Study on the Effect of Silica–Manganese Slag Mixing on the Deterioration Resistance of Concrete under the Action of Salt Freezing

Abstract

The use of silico-manganese slag as a substitute for cement in the preparation of concrete will not only reduce pollution in the atmosphere and on land due to solid waste but also reduce the cost of concrete. To explore this possibility, silico-manganese slag concrete was prepared by using silico-manganese slag as an auxiliary cementitious material instead of ordinary silicate cement. The mechanical properties of the silico-manganese slag concrete were investigated by means of slump and cubic compressive strength tests. The rates of mass loss and strength loss of silico-manganese slag concrete were tested after 25, 50, and 75 cycles. The effect of the silica–manganese slag admixture on the microfine structure and properties of concrete was also investigated using scanning electron microscopy (SEM). Finally, the damage to the silica–manganese slag concrete after numerous salt freezing cycles was predicted using the Weibull model. The maximum enhancement of slump and compressive strength by silica–manganese slag was 17.64% and 11.85%, respectively. The minimum loss of compressive strength after 75 cycles was 9.954%, which was 34.96% lower than that of the basic group. An analysis of the data showed that the optimal substitution rate of silica–manganese slag is 10%. It was observed by means of electron microscope scanning that the matrix structure was denser and had less connected pores and that the most complete hydration reaction occurred with a 10% replacement of silica–manganese slag, where an increase in the number of bladed tobermorite and flocculated C-S-H gels was observed to form a three-dimensional reticulated skeleton structure. We decided to use strength damage as a variable, and the two-parameter Weibull theory was chosen to model the damage. The final comparison of the fitted data with the measured data revealed that the model has a good fitting effect, with a fitting parameter above 0.916. This model can be applied in real-world projects and provides a favorable basis for the study of damage to silica–manganese slag concrete under the action of salt freezing.

1. Introduction

Concrete material is frequently used in the construction industry for the following reasons: it is easy to access the raw materials required to produce concrete, concrete is cost-effective, and it can be used in simple construction processes [1]. It is formed by using cementitious materials, aggregates, and water, as well as, when necessary, the addition of admixtures, additives, and other materials in accordance with certain design ratios, after uniform mixing and blending, maintenance, and molding of the composite material in line with specific requirements. Because of its high compressive strength, durability, and wide range of advantages, it is widely used in military construction, commercial buildings, roads and bridges, water conservancy projects, and other fields [2]. However, with the continuous development of concrete materials and the vigorous promotion of urbanization, the production of metallurgical slag, construction waste, and other by-products is also climbing [3,4]. Most of the treatment of these solid wastes currently takes place in piles and landfills, with a secondary utilization rate of resources of only about 10%. This not only causes land resource tension but also aggravates environmental pollution [5,6,7]. Solid wastes still face the serious challenge of underutilization, and how to improve the utilization of these solid wastes is the focus of the current research.

At present, there are two main ways to prepare manganese metal in the industry: one is to prepare manganese metal via the electrolysis of manganese sulphate solution, and the other is to prepare manganese metal via high-temperature smelting of manganese–iron alloys. Both preparation methods produce waste slag in the preparation process, especially through the high-temperature smelting of ferromanganese alloys, where the high-temperature slag discharged is rapidly cooled by water to form a type of blast furnace slag with potential water hardness and volcanic ash, named silica–manganese slag [8,9,10]. The production of silica–manganese alloys discharges a large amount of silica–manganese slag waste, and currently, silica–manganese slag treatment is primarily carried out by piling up the waste and transporting it to a landfill or reselling it at a low price. Not only does this increase the cost for enterprises, but it also creates harmful elements that penetrate into soil and pollute groundwater [11,12]. Silica–manganese slag contains highly vitreous SiO₂ and Al₂O₃, and its composition is similar to that of cement; thus, it is considered as a raw material in the preparation of concrete, turning waste into treasure [13,14]. Through experimental studies, Allahverdi et al. [15] found that when 35% of cement was replaced with water-quenched manganese slag, the 28 d strength of the concrete was still higher than 35 MPa, and the bulk stability was satisfactory. Kumar et al. [16] ball-milled silica–manganese slag to improve its activity and ground it finely into a cementitious material with physical properties similar to those of cement. They found that its hydration products were hydrated calcium silicate (C-S-H gel) and tobermorite. Frias et al. [17] studied the effect of silica–manganese slag on the erosion resistance of concrete by incorporating 15% silica–manganese slag into cement and curing it under different solutions for 45 d. They concluded that silica–manganese slag improves the resistance of concrete to seawater erosion. Patil et al. [18] performed an experimental study on the use of silica–manganese slag as a non-conventional or human-made aggregate material for road and railroad construction and concluded that when used for rigid pavements, the compressive strength, split tensile strength, and flexural strength (modulus of rupture) of the concrete fabricated from silica–manganese slag aggregates met the design requirements.

In the northwest of China, most concrete buildings are in saline and alkaline environments. The main erosion ions in the soil are SO₄²⁻, Cl⁻, K⁺, and Mg²⁺, and the service life of buildings is greatly reduced when buildings are in this environment for a long time. Whereas freeze–thaw cycles are prevalent in cold regions, the double damage of salt freezing puts the safety of concrete buildings to a serious test [19,20]. Salt freezing damage involves freezing and thawing damage and salt erosion, which interact with each other to influence [21] the concrete matrix structure. The concrete matrix structure belongs in a saturated water state. When the environment repeatedly alternates between high- and low-temperature states, concrete freezing and thawing damage is likely to occur [22,23]. When concrete is in extremely cold saline areas, due to the alternating positive and negative temperature changes, the concrete will continue to freeze and thaw in cycles. When this process is accompanied by salt ion erosion, in the long run, it will result in the external surface mortar falling off and the concrete cracking. This results in the early decommissioning of the concrete structure or the service life of the project being far less than its engineered design life. Engineering disaster cases, which have happened from time to time, have resulted in huge economic and resource losses and even, tragically, human casualties [23,24].

Concrete structure durability damage is the result of multiple factors, and there is little research on the concrete damage law and its modeling under the combined effect of salt erosion and freeze–thaw cycles. Therefore, in this study, we carry out tests related to the analytical investigation of silica–manganese slag concrete under the action of mixed salt–freeze–thaw cycles. The mechanical properties (slump and cubic compressive strength) and salt freeze resistance of silica–manganese slag concrete are investigated using silica–manganese slag instead of cement as the main variable. Finally, a probability distribution is introduced into the study of salt freezing damage in silica–manganese slag concrete, and a damage model for silica–manganese slag concrete under the salt freezing cycle is deduced through the relationship between the damage variables and the relative strength. Our aim is to improve the durability of silica–manganese slag concrete under the salt freezing cycle and to provide a theoretical basis for real-world engineering that can prevent and mitigate disasters in saline areas.

2. Experimental Design and Methods

2.1. Test Materials

We used P-O 42.5 silicate cement produced by the Jinzhou Bohai Cement Plant. The cement had a fineness modulus of 2.63, a specific surface area of 312 m²/kg, and 3.81% burned vectors. The chemical composition of the cement is shown in

Table 1. Chemical composition and content of cement and silica–manganese slag.

Type	Al2O3	SiO2	CaO	MgO	MnO	Fe2O3	SO3
Cement	28.38	39.20	17.56	5.06	7.53	0.48	-
Silica–manganese slag	27.22	23.02	32.33	9.77	-	-	7.66

. The fine aggregate was local river sand from Jinzhou City, with a fineness modulus of 2.6, a mud content of 1.3%, and an apparent density of 2650 kg/m³. The coarse aggregate was the local natural gravel in Jinzhou, with a particle grading of 5–20 mm, an apparent density of 2641 kg/m³, and a crushing index of 8.87%. We used silica–manganese slag produced as waste by a local metallurgical plant in Jinzhou. It was obtained by means of water quenching and cooling, with a fineness modulus of 2.8 and a bulk density of 1500 kg/m³. The chemical composition of the slag is shown in Table 1, and its appearance and morphology are shown in Figure 1.

2.2. Test Apparatus

The mortar mixer, pressure machine, and fully automatic freeze–thaw box used for the test are shown in Figure 2.

2.3. Mixing Ratio

In China’s western salty soil and coastal areas, there is a great amount of SO₄²⁻ and Cl⁻. Concrete materials are often affected by temperature, humidity, and corrosive substances, accelerating their rate of deterioration and seriously affecting personal safety. Our test aimed to investigate the mechanical properties and durability of silica–manganese slag concrete for the western region of China, where buildings are exposed to salt freezing environments. The mixed salt solution used was 5%NaCl + 5%Na₂SO₄.

Concrete with a strength of C40, a sand rate of 37%, and a water/cement ratio of 0.43 was test-mixed according to the “Specification for the Design of Plain Concrete Ratio” (JGJ55-2011) [25]. Silica–manganese slag concrete was prepared via the substitution of silica–manganese slag (5%, 10%, 15%, and 20%) to study the slump, cubic compressive strength, and salt freezing resistance of the concrete. The amounts of coarse aggregate, fine aggregate, and water were 648 kg/m³, 1104 kg/m³, and 195 kg/m³, respectively. The mixes are summarized in

Table 2. Silica–manganese slag concrete mix ratio (kg/m³).

Specimen Number	Cement	Silica–Manganese Slag
C-1	453	0
CS-5	450.35	22.65
CS-10	407.7	45.3
CS-15	385.05	67.95
CS-20	362.4	90.6

Note: C is concrete, and S is silica–manganese slag. E.g., CS-10: concrete specimen with 10% silica–manganese slag substitution rate.

.

The test process was performed in strict accordance with the slow freezing method in the Test Method for Long-Term Performance and Durability of Ordinary Concrete (GB 50082-2009), and 25, 50, and 75 salt freezing cycles were used [26]. Cubic specimens of 100 mm × 100 mm × 100 mm were used for compressive strength, mass loss rate, and strength loss rate tests. A total of 21 100 mm × 100 mm × 100 mm cube specimens were prepared for each group; of these, 3 specimens were used to test the compressive strength of cubes, and 18 specimens were used to test resistance to salt freezing.

2.4. Test Piece Production

The mixer was moistened before the test to prevent the influence of water absorption on the mix ratio of concrete during mixing. In order to ensure that the concrete was evenly mixed, we first introduced the weighed stone and sand into the mixer and mixed them evenly. Then, we added cement and silica–manganese slag and mixed the substance for 1 min. The loading volume was 1/3~1/2 of the whole mixer volume. Finally, we added the mixture evenly into the mixer and mixed it for 2 min. After it was evenly mixed, a plastic template coated with a release agent was poured onto it, and the resulting compound was made to vibrate on the shaking table for compaction. The specimen was disassembled 24 h after forming. After the specimen was disassembled, the surface of the specimen was covered with a plastic sheet to prevent water evaporation and ensure that the surface of the concrete was in a wet state. The specimen was cured in a standard curing room for 28 d.

2.5. Test Procedure

2.5.1. Slump Test Procedure

This procedure was carried out according to the provisions of the Standard for Test Methods of Properties of Ordinary Concrete Mixes (GBT50080-2016) [27], as shown in Figure 3.

2.5.2. Salt Freezing Test Procedure

After the specimen was maintained in this condition for 28 d, we selected a high-quality flat specimen and placed it into a bucket containing 5% NaCl + 5% Na₂SO₄ solution to soak. The height of the liquid level in the bucket was 2 cm higher than the top of the specimen. This was carried out until the material reached a saturated state, as shown in Figure 4a. When immersion was complete, the specimens were removed from the buckets, and the surface was dried with a dry cloth and weighed for recording. Finally, the stainless steel troughs containing the specimens were put into the freeze–thaw box in an orderly manner, and 5% NaCl + 5% Na₂SO₄ salt solution was injected into the troughs with the height of the liquid level 4 cm above the specimens, as shown in Figure 4b. A salt freezing cycle test process was carried out to ensure that the freezing and thawing time was not less than 4 h. During this process, the box temperature was maintained between −20 °C and −18 °C, thawing the freezing and thawing box to maintain the internal temperature of the box at approximately 18–20 °C. At the end of the cycle, the specimen was removed from the freezing and thawing box and the weighing and mechanical properties of the test were carried out.

2.6. Calculation Formula

The calculation formula was selected in accordance with the Test Method for Long-term Performance and Durability of Ordinary Concrete (GB 50082-2009) [26].

Equations (1)–(3) were used for compressive strength, mass loss rate, and strength loss rate:

$$f_S={\displaystyle \frac{F_a}{A}}$$

(1)

Here, f_S is the measured compressive strength of the specimen (MPa); F_a is the maximum load on the specimen; and A is the pressurized area.

$$\Delta W_{\mathrm{n}}={\displaystyle \frac{W_0-W_{\mathrm{n}}}{W_0}}\times 100\%$$

(2)

Here, ΔW_n is the mass loss rate (%) of silica–manganese slag concrete after undergoing n salt freezing cycles; ΔW₀ is the unfrozen and thawed mass of silica–manganese slag concrete (g); and ΔW_n is the mass of manganese slag concrete after undergoing n salt freezing cycles (g).

$$\Delta f_{\mathrm{s}}={\displaystyle \frac{f_0-f_n}{f_0}}\times 100\%$$

(3)

Here, Δf_S is the strength loss rate (%) of silica–manganese slag concrete after undergoing n salt freezing cycles; f₀ is the strength of silica–manganese slag concrete without freezing and thawing (MPa); and f_n is the strength measurement value of manganese slag concrete after undergoing salt freezing cycles (MPa).

3. Effect of Silica–Manganese Slag on Mechanical Properties of Concrete

Table 3. Slump and compressive strength test values.

Specimen Number	Concrete Slump/cm	Compressive Strength/MPa
C-1	8.5	43.05
CS-5	9.5	45.98
CS-10	10.0	48.15
CS-15	9.1	44.32
CS-20	8.8	43.68

shows the test results of the mechanical properties of each group of specimens.

3.1. Influence of Silica–Manganese Slag on Slump

If slump is too great, it will cause segregation of the mixture; if it is too small, it will cause construction difficulties. Slump is therefore an important indicator used to judge the quality of concrete. As can be seen from Figure 5, with an increase in silica–manganese slag substitution, the slumps of concrete were all greater than that of the control C-1, and the slumps increased by 11.76%, 17.64%, 7.06%, and 3.53% when the silica–manganese slag mixing amount was increased incrementally from 5% to 20%, respectively. Due to the slump development law of concrete, this occurs primarily because the particles of the silica–manganese slag are mostly spherical, with a smooth surface and a small specific surface area. This reduces the irregularity of the particles and inter-particle friction; thus, the cementitious material adheres to and covers the surface of the sand in a lower capacity, and the adsorption of water is also reduced, which makes the concrete easier to use [28]. Silica–manganese slag consumes CH crystals to undergo hydration reaction, while the silica–manganese slag not involved in the reaction and the hydrated calcium silicate gel generated by the hydration reaction together fill the pores and microcracks, increase the density of the interface transition zone, and improve the structure of the transition zone of the concrete–cementitious-aggregate interface. Macroscopically, this will improve the slump of the concrete [12]. However, as the replacement rate of silica–manganese slag increases, the tendency to slump increases and then decreases significantly, which is due to the fact that the concrete is too viscous, resulting in a decrease in flow. The internal structure of silica–manganese slag is loose and porous, with larger voids than cement and high water absorption. When the substitution rate is too high, the concrete will rapidly absorb the mixing water added into the concrete during its initial mixing, indirectly reducing the water/cement ratio and leading to a decrease in the slump of the concrete.

3.2. Effect of Silica–Manganese Slag on Compressive Strength

When the amount of silica–manganese slag replaces the admixture of cement by 5% to 20%, the compressive strength of the concrete becomes higher than that of the benchmark concrete. From Figure 6, it can be observed that the highest compressive strength, 48.05 MPa, was obtained with 10% substitution of silica–manganese slag, which is 11.85% higher than that of the basic group. Since the particle size of silica–manganese slag is smaller than that of cement, the slag is mixed into the matrix to improve the aggregate distribution of the matrix, and the hydration of the vitreous silica–manganese slag generates more highly polymerized C-S-H gel, which enhances the bond between the cement paste and the aggregate. Silica–manganese slag can fill in the pores, attaching to the surface of cement particles to form a continuous particle gradation and acting as a lubricant, thereby improving the compressive strength of the material. Silica–manganese slag also contains chemical elements such as Al₂O₃ and SiO₂, and like fly ash, it has a volcanic ash effect [29,30]. Silica–manganese slag is composed of glassy particles. Only after a long hydration reaction, which detaches its shell, can it participate in a secondary hydration reaction. The hydration of dicalcium silicate (C₂S) participates in the hydration reaction to generate Ca(OH)₂, increasing the alkalinity of the solution alkalinity. This stimulates the activity of the silica–manganese slag, thereby increasing its strength [31]. Furthermore, the addition of silica–manganese slag can reduce the brittleness of cement mortar to some extent. This is due to the ability of silica–manganese slag to increase cement’s activity reaction with CH, generated by cement hydration, reducing the CH content and generating a C-S-H gel, thereby reducing brittleness and increasing toughness. When the replacement rate of silica–manganese slag reaches 30% and 40%, its effectiveness in improving compressive strength decreases: at these rates, its strength is only increased by 2.95% and 1.46%, respectively, compared with the basic group. This occurs because the substitution rate of silica–manganese slag is too high, and the alkalinity in the liquid phase is insufficient. Thus, the hydration of the slag is not completed, and the production of cementitious substances is reduced. This leads to a reduction in compressive strength. This also occurs because the increase in the dosage of silica–manganese slag leads to a relative decrease in the cement clinker content, a decrease in the concentration of the hydration products of clinker, a decrease in the strength produced by hydration hardening, and decreased densification of the material. All of these conditions produced a decrease in the compressive capacity of the material.

4. Effect of Silica–Manganese Slag on Salt Freezing Resistance of Concrete

Table 4. Salt freezing resistance test results (%).

Test Group	Mass Loss Rate			Strength Loss Rate
	25 Times	50 Times	75 Times	25 Times	50 Times	75 Times
C-1	−0.423	0.478	0.878	5.416	11.032	15.304
CS-5	−0.290	0.385	0.678	3.341	8.812	11.083
CS-10	−0.223	0.301	0.582	2.127	6.829	9.954
CS-15	−0.274	0.419	0.709	3.728	9.295	11.917
CS-20	−0.337	0.441	0.755	3.858	9.960	12.745

shows the test values for mass loss and strength loss.

4.1. Effect of Silica–Manganese Slag on Mass Loss

As can be seen in Figure 7, the rate of mass loss of the specimens was less than 0 under 25 salt freezing cycles. This indicates that the mass of the specimen has been increased. This occurs because of the generation of osmotic pressure inside the specimen under the salt freezing cycle and due to the material’s absorption of a large amount of water. At the same time, the hydration reaction of the cement hydrate with the sulfate and chloride salts generates products such as gypsum and calcium alumina, which leads to an increase in mass. At the early stage of the salt freezing cycle, the tiny cracks and pores inside the specimen are gradually connected under the action of freezing and expansion, which provides a channel for the salt solution to enter the interior of the specimen. After the salt solution enters the interior of the specimen, it promotes a further hydration reaction in the cement and silica–manganese slag, and the reactants enter and fill the voids of the cement stone, increasing the density of the specimen.

There was a tendency for mass loss to increase in all groups when the number of salt freezing cycles reached 50 and 75. This indicates that, for concrete, repeated freeze–thaw processes are equivalent to repeated tensile and contraction stresses and that the greater the number of salt freeze cycles, the more frequently the stresses are repeated. Thus, the internal damage of concrete gradually accumulates, leading to a decrease in quality in the latter stage of the salt freeze cycle. The highest rate of mass loss was found in the basic group, C-1, with 0.478% and 0.878%, respectively. This was due to the fact that the salt freezing cyclic action caused freezing of the free water inside the concrete material, and the salt solution entered the interior of the specimen, resulting in volume expansion. On the one hand, the volume expansion increases the internal cracks and porosity of the specimen. On the other hand, repeated freezing and thawing will cause fatigue damage to the specimen, thus causing damage to the specimen and decreasing its bearing capacity. At the same time, due to the different coefficients of thermal expansion between the various materials of the concrete, a stress difference is formed at the aggregate bonding surface during the temperature difference process, thus causing further expansion of cracks [32].

Comparing the data, it can be clearly observed that the lowest mass loss rate was achieved when the replacement rate of silica–manganese slag was 10%. Mass loss rate was reduced by 37.03% and 33.71% compared to the basic group, C-1, when the salt freezing cycles were carried out 50 and 75 times. This is due to the fact that the silica–manganese slag plays its most effective role when the secondary hydration reaction consumes calcium hydroxide, reduces the concentration of calcium hydroxide, changes the directional arrangement of calcium hydroxide in the transition zone of the mortar–aggregate interface, and increases the durability performance of the concrete. Microfine silica–manganese slag fills in the pores, internal compactness is enhanced, and corrosive ions are not easily soaked into the concrete when it is immersed in salt solution, which enhances its resistance to freeze–thaw damage.

4.2. Effect of Silica–Manganese Slag on Strength Loss

Figure 8 shows the effect of salt freezing cycles on the strength of the specimens. From the data plot, it can be found that the specimens showed different degrees of strength loss under the action of salt freezing cycles. The basic group, C-1, in particular, showed a strength loss of 15.304% after 75 salt freezing cycles. In the cyclic test mode, freeze–thaw damage and salt solution erosion produced a superimposed effect. As the number of freeze–thaw cycles increases, the tiny cracks and pores within the concrete matrix continue to grow. The main reason for this is that when the temperature is below zero, the water absorbed in the internal pores of the concrete will freeze, and the coarse and fine aggregates and the cementitious material will produce a pressure that makes it impossible to withstand the pressure between the aggregates, causing volumetric expansion, which ultimately leads to cracking. As the number of salt freezes increases, freezing and internal crystallization occur, accelerating the damage to the concrete. When the salt solution becomes flowing water as the temperature rises above freezing, substances such as Ca(OH)₂ in the concrete and the sulfate that intrudes into the interior will undergo a series of reactions to generate calcite. This will expand several times in volume, causing expansion and cracking inside the specimen [33]. In addition, the penetration rate of Cl⁻ is faster than that of SO₄²⁻, and Cl⁻ reacts rapidly with the hydration products of cement to form “Friedel” salt, which makes the concrete loose and greatly reduces its durability [34].

Silica–manganese slag replacement at 10% resulted in the lowest rate of strength loss, with reductions of 60.73%, 38.10%, and 34.96% from the basic group under successive salt freezing cycles. The rate of strength loss in all other groups was less than that of the basic group, indicating that silica–manganese slag can effectively reduce the adverse effects of salt freezing cycles on the specimens. This is because the active substance in the silica–manganese slag can react with the hydration product in cement, Ca(OH)₂, to generate C-S-H gel, which is uniformly distributed in the cement and plays a role in filling the internal pores of the specimen. This improves the pore structure of the specimen to a certain extent [35]. In addition, the incorporation of silica–manganese slag increased the structural compactness and reduced the intrusion of sulfate ions and chloride ions, improving the salt freezing resistance of the specimens. The volcanic ash effect and filling effect of silica–manganese slag improves the internal structure and product composition of the specimen, optimizes the interfacial structure and pore size distribution of the concrete, reduces the porosity, and can block the circulation of the salt solution in the specimen. In addition, silica–manganese slag can replace cement to retard the damage brought about by the precipitation of gypsum, calcovanadate, and salt crystals. The silica–manganese slag not involved in the reaction fills in the voids between the cement particles of the concrete, refining the pore structure, and makes the concrete gradation continuous and structurally dense. This reduces the channels through which the salt solution can penetrate, relieves the hydrostatic and infiltration pressures, reduces the bonding capacity of the concrete to Cl⁻ and SO₄²⁻, and improves the resistance of the concrete to infiltration, thus reducing the expansion of the concrete and improving the resistance of the concrete to salt freezing.

5. Scanning Electron Microscopy Analysis

In Figure 9, it is observed that the hydrates are in bulk and continuous form and are interglued with the hydrated calcium silicate with a reticulated structure (C-S-H gel) and with the calcium hydroxide with a lamellar structure. The hydrated calcium silicate possesses cement-like properties, and its uneroded areas are structurally intact, with no cracks and greater densification, which improves the internal structure of the concrete and increases its strength [36].

As can be seen from Figure 10a,b, after the specimen is subjected to salt freezing cycles, the connection between the aggregate and the cement paste is gradually weakened, the transition zone of the cement–aggregate interface becomes loose and porous, and the aggregate and cement paste in the transition zone of the interface may even be directly separated. In addition, the pore size is gradually increased. This is because salt freezing damage to concrete can be viewed as a special form of freeze–thaw damage to concrete, which is far more severe than the damage produced by water freezing. (1) The water retention and hygroscopicity of salt increases the initial water saturation of concrete to some extent. When the initial water saturation is greater than the critical water saturation, tensile stresses are generated in the concrete; with an increase in the number of salt freezes, the damage to the concrete becomes increasingly severe [37]. (2) When the temperature drops below 0 °C, part of the water inside the concrete freezes, and at the same time, the concentration of the salt solution inside the pores increases and produces a concentration difference, which causes the salt concentration balance inside the concrete to be broken. The free water in the other pores that is not frozen will flow to this place in order to reduce the concentration and establish a new equilibrium. The transfer of pore solution from one capillary pore to another generates osmotic pressure. When the osmotic pressure reaches or exceeds a critical value, it will crack the cement paste. (3) Supercooled water, due to the action of salt, is a type of water that remains liquid and unstable below 0 °C. It is not stable. It will cause icing in the concrete capillaries to accelerate and generate greater internal stresses which, in turn, will cause greater damage to the concrete interior. (4) When the salt solution melts, it needs to absorb heat from the concrete, making the temperature of the concrete in the frozen state drop, causing the formation of a temperature gradient in its interior, generating temperature stresses, and accelerating the rate of salt freezing damage to the concrete [38].

In Figure 10c, it can be seen that the formation of interlocking needle and granular corrosion products in the hydration products weakens the interconnection between the hydration products. As a result, the hydration forms a honeycomb shape, resulting in increased pore size and interconnection and reducing the densification of the hardened cementitious material and the formation of erosion channels. The formation of corrosion products requires the consumption of calcium hydroxide and calcium aluminate hydrate, which ultimately produces very low solubility and swollen, water-absorbing and needle-like crystals of calcium thioaluminate hydrate (AFt). As the number of salt freezing cycles increases, increasing amounts of calcium hydroxide and hydrated calcium aluminate in the hydration products are consumed, generating large amounts of agglomerated AFt. Where corrosion products gather, weak zones are easily formed, leading to the formation of microcracks in the concrete under the action of freezing pressure, crystallization pressure, and osmotic pressure, allowing a large number of corrosion ions to enter the interior of the concrete and the corrosion products to grow further. The interior of the paste becomes loose, more holes are formed, and the connection between the internal pores and crack is further strengthened.

Increasing granular salt crystal accumulation can be seen in Figure 10d. The chloride salt solution reacts with Ca(OH)₂ within the concrete to form CaCl₂⋅Ca(OH)₂⋅nH₂O complex salt. The compound salt is an expansive substance that is mainly concentrated in the surface layer of the concrete. When concrete is subjected to salt freezing, the freezable water inside it freezes and produces a volume expansion of about 9%, which can easily cause spalling of the concrete surface. In addition, because the generation of complex salt consumes a large amount of Ca(OH)₂ in the concrete, preferential access to the specimen inside the chlorine salt involved in the reaction of C₃A and other minerals is generated by the brittle “Friedel” salt, with the brittleness hindering the continuation of hydration and disrupting the equilibrium between C-S-H and Ca(OH)₂. This leads to the decomposition of C-S-H gel, accelerating the collapse of the concrete surface and damaging the structure of the concrete. This produces decomposition of the C-S-H gel, accelerating the collapse of the concrete surface and destroying the concrete structure.

In a single environment, both chloride and sulfate can react with monosulfide-type hydrated calcium thioaluminate (AFm), with the chloride erosion product being “Friedel” salt and the sulfate erosion product being calcovanadate (AFt). However, the erosion products in the composite solution of the two salts has an impact on the factors affecting its erosion in a number of ways, including reactant concentration, erosion time, erosion mode, and the nature of the two ions in the erosion process performance. Cl⁻ diffuses much faster than SO₄²⁻, and because both can react with the hydration products of cement, the erosion products are related to the relative concentrations of SO₄²⁻ and Cl⁻ [39]. At higher concentrations of chloride salts, the “Friedel” salts are produced in large quantities, which improve the internal pore structure of the concrete and slow down the erosion of sulfates. On the contrary, if the concentration of sulfate is high, sulfate reacts with the hydration product AFm to form a large amount of AFt which, in turn, reduces the chemical binding of chloride ions. When the difference between the concentrations of sulfate and chloride salts in solution is small, the reaction pattern can be divided into the following steps [40]. (1) The fast-diffusing chloride ions first react with the hydration products of the cement to form “Friedel” salts. (2) Sulfate ions react with CH to form gypsum, which reduces the pH of the solution. (3) The “Friedel” salts decompose under low-pH conditions. (4) Sulfate ions continue to enter the cementitious matrix to form AFt. Balonisd et al. [41] investigated the relationship between the transformations of Cl-AFm, SO-AFm, and CO₃-AFm by synthesizing “Friedel” salt, as shown in Figure 11. It can be found that the three salts can be converted into each other, and the main controlling factors are the concentrations of Cl⁻ and SO₄²⁻.

As can be seen from Figure 11, there are also differences in the pore structure of aerated concrete prepared with different substitution rates of silica–manganese slag. As can be seen in Figure 12a, the pore wall thickness of the pores is high, and the pore samples are mostly circular, when the silica–manganese slag substitution rate is 0%. It can be seen in Figure 12b that when the substitution rate of silica–manganese slag is 10%, the hydration products that play the role of skeleton support increase, the pore wall is thick, the porosity is low, and most of them are closed pores. Figure 12c shows that the pore size distribution is not uniform during the continuous increase in the silica–manganese slag substitution rate to 20%, and there is an overlapping of pores. Most of these are connected pores. It can be seen that when the replacement rate of silica–manganese slag is too high, the main reason for the decrease in performance is the thinning of the pore wall. After the thickness of the pore wall is reduced, the support force of the specimen becomes weak, and it is easy to form connected pores, thus reducing the mechanical properties of the material.

Figure 13 shows that when the replacement rate of silica–manganese slag is 10%, the hydration products of aerated concrete are dense and there is a large amount of well-crystallized flocculated C-S-H gel. This gel is tightly intertwined with 1~4 µm long rod-shaped calcium alumina and part of the blade-shaped tobermorite. A small amount of the raw materials not involved in the reaction are covered by the hydration products, and the hydration reaction is more complete, forming a denser mesh structure. The concrete specimens, with leaf-shaped and needle-shaped tobermorite crystals and C-S-H gel, are interlaced longitudinally and transversely to form a three-dimensional mesh structure. In the concrete specimens, the leaf-like and needle-like tobermorite crystals and C-S-H gel were interlaced to form a three-dimensional mesh skeleton structure which was connected with the C-S-H gel, playing a more effective role in providing strength and support [42,43]. During the hydration process, SiO₂ in the silica–manganese slag is activated, and the degree of crystallization of the hydrated calcium silicate is increased, producing tobermorite crystals. In addition, it is mainly because silica–manganese slag replaces cement that the content of tricalcium aluminate (C₃A) in cement is reduced. C₃A is the main reactant for the generation of hydrated calcium silicate, and the most important factor is that sodium sulphate can react with hydrated calcium silicate to produce calcium vanadate. Therefore, silica–manganese slag is added to lower the amount of calcium vanadate produced, thereby improving the specimen’s resistance to salt freezing.

6. Damage Modeling of Silica–Manganese Slag Concrete under Salt Freezing Cycle

In predicting the service life of concrete structures, the duration predicted using deterministic models is somewhat ambiguous; thus, it is necessary to accurately predict the service life of concrete by using the probabilistic method. The Weibull model is frequently used to reflect the probability distribution of material failure, and its service life prediction function is widely used in various fields [44].

The cubic compressive strength (S) was chosen to be explored as the value of the damage variable (D). The evolution formula is shown in Equation (4), and the calculated values are shown in

Table 5. Damage degree values.

Groups	Damage Value
	0 Times	25 Times	50 Times	75 Times
C-1	0	0.0542	0.0110	0.0153
CS-5	0	0.0334	0.0881	0.0111
CS-10	0	0.0213	0.0683	0.0995
CS-15	0	0.0373	0.0930	0.0119
CS-20	0	0.0386	0.0996	0.0127

.

$$D=1-{\displaystyle \frac{S_n}{S_0}}$$

(4)

Here, S₀ represents the compressive strength at the initial time, and S_n is the compressive strength after n salt freezing cycles.

The probability density function of the two-parameter Weibull model for different numbers of salt freezing cycles is provided in Equation (5):

$$f(N)=({\displaystyle \frac{\beta }{\eta }}){({\displaystyle \frac{N}{\eta }})}^{\beta -1}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{\eta }})}^{\beta }\right\}}$$

(5)

Integrating with respect to the density allows one to obtain the distribution function of the two-parameter Weibull model, as shown in Equation (6), for different numbers of salt freezing cycles:

$$F(N)=1-{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{\eta }})}^{\beta }\right\}}$$

(6)

The corresponding reliability function for the two-parameter Weibull is Equation (7):

$$R(N)=1-F(N)={\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{\eta }})}^{\beta }\right\}}$$

(7)

The failure rate function for the two-parameter Weibull is Equation (8):

$$r(N)={\displaystyle \frac{f(N)}{R(N)}}={\displaystyle \frac{\beta }{\eta }}{({\displaystyle \frac{N}{\eta }})}^{\beta -1}$$

(8)

Equation (9) calculates the damage degree value and the distribution function:

$$D(N)=F(N)={\displaystyle \int f(N)dN=1-{\mathcal{l}}^{\left\{-{(\frac{N}{\alpha })}^{\beta }\right\}}}$$

(9)

Constant deformation and simultaneous logarithms on both sides of Equation (9) produce the following equation, Equation (10):

$$\ln \left(\ln 1-D\left(N\right)\right)=\beta \ln \eta +\beta \ln \left(N\right)$$

(10)

If we let $Z=\ln \left(\ln 1-D\left(N\right)\right)$, $N=\beta \ln \eta$, $M=\beta$, and $X=\ln \left(N\right)$, the equation for the line is obtained via equivalent substitution, as shown in Equation (11):

$$Z=N+MX$$

(11)

In each equation, η is the value of the degree of material damage, β is the Weibull shape parameter, and N is the number of salt freezing cycles.

The fitting of the damage degree value data with the Weibull distribution yielded a fitting plot, as shown in Figure 14. The relevant parameters are shown in

Table 6. Fitted correlation coefficients.

Groups	Relevant Parameters
	M	N	Fitted Parameter
C-1	−6.1145	1.0052	0.995
CS-5	−7.1269	1.1758	0.930
CS-10	−8.5961	1.4877	0.967
CS-15	−6.9513	1.1539	0.955
CS-20	−6.7551	1.1138	0.916

.

The values of each parameter in the table were brought into Equation (5) to obtain specific expressions for the values of the damage variables and the number of cycles for each mix ratio, as shown in

Table 7. Damage equations.

Groups	Damage Equation
C-1	$f(N)=0.0023{({\displaystyle \frac{N}{438.28}})}^{0.0052}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{438.28}})}^{1.0052}\right\}}$
CS-5	$f(N)=0.0027{({\displaystyle \frac{N}{428.94}})}^{0.1759}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{428.94}})}^{1.1759}\right\}}$
CS-10	$f(N)=0.0046{({\displaystyle \frac{N}{323.15}})}^{0.4877}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{323.15}})}^{1.4877}\right\}}$
CS-15	$f(N)=0.0028{({\displaystyle \frac{N}{413.30}})}^{0.1539}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{413.30}})}^{1.1539}\right\}}$
CS-20	$f(N)=0.0037{({\displaystyle \frac{N}{410.03}})}^{0.1138}{\mathcal{l}}^{\left\{-{({\displaystyle \frac{N}{410.03}})}^{1.1138}\right\}}$

.

Figure 15 shows the results of assigning a value to the number of salt freezing cycles N in Table 7 and comparing it with the measured value.

From Figure 14, it can be seen that the values of the degree of damage of each group of specimens are uniformly distributed on both sides of the fitted curve of the Weibull distribution function, and the distribution characteristics of each group are roughly the same. These values are approximately the same as those of the linear distribution, allowing us to conclude that the model’s fitting correlation is high. As can be seen from Table 6, the fitting parameters of each group are above 0.916, which permits us to conclude that the Weibull model has a good fitting effect. From Figure 15, it can be seen that the error between the measured value and the calculated value is small, indicating that the damage pattern of the specimen corresponding to the test under the action of salt freezing can be better analyzed and predicted with the Weibull distribution function. After the specimens were subjected to the same number of salt freezes, the damage value was smallest when the replacement rate of silica–manganese slag was 10%, which was in line with the tests carried out in the previous section. The damage values of the remaining groups of silica–manganese slag concrete were smaller than those of the basic group C-1. This confirms that silica–manganese slag can improve the resistance of specimens to salt freezing.

7. Conclusions

(1) Silica–manganese slag had an enhancing effect on the slump, compressive strength, and salt freezing resistance of concrete. When the substitution rate was 10%, the slump and compressive strength reached 10.0 cm and 48.15 MPa, respectively; these rates were enhanced by 17.64% and 11.85% compared with the basic group. After the salt freezing cycles reached 75, the mass loss rate and strength loss rate were lower than those in the rest of the groups, reaching 0.582% and 9.954%, respectively; these rates were 33.71% and 34.96% lower than those in the basic group.

(2) With the freezing and thawing cycle playing a supporting role in the main aggregate and because of the continuous dry and wet freezing and the expansion and salt erosion, the structure began to appear separated and broken, leading directly to a loss of strength. However, silica–manganese slag contains Al₂O₃, SiO₂, and other chemical elements, and, like fly ash, it has a volcanic ash effect and an activity effect. Silica–manganese dregs can help reduce the generation of AFt in cement hydration products. Silica–manganese dregs and hydration of cement hydrates can form a supportive skeleton structure, filling the pores and cracks in the internal structure and making the concrete structure more dense. However, silica–manganese slag is not suitable for particularly large replacement rates.

(3) Electron microscope scanning showed that when the substitution rate of silica–manganese slag was 10%, the hydration products of the specimen were mostly tobermorite crystals and C-S-H gels with high support strength, and the pore walls of the air holes were thicker, with fewer connecting pores. The hydration reaction was the most thorough at this rate, and various hydrides were interspersed longitudinally and formed a three-dimensional reticulated skeleton structure. Analyzing the macroscopic data and microscopic images led us to conclude that the recommended replacement rate of silica–manganese slag is 10%.

(4) The fitting coefficients for the two-parameter Weibull model were above 0.916, and the experimental and predicted values had high correlation and agreement, indicating that the model had a good fitting effect. This shows that the established damage model can predict the performance of silica–manganese slag concrete and provide a theoretical basis for subsequent investigations.

(5) Although the mechanical properties and salt freezing resistance of silica–manganese slag concrete were investigated in the tests, in order to study the properties of silica–manganese slag concrete more comprehensively, studies should be carried out on the penetration rate of corrosive ions, the performance of silica–manganese slag concrete under dry and wet cycling, and the performance of silica–manganese slag concrete under high-temperature environments.