Effect of Iron Tailing Powder-Based Ternary Admixture on Acid Corrosion Resistance of Concrete

Exposure of concrete to acidic environments can cause the degradation of concrete elements and seriously affect the durability of concrete. As solid wastes are produced during industrial activity, ITP (iron tailing powder), FA (fly ash), and LS (lithium slag) can be used as admixtures to produce concrete and improve its workability. This paper focuses on the preparation of concrete using a ternary mineral admixture system consisting of ITP, FA, and LS to investigate the acid erosion resistance of concrete in acetic acid solution at different cement replacement rates and different water–binder ratios. The tests were performed by compressive strength analysis, mass analysis, apparent deterioration analysis, and microstructure analysis by mercury intrusion porosimetry and scanning electron microscopy. The results show that when the water–binder ratio is certain and the cement replacement rate is greater than 16%; especially at 20%, the concrete shows strong resistance to acid erosion; when the cement replacement rate is certain and the water–binder ratio is less than 0.47; especially at 0.42, the concrete shows strong resistance to acid erosion. Microstructural analysis shows that the ternary mineral admixture system composed of ITP, FA, and LS promotes the formation of hydration products such as C-S-H and AFt, improves the compactness and compressive strength of concrete, and reduces the connected porosity of concrete, which can obtain good overall performance. In general, concrete prepared with a ternary mineral admixture system consisting of ITP, FA, and LS has better acid erosion resistance than ordinary concrete. The use of different kinds of solid waste powder to replace cement can effectively reduce carbon emissions and protect the environment.


Introduction
Concrete is one of the most extensively used building materials in the world, and its durability is exceptionally significant in structural design, especially in infrastructure design, where the performance of concrete exposed to acidic environments is one of the critical issues of concrete durability [1,2]. Exposure of concrete to acidic environment will reduce the performance of infrastructure, shorten the service life and increase the maintenance cost.
Regarding the improvement of the operational performance of concrete structures in acidic environments, some studies have shown that using different kinds of admixtures and new materials can improve the performance of concrete. In addition, the use of biocide to inhibit the growth of acid bacteria is also one of the effective ways to achieve this [3,4]. The effects of cement type, water-cement ratio (W/C) [5,6], polymer materials, aggregate type, and supplementary cementitious materials (SCMs) [7][8][9] of acid erosion resistance of concrete have been investigated in some previous studies. Alexander et al. [10] studied The P.O 42. 5 Portland cement (OPC) was selected as the binder, meeting the Chinese standard GB 175-2007. At different CRRs, ITP, FA, and LS were used to replace OPC. The chemical compositions and specific surface areas of ITP, FA, and LS are listed in Tables 1 and 2, respectively. The specific surface area was determined by BET method. The particle size distributions and microscopic morphology of ITP, FA, and LS are presented in Figures 1 and 2, respectively. The particle size distribution was determined by Malvern Mastersizer 2000 laser particle size analyzer; the microscopic morphology was determined by ZEISS Gemini 300 scanning electron microscope. The iron tailings sand (ITS) was utilized as a fine aggregate with a fineness modulus of 2.0. The physical properties are provided in Table 3, and the particle gradation of ITS is shown in Figure 3. The iron tailings rock (ITR), with a particle size scope of 5 to 20 mm, was utilized as coarse aggregate. The physical properties of ITR are provided in Table 4, and the particle gradation of ITR is reported in Figure 4. The particle gradation was determined by the Chinese standard GB/T14685-2011. The water reducer was a P-II water reducer. The performance index of water reducer is listed in Table 5. The acid was a 99.5% concentration of concentrated acetic acid. The regular tap water (drinking water) was used for mixing.

Materials
The P.O 42.5 Portland cement (OPC) was selected as the binder, meeting th standard GB 175-2007. At different CRRs, ITP, FA, and LS were used to replace chemical compositions and specific surface areas of ITP, FA, and LS are listed and Table 2, respectively. The specific surface area was determined by BET me particle size distributions and microscopic morphology of ITP, FA, and LS are in Figures 1 and 2, respectively. The particle size distribution was determined by Mastersizer 2000 laser particle size analyzer; the microscopic morphology was de by ZEISS Gemini 300 scanning electron microscope. The iron tailings sand (ITS lized as a fine aggregate with a fineness modulus of 2.0. The physical propertie vided in Table 3, and the particle gradation of ITS is shown in Figure 3. The iro rock (ITR), with a particle size scope of 5 to 20 mm, was utilized as coarse aggr physical properties of ITR are provided in Table 4, and the particle gradation reported in Figure 4. The particle gradation was determined by the Chinese GB/T14685-2011. The water reducer was a P-II water reducer. The performanc water reducer is listed in Table 5. The acid was a 99.5% concentration of con acetic acid. The regular tap water (drinking water) was used for mixing.

Mix Proportion and Preparation of Concrete
The mixed proportions of concrete are listed in Table 6. IFL-0 is the OPC without admixture; specimens IFL-1 to IFL-4 are the ternary systems composed of ITP, FA, and LS with a mixed mass ratio of 2:1:1, a fixed w/b of 0.42 and CRRs of 10%, 20%, 30%, and 40%, respectively; specimens IFL-2, IFL-5 to IFL-7 are the ternary systems composed of ITP, FA, and LS with a mixed mass ratio of 2:1:1, a fixed CRR of 20% and w/b of 0.42, 0.44, 0.46 and 0.48, respectively. For specimen preparation, ITS and ITR were dried in the oven at 105 • C for 24 h. Before the dried ITS and ITR were placed into the blender for 1 min, the weighed OPC, ITP, FA, and LS were put into the blender for 1 min. Finally, the water and water reducer were mixed well, and all of them were dumped into the blender for 2 min. After mixing, the mixtures were put into a 100 mm × 100 mm × 100 mm mold and consolidated using a vibrating table for 30 s. The specimens were cured at a standard curing condition of 20 ± 2 • C and 95 ± 5% relative humidity for 24 h before being tested.

Test Methods
(1) Acid erosion: The acid erosion test consisted of 99.5% concentrated acetic acid being configured into an acidic solution with pH = 3. During the erosion process, the concentration of the saturated solution was adjusted every seven days to keep the concentration of the solution stable. The saturated solution was replaced once every 30 days, and the soaking cycle was 60 days. (2) Compressive strength loss: The Shenzhen Universal testing equipment was used to test the concrete cube's compressive strength in line with Chinese standard GB/T 50081-2019, and the compressive strength loss (CSL) was computed as follows: where f t is the average compressive strength (MPa) of three specimens for the standard curing 28 days and f s is the average compressive strength (MPa) of three specimens for the acid erosion 60 days. the measurement time, the specimens were rinsed with tap water, and then dried at 50 ± 2 • C for 48 h. The mass loss (ML) of concrete was then determined as follows where W t is the average mass (g) of three specimens for the standard curing 28 days and W s is the average mass (g) of three specimens for the acid erosion 60 days. (4) Apparent deterioration: The apparent deterioration test examined soaking concrete specimens in an acetic acid solution that causes apparent changes by recording the apparent changes of acid erosion on concrete specimens in the early, middle, and late stages. (5) MIP: Taking the concrete specimen, a 15 mm thick slice of concrete was cut out with a cutter before a core was drilled and sampled using an electric drill and a hollow drill bit with an inner diameter of 8-14 mm. The sample contained no aggregate. After sampling, the specimen was immersed in anhydrous ethanol for seven days to terminate hydration. Finally, the specimen was baked for three days at a temperature of 50 ± 2 • C to obtain the sample to be tested. The pore distribution was determined using the AutoPore Iv 9510 high performance automatic mercury injection instrument. (6) SEM: Taking the concrete specimen, a 3-5 mm thick slice of concrete was cut out with a cutter before using an electric drill to drill a core sample in the slice. After sampling, the specimen was immersed in anhydrous ethanol for seven days to terminate hydration. Then, the sample was placed in an ultrasonic cleaner to clean, and finally put into an oven at a temperature of 50 ± 2 • C for three days to obtain the sample to be tested. The micromorphology was determined by ZEISS Gemini 300 scanning electron microscope.

Compressive Strength of Concrete at Different CRRs
The 28 d compressive strength of concrete after standard curing with different CRRs is shown in Figure 5. At a certain w/b, the 28 d compressive strength of concrete after standard curing first rises and subsequently drops as CRRs improve. The 28 d compressive strength of the concrete after standard curing reaches a maximum value of 53.3 MPa when the CRR is 20%, compared to IFL-2 with IFL-0, which increases by 16.9%. When the CRRs are 10%, 30%, and 40%, respectively, the 28 d compressive strength after standard curing increases by 16.7%, 11% and 5.5%, respectively, compared with IFL-0. The result indicates that the use of mineral admixtures instead of cement is beneficial to increase the compressive strength of concrete, which is consistent with the conclusion in the literature [31][32][33]. The mechanically ground ITP makes the microstructure denser, and the fine ITP exhibits the filling effect of macroaggregates [23]. Proper blending with FA and LS can improve the particle gradation, disperse in the concrete, fill the spaces inside the microstructure, and increase the compressive strength of the test block. Meanwhile, ITP, FA, and LS contain volcanically active chemical components and react with CH (Ca(OH) 2 ) to form C-S-H gels [24,34], which makes the internal structure of concrete denser and enhances the compressive strength.
The acid erosion 60 d compressive strength of concrete with different CRRs is displayed in Figure 5 At a certain w/b, the acid erosion 60 d compressive strength of concrete specimens first rises and then drops with increasing CRRs. When the CRR is about 16%, the acid erosion 60 d compressive strength is the same as the standard curing 28 d compressive strength of concrete. When the CRR is below 16%, the 28 d compressive strength of concrete after standard curing is better than the acid erosion 60 d compressive strength of concrete. The highest loss of compressive strength of concrete after 60 d of acid erosion is 5.1% at a CRR of 10%. When the CRRs are higher than 16%, the acid erosion 60 d compressive strength of concrete is better than the 28 d compressive strength of concrete after standard curing. The highest rise in compressive strength of concrete in 60 days of acid erosion is 3.4% at a CRR of 20%, indicating stronger resistance to acid erosion. This is consistent with the conclusion in the literature [25] that the presence of supplementary cementitious materials lowers the detrimental effect of acids on concrete. When the CRRs are lower than 16%, the cement content is relatively large, and the acid solution mainly reacts with the hydration products generated by cement hydration, resulting in the loose expansion of the internal structure of concrete and a decrease in concrete compactness, a reduction in compressive strength. When the CRRs are higher than 16% with increasing admixture, the early activity of ITP is low [28], which reduces the hydration rate of cement and the internal pore structure of concrete. As the reaction proceeds, the volcanic ash reaction of ITP, FA, and LS with specific activity occurs [34,35], which consumes the unhydrated particles and fills the connected pores. In addition, the good volcanic ash reaction of FA and LS can improve the pore structure of concrete [24,36], enhance the amount of cementation and the cementation process, and fill capillaries and void cracks in concrete specimens. Therefore, the filling effect increases the compactness of concrete to some extent, strengthens the acid erosion resistance of concrete, and hinders the transmission of erosion media into concrete pores.
pressive strength of concrete is better than the 28 d compressive strength standard curing. The highest rise in compressive strength of concrete i erosion is 3.4% at a CRR of 20%, indicating stronger resistance to acid consistent with the conclusion in the literature [25] that the presence o cementitious materials lowers the detrimental effect of acids on concrete are lower than 16%, the cement content is relatively large, and the acid reacts with the hydration products generated by cement hydration, resu expansion of the internal structure of concrete and a decrease in concre reduction in compressive strength. When the CRRs are higher than 16% admixture, the early activity of ITP is low [28], which reduces the hydrat and the internal pore structure of concrete. As the reaction proceeds, th action of ITP, FA, and LS with specific activity occurs [34,35], which con drated particles and fills the connected pores. In addition, the good volc of FA and LS can improve the pore structure of concrete [24,36], enhan cementation and the cementation process, and fill capillaries and void c specimens. Therefore, the filling effect increases the compactness of con tent, strengthens the acid erosion resistance of concrete, and hinders th erosion media into concrete pores.

Compressive Strength of Concrete at Different w/b
The 28 d compressive strength of concrete after standard curing wit presented in Figure 6. At a certain CRR, the 28 d compressive strength standard curing gradually drops with the improvement of w/b. When th concrete specimen's standard curing 28 d compressive strength reache 53.3 MPa. When the w/b increases from 0.42 to 0.44, 0.46, and 0.48, the strength of concrete after standard curing decreases by 9.2%, 16.5%, an tively, which is consistent with the conclusion in the literature [37] tha strength of concrete drops as w/b increases. As w/b increases, the con water level rises more, porosity increases, and the corresponding capilla resulting in the concrete's decreasing compactness. This phenomenon b vere with the growth of water content, so with the rise of the w/b test bl strength decreases.
The acid erosion 60 d compressive strength of concrete with differ

Compressive Strength of Concrete at Different w/b
The 28 d compressive strength of concrete after standard curing with different w/b is presented in Figure 6. At a certain CRR, the 28 d compressive strength of concrete after standard curing gradually drops with the improvement of w/b. When the w/b is 0.42, the concrete specimen's standard curing 28 d compressive strength reaches a maximum of 53.3 MPa. When the w/b increases from 0.42 to 0.44, 0.46, and 0.48, the 28 d compressive strength of concrete after standard curing decreases by 9.2%, 16.5%, and 20.3%, respectively, which is consistent with the conclusion in the literature [37] that the compressive strength of concrete drops as w/b increases. As w/b increases, the concrete's hardening water level rises more, porosity increases, and the corresponding capillary pores increase, resulting in the concrete's decreasing compactness. This phenomenon becomes more severe with the growth of water content, so with the rise of the w/b test block, compressive strength decreases.
The acid erosion 60 d compressive strength of concrete with different w/b is shown in Figure 6. At a certain CRR, the acid erosion 60 d compressive strength of concrete gradually decreases with the increase in the w/b. When the w/b is about 0.47, the acid erosion 60 d compressive strength is the same as the 28 d compressive strength of concrete after standard curing. When the w/b is less than 0.47, the acid erosion 60 d compressive strength of concrete is better than the 28 d compressive strength of concrete after standard curing, which shows strong resistance to acid erosion. The acid erosion 60 d compressive strength of concrete improved by 3.4% at a w/b of 0.42. When the w/b is higher than 0.47, the acid erosion 60 d compressive strength of concrete is lower than the 28 d compressive strength of concrete after standard curing, and the acid erosion 60 d compressive strength of concrete lost 1.2% at a w/b of 0.48. When the w/b is less than 0.47, the particle size of the incorporated ITP, FA, and LS is very dense, which improves the density and impermeability of the concrete and improves the filling of cement particles. The micro-aggregate effect of ITP improves the hydration environment of cement [23] and the homogeneity of the concrete. Moreover, the addition of admixtures makes up for the defect of the poor bonding surface of cement paste and aggregate [32], which hinders the erosion of concrete by acetic acid solution. As the w/b increases, the greater the degree of diffusion of the erosion medium, the faster the diffusion rate, and the erosion resistance of concrete decreases, resulting in lower compressive strength. When the w/b is too large, the water content of the concrete is high, and the water reducer releases the water in the flocculation structure generated by cement hydration. Excess water in the system occurs, thus significantly increasing the porosity of the concrete structure. An increase in the w/b also makes the internal secondary hydration incomplete. The reduction of hydration products leads to the internal system of the substantial organization becoming thin and the erosion medium more easily invaded, making the compressive strength of tangible decrease. of concrete is better than the 28 d compressive strength of concrete afte which shows strong resistance to acid erosion. The acid erosion 60 d com of concrete improved by 3.4% at a w/b of 0.42. When the w/b is higher erosion 60 d compressive strength of concrete is lower than the 28 d com of concrete after standard curing, and the acid erosion 60 d compressiv crete lost 1.2% at a w/b of 0.48. When the w/b is less than 0.47, the p incorporated ITP, FA, and LS is very dense, which improves the densi bility of the concrete and improves the filling of cement particles. The effect of ITP improves the hydration environment of cement [23] and th the concrete. Moreover, the addition of admixtures makes up for the bonding surface of cement paste and aggregate [32], which hinders the e by acetic acid solution. As the w/b increases, the greater the degree o erosion medium, the faster the diffusion rate, and the erosion resistan creases, resulting in lower compressive strength. When the w/b is too content of the concrete is high, and the water reducer releases the water structure generated by cement hydration. Excess water in the system oc cantly increasing the porosity of the concrete structure. An increase in th the internal secondary hydration incomplete. The reduction of hydrati to the internal system of the substantial organization becoming thin an dium more easily invaded, making the compressive strength of tangibl

Mass Loss of Concrete at Different CRRs
The mass and ML of concrete at different CRRs is exhibited in Tab that the CCRs were 0, 10%, 20%, 30%, and 40%, respectively, and the M 2.49%, 0.75%, −0.12%, 0.32%, and 0.56%, respectively, for a certain w/b. T with a CRR of 20% increased by 0.12%. The result indicates that the con ITP, FA, and LS can improve the mass of concrete after an acid erosion, w with the conclusion in the literature [25]. When the acid comes into con crete, it reacts with the hydration products and produces products that in the mass of the concrete [38]. The mass of concrete in OPC (IFL-0) lo

Mass Loss of Concrete at Different CRRs
The mass and ML of concrete at different CRRs is exhibited in Table 7. It can be seen that the CCRs were 0, 10%, 20%, 30%, and 40%, respectively, and the ML of concrete was 2.49%, 0.75%, −0.12%, 0.32%, and 0.56%, respectively, for a certain w/b. The concrete mass with a CRR of 20% increased by 0.12%. The result indicates that the concrete composed of ITP, FA, and LS can improve the mass of concrete after an acid erosion, which is consistent with the conclusion in the literature [25]. When the acid comes into contact with the concrete, it reacts with the hydration products and produces products that cause an increase in the mass of the concrete [38]. The mass of concrete in OPC (IFL-0) loss was 2.49%. The variation of concrete mass at different CRRs is shown in Figure 7. The mass of admixture concrete has little change, while that of OPC concrete has great change. Compared with the concrete of OPC, the overall ML of the composite admixture was smaller. From the results, the composite admixture reduced the ML of the concrete with certain acid erosion resistance.

Mass Loss of Concrete at Different w/b
The mass and ML of concrete at different w/b are exhibited in Table is 0.42, 0.44, 0.46, and 0.48, respectively, the ML of concrete is −0.12%, 0 0.40%, respectively. It can be seen from the results that the mass loss in increase in w/b. The reason is that the higher w/b, the higher the porosi corrosion degree and the greater ML, which is consistent with the conclu [38]. The variation of concrete mass at different w/b is shown in Figu change in the mass of these four specimens. Compared with the mass concrete, the overall mass loss rate of the composite admixture is smalle acid erosion resistance.

Mass Loss of Concrete at Different w/b
The mass and ML of concrete at different w/b are exhibited in Table 8. When the w/b is 0.42, 0.44, 0.46, and 0.48, respectively, the ML of concrete is −0.12%, 0.20%, 0.24%, and 0.40%, respectively. It can be seen from the results that the mass loss increases with the increase in w/b. The reason is that the higher w/b, the higher the porosity, the higher the corrosion degree and the greater ML, which is consistent with the conclusion in literature [38]. The variation of concrete mass at different w/b is shown in Figure 8. There is no change in the mass of these four specimens. Compared with the mass loss of the OPC concrete, the overall mass loss rate of the composite admixture is smaller and has certain acid erosion resistance.

Apparent Deterioration
The process of concrete test block being soaked by acid solution 9. Deterioration of concrete surfaces occurs due to acid erosion [39] of concrete is shown in Figure 10. In the early erosion, the charac concrete test block were relatively intact. Sanding only appeared in time, in the middle of the erosion, the corners of the concrete test b aged, and the central part was relatively more intact. Because of th surface of the test block appeared rougher, and the overall shape test block surface damage is aggravated by the continuous erosion In the late erosion, the test block surface damage deteriorates, ther of pockmarks and etching pits, and the overall pockmarks are relati is that the erosion medium in acetic acid solution reacts with the h forms soluble salt, which leads to the deterioration of the surface of The overall resistance of the concrete test block to the erosion of the no more bottomless pits and larger cracks appear on the surface.

Apparent Deterioration
The process of concrete test block being soaked by acid solution is displayed in Figure 9. Deterioration of concrete surfaces occurs due to acid erosion [39]. The apparent decline of concrete is shown in Figure 10. In the early erosion, the character and corners of the concrete test block were relatively intact. Sanding only appeared in the surface layer. With time, in the middle of the erosion, the corners of the concrete test block began to be damaged, and the central part was relatively more intact. Because of the loss of the paste, the surface of the test block appeared rougher, and the overall shape became irregular. The test block surface damage is aggravated by the continuous erosion of acetic acid solution. In the late erosion, the test block surface damage deteriorates, there are different degrees of pockmarks and etching pits, and the overall pockmarks are relatively dense. The reason is that the erosion medium in acetic acid solution reacts with the hydration product and forms soluble salt, which leads to the deterioration of the surface of the concrete test block. The overall resistance of the concrete test block to the erosion of the acid solution is good, no more bottomless pits and larger cracks appear on the surface.

Apparent Deterioration
The process of concrete test block being soaked by acid solution is displayed in Figure  9. Deterioration of concrete surfaces occurs due to acid erosion [39]. The apparent decline of concrete is shown in Figure 10. In the early erosion, the character and corners of the concrete test block were relatively intact. Sanding only appeared in the surface layer. With time, in the middle of the erosion, the corners of the concrete test block began to be damaged, and the central part was relatively more intact. Because of the loss of the paste, the surface of the test block appeared rougher, and the overall shape became irregular. The test block surface damage is aggravated by the continuous erosion of acetic acid solution. In the late erosion, the test block surface damage deteriorates, there are different degrees of pockmarks and etching pits, and the overall pockmarks are relatively dense. The reason is that the erosion medium in acetic acid solution reacts with the hydration product and forms soluble salt, which leads to the deterioration of the surface of the concrete test block. The overall resistance of the concrete test block to the erosion of the acid solution is good, no more bottomless pits and larger cracks appear on the surface.

MIP
The particle pore distribution of concrete is displayed in Figure 11. The pore distribution was determined using the AutoPore Iv 9510 high performance automatic mercury injection instrument. The peak strength of IFL-4 is higher than that of IFL-5, implying a high peak with a large CRR. Referring to the method in literature [40], the aperture is divided into species, and the results are shown in Figure 12. IFL-4 dominant pores are 20-50 mm in diameter and account for 39% of the total; IFL-5 prevalent pores are less than 20 mm in diameter and account for 38% of the total. The increase in the number of tiny pores can improve the impermeability of concrete, indicating that IFL-5 is more resistant to acid attack than IFL-4. This finding is corroborated by the previous experimental results obtained within the context of this study, the CSL of IFL-4 is 0.2%, the CSL of IFL-5 is −2.1%, the CSL of IFL-5 is smaller; the ML of IFL-4 is 0.56%, the ML of IFL-5 is 0.2%, and the ML of IFL-5 is smaller. The variation in the pore structure of concrete is an essential indicator of mechanical properties and durability [40]. The microstructure and properties of concrete can be improved by using mineral admixtures.

MIP
The particle pore distribution of concrete is displayed in Figure 11. The pore dist bution was determined using the AutoPore Iv 9510 high performance automatic mercu injection instrument. The peak strength of IFL-4 is higher than that of IFL-5, implying high peak with a large CRR. Referring to the method in literature [40], the aperture divided into species, and the results are shown in Figure 12. IFL-4 dominant pores are 2 50 mm in diameter and account for 39% of the total; IFL-5 prevalent pores are less than mm in diameter and account for 38% of the total. The increase in the number of tiny por can improve the impermeability of concrete, indicating that IFL-5 is more resistant to ac attack than IFL-4. This finding is corroborated by the previous experimental results o tained within the context of this study, the CSL of IFL-4 is 0.2%, the CSL of IFL-5 is −2.1 the CSL of IFL-5 is smaller; the ML of IFL-4 is 0.56%, the ML of IFL-5 is 0.2%, and the M of IFL-5 is smaller. The variation in the pore structure of concrete is an essential indicat of mechanical properties and durability [40]. The microstructure and properties of co crete can be improved by using mineral admixtures. Figure 11. The particle pore distribution of concrete.

MIP
The particle pore distribution of concrete is displayed in Figure 11 bution was determined using the AutoPore Iv 9510 high performance au injection instrument. The peak strength of IFL-4 is higher than that of I high peak with a large CRR. Referring to the method in literature [40 divided into species, and the results are shown in Figure 12. IFL-4 domin 50 mm in diameter and account for 39% of the total; IFL-5 prevalent pore mm in diameter and account for 38% of the total. The increase in the num can improve the impermeability of concrete, indicating that IFL-5 is mor attack than IFL-4. This finding is corroborated by the previous experim tained within the context of this study, the CSL of IFL-4 is 0.2%, the CSL the CSL of IFL-5 is smaller; the ML of IFL-4 is 0.56%, the ML of IFL-5 is 0 of IFL-5 is smaller. The variation in the pore structure of concrete is an e of mechanical properties and durability [40]. The microstructure and p crete can be improved by using mineral admixtures.

SEM
The SEM of the multi-solid waste mineral admixture concrete is presented in Figure 13. In the hardened slurry, it can be observed that a large number of rod-like CH are shown in Figure 13a, clustered C-S-H gels are shown in Figure 13b, the needle-like AFt are presented in Figure 13c. Figure 13d shows reacted FA and unreacted ITP. A large amount of C-S-H gels with Aft crystals makes the concrete internally dense [41], less porous, with uniform pore distribution, and more viscous, which is beneficial to enhance the compressive strength and permeability of concrete. The unreacted ITP and reacted FA are depicted in Figure 13d. This indicates that the hydration ability of FA is a little stronger than that of ITP. The primary role of FA is to enhance the performance of concrete by facilitating secondary hydration reactions. In contrast, the primary function of ITP is the filling effect, which corresponds to the experimental results that the previous admixture can enhance the compressive strength of concrete. Using ITP, FA, and LS as mineral admixtures to replace some OPC, concrete with excellent properties can be made.
Materials 2023, 16, x FOR PEER REVIEW 12 Figure 12. The pore volume ratio of concrete.

SEM
The SEM of the multi-solid waste mineral admixture concrete is presented in Fi 13. In the hardened slurry, it can be observed that a large number of rod-like CH shown in Figure 13a, clustered C-S-H gels are shown in Figure 13b, the needle-like are presented in Figure 13c. Figure 13d shows reacted FA and unreacted ITP. A l amount of C-S-H gels with Aft crystals makes the concrete internally dense [41], les rous, with uniform pore distribution, and more viscous, which is beneficial to enhanc compressive strength and permeability of concrete. The unreacted ITP and reacted FA depicted in Figure 13d. This indicates that the hydration ability of FA is a little stro than that of ITP. The primary role of FA is to enhance the performance of concret facilitating secondary hydration reactions. In contrast, the primary function of ITP i filling effect, which corresponds to the experimental results that the previous admix can enhance the compressive strength of concrete. Using ITP, FA, and LS as minera mixtures to replace some OPC, concrete with excellent properties can be made.

SEM
The SEM of the multi-solid waste mineral admixture concrete is presented in Figure  13. In the hardened slurry, it can be observed that a large number of rod-like CH are shown in Figure 13a, clustered C-S-H gels are shown in Figure 13b, the needle-like AFt are presented in Figure 13c. Figure 13d shows reacted FA and unreacted ITP. A large amount of C-S-H gels with Aft crystals makes the concrete internally dense [41], less porous, with uniform pore distribution, and more viscous, which is beneficial to enhance the compressive strength and permeability of concrete. The unreacted ITP and reacted FA are depicted in Figure 13d. This indicates that the hydration ability of FA is a little stronger than that of ITP. The primary role of FA is to enhance the performance of concrete by facilitating secondary hydration reactions. In contrast, the primary function of ITP is the filling effect, which corresponds to the experimental results that the previous admixture can enhance the compressive strength of concrete. Using ITP, FA, and LS as mineral admixtures to replace some OPC, concrete with excellent properties can be made.

Conclusions
The study focuses on the durability of ITP-FA-LS ternary mineral admixture concrete under acetic acid erosion. The compressive strength, compressive strength loss, and mass loss of concrete were analyzed by different CRRs and different w/b, and the microstructure of concrete was conducted using MIP and SEM. The following conclusions can be

Conclusions
The study focuses on the durability of ITP-FA-LS ternary mineral admixture concrete under acetic acid erosion. The compressive strength, compressive strength loss, and mass loss of concrete were analyzed by different CRRs and different w/b, and the microstructure of concrete was conducted using MIP and SEM. The following conclusions can be derived: (1) When the CRR is 20%, the standard curing 28 d compressive strength of concrete is the highest, which is higher than OPC concrete. When the w/b is 0.42, the standard curing 28 d compressive strength of concrete is the highest. The activity of ITP is significantly increased after grinding. Appropriate mixing with FA and LS can help with the particle gradation and produce a micro-aggregate effect. A volcanic ash reaction occurs, which has a strong hydration reaction ability and makes the compressive strength of concrete increase. (2) Concrete eroded with acetic acid solution after 60 days. When the CRR is greater than 16%, especially at 20%, the concrete strength increase rate after acid erosion 60 d was the largest and showed strong acid erosion resistance. When the w/b is less than 0.47, especially at 0.42, the concrete strength increase rate after acid erosion 60 d is the largest and shows strong acid erosion resistance. The good volcanic ash activity of FA and LS improved the pore structure of concrete, and improved the amount of cementation and cementation process. The filling effect increased the compactness of concrete to a certain extent, strengthened the acid penetration resistance of concrete, and hindered the transport of ions in acetic acid solution in the pores of the concrete. (3) Acidic solution erosion deteriorates the concrete surface. At the same time, there is a loss of mass of concrete, which is minimal when the CRRs is 20% and minimal when the w/b is 0.42; all of them are smaller than the ML of the OPC concrete specimen. The ternary mineral admixture system consisting of ITP, FA, and LS resulted in more delicate pores and a denser structure of concrete, which effectively hindered the erosion of concrete by acetic acid solution. (4) The mineral doping system reduces the permeability of the erosion medium. To a certain extent, it can alleviate the erosion of acid solution concrete through MIP and SEM, which confirms that the concrete has lower internal porosity and better density. The ITP-FA-LS ternary mineral admixture concrete has better acid erosion resistance than OPC concrete.