
CAC 5%
CAC 10%
CAC 15%
100
Corrosion rate (mA/m2)
100
Corrosion rate (mA/m2)
CAC 10%
100
Corrosion rate (mA/m2)
Corrosion rate (mA/m2)
100
(c) 1.0% of chlorides
CAC 5%
10
1
0.1
0.01
10 Corrosion initiantion: 1-2 mA/m2
1
0.1
0.01 0
5
10
15
Number of cycle (weeks)
20
0
5
10
15
20
Number of cycle (weeks)
Fig. 6. Corrosion rate of steel in CAC mixture and OPC mortar depending on chloride concentration in cast for 20 cycles (3 days wet + 4 days dry).
Fig. 7. Representative steels embedded in CAC-mixed mortar containing different concentration of chlorides after 20 wet and dry cycles.
4. Discussion 4.1. Inhibition of CAC mixtures The high resistance of the CAC mixture against chlorideinduced corrosion may arise from increased chloride binding capacity. Since the CAC mixture contains various CA-type hydrations including C12A7, C3A and C4AF, which are all reactive with chloride ions then to be stabilised in the matrix. As the precipitated Ca(OH)2 is still present in hydrations, chlorides can be physically further adsorbed, thereby being subsequently removed from the corrosion process. As seen in Fig. 3, the binding capacity of chlorides in the CAC mixture was always higher than in OPC. It may
imply that the binding of chloride ions in the CAC mixture may be attributed to OPC and CAC hydrations at once; chlorides could be bound by C3A and C4AF formed in OPC paste and simultaneously by C12A7 in CAC one. The more crucial cause of inhibition in CAC mixture may be its ANC to a pH fall. The resistance to the onset of corrosion in concrete is strongly dependent on the buffering capacity, since bound chlorides are released into free by acidification in the vicinity of steel. The buffering for OPC concrete mainly occurs at 11.5–12.5 in the pH, arising from precipitated Ca(OH)2, while the CAC mixtures have the buffering at more acidic region of at 11.5–12.5 and 10.5–11.5 in the pH. In particular, at around 10.5 in the pH, the buffering peaks were highly generated, as seen in Fig. 4.
S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
As a release of bound chlorides usually occurs in a relatively acidic environment, often at 8.5–10.0 in the pH, the buffering of CAC mixture at 10.5–11.5 in the pH would impose an additional inhibitive nature of the cement matrix. Substantially, the CAC mixture may chemically bind more chlorides, which could be kept immobile by increased ANC to acidification of the cement matrix and pore solution. For chlorides in CAC mixture to corrode the steel, a further supply of chlorides is therefore required, implying a mitigated corrosion risk. When it comes to the corrosion initiation, chloride threshold level for CAC mixture exceeded 3.0% by weight of binder, whilst the threshold level for OPC was about 0.5–1.0% by weight of binder. Notwithstanding, the inhibition of CAC mixture is still overwhelmingly high, considering its chloride binding capacity and ANC to a pH fall. The ratio of chloride binding in CAC mixture ranges about 50–60% to total chlorides, and the other chlorides would be formed free and thus reactive in the corrosion process. Although all bound chlorides in the CAC mixture are kept immobile, unreactive due to increased ANC, the resistance to the onset of corrosion in terms of critical chloride threshold value would be approximately double for OPC concrete. However, the chloride threshold for CAC mixture was at least 3 times higher than for OPC. It may suggest that CAC mixture may provide a further inhibitive measure to the steel embedment in concrete, for example, a modification of the passivity on the steel surface. A high level of Al2O3 from CAC may be dissolved in the pore solution in casting of concrete to react with ferrous ions on the steel surface, which may form a passive layer (i.e. sacrificial coating). Thus, further inhibition would result in a dramatic resistance to the onset of corrosion. However, the formation of the layer was not observed in the present study. 4.2. Application of CAC mixture to in-situ Despite the inhibitive measure of CAC mixture, its application to in-situ may be restricted by a lower development of the strength and rapid setting. As seen in Fig. 1, the compressive strength of CAC mixture was always lower than for OPC. In fact, an increase in the CAC content resulted in a decrease in the compressive strength, ranging from 18.5 to 25.3 MPa at 28 days, although the strength for structural concrete must achieve at least 25–30 MPa at the corresponding days to meet most guidelines. A reduction of the strength for CAC mixture may be attributed to a rapid setting and hardening, which subsequently would form thermal cracking. Moreover, a further porosity may be formed in the process of conversion in CAC paste from the hexagonal phase of CA3H6 to the cubic phase of CAH10, in which the process the density of fraction is nevertheless increased [13]. However, the pores generated in the conversion process is limited in gel and capillary pores in the small margin and thus no adverse effect in reducing the strength would take place [14]. Thus, avoiding a rapid setting of fresh CAC mixture must be accompanied to obtain a given target strength by addition of a chemical retarder. The retarded setting of CAC mixture could simultaneously secure the time of casting concrete, as a very short duration of the standing time for fresh concrete, in the range of 1– 3 h, could be problematic in applying into in-situ. Alternatively, a modification of the mix design would enhance a development of the strength, such as a reduced free W/B. As the compressive strength of CAC mixture was in the range of 20–25 MPa, a slight reduction of W/B would meet the target strength. However, CAC mixture may face a barrier in the economic expense due to the high cost of CAC, of which the price cost 5– 10 times higher depending on the portion of Al2O3. In this study, the mix ratio of CAC containing 71.0% of Al2O3 in oxides was 5%, 10% and 15% in total binder, imposing 20–30% increase in the economic expense arising from binder modification. To avoid
233
economic limitation, CAC mixture could be partially used/cast in a high risky zone for corrosion, for example, concrete exposed to tidal/splash zones where corrosion is very severe due to a rapid percolation and supply of oxygen. However, concrete structures built in a seawater or deicer environment are entirely exposed to high chloride-bearing risk. Substantially, a large amount of concrete must be cast with CAC mixtures. To overcome economic limit, steel coating with CAC mixture in in-situ could sort out this problematic limitation with its inhibition effect sustained on the steel surface. As in-situ coating, CAC mixture milk can be sprayed or shot to the steels after placing the steel embedment then to cast a bulk concrete. Unlike polymer-based barrier coatings, there would be no adverse effect such as debondment between coating materials and bulk concrete, due to even lower membrane potential. A rapid setting of CAC mixture may induce, furthermore, no delay of procedures of concrete casting. 5. Conclusion The inhibition of CAC mixture was assessed by monitoring the corrosion rate and potential at different chloride concentration in mortar. Simultaneously, concrete properties for CAC mixture such as a development of the compressive strength and setting time were measured. To investigate the cause of inhibition of CAC mixture, its chloride binding and buffering to a pH fall were determined. The detailed conclusion is given as below: (1). The compressive strength of CAC mixture was always lower than for OPC solely mixed concrete, accounting for 18.5– 25.3 MPa, presumably due to a thermal cracking in the process of rapid setting of fresh concrete. The setting time for the CAC mixture was significantly reduced by an increase in the CAC content in binder, being in the range of 76– 201 min. (2). The CAC mixture showed the higher level of chloride binding, compared to OPC, together with a significant buffering capacity to a pH fall. The chloride binding capacity of CAC mixture was always higher than for OPC at all ages and at all level of total chlorides in cast. In particular, the ANC of CAC mixture to a pH fall further occurred at 10.5–11.5 and 11.8–12.6 in the pH, while OPC had the buffering zone only at 11.8–12.6. The increased ANC of CAC mixture may sustain bound chlorides stabilised in the cement matrix even in the acidic environment. (3). The CAC mixture had a higher resistance to chloride-induced corrosion, compared to OPC, when the corrosion rate and potential were measured by the electrochemical methods. No corrosion risk was observed in CAC mixture at up to 3.0% of chlorides by weight of binder, whilst OPC produced 0.5–1.0% of the critical chloride threshold for the onset of corrosion. This inhibition effect may be attributed to increased binding of chlorides and buffering to a pH fall.
Acknowledgement This work was supported by the research fund of Hanyang University (HY-2013-P). References [1] S. Goni, M.T. Gaztanaga, J.L. Sagrera, M.S. Hernandez, The influence of NaCl on the reactivity of high alumina cement in water: pore solution and solid phase characterization, J. Mater. Res. 9 (6) (1994) 1533–1539. [2] A. Macias, A. Kindness, F.P. Glasser, Corrosion behaviour of steel in high alumina cement mortar cured at 5, 25, and 55 °C: chemical and physical factors, J. Mater. Sci. 31 (9) (1996) 2279–2289.
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[3] Y. Fu, J. Ding, J.J. Beaudoint, Corrosion protection of reinforcement in modified high-alumina cement concrete, ACI Mater. J. 93 (6) (1996) 609–612. [4] G.K. Glass, N.R. Buenfeld, The influence of the steel-concrete interface on the risk of chloride induced corrosion. EPSRC Grant GR/K96328 Report, 2000. [5] T.D. Robson, The characteristics and applications of mixtures of Portland and high-alumina cements, Chem. Ind. 1 (1952) 2–7. [6] H.G. Midgley, A. Midgley, The conversion of high alumina cement, Mag. Concr. Res. 27 (91) (1975) 59–77. [7] G.K. Glass, N.R. Buenfeld, The influence of chloride binding on the chloride induced corrosion risk in reinforced concrete, Corros. Sci. 42 (2) (2000) 329– 344. [8] K.Y. Ann, M.S. Jung, H.-B. Shim, M.C. Shin, Effect of electrochemical treatment in inhibiting the corrosion of steel in concrete, ACI Mater. J. 108 (5) (2011) 485–492.
[9] U.A. Birnin-Yauri, F.P. Glasser, Friedel’s salt, Ca2Al(OH)6(Cl, OH)2H2O: its solid solutions and their role in chloride binding, Cem. Concr. Res. 28 (12) (1998) 1713–1723. [10] K.Y. Ann, H.W. Song, Chloride threshold level for corrosion of steel in concrete, Corros. Sci. 49 (11) (2007) 4113–4133. [11] U. Angst, B. Elsener, C.K. Larsen, Ø. Venesland, Critical chloride content in reinforced concrete, Cem. Concr. Res. 39 (11) (2009) 1122–1138. [12] J.A. Gonzalez, C. Andrade, Effect of carbonation and relative ambient humidity on the corrosion of galvanised rebars embedded in concrete, Br. Corros. J. 17 (1) (1982) 21–28. [13] C. Arya, Calcium aluminate cements in construction. Concrete Society Technical, 1997 Report 46. [14] K.Y. Ann, T.-S. Kim, J.H. Kim, S.-H. Kim, The resistance of high alumina cement against corrosion of steel in concrete, Constr. Build. Mater. 24 (11) (2010) 1502–1510.
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Corrosion behaviour of steel in CAC-mixed concrete containing different concentrations of chloride Sung Ho Jin, Hee Jun Yang, Jun Pil Hwang, Ki Yong Ann ⇑ Department of Civil and Environmental Engineering, Hanyang University, Ansan 426-791, Republic of Korea
h i g h l i g h t s Steel in CAC mixed concrete achieved 100%-inhibition against a chloride environment. CAC mixed concrete imposed higher chloride binding capacity compared to OPC. CAC mixed concrete had increased buffering against a pH fall in the cement matrix. A development of the strength of CAC mixed concrete was lowered, compared to OPC.
a r t i c l e
i n f o
Article history: Received 14 September 2015 Received in revised form 18 December 2015 Accepted 11 February 2016
Keywords: Calcium aluminate cement Corrosion Chloride binding Buffering
a b s t r a c t High aluminate in cement arising from Al2O3 in oxides is presumed to form the higher level of CA-type of hydration, which can subsequently bind chlorides then to reduce the corrosion risk. To maximise the chloride binding in the cement matrix, in this study, a mixture of calcium aluminate cement (CAC) with ordinary Portland cement (OPC) was used as binder. The ratio of CAC to total binder was 5%, 10% and 15%. The resistance of CAC mixture to chloride-induced corrosion was assessed by a monitoring of the corrosion rate, and its binding of chlorides and buffering against a pH fall of the cement matrix were simultaneously measured. As a result, the CAC mixture was very resistive to corrosion; there was no corrosion observed in CAC mixture at exceeding 3.0% of chlorides by weight of binder, whilst OPC produced about 0.5–1.0% of the critical chloride threshold for the onset of corrosion. The inhibitive measure of CAC mixture may arise from increased binding capacity of chlorides and buffering to acidification. In particular, the buffering zone for CAC mixture occurred at 10.5–11.5 and 11.8–12.6 in the pH, at which bound chlorides in the matrix would be kept immobile, unreactive in the corrosion process. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As aluminate-based hydration products such as C3A and C4AF in the cement matrix reacts with chlorides to remove from the pore solution, cement containing the high amount of Al2O3 in oxides may be intuitively thought as an inhibitive binder. Thus, calcium aluminate cement (CAC) containing around 40–50% of Al2O3 may be intuitively thought to enhance the inhibition effect of steel against chloride-induced corrosion. For example, it was reported that the CAC concrete could suppress the electrochemical reaction on the steel surface, then to mitigate the corrosion risk at a give chloride concentration at the depth of the steel [1]. In particular, CAC concrete cured at a lower temperature (i.e. <5 °C) had a low level of the porosity in the cement matrix then to lower percolation
⇑ Corresponding author. E-mail address: [email protected] (K.Y. Ann). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.032 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
of ionic transport and thus corrosion risk [2]. To enhance the effect of inhibition, a modified CAC mixture with zeolite was used for inhibiting corrosion of steel in concrete [3]. More currently, it was shown that a mixture of CAC with ordinary Portland cement (OPC) was very inhibitive to chloride-induced corrosion [4]; the chloride threshold level for CAC mixture ranged from 0.8% to 2.4% by weight of binder, whilst OPC produced 0.2–1.0% of the threshold level for the onset of corrosion. Notwithstanding, the inhibition mechanism of CAC concrete has not been clearly known except for electrochemical measurements, and moreover a hard evidence of the higher prevention of CAC, for example, chloride binding of CAC has not been proved to date. Prior to inhibitive measure of CAC, its concrete properties must be ensured. In fact, CAC concrete usually gains about 80–90% of the ultimate strength within 24 h together with a rapid setting and hardening, accounting for about 3 h [5]. The rapid strengthening of CAC concrete arises from a formation of the hexagonal CAH10
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phase, which may be subsequently converted to the cubic of C3AH6 phase at higher and normal temperatures. In this process, porosities are further formed and in turn the density of the cubic phase is increased, leading to a subsequent reduction of concrete strength [6]. Moreover, when CAC is mixed with OPC in a mix, the concrete may be often be subjected to flash set [5]. Therefore, the use of CAC, in particular, a mixture of CAC with OPC is limited in application into in-situ, unless its problematic sources are removed or/and controlled. Simultaneously, the benefits of CAC mixture in inhibiting corrosion of steel must be promisingly guaranteed by any means, for example, increased chloride binding capacity. Moreover, CAC mixed with OPC may have economic benefit to compensate for the high price of CAC, of which the expense usually costs 5–7 times depending on its type, in terms of the amount of Al2O3 in oxides. In this study, the content of CAC in mix was limited to 5%, 10% and 15% to total weight of binder to produce CAC mixture with OPC to avoid or/and minimise the adverse effect to concrete properties. To ensure concrete properties, the setting time and strength development for the CAC mixture were preliminarily tested. The inhibition effect of CAC mixture was evaluated by electrochemical values in terms of the corrosion rate and potential, which were measured by the anodic polarisation technique. To investigate the inhibition mechanism, the chloride binding capacity of CAC mixture was measured by the water extraction method, and simultaneously its buffering capacity to a pH fall was determined by the resistance of CAC mixture against acidification of the cement matrix. In testing of corrosion, chlorides were mixed in cast to accelerate the corrosion process.
2. Experiments To investigate the inhibitive effect of calcium aluminate cement (CAC) mixture with ordinary Portland cement (OPC), the corrosion resistance was quantitatively measured by the electrochemical techniques and chemistry at chlorides in the cement matrix. Mixtures of CAC were fabricated, being in the range of 5%, 10% and 15% of CAC to total binder content. The oxide composition of CAC and OPC is given in Table 1. In this study, a CAC containing a very high level of Al2O3 was used, accounting for 71.0%, while normal CAC contains 40–50% of Al2O3. Mix proportion for the binder:mixing water:sand (Grade M):10.0 mm gravel was 1.00:0.40:2.45:3.17. The specific gravity of sand gravel was 2.58 and 2.65, respectively and their absorption was simultaneously 1.01%. Mortar specimens were used for testing of corrosion, while cement pastes were used to determine the chloride binding capacity and buffering to acidification in terms of acid neutralisation capacity. For a development of concrete strength, concrete was used at different ages. A free water/binder ratio was kept at 0.4 for all specimens.
2.2. Chloride binding and buffering Cement paste was cast to determine the chloride binding capacity of CAC mixtures rather than mortar and concrete, because aggregates do not affect the chemistry between cement hydration products and chloride ions. Six levels of chlorides were admixed in mixing water as NaCl: 0.0%, 0.5%, 1.0%, 1.5%, 2.0% and 3.0% by weight of binder. After casting the paste, the specimen was rotated at 6.0 rpm for 24 h to avid segregation of chloride ions, and then cured by wrapping in a polythene film at 20 ± 2 °C for 28, 56 and 91 days. Then, the specimen was dried in an oven at 104 °C for 24 h and then crushed/ground to obtain dust sample, which was subsequently sieved into a 300 lm sieve in the fineness. The dust sample was stirred for 5.0 min in 50 °C distilled water to extract the water-soluble chloride. After a further 30 min standing of the sample, the concentration of water soluble chloride ions in the cement paste was measured by the potentiometric titration against silver nitrate, which is taken as free chlorides [7]. Non-soluble chlorides in this procedure were, in turn, considered as bound ones. The buffering capacity of CAC mixture against acidification of the pore solution and cement matrix was determined by the pH measurement of the paste suspension against a given acid concentration. The dust sample was produced after 56 days of curing in the same way with the chloride binding capacity as above. Then, 3.0 g of the dust sample was dissolved in an acid solution consisting of 2.0 M nitric acid and distilled water to produce a 50.0 ml suspension sample. The range of acid concentration was restricted to 20.0 mol/kg binder. The details of acid concentration at a given volume of acid solution in the process of suspension production are given elsewhere [8]. After stirring the suspension for 10.0 min, the pH of the suspension was measured by a pH metre. Then, the acid neutralisation capacity (ANC) was determined at different pH values, as being defined as the acid concentration to a change in the pH in the process of acidifying the suspension. 2.3. Corrosion behaviour The corrosion of steel in concrete was measured by the anodic polarisation technique to quantify the corrosiveness of steel embedment. Mortar specimens were fabricated with a centrally located 10.0 mm diameter mild steel bar and equivalent levels of chloride to testing for chloride binding were simultaneously admixed in mixing water. The two ends of the steel bars were masked using rich cement paste, followed by a further covering with heat shrink insulation to avoid corrosion under the masking. The specimens were cured by wrapping in a polythene film at20 ± 2 °C for 56 days. To accelerate the corrosion process, the specimens were then subjected to a wet and dry cyclic condition consisting of 4 days wet and 3 days dry. During the wet condition, the relative humidity was kept 95% at 50 °C, while for the dry condition, it was 50% RH at 20 °C. The corrosion rate and potential were measured immediately at the completion of every wet cycle to minimise electrical resistance of mortar. In the measurement of the corrosion rate, the corrosion potential was measured by the standard calomel electrode prior to polarisation to determine the corrosion rate. Then, the potential was swept at a low scan rate of about 0.1 mV/s to a potential of 25.0 mV above the corrosion potential to achieve the polarisation resistance of the steel in mortar. The IR drop arising from electrical resistance of mortar was compensated by a current interruption technique. Then, the corrosion rate was calculated by the Ohm’s law as given in Eq. (1).
ICORR ¼
B RP
ð1Þ
where, ICORR indicates the corrosion rate, B for the constant potential (26 mV) and RP for the polarisation resistance, respectively.
2.1. Fundamental properties As concrete properties, a development of the compressive strength for CAC mixture was measured after fabrication of cylindrical specimens (Ø100 200 mm). Concrete specimens were demoulded 24 h after casting and then cured in a 95% humid chamber at 20 ± 2 °C. The setting time was determined by the penetration resistance of fresh concrete. Immediately after casting concrete, gravel in the fresh mix was removed by the 5.0 mm sieve. Then, mortar sample was poured in a cylindrical mould (Ø150 150 mm) to measure the penetration resistance, which was subsequently measured by a set of needles at 2.25, 3.19, 4.54, 7.15, 10.13 and 14.32 mm in the diameter at different time interval. The initial set was defined as the time for the penetration resistance to reach 3.43 MPa, and the final set to 27.46 MPa. The time of set was determined by interpolating the relation between the penetration resistance and time.
3. Results 3.1. Development of strength A development of the compressive strength for CAC mixture at 7, 28, 56 and 91 days is given in Fig. 1. For all concretes, the compressive strength was increased by curing age, irrespective of binder type. However, for CAC mixture, an increase in the CAC content in binder resulted in a decrease in the strength at all ages. For example, the strength for 5% CAC concrete was 25.3 MPa at
Table 1 Chemical physical characteristics of CAC and OPC. Oxides (%)
CAC OPC
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
SO3
28.5 63.8
0.2 22.1
71.0 5.0
0.1 3.0
0.4 1.6
0.2 0.35
– 0.64
– 2.0
Blaine (cm2/g)
Density
3750 3242
3.05 3.12
S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
OPC
5% CAC
10% CAC
15% CAC
Compressive strength (MPa)
40
effect to concrete properties, such as thermal cracking and thus lower strength. In the present study, treatments to secure/retard the setting of fresh concrete were not dealt with, for example, chemical retarder. 3.2. Removal of chlorides
30
20
10
0 0
20
40
60
80
100
Curing age (days) Fig. 1. Development of compressive strength for CAC mixture and OPC concrete with curing age.
28 days, and only 18.5 MPa of the strength was achieved for 15% CAC concrete at the corresponding days. The lower strength for CAC mixture is presumably associated with a very rapid setting of fresh concrete, leading to a high level of hydration heat and thus thermal cracking. At 91 days, however, 15% CAC concrete produced a rapid increase in the compressive strength, accounting for about 29.5 MPa, presumably due to a formation of cubic phase of CAH10, which is often precipitated in CAC paste in a long term then to stabilise the strength of CAC concrete, as the CAC used in this study contains a very high Al2O3 (i.e. 71.0%). The initial and final sets were determined by a monitoring of the penetration resistance of fresh concrete with time, as given in Fig. 2. The penetration resistance was best fitted with time to define the initial and final setting times. The CAC mixture dramatically reduced the setting time, depending on the CAC content in binder. The final set for 5%, 10% and 15% CAC was achieved at 201, 126 and 75 min respectively, while OPC achieved 377 min. A rapid setting for the CAC mixture, in fact, may impose adverse
OPC
CAC 5%
76 min.
126 min.
CAC 10%
CAC 15%
201 min.
377 min.
40
Penetration resistance (MPa)
229
30
Final set : 27.6 MPa
20
The presence of chlorides in the cement matrix was evaluated by chloride binding and buffering against a pH fall. The chloride binding capacity was expressed by the relation between free and bound chlorides at a given total chloride concentration, which was in fact rendered by the Langmuir isotherm, measured at 28, 56 and 91 days to identify the influence of curing age, as seen in Fig. 3. It is evident that an increase in the curing age resulted in an increase in the binding of chlorides, irrespective of binder type. In particular, the binding capacity of chlorides was dramatically increased after 28 days, and the binding capacity at 56 and 91 days was then mostly identical at a lower total chloride concentration. At a higher total chloride concentration exceeding 2.0% and 3.0%, the binding capacity at 91 days was slightly higher than at 56 days. It is notable that a CAC mixture always produced the higher level of chloride binding at a given chloride concentration at all ages. This may be attributed to a further formation of CA-type hydrations in the cement matrix. The high level of Al2O3 in CAC would enhance a formation of C3A or/and other CA-type hydrations, which could subsequently react with chloride ions to form chloro aluminate hydrates (i.e. Friedel’s salt). In fact, an increase in the portion of CAC in the mix resulted in an increase in the chloride binding capacity: for 15% CAC mix, the chloride binding was the highest ranked. The acid neutralisation capacity (ANC) of CAC mixtures was obtained as seen in Fig. 4, which depicts the resistance of hydration products against acidification at different pH values of the suspension. It is seen that OPC produced the higher buffering capacity at around 11.8–12.6 in the pH, at which Ca(OH)2 is presumed to be precipitated in the cement matrix [9]. At the lower pH values, there were only marginal peaks indicating the resistance to a pH fall. For CAC mixtures, the peaks at 11.8–12.6 in the pH were less present, seemingly still imposing the ANC of precipitated Ca(OH)2 to acidification. However, there were many, strong peaks even at lower pH values below 10.5–11.5 in the pH. It has an important implication in determining a chloride removal in the cement matrix. Chlorides bound by hydration products are often released into free at a lower pH, which would often take place at pit nucleation on the surface of steel in concrete. In this process, a number of electrons are released to acidify the cement matrix, thereby leading to a decomposition of chloro aluminate hydrates. Thus, the higher ANC may impose the increased buffering of a pH fall in the cement matrix and in turn bound chlorides could be kept immobile, unreactive in the vicinity of the steel, as being indicative of lower corrosion risk at a given chloride-bearing environment. The increased ANC of CAC mixtures was seemingly associated with CA-type hydration products. 3.3. Corrosion resistance
10
Initial set : 3.5 MPa 0 0
100
200
300
400
Time (min.) Fig. 2. Penetration resistance of fresh concrete with time to determine the setting time.
The corrosion behaviour of steel in CAC mixture was evaluated by the anodic polarisation technique to measure the corrosion rate and potential. As seen in Fig. 5, the corrosion potential was strongly influenced by binder type and chloride concentration in cast. For the lower chloride concentration at 0.0% and 0.5%, all specimens indicated mostly passive, exceeding the corrosion threshold voltage (i.e. 275 mV vs SCE). However, the corrosion potential for OPC specimens dramatically decreased up to about 400 mV, when the chloride concentration exceeded 1.0%. It may imply that OPC may have about 0.5–1.0% of the critical chloride concentration
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S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
(a) OPC
(b) 5% CAC 56 days
91 days
28 days
2.5
2.5
2.0
2.0
Bound chloride (%, binder)
Bound chloride (%, binder)
28 days
1.5
1.0
0.5
28 days: Cb =
1.67 Cf 1 + 0.64 Cf
56 days: Cb =
2.54 Cf 1 + 0.50 Cf
91 days: Cb =
2.25 Cf 1 + 0.13 Cf
0.0 0.0
0.5
1.0
1.5
91 days
1.5
1.0
0.5
28 days: Cb =
3.52 Cf 1 + 1.88 Cf
56 days: Cb =
5.02 Cf 1 + 1.54 Cf
91 days: Cb =
5.83 Cf 1 + 1.76 Cf
0.0 0.0
2.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
Free chloride (%, binder)
(c) 10% CAC
(d) 15% CAC 28 days
56 days
91 days
28 days
2.5
2.5
2.0
2.0
Bound chloride (%, binder)
Bound chloride (%, binder)
56 days
1.5 28 days: Cb =
4.37 Cf 1 + 1.70 Cf
56 days: Cb =
4.73 Cf 1 + 1.17 Cf
91 days: Cb =
5.83 Cf 1 + 1.46 Cf
1.0
0.5
0.0
56 days
91 days
1.5 28 days: Cb =
5.90 Cf 1 + 1.95 Cf
56 days: Cb =
6.62 Cf 1 + 2.13 Cf
91 days: Cb =
4.08 Cf 1 + 0.71 Cf
1.0
0.5
0.0 0.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
0.0
0.5
1.0
1.5
2.0
Free chloride (%, binder)
Fig. 3. Relation between free and bound chlorides at a given total chloride concentration in CAC mixture and OPC paste at 28, 56, 91 days (Rendered by the Langmuir isotherm).
for the onset of corrosion, seemingly being in the range of previous studies [10,11]. For the CAC mixture, however, the corrosion potential kept always higher at all concentrations of chloride, irrespective of the CAC ratio in binder. It means that CAC mixture may keep the cathodic reactivity in mortar passive even at a very high concentration of chlorides. The corrosion rate of steel in CAC mixture was monitored, as seen in Fig. 6. It was observed that the corrosion rate was always in the range for passive zone (<1.0 mA/m2), when chlorides in cast ranged from 0.0% to 0.5%, except for OPC mortar after 10 cycles exposure. For OPC specimens, the corrosion rate at 1.0% of chlorides was highly ranged accounting for 3.1–15.6 mA/m2, at which range the depassivation of steel is known to start [12]. Then, the corrosion rate was rapidly increased with the concentration of chloride. However, the CAC mixture, in particular, 10% and 15% CAC, showed very low corrosion rates at all levels of chloride, indicating that no visible corrosion may takes place, accounting for below 1.0 mA/m2. Even for 5% CAC mixture, the corrosion rate indicated passive, except for at 3.0% of chlorides in cast, which was subsequently recovered to the passive state. The inhibitive nature of CAC mixture may be attributed to increased buffering capacity
of the cement matrix against acidification in the vicinity of the steel. Otherwise, the steel surface in the CAC mixture may achieve further passivity against corrosion arising from CA-type hydrations. After an exposure of the CAC mixed mortar containing different concentrations of chloride ranging 0.0–3.0% to 20 wet and dry cycles, steel embedded in the mortar was obtained by breaking and splitting the specimen. Representative steel rebars at each concentration of chloride in cast are given in Fig. 7. It is evident that all steel rebars were passive in chloride-free mortars, irrespective of binder type. However, for OPC, an increase in the chloride content in cast resulted in an increase in the rust amount on the steel surface, as expected. In particular, 2.0% and 3.0% of chlorides, all surfaces of the steel rebar were severely corroded. For CAC mixed mortar, all steel rebars were kept passive even at a very high chloride concentration, except for the 5% CAC specimen, for which the steel rebar was partially corroded at 3.0% of chloride, at which the corrosion rate exceeded 5.0 mA/m2 at early cycles and subsequently decreased to a small value. It is evident that steel rebars embedded in 10 and 15% CAC mortar were never corroded, implying an guaranteed inhibitive nature.
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S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
(a) OPC
(b) 5% CAC 60
Resistance to a pH fall (mol/kg)
Resistance to a pH fall (mol/kg)
60 50 40 30 20 10 0
50 40 30 20 10 0
13
12
11
10
9
13
12
11
pH of suspension (pH)
(c) 10% CAC
9
10
9
(d) 15% CAC 60
Resistance to a pH fall (mol/kg)
60
Resistance to a pH fall (mol/kg)
10
pH of suspension (pH)
50 40 30 20 10 0
50 40 30 20 10 0
13
12
11
10
9
13
12
11
pH of suspension (pH)
pH of suspension (pH)
Fig. 4. Acid neutralisation capacity a pH fall for CAC mixture and OPC paste.
(a) 0.0% of chlorides
(b) 0.5% of chlorides 10% CAC
15% CAC
0
OPC
-100 -200 -300 -400 -500
10% CAC
15% CAC
0 -100 -200 -300 -400 -500
0
5
10
15
20
5
OPC
10
15
20
Number of cycle (weeks)
(d) 2.0% of chlorides
OPC
5% CAC
10% CAC
15% CAC
0 -100 -200 -300 -400 -500
0
Number of cycle (weeks)
0
5
10
15
Number of cycle (weeks)
(e) 3.0% of chlorides
5% CAC
10% CAC
15% CAC
0
OPC
Corrosion potential (SCE vs. mV)
Corrosion potential (SCE vs. mV)
(c) 1.0% of chlorides
5% CAC
Corrosion potential (SCE vs. mV)
5% CAC
Corrosion potential (SCE vs. mV)
Corrosion potential (SCE vs. mV)
OPC
-100 -200 -300 -400 -500
5% CAC
10% CAC
15% CAC
0 -100 -200
Corrosion potential: -275 mV vs. SCE
-300 -400 -500
0
5
10
15
Number of cycle (weeks)
20
0
5
10
15
20
Number of cycle (weeks)
Fig. 5. Corrosion potential of steel in CAC mixture and OPC mortar depending on chloride concentration in cast for 20 cycles (3 days wet + 4 days dry).
20
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(a) 0.0% of chlorides OPC
(b) 0.5% of chlorides
CAC 5%
CAC 10%
CAC 15%
OPC
CAC 15%
OPC
1
0.1
10
1
0.1
0.01 0
5
10
15
20
OPC
CAC 10%
CAC 15%
10
1
0.1
0.01 0
5
Number of cycle (weeks)
10
15
20
Number of cycle (weeks)
(d) 2.0% of chlorides
CAC 5%
100
Corrosion rate (mA/m2)
10
0.01
0
5
10
15
20
Number of cycle (weeks)
(e) 3.0% of chlorides
CAC 5%
CAC 10%
CAC 15%
OPC
CAC 5%
CAC 10%
CAC 15%
100
Corrosion rate (mA/m2)
100
Corrosion rate (mA/m2)
CAC 10%
100
Corrosion rate (mA/m2)
Corrosion rate (mA/m2)
100
(c) 1.0% of chlorides
CAC 5%
10
1
0.1
0.01
10 Corrosion initiantion: 1-2 mA/m2
1
0.1
0.01 0
5
10
15
Number of cycle (weeks)
20
0
5
10
15
20
Number of cycle (weeks)
Fig. 6. Corrosion rate of steel in CAC mixture and OPC mortar depending on chloride concentration in cast for 20 cycles (3 days wet + 4 days dry).
Fig. 7. Representative steels embedded in CAC-mixed mortar containing different concentration of chlorides after 20 wet and dry cycles.
4. Discussion 4.1. Inhibition of CAC mixtures The high resistance of the CAC mixture against chlorideinduced corrosion may arise from increased chloride binding capacity. Since the CAC mixture contains various CA-type hydrations including C12A7, C3A and C4AF, which are all reactive with chloride ions then to be stabilised in the matrix. As the precipitated Ca(OH)2 is still present in hydrations, chlorides can be physically further adsorbed, thereby being subsequently removed from the corrosion process. As seen in Fig. 3, the binding capacity of chlorides in the CAC mixture was always higher than in OPC. It may
imply that the binding of chloride ions in the CAC mixture may be attributed to OPC and CAC hydrations at once; chlorides could be bound by C3A and C4AF formed in OPC paste and simultaneously by C12A7 in CAC one. The more crucial cause of inhibition in CAC mixture may be its ANC to a pH fall. The resistance to the onset of corrosion in concrete is strongly dependent on the buffering capacity, since bound chlorides are released into free by acidification in the vicinity of steel. The buffering for OPC concrete mainly occurs at 11.5–12.5 in the pH, arising from precipitated Ca(OH)2, while the CAC mixtures have the buffering at more acidic region of at 11.5–12.5 and 10.5–11.5 in the pH. In particular, at around 10.5 in the pH, the buffering peaks were highly generated, as seen in Fig. 4.
S.H. Jin et al. / Construction and Building Materials 110 (2016) 227–234
As a release of bound chlorides usually occurs in a relatively acidic environment, often at 8.5–10.0 in the pH, the buffering of CAC mixture at 10.5–11.5 in the pH would impose an additional inhibitive nature of the cement matrix. Substantially, the CAC mixture may chemically bind more chlorides, which could be kept immobile by increased ANC to acidification of the cement matrix and pore solution. For chlorides in CAC mixture to corrode the steel, a further supply of chlorides is therefore required, implying a mitigated corrosion risk. When it comes to the corrosion initiation, chloride threshold level for CAC mixture exceeded 3.0% by weight of binder, whilst the threshold level for OPC was about 0.5–1.0% by weight of binder. Notwithstanding, the inhibition of CAC mixture is still overwhelmingly high, considering its chloride binding capacity and ANC to a pH fall. The ratio of chloride binding in CAC mixture ranges about 50–60% to total chlorides, and the other chlorides would be formed free and thus reactive in the corrosion process. Although all bound chlorides in the CAC mixture are kept immobile, unreactive due to increased ANC, the resistance to the onset of corrosion in terms of critical chloride threshold value would be approximately double for OPC concrete. However, the chloride threshold for CAC mixture was at least 3 times higher than for OPC. It may suggest that CAC mixture may provide a further inhibitive measure to the steel embedment in concrete, for example, a modification of the passivity on the steel surface. A high level of Al2O3 from CAC may be dissolved in the pore solution in casting of concrete to react with ferrous ions on the steel surface, which may form a passive layer (i.e. sacrificial coating). Thus, further inhibition would result in a dramatic resistance to the onset of corrosion. However, the formation of the layer was not observed in the present study. 4.2. Application of CAC mixture to in-situ Despite the inhibitive measure of CAC mixture, its application to in-situ may be restricted by a lower development of the strength and rapid setting. As seen in Fig. 1, the compressive strength of CAC mixture was always lower than for OPC. In fact, an increase in the CAC content resulted in a decrease in the compressive strength, ranging from 18.5 to 25.3 MPa at 28 days, although the strength for structural concrete must achieve at least 25–30 MPa at the corresponding days to meet most guidelines. A reduction of the strength for CAC mixture may be attributed to a rapid setting and hardening, which subsequently would form thermal cracking. Moreover, a further porosity may be formed in the process of conversion in CAC paste from the hexagonal phase of CA3H6 to the cubic phase of CAH10, in which the process the density of fraction is nevertheless increased [13]. However, the pores generated in the conversion process is limited in gel and capillary pores in the small margin and thus no adverse effect in reducing the strength would take place [14]. Thus, avoiding a rapid setting of fresh CAC mixture must be accompanied to obtain a given target strength by addition of a chemical retarder. The retarded setting of CAC mixture could simultaneously secure the time of casting concrete, as a very short duration of the standing time for fresh concrete, in the range of 1– 3 h, could be problematic in applying into in-situ. Alternatively, a modification of the mix design would enhance a development of the strength, such as a reduced free W/B. As the compressive strength of CAC mixture was in the range of 20–25 MPa, a slight reduction of W/B would meet the target strength. However, CAC mixture may face a barrier in the economic expense due to the high cost of CAC, of which the price cost 5– 10 times higher depending on the portion of Al2O3. In this study, the mix ratio of CAC containing 71.0% of Al2O3 in oxides was 5%, 10% and 15% in total binder, imposing 20–30% increase in the economic expense arising from binder modification. To avoid
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economic limitation, CAC mixture could be partially used/cast in a high risky zone for corrosion, for example, concrete exposed to tidal/splash zones where corrosion is very severe due to a rapid percolation and supply of oxygen. However, concrete structures built in a seawater or deicer environment are entirely exposed to high chloride-bearing risk. Substantially, a large amount of concrete must be cast with CAC mixtures. To overcome economic limit, steel coating with CAC mixture in in-situ could sort out this problematic limitation with its inhibition effect sustained on the steel surface. As in-situ coating, CAC mixture milk can be sprayed or shot to the steels after placing the steel embedment then to cast a bulk concrete. Unlike polymer-based barrier coatings, there would be no adverse effect such as debondment between coating materials and bulk concrete, due to even lower membrane potential. A rapid setting of CAC mixture may induce, furthermore, no delay of procedures of concrete casting. 5. Conclusion The inhibition of CAC mixture was assessed by monitoring the corrosion rate and potential at different chloride concentration in mortar. Simultaneously, concrete properties for CAC mixture such as a development of the compressive strength and setting time were measured. To investigate the cause of inhibition of CAC mixture, its chloride binding and buffering to a pH fall were determined. The detailed conclusion is given as below: (1). The compressive strength of CAC mixture was always lower than for OPC solely mixed concrete, accounting for 18.5– 25.3 MPa, presumably due to a thermal cracking in the process of rapid setting of fresh concrete. The setting time for the CAC mixture was significantly reduced by an increase in the CAC content in binder, being in the range of 76– 201 min. (2). The CAC mixture showed the higher level of chloride binding, compared to OPC, together with a significant buffering capacity to a pH fall. The chloride binding capacity of CAC mixture was always higher than for OPC at all ages and at all level of total chlorides in cast. In particular, the ANC of CAC mixture to a pH fall further occurred at 10.5–11.5 and 11.8–12.6 in the pH, while OPC had the buffering zone only at 11.8–12.6. The increased ANC of CAC mixture may sustain bound chlorides stabilised in the cement matrix even in the acidic environment. (3). The CAC mixture had a higher resistance to chloride-induced corrosion, compared to OPC, when the corrosion rate and potential were measured by the electrochemical methods. No corrosion risk was observed in CAC mixture at up to 3.0% of chlorides by weight of binder, whilst OPC produced 0.5–1.0% of the critical chloride threshold for the onset of corrosion. This inhibition effect may be attributed to increased binding of chlorides and buffering to a pH fall.
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