Influence of magnesium sulphate concentration on durability of concrete containing micro-silica, slag and limestone powder using durability index

Construction and Building Materials 117 (2016) 107–120

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Influence of magnesium sulphate concentration on durability of concrete containing micro-silica, slag and limestone powder using durability index Davood Mostofinejad, Farzaneh Nosouhian ⇑, Hamed Nazari-Monfared Department of Civil Engineering, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran

h i g h l i g h t s
Durability of 36 concrete mix designs was investigated in three sulphate environments.
Durability of the concretes was compared using a proposed durability index.
Replacing 10% cement by micro-silica reduced the durability in magnesium sulphate.
Replacing 15% cement by limestone powder increased the durability of concrete.
The 5% magnesium sulphate solution was the most deteriorating environment.

a r t i c l e

i n f o

Article history: Received 4 October 2015 Received in revised form 17 April 2016 Accepted 24 April 2016

Keywords: Concrete durability Micro-silica Limestone powder Blast furnace slag Magnesium sulphate Concentration

a b s t r a c t Presented experimental study deals with the durability of concretes containing different additives of micro-silica, blast furnace slag and limestone powder as cement replacement with various ratios, exposed to magnesium sulphate environments with different concentrations of 5%, 10% and 14.7% (saturated concentration). Furthermore, the influence of different water-cement (w/c) ratios and also the parameter of time on reducing the compressive strength and increasing the volume of the concrete are investigated in the research process. To do so, 36 mix designs including concretes containing 0, 15 and 30% limestone powder as cement replacement in four conditions of cement replacement with 10% microsilica, 10% blast furnace slag, 20% blast furnace slag and no cement replacement with slag or micro-silica were considered. Three w/c ratios of 0.3, 0.4 and 0.5 were considered and 864 concrete prisms with the size of 70 mm  70 mm  70 mm were produced. Compressive strength of the specimens at 140 and 280 days of sulphate exposure and also volume variations at 70, 140, 210 and 280 days of sulphate exposure were assessed to investigate the durability of different sulphate submerged concretes. In order to compare the durability of the concretes in each environment, a durability index was developed and the most durable concretes were introduced. The 5% magnesium sulphate solution was identified as the most destructive environment and concrete with w/c ratio of 0.3 containing 15% limestone powder and 20% slag was recognized as the most durable concrete. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Although it is more than one century that concrete is used in construction industry, it is not more than several decades that the durability has been precisely studied by researchers. Concrete degradation and early failure of structures in deteriorating environments like coastal areas have attracted researchers’ attention to improve the durability of concretes in harsh environ-

⇑ Corresponding author. E-mail address: [email protected] (F. Nosouhian). http://dx.doi.org/10.1016/j.conbuildmat.2016.04.091 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

ments containing aggressive ions such as chloride and sulphate. Since sulphate ions concentration in seawaters is limited, concrete degradation caused by sulphate in seawater prolongs several years. Hence, accelerated tests utilizing higher sulphate concentrations are required in laboratory scale to investigate sulphate ion influences on concrete. Sulphate ions deteriorates concrete structures by ingression into the concrete body and reacts with its components; gypsum (CaSO42H2O) is produced through reaction of SO2 4 with cement hydration products, i.e. Ca(OH)2 [1]. Calcium sulphate formed as described above can subsequently react with C3A, usually via the formation of mono-sulfo-aluminate, to form ettringite [2].

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developed in concrete specimens; the higher the concentration of sulphate ions, the greater the expansion[25]. It has been reported that SO2 concentration in magnesium sulphate solution affects 4 sulphate attack mechanism; the first product in SO2 4 concentration lower than 4000 ppm is ettringite, while ettringite and gypsum are observed in SO2 concentration between 4000 and 4 7500 ppm and in SO2 concentration higher than 7500 ppm, 4 aggression process due to the magnesium sulphate overcomes other degradation agents [26]. This study aims to simultaneously assess the influence of magnesium sulphate concentration, water-to-cement ratio and incorporating micro-silica, slag and limestone powder as cementitious materials on durability of concrete exposed to sulphate environment. The compressive strength and the expansion of the concrete specimens were measured after their exposure to sulphate. In order to make the results of various mix designs exposed to magnesium sulphate solutions with different concentration comparable, a durability index (DI) has been also developed and examined for the first time.

Furthermore, sulphate ions may react with calcium aluminates hydrate (C-A-H) which leads to formation of ettringite [3]. Formation of gypsum and ettringite is expansive and results in stress increase and concrete cracking [4–6]. Another product that harms concrete is thaumasite which forms in reaction of sulphate ions with calcium hydroxide CH and calcium silicate hydrate C-S-H, in existence of carbonate. Temperatures lower than 10 °C and humidity help the reaction. Thaumasite decreases concrete cohesion and strength and can be formed by all of the sulphate salts [3]. So far, lots of researches on improving concrete durability against sulphate ions have been carried out and pozzolans have been taken into consideration to decrease sulphate exposure damages. Hekal et al. [7] investigated the durability of cement paste containing micro-silica, slag and limestone powder in 10% MgSO4 solution and concluded that cement pastes containing microsilica are less durable in comparison with the paste without micro-silica. On the other hand, an investigation on the influence of slag on formation of thaumasite, ettringite and gypsum in specimens exposed to magnesium sulphate has shown that in specimens with 70% slag cement and 30% ordinary Portland cement, it is mainly ettringite that forms as sulphate attack product; while in specimen made by slag cement only gypsum is formed and no thaumasite or ettringite are produced [8]. Ground granulated blast furnace slag is reported that optimizes pore structure and pore size distribution in concrete, increases compressive strength, enhances durability aspects and also makes Interfacial Transition Zone (ITZ) denser [9]. In cements with limestone powder, mono-carbonate is formed instead of mono-sulphate that causes ettringite not be formed and makes concrete more resistant against sulphate attack [10]. Note that in temperatures lower than 15°C, thaumasite can be produced [10,11]. The addition of 5% limestone has resulted in a higher compressive strength after 28 days than cements with lower or higher limestone content [12]. Ogawa et al. concluded that using blending of supplementary cementitious materials with a suitable amount of limestone powder and a controlled content of calcium sulphate can improve long term sulphate resistance of the cement pastes [13]. Utilizing pozzolans has been reported to decrease the pore volume but increase the surface scaling of concrete due to increased proportion of small diameter pores and associated growth of capillary suction [14]. Many parameters can affect concrete deterioration under sulphate exposure. Sulphate concentration, water to cement ratio [15,16], participant cation [7,17–19], temperature [3,10,20] and pH [21] are some of the factors that influence sulphate attack. Specimen’s size has been also reported as another influencing factor [22]. Previous researches demonstrate that magnesium sulphate environment is more deteriorating in comparison with sodium sulphate, especially in reduction of mass and compressive strength [7,17–19,23]. The investigations on the influences of sodium and magnesium sulphates on the expansion of the mortars made of different types of cement have revealed that expansion increment of the mortars suspended in sodium sulphate has not been constant while it has been uniform in magnesium sulphate solution from the beginning and has been increased with increase of sulphate concentration [24]. The concentration of sulphate ions in the solution has important effect on the expansion and damage

2. Experimental program 2.1. Material properties, concrete mixture proportion and specimens’ details Ordinary Portland cement (type I), 0–4.75 mm fine aggregate (sand) and 4.75– 9.5 mm coarse aggregate (gravel) were used in concrete mix designs. Fineness modulus and bulk specific gravity of the fine aggregates were 2.59 and 2.54, respectively, and bulk specific gravity of the coarse aggregates was 2.70. Water-cement (w/c) ratios of 0.3, 0.4 and 0.5 were considered in concrete mix designs. High resistant concretes with w/c ratios of 0.3 and 0.4 were designed and produced considering ACI 363R [27], ACI 211.4R-93 [28] guidelines and Okamura’s advices [29]. The slump of the concretes was adjusted to 80–100 mm utilizing melamine sulphonated formaldehyde super plasticizer. Different additives of micro-silica, blast furnace slag and limestone powder were considered to be used as cement replacement in concrete. Overall, 36 concretes containing 0%, 15% and 30% limestone powder (LP) were designed with 0%, 10% and 20% blast furnace slag (SL), and 0% and10% silica fume (SF) as cement replacements. Tables 1 and 2 illustrate the chemical and physical properties of the materials used and Table 3 shows the concrete mix proportions.

2.2. Testing A total of 864 concrete prisms with the size of 70 mm  70 mm  70 mm including 36 mix designs were made to assess the deterioration process of concrete in different magnesium sulphate environments. The molds were removed 24 h after casting and the specimens were cured in water for 14 days. Three magnesium sulphate solutions with different concentrations of 5%, 10% and 14.7% were prepared. Among all, 216 cubes were maintained in water as reference specimens and 648 specimens were distributed between three sulphate solutions, equally. Since the concentration of solutions may change over time due to sulphate precipitation, an aeration system containing an air compressor as well as the piping was used to mixing the solutions by circulating air in magnesium sulphate containers. To do so, the entering pipes, which had holes at regular intervals, were laid spirally at the bottom of the containers. The solutions were mixed up frequently using a timer which was turning on the compressor 15 min per hour; so that the air could enter and mix the solutions by pipes. Also, sufficient space between specimens was left using appropriate plastic networks (Fig. 1). These spacers not only let the solution to circulate between the specimens, but also prevented the specimens to stick together. In order to compare the durability of concretes in different environments, compressive strength reduction of sulphate exposed specimens relative to that of the reference specimens was determined at 140 and 280 days exposure period; and also, volume variation of the specimens at four different times of 70, 140, 210 and 280 days of sulphate exposure were examined. Note that 28-day compressive

Table 1 Chemical composition of materials (%). Material

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

K2O

Cement Micro-silica Slag

0.81 75–98 34.6

0.58 0.03–5.78 16.7

21.46 0.06–4.54 1.4

63.95 0.01–0.83 36.1

1.86 0.36–0.52 7.56

1.42 – 1.6

0.26 0.17–0.23 1.1

– 1.15–2.02 1.0

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D. Mostofinejad et al. / Construction and Building Materials 117 (2016) 107–120 Table 2 Physical characteristics of materials. Material

Min. size (lm)

Max. size (lm)

Avg. size (lm)

Density (kg/m3)

Specific surface (m2/kg)

Specific gravity

Cement Micro-silica Limestone powder

– 0.03 –

– 0.77 –

– 0.2 10

– 173 –

300 (blaine) 14000 2000

3.15 2.21 –

strength of different mix designs was assessed using 108 reference specimens. All the experiments were performed in triplicates and the given results are the average values.

3. Results and discussion 3.1. Apparent corrosion of specimens The corrosion of concrete specimens submerged in magnesium solutions getting started by forming some cracks on the edges of the cubes which led to concrete surface crumbling over time, though no crack was seen before 70 days sulphate exposure. By 140 days of exposure, some swelling were produced on specimens

which were different in number according to the w/c and pozzolanic additives-to-cement ratios. The number and size of the swellings increased over time in such a way that exacerbated concretes’ crumbling. Fig. 2 shows a specimen exposed to 14.7% magnesium sulphate solution at 140 and 210 days of exposure. Generally, more apparent corrosion was observed in solutions with higher magnesium sulphate concentration. Fig. 3 shows specimens with the same mix design after 280 days exposure to 5%, 10% and 14.7% magnesium sulphate exposure. Moreover, concretes with lower w/c ratio exhibited less deterioration. Fig. 4 shows two specimens with w/c ratios of 0.3 and 0.5 but the same amount of silica fume (10%) and limestone powder (30%) as cement replacement after 210 days suspension in 14.7% magnesium sulphate solution.

Table 3 Concrete mix designs proportions (kg/m3). Mix design

W/CM

C

SF

SL

LP

CA

FA

W

SP

cconcrete

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

530 450.5 371 477 397.5 318 477 397.5 318 424 344.5 265

0 0 0 53 53 53 0 0 0 0 0 0

0 0 0 0 0 0 53 53 53 106 106 106

0 79.5 159 0 79.5 159 0 79.5 159 0 79.5 159

1106.7 1107.4 1107.6 1106.7 1107.1 1107.4 1106.7 1107.4 1107.6 1106.7 1107.4 1107.6

612.8 600.5 587.7 594.6 581.9 569.3 605.9 594.6 582.8 598.8 588.5 577.8

157.7 159.3 159.6 157.6 158.4 159.1 157.7 159.2 159.6 157.7 159.2 159.6

15.00 11.93 10.60 15.50 13.91 12.32 14.58 10.87 10.34 13.25 10.47 9.54

2422.2 2409.1 2395.5 2404.4 2391.3 2378.1 2414.9 2402.1 2390.3 2406.5 2395.6 2384.5

H13 H14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

400 340 280 360 300 240 360 300 240 320 260 200

0 0 0 40 40 40 0 0 0 0 0 0

0 0 0 0 0 0 40 40 40 80 80 80

0 60 120 0 60 120 0 60 120 0 60 120

1106.6 1106.9 1107.2 1106.6 1106.7 1106.9 1106.6 1106.9 1107.2 1106.6 1106.9 1107.2

717.7 708.2 698.6 704.0 694.3 684.6 712.5 703.7 695.0 707.1 699.2 691.2

162.0 162.6 163.1 161.9 162.2 162.4 162.0 162.5 163.1 162.0 162.5 163.1

10.30 8.80 7.80 10.50 9.80 9.00 10.00 8.30 7.50 9.80 8.50 7.20

2396.6 2386.5 2376.7 2383.0 2373.0 2362.9 2391.1 2381.4 2372.8 2385.5 2377.1 2368.7

H25 H26 H27 H28 H29 H30 H31 H32 H33 H34 H35 H36

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

320 272 224 288 240 192 288 240 192 256 208 160

0 0 0 32 32 32 0 0 0 0 0 0

0 0 0 0 0 0 32 32 32 64 64 64

0 48 96 0 48 96 0 48 96 0 48 96

1107.2 1107.0 1107.5 1106.9 1106.9 1107.2 1108.2 1107.1 1107.3 1107.2 1107.3 1107.6

782.9 775.0 767.5 771.6 740.0 756.4 779.7 771.5 764.4 774.4 767.9 761.7

160.3 163.5 164.2 162.9 163.9 163.4 162.3 163.5 163.7 163.5 163.7 164.4

8.96 8.00 7.20 9.60 8.80 8.00 8.80 7.60 7.20 7.20 6.80 6.25

2379.4 2375.5 2366.4 2371.0 2339.6 2355.0 2379.0 2369.7 2362.6 2372.3 2365.7 2360.0

CM: Cementitious materials. C: Ordinary Portland cement. SF: Micro-silica (silica fume). SL: Slag. LP: Limestone powder. CA: Coarse aggregate. FA: Fine aggregate. W: Water. SP: Super plasticizer. cconcrete: Concrete density.

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Fig. 1. Plastic network used as spacer between specimens.

(a)

(b) (a)

(c) Fig. 3. Specimens with the same mix design after 280 days exposure to (a) 5%, (b) 10% and (c) 14.7% magnesium sulphate solution.

(b) Fig. 2. A specimen submerged in 14.7% magnesium sulphate solution; (a) after 140 days exposure and (b) after 210 days exposure.

This fact reveals the importance of w/c ratio in durability of concrete. Furthermore, in all sulphate concentrations, specimens with the same w/c ratio that had 10% silica fume as cement replacement showed the most deterioration which were worsened by increasing the amount of limestone powder as cement replacement. Fig. 5 shows specimens with w/c of 0.3 and 0.5 and 10% silica fume as cement replacement in different ratios of limestone powder-tocementitious materials (LP/CM), which were exposed to 10% and 14.7% sulphate solutions. It reveals the negative influence of silica fume on concrete durability in magnesium sulphate environment, and the exacerbating effect of limestone powder on apparent deterioration of concretes containing silica fume.

3.2. Compressive strength reduction Figs. 6–8 show the effect of the magnesium sulphate concentration on the compressive strength reduction of the specimens submerged in 5%, 10% and 14.7% magnesium sulphate solutions, respectively, relative to the specimens submerged in water, at 140 and 280 days of sulphate exposure. According to Figs. 6–8, increasing water-to-cementitious materials ratio (W/CM) has increased compressive strength reduction of the mix designs in all sulphate environments. Reduction of strength with increase of w/c can be due to the permeability increase, and consequently, more ingression of sulphate ions into the concrete. Moreover, compressive strength reduction descents with increasing sulphate ion concentration in a specific mix design. Figs. 6–8 also reveal that in all w/c ratios, sulphate exposed concretes containing micro-silica exhibit more compressive strength reduction compared to the concretes with no micro-silica at both

D. Mostofinejad et al. / Construction and Building Materials 117 (2016) 107–120

(a)

(b)

Fig. 4. Specimens with the same amount of silica fume (10%) and limestone powder (30%) as cement replacement after 210 days suspension in 14.7% magnesium sulphate solution; (a) w/c = 0.3 and (b) w/c = 0.5.

I

(a)

(b)

(c)

(a)

(b)

(c)

II

111

respectively. Also, in simultaneous using of limestone powder and silica fume or slag, at a constant amount of limestone powder, the most strength reduction belongs respectively to the concrete containing 10% micro-silica, concrete without micro-silica or slag, concrete containing 20% slag and concrete containing 10% slag. It represents the efficacy of utilizing blast furnace slag in concretes exposed to magnesium sulphate, which is mainly due to the decrease in the amount of the calcium hydroxide in concrete through the reaction with slag. Note that magnesium sulphate reacts with calcium hydroxide of the hydrated cement paste and produces gypsum which deteriorates the matrix of concrete. It is also distinguished from Figs. 6–8 that the most reduction in compressive strength has occurred in concretes with 30% limestone powder, without limestone powder and with 15% limestone powder, respectively. It shows that the concrete with 15% limestone powder replacement of cement is the mix design with the least strength decrease in sulphate environment. This is, in fact, due to the limestone powder participation in hydration reaction which leads to mono-carbonate (mono-carbo-aluminate) formation instead of mono-sulphate (hydrous mono-sulfo-aluminate) which produced during the hydration of ordinary Portland cement. During sulphate attack to concretes made by ordinary Portland cement, mono-sulphate reacts with gypsum and produces ettringite which causes expansion. In concretes made by cement accompanied with limestone powder, ettringite is not produced due to the lack of the mono-sulphate; which consequently makes the cement paste resistant against sulphate. The enhancing influence of using limestone powder, however, is lower than the impact of the cement reduction when 30% of cement is replaced by limestone powder, which eventually decreases the concrete durability. According to Figs. 6–8, reduction of compressive strength has increased in all environments over time; however, the slope of the curves between 140 to 280 days is lower than that between 0 to 140 days. It reveals that destruction effect of the sulphate ions in the first 140 days of exposure has been higher than that in the second 140 days. The most compressive strength reduction is about 37.1% at 140 days and 50.0% at 280 days which belongs to H30 concrete exposed to 5% magnesium sulphate. This specimen contained 30% limestone powder and 10% silica fume as replacement of cement. 3.3. Volume variation (expansion)

Fig. 5. Specimens with 10% silica fume as cement replacement after 280 days suspension in (I) 10% magnesium sulphate solution (w/c = 0.3) and (II) 14.7% magnesium sulphate solution (w/c = 0.5) in different ratios of LP/CM; (a) LP/CM = 0, (b) LP/CM = 0.15 and (c) LP/CM = 0.3.

test times of 140 and 280 days. It represents the negative effect of micro-silica on compressive strength of the concretes in magnesium sulphate environments which is probably due to the conversion of C-S-H to magnesium silicate hydrate (M-S-H) that destroys cement paste, and consequently, decreases the compressive strength. It is noticeable in Figs. 6–8 that the most decrease in compressive strength of the specimens exposed to magnesium sulphate solutions belongs to the concretes without slag, with 20% slag as cement replacement and with 10% slag as cement replacement,

Sulphate exposure deteriorates concrete through sulphate ions reaction with cement hydration products (mainly calcium hydroxide, Ca(OH)2) leading to formation of expansive materials of gypsum (CaSO42H2O) and ettringite (C3ACS
H32). These expansive products cause concrete cracking and consequent deterioration. Thus, sulphate attack is mainly featured through concrete expansion. Furthermore, magnesium ions lead to brucite (Mg(OH)2) formation which causes crystal growth pressures, leading to concrete expansion either [30,31]. These expansive products lead to concrete cracking and consequent deterioration. Hence, the other parameter considered for assessing the durability of concretes in sulphate environment is the volume variation which was calculated from the following expression:

Volume v ariation ð%Þ ¼

V2  V1  100 V1

ð1Þ

where V2 refers to the volume of the specimen at testing time, and V1 is the volume of the same specimen before submerging in sulphate solution. The expansion percents of the specimens submerged in 5%, 10% and 14.7% magnesium sulphate solutions are respectively depicted in Figs. 9–11 at 70, 140, 210 and 280 days of sulphate exposure. The figures reveal that in concretes with constant cement replacement

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30

30

W/CM=0.3 LP/CM=0

20

20

10

10

0

0

30

140

0

280

20

10

10

0

30

0

30

W/CM=0.3 LP/CM=0.30

20

0

W/CM=0.3 LP/CM=0.15

140

0

280

W/CM=0.4 LP/CM=0.15

280

140

280

140

280

140

280

W/CM=0.4 LP/CM=0

0

W/CM=0.4 LP/CM=0.30

40

20

140

30 20

10 10

0

0

140

0

280

0

40

W/CM=0.5 LP/CM=0

30

20

20

10

10 0

W/CM=0.5 LP/CM=0.15

30

0

140

0

280

0

W/CM=0.5 LP/CM=0.30

50 40 30 20 10 0

0

140

280

Fig. 6. Compressive strength reduction (%) in concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 5% magnesium sulphate environment at 140 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the compressive strength reduction in percent.)

materials, increase in the w/c ratio increases the expansion in all sulphate environments. It is due to the increased permeability which has led to more destructive reactions in concrete. It is obvious in the figures that expansion percent of the specimens has increased with increase in exposure time. However, the expansion

percent curves of some of the concretes have descended after 210 days which is because of segregation of concrete surface debris due to the high corrosive environment. The descending of specimens’ volume is displayed on Figs. 10 and 11 to show the crumbling effect of the high concentration magnesium sulphate

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30

20

W/CM=0.3 LP/CM=0

W/CM=0.3 LP/CM=0.15

15

20

10

10

0

5

0

30

140

0

280

30

W/CM=0.3 LP/CM=0.30

20

20

10

10

0

0

30

0

140

0

280

W/CM=0.4 LP/CM=0.15

280

140

280

140

280

140

280

W/CM=0.4 LP/CM=0

0 W/CM=0.4 LP/CM=0.30

40

20

140

30 20

10 10 0

0

140

W/CM=0.5 LP/CM=0.15

20

20

10

10 0

0

30

W/CM=0.5 LP/CM=0

30

0

280

0

140

0

280 50

0

W/CM=0.5 LP/CM=0.30

40 30 20 10 0

0

140

280

Fig. 7. Compressive strength reduction (%) in concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 10% magnesium sulphate environment at 140 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the compressive strength reduction in percent.)

environments on specimens’ expansion, especially in concretes containing limestone powder and micro-silica. Increasing sulphate ion concentration has increased the specimens’ expansion in such a way that has led to crumbling of the specimens’ surface in some mix designs in higher sulphate concentrations. Note that the influence of the sulphate ion concentration on concrete expansion is

opposite to its influence on the compressive strength. It shows that sulphate ion concentration has a dual influence on the durability of concrete. Figs. 9–11 also show that increasing the ratio of silica fume to cementitious materials (SF/CM) has increased the concrete expansion exposed to magnesium sulphate at both 140 and 280 days. It

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25

15

W/CM=0.3 LP/CM=0

20

W/CM=0.3 LP/CM=0.15

10

15 10

5

5 0 0 30

140

0

280

0

30

W/CM=0.3 LP/CM=0.30

20

20

10

10

0

0

20

140 W/CM=0.4 LP/CM=0.15

15

280

140

280

140

280

140

280

W/CM=0.4 LP/CM=0

0 W/CM=0.4 LP/CM=0.30

40 30

10

20

5 0

0

280

140

10

0

140

0

280

30

0

25 W/CM=0.5 LP/CM=0

W/CM=0.5 LP/CM=0.15

20

20

15 10

10

5 0

0

140

0

280 50

0

W/CM=0.5 LP/CM=0.30

40 30 20 10 0

0

140

280

Fig. 8. Compressive strength reduction (%) in concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 14.7% magnesium sulphate environment at 140 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the compressive strength reduction in percent.)

reveals that micro-silica may be an inappropriate component in concretes subjected to magnesium sulphate environment from volume expansion point of view. Note that negative expansions have been mainly due to the surface crumbling of the specimens, which has been more observed in concretes incorporating limestone powder and 10% micro-silica.

Regardless of the specimens with negative expansions due to the high corrosion, the highest expansion (about 5.45%) was observed in 14.7% magnesium sulphate solution and was corresponding to the concrete with w/c ratio of 0.5 containing 10% slag and 30% limestone powder. From the expansion viewpoint, using slag in concretes subjected to magnesium sulphate environments

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1.5

1.5

W/CM=0.3 LP/CM=0

1

1

0.5

0.5

0

0

2

70

140

210

0

280

0

1.5

W/CM=0.3 LP/CM=0.30

1.5

W/CM=0.3 LP/CM=0.15

70

140

210

280

140

210

280

140

210

280

140

210

280

W/CM=0.4 LP/CM=0

1

1 0.5

0.5 0

0

2

70

140

210

0

280

2

W/CM=0.4 LP/CM=0.15

1.5

1

0.5

0.5

0

2

70

140

210

0

280

1.5

0

1.5

W/CM=0.5 LP/CM=0

70

W/CM=0.4 LP/CM=0.30

1.5

1

0

0

70

W/CM=0.5 LP/CM=0.15

1

1 0.5

0.5 0

0

70

140

210

2.5

0

280

0

70

W/CM=0.5 LP/CM=0.30

2 1.5 1 0.5 0

0

70

140

210

280

Fig. 9. Expansion percent of concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 5% magnesium sulphate environment at 70, 140, 210 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the expansion in percent.)

decreases durability. The influence of slag on concrete expansion is opposite of that on compressive strength which shows dual effect of slag on concrete in sulphate environment. It is also specified in Figs. 9–11 that for the specimens in sulphate environments with constant ratios of water-to-cementitious materials (W/CM), slag-to-cementitious materials (SL/CM) and silica

fume-to-cementitious materials (SF/CM), the highest expansions are respectively observed in specimens with 30% limestone powder, without limestone powder, and with 15% limestone powder as cement replacement. It shows that similar to the compressive strength aspect, the optimum amount of limestone powder is about 15% replacement to cement from the expansion viewpoint.

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1.5

1.5

W/CM=0.3 LP/CM=0

1

1

0.5

0.5

0

0

70

140

210

0

70

140

210

280

140

210

280

140

210

280

140

210

280

2.5

2

W/CM=0.3 LP/CM=0.30

1.5

W/CM=0.4 LP/CM=0

2 1.5

1

1

0.5 0

0

280

W/CM=0.3 LP/CM=0.15

0.5 0

1.5

70

140

210

0

280

0

2

W/CM=0.4 LP/CM=0.15

W/CM=0.4 LP/CM=0.30

1.5

1

70

1 0.5

0

0.5

0

2

70

140

210

1

0.5

0.5

0

70

140

210 3

0

280

70

W/CM=0.5 LP/CM=0.15

1.5

1

0

0

2

W/CM=0.5 LP/CM=0

1.5

0

280

0

70

W/CM=0.5 LP/CM=0.30

2.5 2 1.5 1 0.5 0

0

70

140

210

280

Fig. 10. Expansion percent of concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 10% magnesium sulphate environment at 70, 140, 210 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the expansion in percent.)

4. Durability index 4.1. Durability index interpretation Since the influences of compressive strength and expansion parameters on concrete durability were not similar, e.g. the

concrete with highest compressive strength reduction has not necessarily shown the highest expansion, a general expression was developed for determining the most durable concrete in the current study. Earlier indices have presented only one of the concrete durability parameters; e.g. Santhanam et al. assessed the durability of specimens subjected to sodium and magnesium

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2

2

W/CM=0.3 LP/CM=0

1.5

1.5

1

1

0.5

0.5

0

0

2

70

140

210

0

280

W/CM=0.3 LP/CM=0.30

1.5

0

70

140

210

280

140

210

280

140

210

280

W/CM=0.4 LP/CM=0

3 2

1

1

0.5 0

W/CM=0.3 LP/CM=0.15

0

2.5

70

140

210

0

280

W/CM=0.4 LP/CM=0.15

2

0

W/CM=0.4 LP/CM=0.30

3

1.5

70

2

1 1

0.5 0

0

70

140

210

70 W/CM=0.5 LP/CM=0.15

3

2

2

1 0

0

4

W/CM=0.5 LP/CM=0

3

0

280

1

0

70

140

210 5

280

0

0

140

70

210

280

W/CM=0.5 LP/CM=0.30

4 3 2 1 0

0

70

140

210

280

Fig. 11. Expansion percent of concretes with different W/CM, LP/CM, SF/CM and SL/CM, exposed to 14.7% (saturated) magnesium sulphate environment at 70, 140, 210 and 280 days of sulphate exposure. (Note: the horizontal axes show the sulphate exposure time in ‘‘days” and the vertical axes represent the expansion in percent.)

sulphates considering the expansion [32], while Irassar used flexural strength to determine the concrete durability in sulphate environment [33]. In order to define a more general durability index based on the results of this study, the following expression is devised for durability index (DI):

   a1 f 1 a2 f 2 DI ¼ 1000 1  þ max ðf 1 Þ max ðf 2 Þ

ð2Þ

where f1 and f2 refer to the compressive strength reduction (%) and the specimen’s expansion (%), respectively; max (f1) and max (f2) are respectively the maximum amounts of the strength reduction and

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expansion percent of concretes in the same environment at the same specified exposure time; and a1 and a2 are the weight coefficient of each parameter which could be considered between 0 and 1.0 according to importance degree of the corresponding fi. In practice, ai depends on various factors such as concrete application type, constructing costs and casting location, and should be determined based on the previous in-site experiences. Note that summation of ai must be equal 1. In order to achieve a proper range for comparing concretes durability, 1000 has been multiplied in the expression. According to Eq. (1), a concrete with higher durability has a closer DI to 1000. Since ai are determinant factors, DI for the concretes in the current study in all environments with different ai are presented in Table 4 at 280 days of exposure. Three different ai are considered as follows:

I : a1 ¼ 0:50; a2 ¼ 0:50 II : a1 ¼ 0:30; a2 ¼ 0:70 III : a1 ¼ 0:70; a2 ¼ 0:30 As the first approach, coefficients ai were assumed equal (i.e. ai = 0. 5), which means both of the compressive strength reduction (%) and the expansion (%) have an equal influence on concrete

durability. However, the influence of compressive strength reduction (%) and the expansion (%) on durability of concrete are respectively considered higher in the second and third approaches.

4.2. Dividing concretes based on DI In order to specify the durability of concretes in different environments, concretes were arbitrarily divided into four groups; very high durability (VHD) concretes with DI of more than 750, high durability (HD) concretes with DI between 500 and 750, average durability (AD) concretes with DI between 250 and 500, and low durability (LD) concretes with DI of less than 250. It should be noted that since reduction of compressive strength is used in most of the studies to evaluate the concrete durability in harsh environments, and also since the strength reduction is a more reliable parameter than the expansion, because of the volume errors due to the surface crumbling of specimens in sulphate environment, here it was decided to use mode III (a1 = 0.70, a2 = 0.30) to illustrate the durability index. Table 5 presents the concretes with the DI of at least 85% of the highest DI in each environment; i.e. it shows the mixes with DI of at least 632 in 5% magnesium sulphate environment, 663 in 10% magnesium sulphate environment and 749 in 14.7% magnesium sulphate environment. According to

Table 4 Durability indices (DI) of concrete specimens subjected to magnesium sulphate environments at 280 days of sulphate exposure (calculated according to Eq. (1) in three modes of I, II and III as fi various weight coefficient). Environment

1 2 3

5%

10%

Magnesium sulphate

Magnesium sulphate

Magnesium sulphate

II2

III3

I

II

III

I

II

III

597 713 580 440 598 369 525 660 464 605 718 609

616 702 602 418 565 320 496 611 411 619 691 616

578 724 559 463 632 417 554 709 516 591 744 602

556 730 512 504 625 420 588 727 546 624 750 610

556 726 505 493 603 392 550 674 498 604 724 589

556 734 519 515 647 448 626 780 594 645 776 631

682 822 641 614 767 550 733 850 728 723 809 713

689 833 665 643 797 585 719 820 716 706 763 696

675 810 618 584 737 515 747 881 740 740 855 731

H13 H14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24

526 572 479 362 452 173 485 569 417 572 632 558

522 551 470 337 436 175 458 537 364 585 626 566

530 593 489 387 468 171 512 601 470 559 638 550

541 635 498 347 471 183 523 628 453 533 640 438

536 608 493 320 457 195 498 587 401 524 608 399

546 662 503 375 484 171 549 670 506 541 673 477

612 670 592 479 615 283 671 726 617 650 743 639

617 659 604 469 624 346 657 704 583 636 732 621

607 681 581 489 606 219 684 748 651 664 755 656

H25 H26 H27 H28 H29 H30 H31 H32 H33 H34 H35 H36

455 531 423 254 360 0 430 485 310 438 514 390

427 494 402 226 348 0 388 424 227 402 467 341

484 567 443 282 373 0 473 546 393 475 561 439

462 584 425 293 490 90 376 561 262 390 496 266

439 543 393 269 507 125 316 493 157 337 413 165

485 625 456 318 474 54 437 629 367 443 580 367

568 638 540 423 527 0 547 630 336 619 672 508

556 615 532 415 507 0 496 571 203 600 636 446

580 662 548 431 548 0 598 688 470 638 708 569

Avg.

479.58

456.67

502.58

493.8

463.81

523.97

609.36

596.14

622.61

Mix design

I

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12

1

14.70%

Mode I: a1 = 0.50, a2 = 0.50. Mode II: a1 = 0.30, a2 = 0.70. Mode III: a1 = 0.70, a2 = 0.30.

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D. Mostofinejad et al. / Construction and Building Materials 117 (2016) 107–120 Table 5 Durable concretes in sulphate environments at 280 days of sulphate exposure considering mode III. Environment

DIAvg.

DImax

Mix design with DImax

0.85 * DImax

Mix designs with DI P 0.85 * DImax

Very high durable concretes (DI P 750)

5% magnesium sulphate 10% magnesium sulphate 14.7% magnesium sulphate

502.58 523.97 622.61

744 780 881

H11 H8 H8

632 663 749

H2, H5, H8, H11 and H23 H2, H8, H11, H20 and H23 H2, H8, H11 and H23

– H8 and H11 H2, H8, H11 and H23

Table 5, none of the concretes has displayed very high durability in 5% magnesium sulphate environment while concretes with w/c ratio of 0.3 containing 15% limestone powder plus 10% and 20% blast furnace slag (H8 and H11) have respectively produced the most durable concretes in 10% magnesium sulphate solution. In sulphate saturated solution, concretes with w/c ratio of 0.3 containing 15% limestone powder with (H8 and H11) or without (H2) blast furnace slag have respectively shown very high durability. At last, concretes with w/c ratio of 0.4 containing 15% limestone powder and 20% slag without micro-silica (H23) were also in ‘‘very high durability” category. It reveals that increasing w/cratio made concrete more vulnerable in sulphate environments as none of the mix designs with w/c ratio of 0.5 had DI higher than 750. On the other hand, incorporating 15% limestone powder has improved the concrete durability while using micro-silica has decreased the durability (Table 4). It is also noticeable in Table 4 that incorporating slag in concretes with w/c ratio of 0.3 containing 15% limestone powder (H8 and H11) has mostly decreased the DI compared to the mix design without slag (H2); while in concretes with w/c ratio of 0.4 containing 15% limestone powder (H20 and H23), utilizing slag has increased the DI compared to the mix design without slag (H14). It means that the influence of slag on durability of concrete may depends on the w/c ratio. Moreover, Table 5 indicates that magnesium sulphate solutions with higher concentrations have shown lower deteriorating influences on concrete (higher DIAvg.). The above discussions indicate that the proposed durability index can provide a lot of comparative information that here it was only partly referred. 5. Conclusions In this paper, the influence of magnesium sulphate concentration on durability of concretes containing micro-silica, blast furnace slag and limestone powder as cement replacements has been experimentally investigated through testing 36 mix designs subjected to 5%, 10% and 14.7% (saturated) magnesium sulphate environments. Following conclusions can be drawn based on the results of the current study: (1) The 5% magnesium sulphate solution was observed as the most deteriorating environment from the compressive strength reduction viewpoint; while the 14.7% magnesium sulphate solution has exhibited the most destructive environment from expansion aspect. (2) Replacing 10% cement by micro-silica reduced durability of concretes in magnesium sulphate environment. (3) Blast furnace slag had a different influence on concrete durability; replacing cement with slag in concretes subjected to the magnesium sulphate environment increased the durability from the strength viewpoint, while it behaved inversely from the expansion aspect. (4) Utilizing 15% limestone powder as cement replacement increased durability of concretes submerged in all magnesium sulphate solutions during 280 days of sulphate exposure, while it decreased the durability when 30% of cement replaced by limestone powder.

(5) Based on the durability index (DI) proposed in this study, it was specified that the most durable concrete in all three magnesium sulphate environments have the lowest w/c, i.e. w/c = 0.3. The most durable concrete in 5% magnesium sulphate environment, which has been the most deteriorating environment, contains 20% slag and 15% limestone powder as cement replacements (DI = 744), while the concrete that contained 10% slag and 15% limestone powder as cement replacements has been the most durable concrete in 10% (DI = 780) and 14.7% (DI = 881) magnesium sulphate environments.

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