Synergistic effect of metakaolin and fly ash on properties of concrete

Construction and Building Materials 155 (2017) 830–837

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Synergistic effect of metakaolin and fly ash on properties of concrete S. Sujjavanich a,⇑, P. Suwanvitaya a, D. Chaysuwan b, G. Heness c a

Department of Civil Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand c School of Physics & Advanced Materials, University of Technology, Sydney, Australia b

h i g h l i g h t s  Synergy of cement, metakaolin and fly ash on cementitious system was investigated.  Phase composition, microstructure and long term concrete behaviors were studied.  It improved workability, mix uniformity, dense matrix and performance of concrete.  Better understanding of synergy on properties of this ternary system was proposed.

a r t i c l e

i n f o

Article history: Received 17 March 2017 Received in revised form 8 August 2017 Accepted 14 August 2017

Keywords: Synergistic action Ternary system Fly ash Metakaolin Microstructure Durability

a b s t r a c t This paper reports the effects of the interaction between metakaolin and fly ash on the microstructure and property development of concrete. X-ray diffraction analysis revealed unstable hemicarboaluminate and calcium monocarboaluminate compounds in most mixtures during 7–28 day curing periods, the relative amounts depending on the metakaolin to fly ash ratio. The mix with the highest peaks of the monocarboaluminate phase yielded the highest long-term strength. Significant improvements in terms of durability, abrasion resistance, chloride permeability and steel corrosion risk were observed. A proportion of cement:metakaolin:fly ash as 80:10:10 yielded marked improvements on slump, slump loss and long- term strength. Synergistic action in the ternary blend significantly improved the workability of fresh concrete and yielded a more uniform mix, denser microstructure and better performance of the hardened concrete. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Due to its improvement of concrete properties, the use of fly ash as pozzolan in the Thai concrete industry has increased significantly during the past decade. The responsible mechanism for the improvement has been well documented [1–3]. Since the microstructure and properties development of concrete at the time of exposure to the surroundings are critical in determining longterm performance, the typically slow early strength gain by local fly ash concrete probably affects concrete durability significantly, particularly in marine environments. The still-developing microstructure in the early stages of curing provides insufficient chloride penetration resistance, resulting in low protection of steel reinforcement. In an attempt to overcome some of these issues, attention has now turned to the use of ternary systems, containing two types of compatible and reactive pozzolans [4]. The resulting

⇑ Corresponding author. E-mail address: [email protected] (S. Sujjavanich). http://dx.doi.org/10.1016/j.conbuildmat.2017.08.072 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

changes in reactivity rate and hydration processes affected phase compositions and microstructures of the cement pastes [5]. The benefits from the synergy of pozzolans in cementitious systems, such as higher early strength, have been widely reported [6–8]. Some observed specific compounds such as calcium mono- or hemicarboaluminate were reported in fly ash-limestone systems and corresponded well with the better compressive strength improvement [8,9]. Metakaolin, produced by heating and grinding of natural kaolin, has been reported to be a good and effective pozzolanic material [10,11]. The faster strength development and denser microstructure formation during the early stages of curing of this material compensate for the drawbacks in fly ash concrete mentioned earlier as well as improve concrete durability, in particular, chloride permeability [12]. Metakaolin and fly ash are aluminate-rich pozzolans. Improved concrete performance is expected from the combined addition of these two materials to concrete. The pozzolanic reactions from these materials provide additional aluminates to the system which

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react with hydration products and produce more phases containing alumina in addition to CSH and reportedly enhance strength gain [13]. Little information on the actual performance of the combined use of these materials is available [14], although synergistic reactions and the improved products of concrete with other systems, e.g., fly ash-limestone powder system have been widely reported [8]. The combination of different pozzolans in cementitious systems changes reaction processes, phase compositions and microstructure development as well as the behavior of the concrete. Studies on the synergistic effects of cement, metakaolin and fly ash on concrete performance are rare. This study aimed to investigate the effect on microstructure and properties, in both early stages of curing and in the longer term of metakaolin-fly ash concrete to provide a better understanding of this synergy. In the first part, phase development of the cement paste incorporating various ratios of metakaolin and fly ash was investigated using X-ray diffraction (XRD) analysis. For the second part, the investigation focused on concrete strength and durability on 1) abrasion resistance, 2) chloride permeability and 3) steel corrosion risk. These results provided information on specific concrete performance for some applications such as in marine environment. 2. Experimental investigation 2.1. Materials Local metakaolin (MK) and lignite fly ash (FA) were used in this study. Metakaolin was prepared, using the techniques of Sayamipuk [15], by heating raw kaolin at 800 °C for 6 h and grinding in a high-speed ball mill. The chemical compositions and physical properties of the materials are listed in Table 1. Commercial Type I Portland cement interground with 4% limestone powder [16] was used throughout this study. The small amount of limestone powder benefits cement particle size distribution and workability of fresh concrete, furthermore, it provides more nucleation surface for accelerated hydration reactions [9]. The concrete mixes were designed for a 28 day compressive strength of 45 MPa and a slump of 75 ± 25 mm. The total percentage of cement replacement was kept at 20 wt% with ratios of metakaolin to fly ash varied in 5% increments (20:0, 15:5,10:10, 5:15 and 0:20). Crushed limestone with a maximum size of 9 mm and coarse river sand were used as coarse and fine aggregates. The details of the mix proportions are shown in Table 2. No chemical admixture was used in this system. 2.2. Cement paste specimens Paste samples at normal consistency (ASTM C 187) were prepared to investigate the phase development during curing. All specimens were moist cured until the age for microstructure examination. The structural development of a mature fly ashcement paste system (8 months) was used for comparison. 2.3. Concrete specimens Small specimen sizes of 75  75  75 mm3 and 62.5  62.5  300 mm3 were used for compressive and flexural strength tests to reduce the number of batches and hence the variation between batches. All specimens were moistly cured for 24 h after casting, then demolded and water cured at room temperature, 28 °C, until test to simulate the practical condition. For the designed target strength and slump, the water to binder ratios (W/B) were varied between 0.44 and 0.52. Weight loss of 150  150  75 mm3 concrete samples, according to ASTM C944-90a, was used as

Table 1 Compositions and properties of metakaolin, fly ash and cement. Chemical composition (%)/Properties

MK

FA

OPC

SiO2 Al2O3 Fe2O3 CaO SO3 SiO2 + Al2O3+Fe2O3 LOI< Specific gravity Blaine fineness, cm2/g

58.26 35.18 0.97 0.03 0.03 94.41 1.20 2.51 13800

47.65 18.51 10.29 11.94 2.69 76.45 0.5 2.64 4045

21.16 5.09 3.01 66.22 2.42 29.26 0.98 3.15 3320

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an index for abrasion resistance. The Rapid Chloride Permeability Test (ASTM C1202) was conducted at 28 and 56 days curing on 50 mm thick specimens sliced from water-cured cylindrical samples, 100 mm diameter by 200 mm height. For steel corrosion risk investigation, cylindrical specimens (150 mm diameter by 300 mm height) with 9 mm diameter reinforcing steel embedded to a length of 200 mm, were submerged in 3% NaCl solution in order to simulate a marine environment. Direct current voltage of 6 volts was applied throughout the experiment to accelerate the corrosion. Consequently, the corrosion potential was measured, using a copper-copper sulfate half-cell potentiometer. After 90 days and 140 days in the NaCl, specimens were broken and the broken surfaces sprayed with silver nitrate solution for the measurement of chloride penetration depth.

3. Results and discussion 3.1. Effects on the physical properties of fresh concrete The morphological features of the metakaolin and fly ash particles are shown in Fig. 1. The metakaolin particles were fluffy and porous, whereas those of fly ash were spherical and ranged in size from 2 lm to 40 lm. Fig. 2 shows the effects of the admixtures on the slump and slump loss with time of concrete. It can be seen that metakaolin increased the water requirement while fly ash decreased it for the given slump. The reason for this was due to the spherical shape of fly ash compared to that of metakaolin. Due to the same reason slump loss also decreased by increasing fly ash substitution level or by decreasing metakaolin inclusion level. 3.2. Phase development The changes in microstructure with time of the 10:10 metakaolin:fly ash specimens are shown in Fig. 3. A loose and porous structure is observed at an early age, from 3 to 14 days. A decrease in porosity is observed after 14 days. The results of XRD analysis, shown in Figs. 4–7 indicate the decrease of calcium hydroxide content and the formation of calcium hemicarboaluminate, or tetra calcium dialuminium dodecahydroxide hemicarbonate hydroxide n-hydrate Ca4Al2O6(CO3)0.5 (OH)11.5H2O, (peaks labeled ‘4’) and calcium monocarboaluminate, Ca4Al2O6CO3
11H2O (peaks labeled ‘5’). The lower peak intensity of CH after 7 day is somewhat an anomaly, but the phenomenon had been reported [17] and partially explained by the interaction of calcium silicate hydrate and nano sized CH [18,19]. The results present significant changes in the development of the hemi- and monocarboaluminate in the alumina, ferric oxide, monosulfate phase (AFm) of the cement-metakaolin-fly ash system (PC-MK-FA) during 7–28 days. At 7 days, calcium hemicarboaluminate was found in all ternary mixes and binary mix of metakaolin, with a slight increase in the peaks for monocarboaluminate. Of note, this intensity increase appears to be highest in the 10:10 metakaolin-fly ash mix. The formation of both carboaluminate products probably from the small amount of available limestone in the systems did not appear either in cement or in binary cement-fly ash mixtures. The formation of these two products has been widely reported for limestone-cement mixes [13,20,21]. In that system, the hemicarboaluminate formation was found as early as 1–3 days and gradually converted to calcium monocarboaluminate within 7 days [21]. At 14 days, the reduced intensity of the calcium hydroxide peak was evident in the ternary system with high metakaolin percentage. This was due to the pozzolanic activity during the early stages of curing. This was compatible with the results of Wild et al. who found the maximum pozzolanic reaction of metakaolin was at about 14 days [22]. It was observed that the peak intensity of the hemicarboaluminate decreased, but that of the monocarboluminate increased during this early stage of phase development. This supports the previously reported conversion of the hemi- to monocarboluminate phase in the calcite-rich

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Table 2 Mix proportion of concrete. Mix designation

W/B

PC (kg/m3)

MK (kg/m3)

FA (kg/m3)

Water (kg/m3)

Limestone (kg/m3)

Sand (kg/m3)

Control MK:FA, 20:0 MK:FA, 15:5 MK:FA, 10:10 MK:FA, 5:15 MK:FA, 0:20

0.48 0.52 0.51 0.48 0.46 0.44

433 346 346 346 346 346

0 87 65 43 22 0

0 0 22 43 65 87

208 225 221 208 199 189

915 915 915 915 915 915

761 697 709 745 769 798

Fig. 1. Microstructure of pozzolanic materials (a) Metakaolin (b) Fly ash particles.

90

0:00 15:05 5:15

80

Slump, mm

70

20:00 10:10 0:20

60 50 40 30 20 10 0

0

30

60

90

Time, minutes

120

150

Fig. 2. Slump loss with time of concrete with different percentages of MK:FA.

cement-limestone-fly ash system (PC-LP-FA) [8,17]. Evidence of these carboaluminate products filling the capillary pores and improving mechanical behavior in PC-LP-FA systems has been reported [23–25]. Similar mechanisms and results were observed in this PC-MK-FA system. X-ray diffraction patterns in Figs. 4–7 show that as the hydration progressed, the synergistic action of the two pozzolans (MK and FA) in the ternary system increased the calcium hydroxide consumption, compared to the binary system, and improved phase development. Hemicarboaluminate and monocarboaluminate phases decreased and almost disappeared in most mixes at 28 days’ curing, but the synergistic effect continued. Similar to the observation at 7 days, the highest relative intensity of the calcium monocarboaluminate phase was observed in the MK-FA for a 10:10 mix. On further curing, the presence of the phase decreases and is almost completely gone by 91 days (Fig. 7). However, only the MK:FA 5:15 mix yielded this phase. Both local metakaolin and fly ash are rich in alumina (35.18% and 18.51%, respectively) compared to the composition of Portland cement (5.09%). The pozzolanic reaction from these materials resulted partly from the increase in aluminates in the system. Particularly with the addition of the metakaolin, the decreased ratios of SO3/Al2O3(Sˆ/A) and CaO/Al2O3(C/A) (Table 3) and the

time-dependent release of alumina affected the synergistic reactions and the resultant products, similarly to that occurred and reported in limestone-fly ash system by other researchers [8,24]. As the cement pastes mature, evidence to support this was observed in the form of the depletion of calcium hydroxide, increase in densification of the microstructure and development of hydration products on the surface of fly ash particles. The Sˆ/A and C/A ratio-dependency of stable monocarboaluminate and hemicarboaluminate has been reportedin the literature [23,26]. The benefit of additional alumina on the reduction of the Sˆ/A ratio and the carboaluminate products on hardened concrete in the ternary system was also observed in this work as will be discussed in the following section. 3.3. Hardened concrete 3.3.1. Compressive strength As shown in Table 3 and Fig. 8, the strength of the metakaolin concrete increased by about 10–15% both in the early stages of curing and in the long term. This resulted from both physical and chemical effects, as suggested by Wild et al. even with the increase in the W/B [27]. MK-rich ternary mixtures yielded higher early strength than FA-rich mixtures. This was due to the more inertness of FA than that of MK which provided calcium silicate hydrate earlier in the hydration process. Although an amorphous aluminosilicate metakaolin was expected to exhibit pozzolanic properties similar to silica fume, it depended on the purity of the raw kaolin and the thermal process it had undergone [28]. Upon curing, the binary mixture with metakaolin developed higher compressive strengths than that of the control. For the ternary system, higher strength at 7 days was observed for all mixtures with a metakaolin percentage greater than 10. An MK:FA ratio of 10:10 produced the highest long-term strength (91 days). The most pronounced effect was observed on the ternary system’s strength development and the synergistic effect seemed to continue after 91 days. The synergistic effect from the strong chemical interaction between the alumina phases inthe cement mix with a MK:FA ratio of 10:10 probably accounted for the observed strength development as mentioned by Zhang and Ye [24]. The same mix yielded a compressive strength about 24%

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a

c

10:10

b

10:10

10:10

d

10:10

Fig. 3. Structure of metakaolin-fly ash paste (10:10) at ages (a) 3 (b) 14 (c) 28 and (d) 91 days.

Fig. 4. XRD results of metakaolin-fly ash paste of different pozzolan ratios at 7 days.

higher than that of the control mix, as well as 13% and 8% higher than those of the binary mixes. During the first 28 days, the binary PC:MK showed better strength development than the other mixes. Similar performance was observed for flexural strength. Considering the effect of SO3/Al2O3 ratio, the ternary mixture (PC:MK:FA 80:10:10) with the SO3/Al2O3 ratio of 0.234 yielded the highest long-term compressive and flexural strength. 3.3.2. Abrasion resistance Metakaolin-fly ash and fly ash concrete showed significantly higher abrasion resistance than normal concrete at 28 days and

56 days. The resistance changed slightly with pozzolan ratio but did not show any correlation with the gain in strength, as shown in Table 3. This was compatible with the results of microstructure examination where denser microstructure was observed (Fig. 3a & b). 3.3.3. Chloride permeability The effects of the pozzolans on the void system were more pronounced in the changes in chloride permeability, as shown in Fig. 9. Both pozzolans showed improvement in reducing chloride permeability over the control specimens but mixtures richer in metakaolin showed a lower charged passed and thus, a better

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Fig. 5. XRD results of metakaolin-fly ash paste of different pozzolan ratios at 14 days.

Fig. 6. XRD results of metakaolin-fly ash paste of different pozzolan ratios at 28 days.

resistance to chloride permeability than those richer in fly ash. On further curing to 56 days, all binary and ternary mixes yielded very low permeability, according to ASTM C1202. This corresponded well with the moderate level of permeability for the control concrete at the same age. 3.3.4. Corrosion risk The results of the chloride penetration tests are shown in Fig. 10. They indicated a markedly reduced risk of corrosion in all pozzolan mixes compared to that of the control. During the accelerated test, the control mix reached the ASTM-suggested potential difference of 350 mV (ASTM C876) at 8 days. This value indicated the probability of corrosion in the reinforcing steel at 90%. Mixes incorporating the pozzolans, however, performed much better regardless of the type and/or percentage for the ranges studied.

Over the 140 days of test duration, the corrosion risk resistance was still lower than the threshold level and no sign of cracking was observed for any specimens except those of the control concrete. As with the chloride permeability, the mixes higher in MK seem to provide better resistance to degradation through the entire test. The penetration depths from broken specimens after 90 and 140 days of accelerated testing are shown in Table 4 and Fig. 11. The 10:10 MK:FA cement replacement appeared to have no effect on the chloride front depth while the depth in fly ash concrete samples slightly increased. Metakaolin incorporated mixtures showed better performance than that of fly ash from chloride impermeability view point. This may be partly due to the better chloride binding capacity of metakaolin than that of fly ash [29]. This effect correlates with the available alumina content of the

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Fig. 7. XRD results of metakaolin-fly ash paste of different pozzolan ratios at 91 days.

Table 3 Some chemical compound ratios, abrasion resistance, compressive and flexural strength of different mixes. Mix description

W/B

C/A

Sˆ/A

Control MK:FA, 20:0 MK:FA, 15:5 MK:FA, 10:10 MK:FA, 5:15 MK:FA, 0:20

0.48 0.52 0.51 0.48 0.46 0.44

13.010 4.770 5.215 5.738 6.363 7.122

0.475 0.175 0.202 0.234 0.272 0.272

Wt.loss/A, (10

4

g/mm2)

Comp.strength, (MPa)

Flex.strength, (MPa)

28 d

56 d

28 d

56 d

28 d

56 d

7.76 5.11 5.16 5.51 5.56 5.41

7.55 3.44 4.05 4.63 4.50 3.62

46.6 52.3 47.8 48.3 45.0 43.5

47.7 53.5 49.9 55.0 49.6 56.2

5.2 5.9 5.7 5.7 5.2 5.2

5.6 6.0 6.0 6.0 6.0 6.1

Note: 1. C = CaO, A = Al2O3, Sˆ = SO3. 2. Wt. loss/A = Weight loss/abrasive area, Comp. strength = Compressive strength, Flex. strength = Flexural strength.

Fig. 8. Strength at different ages of binary and ternary system concrete.

SCM [30]. The embedded steel in the control was found severely corroded at 90 days, whilst little or no corrosion was found in the pozzolan mixtures (Fig. 12). This confirmed the effectiveness of pozzolan addition in concrete to reduce degradation.

4. Conclusions From this study, the following conclusions can be made.

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Fig. 9. Chloride permeability of concrete mixtures.

Fig. 12. Embedded steel conditions in each concrete type after 90 days of accelerated test.

Fig. 10. Half-cell potential reading of metakaolin concrete.

Table 4 Chloride penetration depth measurement. Mix

Control MK:FA, 20:0 MK:FA, 15:5 MK:FA, 10:10 MK:FA, 5:15 MK:FA, 0:20

Average chloride depth from surface, mm 90 d

140 d

>70 12 11 12 13 19

>70 19 18 17 21 23

1. The combination of alumina-rich metakaolin and fly ash strongly affected phase development. The ratio of SO3/Al2O3(Sˆ/A) strongly influenced the synergistic reaction and the products obtained. 2. Ternary systems substantially improved workability and later stage strength. Slump loss behavior was also improved. The replacement ratio of metakaolin to fly ash of 10:10 appeared to be the most effective for strength development and workability improvement over the control sample. 3. The produced monocarboaluminate fills voids providing a denser and more uniform microstructure of the hardened concrete with improved durability and strength gain.

Fig. 11. Chloride penetration depth of various concretes.

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4. Regardless of the pozzolan type, their effects on long-term void improvement was pronounced. High abrasion resistance, very high level of chloride ingress resistance and marked corrosion risk reduction were observed. MK was more efficient at all these. 5. Synergistic action of MK and FA helped improving long-term strength and durability. This indicates the potential of incorporating fly ash and metakaolin as a low-cost local supplement material for high strength and durable concrete.

[11] [12]

[13]

[14] [15]

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