1. Introduction
The demand for concrete as a major material for engineering and construction is increasing. China is a large country for concrete production and consumption, and the demand for sand, stone, and water as raw materials for concrete preparation is increasing. According to the 2015–2020 China Construction Industry Market Analysis and Investment Strategy Planning Report, China’s total sand and gravel production accounts for about one-third of the world’s total sand and gravel resources. The demand for sand and gravel in China reached 140 × 10
10 t in 2014, and the demand is still increasing and is expected to reach 250 × 10
10 t in 2030. The growing conflict between supply and demand for sand and gravel will lead to an increase in the price of the material. Uncontrolled exploitation leads to serious damage to ecological resources and causes many ecological problems [
1,
2].
As 70% of the earth’s area, the ocean is rich in resources, providing mankind with abundant fisheries, oil, natural gas, and other resources, of which seawater is considered an inexhaustible resource. With a vast sea area and a long and narrow coastline, China’s offshore sea sand is rich in resources, with total resources ranging from 67.96 × 10
10 to 68.49 × 10
10 m
3 [
3]. With the implementation of China’s ocean power and far-sea development strategy and the construction of ports, wharves, and islands, the demand for concrete is bound to increase. Moreover, there is a lack of river sand and freshwater resources in offshore areas. If seawater can be used instead of fresh water, sea sand instead of river sand can not only reduce the transportation cost and the overall construction cost [
4,
5] but also local materials and reduce the damage to the environment caused by mining river sand [
6,
7]. As a result, seawater sea sand concrete was created. In addition, some scholars have shown [
8,
9] that the basic mechanical properties of sea sand and river sand are similar, and the original sea sand is mainly medium-coarse sand compared with river sand, which has the advantages of lower mud content and moderate fineness; however, the content of shells and Cl
− is relatively high. The high content of shells can reduce the effective sand ratio of concrete, thus reducing the ease of concrete, strength, and the expansion and contraction of concrete in the later stage of the expansion and contraction, creep, and other adverse effects. The high content of chloride ions and other corrosive ions in seawater sand, once misused, will surely cause serious consequences, as evidenced by the emergence of the phenomenon of “seawater sand house”. A total of 166 commercial buildings were constructed in Zhoushan from 1994 to 1996, and the phenomenon of “seawater sand house” gradually appeared two years later. “In 2005, the Jinshan Mingzhu District in the suburb of Fenghua City caused the leakage of reinforcing steel due to the illegal use of sea sand [
10]. Therefore, investigating the influence of corrosive ions in seawater sand on concrete structures is a prerequisite to ensure the quality and safety of the project.
Seawater sea sand concrete is a concrete material in which the mixing water is seawater and the fine aggregate is sea sand. Since seawater sea sand inherently contains chloride and sulfate salts [
11], it is directly involved in the hydration reaction of cement when used as a mix. Among them, chloride and sulfate salts will accelerate cement hydration and play an early strength role, but the late strength growth is slow and, ultimately, comparable to the strength of ordinary concrete [
12,
13,
14]. Tixier et al. [
15], in their study of natural seawater sea sand concrete, found that despite the fact that seawater sea sand impeded the strength of the concrete, there was no significant difference in the overall mechanical properties compared to normal concrete. Islam et al. [
16] compared seawater sea sand concrete with ordinary concrete. After the age of curing reached 7d, the strength of seawater sea sand concrete was superior to that of plain concrete regardless of the type of curing (seawater curing and freshwater curing). In view of the existence of corrosive ions in this seawater sea sand, many scholars at home and abroad have proposed methods to solve the problem. For example, the addition of mineral admixture, desalination of seawater sand, the use of corrosion-resistant reinforcement, etc. [
17,
18,
19,
20,
21].
Fly ash is industrial waste residue, and considering China’s annual emissions of more than 400 million tons, the treatment of waste residue requires a large number of sites, as poor treatment will cause environmental pollution and even destroy the ecological environment [
22]. Fly ash as a mineral admixture for concrete not only has the advantages of reducing the heat of hydration of concrete, optimizing the pore structure, anti-freezing, chemical corrosion resistance, etc., but also its own volcanic ash reaction can improve the late strength of concrete [
23]. Scholars believe that, compared with ordinary silicate cement concrete, the addition of mineral admixtures makes the cement gel layer more compact and strengthens the interfacial transition zone (aggregate–cement interface) (ITZ), which, in turn, strengthens the concrete matrix [
24,
25]. Ying Tao Li et al. [
26] conducted an experimental study on seawater, sea sand, and seawater coral sand concrete, and the results showed that the modulus of elasticity of seawater sea sand and seawater coral sand concrete is slightly lower compared with ordinary concrete, and the mechanical properties of seawater sea sand concrete can be improved by adding fly ash and slag. Karthikeyan et al. [
27] formulated C30 grade concrete with sea sand mass replacements of 10%, 20%, 30%, and 40%, adding silica fume as an external admixture. It was shown that the highest strength of concrete was obtained when the replacement rate of sea sand was 30% and silica fume was added at 5%. Manjunath [
28] et al. used granulated blast furnace slag as an admixture into concrete and found, using electron microscope scanning, that the internal structure of slag concrete after high temperatures was denser than that of normal concrete.
Considering that the majority of seawater sea sand concrete is used in marinas in offshore and coastal areas, the working environment is often a marine environment [
29]. Concrete specimens are not only affected by physical factors such as tidal flushing or drying, wetting, freezing, and thawing but also by chemical reactions with salt in seawater. In the northern region of China, 22% of dams and 21% of small and medium-sized hydraulic buildings have freeze–thaw problems; moreover, concrete building damage caused by freeze–thawing is a common phenomenon in the northern region. The dual-factor action of freeze–thaw cycles and corrosive media exacerbates the damage to the durability of concrete, resulting in the premature failure of concrete specimens. It is thus necessary to study the process of seawater sea sand concrete change under seawater freeze–thaw conditions.
At present, to study the effect of seawater freeze–thawing on concrete, the mixing water and corrosion medium are mostly artificial seawater. Some scholars have studied the effect of single ions (CI
− and SO
42−) on concrete [
30,
31]. This is somewhat different from the reality of the marine environment. Based on the above considerations, this paper investigates the mechanical properties and durability of seawater sea sand concrete using the replacement rate of fly ash as the main variable. The effects of freezing and thawing on the appearance, microstructure, and physical phase composition of seawater sea sand concrete were analyzed using SEM electron microscope scanning. Gray system theory was applied to predict the freeze–thaw damage of the seawater sea sand concrete to provide certain theoretical references for related engineering practice.
5. Microanalysis
W
1S
1F
0C and W
1S
1F
0.2C were sampled, and the specimens were scanned using SEM to investigate the damage mechanism of seawater freezing and thawing on the specimens.
Figure 12 shows the microstructure of W
1S
1F
0C and W
1S
1F
0.2C specimens before seawater freeze–thaw. As can be seen in
Figure 11a, a large number of sheet-like Ca(OH)
2 and flocculent C-S-H gels are tightly articulated and have a relatively dense structure; however, there are slight pores in localized positions. This is due to the fact that AFm is a common substance in cement concrete called monosulfur-type calcium sulfoaluminate with the chemical formula 3CaO·Al
2O
3·CaSO
4·12H
2O, and AFm has less structural water compared to AFt. When sulfate ions in seawater and sea sand react with AFm and convert to AFt, the structural water increases and the volume expands, which can cause structural damage to the cement paste. CI
− in seawater and sea sand hydrates with cement to form “Friedel” salt. This substance is brittle and hinders the continuation of hydration when encapsulating the cement paste. In addition, SO
42− promotes the release of CI
− from the “Friedel” salt, which causes secondary damage to the concrete, as shown in
Figure 12b. When mixed with fly ash, the hydration products are interspersed with each other, bonded to each other, and lapped to each other, and there are no obvious holes on the surface. This is because the fineness of fly ash is about a hundred times that of ordinary silicate cement, and such a high fineness is conducive to filling the capillary pores inside the concrete, enhancing the compactness of the concrete and impeding the infiltration of CI
− and SO
42−. Thereby, the performance of the specimens after the incorporation of fly ash is better than the basic group, as shown in
Figure 12c.
The internal condition of the specimen after the freezing and thawing of the W
1S
1F
0C specimen is shown in
Figure 13a–c. It can be noticed that the crack width and number gradually increase with the amount of seawater freezing and thawing. With the increase in the number of seawater freezing and thawing, the capillary pores gradually increase, resulting in the acceleration of the propagation rate of corrosive ions, and the damage of the specimen is gradually aggravated. At the same time, AFt, “Friedel” salt, Mg(OH)
2, and other substances produced by the reaction between erosion ions in seawater and the hydration products of the slurry cover the surface of the unhydrated cementitious material particles to hinder their hydration, resulting in the cement slurry tending to be loose, with a large number of pores in the structure; moreover, there is almost no overlap between the hydration products’ looseness and porosity (
Figure 13d).
Under coupling, a portion of the solution in the internal pores freezes as the temperature decreases. As the water becomes ice, the volume expands, forcing the unfrozen water in the hole to flow and thus generate hydrostatic pressure [
41], as shown in
Figure 14. When the hydrostatic pressure generated is greater than the tensile strength of the concrete pore generated for the cracks, the generation of micro-cracks accelerates the rate of seawater erosion so that the products generated by the reaction will be attached to the cracks. As the erosion time lengthens, the increase in reaction products will further widen the cracks, and freeze–thaw cycles occur simultaneously with seawater erosion. As the number of coupling increases, the cracks inside the specimen will gradually increase in size. When the coupling reaches a certain number of times, the pores will be penetrated by cracks between the pores, causing changes in the quality and strength of the specimen. Seawater freeze–thaw coupling is a typical damage problem of two-factor coupling between corrosion and freeze–thaw. Compared with seawater erosion and freeze–thaw cycle single factor, the coupling effect has a “1 + 1 > 2” damage effect. In essence, it is the superposition of chemical action and physical action, and the two influence each other to accelerate the damage of concrete specimens. Compared with ordinary concrete, seawater sea sand concrete contains chlorine salt and sulfate in its own mixture, and its hydration reaction product originally has “Friedel” salt and Aft; under the coupling effect, seawater erosion products will have a superposition effect, which will strengthen the damage degree of the coupling effect, which leads to a gradual increase in cracks in the specimen.
An EDS energy spectrometer was used to score the interior of the slurry of W
1S
1F
0C for elemental analysis, and the location of the score is shown in
Figure 15a. The elemental composition and content of concrete after seawater freezing and thawing were obtained through an elemental capture of the samples after seawater freezing and thawing, and the spot-scan results are shown in
Figure 15b–d. From
Figure 15b–d, ions such as Ca, O, Al, and Cl can be seen. The hydrides can be introduced roughly as 3CaO·Al
2O
3·3CaSO
4·32H
2O (Aft), 3CaO·Al
2O
3·CaCl
2·10H
2O (“Friedel” salts), and CaSO
4·2H
2O (gypsum). The action of these substances with expansive properties led to the deterioration of the specimens after seawater freezing and thawing. Some support is provided for the interpretation of the internal generators as well as erosion products of seawater sea sand concrete.
As shown in
Figure 16, after mixing fly ash and a freeze–thaw cycle, the hydration products are interspersed with each other and bonded to each other; moreover, the contact at the hydride lap point is better, the pores in the structure are fewer, and the densification of the structure is higher. The reasons for this are as follows: (1) the effect of fly ash on concrete can be divided into the volcanic ash reaction effect, microaggregate effect, and particle morphology effect. Among them, the volcanic ash effect is a chemical reaction and the latter two are physical effects. Studies have shown that fly ash plays a much greater physical than chemical role in the early stages of cement hydration; however, the role of the volcanic ash effect becomes progressively greater as age increases. Fly ash particles are smaller than cement particles, and more than 70% are smooth, intact glass particles that are mixed into the concrete to play the role of lubrication. The appropriate amount of mixing can change the working properties of cement, and the fine particles themselves can be filled into the pores produced by the hydration of cement so that the concrete itself is denser [
42]. (2) Higher aluminum content in fly ash is conducive to reducing the penetration rate of chloride ions in concrete and improving the resistance of concrete to chloride ion penetration. (3) CI
− and cement hydrate can form sodium hydroxide, effectively promote fly ash vitreous network depolymerization and disintegration, and release its internal soluble active silica and alumina, affecting the cement hydration products of the calcium hydroxide in the volcanic ash reaction. This accelerates the speed of the fly ash cement hydration reaction so that the amount of cement hydration products continues to increase; moreover, the hardening of the cement paste porosity continues to decrease, thereby improving the performance of the concrete.
7. Conclusions
(1) Fly ash enhances the cubic compressive strength, flexural strength, and seawater freeze–thaw resistance of seawater sea sand concrete to some extent. The maximum value of compressive strength is 48.31 Mpa, and the maximum improvement of compressive strength is 13.22%. The maximum value of flexural strength is 6.38 Mpa, and the maximum improvement of flexural strength is 17.06%. The minimum value of mass loss was 0.734% and the maximum reduction in mass loss rate was 57.26% after the specimens were subjected to seawater freezing and thawing 75 times. The minimum value of strength loss was 12.89% and the maximum reduction in strength loss rate was 43.14%. A comparison of the data shows that for compressive strength and resistance to seawater freeze–thaw tests, the optimum substitution rate of fly ash is 20%. For flexural strength, the optimum replacement rate of fly ash is 10%.
(2) Compared with single seawater erosion and freezing and thawing, the coupling effect has the damaging effect of “1 + 1 > 2”, and the fly ash can give full play to the “volcanic ash effect” because of its own nature in the more stable chemical properties. When subjected to seawater freezing and thawing, seawater will accelerate the volcanic ash reaction of fly ash.
(3) The cracks and holes inside the seawater sea sand concrete specimens under the microstructure provide natural conditions for seawater intrusion, and, in the process of seawater freezing and thawing, the salt inside the concrete crystallizes. The AFt accumulates continuously, which, ultimately, makes the inside of the concrete brittle and porous; moreover, the cracks dilate, which reduces the service life of the concrete. Regarding the addition of fly ash-generated C-S-H gel and Aft to enhance the degree of structural compactness, the specimen did not appear to have obvious holes, and the addition of fly ash C-S-H gel increased, which is conducive to enhancing the strength and compactness of the interfacial links and enhancing the resistance to seawater freezing and thawing performance.
(4) The basic idea of gray system theory is introduced into the study of seawater sea sand concrete resistance to seawater freezing and thawing, and the damage prediction model established by using the mean value of GM (1, 1) has a high accuracy, which can provide a more reliable damage prediction and evaluation of concrete under the action of seawater erosion.