Cementing efficiencies and synergistic roles of silica fume and nano-silica in sulphate and chloride resistance of concrete

Construction and Building Materials 223 (2019) 965–975

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Cementing efficiencies and synergistic roles of silica fume and nano-silica in sulphate and chloride resistance of concrete L.G. Li a,⇑, J.Y. Zheng b, P.L. Ng c,d, J. Zhu a,⇑, A.K.H. Kwan c a

Guangdong University of Technology, Guangzhou, China Guangzhou Institute of Construction Industry, Guangzhou, China c The University of Hong Kong, Hong Kong, China d Vilnius Gediminas Technical University, Vilnius, Lithuania b

h i g h l i g h t s
Adding MS and/or NS can improve sulphate and chloride resistance of concrete.
Cementing efficiencies and synergistic effects evaluated by new formulas.
Cementing efficiency factors of MS and NS are much higher than 1.0.
Synergistic effect is larger in sulphate resistance than in chloride resistance.

a r t i c l e

i n f o

Article history: Received 11 April 2019 Received in revised form 18 July 2019 Accepted 19 July 2019

Keywords: Cementing efficiency Chloride resistance Nano-silica Silica fume Sulphate resistance Synergistic effect

a b s t r a c t It has been proven by various studies that adding silica fume (SF) alone or nano-silica (NS) alone can provide significant beneficial effects on the durability of concrete. In theory, SF and NS may also be added together, but up to now, such possible combined usage of SF and NS for improving the durability of concrete, especially the sulphate and chloride resistance, has not been deeply explored. In this paper, the individual and synergistic roles of SF and NS in the sulphate and chloride resistance of concrete were investigated by producing a series of concrete mixes containing varying SF, NS and water contents for strength test, sulphate attack test and rapid chloride permeability test. It was found that the combined addition of SF and NS can further enhance the sulphate and chloride resistance to higher than possible with the single addition of SF or NS. From the experimental results, the cementing efficiencies of SF and NS were evaluated, from which it was revealed that there exist certain synergistic effects offered by the combined addition of SF and NS on the sulphate and chloride resistance. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As a byproduct from the production of ferrosilicon alloys and silicon crystals [1], silica fume (SF) has been used widely in highperformance concrete (HPC) due to its high fineness and good pozzolanic reactivity [2–6]. Apart from offering the beneficial effect of strength enhancement [7–10], SF can also effectively improve the durability [11–14]. Moon et al. [15] proved that SF has positive effects for mitigating the strength reduction and dimensional instability of mortar exposed to sulphate attack. Manera et al. [16] showed that the electrical resistivity of concrete containing 10% SF is one order of magnitude higher than that of concrete without SF. Farahani et al. [17] placed concrete specimens into the sea⇑ Corresponding authors. E-mail address: [email protected] (J. Zhu). https://doi.org/10.1016/j.conbuildmat.2019.07.241 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

water tidal zone and demonstrated that increasing the SF content in conjunction with decreasing the water/cementitious materials (W/CM) ratio can reduce the chloride diffusion. Khodabakhshian et al. [18] found that adding marble powder as cement replacement would reduce the sulphate resistance of concrete but such adverse effect can be offset by adding SF. Along with rapid advancement of nanotechnology, many nano-materials have been developed for producing HPC [19–29]. Amongst these, nano-silica (NS), with its ultrafine particle size and high pozzolanic reactivity, is quite possibly the one having the greatest potential for practical applications. Being finer than SF, NS can fill into very small voids to enhance the imperviousness and durability of concrete. Ji [30] carried out water permeability test and showed that the addition of NS can improve the water resistance of concrete. Abd El-Aleem et al. [31] revealed that replacing the cement content by 5% NS can densify the microstruc-

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ture of paste. Du et al. [32] observed that even adding 0.3% NS can substantially reduce the chloride diffusion in concrete. Adak et al. [33] reported that adding 6% NS can appreciably increase the chloride resistance of geopolymer mortar. Ghafari et al. [34] demonstrated that the corrosion rate of steel bars in HPC can be further reduced by adding NS. A nano-material may also be used in conjunction with another supplementary cementitious material (SCM) to further improve the durability. For instance, Supit and Shaikh [35] added 38% fly ash (FA) together with 2% NS and found that both the water sorptivity and chloride permeability can be reduced. Mohseni and Tsavdaridis [36] demonstrated that the combined addition of 25% FA and 1% to 5% nano-Al2O3 can result in denser microstructure and lower water and chloride permeability. Arel and Thomas [37] studied the relative performance of NS and other micro-size SCMs in improving sulphate resistance, and suggested combined usage of a SCM and NS. Li et al. [38] showed that adding SF and NS together can further improve the water, sulphate, chloride and carbonation resistance. Bernal et al. [39] revealed the changes in microstructure due to the addition of SF and NS. Ramezanianpour and Moeini [40] reported that the use of 5% SF and 2% NS together can maximize the chloride resistance. Sharkawi et al. [41] reported that adding both SF and NS can substantially delay steel corrosion in concrete. Massana et al. [42] found that the combined usage of SF and NS can substantially improve the freeze-thaw resistance of concrete. Zhang et al. [43] showed that the combined use of SF and NS can reduce the porosity and improve the microstructure of interfacial transition zones. However, the combined effects of SCM and nano-material are fairly complicated and more systematic investigation is still needed to quantify the combined effects. For evaluating the effectiveness of various SCMs in strength development, it has been proposed to employ the concept of cementing efficiency [44–49]. Basically, the cementing efficiency of a SCM is evaluated in terms of a cementing efficiency factor (CEF), defined as the mass of cement that is replaceable per mass of the SCM added without changing the strength. Smith [44] first proposed this concept and suggested that a CEF of 0.25 for FA may be adopted for preliminary design of FA concrete. Babu and Kumar [50] reported that the CEF of ground granulated blastfurnace slag (GGBS) would decrease from 1.29 to 0.70 as the GGBS content increases from 10% to 80%. Wong and Razak [51] found that the CEF of SF is 2.1 to 3.1 at 28 days and 2.4 to 3.3 at 180 days. Antiohos et al. [52] obtained the CEF of rice husk ash as 0.8 at 28 days. Recently, the authors’ research group [53] investigated the CEFs of SF and NS with respect to the strength of concrete in compression, and for the first time introduced a new factor called the synergistic factor to quantify the synergistic effects of adding both SF and NS together. In the European Standard EN 206 [54], it is clearly stated that the concept of cementing efficiency may also be applied to the durability of concrete. But, when applied to the durability, the CEF should be defined as the mass of cement that is replaceable per mass of the SCM added without changing the durability performance attribute being studied. In the CEN document PD CEN/TR 16639 [55], each SCM is considered as a cement substitute with the CEF denoted by a k-value and prescriptive k-values with ample conservatism are allowed in the concrete mix design and optimisation. Papadakis and Tsimas [47] tested the durability performance of low-calcium FA and high-calcium FA, and obtained their chloride resistance CEFs as 2.5 and 2.0, respectively, and carbonation resistance CEFs as 0.5 and 0.7, respectively. Aponte et al. [56] proved that the chloride resistance CEF of FA is dependent on the W/CM ratio. Gruyaert et al. [57] applied the CEF concept to the durability of GGBS concrete, and compared the CEF concept and the equivalent performance concept regarding their scientific merits. Lollini et al. [58] tested different types of SCMs and found that

the chloride resistance CEFs are about 1.5 for FA and GGBS, about 0.6 for natural pozzolans, and almost negligible for ground limestone. However, this concept has been applied only to the single addition of one SCM. Herein, this concept is extended to the combined addition of one SCM and one nano-material, with the possible synergistic effect among them considered. In this study, with the aim to evaluate the effects of combined usage of SF and NS on the durability of concrete, a total of 24 trial concrete mixes containing different water, SF and NS contents were made for slump cone test, strength test, sulphate attack test and rapid chloride permeability test. While analysing the test results, the CEF concept was extended to apply to the combined effects of SF and NS on the sulphate and chloride resistance of concrete, and the synergistic factor was employed to quantify the synergistic effect of SF and NS. 2. Experimental details 2.1. Materials used Three binder materials were used. They are ordinary Portland cement (OPC), silica fume (SF) and nano-silica (NS). The OPC was in compliance with the Chinese Standard for cement [59] and has strength of class 52.5 and specific gravity of 3.11. The SF was a condensed silica fume in compliance with the Chinese Standard for SF [60] and has particle size of about 0.1 lm and specific gravity of 2.20. The NS was in the form of white colour powder and has particle size falling in the range of 5 to 20 nm and specific gravity of 1.94. The SEM image of SF and the TEM image of NS are depicted in Fig. 1(a) and (b), respectively. Laser diffraction test was employed to measure the particle size distributions of the OPC and SF, and the results obtained are presented in Fig. 2(a). Both coarse and fine aggregates were employed in the concrete mixes. The coarse aggregate employed was crushed granitic rock with maximum aggregate size of 10 mm, specific gravity of 2.68, water absorption of 1.04% and moisture content of 0.11%. The fine aggregate employed was river sand with maximum aggregate size of 5 mm, specific gravity of 2.64, water absorption of 1.10% and moisture content of 0.11%. Mechanical sieving was employed to obtain the particle size distributions of the coarse and fine aggregates, and the results are shown in Fig. 2(b). An aqueous form superplasticizer (SP) was dosed to each of the concrete mixes to attain sufficient workability. The SP was of the polycarboxylate-based type. It has a specific gravity of 1.03 and a solid mass content of 20%.

2.2. Mix proportions The testing programme encompassed 24 trial concrete mixes. Four mix parameters, namely the water/cementitious materials (W/CM) ratio, SF content, NS content and SP dosage, were varied in the following manner. The W/CM ratio by mass was varied from 0.30 to 0.45 in increments of 0.05, the SF content was varied among 0% and 5% of total binder by mass, and the NS content was varied among 0%, 0.5% and 1% of total binder by mass. The choice of SF content and NS content was based on practical considerations of workability and economy of the concrete mixes. To impart adequate workability to the concrete mixes, SP was dosed to each mix to achieve 150 ± 50 mm slump, and the corresponding SP dosage (as percentage of total binder by mass) was first determined by trial mixing [61-63]. During trial mixing, the SP was added in small fractions until the slump reached the target range. Then, the SP dosage so determined was adopted during the formal production of concrete mixes for conducting the various tests. The other two mix parameters: the paste volume and fine/total aggregate ratio, were fixed. The paste volume was set at 30% of the total concrete volume. As a result, the aggregate volume was set constant at 70%. The fine/total aggregate ratio by mass was set at 0.4. Table 1 presents the concrete mix proportions. As tabulated in the first column of Table 1, each concrete mix was annotated by a mix number in the format of A-B-C, which corresponds to: (W/CM ratio)-(SF content in percentage)-(NS content in percentage).

2.3. Concrete mixing In this study, a horizontal single-spindle mixer was used to carry out the concrete mixing. The mixing procedures for a concrete mix that did not contain NS are as follows. Firstly, the water and SP were pre-mixed inside a bucket for one minute. Secondly, the OPC, SF and coarse aggregate were placed into the mixer and dry mixed for one minute. Subsequently, the solution of water and SP was added into the mixer and mixed for two minutes. Lastly, the fine aggregate was added and further mixed for two minutes. On the other hand, for a concrete mix that contained NS, to minimise agglomeration and thoroughly disperse the NS, firstly, the water,

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Cumulative percentage passing (%)

100

80

60 OPC

SF

40

20

(a) SF 0 0.01

0.1

1 Particle size ( m)

10

100

10

100

(a) SF and OPC

(b) NS Fig. 1. Microscopic images of SF and NS. SP and NS were pre-mixed and ultra-sonicated for five minutes. Then, the remaining mixing procedures as described above for a concrete mix with absence of NS were implemented.

Cumulative percentage passing (%)

100

80

60 Fine aggregate 40

20

0 0.01

0.1

2.4. Testing methods The workability of the concrete mixes was evaluated from the slump cone test in accordance with the relevant Chinese Standard [64]. During the test, the drop in height of the concrete mix after lifting of the slump cone to form a patty was taken as the slump value. The sulphate resistance of the concrete mixes was assessed from the sulphate attack test as specified in the relevant Chinese Standard [65]. The test procedures are outlined as follows: Firstly, six 100 mm size cubes were produced from each concrete mix. Three of the cubes were moist cured for 26 days, air dried at 80 °C for 2 days, transferred to the sulphate attack machine as depicted in Fig. 3(a), and subject to one wetting-drying cycle (15 h immersion in 5% Na2SO4 solution + 1 h air drying + 6 h drying at 80 °C + 2 h cooling) in every 24 h for 90 days. After then, the cube strengths of these three specimens were tested and averaged as the cube strength after sulphate attack (f1). Meanwhile, the other three cubes were moist cured for 28 days and air dried for 90 days. After then, the cube strengths of these three specimens were tested and averaged as the cube strength without sulphate attack (f2). During the cube compression test, the loading rate was set at 0.6 ± 0.2 MPa/s. Lastly, the strength loss due to sulphate attack was determined as (f2  f1)/f2 and taken as a measure of sulphate resistance; the larger the strength loss, the lower the sulphate resistance, and vice versa. The chloride resistance of the concrete mixes was evaluated from the rapid chloride permeability test (RCPT) in accordance with the relevant Chinese Standard [65]. The test procedures were very similar to those in American Standard ASTM C1202–19 [66]. To perform the RCPT, three cylinder specimens were made from each concrete mix and moist cured for 28 days. Then, the side surfaces of the cylin-

Coarse aggregate

1 Particle size (mm)

(b) Fine and coarse aggregates Fig. 2. Particle size distribution curves of SF, OPC and aggregates. ders were coated with impervious paint, and the cylinders were placed inside a vacuum machine for 24 h (3 h under vacuum + 1 h immersion in pre-boiled water under vacuum + 20 h immersion in pre-boiled water at atmospheric pressure). After these preparation works, a voltage cell was installed to each specimen, with the positive electrode of the cell made up of 0.3 mol/L NaOH solution and the negative electrode made up of 3% NaCl solution. Then, a 60.0 ± 0.10 V direct current was applied across the voltage cell and the specimen, and an electric flux recorder was employed to record the resulting current over a 6-hour period at 5-min time intervals, as depicted in Fig. 3(b). Finally, the total charge passed throughout the RCPT was calculated by numerically integrating the current–time history record.

3. Experimental results 3.1. Slump and SP dosage Though the SP dosage was adjusted to attain similar slump values within the target range of 150 ± 50 mm, the actual measured slump did vary slightly since the SP was added in small increments.

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Table 1 Mix proportions of concrete mixes. Mix number

OPC (kg/m3)

SF (kg/m3)

NS (kg/m3)

Water (kg/m3)

Coarse aggregate (kg/m3)

Fine aggregate (kg/m3)

0.30-0-0 0.30-0-0.5 0.30-0-1 0.30-5-0 0.30-5-0.5 0.30-5-1

483 480 478 459 456 454

0.0 0.0 0.0 24.1 24.1 24.1

0.0 2.4 4.8 0.0 2.4 4.8

145 143 142 142 141 140

1109

739

0.35-0-0 0.35-0-0.5 0.35-0-1 0.35-5-0 0.35-5-0.5 0.35-5-1

447 445 442 425 422 420

0.0 0.0 0.0 22.3 22.3 22.3

0.0 2.2 4.5 0.0 2.2 4.5

165 165 163 164 163 162

1109

739

0.40-0-0 0.40-0-0.5 0.40-0-1 0.40-5-0 0.40-5-0.5 0.40-5-1

416 414 412 395 393 391

0.0 0.0 0.0 20.8 20.8 20.8

0.0 2.1 4.2 0.0 2.1 4.2

182 181 180 181 179 179

1109

739

0.45-0-0 0.45-0-0.5 0.45-0-1 0.45-5-0 0.45-5-0.5 0.45-5-1

389 387 385 369 368 366

0.0 0.0 0.0 19.4 19.4 19.4

0.0 1.9 3.9 0.0 1.9 3.9

192 191 190 191 190 189

1109

739

Note: In calculating the water content, the moisture content and water absorption of the aggregates as well as the water in the SP have been accounted for.

During the experiment, the slump results were recorded and are listed in the second column of Table 2. It is recorded that the slump results are all within 110 to 188 mm, showing that the slump of every concrete mix has met with the requirement of within 150 ± 50 mm. The SP dosages are summarized in the third column of Table 2. Since the slump was maintained at within 150 ± 50 mm by adjusting the SP dosage, the SP dosage actually better reflects how the various mix parameters affect the workability. From these results, it is noted that at given SF and NS contents, the SP dosage was generally smaller when the W/CM was higher. This phenomenon is in accordance with general expectation. Notably, the SP dosage was increased significantly as the SF and/or NS contents increased. For example, at W/CM ratio = 0.30, adding 1% NS alone increased the SP dosage from 4.5% to 5.4%, adding 5% SF alone increased the SP dosage from 4.5% to 5.2%, and adding 5% SF + 1% NS together increased the SP dosage to 5.9%. The obvious cause of such increases in SP dosage was the ultrafine SF and NS particles had very large specific surface areas, which demanded much larger quantity of SP for their dispersion compared to the cement particles. 3.2. Cube strength without sulphate attack The average cube strength results of the three specimens not subjected to any sulphate attack are presented in the fourth column of Table 2. At given SF and/or NS contents, the cube strength always decreased as the W/CM ratio increased. Such phenomenon was just as expected. Moreover, regardless of the W/CM ratio, the concrete strength increased with the SF and/or NS content. For instance, at W/CM ratio = 0.45, the cube strength with no SF and no NS added was 44.7 MPa, the addition of 1% NS alone increased the cube strength to 59.6 MPa, the addition of 5% SF alone increased the cube strength to 62.8 MPa, and the addition of 5% SF + 1% NS together increased the cube strength to 73.3 MPa. Hence, adding SF and/or NS offers beneficial effect on the strength of concrete.

3.3. Crack appearance and cube strength after sulphate attack After the sulphate attack test, each cube specimen was photographed. But due to space limitation of this paper, only representative photographs of the specimens of concrete mixes 0.45-0-0, 0.45-0-1, 0.45-5-0 and 0.45-5-1 are selected and presented in Fig. 4. Fig. 4(a) shows that with no SF and no NS added, some major cracks were formed on the concrete surfaces and spalling had occurred. Fig. 4(b) and (c) show that with 5% SF or 1% NS added, some minor cracks were formed on the concrete surfaces and no spalling had occurred. Lastly, Fig. 4(d) shows that with 5% SF and 1% NS added together, only a few minor cracks were formed on the concrete surfaces and no spalling had occurred. Hence, it is obvious that adding SF or NS is a promising way of mitigating cracking due to sulphate attack and adding SF and NS together is even better. The average cube strength results of the three specimens subjected to sulphate attack are summarized in the fifth column of Table 2. Similar to the phenomenon shown by the cube strengths without sulphate attack, the cube strengths after sulphate attack decreased with increasing W/CM ratio regardless of the SF and NS contents, and increased with increasing SF content and/or increasing NS content at all W/CM ratios. For easier interpretation, the cube strengths without sulphate attack and the cube strengths after sulphate attack are graphically presented in Figs. 5 and 6, respectively. Comparing the two figures, it can be observed that the cube strength after sulphate attack decreased more rapidly with increasing W/CM ratio than the cube strength without sulphate attack. Hence, the sulphate attack produced more pronounced effect on the concrete strengths at higher W/CM ratio. Moreover, the curves plotted in Fig. 6 for the cube strength after sulphate attack are more widely spaced than the curves plotted in Fig. 5 for the cube strength without sulphate attack. Since the different curves plotted in the same figure are for different SF and NS contents, this indicates that both the SF content and NS content have greater effects on the concrete strength after sulphate attack than on the concrete strength without sulphate attack. In

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concrete. Particularly, the strength loss of concrete containing both SF and NS was always smaller than that of concrete containing only SF or only NS, implying that the usage of SF and NS together is a better way of mitigating damage due to sulphate attack. 3.4. Total charge passed in RCPT

(a) Sulphate aack machine

The RCPT total electrical charge passed (in Coulomb) of each concrete mix was calculated, as presented in the last column of Table 2, and its variation with the W/CM ratio is plotted in Fig. 8. From these results, it is found that at given SF and/or NS contents, the total charge passed increased with the W/CM ratio, indicating that the chloride resistance was lower for concrete with higher W/CM ratio. This observation agrees with those in previous studies by other researchers [38,68,69]. From the figure, it is also noted that the RCPT total charge passed-W/CM ratio curves shift downwards following the sequence of [0% SF + 0% NS], [0% SF + 0.5% NS], [0% SF + 1% NS], [5% SF + 0% NS], [5% SF + 0.5% NS], and [5% SF + 1% NS]. Such downward shifting of the total charge passed-W/CM ratio curves as the SF and/or NS content increases reveals that adding SF and/or NS can enhance the chloride resistance of concrete. It is noteworthy that for concrete with SF and NS added together, the total charge passed was always lower than that of concrete with SF added alone or with NS added alone, showing that the usage of SF and NS together is a better way of enhancing the chloride resistance. 4. Combined effects of SF and NS 4.1. Combined effects on cube strength

(b) RCPT electric flux recorder and voltage cells Fig. 3. Photographs of test equipment.

other words, the SF and NS contents did play important roles in the change in cube strength due to sulphate attack. Comparing the cube strength results listed in the fourth and fifth columns of Table 2, it is apparent that due to damage caused by sulphate attack, the concrete strength values after sulphate attack were consistently lower than those without sulphate attack. To quantify the sulphate resistance, the strength loss due to sulphate attack of each concrete mix was calculated as a percentage, as listed in the sixth column of Table 2, and its variation with the W/CM ratio is plotted in Fig. 7. From these results, it is noted that regardless of the SF content and NS content, the strength loss decreased with decreasing W/CM ratio, showing that the sulphate resistance was better for concrete with lower W/CM ratio. This is consistent with the observations from previous investigation that the sulphate resistance was in general higher at lower W/CM ratios, and vice versa [38,67]. Moreover, it is worth stressing that as observed from Fig. 7, the strength loss-W/CM ratio curve shifts downwards following the sequence of [0% SF + 0% NS], [0% SF + 0.5% NS], [0% SF + 1% NS], [5% SF + 0% NS], [5% SF + 0.5% NS], and [5% SF + 1% NS]. The downward shifting of the strength lossW/CM ratio curves as the SF and/or NS content increases reveals that adding SF and/or NS can enhance the sulphate resistance of

To quantify the combined effects of SF and NS addition on cube strength, the corresponding percentages increase in cube strengths are presented in Tables 3 and 4, for the cases without sulphate attack and after sulphate attack, respectively. With reference to the tabulated values, it is evident that at a given W/CM ratio and with or without sulphate attack, the percentage increase in cube strength by adding 5% SF alone was always larger than that by adding 1% NS alone, whereas the percentage increase in cube strength by adding SF and NS together was always larger than that by adding only SF or only NS. Therefore, the usage of both SF and NS would enable a higher strength to be achieved than the single usage of SF alone or NS alone. Moreover, at given SF and NS contents, the percentage increase in cube strength was generally greater for concrete with a higher W/CM ratio, showing that the positive effect of SF and/or NS on strength was more pronounced at higher W/CM ratios. 4.2. Combined effects on sulphate resistance To quantify the combined effects of SF and NS addition on sulphate resistance, the corresponding percentages decrease in strength losses due to sulphate attack are presented in Table 5. With reference to the tabulated values, it is obvious that at a given W/CM ratio, the percentage decrease in strength loss by adding 5% SF alone was always larger than that by adding 1% NS alone, whereas the percentage decrease in strength loss by adding SF and NS together was always larger than that by adding only SF or only NS. More interestingly, it is also noticed that the percentage decrease in strength loss resulted from addition of both SF and NS was generally greater than the sum of the individual percentages decrease in strength loss resulted from SF addition alone and from NS addition alone. For instance, at W/CM ratio = 0.30, the decrease in strength loss by adding 5% SF was 41.8% and the

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Table 2 Experimental results of concrete mixes. Mix number

Slump (mm)

SP dosage (%)

Cube strength without sulphate attack (MPa)

Cube strength after sulphate attack (MPa)

Strength loss after sulphate attack (%)

RCPT total charge passed (Coulomb)

0.30-0-0 0.30-0-0.5 0.30-0-1 0.30-5-0 0.30-5-0.5 0.30-5-1

140 158 130 140 122 110

4.50 5.00 5.40 5.20 5.58 5.90

105.7 110.9 113.0 113.2 118.1 121.4

86.3 93.2 97.7 101.1 110.8 118.2

18.4 16.0 13.5 10.7 6.2 2.7

1056 913 799 585 435 266

0.35-0-0 0.35-0-0.5 0.35-0-1 0.35-5-0 0.35-5-0.5 0.35-5-1

157 160 150 128 145 129

2.40 2.60 3.00 2.75 3.15 3.35

97.3 102.4 105.4 106.2 108.5 111.6

71.7 76.4 82.1 93.4 99.4 106.6

26.3 25.4 22.1 12.1 8.4 4.5

1732 1455 1375 786 579 395

0.40-0-0 0.40-0-0.5 0.40-0-1 0.40-5-0 0.40-5-0.5 0.40-5-1

181 175 169 181 188 187

0.70 0.85 1.18 0.98 1.35 1.50

68.1 72.0 75.9 80.5 84.9 90.6

42.5 49.4 56.9 64.7 74.4 85.3

37.6 31.4 25.0 19.7 12.4 5.9

2600 2080 1970 883 649 483

0.45-0-0 0.45-0-0.5 0.45-0-1 0.45-5-0 0.45-5-0.5 0.45-5-1

125 120 110 115 169 166

0.30 0.50 0.83 0.64 0.98 1.15

44.7 50.9 59.6 62.8 65.4 73.3

23.4 28.7 39.7 45.7 53.4 65.4

47.8 43.5 33.4 27.1 18.4 10.7

3433 2952 2584 1083 883 665

decrease in strength loss by adding 1% NS was 26.6%, but the decrease in strength loss resulted from combined addition of 5% SF and 1% NS was 85.3%, which was larger than the sum of 41.8% and 26.6% (i.e. 68.4%). This implies that the usage of SF and NS together would allow the SF and NS to play certain synergistic roles in the sulphate resistance. 4.3. Combined effects on chloride resistance To quantify the combined effects of SF and NS addition on chloride resistance, the corresponding percentages decrease in RCPT total charge passed are listed in Table 6. These results indicate that at a given W/CM ratio, the percentage decrease in total charge passed by adding 5% SF alone was always larger than that by adding 1% NS alone, whereas the percentage decrease in total charge passed by adding SF and NS together was always larger than that by adding the SF alone or the NS alone. Meanwhile, it is observed that the percentage decrease in total charge passed resulted from addition of both SF and NS was only occasionally greater than the sum of the individual percentage decreases in total charge passed resulted from single addition of the SF and from single addition of the NS. When the W/CM ratio was equal to 0.30, the decrease in total charge passed by adding 5% SF was 44.6%, and the decrease in total charge passed by adding 1% NS was 24.3%, while the decrease in total charge passed resulted from combined addition of 5% SF and 1% NS was 74.8%, which was larger than the sum of 44.6% and 24.3% (i.e. 68.9%). This reveals that in some cases, the usage of SF and NS together could provide certain synergistic effect on the chloride resistance. 4.4. Mechanism of combined effects From the foregoing analysis, the combined addition of SF and NS clearly demonstrated greater effects on the cube strength, sulphate resistance and chloride resistance than the sum of individual effects. The greater combined effects than the sum of individual effects are synergistic effects that may be exploited for maximizing the performance and cost effectiveness of the concrete produced.

The synergistic effect on the strength of mortar had been observed before and attributed to the microstructure improvement due to combined addition of SF and NS as depicted by SEM images [25]. Synergistic effects on the durability of mortar/concrete had also been studied before [38–43] and attributed to the microstructure densification [39,41,43] and the porosity reduction [42,43]. However, the synergistic effects were found to be dependent on the performance attribute being considered and some other mix parameters, and were so far only qualitatively studied, making direct comparison between the test results obtained by different researchers rather difficult. Hence, further and more in-depth research on the quantification of the synergistic effects, as attempted in the following section, is considered necessary. Nevertheless, the underlying mechanism of the synergistic effects may be explained qualitatively as follows. Firstly, both SF and NS are composed of high purity silica, and they readily react with the calcium hydroxide in the concrete to form additional gel product to enhance the strength and durability. Secondly, NS is finer than SF and SF is finer than cement, and therefore SF and NS have successive filling effect to increase the packing density. The densified microstructure helps to lower the permeability for ingress of deleterious substances. In contrast, if only SF is added, there would be a lack of high fineness materials to fill the voids amongst the SF particles. On the other hand, if only NS is added, there would be a gap between the particle sizes of cement and NS in the overall grading of particles. Finally, the W/CM ratio has a role in the combined effects of SF and NS, because the effectiveness of SF and NS is dependent on the rheology. Further investigations on the changes in porosity, packing density and microstructure due to the combined addition of SF and NS at different W/CM ratios are recommended. Particularly, more microstructural studies, which can help to visualize the actual effects, should be carried out. 5. Cementing efficiencies and synergistic effects of SF and NS Herein, to determine the CEFs of SF and NS with respect to the sulphate and chloride resistance and quantify their synergistic

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(a) Mix no. 0.45-0-0

(b) Mix no. 0.45-0-1

(c) Mix no. 0.45-5-0

(d) Mix no. 0.45-5-1

Fig. 4. Photographs of specimens after subject to sulphate attack.

140

140

Cube strength (MPa)

120

100

80

60

40 0.25

0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

120

Cube strength (MPa)

0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

100

80

60

40

0.30

0.35

0.40

0.45

0.50

W/CM ratio Fig. 5. Plot of cube strength without sulphate attack against W/CM ratio.

20 0.25

0.30

0.35

0.40

0.45

0.50

W/CM ratio Fig. 6. Plot of cube strength after sulphate attack against W/CM ratio.

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by treating the SCM content as equivalent to a cement content equal to the CEF of SCM times the SCM content. The equivalent water/cement ratio, denoted by W/Ceq, is given by the following formula:

Strength loss (%)

60 0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

W=Ceq ¼

40

W C þ kSCM  SCM

ð1Þ

in which W, C and SCM are the water, cement and SCM contents in kg/m3, respectively; and kSCM is the CEF of SCM. Herein, in calculating the equivalent water/cement ratio, the SF or NS content is treated as equivalent to a cement content equal to the CEF of SF times the SF content or the CEF of NS times the NS content. To allow for the synergistic effect when the SF and NS are added together, an additional equivalent cement content equal to the synergistic factor times the SF content times the NS content divided by the total cementitious materials content is added. The equivalent water/cement ratio W/Ceq is given by the following formula:

20

0 0.25

0.30

0.35

0.40

0.45

0.50

W/CM ratio Fig. 7. Plot of strength loss after sulphate attack against W/CM ratio.

W=Ceq ¼

W C þ kSF  SF þ kNS  NS þ kSYN ðSF  NS=TCÞ

ð2Þ

where SF, NS and TC are respectively the SF, NS and total cementitious materials contents with units of kg/m3; kSF is the CEF of SF, kNS is the CEF of NS, and kSYN is the synergistic factor.

RCPT total charge passed (Coulomb)

4000

3000

0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

5.1. CEFs and synergistic factor in sulphate resistance For the purpose of evaluating the CEFs and synergistic factor of SF and NS in sulphate resistance, the strength loss (fsl) is correlated to the equivalent water/cement ratio by the following newly proposed formula:

f s1 ¼ aW=Ceq þ b

2000

1000

0 0.25

0.30

0.35

0.40

0.45

0.50

W/CM ratio Fig. 8. Plot of RCPT total charge passed against W/CM ratio.

effect when used together, new formulas based on Smith [44] and Hobbs [70] are employed. The basic idea of the new formulas is that like the strength, the sulphate resistance and chloride resistance may also be correlated to the equivalent water/cement ratio. In the works of Smith [44] and Hobbs [70], a single type of SCM was considered. The equivalent water/cement ratio is determined

ð3Þ

in which, fsl is the strength loss (%); and a and b are numerical coefficients. To determine the factors kSF, kNS and kSYN, the following steps are carried out. First, the coefficients a and b are evaluated through regression analysis of the strength loss results from the pure cement concrete mixes (without SF or NS added) with different W/CM ratios. Second, at each W/CM ratio, the kSF, kNS and kSYN values are determined through regression analysis of the strength loss results from the group of concrete mixes with the same W/CM ratio (with or without SF or NS added). The values of kSF, kNS and kSYN so obtained are presented in Table 7, which shows that the factors kSF and kNS are both much higher than 1.0, meaning that adding SF and/or NS to replace cement is an effective way of improving the sulphate resistance. Moreover, the factor kNS is larger than the factor kSF, meaning that at the same content, the NS is better than the SF. Regarding the synergistic effect, the synergistic factor kSYN is always positive. This proves that the contribution of SF is larger if NS is also added and the contribution of NS is larger if SF is also added. Lastly, the factors kSF, kNS and kSYN increase altogether as the W/CM ratio increases, indicating that the cementing efficiencies and synergistic effect

Table 3 Effects of SF and NS on cube strength without sulphate attack. Resulted from addition of

Percentage increase in cube strength without sulphate attack W/CM ratio = 0.30

W/CM ratio = 0.35

W/CM ratio = 0.40

W/CM

0.5% NS only 1% NS only 5% SF only

4.9 6.9 7.1

5.2 8.3 9.1

5.7 11.5 18.2

13.9 33.3 40.5

5% SF + 0.5% NS 5% SF + 1% NS

11.7 14.9

11.5 14.7

24.7 33.0

46.3 64.0

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L.G. Li et al. / Construction and Building Materials 223 (2019) 965–975 Table 4 Effects of SF and NS on cube strength after sulphate attack. Resulted from addition of

Percentage increase in cube strength after sulphate attack W/CM ratio = 0.30

W/CM ratio = 0.35

W/CM ratio = 0.40

W/CM ratio = 0.45

0.5% NS only 1% NS only 5% SF only

8.0 13.2 17.1

6.6 14.5 30.3

16.2 33.9 522

22.6 69.7 95.3

5% SF + 0.5% NS 5% SF + 1% NS

28.4 37.0

38.6 48.7

75.1 100.7

128.2 179.5

Table 5 Effects of SF and NS on sulphate resistance. Results from addition of

Percentage decrease in strength loss after sulphate attack W/CM ratio = 0.30

W/CM ratio = 0.35

W/CM ratio = 0.40

W/CM ratio = 0.45

0.5% NS only 1% NS only 5% SF only

13.1 26.6 41.8

3.4 16.0 54.0

16.5 33.5 47.6

9.0 30.1 43.3

5% SF + 0.5% NS 5% SF + 1% NS

66.3 85.3

68.1 82.9

67.0 84.3

61.5 77.6

Table 6 Effects of SF and NS on chloride resistance. Resulted from addition of

Percentage decrease in RCPT total charge passed W/CM ratio = 0.30

W/CM ratio = 0.35

W/CM ratio = 0.40

W/CM ratio = 0.45

0.5% NS only 1% NS only 5% SF only

13.5 24.3 44.6

16.0 20.6 54.6

20.0 24.2 66.0

14.0 24.7 68.5

5% SF + 0.5% NS 5% SF + 1% NS

58.8 74.8

66.6 77.2

75.0 81.4

74.3 80.6

of the SF and NS in sulphate resistance are all greater when the W/ CM ratio is higher. To verify the accuracy of Eq. (3), the predicted strength loss from this equation is plotted against the measured strength loss in Fig. 9. The root mean square error of the prediction is found to be 3.85%, which is small enough to be considered acceptable. It should also be noted that without the incorporation of the synergistic factor kSYN, the equation would not fit the measured results. Hence, when developing a theoretical model for prediction of sulphate resistance, the synergistic effect should not be neglected.

The values of kSF, kNS and kSYN so obtained are listed in Table 8. These results reveal that the factors kSF and kNS are both much higher than 1.0, indicating that adding SF and/or NS to replace cement is a good way of improving the chloride resistance. Moreover, the factor kNS is larger than the factor kSF, meaning that at the same content, the NS is more effective than the SF. Regarding the synergistic effect, the factor kSYN is always positive but is relatively

50

For the purpose of evaluating the CEFs and synergistic factor of SF and NS in chloride resistance, the total charge passed (ftcp) is correlated to the equivalent water/cement ratio by the following newly proposed formula:

f tcp ¼ aW=Ceq þ b

ð4Þ

in which, ftcp is total charge passed (Coulomb); and a and b are numerical coefficients. To determine the factors kSF, kNS and kSYN, similar steps as before are adopted.

Predicted strength loss (%)

5.2. CEFs and synergistic factor in chloride resistance

40

RMS error = 3.85%

30

20 0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

10

Table 7 Cementing efficiencies and synergistic factors in sulphate resistance. W/CM ratio

kSF

kNS

kSYN

0.30 0.35 0.40 0.45

3.34 5.87 6.34 6.50

6.75 9.67 19.98 22.06

190.1 200.8 245.8 286.7

Note: The values of a and b in Eq. (3) are respectively 204.37 and 51.47.

0

0

10

20

30

40

Measured strength loss (%) Fig. 9. Predicted strength loss versus measured strength loss.

50

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L.G. Li et al. / Construction and Building Materials 223 (2019) 965–975

Table 8 Cementing efficiencies and synergistic factors in chloride resistance. W/CM ratio

kSF

kNS

kSYN

0.30 0.35 0.40 0.45

2.78 5.13 7.85 9.87

4.76 7.87 10.59 12.01

86.6 78.8 51.3 37.5

Note: The values of a and b in Eq. (4) are respectively 14,625 and 3935.

Predicted total charge passed (Coulomb)

3500

2800

2100

RMS error = 3.07%

1400 0%SF+0%NS 0%SF+0.5%NS 0%SF+1%NS 5%SF+0%NS 5%SF+0.5%NS 5%SF+1%NS

700

0 0

700

1400

2100

2800

3500

(ii) With both SF and NS added together, the sulphate resistance and chloride resistance were further improved, compared to the cases with SF added alone or with NS added alone, indicating that the combined use of SF and NS have additive and positive effects on the sulphate and chloride resistance. (iii) The CEFs of SF and NS in sulphate and chloride resistance are all much higher than 1.0, proving that both SF and NS have substantial positive effects on sulphate and chloride resistance. In general, the CEFs of NS are higher than those of SF, indicating that at same content, NS is more effective than SF. Moreover, the CEFs of both SF and NS are higher at higher W/CM ratio. (iv) The synergistic effects of the combined usage of SF and NS on sulphate and chloride resistance are for the first time quantified in term of synergistic factors determined using two newly proposed formulas. The synergistic factors so obtained reveal that the synergistic effect is relatively large in sulphate resistance and relatively small in chloride resistance. (v) The small RMS errors of less than 4% when using the proposed equations to predict the sulphate and chloride resistance prove that good accuracies can be achieved if the synergistic effects are considered in the theoretical predictions. Lastly, in order to reveal the mechanism behind the synergistic effects, further investigations on the changes in porosity, packing density and microstructure due to the combined addition of SF and NS at different W/CM ratios are recommended.

Measured total charge passed (Coulomb) Fig. 10. Predicted versus measured total charge passed.

small when compared to that in the previous case of sulphate resistance. Lastly, the cementing efficiency factors kSF and kNS increase altogether with the W/CM ratio but the synergistic factor kSYN decreases as the W/CM ratio increases. To verify the accuracy of Eq. (4), the predicted total charge passed is compared to the measured total charge passed in Fig. 10. The root mean square error of the prediction is only 3.07%, which is quite small and should be acceptable. As before, without incorporation of the synergistic factor kSYN, the equation would not fit the measured results. Hence, when developing a theoretical model for prediction of chloride resistance, the synergistic effect should not be neglected. 6. Conclusions The effects of silica fume (SF) and nano-silica (NS), whether added alone or together, on the sulphate resistance and chloride resistance of concrete have been investigated by subjecting concrete specimens composing of different contents of SF, NS and water to sulphate attack test and rapid chloride permeability test (RCPT). From the test results, the individual and combined effects of the SF and NS have been quantitatively analysed. Moreover, the cementing efficiency factors (CEFs) and synergistic factors of the SF and NS with respect to the sulphate resistance and chloride resistance have been determined by regression analysis of test data using newly proposed formulas. The research findings are summarized hereunder: (i) The use of SF and/or NS increased the SP demand required to achieve the target slump, improved the sulphate resistance as demonstrated by reduced strength loss after sulphate attack, and enhanced the chloride resistance as demonstrated by reduced total charge passed during RCPT.

Declaration of Competing Interest None. Acknowledgement The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (Project Nos. 51608131, 51778150 and 51808134), Featured and Innovative Project for Colleges and Universities of Guangdong Province (Project No. 2017KTSCX061), Pearl River S&T Nova Program of Guangzhou City (Project No. 201906010064), and Guangdong University of Technology via the ‘‘One-Hundred Young Talents Plan” (Project Nos. 220413226 and 220413508) and ‘‘Outstanding Talents Support Program” (Project No. 220411336), and the provision of materials by Conhubform Construction Technology Co. Ltd. (Hong Kong), Chengdu Donglanxing Science & Technology Development Co. Ltd., Score Tech Mortar Co. Ltd., Shanghai MBT & SCG High-Tech Construction Chemicals Co. Ltd. and Xiushan Lonfee New Material Co. Ltd. References [1] V.M. Malhotra, Condensed Silica Fume in Concrete, CRC Press, Boca Raton, 2018. [2] R.N. Swamy, Cement Replacement Materials, Surrey University Press, Surrey, 1986. [3] V.M. Malhotra, Fly ash, slag, silica fume, and rice husk ash in concrete: a review, Concr. Int. 15 (4) (1993) 23–28. [4] J. Yajun, J.H. Cahyadi, Effects of densified silica fume on microstructure and compressive strength of blended cement pastes, Cem. Concr. Res. 33 (10) (2003) 1543–1548. [5] R. Siddique, Utilization of silica fume in concrete: review of hardened properties, Resour. Conserv. Recy. 55 (11) (2011) 923–932. [6] N. De Belie, M. Soutsos, E. Gruyaert, Properties of Fresh and Hardened Concrete Containing Supplementary Cementitious Materials, Springer, Cham, Switzerland, 2018. [7] A. Godman, A. Bentur, Bond effects in high-strength silica fume concretes, ACI Mater. J. 86 (5) (1989) 440–449.

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