Long-term behaviors of concrete under low-concentration sulfate attack subjected to natural variation of environmental climate conditions

Cement and Concrete Research 116 (2019) 217–230

Contents lists available at ScienceDirect

Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres

Long-term behaviors of concrete under low-concentration sulfate attack subjected to natural variation of environmental climate conditions

T

Zhang Zhongyaa, Jin Xiaoguangb, , Luo Weic ⁎

a

School of Civil Engineering, Chongqing University, Chongqing 400045, People's Republic of China Key Laboratory of New Technology for Construction of China in Mountainous Area, Chongqing University, Chongqing 400045, People's Republic of China c National Joint Engineering Research Center of Geohazards Prevention in the Reservoir Areas, Chongqing University, Chongqing 400045, People's Republic of China b

ARTICLE INFO

ABSTRACT

Keywords: Long-term behaviors Concrete Durability Sulfate attack

Accelerated laboratory tests on resistance of concrete exposed to sulfate attack have generally been conducted in high-concentration sulfate solutions or under drastic drying-wetting cycle conditions. However, these accelerated regimes radically alter the nature of sulfate attack mechanisms on concrete under real field exposure situation. To obtain reliable information on the long-term behaviors of concrete under real field conditions, the behaviors of concrete samples under three different exposure regimes, i.e., continuous full immersion, full immersion with general use drying-wetting cycles and full immersion with natural drying-wetting cycles, were investigated in this research. Three different concentrations of sulfate sodium solutions, i.e., 0% water for control, 2.1% for field-like condition and 15% for high-concentration condition, were considered. Physical and mechanical properties, such as mass, expansion, permeability and compressive strength, were tested at regular time intervals during the whole exposure period to describe the associated evolution laws. Microanalysis was also carried out to identify the underlying mechanisms. Results from this study showed that the exposure regime of full immersion in 2.1% sulfate sodium solutions subjected to natural drying-wetting cycles can well reproduce the field exposure condition of concrete under certain sulfate-rich environments. Both concentration and exposure type affect the nature of sulfate attack mechanism on concrete, along with the evolution of physical and mechanical properties. A three-stage evolution model, consisting of the enhancement stage, the incubation stage and the degeneration stage, was observed in the property evolution of concrete samples under the field-like exposure condition. Meanwhile, a distinct coupling physicochemical damage on concrete samples was detected when subjected to drying-wetting cycle exposure. In addition, the effects of water-to-binder ratio and binder type were duly studied.

1. Introduction

sulfate solutions (e.g. [10–12]) or by placing them under drastic dryingwetting cycle conditions (e.g. [13–15]). These exposure conditions are believed to be far away from the condition of concrete exposed to sulfate containing environments on spot [25,26]. Although these accelerated tests on concrete samples can short testing period and saving cost, few of them are able to perfectly forecast the long-term performance and to well assess the durability under field conditions. Meanwhile, it is hard to link these testing data to field performance of concrete materials. Therefore, it is of great significance to evaluate the long-term behaviors of concrete subjected to field exposure conditions. When concrete contacts with sulfate bearing environments, deleterious sulfate ions has access to penetrate into hardened cement paste, due to diffusion by water [16]. Many components of matrix are vulnerable to sulfate ions [17,18]. Calcium hydroxide (CH) reacts with sulfate ions to form gypsum, which can in turn react with unhydrated

Concrete is widely applied in marine and offshore engineering (e.g., offshore oil platforms, piers and offshore wind turbines), due to its reliability for strength and durability when exposed to aggressive environments [1]. However, harsh marine environments may cause serious degeneration on concrete, resulting in an immediate requirement of repair on them. These degeneration processes begin with the ingress of certain pernicious species into the microstructure, such as sulfate ions (SO42−) and chloride ions (Cl−), among others [2–4]. In the past few decades, a high number of research programs have been conducted to capture the nature of sulfate attack mechanisms on cement-based materials and to explore how to mitigate this distress (e.g. [5–9]). These studies were almost carried out in the form of accelerated laboratory tests, either by submerging concrete samples in high-concentration ⁎

Corresponding author. E-mail address: [email protected] (J. Xiaoguang).

https://doi.org/10.1016/j.cemconres.2018.11.017 Received 30 July 2018; Received in revised form 22 October 2018; Accepted 28 November 2018 0008-8846/ © 2018 Elsevier Ltd. All rights reserved.

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

tricalcium aluminate (C3A) phase to form ettringite (AFt) [19–22]. Monocarbonate (AFm) also combines with sulfate ions to form ettringite (AFt) [23]. When the buffering calcium cations (Ca+) are depleted, decalcification of C-S-H gels (calcium silicate hydrate) begins, which may lose their intrinsic cementing property during this process [24]. All of these reactions modify the microstructure and performance of the hardened cement paste, accompanied by macroscopical expansion and microscopic cracking [27,50]. Subsequently, deterioration, softening and disintegration of concrete materials are observed [28,29]. Apart from this chemical sulfate attack (CSA), physical sulfate/salt attack (PSA) involving salt crystallization and phase transformation has currently been identified as an accomplice in service concrete structures/elements [7,8,13,16]. Clearly, existing investigations have obtained some important details on the driving mechanism of sulfate attack on hardened concrete. Unfortunately, there is still dearth of information on long-term performance evolution of concrete under field conditions. In zones of splash and tidal or drying-wetting cycles, natural variation in temperature and relative humidity (RH) aggravates these two damage mechanisms on concrete, resulting in premature deterioration of concrete materials. Thus, it is critical to obtained reliable information on the long-term behaviors of concrete under these field conditions. As per ASTM C1012 [30], 5% sodium sulfate solution was suggested to be used in lab tests on concrete resistance to sulfate attack. However, no detailed explanation about this selected concentration was given. Bellmann et al. [10] investigated the influence of sulfate solution concentration on sulfate resistance of concrete. Results of that paper showed that the performance of concrete under field conditions was different from that suggested by laboratory tests. Since almost complete loss of the alkali ions from the pore solution by diffusing out of the concrete in high-concentration sulfate sodium solutions (e.g., 15% or 30% w/w), precipitation of gypsum rather than ettringite dominated in deterioration mechanism on concrete under this exposure condition. However, this symptom was rare to be detected under real field conditions. This was confirmed by Drimalas [31] through a comprehensive evaluation on deterioration of concrete structures in the state of Texas. Therefore, concentration of sulfate solutions has a significant influence on mechanism of sulfate attack on concrete [25]. High-concentration sulfate solutions can change the deterioration nature of concrete exposed to aggressive sulfate-rich environments [10]. Clearly, a sulfate concentration similar to field conditions is more suitable to be used to study the underlying deterioration mechanism of concrete under real field conditions. Monteiro and Kurtis [26] reported the long-term and non-accelerated laboratory tests on concrete resistance under sulfate exposure for over 40 years, in which 2.1% w/w sulfate sodium solution was adopted. However, expansion of those tested concrete samples was the only measured property [26]. Considering splash and tidal zone with/without sulfate attack, drying-wetting cycles of temperature and relative humidity (RH) were generally performed in tests on concrete resistance to sulfate attack, concurrently to notably reduce test period. Bassuoni and Nehdi [15] adopted a drying-wetting cycle regime consisted of 2 days of immersion in a 5% sodium sulfate solution (pH of 6.0–8.0) at a temperature of 20 ± 2 °C followed by 2 days of drying at 40 °C and 40% RH, to investigate the durability of self-consolidating concrete exposed to sulfate attack. Najjar et al. [8] suggested a similar cycle regime consisted of one week at 20 °C and 80% RH followed by one week at 40 °C and 35% RH to explore the damage mechanism of two-stage concrete exposed to sulfate attack. In the experimental study on resistance of cement mortars under sulfate attacks [32], a cycle regime, composed of full immersion in sulfate solution at 20 °C for 94 h followed by oven drying at 50 °C for 72 h and cooling in air at 20 °C for 2 h, was proposed. Under this cycle regime for 24-weeks exposure, the maximum damage was observed in mortar made with 92% Ordinary Portland Cement (OPC) and 8% silica fume (SF), with a mass loss of 7.03%. Note that a high temperature of 50 °C was designed in drying phase, which is rare under practical climate conditions. However,

temperature significantly affects the rate of chemical reactions, which is the main driving mechanism of chemical sulfate attack [33–35]. In addition, both RH and PH have an influence on these attack processes [36–40]. Although regimes of drying-wetting cycles are diverse and widely available in previous researches, few of them concern the real field exposure conditions of concrete on spot. In fact, these regimes are more drastic than those happen in natural environment. In particular, the high-temperature and low-RH of drying phase may lead to rapid evaporation of moisture, resulting in shrinkage and supersaturation in pore solution containing sulfate ions. This process facilitates salt crystals growth and phase transformation of sulfate within concrete pores, which may generate destructive pressure on pore wall, resulting in subsequent damage on concrete materials [41–44]. However, these unrealistic aspects of accelerated laboratory tests, such as high-concentration solutions or drastic drying-wetting cycles, are often overlooked due to the short test period and high efficiency. To obtain reliable experimental data on durability of concrete under real field conditions, the long-term evolution of concrete performance under a field-like sulfate exposure condition was investigated in current study. The field-like exposure condition was a natural drying-wetting cycle regime similar to the occurrence in nature, such as in splash and tidal zones. In wetting phase, concrete samples were fully-submerged in 2.1% w/w sulfate sodium solutions. This concentration of sodium sulfate solutions was referring to that used by U.S. Bureau of Reclamation [26]. However, it was close to the concentration in the water heavily contaminated by sulfate ions, which ranges from about 0% to 2% w/w [26]. In drying phase, they were taken out from solutions and air-dried in laboratory undergoing natural environmental climate condition (NECC). This exposure regime is designed to simulate the field exposure condition, such as concrete structures in marine and offshore engineering. In addition, comparative trials were conducted under full immersion regime and general use drying-wetting cycles, in pure water or in high-concentration sodium sulfate solutions (15% w/w), respectively. Long-term evolution of mass, expansion, water permeability coefficient and compressive strength were tested and analyzed by different methods. In addition, microanalysis was also carried out to provide evidence about the underlying mechanisms of sulfate attack on concrete. 2. Experimental program 2.1. Materials In this study, general use (GU) Ordinary Portland Cement (OPC, C3A content of 9%, P.O. 42.5) was used as the main cementitious materials (CMs). The physical and chemical properties of the cementitious materials and supplementary cementing materials are provided in Table 1. The total cementitious materials (CM + SCMs) content was kept constant at 464 kg/m3 for all mixtures in current study. The fine aggregate was natural siliceous sand with a fineness modulus of 2.80, a saturated surface dry specific gravity of 2.65 and water absorption of 1.5%. The coarse aggregate was crushed stone with continuous grading and a maximum size of 10 mm, a saturated surface dry specific gravity of 2.68 and water absorption of 0.8%. A polycarboxylate superplasticizer having a water-reducing rate of 21% was used as water reducing admixture. In addition, three water-to-binder ratios (w/b) were considered in this study. And, two common used supplementary cementing materials (SCMs), i.e., fly ash (FA) and silica fume (SF) was used to make binary binders. The mix design proportions of concrete mixtures are listed in Table 2. 2.2. Specimen preparation The casting and curing conditions of the concrete mixtures comply with the GB 50081-2002 [45] and GB/T 50082-2009 [46]. Concrete was mixed in a mechanical mixer and cast in cylindrical (50 mm 218

Cement and Concrete Research 116 (2019) 217–230

94.0 0.1 0.1 0.4 0.4 1.3

0.3 280 2.08

4.7 19,530 2.12

2.5 410 3.17

b

61.8 12.1 9.1 7.6

FA20

SF10

464

464

464

907 890 3.376 162.4 0.35

907 890 3.376 208.8 0.45

907 890 3.376 255.2 0.55

371.2 92.8 907 890 3.376 208.8 0.45

417.6 46.4 907 890 3.376 208.8 0.45

Concentration of solutions (w/w)

diameter and 100 mm height), cubic (100 mm3) and prismy (40 mm × 40 mm × 160 mm) molds to prepare test samples for each mixture. Three replicates were made for each sample to verify reproducibility of results. The concrete samples were demoulded after casting for 24 h, and then cured in a curing chamber for 28 days at standard conditions (22 ± 2 °C [71.6 °F] and 95 ± 3% RH) according to GB 50081-2002 [45] and GB/T 50082-2009 [46]. After standard curing for 28 days, concrete samples were left in the laboratory conditions of 23 °C [73.4 °F] and RH of 70% for one day to eliminate excess moisture, aiming at providing a uniform basis of comparison. Then, the initial mass (m0), initial length (L0), initial compressive strength (σC0), initial elastic modulus (E0), initial permeability and initial porosity were determined before immersion in sulfate sodium solutions.

Continuous full-immersion Full-immersion + drying-wetting cycles (GUa) Full-immersion + drying-wetting cycles (NECC)

PC55

Exposure-I Exposure-II Exposure-III

PC45

Immersion regimes

OPC (kg) FA (kg) Coarse aggregate (kg) Fine aggregate (kg) Water reducing (kg) Water (kg) w/cm (w/w)

PC35

Exposure no.

Materials

Temperature and RH

Table 2 Mix design proportion of concrete mixtures per cubic meter.

2.3. Environmental exposure conditions

Table 3 Details of exposure conditions.

Three types of concrete samples were prepared from each mixture, i.e., cylinder (50 mm diameter and 100 mm height), cube (100 mm3) and prism (40 mm × 40 mm × 160 mm), for different tests. After standard curing of 28-days, these concrete samples were divided into 3 groups and subjected to 3 different exposure conditions (list in Table 3) up to 360 or 720 days, respectively. Part of these submerged samples is shown in Fig. 1. A drying-wetting cycle (15 days) consisted of 8 days of full immersion in sodium sulfate solutions and 7 days of drying in an environmental chamber. This cycle regime is close to that used by Nehdi et al. [7]. Each exposure condition includes 3 types of concentration of sodium sulfate (Na2SO4) solutions, i.e., 0% w/w (water for control, C), 2.1% w/w (low-concentration, LC) and 15% w/w (high-concentration, HC). The container where the samples were placed was made of plastic. The solutions were renewed monthly and the PH was controlled at a range of 6.0–8.0 by titration with diluted sulfuric acid solutions at 219

GU indicates general use, which means the common regime in the literature, e.g., [14,15]. NECC means Natural Environmental Climate Condition.

48.9 23.3 14.9 3.8 0.7 0.2

b

19.7 5.12 2.41 60.5 3.31 3.01

SF

a

FA

720 days 360 days 720 days

OPC

Natural Environmental Climate Condition (NECC ) 20 ± 2 °C, 90 ± 5% RH (wetting)/45 ± 2 °C, 35 ± 5% RH (drying) Natural Environmental Climate Condition (NECC)

SCMs

0% (C), 2.1% (LC) and 15% (HC) 0% (C), 2.1% (LC) and 15% (HC) 0% (C), 2.1% (LC) and 15% (HC)

Chemical compositions (%) Silicon oxide (SiO2) Aluminum oxide (Al2O3) Ferric oxide (Fe2O3) Calcium oxide (CaO) Magnesium oxide (MgO) Sulfur trioxide (SO3) Compound composition of clinker (%) Tricalcium silicate (C3S) Dicalcium silicate (C2S) Tricalcium aluminate (C3A) Tetracalcium aluminoferrite (C4AF) Physical properties Loss on ignition (%) Specific surface area (m2/kg) Specific gravity

CMs

Exposure period

Components/properties (w/w)

PH

Table 1 Chemical compositions and physical properties of cementitious materials and supplementary cementing materials.

6.0–8.0 6.0–8.0 6.0–8.0

Z. Zhongya et al.

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

Fig. 1. Concrete samples are immersed in sulfate sodium solutions.

regular time intervals (7 days). However, this process may slightly increase the sulfate ion concentration in solutions. Thus, relatively diluted sulfuric acid solutions (solute mass fraction less than 10%) were adopted. In this way, the consumed sulfate ions in solutions would be duly replenished. Paglia et al. [47] investigated the impact of two different temperatures (23 °C and 65 °C) on the damage mechanism of sulfate attack on concrete. Conclusions of that paper showed that the damage was mainly controlled by the growth of ettringite at higher temperatures of 65 °C. This implied that temperature might have an influence on the driving mechanism of sulfate attack on concrete. Hossack and Thomas [48] studied the effect of temperature on the sulfate attack of Portland cement blended mortars in sodium sulfate solution. Four constant temperatures (23 °C, 10 °C, 5 °C and 1 °C) were under consideration in that paper. However, concrete materials mainly subject to Natural Environmental Climate Condition in local area. Neither a constant temperature and RH condition nor a specific dryingwetting cycle of temperature and RH can well reproduce the field exposure conditions of concrete structures/elements. On the contrary, some of them, such as drying-wetting cycles or high-concentration erosion solutions, might accelerate or even change the mechanism of sulfate attack on concrete materials (e.g. [46,47]). Unfortunately, the reports associated with this consideration on laboratory tests are rare to date. Thus, a natural variation of Environmental Climate Condition (see Fig. 2) was adopted in current study. The Natural Environmental Climate Condition (NECC), including temperature and relative humidity (RH) during the whole exposure period, was in accordance with the real-time climate information in Chongqing (from November 2015 to November 2017), China from the file of China Meteorological Administration (CMA). The mean temperature and mean RH were 19.2 °C and 73.8%, respectively. The maximum and minimum of temperatures were 42 °C and 1 °C, respectively. The maximum and minimum of RH were 100% and 11%, respectively.

100

Relative Humidity (%)

90 80

Mean 73.8 %

70 60 50 40 Mean 19.2

C

30 0

10

20

30

40

Temperature ( C) Fig. 2. Natural variation of environmental climate conditions (NECC) during the whole exposure period. (Data from the file of China Meteorological Administration)

according to Eq. (1):

Mass Variation at (t ) = mt

m0

m 0 × 100%

(1)

where m0 is the initial mass, mt is the mass of sample at exposure period t (day). 2.4.2. Expansion As per ASTM C1012 [30], the concrete bars with a dimension of 25 mm × 25 mm × 285 mm were casted to the assessment of the expansion of concrete specimen under sulfate attack. However, there is no standard method to address this issue in China. Thus, the length changes of prism samples (40 mm × 40 mm × 160 mm) were measured monthly as a representative of the expansion, by using a high-accuracy digital length comparator. Subsequently, the expansion was calculated according to Eq. (2):

2.4. Tests 2.4.1. Mass variation At regular time intervals, cubic samples were taken out from solutions and then air-dried in the laboratory at temperature of 23 °C [73.4 °F] and RH of 70% until constant mass was reached. Meanwhile, salt efflorescence (if any) and debris were carefully removed from the surface of the samples by a nylon brush. Then the mass of sample at exposure period t, i.e., mt, was measured using a balance with an accuracy of 0.01 g. Subsequently, the mass variation was calculated

Expansion at (t ) = Lt

L0

L0 × 100%

(2)

where L0 is the initial length, Lt is the length of sample at exposure period t (day). Note that expansion was immediately measured after 220

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

removal from solutions.

(portlandite), to form gypsum (CaSO4·2H2O), which can in turn react with cement hydration products of monosulfate, C3A (tricalcium aluminate) or C-A-H (hydrated calcium aluminate) to form ettringite. These products are expansive and may fill micro pores and cracks in matrix, resulting in mass gain. After 600 days of exposure, the mass started to decrease. This is due to the occurrence of slightly surface scaling and internal cracking at later stage when the micro pores and cracks in matrix were filled up with expansive products. At the end of exposure period of 720 days, a final mass gain of 3.45% was reached. In 15% sulfate sodium solution (HC), the mass increased by 3.41% before 480 days of exposure, with a decreasing rate of growth. After 480 days of exposure, mass started to decrease, which is earlier than that in 2.1% sulfate sodium solution (LC). This is due to high-concentration solutions accelerate the chemical reactions between sulfate ions and cement hydration products, leading to rapid formation of expansive products [10,31]. At the end of exposure period of 720 days, a mass loss of 2.04% was reached. This indicates that high-concentration of sulfate sodium solutions aggravate concrete samples, which has been widely confirmed [10,25]. Fig. 3b shows the evolution of expansion of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-I. It can be seen that the expansion of control samples (in water) exhibited a similar trend of time evolution to that of mass. The expansion increased first due to further hydration of cement paste, and then remained a constant of 0.051% after 420 days of exposure in water. Clearly, the expansion of all samples immersed in sulfate sodium solutions kept continuous growth during whole exposure period. It can be seen that higher concentration of sulfate sodium solutions leads to larger expansion of samples, regardless of other influencing factors. The samples immersed in 15% sulfate sodium solution achieved a maximum expansion of 1.38% followed by samples in 2.1% sulfate sodium solution with a final expansion of 0.95%. Generally, expansion of concrete samples takes place due to the formation of expansive products, such as gypsum and ettringite. Thus, it is a prevailing feature of chemical sulfate attack (CSA) on concrete [7]. Although slightly surface scaling occurred at later stage, it did not affect expansion of samples because the chemical reaction between sulfate ions and cement hydration products continued. However, surface scaling is generally regarded as a visual assessment of physical sulfate attack (PSA) on concrete [16]. Therefore, it can be inferred that the mass variation is associated with both CSA and PSA on concrete, but expansion only correlates to CSA. Since the deterioration of concrete under sulfate attack starts with the deleterious ions transportation by water, the water permeability coefficient is critical in the durability assessment of concrete under sulfate-laden environment [49]. Fig. 3c delineates the evolution of permeability of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-I. The permeability of samples in water steadily reduced during whole exposure period. This is due to that further hydration of cement paste refined the microstructure and enhanced the compactness of concrete. For the samples in 2.1% sulfate sodium solution, the permeability kept drop before approximately 540 days of exposure, then the drop rate decreased and finally 49% of initial permeability (1.98 × 10−12 m/s) remained. This is attributed to that on one hand, further hydration and expansive products refined the microstructure and enhanced the compactness of concrete; on the other hand, expansive products might exert a destructive pressure on the pore wall when the pores were filled up. Once the pressure exceeded the tensile strength of concrete, micro-damage of concrete took place due to the initiation of cracks in matrix. These cracks were responsible for the increment of permeability predominantly at later stage. Therefore, in 15% sulfate sodium solution, the evolution of permeability decreased first and then followed by an apparent growth at later stage due to the occurrence of remarkable micro-damages. The final enhancement of permeability reached 4.5% for samples in 15% sulfate sodium solution, which is larger than that of

2.4.3. Permeability The auto-compensated and auto-equilibrated triaxial cell system was used for the water permeability test of concrete samples. Three replicates of cylindrical samples (50 mm diameter and 100 mm height) from different regimes were tested monthly at different exposure period. The water permeability coefficient (K) of the sample was determined according to Darcy's law [49]. 2.4.4. Compressive strength Compressive strength of the cubic samples were measured monthly from three replicates during the whole exposure period. The test method complies with the GB 50081-2002 [45] (Standard for method of mechanical properties on ordinary concrete) and GB/T 50082-2009 [46] (Standard for test methods of long-term performance and durability of ordinary concrete). On the test day, the tested samples were taken out from solutions, and then the surface of each sample was thoroughly washed and wiped. Subsequently the samples were air-dried at lab condition at temperature of 23 °C [73.4 °F] and RH of 70% until a constant mass before tests. The goal is to provide a uniform basis of comparison. 2.4.5. Microanalysis The microstructure and morphology of minerals in deteriorating samples were examined by Scanning Electron Microscopy (SEM) with Energy Dispersive x-ray analysis (EDX) in Center of Electron Microscope of Chongqing University, China. The goal is to identify the underlying mechanisms of damage from micro-scale. At a specified exposure period, the deteriorating samples were taken out from the container. They were air-dried in the laboratory until no water can be seen. Then, some of them were broken up and small debris (diameter less than 11 mm, flat surface) was collected from different portions of samples. The collected debris (no aggregate) was dried at 40 °C for 48 h using a desiccator and then coated with gold in surface before the SEM and EDX testing. 3. Results 3.1. Exposure-I During 720-days exposure period, long-term behaviors of concrete samples were monitored monthly. Exposure-I (full immersion) was in line with previous works [10]. This regime is generally performed to induce pure chemical sulfate attack on concrete [51,52]. The mass and compressive strength were determined from cubic samples. Expansion was measured from prism samples. Cylindrical samples were used for evaluation of water permeability. Note that the variation of compressive strength and permeability was calculated via dividing measured value by initial value. Fig. 3a depicts the mass variation of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-I. The mass of control samples immersed in water increased first and then remained a constant gain of 2.12% after 450 days. The mass gain at early stage is due to water absorption and further hydration of cement paste, since 28 days hydration may not be sufficient. This phenomenon might refine the microstructure and result in a small gain of mass. The mass of samples immersed in sulfate sodium solutions exhibits two distinct stages, an increase stage in early and middle periods and then a decrease stage in later period. In 2.1% sulfate sodium solution (LC), the sample mass increased by 3.92% before 600 days, with a decreasing growth rate. The reasons for mass gain are twofold: one is because of water absorption and further hydration; another one is that sulfate ions first migrated into samples through pores and cracks by diffusion, permeation and capillarity. Then they reacted with the main components of hydrated cement paste, e.g., CH 221

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

(a)

(b)

4

Expansion variation (%)

Mass variation (%)

3 2 1 0

water 2.1 % Na2SO4

-1

1.4 1.2

15 % Na2SO4

1.0 0.8 0.6 0.4 0.2

15 % Na2SO4

-2

water 2.1 % Na2SO4

0.0 0

100

200

300

400

500

600

700

800

0

100

200

Exposure period (days)

(c)

(d) Compressive strength variation

Permeability variation

1.0

0.8

0.6

water 2.1 % Na2SO4

0.4

15 % Na2SO4 0.2 0

100

200

300

400

500

300

400

500

600

700

800

Exposure period (days)

600

700

1.10

1.08

1.06

1.04

water 2.1 % Na2SO4 1.02

15 % Na2SO4

1.00

800

0

100

Exposure period (days)

200

300

400

500

600

700

800

Exposure period (days)

Fig. 3. Variation of (a) mass, (b) expansion, (c) water permeability coefficient and (d) compressive strength of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-I.

PSA would take place in full-immersion regime (exposure-I). Fig. 5 depicts the expansion variation of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-I. The goal is to study the effects of w/b ratio and binder type. As shown in Fig. 5, all mixtures underwent persistent length expansion, and the final expansion degree follows: PC55 > PC45 > PC35 > SF10 > FA20. With respect to the effect of w/b ratio, it is apparent that the concrete mixtures with higher w/b ratios expanded much more compared to those with lower w/b ratios. More specifically, concrete samples have ultima expansion of 1.05%, 0.95% and 0.76% respectively with respect to w/b ratio of 0.55, 0.45 and 0.35. The reason is likely due to that a higher w/b ratio may lead to a higher water permeability and larger volume of pores on shotcrete pastes, thus resulting in vast accumulation of expansive products in these spaces. As for the effect of binder type, concrete mixtures with addition of FA or SF revealed less expansion in comparison with those made by pure OPC. More specifically, concrete samples have ultima expansion of 0.291% and 0.182% respectively with respect to binary binders blended with SF and FA. Therefore, lowing w/b ratio and adding FA or SF seems to be better countermeasures to enhance the resistance of concrete in exposure-I. When concrete is subjected to pure chemical sulfate attack (like exposure-I), both FA and SF additions can enhance the sulfate resistance.

samples in 2.1% sulfate sodium solution. Compressive strength is a critical criterion of durability evaluation on concrete structures/elements in practical engineering. Fig. 3d presents evolution of compressive strength of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-I. For samples immersed in water and 2.1% sulfate sodium solution, compressive strength continued to increase during whole exposure period, having a decreasing rate of growth. The maximum gain of strength in 2.1% sulfate sodium solution is 9.2%, larger than that of control samples in water (6.4%). It owes to the refinement effect of expansive products on concrete pores and cracks. However, the strength of samples in 15% sulfate sodium solution increased by 8.9% before 450 days and then decreased to 5.8% at the end of exposure. The evolution of strength in H-C solutions displays a two-stage model: an initial enhancement and a later deterioration [17]. The enhancement owes to refinement effects of further hydration of cement paste and expansive products, while the deterioration is mainly due to cracking caused by expansive pressures [18,19]. Fig. 4 illustrates SEM and EDX analyses for the deteriorated concrete samples in LC and HC sulfate sodium solutions. Ettringite (see Fig. 4a) and gypsum (see Fig. 4b) were identified as the main expansive products in small debris from deteriorated concrete. Moreover, gypsum was the main product in HC sulfate sodium solutions since abundant traces of gypsum were observed during microscopic scanning. Ettringite was expected to dominate in LC sulfate sodium solutions. This indicates that concentration of salt solutions not only accelerates sulfate attack on concrete, but also changes the deterioration mechanism and kinetics of sulfate attack on concrete materials [10]. In addition, slightly surface scaling was detected in samples immersed in HC sulfate sodium solutions at later stage of exposure. This implies just a negligible effect of

3.2. Exposure-II Exposure-II involves full-immersion in combined with general use drying-wetting cycles. The wetting phase is to submerge samples into different solutions for eight days. In this phase, sulfate ions in solutions penetrate into cement pastes and react with hydration products (e.g., 222

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

Fig. 4. SEM micrograph and EDX spectra from the deteriorated concrete samples in (a) LC and (b) HC sulfate sodium solutions in exposure-I.

360 days. This evolution trend is similar to that in exposure-I, but the deterioration of exposure-II is more serious and rapid. Clearly, dryingwetting cycles notably accelerate sulfate attack on concrete and aggravate cement-based materials under aggressive sulfate-rich environment. Furthermore, more serious surface scaling and flaking of concrete than that in exposure-I were observed, which results in severe loss of mass at later exposure stage. Since filling leads to mass gain and surface scaling results in mass loss, the resulting mass variation is highly affected by the combined distress of PSA and CSA. Under this situation, thus, mass variation seems not to be good assessment criteria of chemical attack on concrete. Fig. 6b shows the expansion variation of PC45 concrete samples in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-II. Expansion of all samples immersed in sulfate sodium solutions increased with time. Expansion variation in LC solution reached 1.603% at the end of exposure period. While in HC solution, the maximum expansion was 2.88% after 360 days of exposure. It can be found that expansion can be a good evaluation of chemical sulfate attack on concrete, but it is not sensitive to surface scaling. Fig. 6c delineates the evolution of permeability of PC45 concrete samples in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-II. The permeability of control samples in water steadily reduced with time due to further hydration of cement paste, and finally reduced to 34.5% of initial permeability. The permeability of samples immersed in sulfate sodium solutions also exhibits two distinct stages, an early drop and a subsequent rise. In LC solution, permeability dropped by 65.5% before 210 days of exposure, and subsequently rise up to 287% of initial permeability from 210 days to 360 days. In HC solution, permeability dropped by 65.5% before 120 days of exposure, and subsequently rose up to 654% of initial permeability from 120 days to 360 days. Note that the maximum drop of permeability was all around 65.5% for different solutions, which indicates a uniform porosity and similar connectivity from a mixture, irrespective of the discrete of cement-based materials. Under sulfate attack and drying-wetting cycles, expansive products such as ettringite filled concrete pores to reduce the pores' volume and connection, resulting in permeability drop at early stage. However, this filling process might cause cracking when concrete pores were filled up, leading to high connectivity, and subsequently a rapid rise of permeability of concrete samples at later stage. Clearly, permeability of concrete is sensitive to the connectivity of pores and cracks. Fig. 6d presents evolution of compressive strength of PC45 concrete samples in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-II. The evolution of compressive strength in sulfate sodium solutions also exhibits two distinct stages, an early enhancement and a subsequent degeneration. In LC solution, strength increased by 9.8% before 210 days of exposure, and then decreased to 86.9% of initial strength from 210 days to 360 days. In HC solution, strength increased by 9.8% before 120 days of exposure, and

1.2

PC35 (w/b = 0.35) PC45 (w/b = 0.45) PC55 (w/b = 0.55) FA20 (w/b = 0.45) SF10 (w/b = 0.45)

Expansion variation (%)

1.0 0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

600

700

800

Exposure period (days) Fig. 5. Expansion variation of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-I.

CH) causing chemical damage on concrete [20]. The drying phase is to place samples in an environmental chamber at temperature of 45 ± 2 °C and RH of 35 ± 5% for seven days. In this phase, moisture evaporation takes place due to the presence of evaporation front which is free to air. The evaporation of moisture might result in a supersaturation of pore solutions since no continuous uptake of salt solutions at all. Thus, salt crystals are able to grow from a supersaturated solution in its pores, simultaneously accompanied by phase transition of salts (e.g., mirabilite to thenardite) [43]. A whole drying-wetting cycle consists of 15 days which is similar to that performed in [7]. The entire test period in this exposure was 360 days, since it was enough to get a mass of useful details. Previous reports have confirmed that dryingwetting cycles can facilitate these diffusion-reaction and evaporationcrystallization phenomena [37]. Thus, various drying-wetting cycle regimes have been conducted in accelerated lab tests to significantly reduce test period (e.g. [8,15,32,37]). Fig. 6a depicts the mass variation of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-II. For control samples in water, the mass variation increased first and then reached a constant gain of around 2.12% after 450 days of exposure. The mass variation of samples immersed in sulfate sodium solutions exhibits two distinct stages, a first increase and a subsequent decrease. For samples in LC sulfate sodium solution, the mass variation first increased by 3.78% at 180 days of exposure, and then decrease to −2.37% from 180 days to 360 days. For samples in HC sulfate sodium solution, the mass variation first increased by 3.05% at 120 days of exposure, and then decrease to −5.74% from 120 days to 223

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

(a)

(b)

2 0 -2 -4

water 2.1 % Na2SO4

-6

15 % Na2SO4

3.0

Expansion variation (%)

Mass variation (%)

4

water 2.1 % Na2SO4

2.5

15 % Na2SO4

2.0 1.5 1.0 0.5 0.0

0

100

200

300

0

400

100

7

(d) water 2.1 % Na2SO4

Permeability variation

6

Compressive strength variation

(c)

15 % Na2SO4

5 4 3 2 1

100

200

300

400

300

400

1.10 1.05 1.00 0.95 0.90

water 2.1 % Na2SO4

0.85

15 % Na2SO4

0.80

0 0

200

Exposure period (days)

Exposure period (days)

300

400

0

Exposure period (days)

100

200

Exposure period (days)

Fig. 6. Variation of (a) mass, (b) expansion, (c) water permeability coefficient and (d) compressive strength of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-II.

then decreased to 81.2% of initial strength from 120 days to 360 days. It is noticeable that the maximum increment of strength of samples in sulfate sodium solutions was higher than that of samples in water. This is due to the refinement effect of sulfate reaction products. It can be found that compressive strength is sensitive at early stage due to concrete pores were filled by expansive products. However, in later stage, cracks initiated and propagated in cement pastes, resulting in degeneration of concrete strength. According to Boyd and Mindess [53], applying a compressive stress on deteriorated concrete tends to close up internal cracks. Hence, compressive strength is not a sensitive indicator for internal cracks at later exposure stage. Clearly, general use drying-wetting cycles accelerate test period and facilitate the damage on concrete under sulfate attack. Meanwhile, SEM and EDX analyses (see Fig. 7) showed that abundant traces of sulfate sodium were identified in deteriorated concrete samples, except for gypsum and ettringite. This is the typical characteristic of PSA due to salt crystallization. Therefore, it can be inferred that frequently used drying-wetting cycles also alter the deterioration mechanism and kinetics of sulfate attack on concrete materials. Furthermore, this common used drying-wetting cycle regime in lab tests is not common in field around the world, especially the high temperature in drying phase. Fig. 8 depicts the mass variation of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-II. Initially, all concrete samples showed a gradual rise in mass, attributed to water imbibition during the hydration process as well as sulfate expansive products. This was followed by a mass decrease, which started at different times depending on w/b ratio and binder type. Clearly, an earlier and sharper drop of mass variation was detected in concrete samples with a higher w/b ratio. Concrete samples suffered from mass loss of 3.39%, 2.37% and 1.45% respectively with respect to w/b ratio of 0.55, 0.45 and 0.35. This is attributed to that samples with

higher w/b ratios possess lower tensile strength, thus showing an early cracking failure. Meanwhile, more serious physical attack on these samples results in serious surface spalling, as shown in Fig. 9. However, the concrete samples with addition of FA and SF suffered from much more loss of mass compared to those made by pure OPC. More specifically, concrete samples with addition of FA and SF showed final mass loss of 4.45% and 6.24%, respectively. This indicates that adding FA or SF seems to be not beneficial in this exposure condition. The reason is likely due to that adding FA or SF can refine the porosity of concrete, thus increasing their capillary action. This improvement may facilitate physical sulfate attack, resulting in more crystallization damage on concrete. This type of attack mainly resulted in surface spalling, thus giving rise to more loss of mass, as shown in Fig. 9. As shown in Fig. 9, concrete samples made with blended binder such as FA or SF suffered from more surface deterioration (e.g. spalling and chipping) along the corners and edges when exposed to exposure-II condition. In addition, low-Al2O3 content in SF blended system was favor to the formation of thaumasite [21]. Therefore, addition of FA or SF in concrete mixtures proved not to be an effective improvement to enhance the sulfate resistance under exposure-II, especially SF addition may attract serious degeneration. 3.3. Exposure-III Exposure-III involves full-immersion in combined with drying-wetting cycles under Natural Environmental Climate Condition (NECC). This exposure regime aims at proposing a field-like exposure condition in lab-scale tests to evaluate the long-term behaviors of concrete under sulfate attack. This is of great significance to the assessment of durability of concrete under aggressive sulfate-rich environments. The Natural Environmental Climate Condition (NECC), including 224

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

Fig. 7. SEM micrograph and EDX spectra showing abundant traces of sulfate sodium from the deteriorated concrete samples in exposure-II.

that this mass variation was not as dramatic as that of samples in HC solution in exposure-II. However, in LC solution, mass variation of samples does not accord with what were reported in previous works [27,28]. Surprisingly, a three-stage evolution of mass variation was observed. Before 270 days of exposure, mass increased by around 3.9%, then remained unchanged from 270 days to 390 days, and finally decreased to 0.62% from 390 days to 720 days. In addition to increase stage and decrease stage, another flat stage was found. The likely reason is that scaling and filling reached equilibrium at this flat stage. Fig. 10b shows the expansion variation of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-III. Expansion of control samples in water gradually increased before 330 days of exposure, and then reached a stable value of 0.052% after 330 days. The reason is obvious that further hydration products lead to slightly expansion of samples. In HC solution, expansion of samples gradually increased until the end of exposure, due to the filling of sulfate reaction products in pores of concrete. In LC solution, expansion variation of samples undergoes a riseflat-rise process, with the maximum expansion of 1.45% obtained. The rise of expansion is attributed to expansive products between sulfate ions and cement hydration products. Additionally, it is coincidental that the occurrence times (from 270 days to 390 days) of flat stages for mass and expansion are almost same. Fig. 10c delineates the evolution of permeability of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-III. The permeability of control samples in water steadily reduced with time due to further hydration, and finally reduced to 34.5% of the initial permeability. The permeability of samples immersed in sulfate sodium solutions also exhibits two distinct stages, an early drop and a subsequent rise. In LC solution, permeability dropped by 65.5% before 330 days of exposure, and subsequently rise up to 312% of initial permeability from 330 days to 720 days. In HC solution, permeability dropped by 65.5% before 210 days of exposure, and subsequently rose up to 674% of initial permeability from 210 days to 720 days. Clearly, the evolution trend is similar to that in exposure-II. However, the evolution of permeability in exposure-III is slower than that in exposure-II, since different drying-wetting cycle regimes were adopted. Fig. 10d presents evolution of compressive strength of PC45 concrete samples in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-III. The evolution of strength of control samples in water was expected to undergo a rise-flat process.

5 4 3

Mass variation (%)

2 1 0 -1 -2

PC35 (w/b = 0.35) PC45 (w/b = 0.45) PC55 (w/b = 0.55) FA20 (w/b = 0.45) SF10 (w/b = 0.45)

-3 -4 -5 -6 -7 0

50

100

150

200

250

300

350

400

Exposure period (days) Fig. 8. Mass variation of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-II.

temperature and relative humidity (RH) during whole exposure period, is in accordance with the real-time climate information in Chongqing, China from the file of CMA. In marine and offshore engineering, cement-based building materials suffer from the changes of temperature and humidity in splash and tidal zones. This drying-wetting cycle is also under natural environmental climate condition in local area. The goal of this consideration is to explore the nature of sulfate attack mechanisms on concrete under field exposure conditions. These lab tests attempt to acquire fundamental performance data of concrete durability exposed to real sulfate-rich environments. Fig. 10a depicts the mass variation of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 720 days of exposure in exposure-III. Mass of control samples in water gradually increased before 360 days of exposure, and then reached a stable value of 2.12% after 360 days. It is clear that this ascribes to further hydration of cement paste, which is in line with that of control samples in water in exposure-I and -II. In HC solution, mass of samples exhibits two distinct stages, a first increase by 3.22% before 210 days and a subsequent decrease to −4.82% from 210 days to 720 days. It is not hard to find 225

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

PC35

PC45

PC55

FA20

SF10

Fig. 9. Final surface morphology of various concrete samples at the end of exposure-II.

Strength increased by around 6% before 330 days, and then remained unchanged. In HC solution, strength evolution of samples exhibits twostage model: first increase and then decrease. In this situation, strength increased by 9.8% before 210 days of exposure, and then decreased to 78.2% of initial strength from 210 days to 720 days. In LC solution, strength evolution of samples exhibits three-stage model: a first

increase stage, then a flat stage and finally a decrease stage. Before 270 days of exposure, strength increased by around 9.8%, then remained unchanged from 270 days to 390 days, and finally decreased to 83.3% of initial strength from 390 days to 720 days. The increase stage owes to the enhancement effect of expansive products, while the decrease stage is attributed to deterioration of concrete in later exposure

4

(a)

2.8

(b)

3

Expansion variation (%)

2

Mass variation (%)

water 2.1 % Na2SO4

2.4

1 0 -1 -2 -3

water 2.1 % Na2SO4

-4

1.6 1.2 0.8 0.4

15 % Na2SO4

-5

15 % Na2SO4

2.0

0.0 0

100

200

300

400

500

600

700

800

0

100

200

Exposure period (days) 7

Permeability variation

6

400

500

600

700

800

600

700

800

(d)

water 2.1 % Na2SO4

Compressive strength variation

(c)

300

Exposure period (days)

15 % Na2SO4

5 4 3 2 1

1.10 1.05 1.00 0.95 0.90 0.85

water 2.1 % Na2SO4

0.80

15 % Na2SO4

0.75

0 0

100

200

300

400

500

600

700

0

800

100

200

300

400

500

Exposure period (days)

Exposure period (days)

Fig. 10. Variation of (a) mass, (b) expansion, (c) water permeability coefficient and (d) compressive strength of PC45 samples (w/b = 0.45) in different concentrations of sulfate sodium solutions after 360 days of exposure in exposure-III. 226

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

trends were similar to those observed in exposure-I. More specifically, the final expansion percentage follows: PC55 (1.85%) > PC45 (1.45%) > PC35 (1.26%) > SF10 (0.213%) > FA20 (0.163%). In addition, adding FA or SF in concrete mixtures can greatly suppress the length expansion compared to those made by pure OPC. It is noteworthy that the three-stage evolution model was inapparent from concrete samples with addition of FA or SF, since the expansion amount was severely suppressed. Overall, w/b ratio and binder type strongly affect the durable performance of concrete when exposed to sulfate solutions. Engineers should be cautious to use blended binders such as FA and SF, since the resulting performance is highly depends on many influencing factors, such as exposure conditions. It is well known that damage evolution affects the durability of the concrete material and subsequently the concrete structures [24]. As a whole, the evolutions of physical and mechanical properties under exposure-III are different from that under exposure-I and -II, with respect to trend or extent. Therefore, the nature of sulfate attack mechanisms on concrete under drying-wetting cycles with Natural Environmental Climate Condition may also be different. In addition, the flat stage requires further attention and verification.

6

(a)

5

Mass variation (%)

4 3 2 1 0

PC35 (w/b = 0.35) PC45 (w/b = 0.45) PC55 (w/b = 0.55) FA20 (w/b = 0.45) SF10 (w/b = 0.45)

-1 -2 -3 -4 0

100

200

300

400

500

600

700

800

Exposure period (days) 2.0

(b)

PC35 (w/b = 0.35) PC45 (w/b = 0.45) PC55 (w/b = 0.55) FA20 (w/b = 0.45) SF10 (w/b = 0.45)

Expansion variation (%)

1.8 1.6 1.4 1.2

4. Discussion 4.1. Effect of exposure condition Clearly, exposure condition strongly affects the attacking mechanism of sulfate on concrete, as well as the resulting evolution law of properties. Firstly, concentration of sulfate sodium solutions influences the evolution of physical and mechanical properties of concrete (see Figs. 3, 6 or 9). On one hand, high-concentration of sulfate sodium solutions provides more sulfate ions and thus accelerates chemical sulfate attack on concrete. On the other hand, precipitation of gypsum dominates in high-concentration of sulfate sodium solutions, which indicates high-concentration changes the mechanism of sulfate attack on concrete. Bellmann et al. [10] also reported that the formation of gypsum takes place in high-concentration of sulfate sodium solution. The transformation of portlandite into gypsum is accompanied by an increase in volume, as the molar volume of gypsum is higher than that of portlandite. Bellmann et al. [25] found that the formation of gypsum can proceed in the highly concentrated test solutions. In current study, the traces of gypsum and ettringite were both identified in deteriorated concrete samples. This well confirms what was reported by Bellmann et al. [10,25]. Therefore, precipitation of gypsum dominates in highconcentration of sulfate sodium solution, but not ettringite. However, the sulfate concentrations under field conditions are usually much lower than that used in laboratory tests [10]. Many field investigations showed that limited gypsum could be detected in deteriorated concrete cores under sulfate-rich environments (e.g. [31,51,54]). Thus, it should be cautiously reconsidered to assess durability and performance of concrete under high-concentration of sulfate attack in lab-scaled tests. Then, cycles of temperature and RH also affect the evolution of physical and mechanical properties of concrete. For instance, Fig. 12 illustrates mass variation of PC45 concrete samples in 2.1% sulfate sodium solutions under different exposure regimes. Here, we define that a more severe and sharp variation of mass means a higher degree of damage on concrete samples. As shown in Fig. 12, the damage degree of exposure-II is the highest followed by exposure-III and exposure-I. This is because that the variation of NECC under exposure-III is more moderate than that of general use dryingwetting cycle under exposure-II. Under exposure-II, thenardite crystallizes in pores near surfaces (subflorescence zone) during drying cycle. Sulfate crystals grow and accumulate to sufficient amounts. Then these crystals exert a crystallization pressure against the pore wall, which leads to the disintegration of the concrete surface layer [32]. During wetting cycle, thenardite may transform to mirabilite, simultaneously accompanied by mirabilite crystallization alone [7]. These processes

1.0 0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

600

700

800

Exposure period (days) Fig. 11. Variation of (a) mass and (b) expansion of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-III.

period. Note that the occurrence times of flat stages for mass, expansion and strength are almost same. In microanalysis, the amount of sulfate sodium crystals was limited in deteriorated concrete samples, except for gypsum and ettringite. This indicates that slight PSA took place under this exposure regime. Since the natural variation of temperature and RH was more moderate than GU drying-wetting cycles, no distinct acceleration of test can be observed. However, this is closer to field exposure condition of concrete. Meanwhile, this environment variation is too moderate to trigger the phase transition between thenardite and mirabilite. In fact, salt crystallization and this phase transition can take place at any possible conditions [41,44]. Therefore, the presence of sulfate sodium crystals is not uncanny due to drying-wetting cycles. Fig. 11 depicts the expansion and mass variation of various concrete samples in 2.1% sulfate sodium solutions after 720 days of exposure in exposure-III. In Fig. 11a, all concrete samples exhibited an early gain and following loss in their mass. The variation trend (increase first and then decrease) is similar to what was observed in exposure-II. More specifically, the final loss of mass follows: SF10 (−3.04%) > FA20 (−2.15%) > PC55 (−1.23%) > PC45 (0.62%) > PC35 (1.35%). More interesting, concrete samples with higher w/b ratios generally gained more mass at the early rise stage. This is likely due to that concrete mixtures with higher w/b ratios produce a more loosened microstructure in system. These mixtures have a larger porosity and a higher permeability, thus resulting in that large amount of pores can be filled by expansive products. Such large volume of filling by sulfate products may result in more gain of mass. In Fig. 11b, all concrete samples underwent continuous length expansion, whose evolution 227

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

this exposure condition, compared with the common two-stage model. Herein, we define these three stages model as follows: the early enhancement stage (from 0 days to around 270 days), the middle incubation stage (from around 270 days to 390 days) and the later degeneration stage (from around 390 days to 720 days). The enhancement stage owes to the refinement and filling effect of sulfate reaction products, such as gypsum and ettringite. The degeneration stage is attributed to swelling and cracking derived from destructive expansive pressures. The filling and cracking phenomena are confirmed by SEM and EDX analyses, as shown in Fig. 14. In addition, the incubation stage (from around 270 days to 390 days) may be an accumulation process of damage on concrete. The likely reason was presented as follows. As known, the destructive pressure generates in small pores by expansive pressures like ettringite. Meanwhile, the filling of single pore can't lead to premature failure of concrete. Only when large amounts of small pores are filled up with expansive products, deterioration of concrete begins. During the formation of expansive products, pores and cracks can be blocked by these products, reducing the connectivity (see Fig. 14a). This may mitigate the further ingress of aggressive ions [15]. On one hand, expansive pressures attempt to exceed the tensile strength of concrete [20], causing cracking (see Fig. 14b–d). On the other, expansive products block pores and cracks, reducing the subsequent supply of aggressive ions [21]. Thus, a competitive mechanism may exist between cracking effect and block effect during the incubation stage. Especially, expansive ettringite is hard to form when the pore is going to be filled up, since the supply of sulfate ions is not in time. Meanwhile, the precursor of ettringite (e.g., portlandite, monocarbonate or C-A-H gels) has been consumed up in this confined region between crystals and pore wall. As a result, the incubation stage is distinct under exposure-III but can't be detected under exposure-II (see Figs. 6 and 8), since sharp dryingwetting cycles not only significantly accelerate the test period, but also result in a coupling damage on concrete. In addition, this stage is also not detected under exposure-I (see Figs. 3 and 5), due to that the attacking rate under full immersion is too slow. This three-stage evolution model is critical in the evaluation of long-term performance and durability design in service concrete structures or constructional engineering under construction. However, further verification tests should be performed to clarify this finding. Besides, the evolution of permeability exhibits a typical two-stage model. The likely reason is that permeability is sensitive to micro-cracks, while cracking occurs at any moment during the whole exposure period.

4

Mass variation (%)

3 2 1 0 -1

Exposure-I Exposure-II Exposure-III

-2 -3 0

100

200

300

400

500

600

700

800

Exposure period (days) Fig. 12. Mass variation of concrete samples (PC45) in 2.1% sulfate sodium solutions under different exposure regimes.

are generally called “physical salt attack (PSA)” in recent studies, and responsible for surface scaling and flaking of concrete [16]. Such repetitive crystallization of thenardite and mirabilite due to cyclic wetting and drying resulted in the deterioration of concrete samples exposed to sulfate attack [14]. However, these processes are slow or even rare under exposure-III. Evaporation of moisture can result in supersaturation in pore solutions of concrete, which is necessary to salt crystallization. Compared with drying phase under exposure-II, evaporation of moisture is limited in drying phase under exposure-III. However, chemical reactions between sulfate ions and cement hydration products take place at any moment. In drying phase, these chemical reactions not only consume CH and gels, but also reduce the concentration of sulfate ions in pore solutions. This in turn mitigates the crystallization of sulfate salt. Meanwhile under exposure-III, the only difference between drying and wetting phase is that the consecutive supply of sulfate ions is cut off in drying phase. Therefore, the surfaces of concrete are still damp where mirabilite can be detected [32]. As time goes on, the dry-wet interface moves into interior samples due to surface evaporation. In fact, the rate of moisture evaporation outdistances the rate of ions diffusion and supply since no sulfate ions supply at all. Thus, the subflorescence zone develops and white efflorescence occurs in or near the surface of concrete samples. This process will stop when a wetting phase is back. However, chemical sulfate attack is the predominant mechanism of sulfate attack on concrete in wetting phase, like full-immersion under exposure-I. Clearly, a concomitance of PSA and CSA on concrete under exposure-II and -III is observed, which is also common in field conditions. The PSA under exposure-II is more severe than that under exposure-III, thus more loss of mass was detected in exposure-II. However, the condition of exposure-III is closer to field exposure condition of concrete. Although a higher time-cost of exposure-III, this regime is suggested to investigate the long-term performance and durability of concrete exposed to aggressive sulfate-rich environments in lab-scale tests.

5. Conclusions In this study, long-term performance evolution of concrete subjected to a field-like exposure regime was investigated in lab-scale tests. The field-like exposure condition consists a wetting phase of full immersion in low-concentration sulfate solution (2.1% w/w) and a drying phase under natural environmental climate cycles. Comparative trials were also conducted under full immersion regime and general use dryingwetting cycles. Several conclusions can be drawn based on the experimental results: ■ Concentration of sulfate sodium solutions alters sulfate attack mechanism on concrete and affects the evolution of physical and mechanical properties of concrete. In high-concentration condition (15% w/w), precipitation of gypsum rather than ettringite is dominating in the driving mechanism of sulfate attack on concrete. Meanwhile, the property evolution of samples in high-concentration conditions often exhibits a two-stage evolution model, i.e., the early enhancement (increase) stage and the later degeneration stage (drop). ■ Water-to-binder ratio and binder type significantly affect the durable performance of concrete when exposed to sulfate solutions. Mixture with a higher w/b ratio reveals better resistance to sulfate

4.2. The three-stage evolution model Under field-like exposure condition (exposure-III in 2.1% sulfate sodium solutions), a three-stage evolution model of mass, expansion and compressive strength are found. Fig. 13 shows the evolutions of mass, expansion and compressive strength versus exposure period in 2.1% sulfate sodium solutions under exposure-III. Clearly, a distinct three-stage evolution model can be found under 228

Cement and Concrete Research 116 (2019) 217–230

Evolution of concrete properties

Z. Zhongya et al.

4 3 2 1 0 1.6 1.2

Mass 120 days

Expansion

0.8 0.4 0.0 1.1 1.0 0.9 0.8 -100

Compressive strength 0

100

200

300

400

500

600

700

800

Exposure period (days) Fig. 13. Evolutions of mass, expansion and compressive strength versus exposure period in 2.1% sulfate sodium solutions under exposure-III. (Data from PC45 samples)

Fig. 14. SEM micrograph showing filling and cracking phenomena at different exposure period.

attack. However, it should be cautious to select appropriate SCMs to make blended binders, since the efficiency of these additions strongly relies on lots of influencing factors such as exposure conditions. ■ Drastic drying-wetting cycles accelerate test period and facilitate the damage on concrete under sulfate attack. This regime also facilitates Physical Salt Attack on concrete, aggravating concrete deterioration. A distinct coupling mechanism of chemical and physical sulfate attack on concrete exposed to general use drying-wetting cycles was observed.

■ The field-like exposure condition can well reproduce the real field exposure conditions of concrete under certain sulfate-rich environments. A three-stage evolution model of physical and mechanical properties of concrete under this condition was observed. Acknowledgments The authors highly appreciate the financial support from National Natural Science Foundation of China (no. 51578091), Natural Science Foundation Project of CQ CSTC (No. cstc2017jcyjA1250), Project (No. 229

Cement and Concrete Research 116 (2019) 217–230

Z. Zhongya et al.

106112017CDJXY200007, 106112017CDJXY200004) supported by the Foundamental Research Funds for the Central Universities.

[27] K. Sotiriadis, E. Nikolopoulou, S. Tsivilis, Sulfate resistance of limestone cement concrete exposed to combined chloride and sulfate environment at low temperature, Cem. Concr. Compos. 34 (8) (2012) 903–910. [28] M. Maes, N.D. Belie, Resistance of concrete and mortar against combined attack of chloride and sodium sulphate, Cem. Concr. Compos. 53 (10) (2014) 59–72. [29] M.S.H. Khan, O. Kayali, U. Troitzsch, Effect of NaOH activation on sulphate resistance of GGBFS and binary blend pastes, Cem. Concr. Compos. 81 (2017) 49–58. [30] ASTM C1012, Standard Test Method for Length Change of Hydraulic-cement Mortars Exposed to a Sulfate Solution, ASTM International, West Conshohocken, PA, 2004. [31] T. Drimalas, Laboratory and Field Evaluations of External Sulfate Attack, PhD thesis University of Texas at Austin, US, 2007. [32] T. Aye, C.T. Oguchi, Resistance of plain and blended cement mortars exposed to severe sulfate attacks, Constr. Build. Mater. 25 (6) (2011) 2988–2996. [33] D. Chen, X. Yu, M. Guo, Y. Liao, F. Ouyang, Study on the mechanical properties of the mortars exposed to the sulfate attack of different concentrations under the triaxial compression with constant confining pressure, Constr. Build. Mater. 146 (2017) 445–454. [34] M. Zhang, J. Chen, Y. Lv, D. Wang, J. Ye, Study on the expansion of concrete under attack of sulfate and sulfate–chloride ions, Constr. Build. Mater. 39 (1) (2013) 26–32. [35] S. Zhutovsky, D.R. Hooton, Accelerated testing of cementitious materials for resistance to physical sulfate attack, Constr. Build. Mater. 145 (2017) 98–106. [36] G. Massaad, E. Rozière, A. Loukili, L. Izoret, Advanced testing and performance specifications for the cementitious materials under external sulfate attacks, Constr. Build. Mater. 127 (2016) 918–931. [37] D. Chena, X. Yu, R. Liu, S. Li, Y. Zhang, Triaxial mechanical behavior of early age concrete: experimental and modelling research, Cem. Concr. Res. 115 (2019) 433–444. [38] H. Mohammadhosseini, J.M. Yatim, A.R.M. Sam, A.S.M.A. Awal, Durability performance of green concrete composites containing waste carpet fibers and palm oil fuel ash, J. Clean. Prod. 144 (2017) 448–458. [39] H. Mohammadhosseini, M.M. Tahir, A.R.M. Sam, N.H.A.S. Lim, M. Samadi, Enhanced performance for aggressive environments of green concrete composites reinforced with waste carpet fibers and palm oil fuel ash, J. Clean. Prod. 185 (2018) 252–265. [40] A. Bonakdar, B. Mobasher, N. Chawla, Diffusivity and micro-hardness of blended cement materials exposed to external sulfate attack, Cem. Concr. Compos. 34 (1) (2012) 76–85. [41] G.W. Scherer, Stress from crystallization of salt, Cem. Concr. Res. 34 (9) (2004) 1613–1624. [42] R.J. Flatt, Salt damage in porous materials: how high supersaturations are generated, J. Cryst. Growth 242 (3–4) (2002) 435–454. [43] R.M. Espinosa, L. Franke, G. Deckelmann, Phase changes of salts in porous materials: crystallization, hydration and deliquescence, Constr. Build. Mater. 22 (8) (2008) 1758–1773. [44] M. Steiger, Crystal growth in porous materials—II: influence of crystal size on the crystallization pressure, J. Cryst. Growth 282 (3) (2005) 455–469. [45] GB 50081-2002, Standard for Method of Mechanical Properties on Ordinary Concrete, China Architecture and Building Press, 2002. [46] GB/T 50082-2009, Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete, China Architecture & Building Press, 2009. [47] C. Paglia, F. Wombacher, H. Böhni, The influence of alkali-free and alkaline shotcrete accelerators within cement systems: I. Characterization of the setting behavior, Cem. Concr. Res. 33 (3) (2001) 387–395. [48] A.M. Hossack, M.D.A. Thomas, The effect of temperature on the rate of sulfate attack of Portland cement blended mortars in Na2SO4 solution, Cem. Concr. Res. 73 (2015) 136–142. [49] X.T. Yu, D. Chen, J.R. Feng, Y. Zhang, Y.D. Liao, Behavior of mortar exposed to different exposure conditions of sulfate attack, Ocean Eng. 157 (2018) 1–12. [50] G.J. Yin, X.B. Zuo, Y.J. Tang, O. Ayinde, J.L. Wang, Numerical simulation on timedependent mechanical behavior of concrete under coupled axial loading and sulfate attack, Ocean Eng. 142 (2017) 115–124. [51] E.G. Moffatt, Performance of 25-year-old silica fume and fly ash lightweight concrete blocks in a harsh marine environment, Cem. Concr. Res. (2018), https://doi. org/10.1016/j.cemconres.2018.07.004. [52] H.A. Alcamand, P.H.R. Borges, F.A. Silva, A.C.C. Trindade, The effect of matrix composition and calcium content on the sulfate durability of metakaolin and metakaolin/slag alkali-activated mortars, Ceram. Int. 44 (2018) 5037–5044. [53] A.J. Boyd, S. Mindess, The use of tension testing to investigate the effect of w/c ratio and cement type on the resistance of concrete to sulfate attack, Cem. Concr. Res. 34 (3) (2004) 373–377. [54] J. Haufe, A. Vollpracht, Tensile strength of concrete exposed to sulfate attack, Cem. Concr. Res. 116 (2019) 81–88.

Conflict of interest The authors declare that they have no conflict of interest. References [1] S.W. Tang, Y. Yao, C. Andrade, Z.J. Li, Recent durability studies on concrete structure, Cem. Concr. Res. 78 (2015) 143–154. [2] N. Shanahan, A. Zayed, Cement composition and sulfate attack: part I, Cem. Concr. Res. 37 (4) (2007) 618–623. [3] P. Brown, R.D. Hooton, B. Clark, Microstructural changes in concretes with sulfate exposure, Cem. Concr. Compos. 26 (8) (2004) 993–999. [4] N.M. Al-Akhras, Durability of metakaolin concrete to sulfate attack, Cem. Concr. Res. 36 (9) (2006) 1727–1734. [5] Harvey Haynes, Sulfate attack on concrete: laboratory versus field experience, Concr. Int. 24 (7) (2002) 65–70. [6] Manu Santhanam, D. Menashi, Jan Olek, Sulfate attack research — whither now? Cem. Concr. Res. 31 (8) (2004) 1275–1296. August. [7] M.L. Nehdi, A.R. Suleiman, A.M. Soliman, Investigation of concrete exposed to dual sulfate attack, Cem. Concr. Res. 64 (2014) 42–53. [8] M.F. Najjar, M.L. Nehdi, A.M. Soliman, T.M. Azabi, Damage mechanisms of twostage concrete exposed to chemical and physical sulfate attack, Constr. Build. Mater. 137 (April 2017) (2017) 141–152. [9] C. Yu, W. Sun, K. Scrivener, Degradation mechanism of slag blended mortars immersed in sodium sulfate solution, Cem. Concr. Res. 72 (2015) 37–47. [10] Frank Bellmann, Bernd Möser, Jochen Stark, Influence of sulfate solution concentration on the formation of gypsum in sulfate resistance test specimen, Cem. Concr. Res. 36 (2) (2006) 358–363. [11] E. Gruyaert, P.V.D. Heede, M. Maes, N.D. Belie, Investigation of the influence of blast-furnace slag on the resistance of concrete against organic acid or sulphate attack by means of accelerated degradation tests, Cem. Concr. Res. 42 (1) (2012) 173–185. [12] F. Shaheen, B. Pradhan, Influence of sulfate ion and associated cation type on steel reinforcement corrosion in concrete powder aqueous solution in the presence of chloride ions, Cem. Concr. Res. (2016) 91. [13] M. Nehdi, M. Hayek, Behavior of blended cement mortars exposed to sulfate solutions cycling in relative humidity, Cem. Concr. Res. 35 (4) (2005) 731–742. [14] M.T. Bassuoni, M.L. Nehdi, Durability of self-consolidating concrete to different exposure regimes of sodium sulfate attack, Mater. Struct. 42 (8) (2009) 1039–1057. [15] M.T. Bassuoni, M.L. Nehdi, Durability of self-consolidating concrete to sulfate attack under combined cyclic environments and flexural loading, Cem. Concr. Res. 39 (3) (2009) 206–226. [16] M.T. Bassuoni, M.M. Rahman, Response of concrete to accelerated physical salt attack exposure, Cem. Concr. Res. (2015) 22. [17] N. Cefis, C. Comi, Chemo-mechanical modelling of the external sulfate attack in concrete, Cem. Concr. Res. 93 (2017) 57–70. [18] A.E. Idiart, C.M. López, I. Carol, Chemo-mechanical analysis of concrete cracking and degradation due to external sulfate attack: a meso-scale model, Cem. Concr. Compos. 33 (3) (2011) 411–423. [19] X. Xi, S. Yang, C. Li, A non-uniform corrosion model and meso-scale fracture modelling of concrete, Cem. Concr. Res. 108 (2018) 87–102. [20] R. Ragoug, O.O. Metalssi, F. Barberon, et al., Durability of cement pastes exposed to external sulfate attack and leaching: physical and chemical aspects, Cem. Concr. Res. 116 (2019) 134–145. [21] W. Müllauer, R.E. Beddoe, D. Heinz, Sulfate attack expansion mechanisms, Cem. Concr. Res. 52 (2013) 208–215. [22] A. Soive, V.Q. Tran, External sulfate attack of cementitious materials: new insights gained through numerical modeling including dissolution/precipitation kinetics and surface complexation, Cem. Concr. Compos. 83 (2017). [23] E. Rozière, A. Loukili, R.E. Hachem, F. Grondin, Durability of concrete exposed to leaching and external sulphate attacks, Cem. Concr. Res. 39 (12) (2009) 1188–1198. [24] J.K. Chen, M.Q. Jiang, J. Zhu, Damage evolution in cement mortar due to erosion of sulphate, Corros. Sci. 50 (9) (2008) 2478–2483. [25] F. Bellmann, W. Erfurt, H.M. Ludwig, Field performance of concrete exposed to sulphate and low pH conditions from natural and industrial sources, Cem. Concr. Compos. 34 (1) (2012) 86–93. [26] P.J.M. Monteiro, K.E. Kurtis, Time to failure for concrete exposed to severe sulfate attack, Cem. Concr. Res. 33 (7) (2003) 987–993.

230