metakaolin blends

Construction and Building Materials 33 (2012) 99–108

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Engineering and durability properties of concretes based on alkali-activated granulated blast furnace slag/metakaolin blends Susan A. Bernal a,b,⇑, Ruby Mejía de Gutiérrez a, John L. Provis b a b

School of Materials Engineering, Composite Materials Group, CENM, Universidad del Valle, Cali, Colombia Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia

a r t i c l e

i n f o

Article history: Received 15 November 2011 Received in revised form 10 January 2012 Accepted 27 January 2012 Available online 28 February 2012 Keywords: Alkali-activated concretes Slag/metakaolin blends Mechanical strength Chloride permeability Carbonation

a b s t r a c t The effects of activation conditions on the engineering properties of alkali-activated slag/metakaolin blends are examined. At high activator concentration, compressive strengths at early age are enhanced by the inclusion of metakaolin in the binder. A similar effect is observed in the flexural strength of the concretes, as dissolution and reaction of metakaolin is favoured under higher-alkalinity activation conditions. Increased metakaolin contents and higher activator concentrations also lead in most cases to reduced water sorptivity and lower chloride permeability. The correlation between the outcome of the rapid chloride permeability test (RCPT) and a directly-measured chloride diffusion coefficient is weak, revealing the limitations of the RCPT when applied to alkali-activated concretes. Accelerated carbonation, induced by exposure to elevated CO2 concentrations, leads to a reduction in compressive strength and an increased permeability; however, there is not a linear relationship between carbonation depth and total porosity, indicating that CO2 diffusion is not the only parameter controlling the carbonation of these materials. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The production of alkali-activated concretes has been a highly active area of academic and industrial research over the past decades, with many studies carried out in order to assess the feasibility of the commercialisation of this technology [1]. These materials differ from traditional Portland cement concretes through the use of a clinker-free binder matrix such as alkali-activated slag or geopolymer. The production of these binders is associated with low energy consumption and low CO2 emission, along with the potential to reach high mechanical strength at early ages of curing, high stability in aggressive environments, and resistance to elevated temperatures, among other potentially beneficial properties [2–9]. Despite the environmental and performance advantages identified as being offered by these systems, studies of alkali-activated binders based on metallurgical slags have often shown a very fast setting time and associated high susceptibility to drying shrinkage [10–12]. Several authors have also noted a higher susceptibility to carbonation compared with conventional Portland cements, as a consequence of the differences in the mechanism of degradation and the binder structures [13–16]. However, the performance of alkali-activated binders is strongly influenced by ⇑ Corresponding author at: Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia. Tel.: +61 3 8344 8755; fax: +61 3 8344 4153. E-mail address: [email protected] (S.A. Bernal). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2012.01.017

the characteristics of the precursors and the activation conditions used. To improve the performance of these binders, numerous recent studies have been focused on the production of mixes based on blends of reactive precursors. The blends usually involve a Ca-rich precursor such as granulated blast furnace slag (GBFS), and an aluminosilicate source such as metakaolin (MK) or low calcium fly ash, to promote the stable coexistence of calcium silicate hydrate (C–S–H) gels formed from the activation of the GBFS, and the geopolymer gel (also described as ‘‘N–A–S–H’’, by analogy with C–S–H) produced from the activation of the aluminosilicate. In some cases, small quantities of Portland clinker are also used as the Ca source in alkali-activated hybrid binder systems [9,17]. There are a relatively smaller number of reports describing the structure and performance of alkali-activated GBFS/MK blends, and these have mainly discussed cases where some GBFS is added to a metakaolin-based binder to enhance strength and microstructure [18–21]. It has been observed [18–22] that in alkali silicateactivated blends of MK with a small to moderate quantity of GBFS, the calcium is released from the slag and participates in the formation of C–S–H gels, as has also been observed in MK/Ca(OH)2 blends [23,24], rather than participating in geopolymer-type alkali aluminosilicate gels. Conversely, the inclusion of a small amount of MK in GBFS-rich alkali silicate-activated binders promotes enhanced workability and a structure mainly composed of coexisting alkali aluminosilicate and Al-substituted calcium silicate hydrate (C–S–H) gels. This

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stable gel coexistence, when it can be achieved, provides performance advantages in terms of permeability and durability, and promotes improved mechanical strength when compared with geopolymers solely based on MK [25,26]. Previous studies [16,27] have assessed the effects of activation conditions on the performance of silicate-activated slag-rich GBFS/MK blends, and revealed that the conventional conditions of formulation of alkali-activated slag binders, with relatively low activator content, do not provide sufficient alkalinity to promote the dissolution of MK to produce a highly stable coexisting geopolymer gel in these systems. Thus, metakaolin inclusion instead led to reduced compressive strengths, along with higher permeability and increased susceptibility to carbonation with the inclusion of higher contents of MK in the binders, when compared with reference alkali-activated slag specimens [16,27]. In order to achieve sufficient alkalinity to promote the dissolution and polycondensation of the MK included in the blended systems, it is also possible to specify formulations with higher alkali content, considering the selection of desired overall (activator + solid precursor) oxide molar ratios [20,25]. Under higher alkalinity conditions than those usually assessed in binders solely based on GBFS, the formation of stable aluminosilicate and calcium silicate-based reaction products (including some crystalline aluminosilicate zeolite products such as gismondine and garronite) is attained. These pastes show moderate setting times, high compressive strengths and improved carbonation performance, compared with reference alkali-activated slag systems [20]. Based on the improved properties identified in pastes and mortars based on activated GBFS/MK blends formulated under these activation conditions, the aim of this study is to assess the engineering properties of concretes produced with those binders. 2. Experimental program 2.1. Materials The primary raw material used in this study is a Colombian granulated blast furnace slag (GBFS) from the factory Acerías Paz del Río. The basicity coefficient (Kb = CaO + MgO/SiO2 + Al2O3) and the quality coefficient (CaO + MgO + Al2O3/ SiO2 + TiO2) based on the chemical composition (Table 1) are 1.01 and 1.88, respectively. Its specific gravity is 2900 kg/m3 and Blaine fineness is 399 m2/kg. The particle size range, determined through laser granulometry, is 0.1–74 lm, with a d50 of 15 lm. Metakaolin was generated in the laboratory by calcination of a Colombian kaolin containing minor quartz and dickite impurities [27]; calcination was carried out at 700 °C in air for 2 h. The particle size range of the MK is 1.8–100 lm, with a d50 of 12.2 lm and 10% of particles finer than 4 lm. Alkaline activating solutions are formulated by blending a commercial sodium silicate solution with 32.4 wt.% SiO2, 13.5 wt.% Na2O and 54.1 wt.% H2O, and 50 wt.% NaOH solution, to reach the desired overall molar ratios. Crushed gravel and river sand are used as coarse and fine aggregates in the manufacture of concrete. The coarse aggregate was of 19 mm maximum size, with a specific gravity of 2790 kg/m3 and absorption of 1.23%. The specific gravity, absorption, and fineness modulus of the sand are 2450 kg/m3, 3.75% and 2.57, respectively. 2.2. Concrete mixes The concretes are produced with total binder (GBFS + MK) contents of 400 kg/ m3. This amount of binder has been identified as being optimal for a range of alkali-activated slag concretes, where lower binder content gives poor strength, but higher binder content can lead to microcracking [28]. The mixes are formulated

Table 1 Oxide composition of the GBFS and MK used, from X-ray fluorescence analysis. LOI is loss on ignition at 1000 °C. Precursor

GBFS MK

Component (mass% as oxide) SiO2

Al2O3

CaO

Fe2O3

MgO

TiO2

Other

LOI

32.29 50.72

16.25 44.63

42.45 2.69

2.35 –

2.87 –

0.50 –

1.38 0.94

1.91 1.02

with overall (activator + solid precursor) SiO2/Al2O3 (S/A) molar ratios of 3.6, 4.0 and 4.4, GBFS/(GBFS + MK) ratios of 0.8, 0.9 and 1.0 (i.e. 20%, 10% and 0% MK respectively), and a constant Na2O/SiO2 molar ratio of 0.25. It is noted that the overall molar ratios do not account for the strong probability of incomplete reaction of the solid precursors, and thus should not be taken as being representative of the molar composition of the binder gel; the ratios are specified for mix design purposes. Table 2 presents the concentrations of the activator, expressed as wt.% Na2O relative to the mass of GBFS + MK, compared to the overall molar ratios. These activation conditions have been identified as enabling alkali silicate-activated GBFS/MK blends to reach high compressive strengths while retaining acceptable paste workability [25]. The activator contents are quite high relative to those which are often assessed in alkali-activated slag systems, but this was found to aid in microstructural development and mechanical performance in the systems studied here. Detailed mix design information for the concretes is given in Table 3. A total water/binder ratio, defined as (free water + water in activators)/(GBFS + MK + anhydrous activator), of 0.47 and a constant GBFS + MK content are used in all mixes. This means that the content of aggregates varies slightly between mixes, which may affect mechanical performance slightly, but is not expected to be a dominant effect, based on preliminary studies in these systems. The concrete specimens are cured at a relative humidity of 90% at 25 ± 5 °C to prevent drying effects or leaching of the alkaline activator. No signs of efflorescence are observed in the cured samples or during any of the tests, which indicates that the microstructure of the samples is sufficiently densified to prevent migration of alkalis to the sample surfaces during curing or testing. 2.3. Tests conducted on concretes Concrete samples are tested to determine compressive strength following the standard procedure ASTM C39/C39M-09a (Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens). Flexural strength is assessed in prismatic samples with dimensions 0.076  0.076  0.28 m, in accordance with the standard procedure ASTM C 293-08 (Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Centre-Point Loading)). These properties are determined after 7 (compressive only), 28, 90 and 180 days of curing. Triplicate samples are tested at each age. Total porosity and absorption are calculated according to the standard procedure ASTM C642-06 (Standard Test Method for Density, Absorption, and Voids in Hardened Concrete), after 28 and 90 days of curing. According to this test method, specimens are dried at 100 °C for 24 h, then submerged in water for 48 h to determine the saturated mass after immersion; then the specimens are boiled at 100 °C for 5 h and the saturated mass after boiling and immersed apparent mass are obtained. These parameters are used to calculate total pore volume and water absorption properties. Capillary sorptivity is also assessed by applying the standard procedure EMPA–SIA 162/1 [29], in which water is allowed to pass into a dried sample through a more gradual process of capillary suction, and the mass of the sample is monitored as a function of time. A measure of the resistance to chloride ion penetration is obtained by testing in accordance with the standard procedure ASTM C1202-05 (Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration) using a PROOVE’it instrument supplied by Germann Instruments. Duplicated samples are tested at each age. The chloride diffusivity coefficient is also measured using the technique described by Mejía et al. [30]. This test method uses saturated concrete disks (immersed for 2 days in a saturated Ca(OH)2 solution) of 1 cm thickness and / = 7.62 cm placed in the centre of a migration testing cell between two solutions: 3.5% NaCl and Ca(OH)2, applying a constant voltage of 12 V for 45 days. The steadystate chloride diffusion coefficient (Deff) is calculated by

Deff ¼

JRTL zFC DE

ðm2 =sÞ

ð1Þ 4

where T is temperature (298 K); F is the Faraday constant (9.6485  10 C/mol); R is the ideal gas constant (8.314 J/(mol K)); DE is the voltage difference (12 V); z is the charge of the chloride ion (1); C is the Cl concentration in the cell containing 3.5% NaCl (512 mol/m3); L is the sample thickness (0.01 m); and J is the steady-state chloride flux, determined from the slope of the first linear region of a plot of the chloride concentration in the downstream cell as a function of time. This test was conducted on samples after 28 days of curing. An accelerated carbonation testing chamber system is used to induce the carbonation of concretes after 28 days of curing, using a CO2 concentration of 3.0 ± 0.2%, a temperature of 20 ± 2 °C, and RH = 65 ± 5%. To perform the carbonation measurements, the cylindrical specimens (/ = 7.62 cm) are removed from the chamber after 250, 500, 750 and 1000 h of exposure. The depth of carbonation is measured by treating the surface of a freshly cleaved specimen with a 1% solution of phenolphthalein in alcohol. According to this test, in the uncarbonated part of the specimen where the concrete is still highly alkaline, purple-red colouration is obtained, while no colouration is observed in the carbonated region. Each reported result represents the average depth of carbonation measured at eight points, using two replicate samples (four points per sample). The properties of uncarbonated samples after 28 days of curing are used as reference values, indicated as 0 h of exposure to CO2. For concretes subjected to the different times of CO2 exposure, the compressive strength and total pore volume are determined as described above.

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S.A. Bernal et al. / Construction and Building Materials 33 (2012) 99–108 Table 2 Correspondence between activation conditions and the overall oxide ratios (precursor + activator) selected for the formulation of the concretes assessed. GBFS/ (GBFS + MK)

Overall SiO2/ Al2O3 ratio

Activator concentration (wt.% Na2O relative to GBFS + MK)

1.0

3.6 4.0 4.4 3.6 4.0 4.4 3.6 4.0 4.4

9.1 9.9 10.6 10.5 11.6 12.5 12 13 14.5

0.9

0.8

3. Results and discussion 3.1. Compressive strength Concretes based on GBFS alone (Fig. 1a) show an increase in compressive strength when formulated with higher overall S/A ratios, corresponding to higher activator concentrations. This effect is more notable in samples formulated with S/A ratios of 4.0 and 4.4, which exhibit 7-day strengths up to twice as high as the specimens formulated with S/A 3.6. The GBFS-only concretes achieved the highest compressive strengths when formulated with an S/A ratio of 4.4, reporting 40 MPa at 7 days, and then a further 83% increase in strength from this time to 180 days. Differences in 180-day strength as a function of activation conditions are more marked (up to 40 MPa difference between the different S/A ratios) than the differences at early age, which is consistent with results reported for pastes and mortars prepared with similar binders [20]. In the mixes studied here, increasing metakaolin content while

maintaining a constant overall S/A ratio requires the use of an increased activator modulus (visible as the ratio of Na silicate to NaOH solutions in the mix designs in Table 1), because the metakaolin is more Al-rich than the slag, and so more Si must be supplied by the solution to compensate for this. To achieve this while holding the overall N/A ratio constant, a higher Na2O content (activator concentration) is used. The good mechanical performance of appropriately formulated alkali-activated slag concretes [4,6,9] is attributed to the strong load-bearing gel structure developed in the binder regions. The early-age reactions in these systems are fundamentally based around alkali-mediated dissolution and precipitation mechanisms [31–34]. Increased activator concentrations (linked to higher S/A ratios in the way the samples are formulated here) provide sufficient alkalinity to the system to generate and maintain a high pH, and favour the dissolution of silica and alumina from the slag [35]. These dissolved species, together with calcium (which is not particularly soluble at elevated pH, but which is released along with the silicate and aluminate species as the slag particles are attacked by the alkali) then begin to precipitate and act as nucleation centres, promoting the formation of calcium silicate hydrate gel [36,37]. Under the activation conditions assessed in this study, the C–S–H structures formed display Ca/Si ratios lower than conventional cements, with a gel structure which is disordered, but appears comparable to the crystalline riversideite (9 Å tobermorite) form of C–S–H according to X-ray diffraction [20]. The C–S–H in alkali-activated slag binders also tends to be enriched in Al compared to the products which are observed in hydrated Portland cement (thus being termed C–A–S–H), due to the higher content of Al in slag than cement and the reduced tendency towards calcium aluminate hydrate formation at the lower Ca concentrations prevailing in alkali-activated systems [38,39]. C–A–S–H which is in

Table 3 Mix designs of the concretes. All quantities are in kg/m3 of fresh concrete. Component

Mixes specified according to overall S/A ratio GBFS only

GBFS MK Sodium silicate solution NaOH (50 wt.% solution) Coarse aggregate Sand Free water

10% MK

20% MK

3.6

4.0

4.4

3.6

4.0

4.4

3.6

4.0

4.4

400 – 28 84 860 860 156

400 – 76 76 844 844 144

400 – 120 68 832 832 132

360 40 80 80 836 836 140

360 40 136 72 816 816 124

360 40 188 64 804 804 112

320 80 132 78 808 808 124

320 80 192 68 792 792 108

320 80 256 60 768 768 88

Fig. 1. Compressive strengths of concretes based on silicate-activated GBFS/MK blends, formulated with GBFS/(GBFS + MK) ratios of: A – 1.0, B – 0.9 and C – 0.8.

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contact with alkaline activating solutions also tends to show significant levels of alkali uptake [16,40], but due to the high concentrations used, the majority of the alkalis remain in the pore solution in the absence of an added aluminosilicate component [41]. Fig. 1b shows that concretes formulated with a GBFS/ (GBFS + MK) ratio of 0.9 exhibit higher compressive strengths at early ages than concretes prepared without MK, reporting strengths exceeding 40 MPa after 7 days of curing, under all the activation conditions tested. Increased S/A ratios generate a further increment of strength of up to 44% from 7 to 28 days of curing; however, only a slight increment in compressive strength is identified from 28 to 180 days, similar to the concretes based solely on GBFS in Fig. 1a. This indicates that under the alkalinity conditions assessed, MK promotes higher early strength development by reacting relatively rapidly under the alkaline conditions prevailing soon after mixing, but is consumed and shows little influence after this time. This is in concordance with the performance of pastes developed using the same binder mix design as studied here, which obtain up to 95% of their final strength at 28 days of curing [20]. The inclusion of 20 wt.% MK (Fig. 1c) also leads to an increase in strength development at early ages compared to concretes without MK (Fig. 1a), but a slight reduction from the 10% MK samples. However, strength development continues for a longer period of time at this high MK content, particularly for the sample with the lowest S/A ratio (3.6), which also contains less Na2O as the Na2O/SiO2 ratio is held constant across all samples. The strength development of concretes based on the activated GBFS/MK blends here differs from previous observations related to blended binders activated under the conventional activation conditions used for alkali-activated slag [16], where the activator concentration was in general lower. In those samples, which were formulated with constant activator modulus and activator concentration as MK was added to replace GBFS in the mix, an increased MK content caused significant reductions in compressive strength,

as a consequence of the incomplete reaction of the MK incorporated into the system. The activation conditions applied here, where the concentration of activation is high and increases in each sample set with increasing MK addition, favour the reaction of MK without hindering the dissolution of Ca2+. The dissolution of the Si and Al species from the MK is promoted by the higher alkalinity at high activator concentrations, but in doing so, the alkalinity is reduced through the deprotonation of hydrated silica (silicic acid) molecules and the consumption of Na by the formation of aluminosilicate reaction products, and so the release of Ca2+ from the slag becomes less restricted. 3.2. Flexural strength The flexural load–deflection curves for concretes based on GBFS show (Fig. 2a) that these materials exhibit improved toughness (related to the area underneath the load–deflection curve) when formulated with increased S/A ratios. Concretes formulated with 10% replacement of GBFS by MK (Fig. 2b) show only slight differences in maximum load at different S/A ratios, and the values reported for all three samples are comparable to the highest obtained in specimens based solely on GBFS. Increasing the content of MK in the binder to 20% (Fig. 2c) does reduce the maximum load and toughness at the higher S/A ratios (4.0 and 4.4), but concretes formulated with an S/A ratio of 3.6 do not exhibit variations in the maximum load when compared with specimens prepared with similar formulation conditions but lower contents of MK. Based on the flexural testing curves, the flexural strength has been calculated via the modulus of rupture (MOR). Concretes based solely on GBFS show (Fig. 3a) an increase in MOR with higher S/A ratios; after 28 days of curing, the MOR is 73% higher at an S/A ratio of 4.4 compared to concretes formulated with an S/A ratio of 3.6. These results are consistent with the reduced toughness and compressive strength observed in these samples. With longer curing

Fig. 2. Representative flexural load–deflection curves of activated concretes after 28 days of curing, with GBFS/(GBFS + MK) ratios of: A – 1.0; B – 0.9; and C – 0.8.

S.A. Bernal et al. / Construction and Building Materials 33 (2012) 99–108

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Fig. 3. Modulus of rupture of activated concretes formulated with GBFS/(GBFS + MK) ratios of: A – 1.0, B – 0.9 and C – 0.8, as a function of the curing duration.

durations, a similar trend in the MOR at higher S/A ratio is identified; however, differences between formulation conditions in the MOR are less remarkable. The inclusion of 10% of MK in the concretes leads to an increment in the MOR at early ages of curing, independent of the activation conditions, which is in good agreement with the higher compressive strengths identified for these concretes compared to those without MK. The specimens with S/A ratios of 4.0 and 3.6 show only slight increments in MOR at extended times of curing; however, concretes with S/A = 4.4 have a clear trend of increasing MOR with longer periods of curing. These trends differ notably from the trends in compressive strength (Fig. 1b), where there is little change in compressive performance after 28 days for any of the specimens, and the S/A 4.4 specimens showed lower strengths at each age than the S/A 4.0 samples. An increased proportion of MK (20 wt.% replacement of GBFS) leads to a slight reduction in MOR at early ages compared with concretes with 10% MK, with MOR values similar to those of the GBFS-only samples formulated with S/A ratios of 3.6 and 4.0. It is interesting to identify that the compressive strengths of the 20% MK samples are, with the exception of the S/A 4.4 mix, similar to those of the 10% MK samples, and notably higher than the GBFS-only samples. This may be related to the elevated activator

concentration used in these specimens, which is expected to influence the relative rates of formation of Ca-rich and aluminosilicaterich reaction products, and given that these contribute in different ways to the early development of compressive and flexural strength. At more advanced ages, the concretes with 20% MK show a remarkable increment in the MOR; these mixes reach the highest MOR values of all the mixes studied, although their compressive strength development is equivalent to the performance of the mixes with either 0% or 10% MK at the same S/A ratio. It has previously been observed that alkali-activated concretes show higher flexural strengths than those based on Portland cement, when the compressive strengths are comparable [4,42], and the values reported here are certainly higher than those identified in standards such as ACI 318-08 as being representative of Portland cement-based concretes. A plot of flexural against compressive performance, with samples tested at different ages shown as distinct symbols, is presented in Fig. 4. The relationship between flexural and compressive strength of the concretes assessed is higher than the recommendation of ACI 318-08, and also the relationship recently proposed by Díaz-Loya et al. for alkali-activated fly ash concretes [43]. This suggests that adopting design criteria used for OPC-based concretes when using alkali-activated GBFS/MK concrete leads to a conservative design, promoting higher flexural strengths than expected for conventional concretes, for the same compressive strength. Similar results have been reported in concretes based solely on alkali-activated slags [44] and fly ash/slag blends [42], which is attributed to the increased tensile strength of alkali-activated binders associated with a highly dense interfacial transition zone between matrix and aggregates when compared with conventional cements. In this case, this performance can be also associated with the increased ductility reported for activated blended concretes, which might be associated with the development of a distinct microstructure compared to binders based on alkali-activated aluminosilicates, promoting increased flexural strength and modifying the flexural–compressive strength relationship. 3.3. Water absorption properties

Fig. 4. Relationship between flexural and compressive performance of the samples tested at different ages, with the relationships specified for Portland cement concrete (ACI 318-08) and for alkali-activated fly ash concretes (AAFA) [42] plotted for comparison. A curve describing the relationship between compressive and flexural strength of the 28-day alkali-activated GBFS/MK (AAS) samples according to a 0.5 power law (MOR28d = 1.02r0:5 c;28d ) is also provided.

3.3.1. Absorption and total porosity Concretes formulated with an S/A ratio of 4.4 (Fig. 5A and B) exhibit the lowest pore volume of the samples studied, independent of the content of MK in the binder. A reduction in the activator concentration, associated with lower S/A ratios, leads to an increment in the porosity of the specimens. This trend is more significant in concretes based solely on slag [45]. The concretes including MK report a pore volume notably lower than that which is identified in slag-based concretes formulated with similar S/A

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Fig. 5. Total porosity (A and B) and water absorption in the first 48 h (C and D) of alkali-activated GBFS/MK blended concretes with 28 (A and C) and 90 days (B and D) of curing.

ratios. At 28 days, the porosity is lower when 10% MK is included in the S/A 4.0 and 4.4 binders than with 20% MK, although this is reversed at greater ages. Curing for 90 days reduces the porosity of all mixes compared with the 28-day data, although the effect is only slight in the S/A 4.4 sample set. Concretes with this high S/A ratio also show the smallest decrease in porosity with the inclusion of MK, at both 28 and 90 days. Considering that at higher contents of MK the activator content is also increasing (Table 2), it is likely that the dissolution of MK and the consequent formation of stable spacefilling reaction products will be favoured in these systems. It is also interesting to note that the ‘‘N–A–S–H’’ type gels formed through the reaction of low-calcium alkali aluminosilicate systems are lower in space-filling character than the C–A–S–H gels formed through the alkali activation of GBFS [46], and so the observed reduction in porosity suggests that the reaction of MK is contributing to the development of both types of gel in these systems.

The total water absorption into concretes within a 48-h period (Fig. 5C and D) provides additional information regarding the connectivity of the pore network. The lowest absorption values at 28 days are observed for concretes formulated with S/A 4.4, and decreased S/A ratios lead in general to significant increments in absorption. This trend is stronger at 28 days than at 90, at which age the metakaolin-containing S/A 4.0 samples show the lowest absorption. This suggests that there are competing effects related to gel structure development (densification) and composition (Ca and Al content) influencing the observed absorption in different ways as a function of sample maturity. When the S/A ratio is reduced, a decrease in absorption is identified in concretes with higher contents of MK, which is consistent with the lower porosity exhibited by these concretes. Longer periods of curing (90 days) lead particularly to lower absorption in the concrete formulated with the lowest S/A ratio (3.6). This is attributed to the ongoing refinement of the porosity in these samples, which contain less

Fig. 6. Capillary sorptivity of 90-day cured concretes based on activated GBFS/MK blend with GBFS/(GBFS + MK) ratios of: A – 1.0; B – 0.9; and C – 0.8.

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alkali than the others studied, and so will require more time to achieve comparable microstructural development. These samples showed quite high porosity, attributable to a less-mature binder with more connected pores, at 28 days. This difference between the samples at 28 and 90 days is much more notable in these absorption data than in the strength or total porosity data, indicating the value of an analytical method which is sensitive to pore connectivity in analysing microstructural development as a function of time in these complex systems. The permeability of concrete plays an important role in determining the ingress of various aggressive ions from the environment, and their movement through the material. It has been reported [47] that absorption values of 3% and total porosity values of 10% indicate concretes with good durability. This suggests that the concretes assessed in this study have the potential to be highly durable; however, broader analysis is necessary to more fully determine the performance of these concretes. 3.3.2. Capillary sorptivity The capillary sorptivity curves of concretes after 90 days of curing are shown in Fig. 6. Total water penetration is lowest in the samples with 10% MK, and the concrete with no MK and S/A 3.6 has the highest water absorption of all samples tested. There is not a strong difference between the final water uptake extents after 1200 h within the sample sets with 10% MK or 20% MK, although the rate of uptake differs markedly between the samples in each set. In particular, concretes formulated with an S/A ratio of 4.4 show slower water absorption when solely based on slag and including 20% MK, while the S/A 4.0 samples containing 10% or 20% MK show the most rapid initial water uptake. This indicates that, although a relatively low volume of permeable pores is reported for the MK-containing S/A 4.0 concretes compared to those with S/A 3.6, it is likely that the capillary pore network is either more connected or less tortuous than the pore networks developed in the other mixes. It is also notable that the standard test duration, 1200 h, was not sufficient to reach saturation in the majority of the samples assessed at 90 days of age. This shows that there is a high degree of refinement and tortuosity in the pore networks, as the capillary action in these materials is present, but slow. This is more notable at 90 days of age than at 28 days, which is consistent with tomography data showing a notable increase in pore network tortuosity as a function of curing time in alkali silicate-activated slag binder systems [46]. The kinetics of the capillary sorption of water into concrete can be described by the capillary absorption coefficient (k; derived from the initial slopes of the sorptivity plots in Fig. 6). The values of this parameter, as determined from capillary sorption curves at 28 and 90 days, are shown in Table 4. The parameter k is related to total capillary porosity, being derived from the kinetics of water sorption in the time leading up to the saturation of the capillary pore network. The results in Table 4 show that after 90 days of curing, there is a substantial reduction in the k values of all samples assessed (by up to 97% relative to 28 days), and that this parameter is lower in concretes formulated with a S/A ratio of 3.6 and 4.4, and a GBFS/(GBFS + MK) ratio of 0.9. This is consistent with the reduced porosity identified in concretes with this metakaolin content (Fig. 5A and B). It is also seen in Table 4 that, for the mixes with 0% or 20% MK, increasing S/A ratio in the 90-day samples gives a decrease in k, which is related to the increased activator content of these binders, as was the case for the total water absorption and total porosity of these samples. The fact that the trend is not as clear at 28 days, or for the 10% MK samples, again brings reference to the interaction and competition between the effects related to gel maturity and to gel composition. To fully deconvolute these effects, more de-

Table 4 Capillary absorption parameters of activated GBFS/MK concretes as function of curing duration. S/A ratio

GBFS/(GBFS + MK)

3.6 4.0 4.4 3.6 4.0 4.4 3.6 4.0 4.4

1.0

0.9

0.8

k (kg/m2 s1/2) 28 days

90 days

0.016 0.005 0.029 0.011 0.023 0.011 0.031 0.022 0.019

0.003 0.002 0.001 0.001 0.009 0.003 0.003 0.002 0.001

tailed analysis of the binder chemistry, including the interaction with aggregate surfaces (in particular the presence or absence of interfacial transition zones), and the evolution of these features as a function of time, will certainly be necessary.

3.4. Resistance to chloride penetration 3.4.1. Rapid chloride permeability test A measure of the permeability of the concretes is able to be obtained according to the charge passed during the rapid chloride permeability test (ASTM C1202), and the values of total transferred charge for 28-day and 90-day cured samples are shown in Fig. 7. Independent of the activation conditions and the content of MK, all concretes tested display a total charge passed value of between 1000 and 2000 coulombs, which is defined according to the classifications of the testing method as corresponding to a low permeability to chlorides. After 28 days of curing (Fig. 7A), concretes based solely on alkali-activated GBFS show a higher charge passed, representing (under the assumptions of the test) higher permeability to chlorides, when formulated with higher activator concentrations. However, this can more realistically be attributed to the diffusion of Na+ ions through the specimens at the increased pore solution alkalinity which is associated with higher S/A ratios [48]. A similar trend is observable in each of the data sets presented; considering the uncertainties inherent in the test method (variability exceeding 12% for a single operator according to [49]), all data sets obtained at 28 and 90 days (Fig. 7A and B) are consistent with this trend. The inclusion of 10% MK in the binder induces a reduction of the total charge passed through 28-day cured specimens by as much as 40% when concretes are formulated with an S/A ratio of 3.6, which is consistent with the lower sorptivity identified in these materials. Further increasing the content of MK in the binders to 20 wt.% mainly influences the mixes with higher S/A ratios, reducing the charge passed by as much as 30% compared with the specimens solely based on activated GBFS. This effect can again be attributed to the reduced capillarity identified in these specimens, and may also be related to the microstructural characteristics of these binders formed by a sodium calcium (alumino)silicate hydrate (N–C–(A)– S–H) type of gel [16,20], reducing the concentration of free Na+ present in the pore solution, and thus also the charge passed. Any Na+ present in the pore solution will counter-diffuse as the Cl ions are electrically driven through the pore network by the imposed electrical field gradient, leading to an increase in the charge passed [28]. Extended times of curing (90 days) give a slight reduction in the charge passed by each concrete mix. This is to some extent consistent with the reduced permeability and sorptivity identified in these specimens at longer times of curing, but the fact that the decrease in charge passed at higher age is much less marked than the decrease in porosity or capillary sorptivity (Fig. 5, Table 4)

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Fig. 7. Rapid chloride permeability test results for (A) 28-day and (B) 90-day cured activated GBFS/MK concretes as function of the GBFS/(GBFS + MK) ratio.

again highlights the dominant role of pore solution chemistry, rather than microstructure, in determining the results of this accelerated test [50,51]. The RCPT values obtained for the concretes assessed in this study are slightly higher than those previously identified in alkali-activated slag concretes with lower activator concentrations [28], despite the reduced capillary sorptivity reported by the specimens studied here compared to those lower-alkali systems. This is also associated with the differences in Na+ concentration in the pore solution rather than being directly related to permeability. The incorporation of MK in the binder does not seem to generate significant variations in charge passed, compared to the data obtained for concretes with 28 days of curing. This is likely to be related to the ongoing formation of aluminosilicate-type gels in these systems leading to binding of some of the extra alkalis which are added (Table 2); these alkalis were more mobile at 28 than 90 days. 3.4.2. Chloride diffusion coefficient Considering the controversies associated with the application of the ASTM C 1202 and the possibly limited applicability of the results obtained when this test is applied to alkali-activated materials, it is expected that using a test method where a lower voltage is used, more ‘realistic’ results regarding the chloride diffusion can be obtained. The direct measurement of chloride penetration, rather than indirect measurement via charge passed, is also desirable. Thus, the test described by Mejía et al. [30] has been utilised in this study. Fig. 8 shows the variation of chloride concentration in the Ca(OH)2 solution in the downstream diffusion cell, as a function of the testing time. It can be seen in Fig. 8 that the highest

permeability to chlorides is reported for concretes solely based on GBFS and with an S/A ratio of 4.4. The inclusion of MK leads in all cases to reduced chloride permeability, especially when concretes are formulated with a lower concentration of activation. This could be attributed to the combined effect of increased diffusion of ionic species as the alkalinity of the pore solution increases, even when lower voltage is applied, and the modified structure formed in those binders. The results do suggest that the accelerated chloride permeability test using a reduced voltage is likely to have some similar limitations in application to alkali-activated materials when compared with the RCPT method, as the migration of ionic species is strongly accelerated and some mechanisms of transport occurring under natural exposure are not well reproduced. However, the direct measurement of chloride flux, rather than the use of charge passed as a proxy for diffusion, is believed to be advantageous. The chloride diffusion coefficients calculated for the concretes assessed are summarised in Table 5. These results are consistent with the variations of k values determined from the sorptivity test in specimens with 28 days of curing (Table 4). The lowest diffusion coefficient is identified in concretes formulated with S/A 3.6 and 20% MK in the binder, suggesting that the inclusion of MK as a secondary aluminosilicate precursor is contributing to the refinement of the pore network and consequently reducing both the sorptivity of the specimens and the ingress of aggressive agents into the material. It is important to note that the chloride diffusion coefficients determined for the concretes assessed are substantially lower than those reported for high performance Portland cement-based concretes including supplementary cementitious materials [30,52,53], which is consistent with the trends observed from the ASTM C1202 test. 3.5. Accelerated carbonation testing There is little existing knowledge about the mechanisms driving the carbonation process in alkali-activated binders; however, a strong dependence on the activator used [14] and activation conditions [15,16], along with concrete design parameters [28], has been identified. Table 5 Chloride diffusion coefficients of selected alkali-activated slag/metakaolin blended concretes. Concrete S/A ratio

GBFS/ (MK + GBFS) ratio

Specimen area (m2)

Chloride flux (mol/m2 s)

Deff (m2/s)

3.6

0.9 0.8 0.9 0.8 1.0 0.8

0.0025 0.0026 0.0025 0.0026 0.0024 0.0025

1.94  107 3.09  107 1.36  106 3.46  106 1.18  105 1.61  106

0.19  1013 0.14  1013 0.37  1013 0.96  1013 4.26  1013 0.88  1013

4.0 Fig. 8. Chloride concentration variation in the downstream cell as a function of testing time of alkali-activated slag/metakaolin blended concretes.

4.4

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107

Fig. 9. Relationship between the carbonation depth and compressive strength (A) and post-carbonation total pore volume (B) of alkali-activated concretes, as a function of the content of MK incorporated in the binder. Samples were selected from those described in Table 1 to hold the Na2O content as close to constant as possible; the 0% MK sample has S/A 4.4, 10% MK has S/A 4.0 and 20% MK has S/A 3.6.

Fig. 9 presents the correlation between the extent of carbonation (expressed as the percentage of the cross-section area, and thus volume, carbonated in 76-mm diameter concrete cylinders) and macroscopic engineering properties: compressive strength (Fig. 9A) and ASTM C642-08 total pore volume (Fig. 9B) of selected concretes formulated with similar concentrations of activation according to Table 1. Compressive strength decreases monotonically as carbonation proceeds, and this decrease appears to proceed roughly linearly. This indicates that the residual strength appears to depend predominantly on the remaining uncarbonated binder regions. The carbonated binder regions contribute some strength, although this strength (which may be estimated to some extent by extrapolating the trend to 100% carbonation) appears to be much lower in the samples with 20% MK than in the other systems. The 0% and 10% MK samples follow approximately the same trend as a function of carbonation depth, and the post-carbonation strength of these binders may be estimated to be around 20 MPa by extrapolation. The very high extent of strength loss in the 20% MK system would thus be concerning in terms of the application of these materials in a practical sense, if the process of structural damage in these binders due to accelerated carbonation testing is assumed to replicate that which is observed under in-service exposure conditions. However, in contrast to this, the relationship between pore volume and carbonation extent is much more similar between the 10% and 20% MK samples (considering the difference in initial porosity) than in the GBFS-only sample. This shows that porosity is not the only parameter controlling the strength loss of the carbonated binders. There must be a convoluting effect due to the binder gel chemistry, which determines the residual levels of strength after accelerated carbonation. This is probably a gel maturity effect, as observed in the strength, water absorption and total porosity tests when comparing the 28-day and 90-day samples for each MK content. The duration of carbonation exposure for the most highly carbonated samples was 1000 h, which is more than 40 days, meaning that the undamaged core of the samples is effectively continuing to cure during this extra period of time. Combined with the faster carbonation, which halts the gel development reaction process as the carbonation front passes into the sample, this provides an explanation for the greater extent of strength loss in the 20% MK samples than in the others tested under accelerated carbonation conditions.

as a function of curing duration and mix design parameters (expressed as overall oxide molar ratios). The concretes assessed have high alkali activator content, which leads to satisfactory early strength development and in most cases high (>50 MPa) final compressive strengths after 90–180 days. Flexural strength at later age is enhanced by the incorporation of metakaolin into the binder mix, and the flexural strengths of all samples fall well above the values predicted by the ACI 318 standard correlation for flexural strength as a function of compressive strength. Almost all durability parameters tested fall within the ranges characteristic of highly durable concretes, assuming that the correlations developed for Portland cement concretes also hold for alkali-activated concretes. Water absorption and accessible pore volume measurements show densification and pore refinement as during the evolution of gel structure with time, although this is less notable at higher MK content. The results of the rapid chloride penetration test are dominated by pore solution chemistry, and show almost no change from 28 to 90 days of curing, contrasting other measures of the pore system. Accelerated carbonation testing shows rather rapid carbonation accompanied by a loss in strength, and this occurs faster at higher metakaolin content, but the full interpretation of the test data also requires consideration of the evolution of the sample structures during the long duration of the tests. The data presented here will be valuable in the design and development of high-performance, durable concretes based on alkali-activated blended binders, and in the selection and application of appropriate testing methodologies for these materials. Acknowledgements This study was sponsored by Universidad del Valle (Colombia), the Center of Excellence of Novel Materials (CENM) and Patrimonio Autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación ‘‘Francisco José de Caldas’’ Contrato RC – No. 275-2011. The participation of JLP was funded by the Australian Research Council (ARC), including partial funding through the Particulate Fluids Processing Centre, a Special Research Centre of the ARC. The authors would like to express their gratitude to Carolina Perea for her support in the data collection for the chloride diffusion coefficient test. References

4. Conclusions Several engineering and durability-related properties of concretes based on alkali silicate-activated blends of granulated blast furnace slag and metakaolin have been assessed, and compared

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