CHLORIDE ION PERMEABILITY
STUDIES OF METAKAOLIN BASED
HIGH PERFORMANCE CONCRETE
Dr.Vaishali. G.Ghorpade1*
Associate Professor in Civil Engineering Dept.
JNTUA College of Engineering
Anantapur-515002
Dr.H.Sudarsana Rao Professor of Civil Engineering Dept.
JNTUA College of Engineering
Anantapur-515002
Abstract
To increase the applications of HPC in India, greater under standing of HPC produced with locally available materials and indigenously produced mineral admixtures is essential. In the present investigation, HPC has been produced with locally available aggregates and metakaolin as the mineral admixture. Various metakaolin based HPC mixes were attained by absolute volume method. Cubes of 150X150X150 mm size were cast and cured for 28 days and then tested for compressive strength. Chloride ion permeability test as per ASTM C 1202 has been conducted on various HPC mixes to measure the permeability values of HPC produced with metakaolin. The experimental results indicate that metakaolin has the ability to considerably reduce the permeability of high performance concrete. The various details about the chloride ion permeability test have been presented in this paper.
Keywords: Metakaolin, High Performance Concrete, Chloride ion Permeability
1. Introduction
The durability of cement concrete is defined as its ability to resist weathering action, chemical attack, or any other process of deterioration. Durable concrete will retain its original form quality, and serviceability when exposed to environment.
One of the main reasons for deterioration of concrete in the past is that too much emphasis is placed on concrete compressive strength rather than on the performance criteria. The deterioration of reinforced concrete structures usually involves the transport of aggressive substances from the surrounding environment followed by physical and chemical actions in its internal structure. The transport of aggressive gases and/or liquids into concrete depends on its permeation characteristics. As the permeation of concrete decreases its durability performance, in terms of physio-chemical degradation, increases. Therefore, permeation of concrete is one of the most critical parameters in the determination of concrete durability in aggressive environments.
Since high resistance to chloride penetration can be directly related to low permeability that dominates the deterioration process in concrete structures, the resistance to chloride penetration is one of the simplest measures to determine the durability of concrete. Therefore, in this study, the rapid chloride permeability test method designated in ASTM C 1202(1997) is adopted. The advantage of adopting this rapid chloride permeability test (RPCT) test is direct cost savings could be quantified when compared to other tests and the brief procedural steps involved significantly reduce the technician time necessary to evaluate a particular concrete.
2. High performance concrete
Any concrete which satisfies certain criteria proposed to overcome limitations of conventional concretes may be called High-Performance-Concrete (HPC). It may include concrete, which provides either substantially improved resistance to environmental influences (durability in service) or substantially increased structural
1
capacity while maintaining adequate durability. There has been a phenomenal increase in the development and use of High-Performance-Concrete in the last decade.
Admixtures play an important role in the production of HPC. Metakaolin is a recently developed man made factory manufactured mineral admixture which has potential for use in the production of HPC. Poon et al. [2006] related the mechanical and durability properties of high performance metakaolin and silica fume concretes to their microstructure characteristics. They reported that metakaolin concrete has superior strength development and similar chloride resistance to silica fume concrete. The use of zeolitic admixtures, a natural pozzolan, was examined by Feng et al. [1990] as was metakaolin, a reactive alumino-silicate pozzolana by Walters and Jones [1991]. The use of 5% and 10% metakaolin was found to improve durability of ordinary concrete. Khatib and Wild [1996] studied the pore size distribution in metakaolin paste and reported that the large pores in the pates decreases with increase in metakaolin content. Wild et al. [1996] presented the mechanical properties of super plasticized metakaolin concrete. Curcio et al. [1998] presented the utility of metakaolin as micro filler in the production of high performance mortars. Palomo et al. [1999] investigated the chemical stability of metakaolin based cement composites. Frias and Cabrera [2000] investigated the relationship between the pore size distribution and degree of hydration of metakaolin based cement pastes. They reported that metakaolin showed the best enhancement on the mechanical properties of young concrete. The present investigation aims to produce high-performance concrete using metakaolin admixture. The durability of produced metakaolin HPC will be ascertained by conducting chloride ion permeability test as per ASTM C-1202 code provisions. Compressive strength test was also conducted as a reference test to give a measure of strength. The details of experimentation, results and discussion are presented in this paper to highlight the potential utility of metakaolin in producing quality high-performance-concrete.
3. Experimental program
Experimental program has been planned to provide sufficient information for ascertaining the quality of metakaolin based high performance concrete. To evaluate the behavior of metakaolin based high performance concrete, both compressive strength and durability aspects have been studied in this investigation. The various parameters studied are given below.
The parameters studied are: -
Aggregate-Binder Ratio (A/B Ratio): 2.0
Water-Binder ratio (W/B ratio): 0.3, 0.35, 0.4, 0.45 and 0.5 Percentage replacement of cement by metakaolin: 0, 10, 20, and 30
Dosage of Super plasticizer – 2.5% by weight of binder (cement + metakaolin)
The various mixes tried in the present investigation along with their nomenclature are presented in Table 2. The details of various materials used in this investigation are given in the following sections.
3.1 . Materials
Portland cement of 53 grade manufactured by Birla Company confirming to IS 12269 was used in this investigation. The specific gravity of the cement was 3.06. The initial and final setting times were found as 40 minutes and 360 minutes respectively. Locally available river sand passing through 4.75 mm IS. Sieve was used. The specific gravity of the sand is found to be 2.68.Crushed granite aggregate available from local sources has been used. To obtain a reasonably good grading, 50% of the aggregate passing through 12.5mm I.S.sieve and retained on 10 mm I.S.sieve and 50% of the aggregate passing through 10 mm I.S.sieve and retained on 6 mm I.S.sieve was used in the production of HPC. In the production of M20 grade concrete, 20mm maximum size coarse aggregate has been used. The specific gravity of coarse aggregate is 2.75. Potable fresh water available from local sources was used for mixing and curing of both HPC mixes and M20 grade concrete. To improve the workability of the HPC mixes, a high range water-reducing agent COMPLAST SP-337 has been used in the present work. The mineral admixture Metakaolin is obtained from the 20 MICRON LIMITED company at Vadodara in Gujarat. The specific gravity of Metakaolin is 2.54. The Metakaolin is in conformity with the general requirements of pozzolana. Various properties of Metakaolin as supplied by the manufacturer are presented in Table 1.
3.2. Casting of test specimens
curing. After 28 days of curing, the test specimens were removed from the curing pond and allowed for drying under shade for an hour before testing.
Similarly, ordinary M20 grade concrete has been designed by I.S. code method to have a proportion of 1: 1.5: 3.3. Cylinder specimens for chloride permeability and cube specimens for compressive strength test were cast for M20 concrete also for comparison purposes. The compressive strength test was conducted on 150X150X150mm cube specimens in 2000 kN AIMIL make digital compression testing machine and the compressive strengths of various HPC mixes are presented in Table 3. The 28-day compressive strength of M20 concrete was also determined and its value is 26.1MPa.
Table 1: Properties of metakaolin
S. No Property Value
1 Specific gravity 2.54
2 Accelerated pozzolanic active index, % of
control 89
3 Residue on 45 sieve, % 1.31
4 Chemical Analysis, % Loss on ignition
Silica (SiO2) Iron oxide (Fe2O3) Aluminium (Al2O3) Calcium oxide (CaO) Magnesium oxide (MgO)
0.70 52.24
0.60 43.18
1.03 0.61
Table 2: Mix proportions for different HPC mixes (For one cubic metre of concrete)
Mix designation
W/B ratio
Cement (Kg)
Metakaolin (Kg)
Coarse Aggregate
(Kg)
Sand (Kg)
Water (litres)
0% Metakaolin
A1 0.3 734.39 0 881.27 587.51 220.32
A2 0.35 708.38 0 850.06 566.71 247.93
A3 0.4 684.15 0 820.98 547.32 273.66
A4 0.45 661.52 0 793.82 529.22 297.68
A5 0.5 640.34 0 768.41 512.27 320.17
10% Metakaolin
B1 0.3 657.72 73.08 876.96 584.64 219.24
B2 0.35 634.54 70.50 846.05 564.03 246.76
B3 0.4 612.93 68.10 817.24 544.83 272.41
B4 0.45 592.75 65.86 790.33 526.88 296.37
B5 0.5 573.85 63.76 765.13 510.09 318.8
20% Metakaolin
C1 0.3 581.79 145.45 872.69 581.79 218.17
C2 0.35 561.38 140.35 842.08 561.38 245.61
C3 0.4 542.35 135.59 813.53 542.35 271.18
C4 0.45 524.57 131.14 786.86 524.57 295.07
C5 0.5 507.92 126.98 761.88 507.92 317.45
30% Metakaolin
D1 0.3 506.61 217.12 868.47 578.69 217.12
D2 0.35 488.92 209.54 838.14 558.76 244.46
D3 0.4 472.42 202.46 809.86 539.91 269.95
D4 0.45 456.99 195.86 783.42 522.28 293.78
Table 3: Compressive Strengths of HPC Mixes:
4. Rapid chloride permeability test (ASTM C –1202)
4.1. Apparatus
In the present work, the Rapid Chloride Permeability Test Apparatus has been used to determine the chloride permeability of both high performance metakaolin concrete mixes and M20 concrete mix. The Rapid Chloride Permeability Test Apparatus consists of the following different components as described in ASTM C-1202. The test set-up consists of two acrylic chambers having grooved recesses on one face and closed at the other end. The specimen can snug-fit into the open faces of the chambers.
One of the cells is filled with NaCl solution (concentration 2.4 M), while the other is filled with 0.3 M NaOH solution. Copper mesh electrodes are mounted in the cells such that they are in contact with the end faces of the specimen. The whole assembly is held together by long threaded rods with wing nuts at both ends. An external voltage cell is used to apply a voltage difference of 60V between the electrodes.
4.2. Preparation of test solutions
NaCl : 140 grams of NaCl salt is mixed with 1000 ml of distilled water to get the sodium chloride solution of 2.4M strength.
NaOH : 12.5 grams of NaOH salt is mixed with 1000 ml of distilled water to obtain the sodium hydroxide solution of 0.3M strength.
4.3. Procedure
The various steps involved in conducting the rapid chloride permeability test on HPC are as given below. The disc shaped specimens cast and cured were taken and snug-fitted into the open faces of the chambers.
One of the cells is filled with NaCl solution (concentration 2.4 M), while the other is filled with 0.3 M NaOH solution.
The chamber containing NaCl solution is connected to the positive terminal and the chamber containing NaOH solution is connected to the negative terminal of the external DC voltage cell.
The external voltage cell is always maintained at 60V of power supply.
The electrochemical cell, constituted by this assembly, results in the rapid migration of chloride ions from the sodium chloride solution to the sodium hydroxide solution, via the pore network offered by the concrete disc shaped specimen. The movement of chloride ions is proportional to the intensity of electric current as measured by an ammeter in the power source. The test is carried out for duration of 6 hours and the current is measured at 15minute intervals. The chloride ion permeability is computed as the total charge passed through by using the formula given below.
Chloride ion permeability, Coulombs = (I0 + I1 + I2 + I3 + I4 + I5 ) mA × 0.001× 60 × 60 Where
I0, I6 are the initial and final currents I1, I2, I3, I4, I5, are the intermediate currents
The chloride ion permeability values of various HPC mixes are presented in Table 4. The chloride ion permeability of M20 grade concrete was also determined for comparison purposes and its value is 3906 coulombs.
W/B ratio
Compressive strength for different % replacements of cement by Metakaolin
(N/mm2)
0% 10% 20% 30% 0.3
0.35 0.4 0.45
0.5
84.6 80.3 79.4 76.9 74.6
95.4 91.7 88.7 85.6 82.9
81.7 79.1 77.7 74.3 72.1
Table 4: Chloride ion permeability of HPC Mixes:
5. Discussion of test results
In this work, all the HPC mixes attained low and moderate chloride permeability as per ASTM C 1202 criteria. All the mixes containing metakaolin showed good resistance to permeability at ages of 28 days. The chloride permeability of M20 grade concrete was found to be 3906 coulombs indicating a high range of permeability to chloride ions when compared to high performance concrete mixes. Based on the results of the present experimental work, a discussion has been presented in the following sections to project the basic objective of utilization of metakaolin in the production of high-performance-concrete.
5.1. Effect of water-binder ratio (W/B) on compressive strength and chloride permeability
The variation of compressive strength of HPC with water-binder ratio is presented in Fig. 1. It can be observed that the compressive strength of HPC decreases with increasing water-binder ratio. This observation is just similar to ordinary concrete and is with in the spirit of Abram’s law for concrete. The Fig.1 shows a decreasing rate of compressive strength with increasing W/B ratio. Similar observation can be made for all percentages of cement replacement with metakaolin. In the present work, a maximum compressive strength of 95.4 N/mm2 has been achieved for a mix with a water-binder ratio of 0.3 and with 10% replacement of cement by metakaolin. The variation of chloride ion permeability with water-binder ratio is presented in Fig. 2. It can be observed from this figure that the chloride permeability of HPC increases with increasing water-binder ratio, which is not desirable from durability point of view. The chloride ion permeability in terms of total charge passed in coulombs is 774 coulombs and 1181 coulombs for W/B ratios of 0.3 and 0.5 respectively for HPC mix containing 10% of metakaolin showing an increasing rate of permeability with increasing W/B ratio. Similar observation can be made for all mixes with different percentages of metakaolin. Accordingly it is suggested that lower water-binder ratios have to be used in the production of high-performance-concrete. It can be noticed that the compressive strengths of all HPC mixes are considerably higher than that of reference M20 grade concrete (26.1 N/mm2) at all water-binder ratios and for all metakaolin contents. It can also be observed that the chloride permeability of HPC mixes is considerably less when compared to that of M20 grade concrete (3906 coulombs) indicating excellent durability characteristics of HPC with metakaolin admixture.
5.2. Effect of metakaolin content on compressive strength and chloride permeability
It can be observed from Fig. 1, that the compressive strength of HPC marginally increased with the replacement of cement by metakaolin up to 10%. Beyond 10% replacement, the compressive strength of HPC mixes decreased. It can be observed that even at 30% cement replacement by metakaolin, the HPC mixes have very high compressive strengths when compared to reference M20 grade concrete and hence can be used for all structural applications. The chloride ion permeability in terms of total charge passed in coulombs attained a maximum value of 1762 Coulombs and a minimum value of 490 Coulombs for 0% and 30% metakaolin content respectively for HPC mixes with W/B ratio of 0.3, showing a decreasing rate of permeability with increasing metakaolin content which is very much desirable indeed. Thus, it can be concluded that with regard to permeability, the metakaolin can be effectively used as partial replacement to cement even up to 30% in the production of durable high performance concrete. The mineral admixtures, which serve as supplementary cementitious materials contain reactive silica that in turn combines with calcium hydroxide liberated during hydration of cement to form additional calcium-silicate-hydrate which otherwise would have leached out increasing the porosity of cement matrix. As the percentage of mineral admixture increases it in turn increases the C-S-H gel and thus increases denseness of the matrix and refines the pore structure. This ultimately decreases the permeability of HPC.
W/B ratio
Chloride ion permeability for different % replacements of cement by Metakaolin
(Coulombs)
0% 10% 20% 30% 0.3
0.35 0.4 0.45
0.5
1762.0 1956.0 2074.0 2155.0 2248.0
774.0 952.0 1017.0 1086.0 1181.0
511.0 573.0 718.0 763.0 801.0
0.30 0.35 0.40 0.45 0.50 65
70 75 80 85 90 95 100
Fig.1 Effect of metakaolin on compressive strength
(A/B = 2.0)
C
om
p
ressive stren
gth
(M
Pa)
Water-Binder ratio
M 0% M 10% M 20% M 30%
0.30 0.35 0.40 0.45 0.50
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Fig.2 Effect of metakaolin on chloride ion permeability
(A/B = 2.0)
Charg
e pa
ss
ed
(coul
ombs
)
Water-Binder ratio
M 0% M 10% M 20% M 30%
6. Conclusions
The present work, investigates the utility of metakaolin in the production of durable high-performance-concrete and the major conclusion of the work are listed below.
High Performance Concrete with a maximum Compressive Strength of 95.4MPa has been produced in the present investigation using 10% metakaolin with a W/B ratio of 0.3 without using any special curing conditions.
The Chloride Ion Permeability increases with increase in W/B ratio. Hence, it is advised to use lower W/B ratios in producing HPC.
The Chloride Ion Permeability value decreased considerably with increase in metakaolin content from 0 to 30% thus indicating improved durability with increasing metakaolin content.
From the above, utility of metakaolin in producing durable high-performance-concrete is presented as a holistic solution to the problem of meeting the higher demands of concrete at lower costs. This also contributes to the reduction of pollution contributed by cement industry. Thus, the use of metakaolin in the production of high-performance-concrete not only improves its strength and but also durability.
References
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[2] Feng, N.Q.,Li, G.Z. and Zang X.W. (1990): High-Strength and Flowing Concrete with a Zeolitic Mineral Admixture. Cement, Concrete and Aggregate, winter, 12 (2), pp. 61-69.
[3] Frias, Moises and Cabrera, Joshep.(2000): Pore Size Distribution and Degree of Hydration of Metakaolin-Cement Pastes. Cement and Concrete Research, 30 (4), pp. 561-569.
[4] Khatib, J. M. and Wild, S. (1996): Pore Size Distribution of Metakaolin Paste. Cement and Concrete Research, 26 (10), pp. 1545-1553. [5] Palomo, A., Blanco-Varela, M. T., Granizo, M. L., Vazquez, T. and Grutzeck, M. W. (1999): Chemical Stability of Cementitious
Materials based on Metakaolin. Cement and Concrete Research, 20 (7), pp. 997-1004.
[6] Poon, C.S., Kou, S.C. and Lam, L. (2006): Compressive strength, chloride diffusivity and pore structure of high performance metakaolin and silica fume concrete. Construction and Building Materials, 20 (10), pp. 858-865.
[7] Walters, G.V. and Jones, T.R. (1991): Effect of Metakaolin on Alkali-Silica Reaction (ASR) in Concrete Manufactured with Reactive Aggregate. Durability of Concrete. Second International Conference, 1991, Montreal, Canada; Ed. by V.M. Malhotra; American Concrete Institute, Detroit, MI, 2, pp 941-953. (ACI SP-126)
