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Article

Comparative Study on the Effects of Five Nano-Metallic Oxide Particles on the Long-Term Mechanical Property and Durability of Cement Mortar

by
Shuangkuai Dong
1,
Songhan Yu
1,
Liangliang Chen
2,3,
Qi Zhuo
1,
Fufei Wu
1,*,
Lilan Xie
1,* and
Liuyan Liu
1
1
School of Materlals and Architectural Engineering (Guizhou School of Emergency Management), Guizhou Normal University, Guiyang 550025, China
2
School of Civil Engineering, Huzhou Vocational and Technical College, Huzhou 313000, China
3
Huzhou Key Laboratory of Green Building Technology, Huzhou 313000, China
*
Authors to whom correspondence should be addressed.
Buildings 2023, 13(3), 619; https://doi.org/10.3390/buildings13030619
Submission received: 30 January 2023 / Revised: 23 February 2023 / Accepted: 24 February 2023 / Published: 26 February 2023
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

:
Nano-metallic oxide particles have been found to be potentially effective microstructural reinforcements for cement mortar and have become a research hotspot in recent years for nano-modification technology of building materials. However, different conclusions have been obtained due to various researchers used different research methods, which have resulted in a deficiency for the performance comparison between different nano-metallic oxide particles. In the present study, the effects of five kinds of nano-metallic oxide particles, namely nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO, on the performance of cement mortar at 28 days and 730 days in terms of mechanical, durability, microstructure, and pore size distribution properties by performing different experiments were investigated. Test results show that the dosage of nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO is 2%, 1%, 1%, 1%, and 2%, respectively, where they can significantly prove the compressive and flexural strengths, decrease the porosity, drying shrinkage, and permeability, and refine the pore size distribution of cement mortar. It can be seen through SEM analysis that nano-metallic oxide particles can promote cement hydration, and also refine the size and distribution of Ca(OH)2 crystal, but the specific principles are different. The analysis concluded that the five kinds of nano-metallic oxide particles can play a filling role in cementitious materials to improve the denseness and surface activity role to promote the hydration of cement particles, thus improving the mechanical properties, durability, and pore size distribution of cementitious materials, with the order of their modification effect on cement-based materials being nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-ZnO > nano-CuO.

1. Introduction

Cement mortar, a composite material, is a nano structured material that ages over time, which is also a widely used building material in the construction industry [1]. With the in-depth application of concrete structures in high-rise buildings, long spans, special service conditions, and harsh environments (erosion areas, undersea tunnels, high ground stress, etc.), new requirements are needed for their alkalinity, construction performance, mechanical properties, and durability [2,3,4]. Therefore, traditional cementitious materials have been unable to meet the needs of some engineering projects, with high strength and high durability as an important development direction of concrete materials. This makes the modification of cementitious materials by nano materials into a hot issue of high necessity for research institutes at home and abroad.
Usually, the methods of preparing nano-materials include the solid phase method, liquid phase method, and gas phase method. Relatively speaking, the precipitation method in the liquid phase method has the advantages of a simple reaction process, low cost, and of convenient popularization and industrial production. Therefore, it is widely used in the production of nano-metal oxides. Nano-metallic oxide particles, as a nano material, have many excellent properties such as size effect [5] and surface and interface effects [6], so they have been often applied in the biological, energy and medical fields. In the modification of cement-based materials, nano-materials such as nano-calcium carbonate [7], multilayer graphene [8], multi-scale steel fiber [9], nano-crystalline graphene [10], and nano-oxide [11,12,13] are mainly used. However, the application of nano-metal oxides in the civil engineering area is not popular, especially in the cement concrete area, which needs to be further improved and supplemented.
Nano-CuO, as a kind of soil stabilizer, can reform soil properties, improve mechanical properties and shear resistance of soft soil, and reduce environmental pollution [14]. As a filling agent for cement composites, nano-CuO can make the internal structure more uniform and compact [15]. Rawdhan [16] compared the effects of different amounts of nano-particles of nano-Fe2O3, nano-CuO, and nano-Al2O3 on the properties of bonded bricks; the results show that the addition of nano-copper particles can increase the bulk density and bearing strength of the bricks, but it can also decrease the water absorption and workability of the new bricks. Nazari [17] studied the effect of different amounts of CuO nano-particles on self-compacting concrete; the experiment shows that nano-CuO particles can improve the compressive strength and reverse the negative impact of superplasticizer on the compressive strength of concrete.
Nano-Al2O3, as an amphoteric oxide, can interact with strong acid or strong alkali to yield aluminate, and its main mineral component is corundum [18]. Most studies found that for cement composite materials, the addition of nano-Al2O3 will shorten the initial and final setting time, and improve its compressive, flexural, tensile, and splitting tensile strength. Furthermore, the super-high specific surface area of nano-Al2O3 makes it more attractive to water than cement particles, resulting in a lower workability of materials [19,20,21]. However, the amount of nano-Al2O3 should be moderate, otherwise the material properties will be reduced. The optimum content of nano-Al2O3 seems to depend on many factors, such as w/b ratio, curing conditions, nano-particle diameter, and the type and content of pozzolanic ash that can be used [22].
Nano-MgO is sintered from magnesite at 700 °C [23]. Compared with Portland cement, it can reduce carbon dioxide emissions due to its lower production temperature. Therefore, nano-MgO is a green low-carbon, soft-soil stabilizer [24,25]. In addition, some studies have found that adding a certain amount of nano-MgO into the material can effectively fill the voids in the cement mortar, thus improving the strength and toughness of the material, and making its microstructure denser than ordinary cement-based materials [26,27]. At the same time, nano-MgO reduces the self-shrinkage of cement mortar, which is beneficial in enhancing its stability [28].
Nano-ZnO, as a nano filler, has no pozzolanic activity and cannot react with CH crystal to produce more hydration products [29,30]. At the same time, nano-ZnO can absorb high-efficiency water-reducing agents and control the workability of composite materials [31]. It has a small size and a large surface effect, which can shorten the early setting time [32]. In addition, nano-ZnO can produce active oxygen, release zinc ions that affect cell membrane, and has an antibacterial effect, so it is often used for air purification, food packaging, and medical implants [33].
Nano-ZrO2, as an inorganic metal oxide, has the advantages of high strength, high toughness, excellent corrosion resistance, and small size, which opens the market for its application in the field of cement composite materials [34,35]. Compared with ordinary cement paste, nano-ZrO2 composite can improve the microstructure of the mixture, as nano-ZrO2 has pozzolanic activity and filling effect, which can reduce the number of pores [36,37]. Moreover, the nucleation effect of nano-ZrO2 can promote the hydration of cement and reduce the orientation and size of the calcium hydroxide crystal, thus improving the wear resistance of cement-based composites [38,39].
In the study of previous literature, the importance of nano-metallic oxide particles in improving various characteristics of cement-based materials has been apparent. Although a significant amount of literature has shown the role of nano-metallic oxide particles in altering the properties of traditional cement-based materials, there is a limited amount of literature on the effects of different types of nano-metallic oxide particles under the same experimental conditions and evaluation indicators. In addition, the vast majority of experimental research on cement-based materials is concentrated on one individual type of nano-metallic oxide particle instead of investigating the comparative studies on the effects of different nano-metallic oxide particles on cement-based materials properties, especially in the long-term. In order to complement and perfect this knowledge gap, this study investigated the comparative effects of five kinds of nano-metallic oxide particles, namely nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO, on the mechanical properties and durability of cement mortar, especially, the flexural strength, compressive strength, drying shrinkage, permeability, porosity, and microstructure which were required to be supplemented and improved with further experimental investigation. In addition, SEM microscopic methods were used to analyze the modification mechanism and differences. The research outcomes can be applied to modify the long-term mechanical properties and durability, thus improving the performance of cement-based materials.

2. Materials and Methods

2.1. Experimental Materials

Ordinary Portland cement (P·O42.5R), produced by the XiNan cement factory, was used in this study. The physical properties of cement are listed in Table 1 and the chemical composition of cement is illustrated in Table 2. Cement mortar was produced by standard sand, which was provided by China’s Xia Men ISO Standard Sand Co., Ltd. in Xiamen City, Fujian Province, China. Five common nano-metallic oxide particles were selected and are shown in Figure 1, which were nano-MgO (NM), nano-Al2O3 (NA), nano-ZrO2 (NR), nano-CuO (NC), and nano-ZnO (NZ), purchased from CS New Materials Co., Ltd. in Beijing, China; their chemical compositions are illustrated in Table 2. The physical properties of five kinds of the nano-metal oxides are shown in Table 3. Superplasticizer was employed to provide a satisfactory workability, which came from Laiyang Hongyang construction admixture factory of China. The water was laboratory tap water.

2.2. Sample Preparation of Cement Mortar with Nano-Metallic Oxide Particles

The tests mainly considered the influence of nano-metallic oxide particles (nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, nano-ZnO), and the dosage (1%, 2%, 4%, 8%) with a water-cement ratio of 0.4 and the polycarboxylic acid water-reducing agent determined at 1% according to the test mix provided in Table 4.
When preparing nano-MgO mortar specimens, first, the nano-MgO particles and the polycarboxylic acid water-reducing agent were added into 150 g of required water in a beaker and stirred evenly with 40 Hz in an ultrasonic water bath for 5 min to mix them well. Meanwhile, cement particles and standard sand were placed in the mixing pot and stirred for 2 min. The nano-MgO particles solution was slowly added after the cement and standard sand had been evenly mixed. Then, the remaining water was added in a beaker, after washing the beaker and glass rod, and poured into the blender again slowly. After all the water requirement was added, stirring continued for 3 min to obtain a uniformly agitated mixture. After stirring, the mortar was then poured into an oiled standard triplet specimen mold (40 mm × 40 mm × 160 mm), and vibrated to make good compaction. Mortar specimens were removed from the mold 24 h later and placed in the standard curing room, where they were maintained under the conditions of humidity above 95% and temperature set at 20 ± 2 °C. However, nano-ZrO2 specimens needed to be cured for 3 days before they could be removed; the other steps are consistent with the preparation of the nano-Al2O3 specimens, nano-ZrO2 specimens, nano-CuO specimens, and nano-ZnO specimens.

2.3. Test Methods

2.3.1. Flexural Strength Test

The flexural strength of cement mortar was determined according to the national standard GB/T 17671-2021 [40] at age of 28 days and 730 days, and a three-point bending test method was used at a loading rate of 100 N/s with a YAW-300D type testing machine (Changchun Kexin Test Instrument Co., Ltd., Changchun, China). The test specimen sizes were 160 mm × 40 mm × 40 mm, and the average value of three specimens in each group was taken as the final value of the group test.

2.3.2. Compressive Strength Test

After the flexural strength test, the test specimens of 160 mm × 40 mm × 40 mm were broken in two, near the middle. The two specimens were tested according to the national standard GB/T 17671-2021 [40] with a contact area of 40 mm × 40 mm. All the test specimens were loaded to failure by a YAW-300D type testing machine with a loading rate of 0.5 Mpa/s, and the average value of six specimens in each group was taken as the final value of the group test.

2.3.3. Porosity Test

Specimens with a size of 10 mm × 40 mm × 40 mm were cut with a size of 160 mm × 40 mm × 40 mm after curing for 28 days and 730 days. First, the test specimen was immersed in water, and then vacuum was applied to exhaust the internal gas. Second, its mass (mx) suspended in water was weighed with a hydrostatic balance and then the test specimen was taken out to measure its saturated surface dry mass (ms). Finally, it was dried in an oven. The drying oven temperature was adjusted to 50 °C. The test specimen was baked to a constant weight, and its mass (md) measured. The porosity calculation expression was as the following Formula (1), and the average value of three specimens in each group was taken as the final value of the group test.
p = ( m s − m d ) / m s − m x × 100 %
where p was the porosity of the specimen, %; ms was the saturated water quality of the specimen, g; md was the completely dry mass of the test piece, g; mx was the floating weight of the saturated water specimen, g.

2.3.4. Drying Shrinkage Test

Specimens with a size of 280 mm × 25 mm ×25 mm, were molded according to the mixing ratio in Table 3, and then the shrinkage value of curing for 28 days and 730 days was tested by referring to the ”cement mortar drying shrinkage experiment method” (JC/T 603-2004) [41]. After natural curing for 1 day, the mortar specimen was dismantled, and immediately moved into a dry environment (temperature 20 ± 3 °C, relative humidity 50 ± 3% for curing. When the specimen was cured in a dry environment for 4 h, the length of mortar measured by a specific length meter was taken as the initial length. After curing for 28 days and 730 days, the length of mortar was tested, and the difference between the length of mortar and the initial length was calculated, then the average value of three specimens in each group was taken as the final value of the group test.

2.3.5. Permeability Test

Specimens with a size of 100 mm × 50 mm (d × h) which were cut with a size of 100 mm × 200 mm after curing for 3 days were taken from the curing room and dried in an oven at 105 °C to keep the quality constant. The side of the specimen was sealed with wax to keep its one-dimensional penetration, then it was immersed in water and the cutting surface was submerged in 10 mm of water. After the specimen was saturated with water, it was taken out and weighed. Finally, the capillary permeability coefficient was calculated according to the following Formula (2), and the average value of three specimens in each group was taken as the final value of the group test.
k = Q 2 / ( A 2 · t )
where k was the capillary permeability coefficient, cm2/s; Q was the amount of water absorbed, cm3; t was the time elapsed, s; A was the area of the specimen in contact with water, cm2.

2.3.6. SEM Experiments

Specimens with a size of 5 mm × 10 mm were cut with a size of 160 mm × 40 mm × 40 mm after curing for 28 days and 730 days. First, the test specimen was immersed in alcohol for termination of hydration. Second, it was dried in an oven at 50 °C for 24 h and then was taken out to be sprayed gold in a vacuum. Finally, the test specimen was tested with a TESCAN MIRA LMS scanner (Tescan, Prague, Czech) and OXFORD Xplore spectrometer. Its resolution was 1.2 nm@30 keV and 3.5 nm@1 keV, acceleration voltage was 200 eV–30 keV and probe beam current was 3pA-20nA; the stability was better than 0.2%/h.

3. Results and Discussion

3.1. Compressive Strength

Figure 2a,b shows the effect of the content of five nano-metallic oxide particles on the compressive strength of cement mortar. From Figure 2, it can be seen that the compressive strength of cement mortar containing nano-metallic oxide particles at the age of 28 days and 730 days, shows a downward trend after first rising, and the change of the peak compressive strength depends on the content of the five nano-metallic oxide particles.
As expected, adding nano-metallic oxide particles resulted in a higher compressive strength at 28 and 730 days than that of control group without the addition of nano-metallic oxide particles. As a control group, the compressive strength is 33.1 MPa at 28 days and 51.2 Mpa at 730 days. The optimum dosage of nano-CuO is 2%, and when the dosage of nano-CuO is less than 2%, the compressive strength of the test specimen increases with the amount of the nano-CuO. Inversely, the compressive strength reduces sharply. The compressive strength of test specimen with 2% nano-CuO is 50.2 Mpa at 28 days and 66.4 Mpa at 730 days, which represent an increase of 51.66% and 28.69%, respectively, compared with the control group. The peak value of the compressive strength of the nano-ZnO group appears at 1%, which is 45.2 Mpa at 28 days and 67.2 Mpa at 730 days, an increase of 36.56% and 31.25% compared with the control group. The peak value of compressive strength of the nano-Al2O3 group appears at 1%, which is 52.1 Mpa at 28 days and 68.6 Mpa at 730 days, which represent an increase of 57.40% and 33.98%, respectively, compared with the control group. The peak values of compressive strength of the nano-MgO group, which is 52.9 Mpa at 28 days and 70.2 Mpa at 730 days, appears at 1%, compared with the control group, the compressive strengths at 28 days and 730 days increase by 59.82% and 37.11%, respectively. The optimum dosage of nano-ZrO2 is 2%, and when the dosage of nano-ZrO2 is more than 2%, the compressive strength of the test specimen reduces with the amount of the nano-ZrO2. Conversely, the compressive strength increases sharply. The compressive strength of the specimen with 2% nano-ZrO2 is 59.9 Mpa at 28 days and 78.9 Mpa at 730 days, which represent an increase of 80.97% and 54.10%, respectively, compared with the control group. In the case of the optimal dosage, the compressive strength of nano-ZnO, nano-Al2O3, and nano-MgO group are very close at the age of 28 days, with a difference of only 2.7 Mpa. While the compressive strength of nano-CuO, nano-ZnO, nano-Al2O3, and nano-MgO group are also very close at the age of 730 days, with a difference of only 3.8 Mpa. The highest compressive strength shows a decreasing trend of nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-CuO > nano-ZnO at 28 days and nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-ZnO > nano-CuO at 730 days, respectively. On adding 1–8% nano-metallic oxide particles, the compressive strengths of cement mortar are higher than that of the control group. These results indicate that nano-metallic oxide particles can significantly improve the compressive strength of cement mortar, which can be ascribed to the decrease of the porosity content, as discussed later.

3.2. Flexural Strength

Figure 3a,b shows the effect of the content of five nano-metallic oxide particles on the flexural strength of cement mortar. Similar to the change trend of compressive strength of cement mortar with the increase of the five nano-metallic oxide particles additive content, namely, the five series also show a downward trend after rising first. Adding 1–8% of nano-metallic oxide particles, the flexural strengths of cement mortar are higher than that of the control group, however, the highest flexural strength depends on the content of the five nano-metallic oxide particles. The optimum dosage appears at 1% for nano-ZnO group, nano-MgO group, and nano-Al2O3 group and 2% for nano-ZrO2 group and nano-CuO group. At 28 days, the peak value of the flexural strength in the ano-ZrO2 group is 9.2 Mpa, in the nano-MgO group 8.9 Mpa, in the nano-Al2O3 group 8.4 Mpa, in the nano-CuO group 7.9 Mpa, in the nano-ZnO group 7.4 Mpa, increases of 33.33%, 28.99%, 21.74%, 14.49%, and 11.59%, respectively, compared with the flexural strength of the control group, which is 6.9 Mpa. At 730 days, the peak value of the flexural strength in the ano-ZrO2 group is 18.9 Mpa, in the nano-MgO group 18.2 Mpa, in the nano-Al2O3 group 17.3Mpa, in the nano-ZnO group 17.1 Mpa, in the nano-CuO group 16.7 Mpa, increases of 36.96%, 31.88%, 25.36%, 23.91% and 21.01%, respectively, compared with the flexural strength of the control group, which is 13.8 Mpa. Therefore, these results also indicate that nano-metallic oxide particles can significantly improve the flexural strength of cement mortar.
In order to understand the enhancement effect of the five nano-metallic oxide particles on the mechanical properties of cement mortar at 28 and 730 days, their peak strengths were plotted and shown in Figure 4. Longitudinal comparison of the five nano-metallic oxide particles shows they mainly act on compressive strength improvement at 28 days and flexural strength improvement at 730 days, which are significantly higher than the compressive strengths at 730 days and the flexural strengths at 28 days, especially the 28 days compressive strength, which has the greatest effect. Therefore, there are differences among the five nano-metallic oxide particles: the compressive strength percentage increase shows a decreasing trend of nano-ZrO2 > nano-MgO > nano-Al2O3 > nano-CuO > nano-ZnO at 28 days, and the compressive strength percentage increase of the nano-ZrO2 group at 730 days is higher than the other nano-metallic oxide particles group.

3.3. Porosity

The size of the porosity of a cementitious material directly reflects the degree of denseness of the cementitious material, and materials with the same porosity can differ in their pore characteristics. After adding nano-CuO, nano-ZrO2, nano-MgO, nano-Al2O3, and nano-ZnO, the porosity of the cement mortar is shown in Figure 5.
From Figure 5, it can be seen that the porosity of cement mortar containing nano-metallic oxide particles at the age of 28 days and 730 days, shows a downward trend and rise with the increase of the content of nano-metallic oxide particles. As expected, adding 1–8% nano-metallic oxide particles resulted in lower porosity at 28 and 730 days than that of the control group without the addition of nano-metallic oxide particles. The minimum porosity appears at 4% for the five nano-metallic oxide particles group, however, the reduction ratio is different, which depends on the characteristics of the nano-metallic oxide particles. At 28 days, the peak value of porosity in the nano-ZrO2 group is 16.3%, in the nano-MgO group 16.9%, in the nano-Al2O3 group 17.2%, in the nano-CuO group 17.6%, in the nano-ZnO group 18.3%, decreases of 16.84%, 13.78%, 12.24%, 10.20% and 6.63%, respectively, compared with the porosity of the control group, which is 19.6%. At 730 days, the peak value of porosity in the nano-ZrO2 group is 6.2%, in the nano-MgO group 6.4%, in the nano-Al2O3 group 6.7%, in the nano-ZnO group 6.9%, in the nano-CuO group 7.3%, decreases of 26.19%, 23.81%, 20.24%, 17.86% and 13.10%, respectively, compared with the porosity of the control group, which is 8.4%. With increasing the five nano-metallic oxide particles dosage above 4%, the improvement in the porosity is weakened, which can be due to the distance between the nano-metallic oxide particles decreasing along with the increasing nano-metallic oxide particles dosage. The Ca(OH)2 crystals cannot grow sufficiently which can be attributed to the limited distance and space and the decreased crystal amount, which results in the mechanical properties and permeability of cement mortar decreasing, and drying shrinkage increasing, so that the pore structure of the cement mortar is loosened relatively. All in all, adding 1–8% nano-metallic oxide particles, can improve the pore structure of cement mortar. First, the nano-metallic oxide particles can act as a nano filler to improve the density of the cement mortar, which results in the porosity of cement mortar being decreased markedly. Second, because nano-metallic oxide particles have high surface activity, which not only act as a catalyst to accelerate cement hydration, but also act as a nucleus in the cement mortar to refine the size and distribution of the Ca(OH)2 crystals. In addition, the most important aspect is that part of the nano-metallic oxide particles, such as, nano-Al2O3 particles can react with Ca(OH)2 crystals to generate Ca[Al(OH)4]2, and nano-MgO particles hydration can form rodlike Mg(OH)2 crystals in the hydration reaction of the cement system; in turn the quantity of Ca(OH)2 crystal is decreased, the compactness of cement mortar is improved, and then the porosity is decreased. Therefore, nano-metallic oxide particles can significantly improve the porosity of cement mortar, which improves the compressive strength, flexural strength, and permeability of cement mortar that can be attributed to the fineness of the nano-metallic oxide particles being smaller than that of cement mortar.

3.4. Drying Shrinkage

Drying shrinkage is a common deformation phenomenon, in which cement mortar contractions attribute to the loss of capillary water, resulting in an increase in tensile stress that may cause cracking, internal warping, and external deflection. Therefore, drying shrinkage is an important index of durability in cement mortar. The drying shrinkage of cement mortar containing the five nano-metallic oxide particles is discussed, as shown in Figure 6.
Figure 6a,b shows that the five nano-metallic oxide particles favor drying shrinkage decreases, and the trend of drying shrinkage of cement mortar along with the amount of nano-metallic oxide particles is varying. The minimum drying shrinkage appears at 8% dosage for the five nano-metallic oxide particles at 28 and 730 days, just as with the mechanical properties and porosity, there is a consistent decreasing trend, but it is relatively slow with the dosage. At 28 days, the minimum drying shrinkage has a decrease of 53.66% for nano-ZrO2 group, 50.00% for nano-MgO group, 46.34% for nano-Al2O3 group, 39.02% for nano-CuO group, and 43.90% for nano-ZnO group, respectively, compared with the drying shrinkage of the control group. At 730 days, the minimum drying shrinkage has a decrease of 53.97% for nano-ZrO2 group, 51.59% for nano-MgO group, 49.21% for nano-Al2O3 group, 44.44% for nano-CuO group, and 47.62% for nano-ZnO group, respectively, compared with the drying shrinkage of the control group. From the drying shrinkage of 28 days and 730 days, it was found that the drying shrinkage value also increases with the extension of the curing age, and the modification effect of the five nano-metallic oxide particles also increases, but the relative increase is small. The longitudinal comparison of drying shrinkage of the five nano-metallic oxide particles shows that the drying shrinkage of the nano-ZrO2 group is lower than that of other groups as a whole, meaning that the filling effect of the nano-ZrO2 on the micropores is better than that of the others. The drying shrinkage shows a decreasing trend of nano-CuO > nano-ZnO > nano-Al2O3 > nano-MgO > nano-ZrO2 at 28 and 730 days, but this difference is relatively small, which is opposite to the mechanical properties trend. On the whole, the larger the dosage of the five nano-metallic oxide particles, the smaller is the drying shrinkage of cement mortar, therefore, as expected, adding nano-metallic oxide particles results in lower drying shrinkage at 28 and 730 days compared to that of control group. This phenomenon shows that the internal micropores of cement mortar can be filled by nano-metallic oxide particles, which changes the distribution law of pore size, increases the ability to hold water, and reduces the drying shrinkage. Meanwhile, nano-metallic oxide particles have a strong water absorption capacity, which can maintain water in the pores and reduce the drying shrinkage of cement mortar. Therefore, the larger the content of nano-metallic oxide particles, the smaller is the drying shrinkage of the cement mortar.

3.5. Permeability

Permeability of cement mortar is the ability of cement mortar to resist the penetration of water, air, and other substances to enter the cement mortar matrix and enable these substances to enter or depart the pores of the cement mortar. The permeability of cement mortar containing the five nano-metallic oxide particles was calculated, as shown in Figure 7.
Figure 7 shows that the five nano-metallic oxide particles favor permeability strengthening, and the trend of permeability of cement mortar with the amount of nano-metallic oxide particles is also varying. The minimum permeability appears at 1% for nano-ZnO group, nano-MgO group, and nano-Al2O3 group and 2% for nano-ZrO2 group and nano-CuO group, just the same as for strength, but it reduces first and then increases with the content, which is contradictory to the trend of strength. At 28 days, the minimum permeability has decreases of 33.70% for nano-ZrO2 group, 27.83% for nano-MgO group, 25.00% for nano-Al2O3 group, 15.87% for nano-CuO group, and 12.83% for nano-ZnO group, respectively, compared with the permeability of the control group. At 730 days, the minimum permeability has a decrease of 34.59% for nano-ZrO2 group, 28.50% for nano-MgO group, 25.00% for nano-Al2O3 group, 16.45% for nano-CuO group, and 19.43% for nano-ZnO group, respectively, compared with the permeability of the control group. Compared with the permeability of cement mortar at 28 and 730 days, it is not difficult to see that the 730 days penetration of cement mortar is nearly 100 times lower than that of 28 days. Therefore, as expected, adding 1–8% nano-metallic oxide particles and advancing the curing age result in lower permeability of cement mortar. This phenomenon is mainly attributed to the decrease of porosity and the increase of compactness of the cement mortar. However, the internal agglomeration phenomenon of nano-metallic oxide particles is extreme in the case of 8% addition owing to the superabundant dosage, as discussed later by SEM, thus the permeability of the cement mortar is decreased dramatically.

3.6. Microstructure Analysis

To study the effect of the five nano-metallic oxide particles on the microstructure of cement mortar, the substitution rate of the five nano-metallic oxide particles was 0%, 1%, 2%, 4%, and 8% and the curing time 730 days. The scanning experimental results are shown in Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13. As can be seen from Figure 8a,b, the interface of the control group is comparatively loose and porous with the size of 5 μm, while with the graph magnified to the size of 1 μm, a significant amount of needle-like ettringite and hydrated calcium silicate gel and a trace of portlandite are detected in the sample; its interface is also porous.
On adding 1–8% nano-Al2O3 particles, the interface of the sample is comparatively dense with only a trace of pores compared to that of the control group, demonstrating nano-Al2O3 particles can play a vital filling role for the micropores of cement mortar. As can be seen in Figure 9a, its hydration products have needle-like ettringite, hydrated calcium silicate gel, calcium aluminate hydrate, short-rod gypsum, combined with the absence of portlandite. From Figure 9b–d, it can be seen that the morphology of hydration products is not obvious with the increase of dosage, but the particle size is larger than that of a single nano-Al2O3 particle, which demonstrates that nano-Al2O3 particles can play the role of the crystal nucleus in hydrolysis. Therefore, when the dosage of nano-Al2O3 particle exceeds 4%, the dispersion effect of the nano-Al2O3 particle is relatively poor due to the agglomeration phenomenon, and some agglomerated particle sizes even reach micron level; this leads to the increase of porosity and the decrease of mechanical properties [42,43,44]. On the whole, nano-Al2O3 particles can accelerate the reaction rate and consume the sulphate SO42− to accelerate the formation of the hydration products, following by the unreacted nano-metal oxides which can crowd the reaction space and stop the reaction [42], thus improving the mechanical properties of the cementitious material. In addition, nano-Al2O3 particles can react with calcium hydroxide to generate calcium aluminate hydrate at early age [43,44], which can accelerate the reaction of the cement and contribute to strength.
On adding 1–8% nano-CuO particles, the interface of the sample is comparatively dense with a small amount of pores compared to that of control group, but it is also worse than that of the nano-Al2O3 group. Thus, the filling role of nano-CuO particles is poorer than that of nano-Al2O3 group, resulting in increased porosity and decreased mechanical properties contrasting with the nano-Al2O3 group. In all nano-CuO groups, needle-like ettringite and nano-CuO particles stick together and can be seen as microaggregate, which can play an important filling role for the micropores of cement mortar. Especially when the dosage is 8%, the agglomeration phenomenon is severe, but the agglomeration particles also adhere to the ettringite, resulting in a poor filling effect, which has a negative influence on the porosity and mechanical properties of the cement mortar. Generally speaking, the nano-CuO particles are unable to participate in the reaction with Ca(OH)2, but can accelerate C–S–H gel formation in the cement system, as a result of the increased crystalline Ca(OH)2 amount at the early ages of hydration [14]. This makes the cement mortar more homogeneous and compact. Consequently, the pore structure of cement mortar is clearly improved by nano-CuO particles, which causes shifting of the distributed pores of the cement mortar from harmful to lower harm, even harmless pores [45]. Therefore, the mechanical properties and durability of cement mortar are improved after adding nano-CuO particles.
On adding 1–8% nano-ZnO particles, the interface of the sample is comparatively dense and only a thimbleful of pores compared to that of the nano-CuO group, but better than that of the nano-CuO group. Thus, the filling role of nano-ZnO particles is superior to nano-CuO at 730 days, resulting in decreased porosity and increased mechanical properties compared with the nano-CuO group. In all the nano-ZnO groups, a small amount of needle-like ettringite and fibrous hydrated calcium silicate gel, combined with the absence of portlandite, are observed. In addition, although the agglomeration phenomenon of nano-ZnO particles is extreme when the dosage is 4%, the interface of the sample is comparatively dense, which is especially obvious when the dosage is 8%. In general, Zn ions in nano-ZnO particles can stimulate the reactions of C3A and produce a small amount of ettringite (Aft) which will help the continually development of cement hydration products [46]. Furthermore, nano-ZnO particles can prolong the cement setting time and have a negative influence on cement mortar early strength, but can improve cement mortar long-term strength [47]. For this reason, the mechanical properties are lower than that of the nano-CuO group at 28 days, but the opposite trend at 730 days is shown.
On adding 1–8% nano-MgO particles, the interface of the sample is relatively dense and porous, but a great deal of hydration products are observed, such as rod-like Mg(OH)2 crystals, needle-like ettringite, and fibrous hydrated calcium silicate gel, which improves the compactness of the interface and decreases the porosity, especially when the dosage is 4%. Just like nano-CuO, nano-ZnO and nano-Al2O3, the agglomeration phenomenon also exists in the nano-MgO group, but the nano-MgO particles are surrounded by hydration product Mg(OH)2 crystals attributed to the hydration of nano-MgO particles, and stuck together with ettringite that can act as microaggregate, and can also bring into play an important filling role for the micropores of the cement mortar. Overall, the nano-MgO particles hydration can form rod-like Mg(OH)2 crystals in the hydration reaction of the cement system, which can grow in the interface transition zone of cement mortar [48]. However, the size of the Mg(OH)2 crystal particle is small, although it can compact the structure of the cement stone that effectively improves the bearing and deformation capacities of the structure, in turn increasing the bonding force in the interface of the cement mortar, although the bonding force is small [49]. Therefore, in this experiment, the mechanical properties of nano-MgO group at 28 and 730 days are lower than those of the nano-ZrO2 group.
On adding 1–8% nano-ZrO2 particles, the interface of the sample is comparatively dense with a small quantity of pores compared to that of other groups, especially when the dosage is 1%. Meanwhile, just like the other nano-metal oxide particles, the agglomeration phenomenon also exists in the nano-ZrO2 group, but its particle size is relatively small. What is more, in the nano-ZrO2 group, a small amount of needle-like ettringite and fibrous hydrated calcium silicate gel and layered portlandite crystal are observed, and needle-like ettringite and nano-ZrO2 particles are stuck together closely, which can be as a microaggregate with the purpose of a filling role for the micropores of the cement mortar. Therefore, nano-ZrO2 particles are easily dispersed in the cement system, which is beneficial to the fine pores and improves compactness, and in turn, improves the mechanical properties and durability of the mortar. All in all, the nano-ZrO2 particles cannot accelerate the hydration process of cement, but they can fill in the pores of cement mortar, which leads to the reduction of the growth space of the calcium hydroxide crystals of the cement, thus the size of portlandite crystals will be reduced [50]. Furthermore, nano-ZrO2 particles can reduce the portlandite crystal orientation to improve the microstructures. Thus, adding high purity (99.9%) and a high fineness value (30 nm) of the nano-ZrO2 particles are important to enhance the mechanical properties of the cement mortars [51]. Therefore, in this experiment, the mechanical properties and durability are superior.

4. Conclusions

In the present research, the effects of five nano-metallic oxide particles (nano-CuO, nano-ZrO2, nano-Al2O3, nano-MgO, and nano-ZnO) on the long-term mechanical properties and durability of cement mortar were discussed, the experimental results can be summarized as follows:
(1) The dosage of nano-MgO, nano-Al2O3, nano-ZrO2, nano-CuO, and nano-ZnO is 2%, 1%, 1%, 1%, and 2%, the 730 days compressive strength increased by 28.69%, 31.25%, 33.98%, 37.11%, and 54.10%, respectively. On the whole, adding the five nano-metallic oxide particles can observably improve the compressive strength of cement mortar, while the nano-ZrO2 group has the best strength enhancement effect on cement mortar and a similar rule applies for the flexural strength.
(2) The 730 days porosity of nano-CuO group, nano-ZnO group, nano-Al2O3 group, nano-MgO group, and nano-ZrO2 group decreased by 26.19%, 23.81%, 20.24%, 17.86%, and 13.10%, respectively. This is mainly because nano-metallic oxide particles can act as a nano filler and they themselves have high surface activity, which can be act as a nucleus in cement mortar to refine the size and distribution of the Ca(OH)2 crystals. Furthermore, due to the decrease of porosity, the permeability also decreased.
(3) The 730 day drying shrinkage of the nano-CuO group, nano-ZnO group, nano-Al2O3 group, nano-MgO group, and nano-ZrO2 group decreased by 44.44%, 47.62%, 49.21%, 51.59%, and 53.97%, respectively. This is mainly because nano-metallic oxide particles have a strong water absorption capacity, which can maintain water in the pores and reduce the drying shrinkage of the cement mortar.
(4) Nano-metallic oxide particles can promote cement hydration and refine the size and distribution of Ca(OH)2 crystals, but the specific principles are different. On the whole, nano-Al2O3 particles can consume sulphate SO42- and react with calcium hydroxide to generate calcium aluminate hydrate at an early age. Nano-CuO particles are unable to participate in the reaction with Ca(OH)2, but can accelerate CSH gel formation in the cement system, as a result of the increased crystalline Ca(OH)2 amount at the early ages of hydration. Zn ions in nano-ZnO particles can stimulate the reactions of C3A and produce a small amount of ettringite (AFt) which will help the continual development of cement hydration products. Nano-MgO particle hydration can form rod-like Mg(OH)2 crystals in the hydration reaction of the cement system, which can grow in the interface transition zone of the cement mortar. The nano-ZrO2 particles cannot accelerate the hydration process of cement, but can fill in the pores of the cement mortar, which leads to the reduction of the growth space for the calcium hydroxide crystals of the cement, thus the size of the portlandite crystals are reduced.
(5) Nano-metal oxide particles can play a filling role and improve the compactness and porosity of the cementitious materials, thus achieving the purpose of improving the long-term mechanical properties and durability of cementitious material.

Author Contributions

Conceptualization, S.D. and F.W.; methodology, Q.Z. and S.Y.; software, L.X. and L.L.; vali dation, Q.Z., L.L., L.X. and L.C.; formal analysis, L.X.; investigation, L.X. and S.Y.; resources, L.X. and S.Y.; data curation, F.W. and S.Y.; writing—original draft preparation, S.D.; writing—review and editing, S.D. and S.Y.; visualization, F.W.; supervision, Q.Z.; project administration, F.W.; funding acquisition, F.W. and L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study is supported by Guizhou Provincial Department of Education General undergraduate University Scientific Research Project (Youth Project Qian Jiaoji [2022] 128), the Special Project of Scientific Research on the Introduction of Talents in Huzhou Vocational and Technical College (No.2022GY04), Guizhou Science and Technology support Plan Project (Qianke Joint Support [2021] General 486). The authors are extremely grateful for the support of the School of Materials and Architectural Engineering of Guizhou normal university.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors appreciate the aid provided by the reviewers and editors to improve the paper.

Conflicts of Interest

The authors have no conflict of interest about this manuscript entitled “Comparative Study on the Effects of Five Nano-metallic Oxide Particles on the Long-term Mechanical Property and Durability of Cement Mortar”, and this manuscript has not been published before.

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Figure 1. Sample of nano-metallic oxide particles: (a) NM; (b) NA; (c) NR; (d) NZ; (e) NC.
Figure 1. Sample of nano-metallic oxide particles: (a) NM; (b) NA; (c) NR; (d) NZ; (e) NC.
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Figure 2. Compressive strength of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
Figure 2. Compressive strength of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
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Figure 3. Flexural strength of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
Figure 3. Flexural strength of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
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Figure 4. The percentage increase of strength at 28 and 730 days.
Figure 4. The percentage increase of strength at 28 and 730 days.
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Figure 5. Porosity of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
Figure 5. Porosity of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
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Figure 6. Drying shrinkage of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
Figure 6. Drying shrinkage of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
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Figure 7. Permeability of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
Figure 7. Permeability of cement mortar with nano-metallic oxide particles: (a) 28 days; (b) 730 days.
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Figure 8. Microstructure of pure cement mortar: (a) 5 μm; (b) 1 μm.
Figure 8. Microstructure of pure cement mortar: (a) 5 μm; (b) 1 μm.
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Figure 9. Microstructure of cement mortar with NA: (a) 1% NA; (b) 2% NA; (c) 4% NA; (d) 8% NA.
Figure 9. Microstructure of cement mortar with NA: (a) 1% NA; (b) 2% NA; (c) 4% NA; (d) 8% NA.
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Figure 10. Microstructure of cement mortar with NC: (a) 1% NC; (b) 2% NC; (c) 4% NC; (d) 8% NC.
Figure 10. Microstructure of cement mortar with NC: (a) 1% NC; (b) 2% NC; (c) 4% NC; (d) 8% NC.
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Figure 11. Microstructure of cement mortar with NZ: (a) 1% NZ; (b) 2% NZ; (c) 4% NZ; (d) 8% NZ.
Figure 11. Microstructure of cement mortar with NZ: (a) 1% NZ; (b) 2% NZ; (c) 4% NZ; (d) 8% NZ.
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Figure 12. Microstructure of cement mortar with NM: (a) 1% NM; (b) 2% NM; (c) 4% NM; (d) 8% NM.
Figure 12. Microstructure of cement mortar with NM: (a) 1% NM; (b) 2% NM; (c) 4% NM; (d) 8% NM.
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Figure 13. Microstructure of cement mortar with NR: (a) 1% NR; (b) 2% NR; (c) 4% NR; (d) 8% NR.
Figure 13. Microstructure of cement mortar with NR: (a) 1% NR; (b) 2% NR; (c) 4% NR; (d) 8% NR.
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Table
Table 1. Physical properties of cement.
Table 1. Physical properties of cement.
Cement TypeStandard ConsistencyDensitySoundness of Cement
P·O42.5R26.8%3.05 g/cm3Qualified
Table
Table 2. Chemical composition of ordinary Portland cement and nano-metal oxides.
Table 2. Chemical composition of ordinary Portland cement and nano-metal oxides.
ElementSiO2Al2O3Fe2O3CaOMgOSO3Na2OK2OZrO2CuOZnO
Cement/wt%27.56.594.8954.872.692.660.460.34---
NM/wt%----99.9------
NA/wt.%-99.9---------
NR/wt.%--------99.9--
NC/wt.%---------99.9-
NZ/wt.%----------99.9
Table
Table 3. The physical properties of nano-metallic oxides.
Table 3. The physical properties of nano-metallic oxides.
NanomaterialsNMNANRNCNZ
ColorWhiteWhiteWhiteBlackWhite
Density(g/cm3)3.583.906.06.495.60
Purity (%)99.999.999.999.999.9
Fineness (nm)3030303030
Table
Table 4. Mixture proportions/g.
Table 4. Mixture proportions/g.
No.CementNCNRNMNANZStandard SandWater
NC-0550.00 1100220
NC-1544.55.5 1100220
NC-2539.011 1100220
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MDPI and ACS Style

Dong, S.; Yu, S.; Chen, L.; Zhuo, Q.; Wu, F.; Xie, L.; Liu, L. Comparative Study on the Effects of Five Nano-Metallic Oxide Particles on the Long-Term Mechanical Property and Durability of Cement Mortar. Buildings 2023, 13, 619. https://doi.org/10.3390/buildings13030619

AMA Style

Dong S, Yu S, Chen L, Zhuo Q, Wu F, Xie L, Liu L. Comparative Study on the Effects of Five Nano-Metallic Oxide Particles on the Long-Term Mechanical Property and Durability of Cement Mortar. Buildings. 2023; 13(3):619. https://doi.org/10.3390/buildings13030619

Chicago/Turabian Style

Dong, Shuangkuai, Songhan Yu, Liangliang Chen, Qi Zhuo, Fufei Wu, Lilan Xie, and Liuyan Liu. 2023. "Comparative Study on the Effects of Five Nano-Metallic Oxide Particles on the Long-Term Mechanical Property and Durability of Cement Mortar" Buildings 13, no. 3: 619. https://doi.org/10.3390/buildings13030619

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