*2.2. Specimen Preparation*

First, the sand, gravel, fly ash, and cement were batched and evenly mixed in a mixer for 60 s, and then the right proportion of water was added to the mixer for another 60 s. Second, blended fibers were evenly distributed into the solid mixture to prevent agglomeration which was mixed for 120 s. After mixing, the mixture was put into a cubic mold with side lengths of 100 mm × 100 mm × 100 mm, distributed evenly around the mold with a spatula to ensure compacting, and finally put on a vibrating table to seal all pores. The specimen was left for 24 h after loading into the mold, and then removed. The specimen was numbered and put into a saturated Ca(OH)2 solution with a relative humidity of 95% and a temperature of 20 ± 2 ◦C to cure for 28 days. The specific steps are shown in Figure 2.

**Figure 2.** The mixing steps of concrete.

#### *2.3. Sulfate Attack Test for Concrete*

After curing for 28 days under standard conditions, the concrete test block was placed in a sodium sulfate solution at a concentration of 5% for long-term immersion. The liquid level was 10 mm above the surface of the test block. In order to ensure the stability of the pH of the solution, the solution is replaced once a month, and the concrete was completely soaked and eroded. The times of sulfate attack were 0 d, 30 d, 60 d, 90 d, 120 d, and 150 d, respectively. The whole test was carried out at 25 ± 2 ◦C.
