Durability of metakaolin Self-Compacting Concrete

Construction and Building Materials 82 (2015) 133–141

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Durability of metakaolin Self-Compacting Concrete Efstratios G. Badogiannis a,⇑, Ioannis P. Sfikas a, Dimitra V. Voukia a, Konstantinos G. Trezos a, Sotirios G. Tsivilis b a b

National Technical University of Athens, School of Civil Engineering, Laboratory of Reinforced Concrete, 5 Heroon Polytechniou, 15773 Athens, Greece National Technical University of Athens, School of Chemical Engineering, Laboratory of Analytical and Inorganic Chemistry, 9 Heroon Polytechniou, 15773 Athens, Greece

h i g h l i g h t s  The durability of SCC mixtures incorporating mk is generally enhanced.  The capillary pore system is more affected by mk than the open pore system.  The chloride penetration resistance is enhanced by more than two classes.  The surface water permeability of the mk concrete appears to be inferior.  The sorptivity and the chloride migration coefficients correlate well with fcc.

a r t i c l e

i n f o

Article history: Received 27 October 2014 Received in revised form 15 February 2015 Accepted 18 February 2015

Keywords: Self-Compacting Concrete Metakaolin Durability Open porosity Sorptivity Water permeability Gas permeability Chloride penetrability

a b s t r a c t The aim of this study is to evaluate the durability of Self-Compacting Concrete (SCC) incorporating metakaolin (mk). For the mixture preparation, cement or limestone powder were replaced by mk at different levels. The estimated properties (open porosity, sorptivity, water and gas permeability, chloride penetrability) were evaluated against a reference mixture (without mk). The incorporation of mk improved durability, but not the near surface water permeability of the concrete. Equations expressing the effect of the replacement level on the examined properties were formulated, whenever appropriate. The most enhancing effect of mk as a replacement material was observed in the chloride penetration resistance. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction After the introduction of EN 206-1:2000 [1], which was recently updated to EN 206:2013 [2], concrete performance has been specified not only on the basis of its mechanical characteristics, but also in terms of its durability properties. Durability improvement can be achieved to a great extent by the incorporation of supplementary cementitious materials (SCM) in the mixture. Due to their high pozzolanic activity and filling effect, SCMs can lead to the production of a more consistent, cohesive and dense concrete, which exhibits enhanced mechanical characteristics [3–8] and reduced permeability [3,6,9–12]. Among other cementitious materials, such as silica fume (sf), granulate blast-furnace slag (ggbs) ⇑ Corresponding author. E-mail addresses: [email protected] (E.G. Badogiannis), gsfikas@gmail. com (I.P. Sfikas), [email protected] (D.V. Voukia), [email protected] (K.G. Trezos), [email protected] (S.G. Tsivilis). http://dx.doi.org/10.1016/j.conbuildmat.2015.02.023 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

and (pulverized) fly ash (pfa), that are frequently used for the production of concrete, metakaolin (mk), is a material with a feasible growing implementation in the concrete industry. Metakaolin, Al2Si2O7, is a known material even before the 1960s, but its application either as a pozzolanic material in cement or as SCM in concrete, gained the interest of researchers only after the early 1980s [13–15]. Metakaolin is a thermally activated pozzolanic material that is obtained by the calcination of kaolinitic clay at moderate temperatures ranging from 650 to 800 °C. The main characteristic of mk is its high reactivity with calcium hydroxide, Ca(OH)2, and its ability to accelerate cement hydration. Compared to other SCMs, like sf or pfa, the pozzolanic action of mk is expected to be more significant due to its high concentration of silica and alumina. Specifically, the calcium to silicon (C/S) ratio of the produced calcium silica hydrates (C–S–H gel) is expected to be higher than the corresponding ratios for sf and pfa, thus leading to a considerably improved concrete microstructure and to the

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ultimate enhancement of the overall performance of concrete, in terms of strength, porosity, permeability, chloride ion diffusivity, freezing and thawing resistance, etc. [17–19]. Moreover, as all other ultrafine pozzolans, mk significantly contributes to filling small pores and voids and, thus, densifies the interfacial transition zone due to the microfiller effect. The latter action is usually referred to as physical action [20]. Additionally to the sustainable performance enhancement of concrete, through its improved durability, mk is also considered as a sustainable and environmental-friendly material, due to the limited CO2 emissions during its production process. Thus, when it is used as a replacement material of Portland cement, a significant reduction of the total CO2 emissions is achieved. The major disadvantage of mk is the high cost of production, compared to cement. However, this could be mainly attributed to its currently low production rates. A higher implementation in the cement and concrete industry, on the basis of its technological and environmental merit, could potentially decrease the production cost of mk and, consecutively, the final cost of mk concrete. 2. Literature review In previous studies that were conducted with regard to the effects of SCMs, and in particular mk, in concrete, a significant enhancement of the mechanical characteristics and the durability properties of the hardened concrete specimens was generally observed [21,22], providing sound arguments regarding the need for further investigation of the effects of mk and its potential for increased future usage. Various researchers have reported a remarkable improvement of the major mechanical properties (such as the compressive and the splitting tensile strengths) of both Normally Vibrated Concrete (NVC) and Self-Compacting Concrete (SCC) mixtures, in which cement was replaced by mk at various percentage levels [9,15,16,19,23–28]. In order to evaluate the durability of concrete, several laboratory methods have been implemented in previous relevant studies, mainly concerning the transport properties of the hardened concrete, i.e. the ability of ions and fluids to move through its pore system. Khatib and Clay (2003) [23] measured the water absorption by capillary rise in NVC specimens incorporating mk as a replacement material of cement up to 20% by mass of total binder (b) and reported that water penetration due to capillary action was reduced for mixtures incorporating mk. Razak et al. (2004) [24] found that a 10% replacement of cement by mk (by weight1) in NVC mixtures significantly decreased the initial surface absorption, the water absorption and the sorptivity of concrete. Furthermore, it was observed that the level of pozzolanic reaction was almost complete during the initial 28 days curing period of mk mixtures. Siddique and Klaus (2009) [15] reported that, for 5 and 10% cement replacement by mk in NVC mixtures, the sorptivity was reduced. In contrast, an increase in the sorptivity was observed for a replacement level of 15%. Another observation was that the compressive strength is inversely related with sorptivity (i.e. higher strength leads to lower sorptivity). The water absorption was reduced for all the examined mixtures incorporating mk. In the same study, it was also reported that mk/b replacement levels higher than 15% were not found to be improving the inner core durability, even though the surface durability properties were enhanced. Badogiannis and Tsivilis (2009) [9] reported that NVC incorporating mk exhibits significantly lower chloride permeability, gas permeability and sorptivity. A high reduction of gas permeability, in the order of 50%, was observed in concrete with 20% cement replacement by mk. Shekarchi et al. (2010) [25] reported that transport 1 All replacement levels in this manuscript are expressed as % by weight of total binder.

properties comprising water penetration, gas permeability, water absorption, electrical resistivity and ionic diffusion of NVC specimens were improved up to 50%, 37%, 28%, 450% and 47%, respectively, for 15% cement replacement by mk. More recently, Güneyisi et al. (2012) [16] reported that the increase of the cement replacement level by mk in NVC mixtures results in a decrease in both the apparent gas permeability of concrete and the water sorptivity. However, it was observed that the magnitude of the maximum reduction was different for each measured property (for a replacement level of 15% and a water-to-cement ratio of 0.25, the reduction reached 52% and 30% for the gas permeability and the water sorptivity, respectively). Ramezanianpour and Bahrami (2012) [27] found that NVC concretes incorporating mk exhibited lower water penetration depth, whereas the replacement of cement by 10% mk resulted in the most enhanced sorptivity, compared to other tested replacement levels in the range from 0% to 15%, regardless of water-to-binder ratio and testing age. Furthermore, exponential relationships between all measured properties (chloride penetrability as measured by passing electric charge, surface resistivity, water penetration depth and sorptivity) and the compressive strength were also reported. Hassan et al. (2012) [19] reported that highly durable SCC mixtures can be produced using a high mk content with an optimum level of cement replacement of 20% (as extracted from a tested range from 0% to 25%). Dinakar et al. (2013) [28] found that the water permeability, the absorption and the chloride penetrability are reduced when the replacement level of cement by mk in high-strength NVC mixtures is gradually increased from 5% to 15%. This enhancing effect was attributed to the filler effect of mk particles, which was considered to substantially reduce the permeability or porosity of the concrete. 3. Research objectives The aim of the present study, which is part of a wider research project on SCC incorporating mineral admixtures [8,10,29–31], is to investigate the effect of mk on the durability of SCC mixtures. For this purpose, mk was used as a replacement material of either cement or limestone powder, at various percentage levels. In total, nine different mixtures of SCC were examined:  one reference SCC mixture that did not incorporate mk;  four SCC mixtures in which a hydraulic material (cement) was replaced at different levels with a finer pozzolanic material (mk), aiming at the investigation of the combined effect of the pozzolanic reactivity of mk and its physical action, regarding packing density enhancement; and  four SCC mixtures in which an inert fine material (limestone powder) was replaced at different levels by a pozzolanic material (mk) of a similar fineness, mainly focusing on the effect of the pozzolanic reactivity of metakaolin. The results regarding the fresh properties and the mechanical characteristics of the aforementioned SCC mixtures were recently published and discussed in another research study [8]. 4. Materials and mixtures 4.1. Constituent materials For the purposes of the current study, metakaolin (mk) of high purity was used. The chemical analysis of the commercial kaolin (k), from which mk was produced, as well as the physical properties of mk are shown in Table 1. An estimation of the kaolin (k) mineralogy was performed, based on the characteristic X-Ray Diffraction (XRD) peaks of each mineral (Fig. 1). The kaolin was found to contain kaolinite and detectable amounts of illite and quartz. A typical Portland composite (CEM II/B-M (P-W-L) 42.5 N) cement (c), conforming to EN 197-1:2011 [32], was used for the production of all SCC mixtures. The specific surface of the cement particles, when calculated by grain size analysis, is 0.70 m2/g.

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E.G. Badogiannis et al. / Construction and Building Materials 82 (2015) 133–141 Table 1 Chemical analysis (% w/w) of kaolin (k) and physical properties of metakaolin (mk). Kaolin chemical analysis (% w/w)

*

Table 2 Physical properties and mineralogical composition (% w/w) of limestone powder (lp).

Metakaolin physical properties

SiO2

Al2O3

CaO

MgO

Fe2O3

L.O.I.

Density (t/m3)

Specific surface area (m2/g)*

47.85

38.20

0.03

0.04

1.29

12.30

2.5

1.41

Physical properties

3

Density (t/m ) Specific surface area (m2/g)* Moisture content (%) *

4.2. Mixture proportions, production and sampling 4.2.1. Mixture proportions In total, nine SCC mixtures, i.e. one reference mixture (RM) and two groups of four mixtures each, were cast and tested (Table 4). In these two groups of mixtures, mk replaces either cement (c) or limestone powder (lp) at different percentage levels. Specifically, in the first group (SCC1 to SCC4), the cement was replaced by gradually higher levels of mk, whereas in the second group (SCC5 to SCC8) mk replaces the incorporated initial quantity of lp (175 kg/m3) at gradually higher levels. The effective water (w) content (tap water at 20 °C) for all specimens was kept constant (210 kg/m3). In the first group of mixtures the water-to-binder ratio, w/b, was kept constant and equal to 0.60, whereas in the second group of mixtures the water-to-cement ratio, w/c, was kept constant and equal to 0.60. For RM, both w/b and w/c ratios were equal to 0.60. 4.2.2. Production and fresh properties For the production of the concrete mixtures a fixed-pan planetary type cylindrical mixer with rotating blades was used and the same mixing procedure was carefully followed for all mixtures. At first, the aggregates were dry-mixed and then lp was added and all ingredients were further dry-mixed. Subsequently,

<18 lm <2 lm

Fineness characteristics

Calculated by grain size analysis.

Limestone powder (lp) of 97.6% calcium carbonate (CaCO3) purity and high fineness was also added. Its physical properties and mineralogical composition are presented in Table 2. It should be mentioned that a limestone powder with this particular fineness was selected aiming to use an inert filler with a comparable fineness to mk. Given its high local availability, this particular filler is not considered to significantly alter the cost of the mixtures, the analysis of which was though outside of the scope of the present study. However, it is clear that a limestone powder with lower fineness would be more cost efficient. Three nominal gradings of locally available crushed calcareous limestone aggregates, conforming to EN 12620:2002 [33], were used: sand (s) 0/4 mm, small gravel (g1) 4/8 mm and medium gravel (g2) 8/16 mm. The physical properties (apparent density, water absorption) of the used aggregates are listed in Table 3. The grading curves of all the fine and coarse materials are shown in Figs. 2 and 3, respectively. It is noticeable that mk and lp present similar gradings, which are finer than cement (Fig. 2), and they have a practically similar specific surface (Tables 1 and 2). The aggregate mixture grading curve is also shown in Fig. 3. Finally, ordinary tap water (w) and polycarboxylic ether superplasticizer (pce), conforming to Tables 11.1 and 11.2 of EN 934-2:2009 [35], were introduced to all mixtures.

Mineralogical composition (% w/w) 97% 15% 2.7 1.27 0.21

CaCO3 MgCO3 SiO2 Fe2O3 Mn2O3

97.5 1.6 0.8 0.1 –

Calculated by grain size analysis.

Table 3 Physical properties of aggregates (calculated by EN 1097-6:2000 [34]). Type

Symbol

Apparent density on an oven dried basis (t/m3)

Water absorption (%)

Sand, 0/4 mm Small gravel, 4/8 mm Medium gravel, 8/16 mm

s g1 g2

2.66 2.66 2.65

0.9 1.1 1.0

the binder (cement and mk) was introduced to the homogeneous dry mixture. Then, 80% of the total water content was added, followed by the rest 20% of the water, in addition with the pce superplasticizer. The batches were cast at an average temperature of 22 °C and an average relative humidity of 50%. The fresh properties of the SCC mixtures, i.e. the workability, the viscosity and the passing ability, were assessed by the Slump-flow, the V-Funnel and the LBox tests, respectively, and classified in accordance with the relevant European Standards [36–38]. The detailed results and discussion regarding the fresh properties have already been published as part of another study on the fresh and mechanical properties of the same mixtures [8] and only the classification is reported here (Table 5), for reasons of completeness.

4.2.3. Sampling Steel moulds were used for the production of the test specimens. For the investigation of durability, four standard cubes, i.e. two 150 mm and two 100 mm edge cubes, and four standard cylinders (diameter: 100 mm, length: 200 mm) were cast per mixture in one lift, without applying any mechanical compaction. The specimens were demoulded after 24 h and were then cured in water, at 20 °C, until the age of testing at 28 days. The cube compressive strength, fcc (N/mm2), has already been examined and discussed as part of another study on the mechanical properties of the same mixtures [8] and the values are only reported here (Table 6), for reasons of completeness. In the aforementioned study, additional compressive strength tests were also performed at 360 days. It has been concluded that, regardless of the replaced material, the enhancing effect on the compressive strength is similar and the

Fig. 1. XRD pattern and mineralogical phases of kaolin (k): (1) kaolinite, (2) illite and (3) quartz.

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E.G. Badogiannis et al. / Construction and Building Materials 82 (2015) 133–141 Table 6 Average value (m) and standard deviation (sd) of the cube compressive strength, fcc (N/mm2), of concrete specimens at 28 and 360 days (as published in Sfikas et al. 2014 [8]). fcc

RM

SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

m28 sd28

62.3 1.1

62.9 0.9

67.2 1.8

73.4 3.5

74.9 3.3

67.6 1.2

81.2 1.0

79.8 0.8

91.0 1.6

m360 sd360

73.7 2.7

74.6 0.9

73.9 2.4

81.0 3.8

81.4 1.2

73.7 3.2

76.0 1.0

82.5 1.6

87.3 1.1

The open porosity (p) was evaluated by weighting a fully saturated 100 mm edge cubic specimen immersed in water and in air. The specimen was weighed again after being oven dried at 105 °C until constant mass. The drying period of the specimens ranged between 4 and 7 days. The open porosity, p (%), was calculated by the combination of the three masses [39]. The sorptivity (S) was, then, assessed by placing one surface of the dry 100 mm edge cubic specimen in marginal contact with water. The weight change due to the water uptake (capillary absorption) was recorded and the sorptivity, S (mm/min0.5), was calculated [40]. The near surface water permeability of concrete was determined by non-steady state tests, which were performed on the side of a 150 mm edge cubic specimen. The specimen was oven-dried at 105 °C until constant mass, with the drying period ranging from 7 to 10 days. After cooling down to ambient temperature, an initial water pressure of 120 kN/mm2 was applied to the side surface of each specimen and the gradual water pressure decrease, as a function of time, ln T (T in s), was recorded at predefined intervals for 15 min. By the extrapolation of the polynomial curve, describing the undisturbed pressure decrease, the time point, ln TPatm (TPatm in s), when the pressure would drop down to the atmospheric pressure, Patm, was estimated. This time point is defined as the pressure equalisation time, higher values of which indicate a less permeable concrete. This method of permeability evaluation was initially proposed by a relevant doctoral thesis [31], and is still under ongoing experimental evaluation. The main advantage of this method is that it mainly evaluates the permeability of the near surface concrete, the quality of which is very significant to the durability of the final product. Gas permeability tests were performed to a 50 mm thick concrete segment that was extracted from the middle zone of a standard cylinder (diameter: 100 mm, length: 200 mm). The segment was oven-dried at 105 °C until constant mass. The drying period of the segments ranged between 4 and 7 days. Then, a modified commercial triaxial cell for 100 mm diameter specimens, operating to maximum cell pressure of 0.7 N/mm2, was used for the determination of the nitrogen dioxide (N2) gas permeability of the specimens. The equipment used, as well as the detailed procedure is described in a previous study [41]. The gas permeability coefficient, Kc (m2), was then estimated [42].

Fig. 2. Grading curves of fine materials.

Fig. 3. Grading curves of aggregates. pozzolanic effect appears to have been already completed at the age of 28 days. The observed increase of the compressive strength at 360 days has been attributed solely to the extended curing. 4.2.4. Durability tests The durability of the SCC mixtures was evaluated on the basis of their water and gas permeability and of the chlorides penetration resistance, at 28 days. The tests that were conducted are further described below.

Table 4 Mixture proportions (kg/m3) and ratios.

*

Description

Symbol

RM

SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

Cement Metakaolin Limestone powder Sand, 0/4 mm Small gravel, 4/8 mm Medium gravel, 8/16 mm Effective water Superplasticizer

c mk lp s g1 g2 w pce

350 0 175 1050 150 350 210 4.5

326 24 175 1050 150 350 210 6.0

313 37 175 1050 150 350 210 5.0

301 49 175 1050 150 350 210 4.9

280 70 175 1050 150 350 210 6.0

350 24 151 1050 150 350 210 4.5

350 37 138 1050 150 350 210 5.5

350 49 126 1050 150 350 210 5.5

350 70 105 1050 150 350 210 6.3

Total weight binder*

W b

2290 350

2291 350

2290 350

2290 350

2291 350

2290 374

2291 387

2291 399

2291 420

Water-to-cement Water-to-binder c replacement level lp replacement level

w/c w/b mk/b mk/{mk + lp}

0.60 0.60 0.0% 0.0%

0.64 0.60 6.9%

0.67 0.60 10.6%

0.70 0.60 14.0%

0.75 0.60 20.0%

0.60 0.56

0.60 0.54

0.60 0.52

0.60 0.50

13.7%

21.1%

28.0%

40.0%

Binder refers to the sum of all cementitious materials, i.e. cement and mk.

Table 5 Fresh concrete test results and classification (as published in Sfikas et al. 2014 [8]). Description

RM

SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

Slump-flow class Slump-flow viscosity class V-funnel viscosity class L-box passing ability class

SF2 VS2 VF1 PA2

SF3 VS1 VF1 PA2

SF2 VS1 VF1 PA2

SF1 VS1 VF1 PA1

SF1 VS1 VF1 PA2

SF1 VS2 VF2 PA2

SF2 VS1 VF1 PA2

SF2 VS1 VF1 PA2

SF1 VS2 VF2 PA2

E.G. Badogiannis et al. / Construction and Building Materials 82 (2015) 133–141

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Table 7 Chloride penetration resistance classes at 28 days (Tang (1996) [49]). Chloride migration coefficient Dnssm  10–12 m2/s

Chloride penetration resistance

>16 8–16 2–8 <2

Not suitable for aggressive environment Moderate Good Very good

The chloride penetrability was estimated by calculating the chloride migration coefficient, Dnssm (10 12 m2/s), using a well-established non-steady-state migration experiment [43]. One 50 mm thick segment, extracted from the middle zone of a cylindrical specimen, was maintained for 24 h under a potential difference, between a cathode solution of 10% sodium chloride (NaCl) by mass and an anode solution of sodium hydroxide (NaOH), 0.3 N. It should be noted that the specimens were not dried and preconditioned in a vacuum container, as suggested by the test method, due to equipment limitations. However, given that all specimens of the present study have received the same pre-treatment, i.e. full saturation until the age of testing, the relative effect of chloride penetration is expected to be comparable. At this point it is worth mentioning that the aim of the vacuum saturation with a carbon hydroxide solution, Ca(OH)2, is to achieve an even ions distribution within the pore solution and, thus, to ensure a uniform chloride penetration throughout the specimen. However, many researchers [44–46] have questioned the effectiveness of this preconditioning method, mainly due to the effect of the pre-drying process on the porosity. Further targeted research, investigating the effect of different preconditioning methods on the resulting chloride penetrability, would be valuable. After the test completion, the determination of the chloride penetration depth was performed by a colorimetric method [47,48]. Specifically, the tested specimen was axially split into two pieces and one of the two fractured surfaces was sprayed with a 0.1 M silver nitrate (AgNO3) solution. The average depth of chloride penetration was determined from the colour change in the area where the presence of chlorides chemically leads to the formation of silver chloride (AgCl). The coefficient Dnssm was further calculated in accordance with NordTest Build 492 method [43] and the mixtures were further classified, as suggested by Tang (1996) [49] (Table 7).

5. Results and discussion In Table 8, the experimental results for the open porosity (p), the sorptivity (S), the water permeability (ln TPatm), the gas permeability coefficient (Kc) and the chloride migration coefficient (Dnssm) are presented. In the following sections, the measured values are being discussed. 5.1. Open porosity The open porosity, p (%), of RM was slightly improved with the replacement of either cement or lp by mk (Fig. 4). The high variation of the estimated values for the first group of mixtures makes it impossible to correlate p with the increasing level of cement replacement by mk. However, it is clear that all values are lower than for the RM. The highest reduction is observed for mixture SCC3 (mk/b = 14.0%). Regarding the second group of mixtures, the improvement is bigger for lower lp replacement levels

(mixture SCC5, mk/{mk + lp} = 13.7%) and the enhancing effect of mk is gradually diminished for higher mk levels of replacement. Although a decreasing correlation is macroscopically observed between the open porosity and the increasing mk/b replacement level, the variation is rather high. On the other hand, a good correlation between the open porosity and the mk/{mk + lp} replacement level is only achieved when the RM is ignored, due to the local minimum for SCC5 and the adverse effect of higher replacement levels. 5.2. Sorptivity The sorptivity, S (mm/min0.5), appears to be more susceptible to the mk addition as a replacement material (Fig. 5) than the open porosity. This may mean that the effect of mk, as a replacement material, is more essential for the capillary pore system of the SCC specimens, which is evaluated by this method, than for the open pore system. The improvement of the sorptivity varies between 6 and 41% when mk replaces cement. A general decreasing trend is observed for higher replacement levels. These findings are not in agreement with the study of Ramenzianpour and Bahrami (2012) [27], where for a similar replacement level (mk/b = 10%) the most enhanced sorptivity (local minimum) was observed. However, other studies [9,28] have confirmed the decreasing trend for higher replacement levels. In the case where mk replaces lp, the improvement varies between 22 and 53% and appears to be systematically higher. The corresponding linear correlations between the sorptivity and the cement or lp replacement levels by mk are shown in Figs. 6 and 7, respectively. It is evident that for the examined ranges of replacement levels, i.e. up to 20% or 40% for mk/b and the mk/{mk + lp} respectively, the correlations do not include any turning point (local minimum). The inverse relation between the sorptivity and the compressive strength, as observed by Siddique and Klaus (2011) [50], is also evident in this study for a range of strengths between approximately 60 and 95 N/ mm2, regardless of the replacement case (Fig. 8). 5.3. Water permeability

Table 8 Durability properties of SCC mixtures.

RM SCC1 SCC2 SCC3 SCC4 SCC5 SCC6 SCC7 SCC8

Fig. 4. Open porosity, p (%).

Open porosity

Sorptivity

Water permeability

Gas permeability coefficient

p %

S mm/min0.5

ln TPatm TPatm in s

Kc 10

19.6 14.8 18.1 12.7 16.6 16.6 18.4 18.4 18.9

0.2435 0.2098 0.1788 0.1549 0.1226 0.1638 0.1461 0.1130 0.1017

7.97 7.66 7.30 7.34 7.04 6.97 6.94 6.48 7.95

5.16 3.39 5.01 3.53 3.69 4.87 5.56 3.58 4.17

17

m2

Chloride migration coefficient Dnssm 10 12 m2/s 21.75 6.58 4.79 2.61 1.42 5.86 3.62 2.47 0.77

Higher water permeability, as it is expressed by the ln TPatm values (Fig. 9), is observed for SCC mixtures incorporating mk. As shown in Fig. 10, the pressure equalisation times appear to be linearly decreasing for higher mk/b replacement levels, thus depicting more permeable surface concrete. Regarding the second group of mixtures, a local minimum is evident for mixture SCC7 (mk/ {mk + lp} = 28%), as illustrated in Fig. 11. However, the value for mixture SCC8 (mk/{mk + lp} = 40%) is considered to be overestimated due to an experimental fault, a hypothesis that can only be confirmed through further research. By excluding this value the linear regression exhibits a very high regression coefficient and is similar to the corresponding linear regression of the values for the first group of mixtures, though slightly steeper.

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Fig. 5. Sorptivity, S (mm/min0.5).

Fig. 9. Pressure equalisation time, ln TPatm (TPatm in s).

Fig. 6. Sorptivity, S (mm/min0.5), versus mk/b replacement level (%).

Fig. 10. Pressure equalisation time, ln TPatm (TPatm in s), versus mk/b replacement level (%).

Fig. 7. Sorptivity, S (mm/min0.5), versus mk/{mk + lp} replacement level (%).

Fig. 11. Pressure equalisation time, ln TPatm (TPatm in s), versus mk/{mk + lp} replacement level (%).

mechanisms are still under ongoing experimental investigation. On the basis of these assumptions, the expected pozzolanic effect of mk on the smaller pores (less than 100 nm) may not be traceable by the applied method. As another general comment on the proposed method, it could be mentioned that the generally high amounts of superplasticizer that are used for the production of SCC produce additional air bubbles near the surface, a fact that may considerably influence the local durability properties. Future research should focus on further evaluation of the aforementioned aspects.

Fig. 8. Sorptivity, S (mm/min0.5), versus compressive strength at 28 days, fcc,28 (N/mm2).

It is considered that, in contrast to the open porosity and the sorptivity, this method only evaluates the bigger pores of concrete near the surface. The effect of the initially applied pressure is also considered to negatively affect the pore system, however the exact

5.4. Gas permeability SCC incorporating mk generally exhibits lower gas permeability coefficients, Kc (10 17 m2), compared to RM (Fig. 12). The measured maximum reduction reached 34% for mixture SCC1 (mk/b = 6.9%). A local maximum was observed for mixtures SCC2 (mk/b = 10.6%) and SCC6 (mk/{mk + lp} = 21.1%). In the aforementioned mixtures the measured gas permeability coefficients were

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similar to the corresponding coefficient for RM. Although a similar influence is observed for the two replacement cases, the improvement is slightly but systematically better for the cement replacement by mk. A decreasing linear correlation was observed between the gas permeability coefficients and the increasing replacement levels of either material by mk, as shown in Figs. 13 and 14. It should be mentioned that the calculated regression coefficients were low (R2 = 0.31 and 0.37 for mk/b and mk/{mk + lp} replacement levels, respectively) and the exclusion of the RM would lead to a variation of the gas permeability coefficients over an average value, rather than a clear increasing or decreasing trend. However, it can be suggested that Kc is indeed affected by the replacement level, as a good correlation with the other measured properties (that also correlate well with the replacement levels) has been observed. Specifically, for the first group of mixtures, the Kc coefficients exhibit linear correlations with both the open porosity (R2 = 0.60) and the sorptivity (R2 = 0.54), whereas for the second group of mixtures the corresponding coefficients

Fig. 15. Chloride migration coefficient, Dnssm (10

12

m2/s).

correlate better with the sorptivity (R2 = 0.54) and the chloride penetration coefficients (R2 = 0.28). The corresponding correlations are not presented here, but can be easily produced through the declared values (Table 8) of the various properties. The fact that, regardless of the replaced material, the Kc coefficients do not correlate with all other properties reveals that the choice of the replaced material (cement or sand) may have different impact on the types of affected pores (larger or capillary pores or both). 5.5. Chloride penetrability

Fig. 12. Gas permeability coefficient, Kc (10

Fig. 13. Gas permeability coefficient, Kc (10 level (%).

Fig. 14. Gas permeability coefficient, Kc (10 ment level (%).

17

17

17

m2).

m2), versus mk/b replacement

m2), versus mk/{mk + lp} replace-

Fig. 15 shows that the replacement of both cement and lp by mk results in a significant decrease on chloride penetrability, as it is expressed by the chloride migration coefficients, Dnssm (10 12 m2/s). Specifically, the chloride migration coefficient for RM (21.75  10 12 m2/s) was reduced by more than 70%, even reaching a total reduction of 96%, and varied from 6.58 to 0.77  10 12 m2/s. This is translated to a total enhancement of the chloride penetration resistance by two classes, in accordance with Tang’s classification (Table 7), even for low replacement levels. A similarly enhancing effect of mk, as a replacement material, was also reported in the study of Dinakar et al. 2013 [28]. The calculated non-steady state chloride migration coefficients appear to be exponentially decreasing for higher mk/b or mk/{mk + lp} replacement levels, as shown in Figs. 16 and 17, respectively. In these Figures, the exponential decrease is expressed as a function of the chloride migration coefficient of RM, Dnssm,RM. Finally, opposite to the findings of Ramezanianpour and Bahrami (2012) [27], it was observed (Fig. 18) that the chloride migration coefficient and the compressive strength correlate well, only when the mixtures incorporating mk are taken into account in the exponential regression. It should be noted that, although in the aforementioned study a different chloride penetration assessment method was used (electric charge measurements, in accordance with ASTM C 1202-09 [51]), the two methods are known [29] to

Fig. 16. Chloride migration coefficient, Dnssm (10 ment level (%).

12

m2/s), versus mk/b replace-

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References

Fig. 17. Chloride migration coefficient, Dnssm (10 replacement level (%).

Fig. 18. Chloride migration coefficient, Dnssm (10 strength at 28 days, fcc,28 (N/mm2).

12

12

m2/s), versus mk/{mk + lp}

m2/s), versus compressive

correlate very well for SCC mixtures regardless of the mineral admixture content, and this difference should not be expected.

6. Conclusions This study aimed at investigating the effect of cement and lp replacement by mk, at different levels, on the durability properties of SCC mixtures.  An enhancing effect on the open porosity is generally observed for both replacement cases, regardless of the level of replacement. Lower porosities were measured for higher mk/b levels and lower mk/{mk + lp} levels.  Regardless of the replaced material, the sorptivity is inversely correlated to the replacement level and to the compressive strength, with linear equations.  The effect of mk as a replacement material seems to be more essential for the capillary pore system than for the open pore system.  Higher replacement levels by mk are not enhancing near surface water permeability.  Metakaolin SCC generally exhibits lower gas permeability compared to the reference concrete mixture. Lower gas permeability coefficients were calculated for higher replacement levels of either cement or lp by mk.  The replacement of either cement or lp by gradually higher mk levels results in an exponential decrease of the non-steady state chloride migration coefficient and an enhancement of the chloride penetration resistance by more than two classes. The chloride migration coefficients correlate very well with the compressive strength for all SCC mixtures incorporating mk.

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