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Effects of calcareous fly ash in blended cements on chloride ions migration and strength of air entrained concrete
Construction and Building Materials 126 (2016) 1044–1053
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Effects of calcareous fly ash in blended cements on chloride ions migration and strength of air entrained concrete Mariusz Da˛browski ⇑, Michał A. Glinicki, Karolina Gibas, Daria Józ´wiak-Niedz´wiedzka ´ skiego 5B, 02-106 Warszawa, Poland Institute of Fundamental Technological Research Polish Academy of Sciences, Pawin
h i g h l i g h t s Calcareous fly ash was used as a major constituent of binary and ternary cements. Air void stability in air entrained concrete was investigated by image analysis. Air voids characteristics were stable and adequate for frost resistance. Moderate increase of compressive strength and no change of elastic modulus was observed. A relationship between the chloride migration and the pore size distribution was found.
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Article history: Received 15 April 2016 Received in revised form 14 July 2016 Accepted 27 August 2016
Keywords: Air entrained concrete Air void characteristics Calcareous fly ash Chloride ion migration Multicomponent cements Pore size distribution Strength
a b s t r a c t The influence of blended cements containing calcareous fly ash (W) on the strength and permeability of air entrained concrete was studied. Moderately active calcareous fly ash of CaO content of 26% and loss on ignition of 2%, obtained from lignite fired power station in Poland, was selected. Ternary cement compositions including siliceous fly ash (V) and ground granulated blast furnace slag (S) were also studied. Air void analysis in hardened concrete was used to evaluate the effectiveness of air entraining process. Pore size distribution was characterized by mercury intrusion porosimetry (MIP). The presence of calcareous fly ash in cement resulted in a moderate increase in the compressive strength of concrete and an increase of the category of resistance to chloride ion migration. Test results revealed a linear relationship between the capillary porosity measured by MIP and the chloride migration coefficient (Dnssm). The permeability was lower for increased clinker replacement level which was due to formation of smaller diameters of the capillary pores. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Civil engineering structures in cold climates must exhibit adequate durability for frequent freezing and thawing in winter. Hence a well known solution to achieve the adequate frost resistance is air-entrainment of concrete mixture. This process is however very sensitive for changes of physical and chemical properties of cement, particularly with mineral additions [1]. Massive parts of structures require use of low heat binders which are readily obtained by application of mineral addition to cement or concrete mix. In such a case frequently used mineral addition is siliceous fly ash, which properties are restrictively regulated by EN 206 and EN 450-1 standards. Another type of fly ash with significant content of calcium compounds is in quite limited use in Europe as cement or ⇑ Corresponding author. E-mail address: [email protected] (M. Da˛browski). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.115 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
concrete addition. There are several reasons, one of them being a relatively large variability of physical properties and chemical composition of calcareous fly ash (W). The above-mentioned European standards do not cover the use of fly ash of increased content of calcium. The US practice is different. Following a common acceptance and use of specifications based on ASTM C618, so called high calcium fly ash is readily applied as an addition to concrete mix. The issue of variability of properties of calcareous fly ash is brought forward particularly in the case of air entrained concrete designed for high freeze-thaw durability. This is because the air entrainment process is highly sensitive to fineness and unburned carbon content of fly ash. The concept of increased homogeneity by factory grinding of cement with W fly ash and other mineral addition is thought to be a better option than adding directly to concrete mixture [2]. The influence of calcareous fly ash on cement properties was investigated mainly in Greece and Turkey [3,4]. Obtained results
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were strongly related to chemical composition and mineralogical compounds of W ash. The study by Antiohos at al. [5–7] showed influence of increase active silica content to a higher degree of hydration of ingredients from cement and fly ash. This is revealed by an increase of compressive strength after prolonged curing time >28 days. The use of W fly ash in cement matrix (up to 43% replacement) had a positive influence on concrete impermeability in chloride ions environment, especially after extended period of curing time of specimens [8–10]. The review presented by Bentur [11] showed the effectiveness of mineral additives in respect to chloride ion penetration in concrete. In comparison to other mineral additions W fly ash can be perform better than V fly ash and natural pozzolan. To justify it a key issue is a better understanding of ion transport mechanism in concrete with calcareous fly ash. Many researchers consider the transport of aggressive media in concrete as mainly assigned to porosity of interfacial transition zone (ITZ) [12] and presence of microcracks [13]. Changes of cement composition [14] and aggregate type [15], however, have a greater influence on the ITZ than microcracks. Diamond at all [16] found that permeability of cement matrix alone had more significant impact on the ion ingress than the porosity of ITZ. The same conclusion was presented in a study of Leemann at all [17], in which the chloride migration coefficient was compared with porosity of ITZ for concrete made with blended cements. The most common method to determine the pore size distribution of cement matrix is the mercury intrusion porosimetry (MIP). Despite certain doubts related to interpretation of the MIP method is a very useful for porosity comparison [18]. Based on results obtained by this method it was shown that the biggest capillary pores from range 0.05 lm to 1 lm are responsible for ions transport properties in cement matrix [19]. However Wong at all [20] concluded, that the mechanism of moisture and gas transport in air-entrained concrete is strongly influenced by the humidity of tested specimens. Entrained air voids in concrete increase the permeability indices, especially for dry specimens. The relationship between capillary pore microstructure and chloride migration coefficient of non air-entrained concrete was a subject of a study published by Yang [21]. Pore size parameters obtained from MIP test were used to assess the chloride ingress into Portland cement concrete with different w/c ratios. A linear relationship between the chloride migration coefficient and the total volume of capillary pores was obtained. The chloride migration coefficient was found related to a critical pore size. Both relationships suggest an increase of chloride migration coefficient for increased values of pore size parameters. The pore size distribution in concrete with W fly ash in relation to chloride ion migration coefficient was not a subject of systematic study. Few studies revealed a positive impact of W fly ash addition to concrete after prolonged curing periods (>28 days) on a decrease of total pore volume compared to reference concrete without additives [22,23]. Therefore the purpose of current research is determine the effectiveness of blended cement with W fly ash on the strength properties and chloride ingress into air-entrained concrete by a comprehensive characterization of microstructure. 2. Experimental program 2.1. Materials and specimens 2.1.1. Blended cement with calcareous fly ash The new blended cements were produced by co-miling of Portland clinker and gypsum with mineral addition of calcareous fly ash, siliceous fly ash and ground granulated blast furnace slag, respectively. Chemical composition of individual components are
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shown in Table 1. One should pay attention to very low amount of loss of ignition (LOI) (associated with low presence of unburned carbon particles) less than 2% for both types of fly ashes. The composition of produced blended cement is presented in Table 2. The amount of gypsum was calculated according to SO3 amount in particle component of blended cement in Table 1. The grinding time was adjusted to obtain the Blaine surface area close to ordinary cement CEM I. The European standard BS-EN 197-1 allows the use of used W fly ash in blended cement CEM II, but it is not included in cement type CEM V. 2.1.2. Air-entrained mixtures Air entrained reference concrete was prepared with CEM I cement of the composition presented in Table 2. The prescriptive standard limits of cement content and w/c ratio adequate for XF4 exposure class were assumed. Other concrete mixtures were designed using blended cements and the same w/c ratio. Quartz sand (0–2 mm), granodiorite (2–16 mm) and limestone (2–16 mm) crushed aggregates were used. Both aggregates represent different types of rocks. Granodiorite is an igneous rock similar to granite and it is often used for production of structural, frost resistant concrete. Limestone aggregates are produced using a sedimentary rock of CaCO3 >95%. Nonetheless limestone has similar specific gravity, water absorption and low porosity as granodiorite aggregates (Table 3). Significant differences of physical properties are related to mechanical properties (compressive strength and Boehme abrasion). Moreover the surface texture of grains of limestone and granodiorite aggregates is different, which can cause differences in ITZ in concrete [15]. During concrete mixing the chemical admixtures were applied in two steps. The first step was to add air-entraining admixture (AEA) to achieve the entrained air volume of 6–7% tested by pressure method. After that a polycarboxylate-based superplasticizer was used to achieve the slump in the range 50–100 mm. The water content in chemical admixture was less than 1 kg/m3, hence was not taken into account in the mix design. Concrete mixture design is presented in Table 4. The standard specimens were manufactured using a laboratory mixer. The specimens were cured in water at a constant temperature 20 ± 2 °C until testing. 2.2. Test methods The air content of fresh mix was determined using the pressure method according to BS-EN 12350-7. The slump and the mix density were determined using standard methods. Compressive strength measurements were conducted for three 100 mm cubic specimens according to BS-EN 12390-3. Three prisms with dimensions 500 100 100 mm were used to determine the dynamic modulus of elasticity using a resonant method. Measurements were performed using GrindoSonic MK5 with piezoelectric detector. Immediately before testing the specimen was dried by a paper towel than was excited into vibration by the means of a light tap of hammer. On the basis of the resonant vibration frequency measurement the dynamic modulus of elasticity for three specimens was calculated. Measurements were performed after 28 and 90 days of water curing of specimens. Air void characteristics in hardened concrete were determined using a system of automatic image analysis [25] based on Nikon SMZ800, Marzhasuer Scan scanning table and Image Pro Plus software. Tests were performed on polished concrete specimens 100 100 25 mm cut from 150 mm cube specimens. The automatic measurement procedure was designed to comply with the requirements imposed by BS-EN 480-11:2008. Results of measurements were available as a set of standard parameters for air void microstructure characterization: spacing factor – L [mm]; specific surface – a [1/mm]; air content – A [%]; content of air voids with
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Table 1 Chemical composition of cement constituents (XRF analysis; CaOf – glycol method; LOI – oven up to 1000 °C; SO3 – mass method) [24]. Cement constituents
Contents [%]
Portland clinker W V S
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
K2O
Na2O
CaOfree
LOI
66.7 26.0 3.4 45.6
22.7 40.9 52.3 37.0
4.9 19.0 27.5 6.8
2.4 4.3 6.2 1.6
1.0 1.7 2.7 5.5
0.4 3.9 0.4 1.2
0.39 0.14 1.2 0.1
0.42 0.13 3.30 0.50
– 1.07 – –
0.4 2.1 1.9 0.7
Table 2 Composition and physical properties of blended cement. Cement type
Constituents
CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)”
Clinker [%]
HCFA [%]
FA [%]
Slag [%]
Gypsum[%]
94.5 67.4 66.6 47.9
– 28.9 14.3 23.9
– – 14.3 –
– – – 23.9
5.5 3.7 4.8 4.2
Table 3 Physical properties of coarse aggregate (manufacturer data).
Density [g/cm3] Porosity [%] Water absorption [%] Compressive strength [MPa] Boehme abrasion [mm] Freeze-thaw resistance (25 cycle)
Granodiorite
Limestone
2.69 1.1 0.32 186 2.85 +
2.68 2.1 0.30 101 4.12 +
Surface area (Blaine) [cm2/g]
SO3 content by mass [%]
3850 3750 3750 3800
2.82 3.13 3.13 3.33
were based on the voltage magnitude. Temperature of anolite measured at the beginning and at the end of test and the depth of chloride ion penetration. The non-steady-state migration coefficient (Dnssm) was calculated from the Fick’s second law [26]. 3. Test results 3.1. Air void characteristics The air content in fresh mix was between 6.4% to 7.2% for all mixes (Table 5). However it was achieved using a variable content of admixtures, particularly the air entraining admixture (Table 4). The observed relationship between the amount of AEA and the content of W fly ash is approximately linear up to 29% of replacement of clinker in blended cements (Fig. 1). Significant differences AEA content between concrete mixes did not affect to air void characteristics given in Table 5. Low spacing factor values up to 0.19 mm were found. It was possible due to a high amount of small air bubbles in concrete as seen in Fig. 2. Air voids smaller than 300 lm (A300) represented approximately half of all the air voids in hardened concrete. A comparison of air content measured in fresh concrete mix and in hardened concrete revealed a systematic decrease by 0.5–0.9%. A small reduction of total air content was consistent with rather low slump of concrete mixes. This phenomenon was observed in Glinicki and Da˛browski research [27]. No significant influence of different types of aggregates on air voids characteristics was found. The largest difference in the fresh and hardened air content of 1.25% was observed in W-V limestone concrete. It was related to
diameter less than 0.3 mm – A300 [%]. The magnification set at 30 meant that the pixel size was about 2.76 lm [25]. The tests were conducted on two samples for each concrete mix. Mercury intrusion porosity (MIP) measurements were carried out using small cores drilled from concrete specimens after air voids analysis (Ø = 14 mm; h = 25 mm; weight 3 g) and single, large grain of aggregates (Ø 16 mm). Specimens were dried at 35 °C until a constant weight to avoid microcracks and then they were kept in sealed containers until the day of the test. The size of specimens for MIP analysis was linked to the size of measurement container of Quantachrome POREMASTER 60 mercury porosimeter. Porosimeter could detect the pores as small as 5 nm with the maximum pressure of 414 MPa. Measurements were performed on concrete specimens after 90 days of water curing. Rapid chloride migration test was applied to determine the non-steady state migration coefficient according to Nordtest Method NT Build 492. The test was conducted on 50 mm high disk specimens cut from cylinders of 100 mm in diameter and 200 mm in height. Measurements were performed after 28 and 90 days of water curing of specimens. The evaluation criteria for concretes
Table 4 Composition of air-entrained concrete mixes. Mix type
Cement type
Cement [kg/m3]
Water [kg/m3]
Fine aggregate [kg/m3]
Granodiorite aggregate [kg/m3]
Limestone aggregate [kg/m3]
AEA [% by weight]
SP [% by weight]
W0-G W29-G W14-V14-G W24-S24-G W0-L W29-L W14-V14-L W24-S24-L
CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)” CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)”
340
153
598
1209
–
605
–
1203
0.06 0.20 0.12 0.15 0.05 0.22 0.11 0.19
0.06 0.26 0.15 0.29 0.12 0.35 0.50 0.38
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Table 5 Properties of concrete mixture and quantitative characteristics of air voids in hardened concrete (A – total volume of air voids; a – the specific surface of air voids; L – the spacing factor; A300 – volume of micropores). Mix type
Fresh concrete
W0-G W29-G W14-V14-G W24-S24-G W0-L W29-L W14-V14-L W24-S24-L
Hardened concrete
Air content [%]
Slump [mm]
Density [kg/m3]
A [%]
a. [mm 1]
L [mm]
A300 [%]
6.6 7.0 6.8 6.5 6.8 7.2 6.4 6.6
50 80 40 50 60 60 90 70
2281 2291 2288 2292 2283 2281 2289 2291
6.03 6.64 6.47 5.98 5.95 6.44 5.15 5.89
29.52 33.82 37.83 35.06 30.13 32.62 25.98 34.35
0.12 0.11 0.09 0.11 0.13 0.12 0.19 0.10
3.09 3.81 4.15 2.99 2.90 3.21 2.35 3.48
Fig. 1. The relationship between the content of W ash in blended cements and the content of AEA necessary to obtain the target fresh air content.
Fig. 2. Distribution of air voids (white color) visible at the cross-section of concrete specimen W29_G.
the decreased spacing surface of air voids. Such results indicate same instability of air voids in the mix, meaning a loss of some small air bubbles during compaction of concrete. That resulted in increased air voids spacing factor in hardened concrete. 3.2. Compressive strength and elastic modulus The compressive strength of reference concrete with both types of aggregates was similar, in the range from 46 MPa to 48 MPa
(Fig. 3). No decrease of 28 day compressive strength was found when using CEM II/B-W (29% of W fly ash) cement in comparison to the reference concrete. At the age of 90 days the compressive strength of concrete with CEM II/B-W was approximately 20% higher than for the reference concrete. The 28 day compressive strength of concrete with other blended cements was lower than for the reference concrete by 10–18%. However a relative increase in the compressive strength by 8–15% was observed for extended curing time. The influence of different aggregate type on the compressive strength and dynamic modulus of elasticity was not observed. The dynamic modulus of elasticity of 40–44 GPa was found for reference concrete and all concrete series with blended cements (Fig. 4). Similar to compressive strength, the dynamic modulus of elasticity of concrete with blended cements increased after prolonged curing time. However, the observed increase of dynamic modulus of elasticity was small, from 4% to 8%. No influence of aggregate type on the dynamic modulus of elasticity of concrete was observed. 3.3. Coefficient of chloride migration Chloride migration coefficient values obtained after 28 and 90 days of water curing ranged from 2.61 10 12 to 12.38 10 12m2/s (Fig. 5). The criteria for evaluating the resistance of concrete against chloride penetration were proposed by Tang Luping [26]. The category of ‘‘very good” resistance is associated with Dnssm < 2 10 12m2/s and the category of ‘‘good” resistance is for Dnssm from 2 10 12m2/s to 8 10 12m2/s. However the
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Fig. 3. Compressive strength versus curing time of concrete: 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
Fig. 4. The influence of curing time on the dynamic modulus of elasticity of concrete curing 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
Fig. 5. The influence of concrete composition and curing time on chloride migration coefficient Dnssm – curing time of concrete: 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
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criteria are not yet well established for specific structural applications in European countries. The use of blended cement with W fly ash caused a decrease of chloride permeability appointed by chloride migration test. Effect of chloride ions binding during chloride migration test is considered negligible because test time is too short to occur interacting with the cement matrix. As it could be expected a significant effect of blended cement use on Dnssm decrease was observed after extended curing. At 90 days of curing time a decrease of Dnssm by 20–75% was observed in comparison to the reference concrete for both types of aggregates. The impact of aggregate type on Dnssm was observed for 28 days of water curing of concrete. Granodiorite coarse aggregate had no effect on Dnssm for concrete made with all of blended cement; the results were similar to reference concrete without mineral additives. However, after the same period of curing time a decrease of Dnssm by approximately 25–45% was observed for concrete with limestone aggregates. 3.4. Pore size distribution Pore size distribution of aggregate grains is considered to draw attention to microstructural differences between used rocks. For porosity determination using MIP aggregate grains of the same size were selected. Pore size distribution in aggregate grains is presented in Fig. 6. The total pore volume for granodiorite grain is 0.003 cm3/g and it is four times lower than obtained for limestone grain, although manufacturer date did not show such high differences. Main differences between pore size distribution for both types of aggregates are associated with volume fraction of small pores (<0.01 lm) and presence of large capillary pores (>1 lm). The second value is important for concrete technology, because connected capillary pore network may cause higher water absorption of aggregate grains and subsequent increased demand for chemical admixture above. Limestone aggregate possess approximately 70% of all capillary pores in this range >1 lm, while granodiorite aggregates only 23%. The volume fraction of small pores <0.01 lm in granodiorite grain was 60% and it was 3 times higher than for limestone grain.
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Results of MIP measurement on concrete with blended cements are presented in Figs. 7 and 8. The total volume of pores was within the range 0.029–0.048 cm3/g. The use of blended cement with mixture of additives caused a decrease of total pore volume in comparison to reference concrete regardless of aggregate type. Only for concrete containing cement with 29% of W ash (CEM II/B-W) a strong influence of aggregate type was found. The use of this type of cement with limestone aggregate resulted in the highest total pore volume, higher than in the reference concrete. However concrete with granodiorite aggregate and CEM II/B-W exhibited a decrease of total pore volume in comparison to the reference concrete. Although all cylindrical specimen have the same dimensions, the values of total pore volume are biased by a variable proportion of cement paste to coarse aggregate content. This is expected due to small size of specimens. Therefore a more important parameter to describe a pore size distribution in the investigation specimens, selected by authors of research, ranges of porosity. Six characteristic ranges of pore size were selected. The analysis of MIP results to obtain percentage volume fraction of porosity in selected ranges showed that the most frequently the pores were within 0.01–1 lm range. Such pores considered as capillary pores represent approximately 80% all pores detected in concrete specimens (Figs. 7b and 8b). An increase of non-clinker constituent content in cement causes a decrease of percentage volume fraction of capillary pores within range 0.1–1 lm and simultaneously increase percentage volume fraction of smallest pores within the range 0.01–0.1 lm. It is known the range of porosity measured by MIP underestimates the true pore size by several orders of magnitude as a result of ink-bottle effects [18]. However the results are considered appropriate to show relative differences in concrete microstructure resulting from the use of supplementary cementitious materials. For the performed analysis only capillary pores smaller than 1 lm (calculated from Washburn model) were considered, thus the size was too small for common air voids resulting from the air entraining admixtures use. A similar total air content and other parameters characterizing the air voids coming from the
Fig. 6. Pore size distribution in aggregate grains a) total pore range; b) percentage of pores for selected ranges of pore diameter.
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Fig. 7. Pore size distribution in concrete with granodiorite aggregate: a) total pore range; b) percentage of pores for selected ranges of pore diameter.
Fig. 8. Pore size distribution in concrete with limestone aggregate: a) total pore range; b) percentage of pores for selected ranges of pore diameter.
air-entrainment suggest that observed changes do not result from the presence of spherical pores (air voids). 4. Discussion To achieve the target air content in concrete mixes the content of AEA was significantly increased. Such an increase was found to be proportional to the content of calcareous fly ash in blended cements. The effect of high carbon content in fly ash on the air entrainment is known [28]. It cannot explain the observed effect due to rather low loss on ignition (LOI) of calcareous fly ash used. It is probably the morphology of at least some part of ash particles, that increase AEA absorption. Such
an explanation is also supported by [27], showing a clear increasing tendency of the foam index with increasing the specific surface of calcareous fly ash. In spite of increasing content of AEA the air void system in the mixes was quite stable. The differences in the air content measured in the fresh mix and in the hardened concrete were lower than 1%. It is important to stress that the use of calcareous fly ash as one of the main constituents of blended cement was not found to induce instability or damage of the entrained air void system as its former use as an addition to concrete mix [27]. The instability of the air void system exhibited in the latter case was considered as the major obstacle for achieving the proper air void system for frost resistant concrete [1,29].
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To achieve the target slump and the target air content of all the mixes the content of chemical admixtures was adjusted properly. The required content of admixtures was slightly higher for limestone aggregate concrete (Fig. 9), probably due to increased surface porosity of aggregate grains as shown by MIP results (Fig. 6). An increased aggregate porosity could be held responsible for surface absorption of air entraining admixture and increased water demand for surface wetting of aggregate particles. Characterization of entrained air voids permitted to determine the specific surface of air void system within the range from 26 to 38 mm 1 and the spacing factor well below 0.20 mm. The air void system described by the parameters is usually considered adequate for high freeze-thaw and salt scaling resistance of cement paste in concrete. The microvoids content A300 obtained by method according to BS-EN 480-11 is quite high. This is also an indication of potentially effective air void system in concrete designed for bridges and pavements in the cold climate regions. Because of similar characteristics of entrained air voids in concrete, its relative influence on the compressive strength should be negligible. Therefore the observed differences in the compressive strength can be attributed to the strength of the cement paste and the strength of aggregate-paste interface. The effect of the aggregate type is not found significant, although some earlier investigations suggested major changes when using limestone aggregate [15,30]. So the observed differences in compressive strength are solely attributed to the strength of cement paste resulting from cement hydration and both pozzolanic and hydraulic reaction of other mineral constituents of blended cements. Contrary to other types of calcareous fly ash, the fly ash used is expected to exhibit rather high pozzolanic activity due to a relatively high content of reactive silica (max value 35% [31]) compared with Greek calcareous fly ashes (max value 29% [6]). It was noted by Baran and all [32] that the activity index of such fly ash has average value 104% after 28 day of curing (according to BS-EN 450-1). It was much higher than for Turkish calcareous ashes [4]. Pozzolanic activity test of calcareous fly ashes form Greece [6] was carried out using the method recommended by RILEM (TC FAB-67). This test method involves the determination of the glass content by dissolution part of calcareous fly ash in hydrochloric acid and potassium hydroxide. Hence it is difficult to compare results with the results on calcareous fly ash presented in this paper, where we used compressive strength tests to determine the activity index. The hydraulic activity of calcareous fly ash is defined in an indirect way by a comparison of the compressive strength efficiency on
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standard mortar specimens. Here the results reported in [33] provide for moderate hydraulic activity far below 10 MPa as specified by BS-EN 197-1. However, its clearly beneficial effect of the strength development is observed and can be attributed to joint result of pozzolanic and hydraulic activity. Such synergy effects are much less evident when using such fly ash just as an additive to concrete mix. In spite of the highest content of non-clinker constituents in ‘‘CEM V” the concrete strength development was quite similar to the case of using blended ashes in cement. The advantageous interaction of calcareous fly ash and slag in blended cement calls for further attention. The microstructural studies of such ternary cement systems revealed an increased amount of weight of loss up to 400 °C (Termogravimetric method), related to concentration of main hydration product of hydraulic and pozzolanic reaction (C-S-H; AFt; AFm) in comparison to paste with only Portland clinker and slag cements [34]. A lack of significant effect of new blended cements on the elastic properties of air entrained concrete is in accordance with the observations related to the strength and the porosity. As shown by Popovics [35] the prediction of elastic properties of concrete can be quite accurate using simple models based on the composite average concept. Considering a constant paste and aggregate volume and similar elastic properties of aggregates, eventual changes of elastic properties can be attributed only to the paste properties. Neither total volume of capillary pores nor the entrained air void content was differing significantly. Therefore no significant differences of elastic properties of cement paste could be expected, except these related to the increased hydration (and reaction) degree for prolonged curing time. In conclusion: no detrimental effects of new blended cements were observed in respect to the elastic properties of air entrained concrete. The effect of blended W cements on chloride ion permeability was more significant than on the compressive strength. The most important finding is illustrated in Fig. 10. The relationship confirms the lack of impact of the ITZ on the transport of liquid media in concrete specimens. Obviously the significance of the result illustrated in Fig. 10 must be understood within the limits of MIP applicability to concrete studies. The concerns related to the damaging action of high pressure mercury intrusion do not allow an exact determination of pore size, neither the Washburn model is fully adequate for cement paste characterization. Within the scope of the performed investigation the MIP results provide a good comparative indication of the differences in chloride ion permeability. So provided that the entrained air void system is composed of distinct voids of similar size and spacing, it is the specific range
Fig. 9. Chemical admixture content (sum of superplasticizer and air entraining admixture) in concrete with granodiorite and limestone aggregates.
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References
Fig. 10. Relationship between capillary porosity and Dnssm.
of capillary pores estimated by Washburn equation 0.1–1 mm that is found to be a major controlling factor of chloride ion migration in new blended W cement concretes. The observed lack of effects induced by the change of aggregate type supports earlier observations by Diamond [16] for the case of air entrained concretes made of blended W cements. 5. Conclusions The following conclusions can be drawn: A linear relationship between the dose of AEA and content of calcareous fly ash in blended cement necessary to achieve proper air void system in air-entrained concrete was found in spite of the unburned carbon content as low as 2%. A compatible set of air entraining admixture and superplasticizer allowed for a stable air void system in concrete containing blended W cements with a clinker factor reduced down to 48%. The use of blended cement with W, V and S constituents caused an increase of the compressive strength after 90 days of water curing approximately by 8–20% relative to the reference concrete with Portland cement clinker, regardless of aggregate type. The improved resistance of air entrained concrete to chloride ion migration was more significant for limestone aggregate than for granodiorite aggregate. An increase of non-clinker constituent content in blended cement caused a decrease of volume fraction of capillary pores within estimated range 0.1–1 lm and a simultaneously increase of percentage volume fraction of smaller pores within estimated range 0.01–0.1 lm after 90 days of water curing of concrete, regardless of aggregate type. It concerned the estimated pore size ranges calculated from Washburn equation. A clear relationship between the none-steady state chloride migration coefficient and the capillary porosity obtained by MIP measurement of air-entrained concrete with blended cement with W, V and S constituents was found regardless of aggregate type. An increase of volume fraction of capillary pores within estimated range 0.1–1 lm caused an increase of Dnssm from 2 to 10 10 12m2/s. Acknowledgment The research is a part of the research project ‘‘Innovative cement based materials and concrete with high calcium fly ashes” co-financed by the European Union from the European Regional Development Fund. Project number: POIG 01.01.02-24-005/09.
[1] K.H. Pedersen, A.D. Jensen, M.S. Skjrˇth-Rasmussen, K. Dam-Johansen, A review of the interference of carbon containing fly ash with air entrainment in concrete”, Prog. Energy Combust. Sci. 34 (2008) 135–154. [2] M. Da˛browski, M.A. Glinicki, Influence of aggregate type on the durability of concrete made of blended cements with calcareous fly ash, in: Proc. Int. Symp. Brittle Matrix Compos., vol. 10, Warsaw, 2012, pp. 305–313. [3] V.G. Papadakis, Effect of fly ash on Portland cement systems. Part II. Highcalcium fly ash, Cem. Concr. Res. 30 (2000) 1647–1654. [4] C.D. Atis, A. Kilic, U.K. Sevim, Strength and shrinkage properties of mortar containing a nonstandard high-calcium fly ash, Cem. Concr. Res. 34 (2004) 99–102. [5] S. Antiohos, S. Tsimas, Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems, Cement Concr. Compos. 27 (2005) 171–181. [6] S.K. Antiohos, A. Papageorgiou, V.G. Papadakis, S. Tsimas, Influence of quicklime addition on the mechanical properties and hydration degree of blended cements containing different fly ashes, Constr. Build. Mater. 22 (2008) 1191–1200. [7] S. Antiohos, K. Maganari, S. Tsimas, Evaluation of blends of high and low calcium fly ashes for use as supplementary cementing materials, Cement Concr. Compos. 27 (2005) 349–356. [8] H. Yazici, The effect of silica fume and high-volume Class C fly ash on mechanical properties, chloride penetration and freeze–thaw resistance of self-compacting concrete, Constr. Build. Mater. 22 (2008) 456–462. [9] K. Gibas, M.A. Glinicki, G. Nowowiejski, Evaluation of impermaeability of concrete containing calcareous fly ash in respect to environmental media, Roads and Bridges - Drogi i Mosty 12 (2) (2013) 159–171. [10] D.F. Aponte, M. Barra, E. Vazquez, Durability and cementing efficiency of fly ash in concretes, Constr. Build. Mater. 30 (2012) 537–546. [11] A. Bentur, Impact of additions: indicators for durability and strength performance, in: International RILEM Workshop on Performance Based Evaluation and Indicators for Concrete Durability, Madrid, 2006, pp. 297– 309. [12] A. Hussin, C. Poole, Petrography evidence of the interfacial transition zone (ITZ) in the normal strength concrete containing granitic and limestone aggregates, Constr. Build. Mater. 25 (2011) 2298–2303. [13] J. Zheng, H.S. Wong, N. Buenfeld, Assessing the influence of ITZ on the steadystate chloride diffusivity of concrete using a numerical model, Cem. Concr. Res. 39 (2009) 805–813. [14] M.A. Glinicki, D. Józ´wiak-Niedz´wiedzka, K. Gibas, M. Da˛browski, Influence of Blended Cements with Calcareous Fly Ash on Chloride Ion Migration and Carbonation Resistance of Concrete for Durable Structures, MATERIALS, ISSN: 1996–1944, doi: 10.3390/ma9010018, vol. 9 (Special Issue Utilisation of ByProduct Materials in Concrete), No.1, 2016, 18-1-15. [15] W.A. Tasong, C.J. Lynsdale, J.C. Cripps, Aggregate-cement paste interface Part I. Influence of aggregate geochemistry, Cem. Concr. Res. 29 (1999) 1019– 1025. [16] S. Diamond, J. Huang, The ITZ in concrete – a different view based on image analysis and SEM observation, Cement Concr. Compos. 23 (2001) 179–188. [17] A. Leemann, R. Loser, B. Münch, Influence of cement type on ITZ porosity and chloride resistance of self-compacting concrete, Cement Concr. Compos. 32 (2010) 116–120. [18] S. Diamond, Mercury porosimetry an inappropriate method for the measurement of pore size distributions in cement-based materials, Cem. Concr. Res. 30 (2000) 1517–1525. [19] Rilem report 12, in: J. Kropp, H.K. Hilsdorf (Eds.). Performance criteria for Concrete Durability, 1995. [20] H.S. Wong, A.M. Pappas, R.W. Zimmerman, N.R. Buenfeld, Effect of entrained air voids on the microstructure and mass transport properties of concrete, Cem. Concr. Res. 41 (2011) 1067–1077. [21] C.C. Yang, On the relationship between pore structure and chloride diffusivity from accelerated chloride migration test in cement-based materials, Cem. Concr. Res. 36 (2006) 1304–1311. [22] S. Antiohos, A. Papageorgiou, S. Tsimas, Activation of fly ash cementitious systems in the presence of quicklime. Part II: nature of hydration products. Porosity and microstructure development, Cem. Concr. Res. 36 (2006) 2123– 2131. [23] Z. Yu, G. Ye, The pore structure of cement paste blended with fly ash, Constr. Build. Mater. 45 (2013) 30–35. [24] D. Dziuk, Z. Giergiczny, A. Garbacik, Calcareous fly ash as a main constituent of common cements, Roads and Bridges - Drogi i Mosty 12 (1) (2013) 57–71. [25] M.A. Glinicki, Methods of qualitative and quantitative assessment of concrete air entrainment, Cem. Wapno Beton 19 (81) (2014) 359–369. [26] L. Tang, Chloride Transport in Concrete – Measurement and Prediction, Chalmers University of Technology, Department of Building Materials, Goteborg, 1996. [27] M.A. Glinicki, M. Da˛browski, Influence of high calcium fly ash on air entrainment of mix and air voids characteristics in hardened concrete, in: International Conference of Ashes from Power Industry, 2010, pp. 77–92 (in Polish) . [28] I. Kulaots, R.H. Hurt, E.M. Suuberg, Size distribution of unburned carbon in coal fly ash and its implications, Fuel 83 (2004) 223–230.
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Standards [36] BS-EN 206:2014 Concrete. Specification, performance, production and conformity. [37] BS-EN 450-1:2012 Fly ash for concrete. Definition. specifications and conformity criteria. [38] ASTM C618: 2015 Standard specification for coal fly ash and raw or calcined Natural pozzolan for Use in Concrete. [39] BS-EN 197-1:2011 Cement. Composition. specifications and conformity criteria for common cements. [40] BS-EN 12390-3:2009 Testing hardened concrete. Compressive strength of test specimens. [41] BS-EN 480-11:2005 Admixtures for concrete. mortar and grout. Test methods. Determination of air void characteristics in hardened concrete. [42] NT BUILD 492:1999-11. Concrete. mortar and cement-based repair materials: Chloride migration coefficient from non-steady-state migration experiments. [43] RILEM (TC FAB-67) Test methods for determining the properties of fly ash, Materials and Structures, 22, 130, 1989, 299–308.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of calcareous fly ash in blended cements on chloride ions migration and strength of air entrained concrete Mariusz Da˛browski ⇑, Michał A. Glinicki, Karolina Gibas, Daria Józ´wiak-Niedz´wiedzka ´ skiego 5B, 02-106 Warszawa, Poland Institute of Fundamental Technological Research Polish Academy of Sciences, Pawin
h i g h l i g h t s Calcareous fly ash was used as a major constituent of binary and ternary cements. Air void stability in air entrained concrete was investigated by image analysis. Air voids characteristics were stable and adequate for frost resistance. Moderate increase of compressive strength and no change of elastic modulus was observed. A relationship between the chloride migration and the pore size distribution was found.
a r t i c l e
i n f o
Article history: Received 15 April 2016 Received in revised form 14 July 2016 Accepted 27 August 2016
Keywords: Air entrained concrete Air void characteristics Calcareous fly ash Chloride ion migration Multicomponent cements Pore size distribution Strength
a b s t r a c t The influence of blended cements containing calcareous fly ash (W) on the strength and permeability of air entrained concrete was studied. Moderately active calcareous fly ash of CaO content of 26% and loss on ignition of 2%, obtained from lignite fired power station in Poland, was selected. Ternary cement compositions including siliceous fly ash (V) and ground granulated blast furnace slag (S) were also studied. Air void analysis in hardened concrete was used to evaluate the effectiveness of air entraining process. Pore size distribution was characterized by mercury intrusion porosimetry (MIP). The presence of calcareous fly ash in cement resulted in a moderate increase in the compressive strength of concrete and an increase of the category of resistance to chloride ion migration. Test results revealed a linear relationship between the capillary porosity measured by MIP and the chloride migration coefficient (Dnssm). The permeability was lower for increased clinker replacement level which was due to formation of smaller diameters of the capillary pores. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Civil engineering structures in cold climates must exhibit adequate durability for frequent freezing and thawing in winter. Hence a well known solution to achieve the adequate frost resistance is air-entrainment of concrete mixture. This process is however very sensitive for changes of physical and chemical properties of cement, particularly with mineral additions [1]. Massive parts of structures require use of low heat binders which are readily obtained by application of mineral addition to cement or concrete mix. In such a case frequently used mineral addition is siliceous fly ash, which properties are restrictively regulated by EN 206 and EN 450-1 standards. Another type of fly ash with significant content of calcium compounds is in quite limited use in Europe as cement or ⇑ Corresponding author. E-mail address: [email protected] (M. Da˛browski). http://dx.doi.org/10.1016/j.conbuildmat.2016.08.115 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
concrete addition. There are several reasons, one of them being a relatively large variability of physical properties and chemical composition of calcareous fly ash (W). The above-mentioned European standards do not cover the use of fly ash of increased content of calcium. The US practice is different. Following a common acceptance and use of specifications based on ASTM C618, so called high calcium fly ash is readily applied as an addition to concrete mix. The issue of variability of properties of calcareous fly ash is brought forward particularly in the case of air entrained concrete designed for high freeze-thaw durability. This is because the air entrainment process is highly sensitive to fineness and unburned carbon content of fly ash. The concept of increased homogeneity by factory grinding of cement with W fly ash and other mineral addition is thought to be a better option than adding directly to concrete mixture [2]. The influence of calcareous fly ash on cement properties was investigated mainly in Greece and Turkey [3,4]. Obtained results
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were strongly related to chemical composition and mineralogical compounds of W ash. The study by Antiohos at al. [5–7] showed influence of increase active silica content to a higher degree of hydration of ingredients from cement and fly ash. This is revealed by an increase of compressive strength after prolonged curing time >28 days. The use of W fly ash in cement matrix (up to 43% replacement) had a positive influence on concrete impermeability in chloride ions environment, especially after extended period of curing time of specimens [8–10]. The review presented by Bentur [11] showed the effectiveness of mineral additives in respect to chloride ion penetration in concrete. In comparison to other mineral additions W fly ash can be perform better than V fly ash and natural pozzolan. To justify it a key issue is a better understanding of ion transport mechanism in concrete with calcareous fly ash. Many researchers consider the transport of aggressive media in concrete as mainly assigned to porosity of interfacial transition zone (ITZ) [12] and presence of microcracks [13]. Changes of cement composition [14] and aggregate type [15], however, have a greater influence on the ITZ than microcracks. Diamond at all [16] found that permeability of cement matrix alone had more significant impact on the ion ingress than the porosity of ITZ. The same conclusion was presented in a study of Leemann at all [17], in which the chloride migration coefficient was compared with porosity of ITZ for concrete made with blended cements. The most common method to determine the pore size distribution of cement matrix is the mercury intrusion porosimetry (MIP). Despite certain doubts related to interpretation of the MIP method is a very useful for porosity comparison [18]. Based on results obtained by this method it was shown that the biggest capillary pores from range 0.05 lm to 1 lm are responsible for ions transport properties in cement matrix [19]. However Wong at all [20] concluded, that the mechanism of moisture and gas transport in air-entrained concrete is strongly influenced by the humidity of tested specimens. Entrained air voids in concrete increase the permeability indices, especially for dry specimens. The relationship between capillary pore microstructure and chloride migration coefficient of non air-entrained concrete was a subject of a study published by Yang [21]. Pore size parameters obtained from MIP test were used to assess the chloride ingress into Portland cement concrete with different w/c ratios. A linear relationship between the chloride migration coefficient and the total volume of capillary pores was obtained. The chloride migration coefficient was found related to a critical pore size. Both relationships suggest an increase of chloride migration coefficient for increased values of pore size parameters. The pore size distribution in concrete with W fly ash in relation to chloride ion migration coefficient was not a subject of systematic study. Few studies revealed a positive impact of W fly ash addition to concrete after prolonged curing periods (>28 days) on a decrease of total pore volume compared to reference concrete without additives [22,23]. Therefore the purpose of current research is determine the effectiveness of blended cement with W fly ash on the strength properties and chloride ingress into air-entrained concrete by a comprehensive characterization of microstructure. 2. Experimental program 2.1. Materials and specimens 2.1.1. Blended cement with calcareous fly ash The new blended cements were produced by co-miling of Portland clinker and gypsum with mineral addition of calcareous fly ash, siliceous fly ash and ground granulated blast furnace slag, respectively. Chemical composition of individual components are
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shown in Table 1. One should pay attention to very low amount of loss of ignition (LOI) (associated with low presence of unburned carbon particles) less than 2% for both types of fly ashes. The composition of produced blended cement is presented in Table 2. The amount of gypsum was calculated according to SO3 amount in particle component of blended cement in Table 1. The grinding time was adjusted to obtain the Blaine surface area close to ordinary cement CEM I. The European standard BS-EN 197-1 allows the use of used W fly ash in blended cement CEM II, but it is not included in cement type CEM V. 2.1.2. Air-entrained mixtures Air entrained reference concrete was prepared with CEM I cement of the composition presented in Table 2. The prescriptive standard limits of cement content and w/c ratio adequate for XF4 exposure class were assumed. Other concrete mixtures were designed using blended cements and the same w/c ratio. Quartz sand (0–2 mm), granodiorite (2–16 mm) and limestone (2–16 mm) crushed aggregates were used. Both aggregates represent different types of rocks. Granodiorite is an igneous rock similar to granite and it is often used for production of structural, frost resistant concrete. Limestone aggregates are produced using a sedimentary rock of CaCO3 >95%. Nonetheless limestone has similar specific gravity, water absorption and low porosity as granodiorite aggregates (Table 3). Significant differences of physical properties are related to mechanical properties (compressive strength and Boehme abrasion). Moreover the surface texture of grains of limestone and granodiorite aggregates is different, which can cause differences in ITZ in concrete [15]. During concrete mixing the chemical admixtures were applied in two steps. The first step was to add air-entraining admixture (AEA) to achieve the entrained air volume of 6–7% tested by pressure method. After that a polycarboxylate-based superplasticizer was used to achieve the slump in the range 50–100 mm. The water content in chemical admixture was less than 1 kg/m3, hence was not taken into account in the mix design. Concrete mixture design is presented in Table 4. The standard specimens were manufactured using a laboratory mixer. The specimens were cured in water at a constant temperature 20 ± 2 °C until testing. 2.2. Test methods The air content of fresh mix was determined using the pressure method according to BS-EN 12350-7. The slump and the mix density were determined using standard methods. Compressive strength measurements were conducted for three 100 mm cubic specimens according to BS-EN 12390-3. Three prisms with dimensions 500 100 100 mm were used to determine the dynamic modulus of elasticity using a resonant method. Measurements were performed using GrindoSonic MK5 with piezoelectric detector. Immediately before testing the specimen was dried by a paper towel than was excited into vibration by the means of a light tap of hammer. On the basis of the resonant vibration frequency measurement the dynamic modulus of elasticity for three specimens was calculated. Measurements were performed after 28 and 90 days of water curing of specimens. Air void characteristics in hardened concrete were determined using a system of automatic image analysis [25] based on Nikon SMZ800, Marzhasuer Scan scanning table and Image Pro Plus software. Tests were performed on polished concrete specimens 100 100 25 mm cut from 150 mm cube specimens. The automatic measurement procedure was designed to comply with the requirements imposed by BS-EN 480-11:2008. Results of measurements were available as a set of standard parameters for air void microstructure characterization: spacing factor – L [mm]; specific surface – a [1/mm]; air content – A [%]; content of air voids with
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Table 1 Chemical composition of cement constituents (XRF analysis; CaOf – glycol method; LOI – oven up to 1000 °C; SO3 – mass method) [24]. Cement constituents
Contents [%]
Portland clinker W V S
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
K2O
Na2O
CaOfree
LOI
66.7 26.0 3.4 45.6
22.7 40.9 52.3 37.0
4.9 19.0 27.5 6.8
2.4 4.3 6.2 1.6
1.0 1.7 2.7 5.5
0.4 3.9 0.4 1.2
0.39 0.14 1.2 0.1
0.42 0.13 3.30 0.50
– 1.07 – –
0.4 2.1 1.9 0.7
Table 2 Composition and physical properties of blended cement. Cement type
Constituents
CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)”
Clinker [%]
HCFA [%]
FA [%]
Slag [%]
Gypsum[%]
94.5 67.4 66.6 47.9
– 28.9 14.3 23.9
– – 14.3 –
– – – 23.9
5.5 3.7 4.8 4.2
Table 3 Physical properties of coarse aggregate (manufacturer data).
Density [g/cm3] Porosity [%] Water absorption [%] Compressive strength [MPa] Boehme abrasion [mm] Freeze-thaw resistance (25 cycle)
Granodiorite
Limestone
2.69 1.1 0.32 186 2.85 +
2.68 2.1 0.30 101 4.12 +
Surface area (Blaine) [cm2/g]
SO3 content by mass [%]
3850 3750 3750 3800
2.82 3.13 3.13 3.33
were based on the voltage magnitude. Temperature of anolite measured at the beginning and at the end of test and the depth of chloride ion penetration. The non-steady-state migration coefficient (Dnssm) was calculated from the Fick’s second law [26]. 3. Test results 3.1. Air void characteristics The air content in fresh mix was between 6.4% to 7.2% for all mixes (Table 5). However it was achieved using a variable content of admixtures, particularly the air entraining admixture (Table 4). The observed relationship between the amount of AEA and the content of W fly ash is approximately linear up to 29% of replacement of clinker in blended cements (Fig. 1). Significant differences AEA content between concrete mixes did not affect to air void characteristics given in Table 5. Low spacing factor values up to 0.19 mm were found. It was possible due to a high amount of small air bubbles in concrete as seen in Fig. 2. Air voids smaller than 300 lm (A300) represented approximately half of all the air voids in hardened concrete. A comparison of air content measured in fresh concrete mix and in hardened concrete revealed a systematic decrease by 0.5–0.9%. A small reduction of total air content was consistent with rather low slump of concrete mixes. This phenomenon was observed in Glinicki and Da˛browski research [27]. No significant influence of different types of aggregates on air voids characteristics was found. The largest difference in the fresh and hardened air content of 1.25% was observed in W-V limestone concrete. It was related to
diameter less than 0.3 mm – A300 [%]. The magnification set at 30 meant that the pixel size was about 2.76 lm [25]. The tests were conducted on two samples for each concrete mix. Mercury intrusion porosity (MIP) measurements were carried out using small cores drilled from concrete specimens after air voids analysis (Ø = 14 mm; h = 25 mm; weight 3 g) and single, large grain of aggregates (Ø 16 mm). Specimens were dried at 35 °C until a constant weight to avoid microcracks and then they were kept in sealed containers until the day of the test. The size of specimens for MIP analysis was linked to the size of measurement container of Quantachrome POREMASTER 60 mercury porosimeter. Porosimeter could detect the pores as small as 5 nm with the maximum pressure of 414 MPa. Measurements were performed on concrete specimens after 90 days of water curing. Rapid chloride migration test was applied to determine the non-steady state migration coefficient according to Nordtest Method NT Build 492. The test was conducted on 50 mm high disk specimens cut from cylinders of 100 mm in diameter and 200 mm in height. Measurements were performed after 28 and 90 days of water curing of specimens. The evaluation criteria for concretes
Table 4 Composition of air-entrained concrete mixes. Mix type
Cement type
Cement [kg/m3]
Water [kg/m3]
Fine aggregate [kg/m3]
Granodiorite aggregate [kg/m3]
Limestone aggregate [kg/m3]
AEA [% by weight]
SP [% by weight]
W0-G W29-G W14-V14-G W24-S24-G W0-L W29-L W14-V14-L W24-S24-L
CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)” CEM I CEM II/B-W CEM II/B-M (V-W) ‘‘CEM V/A (S-W)”
340
153
598
1209
–
605
–
1203
0.06 0.20 0.12 0.15 0.05 0.22 0.11 0.19
0.06 0.26 0.15 0.29 0.12 0.35 0.50 0.38
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Table 5 Properties of concrete mixture and quantitative characteristics of air voids in hardened concrete (A – total volume of air voids; a – the specific surface of air voids; L – the spacing factor; A300 – volume of micropores). Mix type
Fresh concrete
W0-G W29-G W14-V14-G W24-S24-G W0-L W29-L W14-V14-L W24-S24-L
Hardened concrete
Air content [%]
Slump [mm]
Density [kg/m3]
A [%]
a. [mm 1]
L [mm]
A300 [%]
6.6 7.0 6.8 6.5 6.8 7.2 6.4 6.6
50 80 40 50 60 60 90 70
2281 2291 2288 2292 2283 2281 2289 2291
6.03 6.64 6.47 5.98 5.95 6.44 5.15 5.89
29.52 33.82 37.83 35.06 30.13 32.62 25.98 34.35
0.12 0.11 0.09 0.11 0.13 0.12 0.19 0.10
3.09 3.81 4.15 2.99 2.90 3.21 2.35 3.48
Fig. 1. The relationship between the content of W ash in blended cements and the content of AEA necessary to obtain the target fresh air content.
Fig. 2. Distribution of air voids (white color) visible at the cross-section of concrete specimen W29_G.
the decreased spacing surface of air voids. Such results indicate same instability of air voids in the mix, meaning a loss of some small air bubbles during compaction of concrete. That resulted in increased air voids spacing factor in hardened concrete. 3.2. Compressive strength and elastic modulus The compressive strength of reference concrete with both types of aggregates was similar, in the range from 46 MPa to 48 MPa
(Fig. 3). No decrease of 28 day compressive strength was found when using CEM II/B-W (29% of W fly ash) cement in comparison to the reference concrete. At the age of 90 days the compressive strength of concrete with CEM II/B-W was approximately 20% higher than for the reference concrete. The 28 day compressive strength of concrete with other blended cements was lower than for the reference concrete by 10–18%. However a relative increase in the compressive strength by 8–15% was observed for extended curing time. The influence of different aggregate type on the compressive strength and dynamic modulus of elasticity was not observed. The dynamic modulus of elasticity of 40–44 GPa was found for reference concrete and all concrete series with blended cements (Fig. 4). Similar to compressive strength, the dynamic modulus of elasticity of concrete with blended cements increased after prolonged curing time. However, the observed increase of dynamic modulus of elasticity was small, from 4% to 8%. No influence of aggregate type on the dynamic modulus of elasticity of concrete was observed. 3.3. Coefficient of chloride migration Chloride migration coefficient values obtained after 28 and 90 days of water curing ranged from 2.61 10 12 to 12.38 10 12m2/s (Fig. 5). The criteria for evaluating the resistance of concrete against chloride penetration were proposed by Tang Luping [26]. The category of ‘‘very good” resistance is associated with Dnssm < 2 10 12m2/s and the category of ‘‘good” resistance is for Dnssm from 2 10 12m2/s to 8 10 12m2/s. However the
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Fig. 3. Compressive strength versus curing time of concrete: 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
Fig. 4. The influence of curing time on the dynamic modulus of elasticity of concrete curing 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
Fig. 5. The influence of concrete composition and curing time on chloride migration coefficient Dnssm – curing time of concrete: 28 and 90 days: a) concrete with granodiorite aggregate; b) concrete with limestone aggregate.
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criteria are not yet well established for specific structural applications in European countries. The use of blended cement with W fly ash caused a decrease of chloride permeability appointed by chloride migration test. Effect of chloride ions binding during chloride migration test is considered negligible because test time is too short to occur interacting with the cement matrix. As it could be expected a significant effect of blended cement use on Dnssm decrease was observed after extended curing. At 90 days of curing time a decrease of Dnssm by 20–75% was observed in comparison to the reference concrete for both types of aggregates. The impact of aggregate type on Dnssm was observed for 28 days of water curing of concrete. Granodiorite coarse aggregate had no effect on Dnssm for concrete made with all of blended cement; the results were similar to reference concrete without mineral additives. However, after the same period of curing time a decrease of Dnssm by approximately 25–45% was observed for concrete with limestone aggregates. 3.4. Pore size distribution Pore size distribution of aggregate grains is considered to draw attention to microstructural differences between used rocks. For porosity determination using MIP aggregate grains of the same size were selected. Pore size distribution in aggregate grains is presented in Fig. 6. The total pore volume for granodiorite grain is 0.003 cm3/g and it is four times lower than obtained for limestone grain, although manufacturer date did not show such high differences. Main differences between pore size distribution for both types of aggregates are associated with volume fraction of small pores (<0.01 lm) and presence of large capillary pores (>1 lm). The second value is important for concrete technology, because connected capillary pore network may cause higher water absorption of aggregate grains and subsequent increased demand for chemical admixture above. Limestone aggregate possess approximately 70% of all capillary pores in this range >1 lm, while granodiorite aggregates only 23%. The volume fraction of small pores <0.01 lm in granodiorite grain was 60% and it was 3 times higher than for limestone grain.
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Results of MIP measurement on concrete with blended cements are presented in Figs. 7 and 8. The total volume of pores was within the range 0.029–0.048 cm3/g. The use of blended cement with mixture of additives caused a decrease of total pore volume in comparison to reference concrete regardless of aggregate type. Only for concrete containing cement with 29% of W ash (CEM II/B-W) a strong influence of aggregate type was found. The use of this type of cement with limestone aggregate resulted in the highest total pore volume, higher than in the reference concrete. However concrete with granodiorite aggregate and CEM II/B-W exhibited a decrease of total pore volume in comparison to the reference concrete. Although all cylindrical specimen have the same dimensions, the values of total pore volume are biased by a variable proportion of cement paste to coarse aggregate content. This is expected due to small size of specimens. Therefore a more important parameter to describe a pore size distribution in the investigation specimens, selected by authors of research, ranges of porosity. Six characteristic ranges of pore size were selected. The analysis of MIP results to obtain percentage volume fraction of porosity in selected ranges showed that the most frequently the pores were within 0.01–1 lm range. Such pores considered as capillary pores represent approximately 80% all pores detected in concrete specimens (Figs. 7b and 8b). An increase of non-clinker constituent content in cement causes a decrease of percentage volume fraction of capillary pores within range 0.1–1 lm and simultaneously increase percentage volume fraction of smallest pores within the range 0.01–0.1 lm. It is known the range of porosity measured by MIP underestimates the true pore size by several orders of magnitude as a result of ink-bottle effects [18]. However the results are considered appropriate to show relative differences in concrete microstructure resulting from the use of supplementary cementitious materials. For the performed analysis only capillary pores smaller than 1 lm (calculated from Washburn model) were considered, thus the size was too small for common air voids resulting from the air entraining admixtures use. A similar total air content and other parameters characterizing the air voids coming from the
Fig. 6. Pore size distribution in aggregate grains a) total pore range; b) percentage of pores for selected ranges of pore diameter.
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Fig. 7. Pore size distribution in concrete with granodiorite aggregate: a) total pore range; b) percentage of pores for selected ranges of pore diameter.
Fig. 8. Pore size distribution in concrete with limestone aggregate: a) total pore range; b) percentage of pores for selected ranges of pore diameter.
air-entrainment suggest that observed changes do not result from the presence of spherical pores (air voids). 4. Discussion To achieve the target air content in concrete mixes the content of AEA was significantly increased. Such an increase was found to be proportional to the content of calcareous fly ash in blended cements. The effect of high carbon content in fly ash on the air entrainment is known [28]. It cannot explain the observed effect due to rather low loss on ignition (LOI) of calcareous fly ash used. It is probably the morphology of at least some part of ash particles, that increase AEA absorption. Such
an explanation is also supported by [27], showing a clear increasing tendency of the foam index with increasing the specific surface of calcareous fly ash. In spite of increasing content of AEA the air void system in the mixes was quite stable. The differences in the air content measured in the fresh mix and in the hardened concrete were lower than 1%. It is important to stress that the use of calcareous fly ash as one of the main constituents of blended cement was not found to induce instability or damage of the entrained air void system as its former use as an addition to concrete mix [27]. The instability of the air void system exhibited in the latter case was considered as the major obstacle for achieving the proper air void system for frost resistant concrete [1,29].
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To achieve the target slump and the target air content of all the mixes the content of chemical admixtures was adjusted properly. The required content of admixtures was slightly higher for limestone aggregate concrete (Fig. 9), probably due to increased surface porosity of aggregate grains as shown by MIP results (Fig. 6). An increased aggregate porosity could be held responsible for surface absorption of air entraining admixture and increased water demand for surface wetting of aggregate particles. Characterization of entrained air voids permitted to determine the specific surface of air void system within the range from 26 to 38 mm 1 and the spacing factor well below 0.20 mm. The air void system described by the parameters is usually considered adequate for high freeze-thaw and salt scaling resistance of cement paste in concrete. The microvoids content A300 obtained by method according to BS-EN 480-11 is quite high. This is also an indication of potentially effective air void system in concrete designed for bridges and pavements in the cold climate regions. Because of similar characteristics of entrained air voids in concrete, its relative influence on the compressive strength should be negligible. Therefore the observed differences in the compressive strength can be attributed to the strength of the cement paste and the strength of aggregate-paste interface. The effect of the aggregate type is not found significant, although some earlier investigations suggested major changes when using limestone aggregate [15,30]. So the observed differences in compressive strength are solely attributed to the strength of cement paste resulting from cement hydration and both pozzolanic and hydraulic reaction of other mineral constituents of blended cements. Contrary to other types of calcareous fly ash, the fly ash used is expected to exhibit rather high pozzolanic activity due to a relatively high content of reactive silica (max value 35% [31]) compared with Greek calcareous fly ashes (max value 29% [6]). It was noted by Baran and all [32] that the activity index of such fly ash has average value 104% after 28 day of curing (according to BS-EN 450-1). It was much higher than for Turkish calcareous ashes [4]. Pozzolanic activity test of calcareous fly ashes form Greece [6] was carried out using the method recommended by RILEM (TC FAB-67). This test method involves the determination of the glass content by dissolution part of calcareous fly ash in hydrochloric acid and potassium hydroxide. Hence it is difficult to compare results with the results on calcareous fly ash presented in this paper, where we used compressive strength tests to determine the activity index. The hydraulic activity of calcareous fly ash is defined in an indirect way by a comparison of the compressive strength efficiency on
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standard mortar specimens. Here the results reported in [33] provide for moderate hydraulic activity far below 10 MPa as specified by BS-EN 197-1. However, its clearly beneficial effect of the strength development is observed and can be attributed to joint result of pozzolanic and hydraulic activity. Such synergy effects are much less evident when using such fly ash just as an additive to concrete mix. In spite of the highest content of non-clinker constituents in ‘‘CEM V” the concrete strength development was quite similar to the case of using blended ashes in cement. The advantageous interaction of calcareous fly ash and slag in blended cement calls for further attention. The microstructural studies of such ternary cement systems revealed an increased amount of weight of loss up to 400 °C (Termogravimetric method), related to concentration of main hydration product of hydraulic and pozzolanic reaction (C-S-H; AFt; AFm) in comparison to paste with only Portland clinker and slag cements [34]. A lack of significant effect of new blended cements on the elastic properties of air entrained concrete is in accordance with the observations related to the strength and the porosity. As shown by Popovics [35] the prediction of elastic properties of concrete can be quite accurate using simple models based on the composite average concept. Considering a constant paste and aggregate volume and similar elastic properties of aggregates, eventual changes of elastic properties can be attributed only to the paste properties. Neither total volume of capillary pores nor the entrained air void content was differing significantly. Therefore no significant differences of elastic properties of cement paste could be expected, except these related to the increased hydration (and reaction) degree for prolonged curing time. In conclusion: no detrimental effects of new blended cements were observed in respect to the elastic properties of air entrained concrete. The effect of blended W cements on chloride ion permeability was more significant than on the compressive strength. The most important finding is illustrated in Fig. 10. The relationship confirms the lack of impact of the ITZ on the transport of liquid media in concrete specimens. Obviously the significance of the result illustrated in Fig. 10 must be understood within the limits of MIP applicability to concrete studies. The concerns related to the damaging action of high pressure mercury intrusion do not allow an exact determination of pore size, neither the Washburn model is fully adequate for cement paste characterization. Within the scope of the performed investigation the MIP results provide a good comparative indication of the differences in chloride ion permeability. So provided that the entrained air void system is composed of distinct voids of similar size and spacing, it is the specific range
Fig. 9. Chemical admixture content (sum of superplasticizer and air entraining admixture) in concrete with granodiorite and limestone aggregates.
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References
Fig. 10. Relationship between capillary porosity and Dnssm.
of capillary pores estimated by Washburn equation 0.1–1 mm that is found to be a major controlling factor of chloride ion migration in new blended W cement concretes. The observed lack of effects induced by the change of aggregate type supports earlier observations by Diamond [16] for the case of air entrained concretes made of blended W cements. 5. Conclusions The following conclusions can be drawn: A linear relationship between the dose of AEA and content of calcareous fly ash in blended cement necessary to achieve proper air void system in air-entrained concrete was found in spite of the unburned carbon content as low as 2%. A compatible set of air entraining admixture and superplasticizer allowed for a stable air void system in concrete containing blended W cements with a clinker factor reduced down to 48%. The use of blended cement with W, V and S constituents caused an increase of the compressive strength after 90 days of water curing approximately by 8–20% relative to the reference concrete with Portland cement clinker, regardless of aggregate type. The improved resistance of air entrained concrete to chloride ion migration was more significant for limestone aggregate than for granodiorite aggregate. An increase of non-clinker constituent content in blended cement caused a decrease of volume fraction of capillary pores within estimated range 0.1–1 lm and a simultaneously increase of percentage volume fraction of smaller pores within estimated range 0.01–0.1 lm after 90 days of water curing of concrete, regardless of aggregate type. It concerned the estimated pore size ranges calculated from Washburn equation. A clear relationship between the none-steady state chloride migration coefficient and the capillary porosity obtained by MIP measurement of air-entrained concrete with blended cement with W, V and S constituents was found regardless of aggregate type. An increase of volume fraction of capillary pores within estimated range 0.1–1 lm caused an increase of Dnssm from 2 to 10 10 12m2/s. Acknowledgment The research is a part of the research project ‘‘Innovative cement based materials and concrete with high calcium fly ashes” co-financed by the European Union from the European Regional Development Fund. Project number: POIG 01.01.02-24-005/09.
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