Effects of Recycled Fine Aggregates and Inorganic Crystalline Materials on the Strength and Pore Structures of Cement-Based Composites

: Concrete is porous; the partial pores in the internal structure of concrete are generated by hydration products, such as calcium hydroxide, dissolved in water. External harmful substances in the form of gases or aqueous solutions can penetrate concrete. The destruction of the internal structure of concrete leads to problems such as shortening of the service life of concrete as well as the corrosion and poor durability of steel. To improve the pore structure of concrete, a material can be added to concrete mixtures to cause the secondary hydration of the hydration products of cement. This reaction is expected to reduce the pore volume and increase the density of concrete. For existing concrete structures, inorganic crystalline materials can be used to protect the surface and reduce the intrusion of external harmful substances. In this study, the water–binder ratio was 0.4 and 0.6. Three inorganic crystalline materials and recycled ﬁne aggregates (0%, 10%, 20%, and 30% replacement of natural aggregates by weight) were used in the same cement-based composites. The results indicated that all specimens had a high total charge-passed value, and inorganic crystalline material C provided superior protection for green cement-based composites. replacement of natural aggregates recycled aggregates. This result is consistent with of the saturated water and initial surface water


Introduction
Concrete is currently the most widely used construction material; however, its service life varies in different environments. Long-term corrosive environments containing materials such as carbon dioxide, chloride, and sulfate can damage or deteriorate concrete [1][2][3][4][5][6][7] and shorten its service life. Steel corrosion is a common failure phenomenon observed in concrete structures. The expansion of corrosion products produces internal tension, which causes the protective layer to crack and accelerates the penetration of harmful substances into the concrete, leading to the insufficient durability of reinforced concrete structures. Taiwan's older reinforced concrete structures are due for demolition or reinforcement. Maintenance and reinforcement technology for concrete structures is a crucial topic in construction engineering. The unsuitable repair of concrete structures affects the social and economic development of an area as well as human safety. The main hazards of concrete cracks are affecting the bearing capacity and safety of the structure, affecting the waterproof of the structure, and affecting the durability and service life of the structure.
Traditional concrete has many negative environmental effects. The concept of green concrete has been developed to reduce the impact of concrete on the environment. Green concrete can be produced using recyclable and renewable resources to reduce environmental pollution. Moreover, green concrete can coexist with the natural environment to achieve sustainable development. The term "green" encompasses energy conservation and environmental sustainability [8][9][10][11]. The addition of recycled materials to concrete to  Table 2. Coating material with common mechanisms [23].

Type Species Characteristics
Coatings or sealers Acrylic Butadiene copolymer Epoxy resin Polyester resin Polyethylene copolymer Polyurethane They can modify appearances They can be isolated from liquid and gas invasion They are often used to protect the outermost layer Their subsequent maintenance is difficult Their wear resistance is poor Pore-lining treatment Silicones Siloxane Silane Silicone resins They are unfavorable in high-temperature environments It can prevent erosion through water or chloride ions It can be used in road structure protection. Corrosion and wear resistance are poor in this treatment It does not affect the appearance of the concrete surface Pore-blocking treatment Silicate Silicofluoride Crystal growth materials It enables resistance to liquid, gas, chemical attack, and abrasion The protective effect depends on the porosity of concrete The pore has good adhesion and long-term effect. Such coatings are the first protective layer of the concrete substrate.

Rendering
Plain and polymer-modified cement-based mortars The coating provides a barrier effect of certain thickness. It can reduce the passing rate of moisture It allows the development of resistance to sulfate attack It has a wide range of applications Cement mortar, which is a typical porous composite material, has varying pore size and shape. Pores are scattered in cement mortar and are an important factor affecting the ion transfer rate. According to their sizes, pores are subdivided into compaction, entrapped air, capillary, and gel pores [24]. The surface of cement mortar has the ability to transfer ions. When the contact angle between cement mortar and the atmosphere is small, water from the air can easily invade the surface of cement mortar along the capillary pores. Moreover, external harmful substances can be transported through water into cement mortar, affecting its mechanical properties and durability. According to [25], the crucial processes in cement mortar transmission include water transmission under hydrostatic pressure, water transmission caused by the capillary phenomenon, particle diffusion caused by the concentration gradient, and particle transmission caused by water movement. The transportation properties of external substances in cement mortar are mainly dependent on its pore structure; thus, pore size, volume, and connectivity in cement mortar affect the aforementioned properties.
An effective method of maintaining the durability of concrete is controlling the pore transport characteristics of materials, which mainly depend on the pore structure and pore size in concrete. The pore structure of concrete has a considerable influence on its strength and permeability. The pores in water-based composite materials can be divided into three categories according to their size [23]: (1) pores larger than 50 nm, which are called macropores; (2) pores 2-50 nm large, which are called mesopores; and (3) pores smaller than 2 nm, which are called micropores. Macropores and mesopores are generally called capillary pores, and they have a considerable influence on the physical properties of materials. Small pores are generally called colloidal pores and have a considerable influence on the drying shrinkage and creep behavior of materials. Thus, pore size, distribution, volume, and continuity in concrete affect the transmission characteristics of external substances that enter the concrete [26]. To improve the pore structure of concrete, in addition to water as well as coarse and fine materials that meet different specifications and standards, Portland cement or other mineral admixtures are added to concrete to increase its compactness and reduce its permeability [27].
In this study, construction waste is used as aggregate, and waste concrete is recycled as recycled aggregate. The practice of green concrete has greatly improved the environmental protection. In order to improve the durability of green concrete, inorganic crystalline coating is used. Inorganic crystalline coating can effectively reduce the pores of concrete, and also greatly reduce the probability of concrete failure. Thus, concrete protection affects its durability. In addition to changing the composition of concrete materials (e.g., by using pozzolanic materials), external water vapor can be prevented from being absorbed by the capillary pores of concrete to change the pore size and block tortuous paths [28][29][30][31][32][33][34]. The paper discusses the protective effect of the coating with mortar. The effect of the coating on the pore protection can be understood through mortar. Three types of inorganic crystalline materials were used in this study to test the mechanical properties, permeability, and microproperties of mortars containing recycled aggregates. The results of this study can serve as a reference for future research regarding which material provides superior surface protection for concrete.

Cement
In this study, Portland I cement from Taiwan Cement Company (Hualien, Taiwan) was adopted. The characteristics of this cement meet the specifications of the National Standards of the Republic of China (CNS) 61.

Natural Fine Aggregates
Natural fine aggregates were obtained from the Lanyang River (Yilan, Taiwan). According to the American Society for Testing and Materials (ASTM) C33 [35], the fineness Crystals 2021, 11, 587 5 of 26 modulus (FM) of these aggregates is 2.8. According to ASTM C128 [36], the specific gravity [saturated surface dry (SSD)] and water absorption of the aforementioned aggregates are 2.56% and 1.85%, respectively.

Recycled Fine Aggregates
The recycled fine aggregates used in this study were obtained from Zunhong Environmental Protection Co., Ltd. (Keelung, Taiwan). According to ASTM C33 [35], the FM of these aggregates is 2.43. According to ASTM C128 [36], the SSD specific gravity and water absorption of the aforementioned aggregates are 2.42% and 6.16%, respectively.

Mix Design and Test Methods
For the setting of the mortar number, the first code is the W/C of mortar specimens and the second code is the crystalline material used, which is set to T, K, and C, as presented in Table 3. The T crystalline material was obtained from Kryton International Inc. (Vancouver, BC, Canada); the K crystalline material was procured from KÖSTER BAUCHEMIE AG (Aurich, Germany); and the C crystalline material was obtained from Jiawurong Industrial Co., Ltd (Taipei, Taiwan). Under the action of water, the active chemicals contained in cement-based permeable crystalline waterproof materials are brought into the surface pores of the structure through the erosion of surface water to the internal structure, and they react with free calcium oxide in mortar to form water-insoluble calcium sulfoaluminate (3CaO·Al 2 O 3 ·CaSO 4 ·32H 2 O) permeable crystals. The crystal absorbs water and expands in the pores of the structure, which makes the surface layer of the mortar structure gradually form a dense impermeability area, and it greatly improves the impermeability of the whole structure. The third code involves the replacement of 10%, 20%, or 30% of the natural aggregates with recycled fine aggregates and the control factor group. The first code is related to the control factor group. In accordance with the research goal and the examination of the relevant literature, water-cement ratios of 0.4 and 0.6 were used for the cement mortar in the pilot test.  Table 4 presents the experimental variables. Three types of inorganic crystalline materials (T, K, and C) were used to coat the surface of the test body. Then, recycled fine aggregates were used to replace 0%, 10%, 20%, and 30% of the natural aggregates. The N mark indicates an unsealed sample. The test age was set to 7, 28, and 56 days. Three specimens of each mixture were required for each test in this study. The designed proportions of materials in the cement mortar are presented in Table 5. The design mix ratio of cement mortar depends on the amount of fine aggregates and T, K, C, or N coating.
The main test items and reference standards were divided into three categories: mechanical properties, permeability, and characterization properties. The mechanical properties of φ 10 × 20 mortar were tested according to ASTM C39m-12 [37]. According to ASTM C642-13 [38], φ 10 × 5 mortar was prepared for an absorption test. The φ 10 × 5 mortar was prepared according to BS 1881 [39] for an initial surface water absorption test. This mortar was prepared according to ASTM C1202 [40] for a rapid chloride penetration test (RCPT). Moreover, according to ASTM d4404-10 [41], 1 × 1 × 1 mortar was prepared for a mercury intrusion porosimetry (MIP) test. The coating is protected by coating on the outer layer of mortar specimen, as shown in Figure 1.

Compressive Strength
The test results are displayed in Figures 2 and 3. When the water-binder ratio was low, C was used as a coating material and recycled fine material was used to replace 0%. The strength of 7 days, 28 days, and 56 days is 24.83%, 32.89%, and 41.99% higher than that of recycled fine material. The compressive strength was higher at a low water-binder ratio than at a high water-binder ratio for all the test groups. Moreover, the compactness was higher at a low water-binder ratio than at a high water-binder ratio. At the same water-binder ratio, material C had higher compressive strength than did the other materials for all curing ages. The initial compressive strength of material C was marginally greater than those of the other materials. For the same water-binder ratio and coating material, the effects of different proportions of recycled aggregate substitution on the compressive strength were compared. The highest compressive strength was obtained when material C was used as the coating material and the amount of replacement was 0%. Thus, the surface protection can be increased using C with 0% recycled substitution at a low waterbinder ratio.
mark indicates an unsealed sample. The test age was set to 7, 28, and 56 days. Three specimens of each mixture were required for each test in this study. The designed proportions of materials in the cement mortar are presented in Table 5. The design mix ratio of cement mortar depends on the amount of fine aggregates and T, K, C, or N coating.
The main test items and reference standards were divided into three categories: mechanical properties, permeability, and characterization properties. The mechanical properties of ϕ 10 × 20 mortar were tested according to ASTM C39m-12 [37]. According to ASTM C642-13 [38], ϕ 10 × 5 mortar was prepared for an absorption test. The ϕ 10 × 5 mortar was prepared according to BS 1881 [39] for an initial surface water absorption test. This mortar was prepared according to ASTM C1202 [40] for a rapid chloride penetration test (RCPT). Moreover, according to ASTM d4404-10 [41], 1 × 1 × 1 mortar was prepared for a mercury intrusion porosimetry (MIP) test. The coating is protected by coating on the outer layer of mortar specimen, as shown in Figure 1.   The 28-day compressive strength reached 32.12 MPa. The strength requirement was 30 MPa when the water-binder ratio was low, C was used as the coating material, and the amount of recycled aggregate substitution was 20%. When the water-binder ratio was high and the other conditions were unchanged, the compressive strength reached 30 MPa in 56 days. When the water-binder ratio was low, the 28-day strength of material C was 4.92% higher than that of material K and 18.96% higher than that of material T. When the water-binder ratio was high, the 56-day strength of C was 0.11% higher than that of K and 13.88% higher than that of T.
Concrete coatings are widely used to improve the durability of reinforced concrete structures in order to prevent and control reinforcement corrosion in chlorides containing environment. Cement-based coatings, as well as organic-based coatings, act as a physical barrier to the penetration of water, ions, and gases [42]. The coating can reduce the pores on the surface of mortar, improve the compactness, and indirectly improve the compressive strength of mortar.
When the water-binder ratio was low, the 28-day compressive strengths of materials C, K, and T were 21.33%, 17.26%, and 2.92% higher than those of the uncoated samples, respectively, when the recycled fine aggregates replaced 20% of the natural aggregates. When the water-binder ratio was high and the other conditions remained unchanged, the 28-day compressive strengths of the C, K, and T coatings were 28.83%, 28.18%, and 20.45% higher than those of the uncoated samples, respectively, when the recycled fine aggregates replaced 20% of the natural aggregates. The aforementioned results indicate that the compressive strength increased by a greater extent under a high water-binder ratio than under a low water-binder ratio. and 13.88% higher than that of T.
Concrete coatings are widely used to improve the durability of reinforced concrete structures in order to prevent and control reinforcement corrosion in chlorides containing environment. Cement-based coatings, as well as organic-based coatings, act as a physical barrier to the penetration of water, ions, and gases. [42] The coating can reduce the pores on the surface of mortar, improve the compactness, and indirectly improve the compressive strength of mortar.

Absorption Test
The porosity of the specimens was measured under oven-dry conditions and SSD. The saturated water absorption decreased with an increase in specimen age because the pores of the specimens were gradually filled in during the curing and hydration reaction and the compactness of the specimens increased. The results of the porosity test are illustrated in Figures 4 and 5. The saturated water absorption was lower under a low water-binder ratio than under a higher water-binder ratio. Thus, the compactness was higher at a low water-binder ratio than it was at a high water-binder ratio. Under the same water-binder ratio, the saturated water absorption of C was lower than that of the other materials. This result indicates that C is superior to the other adopted inorganic crystalline materials. At the same water-binder ratio and with C as the coating material, the saturated water absorption was the lowest for 0% recycled aggregate substitution, indicating that the compactness decreased with an increase in recycled aggregate substitution. The aforementioned results indicate that the best densification and surface protection effect can be obtained under a low water-binder ratio, 0% recycled aggregate substitution, and the use of C as a coating material.   When the water-binder ratio was low, the 28-day compressive C, K, and T were 21.33%, 17.26%, and 2.92% higher than those of t respectively, when the recycled fine aggregates replaced 20% of th When the water-binder ratio was high and the other conditions r the 28-day compressive strengths of the C, K, and T coatings were 20.45% higher than those of the uncoated samples, respectively, w aggregates replaced 20% of the natural aggregates. The aforement that the compressive strength increased by a greater extent under ratio than under a low water-binder ratio.

Absorption Test
The porosity of the specimens was measured under oven-dry The saturated water absorption decreased with an increase in spec pores of the specimens were gradually filled in during the curing an and the compactness of the specimens increased. The results of th lustrated in Figures 4 and 5. The saturated water absorption was lo ter-binder ratio than under a higher water-binder ratio. Thus, t higher at a low water-binder ratio than it was at a high water-bi same water-binder ratio, the saturated water absorption of C was l other materials. This result indicates that C is superior to the oth crystalline materials. At the same water-binder ratio and with C as When the water-binder ratio was low, the 28-day water absorption of the samples coated with K, T, and C was 20.05%, 16.20%, and 11.06% lower than that of the uncoated samples, respectively. When the water-binder ratio was high, the 28-day water absorption of the uncoated samples was 56.67%, 47.23%, and 39.94% higher than that of the samples coated with K, T, and C, respectively. The aforementioned results indicate that the water absorption was lower at a high water-binder ratio than at a low water-binder ratio. Moreover, the water absorption of the samples coated with T was lower than that of the control group.

Initial Surface Water Absorption Test
The surfaces of the specimens were mostly composed of capillary pores. The number of capillary pores and the compactness of the specimens could be inferred from the results of the surface water absorption test. The test results are displayed in Figures 6-11. The surface water absorption was lower at a low water-binder ratio than at a high water-binder ratio, indicating that the number of capillary pores and the compactness were

Initial Surface Water Absorption Test
The surfaces of the specimens were mostly composed of capillary pores. The number of capillary pores and the compactness of the specimens could be inferred from the results of the surface water absorption test. The test results are displayed in Figures 6-11. The surface water absorption was lower at a low water-binder ratio than at a high water-binder ratio, indicating that the number of capillary pores and the compactness were higher when the water-binder ratio was low. Under the same water-binder ratio, the surface water absorption of C was the lowest. higher when the water-binder ratio was low. Under the same water-binder ratio, the surface water absorption of C was the lowest.       . Initial surface water absorption of cement mortar after 28 days when W/C = 0.6. Figure 9. Initial surface water absorption of cement mortar after 28 days when W/C = 0.6.  . Initial surface water absorption of cement mortar after 56 days when W/C = 0.6. Figure 11. Initial surface water absorption of cement mortar after 56 days when W/C = 0.6.
The aforementioned results indicate that the best surface protection effect can be achieved when using C as the coating material. At the same water-binder ratio and with C as the coating material, the surface water absorption increased with the amount of recycled aggregates. Thus, a superior densification effect cannot be achieved by increasing the amount of substituted recycled materials. The best densification and surface protection effect was achieved when the water-binder ratio was low, C was used as the coating material, and the amount of substitution with recycled materials was 0%. This result corresponded with the results of the saturated water absorption test; thus, the accuracy of the aforementioned result was verified.
As displayed in Figures 6 and 7, when the water-binder ratio was low, we used 20% recycled fine material, and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C after 7 days was 37.5% and 79.17% lower than that of the K and T coatings, respectively. When the water-binder ratio was high, we used 20% recycled fine material, and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C after 7 days was 48.15% and 70.37% lower than that of materials K and T, respectively. Under both the high and low water-binder ratios, recycled fine aggregates could replace 20% of the natural aggregates. The initial surface water absorption was highest for material C, followed by materials K and T. Material C could effectively reduce the surface water absorption of the specimens on which it was coated.
As illustrated in Figures 8 and 9, when the water-binder ratio was low, we used 20% recycled fine material and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C after 28 days was 47.62% and 90.48% lower than that of materials K and T coatings, respectively. When the water-binder ratio was high, we used 20% recycled fine material and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C after 28 days was 54.17% and 79.17% lower than that of materials K and T, respectively. Under both the high and low water-binder ratios, recycled fine aggregates could replace 20% of the natural aggregates. The surface water absorption was highest for material C, followed by materials K and T. Material C effectively reduced the surface water absorption of the specimens on which it was coated.
As shown in Figures 10 and 11, when the water-binder ratio was low, we used 20% recycled fine material, and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C after 56 days was 47.37% and 10.53% lower than that of materials K and T, respectively. When the water-binder ratio was high, we used 20% recycled fine material and the detection time was 30 min. Under the aforementioned conditions, the surface water absorption of material C coating after 56 days was 54.55% and 86.36% lower than that of materials K and T, respectively. Under both the high and low water-binder ratios, recycled aggregates could replace 20% of the natural aggregates. Surface water absorption was highest for material C, followed by materials K and T. Material C effectively reduced the surface water absorption of the samples coated with it.

Rapid Chloride Penetration Test
According to the standard evaluation of chloride charge flux in the RCPT, cumulative charges of 4000, 2000-4000, 1000-2000, 100-1000, and <100 Coulombs indicate high chloride permeability, medium chloride permeability, low chloride permeability, extremely low chloride permeability, and chloride-free permeation, respectively. The results obtained in the RCPT at the water-binder ratios of 0.4 and 0.6 are displayed in Figures 12 and 13, respectively. The results indicate that the anti-ion permeability was higher at the low water-binder ratio than at the high water-binder ratio. The specimens coated with C had the lowest cumulative total through charge. Material C had high resistance to chloride-ion penetration at both the high and low water-binder ratios. C exhibited the best protective effect among the coating materials, and its chloride-ion penetration resistance was the highest for 0% replacement of natural aggregates with recycled aggregates. This result is consistent with those of the saturated water absorption and initial surface water absorption tests. the chloride charge fluxes of the samples coated with materials C, K, and T were 21.58% 15.29%, and 4.44% lower than those of the uncoated samples, respectively.
As depicted in Figure 13, when the water-binder ratio was high, the recycled ag gregates could replace 20% of the natural aggregates. The chloride charge flux of materi C was 1.41% and 24.36% lower than those of materials K and T, respectively. The chlorid charge fluxes of the samples coated with materials C, K, and T were 58.86%, 56.66%, an 27.75% lower than those of the uncoated samples, respectively.
The resistance to chloride-ion charge flux was highest for material C, followed b materials K and T. Material C effectively resisted chloride-ion penetration.  As displayed in Figure 12, when the water-binder ratio was low, the recycled aggregates could replace 20% of the natural aggregates. The chloride charge flux of material C was 5.45% and 16.41% lower than those of materials K and T, respectively. Moreover, the chloride charge fluxes of the samples coated with materials C, K, and T were 21.58%, 15.29%, and 4.44% lower than those of the uncoated samples, respectively.
As depicted in Figure 13, when the water-binder ratio was high, the recycled aggregates could replace 20% of the natural aggregates. The chloride charge flux of material C was 1.41% and 24.36% lower than those of materials K and T, respectively. The chloride charge fluxes of the samples coated with materials C, K, and T were 58.86%, 56.66%, and 27.75% lower than those of the uncoated samples, respectively.
The resistance to chloride-ion charge flux was highest for material C, followed by materials K and T. Material C effectively resisted chloride-ion penetration.

MIP Test
The MIP test can be used to determine the distribution of pores in specimens as well as calculate the gel porosity, capillary porosity, and total porosity of specimens. The total pore volume is the pore volume per unit weight of a specimen, and the unit of total pore volume is mL/g. The total pore volume comprises the colloid and capillary pores. Colloid pores, which have a diameter of less than 100 nm, control the stability and durability of the specimen. Capillary pores, which have a diameter of more than 100 nm, control the strength and permeability of a specimen.
The total, colloidal, and capillary pore volumes of the different coating and sealing materials were investigated using water-binder ratios of 0.4 and 0.6 and different proportions of recyclable aggregate replacement. The influence of pozzolan on the cement matrix was also examined. The results of the MIP test are illustrated in Figures 14 and 15. The pore volume of mercury intrusion was smaller at a water-binder ratio of 0.4 than at a water-binder ratio of 0.6. For the specimens containing inorganic crystalline materials, 20% substitution of recyclable aggregates was observed. This result indicated that coating material K had superior pore repairing ability to the other coating materials.
As displayed in Figures 14 and 15, when the water-binder ratio was 0.4, recycled fine aggregates could replace 20% of the natural aggregates. The total pore volume of the material C was 36.08% and 86.87% higher than those of materials K and T, respectively. Compared with the uncoated samples, the total porosities of the samples coated with K, T, and C were 107.38%, 61.45%, and 32.56% lower, respectively. The total pore volume was highest for material K, followed by materials T and C. Material K effectively reduced the total porosity. When the water-binder ratio was 0.6, recycled fine aggregates could replace 20% of the natural aggregates. The total pore volume of material C was 2.59% and 81% lower than those of materials K and T, respectively. Compared with the uncoated samples, the total porosities of the samples coated with T, C, and K were 80.4%, 43.9%, and 40.27% lower, respectively. The total pore volume was largest for material T, followed by materials C and K. Material T effectively reduced the total porosity.

MIP Test
The MIP test can be used to determine the distribution of pores in specimens as well as calculate the gel porosity, capillary porosity, and total porosity of specimens. The total pore volume is the pore volume per unit weight of a specimen, and the unit of total pore volume is mL/g. The total pore volume comprises the colloid and capillary pores. Colloid pores, which have a diameter of less than 100 nm, control the stability and durability of the specimen. Capillary pores, which have a diameter of more than 100 nm, control the strength and permeability of a specimen.
The total, colloidal, and capillary pore volumes of the different coating and sealing materials were investigated using water-binder ratios of 0.4 and 0.6 and different proportions of recyclable aggregate replacement. The influence of pozzolan on the cement matrix was also examined. The results of the MIP test are illustrated in Figures 14 and 15. The pore volume of mercury intrusion was smaller at a water-binder ratio of 0.4 than at a water-binder ratio of 0.6. For the specimens containing inorganic crystalline materials, 20% substitution of recyclable aggregates was observed. This result indicated that coating material K had superior pore repairing ability to the other coating materials.
As displayed in Figures 14 and 15, when the water-binder ratio was 0.4, recycled fine aggregates could replace 20% of the natural aggregates. The total pore volume of the material C was 36.08% and 86.87% higher than those of materials K and T, respectively. Compared with the uncoated samples, the total porosities of the samples coated with K, T, and C were 107.38%, 61.45%, and 32.56% lower, respectively. The total pore volume was highest for material K, followed by materials T and C. Material K effectively reduced the total porosity. When the water-binder ratio was 0.6, recycled fine aggregates could replace 20% of the natural aggregates. The total pore volume of material C was 2.59% and 81% lower than those of materials K and T, respectively. Compared with the uncoated samples, the total porosities of the samples coated with T, C, and K were 80.4%, 43.9%, and 40.27% lower, respectively. The total pore volume was largest for material T, followed by materials C and K. Material T effectively reduced the total porosity.

Scanning Electron Microscopy
Coatings can form an insoluble crystal structure in the interconnected pores other voids of concrete. This crystal structure becomes a permanent and integral pa the concrete matrix. Even under a strong hydrostatic pressure, the coating prevents w and other liquids from penetrating into the concrete and protects the concrete from verse environmental conditions. In this study, dense reticular veins of crystals coul

Scanning Electron Microscopy
Coatings can form an insoluble crystal structure in the interconnected pores and other voids of concrete. This crystal structure becomes a permanent and integral part of the concrete matrix. Even under a strong hydrostatic pressure, the coating prevents water and other liquids from penetrating into the concrete and protects the concrete from adverse environmental conditions. In this study, dense reticular veins of crystals could be observed in scanning electron microscopy (SEM) images obtained under 3000× magnification.
When material C was used and 20% of the natural aggregates were replaced with recycled aggregates, needle-like structures could be observed 5 mm below the substrate surface under a scanning electron microscope, as displayed in Figures 16 and 17. These needle-like structures were produced by chemical reactions between inorganic coating material and the substrate. X-ray diffraction analysis indicated that the needles consisted of C-S-H colloids or calcium carbonate. As depicted in Figures 18 and 19, no needlelike structures were observed 20 mm below the coating, indicating that the amount of needle-like structures formed decreased with an increase in depth from the coating surface. Material C effectively produced dense network crystals, which effectively resisted chlorideion penetration as well as reduced water absorption and initial surface water absorption. When material C was used and 20% of the natural aggregates were replaced wit recycled aggregates, needle-like structures could be observed 5 mm below the substrat surface under a scanning electron microscope, as displayed in Figures 16 and 17. Thes needle-like structures were produced by chemical reactions between inorganic coatin material and the substrate. X-ray diffraction analysis indicated that the needles consiste of C-S-H colloids or calcium carbonate. As depicted in Figures 18 and 19, no needle-lik structures were observed 20 mm below the coating, indicating that the amount of nee dle-like structures formed decreased with an increase in depth from the coating surfac Material C effectively produced dense network crystals, which effectively resisted chlo ride-ion penetration as well as reduced water absorption and initial surface water ab sorption.   When material C was used and 20% of the natural aggregates were replaced with recycled aggregates, needle-like structures could be observed 5 mm below the substrate surface under a scanning electron microscope, as displayed in Figures 16 and 17. These needle-like structures were produced by chemical reactions between inorganic coating material and the substrate. X-ray diffraction analysis indicated that the needles consisted of C-S-H colloids or calcium carbonate. As depicted in Figures 18 and 19, no needle-like structures were observed 20 mm below the coating, indicating that the amount of needle-like structures formed decreased with an increase in depth from the coating surface. Material C effectively produced dense network crystals, which effectively resisted chloride-ion penetration as well as reduced water absorption and initial surface water absorption.

Conclusions
The results of a compressive strength test indicated that specimens coated with material C and containing natural fine aggregates had the highest compressive strength at a low water-binder ratio. This result was confirmed by the results of a permeability test and SEM. The porosity of mortar directly affected the compactness and compressive strength of the specimens. When C was used as the coating material and 20% of the natural aggregates were replaced with recycled fine material, the 28-day compressive strength reached 32.12 MPa, which meets the strength requirement of 30 MPa.
The results of an absorption test indicated that the lower the water absorption, the lower was the internal porosity of the specimens. The best densification effect was achieved at a low water-binder ratio for the specimens coated with C and containing natural fine aggregates. When the water-binder ratio was low, specimens were cured for 28 days, and 20% of the natural aggregates were replaced with recycled aggregates, the water absorption of material C was lower than that of materials K and T.
The results of the initial surface water absorption test and absorption test corresponded to each other. These tests focused on the absorption of cement mortar, and the compactness of the mortar increased with its age. The results of the compressive strength test indicated that the compactness of cement mortar had a positive relationship with its compressive strength.
The results of the RCPT indicated that the lower the porosity, the higher the resistance to chloride ions. Material C effectively resisted chloride-ion penetration. More-

Conclusions
The results of a compressive strength test indicated that specimens coated with material C and containing natural fine aggregates had the highest compressive strength at a low water-binder ratio. This result was confirmed by the results of a permeability test and SEM. The porosity of mortar directly affected the compactness and compressive strength of the specimens. When C was used as the coating material and 20% of the natural aggregates were replaced with recycled fine material, the 28-day compressive strength reached 32.12 MPa, which meets the strength requirement of 30 MPa.
The results of an absorption test indicated that the lower the water absorption, the lower was the internal porosity of the specimens. The best densification effect was achieved at a low water-binder ratio for the specimens coated with C and containing natural fine aggregates. When the water-binder ratio was low, specimens were cured for 28 days, and 20% of the natural aggregates were replaced with recycled aggregates, the water absorption of material C was lower than that of materials K and T.
The results of the initial surface water absorption test and absorption test corresponded to each other. These tests focused on the absorption of cement mortar, and the compactness of the mortar increased with its age. The results of the compressive strength test indicated that the compactness of cement mortar had a positive relationship with its compressive strength.
The results of the RCPT indicated that the lower the porosity, the higher the resistance to chloride ions. Material C effectively resisted chloride-ion penetration. More- Figure 19. SEM image of the microstructures 20 mm below the inner coating of the substrate when W/C = 0.6 (3000×; 4C2, 56 days).

Conclusions
The results of a compressive strength test indicated that specimens coated with material C and containing natural fine aggregates had the highest compressive strength at a low water-binder ratio. This result was confirmed by the results of a permeability test and SEM. The porosity of mortar directly affected the compactness and compressive strength of the specimens. When C was used as the coating material and 20% of the natural aggregates were replaced with recycled fine material, the 28-day compressive strength reached 32.12 MPa, which meets the strength requirement of 30 MPa.
The results of an absorption test indicated that the lower the water absorption, the lower was the internal porosity of the specimens. The best densification effect was achieved at a low water-binder ratio for the specimens coated with C and containing natural fine aggregates. When the water-binder ratio was low, specimens were cured for 28 days, and 20% of the natural aggregates were replaced with recycled aggregates, the water absorption of material C was lower than that of materials K and T.
The results of the initial surface water absorption test and absorption test corresponded to each other. These tests focused on the absorption of cement mortar, and the compactness of the mortar increased with its age. The results of the compressive strength test indicated that the compactness of cement mortar had a positive relationship with its compressive strength.
The results of the RCPT indicated that the lower the porosity, the higher the resistance to chloride ions. Material C effectively resisted chloride-ion penetration. Moreover, the lower the cumulative passing capacity, the higher the resistance to chloride-ion penetration.
The MIP test can be used to examine the changes in the internal porosity of specimens as well as the distribution and amount of capillary and colloidal pores. The results obtained for the mechanical property and permeability tests indicated that the number of pores directly affected the compactness of specimens. The results also indicated that coating material K had good pore repair ability.
SEM analysis indicated that when material C was used, recycled aggregates replaced 20% of the natural aggregates. Needle-like structures were observed 5 mm below the substrate surface at a high and low water-binder ratio. No obvious acicular structure was observed 20 mm below the substrate surface.