In order to avoid the influence of admixtures in cement on the test, P.I.42.5 pure Portland cement is adopted, and its properties are shown in Table 1. The mineral composition of cement is C₃S (59.6%), C₂S (19.5%), C₃A (7.2%), and C₄AF (8.3%).

The micro powder of metal tailings comes from a copper zinc mine in Fujian, China. It is fine-grained tailings screened by drying (45 μm sieve residue is 6.45%). The main component of tailings is SiO₂, as well as some chemical components such as iron, aluminum, copper, and zinc. An S95-grade slag is selected as slag powder, and its main components are SiO₂, CaO, Al₂O₃, etc. See Tables 2, 3 for specific technical indexes and the chemical composition of metal tailings powder and slag powder.

The coarse and fine aggregates used in the experiment are natural aggregates, and the indexes meet the national standards, as shown in Table 4.

Polycarboxylic acid superplasticizer is used as the admixture, with a solid content of about 20%.

Tap water is used for concrete mixing, and the chemical composition in tap water contains very little chloride ion, which will not affect the corrosion test.

In order to explore the action law of metal tailings powder on the sulfate resistance of concrete, the compound admixture of metal tailings powder and slag powder is adopted. The proportion of metal tailings powder in the compound admixture is 0%, 30%, 50%, 70%, and 100%. Two kinds of common concrete, C30 and C50, are prepared. Based on the consideration of low carbon and environmental protection, a low cement clinker and low water binder ratio system are adopted. The cement in C30 concrete is only 30% of the cementitious material, and the basic water binder ratio is 0.40. The cement in C50 concrete is 40% of the cementitious material, and the basic water binder ratio is 0.29. Because it is a low cement clinker system, through trial mixing, the basic water binder ratio is slightly lower than that of ordinary concrete. The water binder ratio of concrete decreases with the increase of the content of nonactive admixture, metal tailings powder, so as to ensure that the strength can meet the design grade. The specific proportion is shown in Table 5.

Place the test piece (100 mm × 100 mm × 100 mm) standard curing for 26 days, take it out of the standard curing room, put it into the drying oven, and dry it at (80 ± 5)°C for 48 h. Take it out after drying, cool it to room temperature, and put the test block into a sulfate wetting–drying cycle test chamber. Prepare 5% Na₂SO₄ solution as corrosion solution, and replace the sulfate solution every other month. The wetting–drying cycle is 24 h, including soaking for 16 h, drying for 6 h at a drying temperature of 80°C, and cooling for 2 h. The sulfate wetting–drying cycle experiment equipment is a NELD-LSC automatic sulfate wetting–drying cycle test machine. The concrete quality, ultrasonic velocity, and compressive strength of concrete were tested every 30 wetting–drying cycles. When the compressive strength of the concrete decreased to 75% before the test, it was regarded as sulfate erosion failure, and the test was stopped.

The sulfate wetting–drying cycle experiment of concrete with different mix proportions was carried out to compare the concrete quality, ultrasonic velocity, and compressive strength after different wetting–drying cycles, so as to study the influence of different metal tailings powder content on the sulfate corrosion resistance.

The relative compressive strength factor P is defined and calculated as follows: P = P i − P 0 P 0 × 100 % (1) P is the relative intensity factor; P ₀ is the compressive strength of the test piece before sulfate wetting–drying cycle, MPa. P _i is the compressive strength of the test piece after the ith cycle sulfate wetting–drying, MPa. The compressive strength of each group is tested after different sulfate wetting–drying cycles, as shown in Figure 1. The relative compressive strength factor is calculated, and the results are shown in Table 6.

In practice, testing the compressive strength needs to consume a large number of test blocks and cannot reflect the sulfate corrosion process of concrete in structural service. Therefore, some nondestructive testing indicators such as quality and ultrasonic velocity should be developed. The relative quality factor M is defined and calculated as follows: M = m i − m 0 m 0 × 100 % (2) M is the relative quality factor, M ₀ is the mass of the test piece before the sulfate wetting–drying cycle, g. M _i is the mass of the test piece after the ith sulfate wetting–drying cycle, g. Test the mass of each group concrete after different sulfate wetting-drying cycles, and calculate the relative quality factor. The results are shown in Table 7.

The relative ultrasonic velocity factor V is defined, and the calculation formula is as follows: V = V i − V 0 V 0 × 100 % (3) V is the relative wave velocity factor, V ₀ is the ultrasonic velocity of the test piece before the sulfate wetting–drying cycle, m/s; V _i is the ultrasonic velocity of the test piece after the ith sulfate wetting–drying cycle, m/s. Test the ultrasonic velocity of each group concrete after different sulfate wetting–drying cycles, and calculate the relative ultrasonic velocity factor. The results are shown in Table 8.

The relative quality factor, relative ultrasonic velocity factor, and relative compressive strength factor of concrete specimens first increase and then decrease with the increase of sulfate dry and wet corrosion cycles. At the initial stage of corrosion, sodium sulfate solution continuously enters the concrete block through the pores. These sodium sulfate react with calcium hydroxide in the concrete to generate ettringite, gypsum, and other erosion products. In addition, the sodium sulfate solution continuously crystallizes inside, and these substances fill the small pores in the concrete. Therefore, the quality of the block has increased compared with that before corrosion, the interior is more dense, the ultrasonic velocity increases, and the compressive strength continues to improve. With the continuous advancement of the sulfate corrosion process, more and more ettringite, gypsum, and sodium sulfate crystals continue to accumulate in the concrete. Compared with the original hydration products, the expansion rate of these products increases, resulting in the continuous increase of internal volume, and micro cracks slowly occur in the concrete block. These micro cracks continue to expand and extend, resulting in the peeling off of the skin around the block, as shown in Figure 2. The relative quality factor, relative ultrasonic velocity factor, and relative compressive strength factor of concrete are reduced. When the expansion stress produced by erosion products is greater than the internal tensile strength of concrete, the micro cracks develop to the degree of penetration, and the concrete is corroded.

According to the standard, when the compressive strength decreases to 75% of the initial strength before corrosion, it is regarded as sulfate corrosion failure. The sulfate corrosion resistance grade of C30-0 to C30-100 groups is KS120. The sulfate resistance grade of C50-0, C50-70, and C50-100 groups is KS150, while the sulfate resistance grade of concrete with 30% and 50% of metal tailings powder accounting for mineral admixtures is KS180, which shows that the appropriate amount of metal tailings powder can improve the sulfate resistance of concrete, which is obvious in the C50 concrete. Taking C30-30 and C50-30 groups, in which the content of metal tailings powder accounts for 30% of mineral admixtures, as an example, the mass, ultrasonic velocity, and compressive strength are the smallest, and the sulfate corrosion resistance is relatively strong.

As the indicator of sulfate corrosion in the national standard is compressive strength, but in the actual test process, the compressive strength test requires a large number of concrete blocks, it is difficult to test the accurate compressive strength in the actual application of a typical environment. Nondestructive corrosion factors such as ultrasonic velocity are an important supplement to the study of metal tailings powder concrete sulfate resistance. The three corrosion indexes of relative quality factor, relative ultrasonic velocity factor, and relative compressive strength factor have been defined earlier, and the relative relationship of the three corrosion factors has been established. See Figure 3 to study the internal relationship and mutual characterization law of the three corrosion factors.

It can be seen from the above figure that the correspondence between the relative quality factor, the relative ultrasonic velocity factor, and the relative compressive strength factor is not obvious, so the change of the quality cannot reflect the change of the compressive strength after a certain number of wetting–drying cycles. The change of quality slightly lags behind the change of ultrasonic wave velocity and compressive strength. This is because the expansion, falling off, and filling are carried out together, and fine falling-off particles are locked in the concrete block, resulting in a small increase in the quality of concrete specimen when the ultrasonic velocity and compressive strength are reduced. However, there is a strong correlation between relative ultrasonic velocity factor and relative compressive strength factor, showing a linear relationship, and the correlation reliability is as high as 0.999. Ultrasonic nondestructive testing can be used to characterize the change of compressive strength, and the corresponding relationship is as follows. P = 2.365 V − 0.0274 With the above formula, the concrete compressive strength can be converted by ultrasonic velocity, so as to understand the corrosion degree of concrete subjected to sulfate wetting–drying cycles.

In order to explore the corrosion deterioration mechanism of metal tailings powder concrete after sulfate wetting–drying cycle, the C50-0 group with single slag powder and C50-50 group with strong sulfate corrosion resistance are selected to study the micro morphology and products of concrete before corrosion and after corrosion damage by SEM and XRD. Figures 4 and 5 shows the SEM image of concrete before and after corrosion. The image is of ×5,000 magnification.

Compared with the micro morphology of the concrete before and after corrosion, the internal structure of the concrete before corrosion is very dense. After sulfate wetting–drying cycle damage, there are many flake gypsum and some needle rod ettringite in the concrete. The volume of these corrosion products expands, the expansion stress is greater than the tensile stress in the concrete, and cracks appear in the concrete, resulting in the damage of the micro morphology. The macroscopic performance is that the compressive strength of concrete decreases to 75% of the initial strength.

Figure 6 is the XRD spectrum of concrete before and after the sulfate wetting–drying cycle corrosion damage. From the spectrum, excluding the influence of silica, there is an obvious Ca(OH)₂ diffraction peak before corrosion. But after corrosion damage, the Ca(OH)₂ diffraction peak no longer exists. There are obvious ettringite and gypsum diffraction peaks after corrosion, which indicates that the sulfate corrosion of concrete is mainly due to the sodium sulfate solution entering the concrete and reacting with the internal Ca(OH)₂. The production reaction formula of ettringite is as follows: N a 2 S O 4 · 10 H 2 O + Ca ( OH ) 2 → CaS O 4 · 2 H 2 O + 2 NaOH + 8 H 2 O 3 ( CaS O 4 · 2 H 2 O ) + 4 CaO · A l 2 O 3 · 12 H 2 O + 14 H 2 O → 3 CaO · A l 2 O 3 · 3 CaS O 4 · 32 H 2 O + Ca ( OH ) 2 Gypsum formation reaction is as follows: Ca ( OH ) 2 + N a 2 S O 4 → C a 2 + + S O 4 2 − + N a + + O H − C a 2 + + S O 4 2 − + H 2 O → CaS O 4 · 2 H 2 O Proper addition of metal tailings powder can increase the sulfate corrosion resistance of concrete. Combined with the analysis of SEM and XRD test results, the Ca(OH)₂ diffraction peak intensity of concrete with metal tailings powder accounting for 50% of mineral admixture before corrosion is significantly lower than that of the C50-0 group with slag powder alone. XRD analysis after corrosion shows that the diffraction peak intensity of ettringite and gypsum in the C50-50 group is significantly lower than that in the C50-0 group, which shows that the addition of metal tailings powder can effectively reduce the generation of sulfate corrosion products, so as to improve the ability of concrete to resist sulfate corrosion.

Nuclear magnetic resonance (NMR) is used to test the porosity of the two kinds of concrete, and the harmless holes (below 0.02 μm), less harmful holes (0.02−0.1 μm), harmful holes (0.1−0.2 μm), and multi harmful holes (above 0.2 μm) are calculated, as shown in Table 9. It can be found that the addition of metal tailings powder will increase the number of less harmful holes and harmless holes in concrete. The introduction of these holes can effectively reduce the stress formed by the expansion of sulfate corrosion products and improve the ability of concrete to resist sulfate corrosion.

The results show that an appropriate amount of inactive metal tailings powder can reduce the content of Ca(OH)₂ in concrete, so as to reduce the formation of ettringite, gypsum, and other corrosion products. At the same time, the metal tailings powder will optimize the pore structure and reduce the expansion stress of corrosion products.