Durability of concrete under sulfate attack exposed to freeze–thaw cycles

Furthermore, the compressive strength loss and RDME loss of concrete in sodium sulfate solution is less than that in water during the initial freeze-thaw cycles, but the damage is more s

Lei Jiang, Ditao Niu, Lidong Yuan, Qiannan Fei

DOI: doi: 10.1016/j.coldregions.2014.12.006

To appear in: Cold Regions Science and Technology

Received date: 23 July 2013

Revised date: 6 December 2014

Accepted date: 12 December 2014

Please cite this article as: Jiang, Lei, Niu, Ditao, Yuan, Lidong, Fei, Qiannan, Durability

of concrete under sulfate attack exposed to freeze-thaw cycles, Cold Regions Science and Technology (2014), doi: 10.1016/j.coldregions.2014.12.006

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Durability of concrete under sulfate attack exposed to

freeze-thaw cycles

Lei Jiang a,b

, Ditao Niub*, Lidong Yuanb, Qiannan Feib

a

College of Civil Engineering and Architecture, Anyang Normal University, Anyang 455000,China

b College of Civil Engineering and Architecture, Xi’an University of Architecture and Technology, Xi’an 710055,China

* Corresponding author Tel: +86 139 9113 1565; E-mail: niuditao@163.com

Address: College of Civil engineering, Xi’an University of Architecture and Technology, No 13 Yanta Road, Xi’an, Shaanxi

710055, P.R China

Abstract: Properties of concrete (with additional 20% fly ash) subjected to freeze-thaw cycles in water

and in sulfate solutions were investigated in this paper The corrosion solutions include two types, namely, 5% sodium sulfate solution and 5% magnesium sulfate solution Through the experiment, visual examination was conducted to evaluate the surface damage The deterioration considering the weight loss, relative dynamic modulus of elastically (RDME) loss and compressive strength loss of concrete under the coupling effect were also investigated To identify the products formed by sulfate attack, analytical techniques, including X-ray diffraction, scanning electron microscopy and thermal analysis were performed on the selected samples after freeze-thaw circulations Test results show that freeze-thaw cycles and sulfate attack affected each other On the one hand, a lower temperature during freeze-thaw cycles slows down the diffusion of sulfate ions in concrete On the other hand, sulfate attack accelerates the formation of microcracks in concrete, which leads to more severe damage under freeze-thaw cycles The rate of damage in concrete is significantly dependent on the types of sulfate solutions, and the concrete deterioration by magnesium sulfate covers the most aggressive corrosion subjected to freeze-thaw cycles Furthermore, the compressive strength loss and RDME loss of concrete in sodium sulfate solution is less than that in water during the initial freeze-thaw cycles, but the damage is more severe in further test period

Keywords: Concrete; Freeze-thaw cycles; Sulfate attack; Microstructure

1 Introduction

External sulfate attack on cement-based materials and the deterioration of concrete constructions resulting from freeze-thaw damage are the severe problems affecting the durability and service life of concrete structures (Skalny et al., 2002; Mehta, 1991) According to the related reports (Liu, 2001; Wang et al., 2001), there exist a lot of sulfate-rich soils and more than 1000 salt lakes are scattered in Northwest China In such regions, a large number of concrete structures are severely deteriorated and their service life is largely shortened, mainly because of the sulfate attack and freeze-thaw damage Thus, studying the damage process of the concretes in these areas for a rapid development of the infrastructure in Northwest China is necessary

Many studies on the damage process of concrete exposed to sulfate attack

is significantly slower than that of PC According to Yu et al (2005; 2008), the freeze-thaw durability of concrete is visibly reduced exposed to the flexural stress in salt solution and the stress, and chemical attack accelerates the damage process Zhang et al (2011) studied the frost resistance of concrete under the action of magnesium sulfate attack, indicating that magnesium sulfate solution can significantly alleviate the freeze-thaw damage for the high-strength concrete (HSC) with a higher water binder ratio and a lower content of mineral admixtures because of lowering effect of the magnesium sulfate solution freezing point Zheng et al (2010) claimed that not only physical changes but also chemical reactions occur in concrete under the combined actions of multi-aggressive (3% sodium chloride and 5% sodium sulfate) and freeze-thaw environment, which can exacerbate the deterioration of the mechanical properties of concrete

Most researches have focused on the single environmental factor affecting the deterioration of concretes, and little information is available about the interaction between freeze-thaw cycles and sulfate attack Furthermore, the microstructural features of concrete under the coupling effect require further investigation In this paper, basic experimental research on the performance of concrete subjected to freeze-thaw cycles in water and in the sulfate solutions was conducted based on the macroscopic and microscopic test The effect of sulfate solution on the deterioration

of concrete under freeze-thaw cycles was also analyzed

2 Experimental details

2.1 Materials and mix proportions

A Chinese standard Ordinary Portland Cement (OPC) of PO 42.5R produced by the Cement Factory of Tongchuan was adopted Grade II fly ash from the Weihe Power Station, river sand with a fineness modulus of 2.69 and coarse aggregate of crushed basalt stone with a diameter of 5 mm to 16 mm were used in the test A naphthalene-type superplasticizer was used, and the dosage was adjusted to keep the slump of fresh mixed concrete in the range of 50 mm to 120 mm The SJ-3 air-entraining agent provided by Tongji University is a type of gleditsia sinensis made from plants The air content of fresh mixed concrete was measured according to

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ASTM C231 The chemical composition of cement and fly ash is shown in Table 1

In this experiment, the water-binder ratio (W/B) was 0.45, and the concrete with

20 wt.% replacements of cement with fly ash was used According to the related investigations (Al-Dulaijian et al., 2003; Jin et al., 2007; Chen et al., 2012), about 20 wt.% of cement is replaced by fly ash, the resistance property of concrete to sulfate attack could be improved effectively The mixture proportion and corresponding compressive strength of the concrete are presented in Table 2 The mixture proportion used in the paper was developed for the laboratory investigation

Table 1 Chemical composition of OPC and fly ash

Insert

Table1 Table 2 Mix proportion and compressive strength of concrete

Insert

Table2 2.2 Specimens preparation and curing conditions

test specimens were fabricated from a single batch of concrete The components of the concrete mixture were batched by weight, the cement was premixed with fly ash, sand and coarse aggregate before adding the water and the admixtures for 1 minute Then, the entire amount of mixing water with the dissolved superplasticizer and air entraining agents were added and mixed for 3 minutes Finally, concrete mixture was mixed for another 2 minutes The concrete specimens were cast in steel moulds and compacted on a vibration table All specimens were demolded after 24 hours of casting and were cured in a condition of 20 ± 3 °C and 95% relative humidity

2.3 Test methods

2.3.1 Freezing and thawing cycle tests

In this study, the cyclic freeze-thaw test was conducted in the accelerated freeze-thaw testing apparatus according to GB/T50082 (2009) The temperature of the concrete samples was controlled by a Pt (platinum thermal) sensor embedded in the center of a 100 mm×100 mm×400 mm prism specimen The temperature of the sample center ranged from –18 ± 2 °C to 5 ± 2 °C In a single freeze-thaw cycle, the temperature of the specimens cools from 3 °C to −16 °C and then warms to 3 °C all within approximately 2.5 hours to 4 hours The freezing time should not exceed three quarters of the freezing and thawing time The time of the temperature cools from

3 °C to −16 °C should not be less than one half of the freezing time, and the time of the temperature warms from −16 °C to 3 °C should not be less than one half of the thawing time In the test, specimens were frozen and thawed in the liquid solution The solution test for freeze-thaw cycles includes the following types: water, sodium sulfate and magnesium sulfate solution In this study, concentrations of 5% sodium sulfate solution and 5% magnesium sulfate solution (by mass) were used (Zhang et al., 2011; Shanahan et al., 2007) The specimens were cured for 86 days, and then immersed in the three solutions for 4 days, respectively At the age of 90

The following Eq (1) was used to calculate the weight loss:

n ( 0 n ) / 0 100

the average weight of concrete specimens at every 25 freeze-thaw cycles in water or

in the sulfate solutions (kg)

The dynamic modulus of elasticity (DME), which was determined by Eq (2), was measured with a high-accuracy nonmetal ultrasonic analyzer as the initial value The RDME, which was determined by Eq (3), is the ratio of DME value to the initial DME value after every 25 freeze-thaw cycles

RDME

  (3)

concrete specimens at every 25 freeze-thaw cycles (m/s)

The measurement of compressive strength property was conducted according to the GB/T50081 (2002) method The following Eq (4) was used to calculate the compressive strength:

f F A (4)

According to the test procedure, the deterioration of the specimens was investigated by determining the RDME loss, weight loss and compressive strength loss The specimen was considered to be a failure if the RDME dropped to 60%, the weight loss exceeded 5% or the compressive strength dropped to 75%

2.3.2 Analytical techniques

The deteriorated surfaces of selected samples were examined by scanning electron microscopy (SEM) and equipped with an energy dispersive spectroscopy

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(EDS) detector In addition, X-ray diffraction (XRD) (Cu-Kα) was performed on samples to identify any compounds formed during the exposure to sulfate solutions Finally, thermal analysis including thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was conducted on powder samples collected from the surface (0 mm to 5 mm depth from the exposed surface) of selected specimens to analyze the corrosion products

3 Results and discussion

3.1 Surface damage

Fig 1 shows examples of the surface deterioration of concrete subjected to 300 freeze-thaw cycles in water and in sulfate solutions The degree of surface deterioration is obviously different in the three types of solutions Concrete specimen exposed to the sodium sulfate solution exhibits slight attack with only a surface layer

of mortar scaled While the specimen in water suffers loss of surface material from the corners and edges, the surface becomes uneven, which shows that the damage is worse than that in the sodium sulfate solution The coarse aggregate exposed in some severely scaled specimens immersed in the magnesium sulfate solution even peeled off Visual inspection revealed that the specimen exposed to magnesium sulfate solution is more severely damaged than that exposed to other solutions The results corroborate the findings of other researchers (Rasheeduzzafar et al., 1994; Al-Amoudi

et al., 1995) However, these studies have been conducted on cement paste or mortar without considering the action of freezing-thaw damage

Insert Fig.1 Fig.1 Surface damage of concrete subjected to 300 freeze-thaw cycles in water and in sulfate solutions

3.2 Weight loss

As shown in Fig 2, the weight loss of concrete specimens exposed to the magnesium sulfate solution exhibits three distinct stages In Stage I, from the initial immersion to 100 freeze-thaw cycles, the weight loss increases gradually In Stage II, from 100 to 150 freeze-thaw cycles, the weight loss decreases gradually In Stage III, from 150 to 400 freeze-thaw cycles, the weight loss increases from 0.32% to 1.69% The weight loss of concrete specimens exposed in the sodium sulfate solution also shows a similar trend The reason that Stage II occurs is that sulfate solution chemical reaction leads to the formation of ettringite and gypsum, increasing the weight of concrete to a certain extent However, sulfate attack accelerates the formation of microcracks in concrete under freeze-thaw cycles, which increases the ingress of sulfate ions into concrete to a certain extent and leads to more serious deterioration in Stage III

The deterioration because of magnesium sulfate covers more aggressive corrosion on concrete Firstly, the Ca ion in the calcium silicate hydrate (C-S-H) can

be replaced by the Mg ion, leading to the formation of non-cementitious magnesium silicate hydrate (M-S-H) and the loss of the cementitious structure (Shi et al 2011)

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Secondly, the formation of ettringite and gypsum due to the sulfate ion cause expansion and cracks and thus significantly lead to the deterioration of concrete Because of the simultaneous significant decomposition of the C-S-H that accompanies the formation of ettringite and gypsum, the corrosive action of magnesium sulfate is greater than that of sodium sulfate Therefore, the weight loss of concrete in the magnesium sulfate solution is larger than that in water and in sodium sulfate solution exposed to freeze-thaw cycles The results agree with those suggested by Miao et al (2001) and Mu et al (2001) However, the results disagree with those reported by Zhang et al (2011), which showed that the weight loss of the HSC with a higher water binder ratio and a lower content of mineral admixtures in the magnesium sulfate solution is less than that in water

Sulfate solution has both positive and negative effects on the concrete subjected

to freeze-thaw cycles The positive effect is that sulfate solution permeates into the pores and the concentration of pore solution increases, which causes the freezing point of pore solution to drop (Mu et al., 2001) The effect of lowering freezing point results in concrete damaged lightly when exposed to freeze-thaw cycles The negative effect is that sulfate attack results in more severe damage in concrete as freeze-thaw cycles increase Obviously, the positive effect is dominant when concrete is exposed

to the sodium sulfate solution, which leads to a more moderate degradation than that

in water On the contrary, the negative effect is dominant in the magnesium sulfate solution

Insert Fig.2 Fig.2 Weight loss in concrete subjected to freeze-thaw cycles in water and in sulfate solutions.

3.3 Relative dynamic modulus of elasticity loss

As shown in Fig 3, the RDME loss of concrete specimens in the sodium sulfate and magnesium sulfate solutions exhibit the following three stages: (I) decreased stage, from the initial immersion to 75 freeze-thaw cycles, (II) slowly decreased stage, from 75 to 200 freeze-thaw cycles and (III) accelerating decreased stage, from 200 to

400 freeze-thaw cycles Corrosion products modify the microstructure of concrete because of their expansion, thus the RDME loss is reduced in Stage II However, sulfate attack results in the formation of more microcracks in concrete during later freeze-thaw cycles, which accelerates the RDME loss in Stage III The experimental results show that the RDME loss of concrete in the magnesium sulfate solution is larger than that in water and in sodium sulfate solution The results agree with the weight loss presented in Fig 2 Obviously, the positive effect is dominant in the sodium sulfate solution during the initial 300 freeze-thaw cycles, and the decline in the RDME loss of concrete is slower than that in water As sulfate attack occurs continuously, the RDME loss of concrete exposed to the sodium sulfate solution is greater in further freeze-thaw cycles

Insert Fig.3 Fig.3 RDME loss in concrete subjected to freeze-thaw cycles in water and in sulfate solutions.

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3.4 Compressive strength loss

The extent of reduction in compressive strength is greatest in the magnesium sulfate solution, as shown in Fig 4 After 400 freeze-thaw cycles, the compressive strength loss ratios of concrete in 5% sodium sulfate solution, 5% magnesium sulfate solution and water are 21.53%, 26.62% and 19.91%, respectively As illustrated in Fig

4, the compressive strength loss of concrete in the two sulfate solutions exhibit the following three periods: (I) decreased period, (II) steady period and (III) accelerating decreased period The compressive strength loss of concrete tested in water is greater than that of concrete tested in the sodium sulfate solution in the first 250 freeze-thaw cycles The damage rate of concrete in the sodium sulfate solution has been greatly accelerated after 100 freeze-thaw cycles, and the compressive strength loss is greater than that in water after 250 cycles The results of compressive strength loss agree with those of RDME loss shown in Fig 3 More importantly, both the positive and negative effects of sulfate solution significantly affect the frost resistance of concrete

Insert Fig.4 Fig.4 Compressive strength loss in concrete subjected to freeze-thaw cycles in water and in sulfate solutions

3.5 Microscopic test analysis

Figs 5 and 6 present the microstructure observation of concrete subjected to freeze-thaw cycles in the magnesium sulfate solution As shown in Fig 5a, needle-like crystals can be observed in the pores The EDS technique was used to characterise the chemical compositions of the needle crystals, and the results are reported in Fig 5b Careful observation of the EDS spectra showed that needle crystals consist of the elements Al, Si, S, Ca and Mg, which indicated that the needle crystals are ettringite crystals The presence of Mg is due to the precipitation of the used magnesium sulfate salt or to the brucite (product of the reaction of portlandite with magnesium sulfate) As shown in Fig 6a, short columnar crystals can be observed in the pores The EDS spectra of these crystals indicated that they are gypsum, which detected the elements Si, S, Ca and Mg (Fig 6b)

Insert Fig.5 Fig.5. Needle crystals in concrete subjected to freeze-thaw cycles exposed to magnesium sulfate

solution (a) SEM and (b) EDS

Insert Fig.6 Fig.6. Short columniation shape crystals in concrete subjected to freeze-thaw cycles exposed to magnesium sulfate solution (a) SEM and (b) EDS

Fig 7a shows the XRD patterns of tested samples exposed to the magnesium sulfate solution subjected to freeze-thaw cycles The degradation material was found

The ettringite and gypsum peaks in concrete are strong, which indicates that they are main corrosion products The results agree with those suggested by Jin et al (2007)

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Portlandite most probably has reacted with magnesium sulfate to form ettringite, gypsum and brucite, all of which are found in the degradation products The concentrations of gypsum and ettringite after 300 freeze-thaw cycles are higher than

decreases, showing that the deterioration rate of concrete increases as freeze-thaw cycles increase

Fig 7b shows the XRD patterns of tested samples exposed to the sodium sulfate solution subjected to freeze-thaw cycles Obviously, gypsum and ettringite are main corrosion products However, the concentrations of gypsum and ettringite in concrete exposed to the sodium sulfate solution are lower than that in the magnesium sulfatesolution, indicating that the damage of concrete is more severe in the magnesium sulfate solution

Insert Fig.7 Fig.7. XRD patterns of concrete subjected to freeze-thaw cycles in sulfate solutions (a) magnesium sulfate solution and (b) sodium sulfate solution (E:ettringite; G:gypsum; P:portlandite; Q:quartz; C:calcite; B:brucite)

Fig 8 presents the TG/DSC curve of the degradation product of concrete exposed to the magnesium sulfate solution subjected to freeze-thaw cycles The DSC plots data from all the samples measured, displaying three typical endothermic peaks that lie in the ranges 100 °C to 120 °C, 140 °C to 150 °C and 450 °C to 460 °C Similar studies (Gao et al., 2013; Kresten et al., 1975) have reported that these three temperature stages correspond to the dehydration and decomposition of ettringite, dehydration and decomposition of gypsum, and decomposition of portlandite, respectively Another extremely small endothermic peak at approximately 400 °C is attributed to the dehydroxylation of brucite The concentrations of gypsum and ettringite obtained from the TG/DSC curve after 200 freeze-thaw cycles are higher than those after 100 cycles, which confirms the XRD observations

Insert Fig.8 Fig.8 TG/DSC curves of deterioration products of concrete exposed to magnesium sulfate solution

subjected to freeze-thaw cycles (a) 100 cycles and (b) 200 cycles

4 Conclusions

Sulfate solution has both positive and negative effects on the concrete subjected

to freeze-thaw cycles The positive effect is that sulfate solution drops the freezing point of concrete pore solution and restrains the sulfate attack when subjected to freeze-thaw cycles The negative effect is that the chemical reaction of sulfate solution leads to the formation of expansion products, which results in the microcracks in concrete and accelerates the damage process

Because of the simultaneous significant dissolution of cement hydrates that accompanies the formation of ettringite and gypsum, the deterioration by magnesium sulfate covers the most aggressive corrosion on concrete subjected to freeze-thaw cycles, and the negative effect is dominant The interaction effect of the sodium

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sulfate attack and freeze-thaw cycles at the early test period is different from that at the latter period The compressive strength loss and the RDME loss of concrete in 5% sodium sulfate solution was found to be less than the loss rates in water during the initial freeze-thaw cycles because the positive effect is dominant In further freeze-thaw cycles, the negative effect of sulfate solution is more obvious and the damage of concrete is more severe The weight loss of concrete in the sodium sulfate solution subjected to freeze-thaw cycles is less than that in water in the entire experiment process Additionally, the main corrosion products of concrete in sulfate solutions subjected to freeze-thaw cycles were confirmed to be ettringite and gypsum

A small quantity of brucite still exists in the corrosion products when tested in the magnesium sulfate solution

Acknowledgements

This project was supported by the Durability and Life Forecast of Shotcrete Tunnel Structure Fund (Chinese, No 51278403) and supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT13089)

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1 Fig.1 Surface damage of concrete subjected to 300 freeze-thaw cycles in water and in sulfate

solutions

2 Fig.2 Weight loss in concrete subjected to freeze-thaw cycles in water and in sulfate solutions

3 Fig.3 RDME loss in concrete subjected to freeze-thaw cycles in water and in sulfate solutions

4 Fig.4 Compressive strength loss in concrete subjected to freeze-thaw cycles in water and in

sulfate solutions

5 Fig.5(a) SEM of needle crystals in concrete subjected to 200 freeze-thaw cycles exposed to

magnesium sulfate solution

6 Fig.5(b) EDS of needle crystals in concrete subjected to 200 freeze-thaw cycles exposed to

magnesium sulfate solution

7 Fig.6(a) SEM of Short columniation shape crystal in concrete subjected to 200 freeze-thaw cycles

exposed to magnesium sulfate solution

8 Fig.6(b) EDS of Short columniation shape crystal in concrete subjected to 200 freeze-thaw cycles

exposed to magnesium sulfate solution

9 Fig.7(a) XRD patterns of concrete subjected to freeze-thaw cycles in magnesium sulfate solution

10 Fig.7(b) XRD patterns of concrete subjected to freeze-thaw cycles in sodium sulfate solution

11 Fig.8(a) TG/DSC curves of deterioration products of concrete exposed to magnesium sulfate

solution subjected to 100 freeze-thaw cycles

12 Fig.8(b) TG/DSC curves of deterioration products of concrete exposed to magnesium sulfate

solution subjected to 200 freeze-thaw cycles

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Fig 1

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Fig 2