**1. Introduction**

Sulfate attack is an important factor affecting the service life and durability of concrete buildings and structures [1–4]. Soils containing a large amount of sulfate are common in saline areas of Northwest China, seawater in coastal areas, and groundwater. As a concrete structure is immersed in a sulfate solution for a long time, a large amount of sulfate ions from the solution is absorbed into the concrete, which reacts with hydration products to form ettringite precipitation. This gradually generates stress on the inner walls of pores, leading to deformation of and damage to the concrete structure [5–7]. This, severely affects the safety and service life of the concrete structure, and causes extensive economic losses [8,9]. Therefore, studying the damage degradation process of concrete under sulfate attack and the damage model of sulfate attack is helpful to delay erosion damage to concrete structures and evaluate their service states. At present, many scholars have carried out a great deal of research on the sulfate corrosion resistance of concrete and

**Citation:** Wu, Q.; Ma, Q.; Huang, X. Mechanical Properties and Damage Evolution of Concrete Materials Considering Sulfate Attack. *Materials* **2021**, *14*, 2343. https://doi.org/ 10.3390/ma14092343

Academic Editor: Krzysztof Schabowicz

Received: 7 April 2021 Accepted: 28 April 2021 Published: 30 April 2021

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have achieved fruitful results [10–14]. The right amount of fiber can inhibit the generation and expansion of microcracks in concrete. This enhances mechanical properties [15] and improves the durability of concrete, and shows that the effect of mixed fiber is better than that of single fiber [16–18]. In addition, adding an appropriate amount of fly ash to replace cement in fiber concrete can improve its durability [19–21].

At present, scholars at home and abroad have primarily studied the change law and erosion mechanisms of the macroscopic mechanical properties of concrete under sulfate attack [22,23]. At the same time, most scholars have established damage models that take compressive strength [24], split tensile strength, or relative dynamic elastic modulus as a single evaluation index of the damage variable to measure the damage caused by sulfate erosion of concrete. However, little attention has been devoted to the relationship between the amount of damage of each evaluation index [25,26]. Moreover, a single factor model with the time of sulfate erosion as the main variable was also established. Bao et al. [27] conducted tensile tests on concrete after sulfate erosion and established an evolution model with crack number density as the damage degree; they found that this model could better reflect the evolution law of concrete erosion damage. An et al. [28] conducted sulfate erosion tests on recycled concrete and established a parabolic damage model with *E*r as the damage quantity and found that it could better reflect the evolution law of erosion damage. Wu et al. [29] studied the relationship between ITZ (interface transition zone)and the damage evolution of concrete under the action of sodium sulfate erosion. They found that the influence of ITZ on the damage evolution of concrete was related to binder composition and immersion time. Xiao et al. [30] carried out a sulfate freeze–thaw coupling test on concrete and established a damage equation using the two-factor Weibull distribution model; they found that the damage fitted by the Weibull damage equation had a good correlation with RAC. Most scholars established concrete damage models that act on single factors; however, the performance of concrete subjected to sulfate erosion is the result of the combined actions of erosion time and the constituent materials [31], as such establishing a damage model based on both erosion time and the constituent materials should be considered.

In this paper, fly ash content and erosion time are taken as the main variables, while the uniaxial compressive strength test, split tensile test, ultrasonic testing test, scanning electron microscopy (SEM) and X-ray diffraction (XRD) were carried out on concrete under sulfate attack. First, the rate of mass change, relative dynamic modulus of elasticity, corrosion resistance coefficient of compressive strength, and the corrosion resistance coefficient of splitting tensile strength of concrete of different erosion times were analyzed. The erosion damage of concrete caused by sulfate attack was defined by the strength damage and wave velocity damage. Considering the superimposed effect of fly ash and erosion time, the influence of fly ash and sulfate erosion time on the expansion of concrete damage was discussed. Based on fly ash content and sulfate erosion time, a concrete composite erosion damage model was established. Finally, our study reveals the microstructure change law of concrete subjected to sulfate attack, aiming to provide theoretical support and an experimental basis for durability research of concrete structures in areas where sulfate attack occurs.

#### **2. Materials and Methods**

## *2.1. Raw Materials and Concrete Mixture Proportions*

This study used Portland cement (P•O 42.5) produced by Bagongshan Cement Plant (Huainan, China), which conforms with the standard for Common Portland cement [32]. The chemical composition of the cement are given in Table 1. The fly ash employed was a secondary class fly ash and was purchased from Luoyang Yizhou Plastic Technology Co., Ltd. (Luoyang, China), which conforms to the standard for fly ash used for cement and concrete [33]. The chemical composition of the fly ash are listed in Table 1. Fly ash was used as a partial replacement for cement and its mixing amounts were 0, 10, and 20% (*ψ*, mass fraction) of the total cementitious material, respectively. The coarse aggregate

used in the test was continuous grading crushed rock of 5–25 mm. The fine aggregate was river sand with fineness modulus of 2.6. The water used was fresh, potable laboratory tap water. In the experiments, 6-mm basalt fiber with a volume ratio of 0.1% and 12-mm polypropylene fiber with a volume ratio of 0.2% were used. Their appearances are shown in Figure 1. The physical and mechanical properties are listed in Table 2. A sodium sulfate solution was prepared using chemical analytical reagents.

**Figure 1.** The appearance of fiber. (**a**) Polypropylene fiber and (**b**) basalt fiber.

According to the JGJ 55-2011 specification for the mix proportion of ordinary concrete [34], concrete with a strength grade of C30 was designed. Each group of concrete needed to have 3 parallel samples, and there were 108 concrete samples in total. In addition, the water-to-binder ratio (W/B) was 0.6. The mix ratios of concrete specimens are shown in Table 3.



**Table 2.** Physical and mechanical properties of fibers [37].


**Table 3.** Mix proportions of concrete.


Notes: FA—fly ash; 0.0, 10, 20—fly ash content of 0.0%, 10%, 20%, respectively.

The types and dosages of concrete and fiber designed in this paper were all carried out on the basis of the research done in [35,37]. Basalt fibers are high elastic fibers and polypropylene fibers are low elastic fiber. The two kinds of fibers play different roles in the process of concrete stress. The correct amount of a fiber disperses evenly within a concrete, enhances the crack resistance effect and improves the interface characteristic of the concrete. It also inhibits the internal stress of concrete cracking after preliminary initiation and, further, reduces the brittleness of concrete, improves the effect of crack resistance, and improves the tensile strength of hybrid-fiber-reinforced concrete materials.
