Study on the Effect of Interfacial Modification on the Properties of Super Standard Mica Sand Cement-Based Materials

: Mica is a harmful substance in sand and occurs frequently. The application of super standard mica sand is a difficult problem in large-scale engineering. In this work, the effects of an interface modifier, mineral admixture, and a curing system on the properties of cement-based materials with super standard mica sand were studied. The strength of cement-based materials linearly decreases with the mica content in sand. When the mica content in sand exceeds 6%, the compressive strength of mortar and concrete at 28 d decreases by more than 22.3% and 33.5%, respectively. By adding the silane coupling agent (SCA) of 50% mica mass and curing in natural conditions, the compressive strength of mortar increases by 10.9%. The cement-based materials with the SCA are more suitable for curing in natural conditions, and the performance of the SCA will not be affected by adding appropriate amounts of mineral admixture. The drying shrinkage strain of the concrete, with the sand containing high mica content modified by SCA, is reduced by 10.5%, and the diffusion of chloride ions in concrete is reduced. The XRD results show that the addition of the interfacial agent does not change the hydration products. The MIP and SEM results show that the SCA can form a bridge structure between the hydration products and the mica, improve the bonding strength of the interface zone, and reduce the number of harmful pores.


Introduction
Natural sand resources have become scarce resources and manufactured sand has become the main source of construction sand [1,2].Mica is a significant mineral that forms rocks.Due to the special development of mica layers, the mica is often separated from rocks in the form of plates, sheets, and columns during the process of sand making [3][4][5].Sand with high mica content has flaking characteristics and a smooth surface, which means that it increases the water consumption of mortar or concrete, resulting in lower strength and insufficient durability [6][7][8].For this purpose, the standard requires the mica content in sand to be no more than 2% [9], which limits the application of super standard mica sand.In order to fully understand the influence of the mica in sand on the properties of cement-based materials, researchers have conducted a series of studies on the application of the super standard mica sand.Hoon et al. [10] pointed out that the mica content and particle size in natural sand would adversely affect the working performance, density, strength, and durability of concrete.Dewar [11] stated that a rise in free mica content in sand would lead to an increase in water consumption per unit and a decrease in the strength of concrete.According to the research of James [12], the performance of concrete was negatively impacted by free mica in sand.Wakizaka et al. [13] analyzed the effect of the mica content in sand on concrete.In 1990, the China Institute of Water Resources and Hydropower Research carried out a systematic study on the effects of free mica content in natural sand on the performance of concrete, and the results showed that the increase in free mica content increased the water consumption per unit of concrete, which had an adverse impact on the strength and durability [14].Li et al. [15,16] found that the mica in the Gshuling sand of Three Gorges affected the discharge of bubbles inside the concrete, resulting in a decline in the compactness, strength, and durability of the concrete.Zhong et al. [17] studied the influence of mica content change on the properties of concrete by mixing the manufactured sand with natural sand.The results revealed that the freezeresistance of concrete was not affected by mica when the mica content in mixed sand was controlled within 3.5%.The flaky structure and smooth surface of the mica have an impact on the bond strength, which is the main reason for the decrease in the frost resistance of concrete.Li et al. [18,19] investigated the influence of stone powder with high mica content on the properties of mortar and concrete.The results showed that when the mica content of the powder was 0~9%, the strength of concrete at 28 d decreased by about 10% compared with the reference concrete, and the strength decreased by 2.2% after 50 freezethaw cycles.Mao et al. [20] studied the effect of mica content on the rheological properties of mortar.The results showed that when the mica content in sand was greater than 2.5%, the fluidity of mortar significantly decreased.They believed that the sheet-like structure of the mica particles caused a decrease in the fluidity, viscosity, and thixotropy of mortar.Tong et al. [21] optimized the mica sorting measures in sand.The results showed that the performance of the concrete mixed with sorted sand was obviously improved, but the cost of the mica sorted sand is too high.Li et al. [22,23] carried out an experimental study on the modification of the sand with excessive mica content by adding a reinforcement agent.The reinforcement agent was shown to be capable of forming a dense interface zone between the mica and cement hydration products through an SEM analysis of the modified hardened microstructure of mortar.
It is a common method to use interfacial modifiers to improve the interfacial bonding properties of cement-based materials.Mazen et al. [24] used the nano-ZnO to modify the interface of recycled aggregate concrete.He et al. [25][26][27] used four different types of solutions to soak the brick-concrete recycled aggregate to improve the strength of the interface zone, and the results showed that the application performance of the brickconcrete recycled aggregate, after soaking in a water glass solution, was significantly improved in cement-based materials.Hu et al. [28] used SCA to modify the interface of recycled aggregate.Zhang et al. [29] studied the improvement of SCA on the volume stability of cement-based materials.Zhang et al. [30] analyzed the mechanism of improving the microstructure of cement-based materials by SCA.The application of SCA to the modification of the super standard mica sand interface has not been reported.
In summary, the existing research on the mica sand mainly focused on the influence of the mica content on the properties of cement-based materials.In the engineering application of the mica sand, the research is mainly carried out from the perspective of reducing the mica content.It is rare to study the application of the mica sand without reducing the mica content in sand.The sand production from the solid waste in the production process is common in large-scale engineering construction, and the phenomenon of the excessive mica sand is more frequent.From the perspective of economy and environmental protection, it is urgent to carry out research on the application of excessive mica sand in a way that does not reduce the mica content.Based on this, this study focused on the mechanical strength, volume stability, and durability of the cement-based materials containing the sand with excessive mica.Meanwhile, an interface modifier was used to modify the surface of the sand with excessive mica, so as to reduce the deterioration effect of excessive mica on the performance of cement-based materials.The variety, dosage, and curing method of the modifiers were also optimized.

Raw Materials
The raw materials and test procedures used in this study are shown in Figure 1.Raw materials include the cement, fly ash, ceramic powder, mica, ISO sand, manufactured sand, coarse aggregate, HPMC, KH-570 solution, and water reducing agent.Meanwhile, the tap water was used throughout the experiment.

Raw Materials
The raw materials and test procedures used in this study are shown in Figure 1.Raw materials include the cement, fly ash, ceramic powder, mica, ISO sand, manufactured sand, coarse aggregate, HPMC, KH-570 solution, and water reducing agent.Meanwhile, the tap water was used throughout the experiment.1.The fly ash is F type and class II, which has a density of 2.2 g/cm 3 , a specific surface area of 430 m 2 /kg, a water requirement ratio of 101%, and a 28 d activity index of 89%.The ceramic powder is taken from a ceramic polishing processing enterprise, with a gray color and a density of 2.36 g/cm 3 .The particle size distribution is shown in Figure 2, and it can be seen that the particle size of the ceramic powder is between 0-50 μm.The chemical compositions of cement, fly ash, and ceramic powder are shown in Table 2.The fine aggregate was manufactured sand with a fineness modulus of 2.8, an apparent density of 2.65 g/cm 3 , and a bulk density of 1.57 g/cm 3 .The standard sand used was the Chinese ISO standard sand produced by Xiamen ISO Standard Sand Co., Ltd., Xiamen, China.The coarse aggregate stone selected was graded gravel with a particle size of 5~20 mm, an apparent density of 2.71 g/cm 3 , a bulk density of 1.52 g/cm 3 , a void ratio of 43.9%, a crushing value of 8.5%, and a mud content of 0. The mica used was the Muscovite mica with an apparent density of 2.6 g/cm 3 .The particle size distribution of the mica is shown in Table 3.The superplasticizer used was a PCA polycarboxylic acid superplasticizer.The solid content of the superplasticizer is 17.5% and the water reduction rate is 21.5%.  1.The fly ash is F type and class II, which has a density of 2.2 g/cm 3 , a specific surface area of 430 m 2 /kg, a water requirement ratio of 101%, and a 28 d activity index of 89%.The ceramic powder is taken from a ceramic polishing processing enterprise, with a gray color and a density of 2.36 g/cm 3 .The particle size distribution is shown in Figure 2, and it can be seen that the particle size of the ceramic powder is between 0-50 µm.The chemical compositions of cement, fly ash, and ceramic powder are shown in Table 2.The fine aggregate was manufactured sand with a fineness modulus of 2.8, an apparent density of 2.65 g/cm 3 , and a bulk density of 1.57 g/cm 3 .The standard sand used was the Chinese ISO standard sand produced by Xiamen ISO Standard Sand Co., Ltd., Xiamen, China.The coarse aggregate stone selected was graded gravel with a particle size of 5~20 mm, an apparent density of 2.71 g/cm 3 , a bulk density of 1.52 g/cm 3 , a void ratio of 43.9%, a crushing value of 8.5%, and a mud content of 0. The mica used was the Muscovite mica with an apparent density of 2.6 g/cm 3 .The particle size distribution of the mica is shown in Table 3.The superplasticizer used was a PCA polycarboxylic acid superplasticizer.The solid content of the superplasticizer is 17.5% and the water reduction rate is 21.5%.The molecular formula of SCA is generally Y-R-Si(OR)3 (Y-organic functional group, SiOR-silanoxy group).Silanoxy groups are reactive to inorganic substances, and organic functional groups are reactive or compatible to organic substances [31].Therefore, when the SCA is between the inorganic and organic interfaces, the binding layer of the organic matrix-SCA-inorganic matrix can be formed [32].This product is a SCA KH-570 colorless transparent liquid, soluble in a variety of solvents and hydrolysis, and can be cured to form insoluble polysiloxane [33].It is mainly used to improve the adhesion of an organic material's or inorganic material's surface, to increase the water resistance, and to reduce the curing temperature [34].Therefore, in this study, the SCA KH-570 was selected as the interface modifier of the super standard mica sand.The KH-570 clear liquid has an SCA content greater than 98% and a density of 1.043 g/cm 3 , which is obtained from Kangjin New material Co., Ltd., Lechang, China.
Acrylic acid (AA) is an important organic compound, chemical formula C3H4O2, and is a colorless liquid, miscible with water, with chemical activity, and capable of easy polymerization in the air.With excellent adhesion and weather resistance, the functional   The molecular formula of SCA is generally Y-R-Si(OR) 3 (Y-organic functional group, SiOR-silanoxy group).Silanoxy groups are reactive to inorganic substances, and organic functional groups are reactive or compatible to organic substances [31].Therefore, when the SCA is between the inorganic and organic interfaces, the binding layer of the organic matrix-SCA-inorganic matrix can be formed [32].This product is a SCA KH-570 colorless transparent liquid, soluble in a variety of solvents and hydrolysis, and can be cured to form insoluble polysiloxane [33].It is mainly used to improve the adhesion of an organic material's or inorganic material's surface, to increase the water resistance, and to reduce the curing temperature [34].Therefore, in this study, the SCA KH-570 was selected as the interface modifier of the super standard mica sand.The KH-570 clear liquid has an SCA content greater than 98% and a density of 1.043 g/cm 3 , which is obtained from Kangjin New material Co., Ltd., Dongguan, China.
Acrylic acid (AA) is an important organic compound, chemical formula C 3 H 4 O 2 , and is a colorless liquid, miscible with water, with chemical activity, and capable of easy polymerization in the air.With excellent adhesion and weather resistance, the functional interface agent has a wide range of applications in the architectural coating industry [35].This study uses different interface modifiers of the super standard mica sand to select the best interface modifier.The AA is an anionic emulsion with a solid content of 48%, which is obtained from Jitian Chemical Co., Ltd., Shenzhen, China.
HPMC is one of the non-ionic cellulose mixed ethers.HPMC has the characteristics of excellent film formation, dispersion, and adhesion.It can effectively improve the strength and bonding force of the cement-based materials [36].HPMC is a white powder with a viscosity of 20 s, which is obtained from Ruitai Chemical Co., Ltd., Nantong, China.
The test process includes weighing, mixing, molding, and curing, followed by a strength test, a deformation test, a chloride ion content test at the corresponding age, and finally microscopic tests of the samples.

Preparation of Mortar
The mix proportions of mortar are listed in Table 4.The SL352-2020 standard states that the maximum content of mica in sand cannot exceed 2% [9].Considering the current situation, where the mica content in manufactured sand is mostly concentrated at 2-7% [37], the content of mica in the sand in this study was set to 0, 2%, 4%, 6%, and 8%, respectively.The water to binder ratio is 0.5.The fluidity of the mortar is adjusted to 180 ± 10 mm by the water reducing agent.The fresh mortar was poured in the mold with a size of 40 mm × 40 mm × 160 mm and cured for 24 h.Then, the specimens were placed in a water bath with a temperature of (20 ± 2 • C) for curing via the standard curing method.Another curing method for mortar is natural curing.During the test in July, the indoor temperature is between 20-35 • C, and the relative humidity is between 50-70%.Compared with the standard curing, the temperature is higher, and the relative humidity is slightly lower.Two groups of standard curing specimens were formed for each numbered mortar, and two groups of natural curing specimens were also formed for the mortars with the numbers of BM4 and BM4SCA30.

Preparation of Concrete
The mix proportions of concrete are listed in Table 5.The water to binder ratio is 0.42 and the sand rate is 38%.The contents of fly ash and ceramic powder are calculated based on cement, and the content of each is 20%.The fresh concrete was poured in a 100 mm × 100 mm × 100 mm cubic mold to test the compressive strength and chloride ion natural diffusion, a 100 mm × 100 mm × 400 mm mold to test the frost resistance, and a 100 mm × 100 mm × 515 mm mold to test the deformation performance.The first curing method of concrete is standard curing, with a temperature of (20 ± 2 • C) and a relative humidity of greater than 95%.The curing environment of the natural curing concrete is consistent with the natural curing method of the mortar.Three groups of standard curing Buildings 2024, 14, 1665 6 of 21 specimens were formed for each numbered concrete, and the strength test and chloride ion diffusion test were carried out, respectively.

Strength Test
The strengths of the mortar were tested at 3 d and 28 d, respectively.The loading rates for the flexural strength and compressive strength tests of mortar are 50 N/s ± 10 N/s and 2400 N/s ± 200 N/s, respectively.The compressive strength and flexural strength were tested according to the Chinese standard GB17671-2021 [38].The mechanical strength was tested at 7 d and 28 d, respectively.The universal testing machine carries out the compressive strength test on the concrete specimen, and loads the concrete specimen at the rate of 0.3~0.5 MPa/s until the specimen is damaged.The compressive strength of concrete was tested according to the Chinese standard GB/T 50081-2019 [39].

Dry Shrinkage Deformation Test
The dry shrinkage test begins to measure the first length of the specimens at 24 h after the mold is removed, and then it is placed in a standard dry shrinkage curing box for curing and its dry shrinkage deformation is measured regularly.The dry shrinkage deformation of 1 d, 3 d, 7 d, 14 d, 28 d, 45 d, 60 d, 90 d, 120 d, and 150 d were tested, respectively.The test method of the dry shrinkage deformation test is shown in GB/T50082-2009 [40].

Chloride Ion Diffusion Test
After curing for 28 d, the other five surfaces of concrete, excluding the chloride ion invasion surface, are sealed with the epoxy resin glue.After the sealing surface was cured, the specimens were immersed in a 0.5 mol/L NaCl solution for 180 d, and the solution was replaced every 30 d.After 180 d, the specimens were taken out from the intrusion surface.Each layer is 5 mm and the amount of the powder taken is not less than 5 g for the chloride ion content test.The chloride ion test of concrete was carried out following the technical specification for tests and the inspection of concrete for water transportation engineering (JTS/T 236-2019) [41].The titration of chloride ion in this study was performed using a ZDJ-4A automatic potentiometric titrator produced by Lei Ci Co., Ltd., Shanghai, China.Each sample was titrated 3 times, and the average value was taken as the final result.

XRD, SEM, and MIP Tests
XRD patterns of the mortar and concrete at 28 d were obtained by the D8 A25 X-ray diffractometer produced by BRUKER AXS, Germany.Cu-Kα was used in the XRD test, the voltage was 40 kV, the current was 30 mA, the scanning angle ranged from 5 • to 70 • , and the scanning rate was 10 • /min.The samples for the XRD test were the mortar and the concrete powder after removing the coarse aggregate, and all samples were screened at 0.16 mm.
In order to conduct the SEM and MIP tests, the mortar blocks were selected after the compressive strength test and immediately dried in an oven of 60 • C. In the SEM test, the flaky fragments of mortar blocks were selected, and the SEM images were observed by Zeiss Sigma 300 produced by Carl Zeiss AG, Oberkochen, Germany.In the MIP test, the dried mortar blocks with a size of 3~5 mm were selected.The surface tension of mercury is 485 erg/cm 2 , the contact angle of mercury is 130 • , and the maximum pressure is 400 MPa.

Effect of Mica Content on Mechanical Strength of Mortar and Concrete
Figure 3 shows the changes in the flexural strength and compressive strength of the mortar with mica content.It can be seen that both the flexural strength and compressive strength of mortar decrease with the increase in the mica content, and the linear relationship is obvious.The linear fitting is performed based on the test results, which is shown in Figure 3, and the minimum linear correlation coefficient is 0.93.Furthermore, when the mica content is 6%, the flexural strength at 3 d and 28 d decreases by 19.6% and 15.6%, respectively, and the compressive strength at 3 d and 28 d decreases by 39.1% and 22.3%, respectively.When the mica content in sand is 8%, the flexural strength at 3 d and 28 d decreases by 26.5% and 23.9%, respectively, and the compressive strength at 3 d and 28 d decreases by 43.9% and 23.2%, respectively.The results above reveal that the compressive strength of mortar cannot meet the strength requirements of the strength grade of P•O 42.5 cement.
and the scanning rate was 10°/min.The samples for the XRD test were the mortar and the concrete powder after removing the coarse aggregate, and all samples were screened at 0.16 mm.
In order to conduct the SEM and MIP tests, the mortar blocks were selected after the compressive strength test and immediately dried in an oven of 60 °C.In the SEM test, the flaky fragments of mortar blocks were selected, and the SEM images were observed by Zeiss Sigma 300 produced by Carl Zeiss AG, Oberkochen, Germany.In the MIP test, the dried mortar blocks with a size of 3~5 mm were selected.The surface tension of mercury is 485 erg/cm 2 , the contact angle of mercury is 130°, and the maximum pressure is 400 MPa.  Figure 4 shows the changes in the compressive strength of the concrete with mica content.It can be seen that the compressive strength of concrete decreases with the increase in mica content, and the strength changes linearly with the increase in mica content.The linear fitting is performed based on the test results, which has the linear correlation coefficient of 0.99, and the results are shown in Figure 4. Compared with the CF30M0 group without mica, the compressive strength of concrete at 7 d and 28 d decreases by 21.9% and 33.5%, respectively, when the mica content in sand is 6%.When the mica content in sand is 8%, the compressive strength of concrete at 7 d and 28 d decreases by 20.2% Figure 4 shows the changes in the compressive strength of the concrete with mica content.It can be seen that the compressive strength of concrete decreases with the increase in mica content, and the strength changes linearly with the increase in mica content.The linear fitting is performed based on the test results, which has the linear correlation coefficient of 0.99, and the results are shown in Figure 4. Compared with the CF30M0 group without mica, the compressive strength of concrete at 7 d and 28 d decreases by 21.9% and 33.5%, respectively, when the mica content in sand is 6%.When the mica content in sand is 8%, the compressive strength of concrete at 7 d and 28 d decreases by 20.2% and 33%, respectively, and the compressive strength at 28 d does not satisfy the minimum compressive strength requirements of the strength grade of C30 concrete.

Effect of Mica Content on Mechanical Strength of Mortar and Concrete
where P is the compressive or flexural strength of the corresponding age, MPa; a and b are the fitting parameters according to the test results; and C M is the mica content in sand, %.
Equation ( 1) is structured according to the general equation obtained from the test results of this study.It is suitable for the strength prediction of cement-based materials with a mix similar to that of this study, but its universality needs to be further verified.According to the results above, if the mica content in sand exceeds 6%, the mechanical strength of the mortar and concrete will decrease sharply, which is not conducive to the control of the engineering quality.The bond strength of the interface zone is affected by the smooth surface of mica [42,43].This is because the hydration products are not easily adsorbed on the smooth surface of mica, thus forming a defective interface zone between the mica and hydration products [23].The more mica content, the greater the influence of the interface zone, and the more the flexural strength and compressive strength of the mortar will decrease [37].It is not recommended to use the sand with a high mica content in engineering if there are no measures taken to reduce the impact of the mica on the strength of the mortar or concrete.

Effect of Interfacial Modifier on Strength of Mortar
In order to realize the direct application of super standard mica sand in engineering, the first task is to solve the influence of the high mica content in sand on the strength of cement-based materials.According to the literature [23,[44][45][46] and engineering practice, the optimal selection of the SCA, AA, and HPMC super standard mica sand interface modifier was carried out, and the optimal dosage was determined.The optimization results of the modifier varieties and modifier contents are shown in Figure 5 and Figure 6, respectively.According to the fitting test results in Figures 3 and 4, it can be seen that the strength of the cement-based materials produced by mixing with the super standard mica sand at the corresponding age have a linear relationship with the mica content, and the general equation can be expressed by Equation (1).
where P is the compressive or flexural strength of the corresponding age, MPa; a and b are the fitting parameters according to the test results; and M C is the mica content in sand, %.Equation ( 1) is structured according to the general equation obtained from the test results of this study.It is suitable for the strength prediction of cement-based materials with a mix similar to that of this study, but its universality needs to be further verified.
According to the results above, if the mica content in sand exceeds 6%, the mechanical strength of the mortar and concrete will decrease sharply, which is not conducive to the control of the engineering quality.The bond strength of the interface zone is affected by the smooth surface of mica [42,43].This is because the hydration products are not easily adsorbed on the smooth surface of mica, thus forming a defective interface zone between the mica and hydration products [23].The more mica content, the greater the influence of the interface zone, and the more the flexural strength and compressive strength of the mortar will decrease [37].It is not recommended to use the sand with a high mica content in engineering if there are no measures taken to reduce the impact of the mica on the strength of the mortar or concrete.

Effect of Interfacial Modifier on Strength of Mortar
In order to realize the direct application of super standard mica sand in engineering, the first task is to solve the influence of the high mica content in sand on the strength of cement-based materials.According to the literature [23,[44][45][46] and engineering practice, the optimal selection of the SCA, AA, and HPMC super standard mica sand interface modifier was carried out, and the optimal dosage was determined.The optimization results of the modifier varieties and modifier contents are shown in Figures 5 and 6, respectively.
13.1% and 1.8%, respectively, and the compressive strength at 3 d and 28 d decreases by 6.3% and 5.9%, respectively.The results above show that the SCA has the best modification effect on the sand with excessive mica.This is because the SiOR of SCA can be directionally adsorbed on the mica or the hydration products [37], forming a bridge structure between the mica and hydration products, and improving the interface zone between the mica and hydration products [29,47].The research mentioned above indicates that the addition of the SCA can mitigate the negative impact of the excessive mica content in sand on the mechanical strength of the cement-based materials.The optimal dosage of the SCA was investigated and the results are illustrated in Figure 6.When the mica content in sand is at 4%, both the flexural 13.1% and 1.8%, respectively, and the compressive strength at 3 d and 28 d decreases by 6.3% and 5.9%, respectively.The results above show that the SCA has the best modification effect on the sand with excessive mica.This is because the SiOR of SCA can be directionally adsorbed on the mica or the hydration products [37], forming a bridge structure between the mica and hydration products, and improving the interface zone between the mica and hydration products [29,47].The research mentioned above indicates that the addition of the SCA can mitigate the negative impact of the excessive mica content in sand on the mechanical strength of the cement-based materials.The optimal dosage of the SCA was investigated and the results are illustrated in Figure 6.When the mica content in sand is at 4%, both the flexural As shown in Figure 5, when the mica content in sand is 4% and the content of the SCA is 30 g, that is, 50% of the mica mass, the flexural strength of mortar at 3 d and 28 d increases by 7.2% and 7.6%, respectively, and the compressive strength of mortar at 3 d and 28 d increases by 20.1% and 10.9%, respectively.After adding 30 g of acrylic interface modifier, the flexural strength at 3 d and 28 d increases by 0.6% and 2.7%, respectively, and the compressive strength at 3 d and 28 d increases by 6% and −0.7%, respectively.After adding 5 g of the HPMC modifier, the flexural strength at 3 d and 28 d decreases by 13.1% and 1.8%, respectively, and the compressive strength at 3 d and 28 d decreases by 6.3% and 5.9%, respectively.The results above show that the SCA has the best modification effect on the sand with excessive mica.This is because the SiOR of SCA can be directionally adsorbed on the mica or the hydration products [37], forming a bridge structure between the mica and hydration products, and improving the interface zone between the mica and hydration products [29,47].
The research mentioned above indicates that the addition of the SCA can mitigate the negative impact of the excessive mica content in sand on the mechanical strength of the cement-based materials.The optimal dosage of the SCA was investigated and the results are illustrated in Figure 6.When the mica content in sand is at 4%, both the flexural strength and compressive strength of mortar exhibit an initial increase, followed by a decrease with the increase in the content of the SCA.This trend aligns with the results reported by Wang [48], indicating a consistency with prior research.When the content of the SCA is 30 g, the mortar exhibits the highest flexural strength and compressive strength.Compared with the MB4SCA0 group without the addition of the SCA, the flexural strength at 3 d and 28 d increases by 7.2% and 7.6%, respectively, while the compressive strength at 3 d and 28 d increases by 19.3% and 10.9%, respectively.This indicates that there is an optimal dosage of the SCA for the modification of the mica-containing manufactured sand.In this study, the optimal dosage of the SCA is determined to be 50% by the mass of the mica in sand.When the dosage of the SCA increases, it forms a chemical bond in the interface zone, thereby enhancing the bonding strength of the interface zone, which helps reduce the deterioration of the strength of the cement-based materials caused by the super standard mica in sand.However, when the dosage of the SCA exceeds a certain amount, the condensation reaction between the SCA and the mica or hydration products releases water [49].With an increase in the SCA dosage, the number of the chemical bonds and water generated increases.The water accumulated in the interface zone leads to an increase in the water to cement ratio, thereby reducing the bonding strength of the hydration products.When the increase in the interface bonding strength resulting from the addition of the SCA is less than the decrease in bonding strength resulting from the increased water content, the SCA has a negative impact.Therefore, there exists an optimal dosage when using SCA as an interface modifier for the super standard mica sand.

Effect of Curing Method on Strength of Mortar
The interface modifier is an organic substance and its effectiveness in modifying the interface of the sand with excessive mica is not only related to the dosage of the interface agent, but also related to the curing method [50].In order to maximize the effectiveness of the interface modifier, comparative experimental studies were conducted between the standard curing and natural curing.The effect of the two curing methods on the strength of the mortar is shown in Figure 7.
Buildings 2024, 14, x FOR PEER REVIEW 10 of 21 strength and compressive strength of mortar exhibit an initial increase, followed by a decrease with the increase in the content of the SCA.This trend aligns with the results reported by Wang [48], indicating a consistency with prior research.When the content of the SCA is 30 g, the mortar exhibits the highest flexural strength and compressive strength.
Compared with the MB4SCA0 group without the addition of the SCA, the flexural strength at 3 d and 28 d increases by 7.2% and 7.6%, respectively, while the compressive strength at 3 d and 28 d increases by 19.3% and 10.9%, respectively.This indicates that there is an optimal dosage of the SCA for the modification of the mica -containing manufactured sand.In this study, the optimal dosage of the SCA is determined to be 50% by the mass of the mica in sand.When the dosage of the SCA increases, it forms a chemical bond in the interface zone, thereby enhancing the bonding strength of the interface zone, which helps reduce the deterioration of the strength of the cement-based materials caused by the super standard mica in sand.However, when the dosage of the SCA exceeds a certain amount, the condensation reaction between the SCA and the mica or hydration products releases water [49].With an increase in the SCA dosage, the number of the chemical bonds and water generated increases.The water accumulated in the interface zone leads to an increase in the water to cement ratio, thereby reducing the bonding strength of the hydration products.When the increase in the interface bonding strength resulting from the addition of the SCA is less than the decrease in bonding strength resulting from the increased water content, the SCA has a negative impact.Therefore, there exists an optimal dosage when using SCA as an interface modifier for the super standard mica sand.

Effect of Curing Method on Strength of Mortar
The interface modifier is an organic substance and its effectiveness in modifying the interface of the sand with excessive mica is not only related to the dosage of the interface agent, but also related to the curing method [50].In order to maximize the effectiveness of the interface modifier, comparative experimental studies were conducted between the standard curing and natural curing.The effect of the two curing methods on the strength of the mortar is shown in Figure 7.It can be seen from Figure 7 that the strength of both 3 d and 28 d is the highest in MB4SCA30-K group under natural curing conditions.The compressive strength of MB4SCA-K group at 3 d and 28 d are 28.1 MPa and 38.8 MPa, and the flexural strength at 3 d and 28 d are 4.6 MPa and 6.7 MPa, respectively.Based on the strength results of MB4SCA-K group at the corresponding ages, the strength of other groups decreases compared with that of the base group, and the proportion of decline is shown in Figure 7.
Furthermore, compared with the base group, the MB4-K group shows a decrease of 3.7% and 14.9% in flexural strength at 3 d and 28 d, respectively, and a decrease of 8.5% and 27.8% in compressive strength, respectively.Similarly, the MB4SCA30-B group shows a decrease of 4.3% and 4.5% in flexural strength at 3 d and 28 d, respectively, and a decrease of 6.4% and 9.5% in compressive strength, respectively.This indicates that the strength of the mortar containing the SCA increases at all ages regardless of whether it undergoes standard curing or natural curing.Moreover, the improvement in strength is more pronounced during natural curing, indicating that natural curing enhances the effectiveness of the SCA in the modified mortar.This can be attributed to the organic nature of the components in the interface agent and the release of water during the condensation reaction [51].The released water can better evaporate in the air, leading to a better film formation [52].In addition, the water released from the condensation reaction is beneficial for further cement hydration, thereby improving the bonding strength in the interface zone [53].Compared with the results of the two curing methods in MB4 group, it can be seen that the flexural strength and compressive strength increase by 6.5% and 17.1%, respectively, during the natural curing at 3 d and 28 d decrease by 4.4% and 11.6%, respectively.The results of MB4-K and MB4-B show that the standard curing is beneficial to the improvement of the later strength.This is mainly because the standard curing provides enough water for cement hydration, which is conducive to the development of cement hydration.

Effect of Mineral Admixtures on Compressive Strength of Concrete with Excessive Mica
Suitable mineral admixtures have the potential to enhance the performance of concrete.In this study, fly ash and ceramic powder were chosen as mineral admixtures to substitute 20% of the cement in the C30M4 group.The effect of these mineral admixtures on the compressive strength of the concrete prepared using the sand containing excessive mica was investigated, and is shown in Figure 8.

Dry Shrinkage Deformation of Concrete
Figure 9 presents the effects of mica, mineral admixture, and interfacial agent on the dry shrinkage deformation of concrete.It can be observed that the dry shrinkage deformation of concrete in each group increases with time.The deformation growth is faster during the initial 45 d, followed by a gradual stabilization of the deformation.Furthermore, the dry shrinkage deformation of concrete increases with the increase in mica content.The drying shrinkage strain of the CF30M0, CF30M2, CF30M4, and CF30M6 groups at 150 d is 415.45 × 10 −6 , 443.32 × 10 −6 , 486.39 × 10 −6 , and 519.36 × 10 −6 , respectively.The dry shrinkage strain of the CF30M2, CF30M4, and CF30M6 groups is 6.7%, 17.1%, and 25% larger than that of the CF30M0 group, respectively.It can be concluded that the higher the mica content, the greater the dry shrinkage deformation of concrete, which is consistent with the results of Li [23].This is because a higher mica content leads to more micro-defects and pores inside the concrete, leading to more water losses during the curing stages The compressive strength of the C30M4 group at 7 d increases by 13.4%, 10.1%, and 8.8% compared with that of the group modified with the SCA, fly ash, and ceramic powder.The compressive strength of the CF30M4SCA30 group at 28 d increases by 7.3%, 2.6%, and 12.8% compared with the group without fly ash and ceramic powder.The results above indicate that the addition of the mineral admixtures and SCA at an early age significantly reduces the compressive strength of concrete.This is mainly because the hydration of mineral admixtures is slow, resulting in fewer hydration products at an early stage.Similarly, the enhancement of the SCA requires sufficient time to form chemical bonds.As cement hy-dration proceeds, the hydration activity of mineral admixtures gradually becomes effective, and the hydration products of cement are continuously compacted.Meanwhile, the SCA gradually forms a bridging structure in the interface zone, strengthening the bond in the interface zone.Furthermore, the water generated by the condensation reaction of the SCA favors the further hydration of admixtures.Therefore, the compressive strength of concrete in the CF30M4SCA30 group, which contains both the SCA and fly ash, is the highest at 28 d.To better improve the deteriorating characteristic of mechanical strength caused by excessive mica in sand, the combined use of appropriate amounts of fly ash admixture and SCA in sand with excessive mica is more effective.In addition, it is evident that the compressive strength of the concrete containing fly ash under the same curing condition is significantly higher than that of the concrete containing ceramic powder.This can be attributed to the activity of the fly ash.As can be seen from the composition results in Table 2, the Al 2 O 3 content in fly ash is significantly higher than that of ceramic powder, and Al 2 O 3 is the main active component of mineral admixtures [54], so it can be determined that the activity of fly ash is higher than that of ceramic powder.

Dry Shrinkage Deformation of Concrete
Figure 9 presents the effects of mica, mineral admixture, and interfacial agent on the dry shrinkage deformation of concrete.It can be observed that the dry shrinkage deformation of concrete in each group increases with time.The deformation growth is faster during the initial 45 d, followed by a gradual stabilization of the deformation.Furthermore, the dry shrinkage deformation of concrete increases with the increase in mica content.The drying shrinkage strain of the CF30M0, CF30M2, CF30M4, and CF30M6 groups at 150 d is 415.45 × 10 −6 , 443.32 × 10 −6 , 486.39 × 10 −6 , and 519.36 × 10 −6 , respectively.The dry shrinkage strain of the CF30M2, CF30M4, and CF30M6 groups is 6.7%, 17.1%, and 25% larger than that of the CF30M0 group, respectively.It can be concluded that the higher the mica content, the greater the dry shrinkage deformation of concrete, which is consistent with the results of Li [23].This is because a higher mica content leads to more micro-defects and pores inside the concrete, leading to more water losses during the curing stages and consequently a larger shrinkage [55].Therefore, the adverse effect of excessive dry shrinkage deformation caused by the high mica content in sand needs to be considered during engineering construction.

Dry Shrinkage Deformation of Concrete
Figure 9 presents the effects of mica, mineral admixture, and interfacial agent on the dry shrinkage deformation of concrete.It can be observed that the dry shrinkage deformation of concrete in each group increases with time.The deformation growth is faster during the initial 45 d, followed by a gradual stabilization of the deformation.Furthermore, the dry shrinkage deformation of concrete increases with the increase in mica content.The drying shrinkage strain of the CF30M0, CF30M2, CF30M4, and CF30M6 groups at 150 d is 415.45 × 10 −6 , 443.32 × 10 −6 , 486.39 × 10 −6 , and 519.36 × 10 −6 , respectively.The dry shrinkage strain of the CF30M2, CF30M4, and CF30M6 groups is 6.7%, 17.1%, and 25% larger than that of the CF30M0 group, respectively.It can be concluded that the higher the mica content, the greater the dry shrinkage deformation of concrete, which is consistent with the results of Li [23].This is because a higher mica content leads to more micro-defects and pores inside the concrete, leading to more water losses during the curing stages and consequently a larger shrinkage [55].Therefore, the adverse effect of excessive dry shrinkage deformation caused by the high mica content in sand needs to be considered during engineering construction.It can also be observed from Figure 9 that the dry shrinkage deformation of the C30M0 group is greater than those in the CF30M0 group at each age, indicating that the addition of fly ash is beneficial for controlling the dry shrinkage [56].Comparing the dry shrinkage of the concrete containing fly ash (CF30M4) with that of the concrete containing ceramic powder (CC30M4), it can be found that before 7 d, the dry shrinkage deformation of the CC30M4 group is larger, while after that, the drying shrinkage of the CF30M4 group is larger.Meanwhile, at 150 d, the relative deviation of the dry shrinkage strain between the two groups is only 2.9%.This demonstrates that the influence of the same contents of fly ash and ceramic powder on the drying shrinkage strain is similar.
The concrete containing the SCA in the CF30M4SCA30 group exhibits a dry shrinkage deformation close to that of the CF30M2 group, which is 4.8% larger than that of the CF30M0 group.The dry shrinkage strain of the CF30SCA30 group is 10.5% less than that of the CF30M4 group.This reveals that the incorporation of the SCA has a significant improvement effect on the dry shrinkage strain.This improvement is mainly attributed to the enhancement of the microstructure in the interfacial zone of concrete and the reduction of the porosity of concrete due to the addition of the SCA [47].

Chloride Ion Diffusion
Figure 10 shows the chloride ion diffusion of concrete after natural diffusion in a NaCl solution of 0.5 mol/L for 180 d.The chloride ion content increases with the increase in mica content.This is because the higher the mica content in sand, the more defects in the concrete interface zone and the more chloride ion transport channels [57].
Buildings 2024, 14, x FOR PEER REVIEW 13 of 21 It can also be observed from Figure 9 that the dry shrinkage deformation of the C30M0 group is greater than those in the CF30M0 group at each age, indicating that the addition of fly ash is beneficial for controlling the dry shrinkage [56].Comparing the dry shrinkage of the concrete containing fly ash (CF30M4) with that of the concrete containing ceramic powder (CC30M4), it can be found that before 7 d, the dry shrinkage deformation of the CC30M4 group is larger, while after that, the drying shrinkage of the CF30M4 group is larger.Meanwhile, at 150 d, the relative deviation of the dry shrinkage strain between the two groups is only 2.9%.This demonstrates that the influence of the same contents of fly ash and ceramic powder on the drying shrinkage strain is similar.
The concrete containing the SCA in the CF30M4SCA30 group exhibits a dry shrinkage deformation close to that of the CF30M2 group, which is 4.8% larger than that of the CF30M0 group.The dry shrinkage strain of the CF30SCA30 group is 10.5% less than that of the CF30M4 group.This reveals that the incorporation of the SCA has a significant improvement effect on the dry shrinkage strain.This improvement is mainly attributed to the enhancement of the microstructure in the interfacial zone of concrete and the reduction of the porosity of concrete due to the addition of the SCA [47].

Chloride Ion Diffusion
Figure 10 shows the chloride ion diffusion of concrete after natural diffusion in a NaCl solution of 0.5 mol/L for 180 d.The chloride ion content increases with the increase in mica content.This is because the higher the mica content in sand, the more defects in the concrete interface zone and the more chloride ion transport channels [57].It can be seen from the chloride ion diffusion results of CF30M4 and CC30M4 groups in Figure 10 that the concrete containing the ceramic powder has a higher chloride ion content than the concrete containing fly ash.This is mainly attributed to the lower reactivity of the ceramic powder [58].As can be seen from Figure 2, the particle size of ceramic powder is small and concentrated, and its ability to fill the pores of concrete tightly is not as good as that of fly ash.At the same time, from the compositions of the mineral mixtures in Table 2, it can be seen that the active components of ceramic powder are also lower than those of fly ash, and the secondary reaction has a less dense filling effect on concrete than fly ash.The hydration products of fly ash produced by the secondary hydration reaction not only refine the pores, but also adsorb a certain amount of chloride ions [59].Therefore, for this study, fly ash has a better effect on the performance improvement of the mica super standard sand cement-based materials.It can be seen from the chloride ion diffusion results of CF30M4 and CC30M4 groups in Figure 10 that the concrete containing the ceramic powder has a higher chloride ion content than the concrete containing fly ash.This is mainly attributed to the lower reactivity of the ceramic powder [58].As can be seen from Figure 2, the particle size of ceramic powder is small and concentrated, and its ability to fill the pores of concrete tightly is not as good as that of fly ash.At the same time, from the compositions of the mineral mixtures in Table 2, it can be seen that the active components of ceramic powder are also lower than those of fly ash, and the secondary reaction has a less dense filling effect on concrete than fly ash.The hydration products of fly ash produced by the secondary hydration reaction not only refine the pores, but also adsorb a certain amount of chloride ions [59].Therefore, for this study, fly ash has a better effect on the performance improvement of the mica super standard sand cement-based materials.
Comparing the chloride ion content of the concrete in the groups of the CF30M4 and CF30M4SCA30 in Figure 10, it can be seen that the addition of the SCA can reduce the transportation of chloride ions in concrete.The main reason is that the addition of SCA improves the microstructure of the concrete interface zone [60].Because SCA can establish a bridging structure between the mica and cement hydration products, the strength of the interface zone is increased and the micro-defects in the interface zone are reduced [23].In addition, the water molecules released by SCA during the formation of hydroxyl group promote the hydration of cement and fly ash, further enhance the compactness of the interface zone, reduce the number of connected open pores, and thus reduce the transport channels of chloride ions [61].

XRD
Figure 11 presents the XRD patterns of the mortar and concrete containing the mica sand with and without interface modifier treatment.As can be seen from the XRD results, the XRD patterns of the BM0 group without the mica sand have no diffraction peak at about 9 • .For the other seven groups containing the sand with 4% mica, an obvious diffraction peak can be observed at about 9 • , which corresponds to the previous research proving the diffraction peak near 9 • of the Muscovite [62]. Figure 11 shows that the mica exists stably in cement-based materials.Comparing the XRD patterns of BM4, BM4HPMC, BM4SCA30-B, BM4SCA30-K, and BM4SCA50-K in Figure 11, there is no significant difference in the position and intensity of the diffraction peaks.The cement-based materials with the interfacial modifiers of HPMC and SCA do not form the new crystal phases.The XRD patterns of BM4SCA30-B and BM4SCA30-K show no difference, indicating that the change in curing conditions cannot promote the reaction of the SCA with hydration products to form a new crystal phase.By comparing the XRD patterns of the CF30M4 and CC30M4 groups, it can be seen that the Ca(OH) 2 diffraction peak near 50 • of the CC30M4 group is significantly higher than that of the CF30M4 group, and the Ca(OH) 2 diffraction peak near 50 • in the CC30M4 group is similar to that of the BM4 group.The results show that the activity of ceramic powder is lower than that of fly ash, because the Ca(OH) 2 diffraction peak in concrete doped with fly ash is low, which is due to the fact that the active ingredient in fly ash reacts with Ca(OH) 2 and is partially consumed [63], resulting in a significant weakening of the Ca(OH) 2 diffraction peak [64].This result also confirms the correctness of the influence of admixtures on the strength, as detailed in Section 3.4.As shown in Figure 11, the XRD patterns of the CC30M4 group has a strong diffraction peak at about 29 • .After acquiring the relevant data [65], it can be seen that the diffraction peak may be generated by CaCO 3 , Ca(OH) 2 or other crystalline minerals with unknown compositions.
Comparing the chloride ion content of the concrete in the groups of the CF30M4 and CF30M4SCA30 in Figure 10, it can be seen that the addition of the SCA can reduce the transportation of chloride ions in concrete.The main reason is that the addition of SCA improves the microstructure of the concrete interface zone [60].Because SCA can establish a bridging structure between the mica and cement hydration products, the strength of the interface zone is increased and the micro-defects in the interface zone are reduced [23].In addition, the water molecules released by SCA during the formation of hydroxyl group promote the hydration of cement and fly ash, further enhance the compactness of the interface zone, reduce the number of connected open pores, and thus reduce the tra nsport channels of chloride ions [61].

XRD
Figure 11 presents the XRD patterns of the mortar and concrete containing the mica sand with and without interface modifier treatment.As can be seen from the XRD results , the XRD patterns of the BM0 group without the mica sand ha ve no diffraction peak at about 9°.For the other seven groups containing the sand with 4% mica, an obvious diffraction peak can be observed at about 9°, which corresponds to the previous research proving the diffraction peak near 9° of the Muscovite [62]. Figure 11 shows that the mica exists stably in cement-based materials.Comparing the XRD patterns of BM4, BM4HPMC, BM4SCA30-B, BM4SCA30-K, and BM4SCA50-K in Figure 11, there is no significant difference in the position and intensity of the diffraction peaks.The cement-based materials with the interfacial modifiers of HPMC and SCA do not form the new crystal phases.The XRD patterns of BM4SCA30-B and BM4SCA30-K show no difference, indicating that the change in curing conditions cannot promote the reaction of the SCA with hydration products to form a new crystal phase.By comparing the XRD patterns of the CF30M4 and CC30M4 groups, it can be seen that the Ca(OH)2 diffraction peak near 50° of the CC30M4 group is significantly higher than that of the CF30M4 group, and the Ca(OH)2 diffraction peak near 50° in the CC30M4 group is similar to that of the BM4 group.The results show that the activity of ceramic powder is lower than that of fly ash, because the Ca(OH)2 diffraction peak in concrete doped with fly ash is low, which is due to the fact tha t the active ingredient in fly ash reacts with Ca(OH)2 and is partially consumed [63], resulting in a significant weakening of the Ca(OH)2 diffraction peak [64].This result also confirms the correctness of the influence of admixtures on the strength, as detailed in Section 3.4.As shown in Figure 11, the XRD patterns of the CC30M4 group has a strong diffraction peak at about 29°.After acquiring the relevant data [65], it can be seen that the diffraction peak may be generated by CaCO3, Ca(OH)2 or other crystalline minerals with unknown compositions.

MIP
Figure 12 shows the pore diameter distribution and cumulative pore volume of the mortar containing the sand with 4% mica and the interfacial modifier.The total pores can be classified into four categories [66]: gel pores (less than 10 nm), medium capillary pores (10-50 nm), large capillary pores (ranging from 50 to 10,000 nm), and air voids (larger than 10,000 nm).
Figure 13 exhibits the pore volume fraction distribution of the mortar containin sand with 4% mica and the interfacial modifier.In general, the pores with a diam larger than 50 nm are considered to be harmful for the durability of cement-based m rials [67,68].Therefore, large capillary pores (50-10,000 nm) and air voids (>10,000 nm harmful pores in cement-based materials.The higher the total amount of large cap pores and air voids, the more defects in cement-based materials, which will damag durability of cement-based materials.As can be seen from Figure 13, the pores gr than 50 nm in the MB4 group containing the sand with 4% mica account for 84.2% o total volume, which is 10.4% higher than that of the MB0 group.The MB4SCA30 gro a mortar modified by the SCA, and its pores above 50 nm account for 70.4% of the volume, which is 13.8% less than that of the MB4 group and 3.4% less than that of the group.The test results show that the SCA has a good effect on the interface modific of the sand with high mica content.Figure 13 exhibits the pore volume fraction distribution of the mortar containing the sand with 4% mica and the interfacial modifier.In general, the pores with a diameter larger than 50 nm are considered to be harmful for the durability of cement-based materials [67,68].Therefore, large capillary pores (50-10,000 nm) and air voids (>10,000 nm) are harmful pores in cement-based materials.The higher the total amount of large capillary pores and air voids, the more defects in cement-based materials, which will damage the durability of cement-based materials.As can be seen from Figure 13, the pores greater than 50 nm in the MB4 group containing the sand with 4% mica account for 84.2% of the total volume, which is 10.4% higher than that of the MB0 group.The MB4SCA30 group is a mortar modified by the SCA, and its pores above 50 nm account for 70.4% of the total volume, which is 13.8% less than that of the MB4 group and 3.4% less than that of the MB0 group.The test results show that the SCA has a good effect on the interface modification of the sand with high mica content.

MIP
Figure 12 shows the pore diameter distribution and cumulative pore volume of the mortar containing the sand with 4% mica and the interfacial modifier.The total pores can be classified into four categories [66]: gel pores (less than 10 nm), medium capillary pores (10-50 nm), large capillary pores (ranging from 50 to 10,000 nm), and air voids (larger than 10,000 nm).
Figure 13 exhibits the pore volume fraction distribution of the mortar containing the sand with 4% mica and the interfacial modifier.In general, the pores with a diameter larger than 50 nm are considered to be harmful for the durability of cement-based materials [67,68].Therefore, large capillary pores (50-10,000 nm) and air voids (>10,000 nm) are harmful pores in cement-based materials.The higher the total amount of large capillary pores and air voids, the more defects in cement-based materials, which will damage the durability of cement-based materials.As can be seen from Figure 13, the pores greater than 50 nm in the MB4 group containing the sand with 4% mica account for 84.2% of the total volume, which is 10.4% higher than that of the MB0 group.The MB4SCA30 group is a mortar modified by the SCA, and its pores above 50 nm account for 70.4% of the total volume, which is 13.8% less than that of the MB4 group and 3.4% less than that of the MB0 group.The test results show that the SCA has a good effect on the interface modification of the sand with high mica content.3.9.SEM Figure 14 shows the microstructure morphology of the mortar containing the sand with 4% mica and the interfacial modifier.Figure 14a presents the microstructure of the MB0 group.It can be observed that the cement hydration products of the MB0 group are evenly distributed.There are a few weak areas, microcracks, and small pores in the interface zone between the hydration products and fine aggregate.As shown in Figure 14b, the MB4 group shows that the obvious microcracks exist around the mica particles and the hydration products cannot be observed in the exposed mica.Moreover, in Figure 14b, the layered structure of mica particles can be clearly seen.From Figure 14c, the microstructure of the MB4SCA30 group shows that a large number of flocculent C-S-H gels are formed around the mica, and the number of holes and microcracks are significantly reduced.It can be concluded that the SCA can improve the interface zone between the mica and hydration products.Figure 14d is the microstructure of the MB4HPMC group.It can be found that a large number of hydration products are distributed around the mica, and the interface zone between the hydration products and mica has a certain improvement compared with the interface zone in Figure 14b.However, more microcracks and holes formed around the interface binding zone.In addition, no filamentous junctions can be found between the hydrated products and mica.
Buildings 2024, 14, x FOR PEER REVIEW 16 . Pore compositions of the mortar containing the sand with 4% mica and interfacial modifier.

SEM
Figure 14 shows the microstructure morphology of the mortar containing the sand with 4% mica and the interfacial modifier.Figure 14a presents the microstructure of the MB0 group.It can be observed that the cement hydration products of the MB0 group are evenly distributed.There are a few weak areas, microcracks, and small pores in the interface zone between the hydration products and fine aggregate.As shown in Figure 14b, the MB4 group shows that the obvious microcracks exist around the mica particles and the hydration products cannot be observed in the exposed mica.Moreover, in Figure 14b, the layered structure of mica particles can be clearly seen.From Figure 14c, the microstructure of the MB4SCA30 group shows that a large number of flocculent C-S-H gels are formed around the mica, and the number of holes and microcracks are significantly reduced.It can be concluded that the SCA can improve the interface zone between the mica and hydration products.Figure 14d is the microstructure of the MB4HPMC group.It can be found that a large number of hydration products are distributed around the mica, and the interface zone between the hydration products and mica has a certain improvement compared with the interface zone in Figure 14b.However, more microcracks and holes formed around the interface binding zone.In addition, no filamentous junctions can be found between the hydrated products and mica.

Interface Modification Mechanism
The interface modification mechanism of the SCA on the concrete with excessive mica is illustrated in Figure 15.The SCA first undergoes the hydrolysis reaction by mixing water and dehydration condensation to form the silane oligomers [69].Then, the hydroxyl group in the silane oligomers forms hydrogen bonds with the hydroxyl groups on the surface of the mica or C-S-H gel, thereby forming a bridging structure in the mica or hydration products [47,70].Similarly, the other end of the SCA can also form hydrogen bonds with the hydroxyl groups on the surface of the mica or C-S-H gel, thereby forming a mica particle-SCA-C-S-H gel system.With further condensation reactions, some hydroxyl groups will release water, and the bonding strength between the SCA and mica or C-S-H gel transforms from the van der Waals force to the chemical force [71], thereby enhancing the bonding strength of the interface zone.As part of the water is released during the condensation reaction process, this provides hydration conditions for the secondary reaction of the unhydrated cement particles or mineral admixtures [72], which further improves the bonding strength of the interface zone.However, if the released water is not promptly discharged or cannot participate in chemical reactions, it will accumulate around the interface zone, increasing the water to cement ratio of the interface zone and adversely affecting the interface structure [73].Therefore, when using the SCA to modify the interface of the high mica content sand, one should not only pay attention to the amount of SCA, but also adopt a reasonable curing method, so as to give full play to the effect of SCA on the interface modification of mica.

Interface Modification Mechanism
The interface modification mechanism of the SCA on the concrete with excessive mica is illustrated in Figure 15.The SCA first undergoes the hydrolysis reaction by mixing water and dehydration condensation to form the silane oligomers [69].Then, the hydroxyl group in the silane oligomers forms hydrogen bonds with the hydroxyl groups on the surface of the mica or C-S-H gel, thereby forming a bridging structure in the mica or hydration products [47,70].Similarly, the other end of the SCA can also form hydrogen bonds with the hydroxyl groups on the surface of the mica or C-S-H gel, thereby formin g a mica particle-SCA-C-S-H gel system.With further condensation reactions, some hydroxyl groups will release water, and the bonding strength between the SCA and mica or C-S-H gel transforms from the van der Waals force to the chemical force [71], thereby enhancing the bonding strength of the interface zone.As part of the water is released during the condensation reaction process, this provides hydration conditions for the secondary reaction of the unhydrated cement particles or mineral admixtures [72], which further improves the bonding strength of the interface zone.However, if the released water is not promptly discharged or cannot participate in chemical reactions, it will accumulate around the interface zone, increasing the water to cement ratio of the interface zone and adversely affecting the interface structure [73].Therefore, when using the SCA to modify the interface of the high mica content sand, one should not only pay attention to the amount of SCA, but also adopt a reasonable curing method, so as to give full play to the effect of SCA on the interface modification of mica.

Conclusions
This study investigated the mechanical strength, dry shrinkage, and chloride ion diffusion of the mortar and concrete prepared using the sand with excessive mica, and added the interface modifier and mineral admixtures.The following conclusions are obtained: (1) The mechanical strength of mortar and concrete decreases with an increase in the mica content in sand, and the strength of cement-based materials linearly decreases with the mica content in sand.When the mica content in sand exceeds 6%, the compressive strength of mortar and concrete decreases by 22.3% and 33.5%, respectively, and the drying shrinkage strain of concrete increases by 25%.(2) The addition of SCA is an effective way to solve the deterioration of the mortar and concrete containing the mica.Adding the SCA with 50% mica weight, the compressive strength of mortar at 28 d can be increased by 10.9% under natural curing temperature.

Conclusions
This study investigated the mechanical strength, dry shrinkage, and chloride ion diffusion of the mortar and concrete prepared using the sand with excessive mica, and added the interface modifier and mineral admixtures.The following conclusions are obtained: (1) The mechanical strength of mortar and concrete decreases with an increase in the mica content in sand, and the strength of cement-based materials linearly decreases with the mica content in sand.When the mica content in sand exceeds 6%, the compressive strength of mortar and concrete decreases by 22.3% and 33.5%, respectively, and the drying shrinkage strain of concrete increases by 25%.(2) The addition of SCA is an effective way to solve the deterioration of the mortar and concrete containing the mica.Adding the SCA with 50% mica weight, the compressive strength of mortar at 28 d can be increased by 10.9% under natural curing temperature.(3) Natural curing is beneficial to the interfacial modification of the mica by the SCA.The compressive strength of the mortar mixed with the SCA under natural curing at 28 d is 10.7% higher than that under standard curing.
(4) The coupling of the mineral admixture and SCA can improve the deterioration of mica on the strength of concrete.The compressive strength at 28 d increases by 4.7% when only 20% fly ash is added to the concrete with 4% mica.The compressive strength of concrete at 28 d increases by 7.3% after adding 20% fly ash and 50% SCA of the mica weight.(5) The higher the content of the mica in sand, the greater the dry shrinkage strain of concrete and the faster the diffusion of chloride ions.The dry shrinkage strain of the mica on concrete and the diffusion of chloride ions can be reduced by adding 50% SCA.(6) The hydration products are not changed when the interfacial agent is added in the super standard mica sand cement-based materials.The SCA forms a bridge structure between the mica particles and the hydration products, reduces the number of harmful pores in cement-based materials, improves the microstructure of the binding zone around the mica particles, and increases the bonding force in the interface zone around the mica.

Figure 1 .
Figure 1.Raw materials and test procedures.The cement used was P•O 42.5 ordinary Portland cement, whose main technical indicators are shown in Table1.The fly ash is F type and class II, which has a density of 2.2 g/cm 3 , a specific surface area of 430 m 2 /kg, a water requirement ratio of 101%, and a 28 d activity index of 89%.The ceramic powder is taken from a ceramic polishing processing enterprise, with a gray color and a density of 2.36 g/cm3 .The particle size distribution is shown in Figure2, and it can be seen that the particle size of the ceramic powder is between 0-50 μm.The chemical compositions of cement, fly ash, and ceramic powder are shown in Table2.The fine aggregate was manufactured sand with a fineness modulus of 2.8, an apparent density of 2.65 g/cm3 , and a bulk density of 1.57 g/cm3 .The standard sand used was the Chinese ISO standard sand produced by Xiamen ISO Standard Sand Co., Ltd., Xiamen, China.The coarse aggregate stone selected was graded gravel with a particle size of 5~20 mm, an apparent density of 2.71 g/cm 3 , a bulk density of 1.52 g/cm 3 , a void ratio of 43.9%, a crushing value of 8.5%, and a mud content of 0. The mica used was the Muscovite mica with an apparent density of 2.6 g/cm3 .The particle size distribution of the mica is shown in Table3.The superplasticizer used was a PCA polycarboxylic acid superplasticizer.The solid content of the superplasticizer is 17.5% and the water reduction rate is 21.5%.

Figure 1 .
Figure 1.Raw materials and test procedures.The cement used was P•O 42.5 ordinary Portland cement, whose main technical indicators are shown in Table1.The fly ash is F type and class II, which has a density of 2.2 g/cm 3 , a specific surface area of 430 m 2 /kg, a water requirement ratio of 101%, and a 28 d activity index of 89%.The ceramic powder is taken from a ceramic polishing processing enterprise, with a gray color and a density of 2.36 g/cm3 .The particle size distribution is shown in Figure2, and it can be seen that the particle size of the ceramic powder is between 0-50 µm.The chemical compositions of cement, fly ash, and ceramic powder are shown in Table2.The fine aggregate was manufactured sand with a fineness modulus of 2.8, an apparent density of 2.65 g/cm3 , and a bulk density of 1.57 g/cm3 .The standard sand used was the Chinese ISO standard sand produced by Xiamen ISO Standard Sand Co., Ltd., Xiamen, China.The coarse aggregate stone selected was graded gravel with a particle size of 5~20 mm, an apparent density of 2.71 g/cm 3 , a bulk density of 1.52 g/cm3 , a void ratio of 43.9%, a crushing value of 8.5%, and a mud content of 0. The mica used was the Muscovite mica with an apparent density of 2.6 g/cm3 .The particle size distribution of the mica is shown in Table3.The superplasticizer used was a PCA polycarboxylic acid superplasticizer.The solid content of the superplasticizer is 17.5% and the water reduction rate is 21.5%.

Figure 2 .
Figure 2. Particle size distribution of cement, fly ash, and ceramic powder.

Figure 2 .
Figure 2. Particle size distribution of cement, fly ash, and ceramic powder.

Figure 3
Figure3shows the changes in the flexural strength and compressive strength of the mortar with mica content.It can be seen that both the flexural strength and compressive strength of mortar decrease with the increase in the mica content, and the linear relationship is obvious.The linear fitting is performed based on the test results, which is shown in Figure3, and the minimum linear correlation coefficient is 0.93.Furthermore, when the mica content is 6%, the flexural strength at 3 d and 28 d decreases by 19.6% and 15.6%, respectively, and the compressive strength at 3 d and 28 d decreases by 39.1% and 22.3%, respectively.When the mica content in sand is 8%, the flexural strength at 3 d and 28 d decreases by 26.5% and 23.9%, respectively, and the compressive strength at 3 d and 28 d decreases by 43.9% and 23.2%, respectively.The results above reveal that the compressive strength of mortar cannot meet the strength requirements of the strength grade of P•O 42.5 cement.

Figure 3 .
Figure 3.The changes in mechanical strength of the mortar with mica content: (a) Flexural strength; (b) Compressive strength.

Figure 3 .
Figure 3.The changes in mechanical strength of the mortar with mica content: (a) Flexural strength; (b) Compressive strength.

Figure 4 .
Figure 4. Changes in compressive strength of the concrete with mica content.

Figure 4 .
Figure 4. Changes in compressive strength of the concrete with mica content.

Figure 6 .
Figure 6.Effect of the SCA on strength of mortar: (a) Flexural strength; (b) Compressive strength.

Figure 6 .
Figure 6.Effect of the SCA on strength of mortar: (a) Flexural strength; (b) Compressive strength.

Figure 6 .
Figure 6.Effect of the SCA on strength of mortar: (a) Flexural strength; (b) Compressive strength.

Figure 7 .
Figure 7. Effect of curing methods on mechanical strength of mortar: (a) Flexural strength; (b) Compressive strength.It can be seen from Figure 7 that the strength of both 3 d and 28 d is the highest in MB4SCA30-K group under natural curing conditions.The compressive strength of MB4SCA-K group at 3 d and 28 d are 28.1 MPa and 38.8 MPa, and the flexural strength at 3 d and 28 d are 4.6 MPa and 6.7 MPa, respectively.Based on the strength results of MB4SCA-K group at the corresponding ages, the strength of other groups decreases

Figure 7 .
Figure 7. Effect of curing methods on mechanical strength of mortar: (a) Flexural strength; (b) Compressive strength.

Figure 8 .
Figure 8.Effect of mineral admixtures on compressive strength of the concrete with excessive mica content.

Figure 8 .
Figure 8.Effect of mineral admixtures on compressive strength of the concrete with excessive mica content.

Buildings 2024 , 21 Figure 8 .
Figure 8.Effect of mineral admixtures on compressive strength of the concrete with excessive mica content.

Figure 9 .
Figure 9. Effects of mica, mineral admixtures, and interfacial modifiers on the dry shrinkage of concrete.

Figure 9 .
Figure 9. Effects of mica, mineral admixtures, and interfacial modifiers on the dry shrinkage of concrete.

Figure 10 .
Figure 10.Chloride ion diffusion of concrete after natural diffusion.

Figure 10 .
Figure 10.Chloride ion diffusion of concrete after natural diffusion.

Figure 11 .
Figure 11.XRD patterns of the mortar and concrete containing the mica sand with and without interface modifier treatment.

Figure 12 .
Figure 12.MIP results of the mortar containing the sand with 4% mica and interfacial modifi Pore diameter distribution; (b) Cumulative pore volume.

Figure 12 .
Figure 12.MIP results of the mortar containing the sand with 4% mica and interfacial modifier: (a) Pore diameter distribution; (b) Cumulative pore volume.

Figure 11 .
Figure 11.XRD patterns of the mortar and concrete containing the mica sand with and without interface modifier treatment.

Figure 12 .
Figure 12.MIP results of the mortar containing the sand with 4% mica and interfacial modifier: (a) Pore diameter distribution; (b) Cumulative pore volume.

Figure 13 .
Figure 13.Pore compositions of the mortar containing the sand with 4% mica and interfacial modifier.

Figure 15 .
Figure 15.Enhancement mechanism of the SCA interface modifier.

Figure 15 .
Figure 15.Enhancement mechanism of the SCA interface modifier.

Table 1 .
Main technical properties of cement.

Table 2 .
Chemical compositions of cement, fly ash, and ceramic powder.

Table 3 .
Particle size distribution of mica.

Table 1 .
Main technical properties of cement.

Table 2 .
Chemical compositions of cement, fly ash, and ceramic powder.

Table 3 .
Particle size distribution of mica.

Table 4 .
Mix proportions of mortar.

Table 5 .
Mix proportions of concrete.