In this study, the mechanical behaviors, failure characteristics, and microstructure of concrete containing fly ash (FA) against combined freeze-thaw cycles and sulfate attack were studied compared with normal concrete, and the formation rates of corrosion products during coupling cycles were investigated. Results showed that, during the coupling action of freeze-thaw cycles and sodium sulfate solution, concrete containing 10% fly ash exposed in 5% sodium sulfate solution exhibited better freeze-thaw resistance. Meanwhile, the variation of compressive strength of concrete during the coupling cycles could be divided into two stages, including the strength enhancement stage and the strength reduction stage. Moreover, the proportion of micropores and capillary pores decreased obviously during combined freeze-thaw cycles and sulfate attack, and excessive concentration of sodium sulfate solution led to more macropores after high-frequency freeze-thaw cycles.
There are numerous factors that affect the durability and service life of concrete, such as carbonation, sulfate corrosion, alkali polymerization expansion, cold and hot cycle, freeze-thaw cycle, dry-wet cycle, and reinforcement corrosion [
Numerous works of deterioration of macroscopic properties of concrete exposed to freeze-thaw cycles in salt solution have been reported. Li et al. [
These studies mainly focus on the macroperformance of concrete in sulfate solution subjected to freeze-thaw cycles. However, the change of macroperformance is essentially brought about by the change of the microstructure [
Fly ash and sulfate solution have both positive and negative effect on the resistance of concrete subjected to freeze-thaw cycle. Though concrete suffered from sulfate attack, it drops the freezing point of the pore solution [
Ordinary Portland cement (OPC) of 42.5 was used as the cementitious material. The continuous grading coarse aggregate ranges from 5 mm to 30 mm. And the fine aggregate used in this work was natural river sand with fineness modulus of 3.4. Second-grade fly ash was used in this study. All of these raw materials were chosen in accordance with Chinese standards and requirements of extreme environment. The air content of concrete was 1.8%, and the C3A content of cement was 7.02%. W/C ratio of 0.4 was adopted for each case, and to improve the workability of concrete, naphthalene superplasticizer with different dosages by the cement weight was used in this work to keep the slump in the range of 50−120 mm. Tap water was used for mixing concrete. And their properties are listed in Tables
Properties and chemical composition of ordinary Portland cement.
MgO (%) | Al2O3(%) | CaO (%) | Fe2O3(%) | SiO2 | SO3(%) | Loss on ignition (%) | Cl−1(%) | Surface area (m2) | Fineness (%) | Setting time (min) | Flexural strength (MPa) | Compressive strength (MPa) | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Initial setting time | Final setting time | 3 d | 28 d | 3 d | 28 d | ||||||||||
1.06 | 5.13 | 64.37 | 5.25 | 21.66 | 2.03 | 1.19 | ≤0.06 | 300 | 3.8 | ≥45 | ≤180 | 4.5 | 7.0 | 18.0 | 36.0 |
Properties and chemical composition of fly ash.
Fineness ( |
Density (g/cm3) | Specific surface area (m2/kg) | Loss | SO₃(%) | CaO (%) |
---|---|---|---|---|---|
43 | 2.12 | 360 | 2.34 | 2.14 | 16 |
The optimum mix ratio was determined by the experiment, and the actual mix proportion of specimens investigated in this work is shown in Table
Mix proportions in kg/m3 (except w/c) of concretes investigated in this study.
Cement | Fly ash | Coarse aggregates | Fine aggregates | Water w/c = 0.4 | Superplasticizer (%) | |
---|---|---|---|---|---|---|
% | kg | |||||
362.5 | — | — | 1211.2 | 681.3 | 145 | 1 |
326.25 | 10 | 36.25 | 1211.2 | 681.3 | 145 | 0.7 |
290 | 20 | 72.5 | 1211.2 | 681.3 | 145 | 0.5 |
To investigate the corrosion damage of fly ash concrete during freeze-thaw cycles, three groups of sodium sulfate concentration (2%, 5%, and 10%) were prepared to stimulate the actual environment. For each group, concrete mixtures with three different FA contents (i.e., 0% 10%, and 20%) as partial replacement of cement and all specimens were immersed in the corresponding sodium sulfate solution and placed in a freeze-thaw test machine to conduct the quick freeze-thaw cycle test. The freeze-thaw cycle was 0, 50, 100, 150, and 200 times. During each freeze-thaw cycle, the specimens were cooled from 20 ± 2°C to −20 ± 2°C within 2 h and warmed from −20 ± 2°C to 20 ± 2°C within 1 h. And the run time per cycle lasts for a total of 3hours, which is a reasonable range according to the test code for hydraulic concrete (SL352-2006) of China (see Figure
Temperature-time curve during the freeze-thaw cycle.
Experimental specimens.
Sodium sulfate concentration (%) | Freezing-thawing cycle ( |
Fly ash (%) | ||||
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0 | 50 | 100 | 150 | 200 | ||
2 | A | A1 | A2 | A3 | A4 | 10 |
B1 | B11 | B21 | B31 | B41 | 0 | |
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5 | B2 | B12 | B22 | B32 | B42 | 10 |
B3 | B13 | B23 | B33 | B43 | 20 | |
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10 | C | C1 | C2 | C3 | C4 | 10 |
To evaluate durability of freeze-thaw and sulfate resistance of concrete with and without fly ash, 100 mm cubic specimens cured for 28 days were immersed in specified solution (i.e., 2%, 5%, and 10% sodium sulfate by wt.%) for rapid freeze-thaw test. According to the Chinese standard GB/T50082-2009, the standard freeze-thaw cycle time was set to 3 h, and the specific process is as follows: firstly, three concrete specimens in the same group were put into the test barrel, and then 0.8 L Na2SO4 solution with the corresponding concentration was poured into the barrel; subsequently, the test barrel was located into the rapid freeze-thaw test machine (KDR-V9) for freeze-thaw test (including 50, 100, 150, and 200 cycles), and finally the macromechanical tests and microexperiments were carried out after every 50 freeze-thaw cycles, respectively. The mass and dynamic elastic modulus of the cubic specimens after every designated freeze-thaw cycle were measured.
Compressive strength of specimens was tested using a YAW2000A electrohydraulic servo pressure testing machine according to the GB/T50081-2002. The tests were carried out in a stress-controlled mode with a loading rate of 0.5 MPa/s.
In this paper, to reveal the deterioration mechanism of fly ash concrete from the micropoint of view under the coupling action of freeze-thaw cycle and sulfate attack, microstructure characterization of specimens was investigated by the following methods: the microcracks and corrosion products in concrete were qualitatively observed and analyzed by SEM (scanning electron microscope analysis) and EDS (energy dispersive spectroscopy); the internal components of concrete were analyzed by XRD (X-ray diffraction analysis); the pore size distribution and porosity of concrete were explored by MIP (mercury intrusion analysis); and TG-DSC (thermal analysis) was used to quantitatively analyze the corrosion products of concrete. Broken pieces were taken from compressive strength test and immersed in ethanol to prevent hydration. Before microtest, the specimens were removed from ethanol and dried at 60°C for 8 h.
Because all specimens completely immersed in Na2SO4 solution for freeze-thaw test, every side became rough due to external exfoliation and internal porosity growth under the coupling action of freeze-thaw and sulfate attack. And the surface conditions of fly ash concrete specimens after combined freeze-thaw cycles and sulfate attack are shown in Figure
Morphology of fly ash concrete after coupling cycles.
At the beginning of freeze-thaw cycles, 5% sodium sulfate solution reduced erosion deterioration of concrete specimens caused by freeze-thaw cycles [
Additionally, it was noted that concrete specimens containing 10% fly ash in 2% Na2SO4 and 20% fly ash in 5% Na2SO4 only suffered 150 freeze-thaw cycles, compared with 200 under other conditions. This indicates that a certain concentration of Na2SO4 solution improved freeze-thaw resistance of concrete, but excessive fly ash content reduced this ability. Therefore, it could be concluded that 10% fly ash and 5% Na2SO4 seem to be the optimum dosage for improving the freeze-thaw resistance of concrete, which is consistent with the research result of Li et al. [
The saturated-surface dry mass was obtained after each freeze-thaw cycle, and the mass loss rate was calculated according to the following equation:
During freeze-thaw cycles, the surface of concrete is damaged (see Figure
The relationship between mass loss rate and coupling cycles.
Figure
The relative dynamic elastic modulus of the specimen after each freeze-thaw cycle was measured and calculated according to equation (
Compared with mass loss rate, RDEM would be more reasonably reflected by the damage of fly ash concrete exposed to combined freeze-thaw cycles and sulfate attack. The influence of coupling action on RDEM was different from that of freeze-thaw or sulfate attack; it was more complex and changeable.
It can be seen from Figure
The relationship between RDEM and coupling cycles.
For the same concentration of salt solution (5%), the RDEM exhibited an upward trend in the early stage of freeze-thaw cycle and then decreased rapidly (Figure
For the same content of fly ash (10%), the RDEM continuously increased with the accumulation of concentration of salt solution before 100 freeze-thaw cycles and then decreased rapidly (Figure
The failure morphology for axial compression every 50 coupling cycles is shown in Figure
Compressive failure morphology of concrete with different contents of fly ash after coupling cycle.
SEM images of the concrete specimen (FA-fly ash and N-Na2SO4 solution).
The compressive strength curves of specimens under different coupling actions of freeze-thaw and sulfate attack are plotted in Figure
The relationship between compressive strength and coupling cycles.
For the same concentration of sodium sulfate solution, the compressive strength of concrete with different contents of fly ash decreased with the increase in number of freeze-thaw cycles, and 10% fly ash concrete exhibited good freeze-thaw resistance, in which strength increased by 10.69% after 50 freeze-thaw cycles, compared with noncycles. This indicates that filling effect of corrosion products and crystalline salts enhanced strength of concrete at the initial stage of freeze-thaw. Additionally, appropriate fly ash improved interface compactness and reduced the content of Ca(OH)2 in this early cycle, slowing down migration rate of sulfate ions and corrosion reaction intensity to decrease the reaction products. On the contrary, high-content fly ash decreased compressive strength in later cycles.
For the same content of fly ash, the compressive strength of concrete decreased by the increase of freeze-thaw cycles at different concentrations of sodium sulfate solution. Besides, it can be seen that concrete specimens in 5% Na2SO4 solution were denser than those in 2% Na2SO4 solution due to more salt crystallization and corrosion products filling their original defects. Compressive strength of concrete in 5% Na2SO4 solution increased by 10.68%, while that in 2% Na2SO4 solution decreased by 11.43%, compared with non-freeze-thaw cycle. This reveals that moderate concentration of Na2SO4 solution inhibited freeze-thaw action in the early stage, but excessive concentration of Na2SO4 solution (10%) made compressive strength of concrete to enter the decline stage ahead of time. And concrete suffers more serious damage in the salt-eroded environment after high-frequency freeze-thaw cycles.
The damage variable
Relationship between
Additionally, it is obvious from Figure
Figure
It is obvious from Figure
According to SEM, the erosion products in zones I, II, and III were judged to be ettringite (AFt) by appearance (Figure
Element content of corrosion products.
Element | Content (%) | SD (%) | Atom (%) | |
---|---|---|---|---|
I | C | 7.63 | 0.2 | 15.21 |
O | 29.02 | 0.93 | 43.44 | |
Na | 1.43 | 0.39 | 1.49 | |
Al | 1.78 | 0.31 | 1.58 | |
Si | 6.51 | 0.33 | 5.55 | |
S | 4.65 | 0.3 | 3.48 | |
Ca | 4.65 | 0.3 | 3.48 | |
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II | C | 10.09 | 0.25 | 17.89 |
O | 37.66 | 0.75 | 50.14 | |
Na | 0.89 | 0.39 | 0.83 | |
Al | 3.99 | 0.31 | 3.15 | |
Si | 9.45 | 0.34 | 7.17 | |
S | 5.07 | 0.32 | 3.37 | |
Ca | 32.84 | 0.69 | 17.45 | |
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III | C | 11.78 | 0.36 | 20.83 |
O | 35.85 | 1.21 | 47.60 | |
Na | 0.54 | 0.59 | 0.50 | |
Al | 3.03 | 0.48 | 2.39 | |
Si | 9.89 | 0.51 | 7.48 | |
S | 4.37 | 0.49 | 2.90 | |
Ca | 34.54 | 1.05 | 18.41 |
For further qualitative analysis of products, XRD test was conducted. As shown in Figure
XRD analysis of concrete.
To identify the pore distribution inside the concrete, MIP analysis was conducted on the concrete specimens containing different fly ash contents exposed to combined freeze-thaw cycles and sulfate attack. The pore size can be divided into four grades: micropores (<10 nm), mesopores (10–50 nm), capillary pores (50 nm–1
Pore characteristics of specimens after coupling cycle.
Parameters | Coupling cycle ( | ||
---|---|---|---|
50 | 150 | 200 | |
Porosity (%) | 11.1496 | 12.1653 | 12.546 |
Total pore volume (ml/g) | 0.0513 | 0.0562 | 0.0588 |
Total pore area (m2/g) | 5.035 | 4.954 | 5.805 |
Average pore size (nm) | 40.8 | 45.4 | 40.5 |
Median pore diameter (volume/nm3) | 81 | 78.1 | 81.4 |
Median pore diameter (area/nm2) | 14.5 | 19 | 15.5 |
Pore characteristics of concrete under different conditions after 150 coupling cycles.
Parameters | 10% FA-2% N | 0% FA-5% N | 10% FA-5% N | 20% FA-5% N | 10% FA-10% N |
---|---|---|---|---|---|
Porosity (%) | 12.1225 | 12.44 | 12.1653 | 15.9487 | 11.4801 |
Total pore volume (ml/g) | 0.0561 | 0.058 | 0.0562 | 0.0788 | 0.0524 |
Total pore area (m2/g) | 5.161 | 6.25 | 4.954 | 7.349 | 4.233 |
Average pore size (nm) | 43.5 | 37.1 | 45.4 | 42.9 | 49.6 |
Median pore diameter (volume/nm3) | 64.3 | 66.8 | 78.1 | 66.4 | 76.1 |
Median pore diameter (area/nm2) | 20.2 | 15.2 | 19 | 20 | 22.7 |
Relationship between pore size distribution and coupling cycles.
Relationship between pore size distribution and dosage of fly ash and salt solution.
Cumulative pore volume of concrete during coupling cycle.
Mechanical strength is affected by the pore structure in a large extent, and the relationship between compressive strength, damage factor
Analysis of macroscopic properties and microstructure.
The macroscopic properties and microstructure were also affected by corrosion products during coupling cycles; as shown in Figure
Thermal analysis curves of concrete in sodium sulfate solution after coupling cycles.
The relationship between mass loss rate and coupling cycle during the dehydration stage of corrosion products (60°C–130°C).
It demonstrated that when the content of fly ash was constant, (1) the content of corrosion products increased linearly with the increase in number of freeze-thaw cycles, (2) the formation rate of corrosion products before 150 freeze-thaw cycles was higher than that after this cycle, resulting from the reduction of Ca(OH)2 in the later period, and (3) the content of corrosion products increased linearly with the accumulation of concentration of sodium sulfate solution at the same freeze-thaw cycles (the specimen in 10% Na2SO4 solution after 50 freeze-thaw cycles was an exception. High concentration of salt solution produced more sodium sulfate crystals, blocking the inner pore of concrete, affecting the further inward migration of SO42−, and thus reducing the formation of corrosion products). When the concentration of salt solution was constant, the content of corrosion products increased with the increasing content of fly ash after 150 freeze-thaw cycles. This is because when fly ash content was up to 20%, the freeze-thaw resistance of concrete becomes weak, and microcracks and voids increased the invasion of SO42− resulting in the accumulation of corrosion products, which was consistent with the conclusion of mass loss rate in macroanalysis in this paper.
The corrosion of concrete under the coupling action of freeze-thaw cycle and sulfate attack is a process from the surface to the interior. To further investigate the internal chemical corrosion, we calculated average formation rates of corrosion products during freeze-thaw cycles, respectively, and the results are shown in Table
Average formation rates of corrosion products during freeze-thaw cycles.
Na2SO4 (%) | Average formation rates (%) | Fly ash (%) | |
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50∼150 ( |
150∼200 ( | ||
2% | 0.550 | — | 10 |
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5% | 0.414 | 0.314 | 0 |
0.479 | 0.156 | 10 | |
0.415 | — | 20 | |
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10% | 0.874 | 0.380 | 10 |
The freeze-thaw resistance of concrete exposed to the combined action of freeze-thaw cycles and sulfate attack was evaluated by using damage coefficient of freeze-thaw durability. The calculation formula is as follows:
Relationship between DF and coupling cycles.
Figure
DF versus compressive strength before and after the coupling cycles.
DF versus porosity of the typical specimen.
In this paper, the mechanical properties and microstructures of concrete under the coupling action of freeze-thaw cycles and sulfate attack were studied by multiscale experiments. Based on the obtained results, the following conclusions are drawn: During the coupling process of freeze-thaw cycle and sulfate erosion, the filling effect of fly ash and corrosion products is dominant in the rising stage of compressive strength, while the joint force is dominant in the falling stage. The corrosion products are mainly ettringite and gypsum during coupling cycle. And the content of ettringite is higher than gypsum, and its formation is also earlier than gypsum. The formation rate of corrosion products in the early stage of freeze-thaw cycles is faster than that in the later stage. Adding 10% fly ash into concrete and exposing it to 5% sodium sulfate solution can effectively improve the resistance to freeze-thaw cycle.
All data used in this article are available from the corresponding author by request.
The authors declare that they have no conflicts of interest.
The authors thank the National Natural Science Foundation of China (Grant nos. 51379015 and 51579013) and the Fundamental Research Funds for the Central Universities, CHD (Grant no. 300102289303) for providing financial support for this project.