2.4.4. Microanalysis

SEM and XRD tests were carried out using an S-3400N scanning electron microscope and a SMARTLAB9KW X-ray diffractometer from Yunnan Langlue Technology Co., Ltd. The microstructures, morphologies, and erosion products of minerals in the eroded samples were tested. The purpose was to analyze the erosion damage mechanism from a microscopic point of view. When a specified time of sulfate attack was reached, the concrete sample was taken out of the solution, left to air dry at room temperature, and broken into smaller parts for testing. After being placed in an oven and dried at 40 ◦C for 24 h, some samples were taken out, their surfaces were plated with gold on the surface, and then subjected to SEM tests. At the same time, the remaining test samples were crushed into powder and analyzed using XRD.

#### **3. Experimental Results and Discussion**

#### *3.1. Characteristics of Apparent Degradation*

The appearance and damage patterns of the concrete samples with different fly ash contents that were completely immersed in 5% sodium sulfate solution for different times are shown in Figure 6.

**Figure 6.** Deformation appearances of concrete samples subjected to sulfate attack. (**a**) FA0-D0; (**b**) FA0-D30; (**c**) FA0-D60; (**d**) FA0-D90; (**e**) FA0-D120; (**f**) FA0-D150; (**g**) FA10-D0; (**h**) FA10-D30; (**i**) FA10-D60; (**j**) FA10-D90; (**k**) FA10-D120; (**l**) FA10-D150; (**m**) FA20-D0; (**n**) FA20-D30; (**o**) FA20-D60; (**p**) FA20-D90; (**q**) FA20-D120; and (**r**) FA20-D150.

> The surface of the concrete specimens changed during erosion in the sodium sulfate solution with the increase in erosion time. The surface of various concretes eroded by 0-d sulfate were relatively flat and smooth. After 30 d of sulfate attack, minor defects began to appear on various concrete surfaces. After 60 d of sulfate attack, the surfaces of various concrete samples went from being relatively smooth surfaces to rough surfaces, and the mortar gradually peeled off. After 120 d of sulfate attack, the corners of the concrete of the FA0 group and FA10 group fell off dropped slightly and the surface mortar peeled off, while the surface mortar of the concrete specimens from the FA20 group appeared to

have mostly peeled off. After 150 d of sulfate attack, cracks occurred at the corners and ends of the concrete in the FA0 and FA10 groups and the mortar peeled off; the corners of the concrete specimens in the FA20 group developed cracks. The more mortar completely peeled off of the surface of the entire concrete specimen, the more severe the deterioration. The reason for this is that the structure of a concrete test block was seriously damaged by sulfate erosion and the surface became brittle. A large amount of sulfate gathered in the concrete, and sulfate crystals precipitated after water evaporation [40,41].
