Mechanical Properties of High Strength Concrete Containing Nano SiO 2 Made from Rice Husk Ash in Southern Vietnam

: This paper presents the experimental results of the production of Nano-SiO 2 (NS) from rice husk ash (RHA) and the engineering properties of High Strength Concrete (HSC) containing various NS contents. Firstly, the mesoporous silica nanoparticles were effectively modulated from RHA using NaOH solution, and subsequently precipitated with HCl solution until the pH value reached 3. The optimum synthesis for the manufacture of SiO 2 nanoparticles in the weight ratio of RHA/NaOH was 1:2.4, and the product was calcined at 550 ◦ C for 2 h. The EDX, XRD, SEM, TEM, FT-IR, and BET techniques were used to characterize the NS products. Results revealed that the characteristics of the obtained NS were satisfactory for civil engineering materials. Secondly, the HSC was manufactured with the aforementioned NS contents. NS particles were added to HSC at various replacements of 0, 0.5, 1.0, 1.5, 2.0, and 2.5% by the mass of the binder. The water-to-binder ratio was remained at 0.3 for all mixes. The specimens were cured for 3, 7, 28, 25 days under 25 ± 2 ◦ C and a relative humidity of 95% before testing compressive and ﬂexural strengths. Chloride ion permeability was investigated at 28 and 56 days. Results indicated that the addition of NS dramatically enhanced compressive strength, ﬂexural strength, chloride ion resistance, and reduced chloride ion permeability compared to control concrete. The optimal NS content was found at 1.5%, which yielded the highest strength and lowest chloride ion permeability. Next, the development of ﬂexural and compressive strengths with an age curing of 3–28 days can be analytically described by a logarithmic equation with R 2 ≥ 0.74. The ACI code was used, and the compressive strength at t-day was determined based on 28 days with R 2 ≥ 0.95. The study is expected to solve the redundancy of waste RHA in southern Vietnam by making RHA a helpful additive when producing high-strength concrete and contributing meaningfully to a sustainable environment.


Introduction
Nano SiO 2 (NS) is well-known for its many uses in the past two decades, including in catalytic materials, dielectric materials, gas adsorbents, heavy metal ion adsorption, and inorganic carriers [1]. Besides this, it has been noted that most NS materials used in civil engineering were supplied by commercial companies, mainly from China and European countries [2]. Due to its being produced in a factory, the main qualities of commercial NS are high purity and uniformity; however, it is expensive and causes difficulty in wide use in actual construction. Thus, the current trend of locating a readily available, low-cost, silicon-rich source of materials for use as a concrete additive merits consideration and has attracted the attention academic scientific and technologists.
The rice-growing regions of the world, for example, China, India, Bangladesh, Brazil, and the far-East countries, increases daily, resulting in agricultural waste. Among them, increase the workability of the concrete mixtures [29]. The HSC used in the experiment included nano SiO 2 , micro SiO 2 , and nano + Micro SiO 2 , all of which were exposed to high temperatures. Subsequent studies found the following: The workability of NS concrete significantly decreased, and NS mixes require substantially more water. The amount of NS utilized was 5%, which was relatively high and increased water consumption. Additionally, most NS particles did not interact with the hydration products, which should be explored further. It is also worth noting that the NS was in an amorphous powdered condition, making mixing extremely difficult [30]. In conclusion, there was a substantial shift in the early interfacial transition zone structure when 3.0% of cement was substituted with NS. In addition, the presence of NS caused an increase in hydration heat and a decrease in CH content. Nano TiO 2 can also extend the time it takes for cement to hydrate. Shahbazpanahi et al. [31] reported that, with 0.5% content added to cement, a combination of 30% natural and 70% recycled coarse aggregates can produce sustainable concrete.
Concerning chloride ion permeability in concrete, recent studies have investigated the effect of Nanoparticles on chloride penetration [19,[32][33][34][35][36]. Li [33] added nano-SiO 2 , nano-CaCO 3 , and multi-walled carbon nanotubes to reinforced concrete and concluded that nanomaterials significantly decrease the chloride diffusion coefficients. Zhang and Li [19] revealed the pore structure and the resistance to chloride penetration that concrete possesses by adding nano-particles to the plain concrete. Similarly, autoclaved concrete significantly reduces the porosity and increases the chloride resistance by using XRD, TG-DTG, SEM, and MIP tests. However, to date, there is no research related to penetration chloride in HSC.
Based on the literature reviews mentioned earlier, the objective of this paper focused on producing NS from waste RHA, obtained in Southern Vietnam. The Sol-gel technique was employed to produce NS. Then, the physical and chemical properties of the obtained Nano-SiO 2 were tested and evaluated by various techniques, such as EDX, XRD, SEM, TEM, FT-IR, and BET. Secondly, the obtained NS was used as an additive replacement in HSC with various contents. To do this, HSC was prepared with the different replacement levels of 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% according to binder weight, maintaining the original silica fume (SF) content of 8% for all mixes. The water-to-binder ratio was kept at 0.3 for all the mixes. The mechanical and durability properties of HSC, such as compressive strength, flexural strength, and chloride ion permeability, were reported. The specimens were tested at 3, 7, 28, and 56 days of curing for compressive and flexural strengths, and 28 and 56 days for chloride ion permeability. Thirdly, based on the obtained results, some correlations and experimental coefficients among the obtained data were proposed, derived from this study. The results obtained from this study are expected to elucidate how NS could successfully be produced from waste RHA, reduce the agricultural waste, and help to successfully manufacture HSC. In addition, the results should also help to use waste material in construction materials, enriching the source of building materials and contributing to protecting the environment.   Tables 4 and 5 detail the physical characteristics and sieve analysis, respectively, following ASTM C33 [37] and ASTM C29 [38]. The coarse aggregate was a crushed stone collected from a local quarry and initially formed of basalt stone. The physical properties of coarse aggregate are presented in Table 6. The grain size distribution of coarse aggregate with a maximum dominant of 9.5 mm was shown in Table 7.

. Superplasticizers
Sika Viscocrete 3000-20 is a superplasticizer provided by the Sika Group. Its exceptional highwater reduction capacity allows for good fluidity while retaining optimal adhesion of the mixture. According to TCVN 8826:2011 [39], this admixture is suitable, with the established criteria, for chemical additions to concrete. Sika Viscocrete 3000-20, a 3rd generation polymer-based high-tech superplasticizer with excellent porosity and simple permeability of concrete, was utilized in the experiment.

Silica Fume
Silica Fume (SF) was provided by a local commercial company, and the chemical composition is reported in Table 8, following ASTM C1240-04 [40]. The specific gravity of SF is 2.2. SF is an amorphous and highly reactive pozzolan. The RHA used in this study was provided by a local commercial company in Southern Vietnam. RHA is a by-product material produced from manufacturing puffed rice, containing a large amount of iron oxide and silica, and the chemical compositions of rice husk. To obtain NS, RHA was first ground and sieved by a 2.0 mm sieve; the passing was collected and dried at 105 • C until a constant mass was reached. Then, 100 g RHA was added into a 1 L cup containing 6M NaOH solution, then stirred, swelled and heated for 4 h at 100 • C. To collect the solution, 4M HCl was added until the solution reached the value of pH~3. Next, the solution was filtered by distilled water and ethanol, then the solution was dried at 100 • C overnight and heated at 550 • C for 2 h to obtain NS. The efficiency of the NS-splitting process is determined by weighing the final product in 100 g of RHA, where more than 85% was found. The detailed properties of NS are presented in the following sections. Figure 1 illustrates the procedure of producing NS from RHA.

. Superplasticizers
Sika Viscocrete 3000-20 is a superplasticizer provided by the Sika Group. Its exceptional highwater reduction capacity allows for good fluidity while retaining optimal adhesion of the mixture. According to TCVN 8826:2011 [39], this admixture is suitable, with the established criteria, for chemical additions to concrete. Sika Viscocrete 3000-20, a 3rd generation polymer-based high-tech superplasticizer with excellent porosity and simple permeability of concrete, was utilized in the experiment.

Silica Fume
Silica Fume (SF) was provided by a local commercial company, and the chemical composition is reported in Table 8, following ASTM C1240-04 [40]. The specific gravity of SF is 2.2. SF is an amorphous and highly reactive pozzolan. The RHA used in this study was provided by a local commercial company in Southern Vietnam. RHA is a by-product material produced from manufacturing puffed rice, containing a large amount of iron oxide and silica, and the chemical compositions of rice husk. To obtain NS, RHA was first ground and sieved by a 2.0 mm sieve; the passing was collected and dried at 105 °C until a constant mass was reached. Then, 100 g RHA was added into a 1 L cup containing 6M NaOH solution, then stirred, swelled and heated for 4 h at 100 °C. To collect the solution, 4M HCl was added until the solution reached the value of pH~3. Next, the solution was filtered by distilled water and ethanol, then the solution was dried at 100 °C overnight and heated at 550 °C for 2 h to obtain NS. The efficiency of the NS-splitting process is determined by weighing the final product in 100 g of RHA, where more than 85% was found. The detailed properties of NS are presented in the following sections. Figure 1 illustrates the procedure of producing NS from RHA.

Mix Proportions and Sample Preparation
In theory, while developing HSC components using NS, it is crucial to ensure that the necessary requirements, such as concrete strength, mix flexibility, and material, are met.

Design Standards and Techniques
The study used the method of ACI 211.4R-08 [41] to design the HSC component. The ACI method: The component-designing phases of HSC utilizing NS were carried out according to the manufacturer's specifications. The slump of the concrete mixture was modified to ensure the needed slump.

Mix Proportions
The ACI specification was used to calculate and design the concrete composition with a specific strength of 60 MPa. SF was used in concrete to improve the strength and reduce the cement content [42]. The original binder was made of 92% cement and 8% SF for the control mix, referring to previous studies [42][43][44], Then, NS was utilized in various ratios in the gradation components, including 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% of the total quantity of the binder. The water-to-binder ratio was kept at 0.3 for all the mixes. The mix proportion of six mixes is presented in Table 9. It can be seen that the superplasticizer (SP) gradually increased to control the slump value of mixtures at 4 ± 1 cm.

Specimen Preparation and Testing Procedures
NS has a high surface area and nanoscale particle size, and is difficult to disperse in concrete mixes. The following steps are experimental mixing procedures for making homogeneous, stable concrete, as presented in Figure 2. There are five steps to mixing the sample, including: Step 1: NS was combined with 60% water and vigorously stirred to evenly distribute the NS particles; Step 2: A combination of sand, crushed stone, cement, and silica fume was mixed for three minutes; Step 3: A total of 20% water was added to the sand, crushed stone and cement mixture, and stirred thoroughly for one minute; Step 4: the mixture containing NS and 60% water in step 1 was added to the Step 3 mixture and mixed for minutes; Step 5: The solution containing the remaining 20% water and superplasticizer was incorporated in the Step 4 mixture for 3 min until homogenous; Step 6: Mixer was temporarily stopped for 2 min to enable the superplastic ingredient to react, which will improve the result; Step 7: The mixture was continuously mixed for another 3 min to avoid slumps and guarantee homogeneity.
A 60-L mixer was utilized to mix the composition. A compressive strength test on a cylindrical specimen with a dimension of 150 mm × 300 mm (d × h), a bending test with a beam of 150 mm × 150 mm × 600 mm, and a chlorine ion permeability with a specimen of 100 mm × 200 mm (d × h) are among the available test specimens. The inside surface of the Crystals 2021, 11, 932 7 of 21 mold should be smooth, clean, and lubricated before sampling. Samples were compacted using a vibrator with a frequency of 2800 ÷ 3000 rpm and an amplitude of 0.35 ÷ 0.5 mm. Then, they were cured in a room at 25 ± 2 • C for a minimum of 24 h. Finally, the molds were removed and soaked in water. The compressive and flexural strengths were tested at 3, 7, 28, and 56 days. The chloride ion permeability was conducted at 28 and 56 days. All the tests were conducted in triplicate with specimens.

Specimen Preparation and Testing Procedures
NS has a high surface area and nanoscale particle size, and is difficult to disperse in concrete mixes. The following steps are experimental mixing procedures for making homogeneous, stable concrete, as presented in Figure 2. There are five steps to mixing the sample, including:

HSC Compressive and Flexural Strength Tests
Experiments were conducted at the Ho Chi Minh City University of Technology's Laboratory of Building Materials LAS -XD 238-Research Center for Industrial Technology and Equipment, Vietnam. Compressive strength and flexural tensile strength tests are performed according to ASTM C39 and ASTM C78 [45], respectively, after molding and curing. The load incensement speed is 0.3 MPa/s, and the testing instrument is a San 3000 electronic compressor with a maximum load of 3000 kN, as shown in Figure 3. Step 1: NS was combined with 60% water and vigorously stirred to evenly distribute the NS particles; Step 2: A combination of sand, crushed stone, cement, and silica fume was mixed for three minutes; Step 3: A total of 20% water was added to the sand, crushed stone and cement mixture, and stirred thoroughly for one minute; Step 4: the mixture containing NS and 60% water in step 1 was added to the Step 3 mixture and mixed for minutes; Step 5: The solution containing the remaining 20% water and superplasticizer was incorporated in the Step 4 mixture for 3 min until homogenous; Step 6: Mixer was temporarily stopped for 2 min to enable the superplastic ingredient to react, which will improve the result; Step 7: The mixture was continuously mixed for another 3 min to avoid slumps and guarantee homogeneity.
A 60-L mixer was utilized to mix the composition. A compressive strength test on a cylindrical specimen with a dimension of 150 mm × 300 mm (d × h), a bending test with a beam of 150 mm × 150 mm × 600 mm, and a chlorine ion permeability with a specimen of 100 mm × 200 mm (d × h) are among the available test specimens. The inside surface of the mold should be smooth, clean, and lubricated before sampling. Samples were compacted using a vibrator with a frequency of 2800 ÷ 3000 rpm and an amplitude of 0.35 ÷ 0.5 mm. Then, they were cured in a room at 25 ± 2 °C for a minimum of 24 h. Finally, the molds were removed and soaked in water. The compressive and flexural strengths were tested at 3, 7, 28, and 56 days. The chloride ion permeability was conducted at 28 and 56 days. All the tests were conducted in triplicate with specimens.

HSC Compressive and Flexural Strength Tests
Experiments were conducted at the Ho Chi Minh City University of Technology's Laboratory of Building Materials LAS -XD 238-Research Center for Industrial Technology and Equipment, Vietnam. Compressive strength and flexural tensile strength tests are performed according to ASTM C39 and ASTM C78 [45], respectively, after molding and curing. The load incensement speed is 0.3 MPa/s, and the testing instrument is a San 3000 electronic compressor with a maximum load of 3000 kN, as shown in Figure 3.

Experiment to Determine Chloride Ion Permeability of HSC
The chlorine ion permeability test was conducted at the Construction Materials Laboratory-LAS-XD 143 Southern Institute of Water Resources Research, Vietnam. The specimens with 100 mm × 200 mm cylinders were used for each kind of concrete to evaluate the chloride ion permeability at 28 days and 56 days. The ASTM C1202 [46] was applied to determine concrete's resistance to chloride ion permeability, as plotted in Figure 4.

Experiment to Determine Chloride Ion Permeability of HSC
The chlorine ion permeability test was conducted at the Construction Materials Laboratory-LAS-XD 143 Southern Institute of Water Resources Research, Vietnam. The specimens with 100 mm × 200 mm cylinders were used for each kind of concrete to evaluate the chloride ion permeability at 28 days and 56 days. The ASTM C1202 [46] was applied to determine concrete's resistance to chloride ion permeability, as plotted in Figure 4.

Nano-SiO2 Modulation
As shown in Figure 5, RHA was the burnt waste of the steam boiler, obtained from a factory located in Long An province, southern Vietnam. The factory burnt around 20 tons of rice husk and produced approximately 4 tons of RHA per day. This study's first aim was to find new additives for civil engineering materials based on the waste obtained by RHA. As discussed in the earlier section, NS is a material that can potentially be extracted from RHA. The Sol-gel technique was proposed to produce NS materials with the cheapest cost and most potent tool [47]. Several major parameters influence the separation of Nana SiO2 from RHA during the NS manufacturing process. The production process of NS is presented in Figure 1. Figure 6 illustrates the detailed procedures involved in the production of nano SiO2. Then, the EDX, XDR, SEM, TEM, and BET methods evaluated the physical and chemical characteristics of the obtained NS. As shown in Figure 5, RHA was the burnt waste of the steam boiler, obtained from a factory located in Long An province, southern Vietnam. The factory burnt around 20 tons of rice husk and produced approximately 4 tons of RHA per day. This study's first aim was to find new additives for civil engineering materials based on the waste obtained by RHA. As discussed in the earlier section, NS is a material that can potentially be extracted from RHA. The Sol-gel technique was proposed to produce NS materials with the cheapest cost and most potent tool [47].

Nano-SiO2 Modulation
As shown in Figure 5, RHA was the burnt waste of the steam boiler, obtained from a factory located in Long An province, southern Vietnam. The factory burnt around 20 tons of rice husk and produced approximately 4 tons of RHA per day. This study's first aim was to find new additives for civil engineering materials based on the waste obtained by RHA. As discussed in the earlier section, NS is a material that can potentially be extracted from RHA. The Sol-gel technique was proposed to produce NS materials with the cheapest cost and most potent tool [47]. Several major parameters influence the separation of Nana SiO2 from RHA during the NS manufacturing process. The production process of NS is presented in Figure 1. Figure 6 illustrates the detailed procedures involved in the production of nano SiO2. Then, the EDX, XDR, SEM, TEM, and BET methods evaluated the physical and chemical characteristics of the obtained NS. Several major parameters influence the separation of Nana SiO 2 from RHA during the NS manufacturing process. The production process of NS is presented in Figure 1. Figure 6 illustrates the detailed procedures involved in the production of nano SiO 2 . Then, the EDX, XDR, SEM, TEM, and BET methods evaluated the physical and chemical characteristics of the obtained NS.

Energy-Dispersive X-ray Spectroscopy (EDX)
EDX is an analytical technique used for the elemental analysis or chemical characterization of a sample. The principle of this technique is based on an interaction between some source of X-ray excitation and a sample. Figure 7 plotted the result of the EDX spectrum. The results indicated that the NS particles were composed of Si and O, with 28.78% Crystals 2021, 11, 932 9 of 21 and 57.92%, respectively. The Si-O ratio was found to be approximately 1:2. Furthermore, the carbon atom, C, was also obtained due to the incomplete combustion of RHA. The obtained results also indicated that the process of preparing Nano-SiO 2 was pure, and suitable for use as an adsorbent and other related applications, especially for additives in civil engineering.

Energy-Dispersive X-ray Spectroscopy (EDX)
EDX is an analytical technique used for the elemental analysis or chemical characterization of a sample. The principle of this technique is based on an interaction between some source of X-ray excitation and a sample. Figure 7 plotted the result of the EDX spectrum. The results indicated that the NS particles were composed of Si and O, with 28.78% and 57.92%, respectively. The Si-O ratio was found to be approximately 1:2. Furthermore, the carbon atom, C, was also obtained due to the incomplete combustion of RHA. The obtained results also indicated that the process of preparing Nano-SiO2 was pure, and suitable for use as an adsorbent and other related applications, especially for additives in Figure 6. Process of nano SiO 2 (NS) preparation.

X-ray Diffraction (XRD)
X-ray diffraction (XRD), a versatile and nondestructive analytical technique, was used to quickly obtain detailed phase and structural information of crystalline materials. XRD is a technique that employs X-ray beams to generate diffraction maxima and minima on solid crystal surfaces owing to the periodicity of the crystal structure. The method of X-ray diffraction (commonly shortened to X-ray diffraction) is used to study the structure of solids, materials, and other objects. In this study, the specimens were tested at the Institute of Applied Materials Science, Vietnam. The result of the XRD pattern of the Nano-SiO 2 sample is plotted in Figure 8. It can be seen from this figure that the sample mainly contains the SiO 2 crystalline phase of monoclinic lattice, and the peak result corresponds to an angle of 2θ, about 19.76 • . Furthermore, Figure 8 also indicated that the sample contains not only crystalline phase SiO 2 , but also a little amorphous SiO 2 .

X-ray Diffraction (XRD)
X-ray diffraction (XRD), a versatile and nondestructive analytical technique, was used to quickly obtain detailed phase and structural information of crystalline materials. XRD is a technique that employs X-ray beams to generate diffraction maxima and minima on solid crystal surfaces owing to the periodicity of the crystal structure. The method of X-ray diffraction (commonly shortened to X-ray diffraction) is used to study the structure of solids, materials, and other objects. In this study, the specimens were tested at the Institute of Applied Materials Science, Vietnam. The result of the XRD pattern of the Nano-SiO2 sample is plotted in Figure 8. It can be seen from this figure that the sample mainly contains the SiO2 crystalline phase of monoclinic lattice, and the peak result corresponds to an angle of 2θ, about 19.76°. Furthermore, Figure 8 also indicated that the sample contains not only crystalline phase SiO2, but also a little amorphous SiO2. SEM is an electron microscope that scans a material with a concentrated stream of electrons to create a picture. When the electrons contact the atoms in a sample, they pro-  X-ray diffraction (XRD), a versatile and nondestructive analytical technique, was used to quickly obtain detailed phase and structural information of crystalline materials. XRD is a technique that employs X-ray beams to generate diffraction maxima and minima on solid crystal surfaces owing to the periodicity of the crystal structure. The method of X-ray diffraction (commonly shortened to X-ray diffraction) is used to study the structure of solids, materials, and other objects. In this study, the specimens were tested at the Institute of Applied Materials Science, Vietnam. The result of the XRD pattern of the Nano-SiO2 sample is plotted in Figure 8. It can be seen from this figure that the sample mainly contains the SiO2 crystalline phase of monoclinic lattice, and the peak result corresponds to an angle of 2θ, about 19.76°. Furthermore, Figure 8 also indicated that the sample contains not only crystalline phase SiO2, but also a little amorphous SiO2. SEM is an electron microscope that scans a material with a concentrated stream of electrons to create a picture. When the electrons contact the atoms in a sample, they produce various signals that may be detected and include information about the sample, such as surface topography and composition. In most cases, the electron beam is scanned in a scanning field, and the position of the beam is combined with the received signal to form a picture. SEM has a resolution of more than 1 nanometer. Specimens can be examined in various environments, including high vacuum, low vacuum, moist conditions, and a wide range of temperatures. A picture depicting the surface is created by scanning the sample

Scanning Electron Microscope (SEM)
SEM is an electron microscope that scans a material with a concentrated stream of electrons to create a picture. When the electrons contact the atoms in a sample, they produce various signals that may be detected and include information about the sample, such as surface topography and composition. In most cases, the electron beam is scanned in a scanning field, and the position of the beam is combined with the received signal to form a picture. SEM has a resolution of more than 1 nanometer. Specimens can be examined in various environments, including high vacuum, low vacuum, moist conditions, and a wide range of temperatures. A picture depicting the surface is created by scanning the sample and detecting secondary electrons. In this work, the NS specimen was tested by SEM at the National Institute of Hygiene and Epidemiology, in Vietnam, as shown in Figure 9. and detecting secondary electrons. In this work, the NS specimen was tested by SEM at the National Institute of Hygiene and Epidemiology, in Vietnam, as shown in Figure 9.

Transmission Electron Microscopy (TEM)
The nano SiO2 sample utilized in the study was analyzed using TEM at Vietnam is National Institute of Hygiene and Epidemiology, Vietnam. Figure 10 shows the TEM image of NS. Looking at the data plotted in Figures 9 and 10, an apparent crystalline grain, microscopic particles (about 10 to 15 nm), and relatively uniform dispersion were obtained.

Brunauer Emmett Teller (BET)
The BET theory seeks to explain the physical adsorption of gas molecules on a solid surface, and serves as the foundation for a crucial analytical approach to determining the specific surface area of materials. The nano SiO2 materials used in the study have a surface area of about 258.3 m 2 /g, as plotted in Figure 11. Furthermore, the results indicated that the NS was found to be pure and appropriate for use as an adsorbent and for other related applications, particularly as a civil engineering additive. The physical properties of NS made from RHA are presented in Table 10.

Transmission Electron Microscopy (TEM)
The nano SiO 2 sample utilized in the study was analyzed using TEM at Vietnam is National Institute of Hygiene and Epidemiology, Vietnam. Figure 10 shows the TEM image of NS. Looking at the data plotted in Figures 9 and 10, an apparent crystalline grain, microscopic particles (about 10 to 15 nm), and relatively uniform dispersion were obtained. and detecting secondary electrons. In this work, the NS specimen was tested by SEM at the National Institute of Hygiene and Epidemiology, in Vietnam, as shown in Figure 9.

Transmission Electron Microscopy (TEM)
The nano SiO2 sample utilized in the study was analyzed using TEM at Vietnam is National Institute of Hygiene and Epidemiology, Vietnam. Figure 10 shows the TEM image of NS. Looking at the data plotted in Figures 9 and 10, an apparent crystalline grain, microscopic particles (about 10 to 15 nm), and relatively uniform dispersion were obtained.

Brunauer Emmett Teller (BET)
The BET theory seeks to explain the physical adsorption of gas molecules on a solid surface, and serves as the foundation for a crucial analytical approach to determining the specific surface area of materials. The nano SiO2 materials used in the study have a surface area of about 258.3 m 2 /g, as plotted in Figure 11. Furthermore, the results indicated that the NS was found to be pure and appropriate for use as an adsorbent and for other related applications, particularly as a civil engineering additive. The physical properties of NS made from RHA are presented in Table 10.

Brunauer Emmett Teller (BET)
The BET theory seeks to explain the physical adsorption of gas molecules on a solid surface, and serves as the foundation for a crucial analytical approach to determining the specific surface area of materials. The nano SiO 2 materials used in the study have a surface area of about 258.3 m 2 /g, as plotted in Figure 11. Furthermore, the results indicated that the NS was found to be pure and appropriate for use as an adsorbent and for other related applications, particularly as a civil engineering additive. The physical properties of NS made from RHA are presented in Table 10.
Similarly, the characteristics of NS were also obtained from the findings of the previous studies [48,49]. A crucial aspect of the NS material made from the RHA is its excellent absorption properties based on the characteristics mentioned earlier. This facilitates the mineralization process when used as an additive for concrete.

Compressive Strength and Flexural Strengths at Early Curing Age
Firstly, a series of compressive and flexural tests were conducted in this study. The curing age was chosen as 3 days. Six NS contents replaced cement at various amounts such as 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%. The 0% NS concrete sample was known as the control specimen. Figures 12 and 13 present the compressive and flexural strength results for different replacement levels of NS, ranging from 0 to 2.5%, by weight. Looking at the data presented in these figures, the NS significantly improves the compressive and flexural strengths. The compressive and flexural strengths are in the range of 48.15-58.25 MPa and 5.68-6.77 MPa, respectively. This demonstrates that NS has a greater impact on flexural strength than compressive strength in the HSC combination. For instance, the flexural and compressive strengths of concrete grow considerably with the NS ratio from 0 to 1.5%, reaching 19.19% and 20.97%, respectively. Certain studies [50][51][52][53][54][55] also confirmed the tendency of this increase in strength. A previous study reported by Naji Givi et al. [51] found that concrete with an NS component of 1.0 ÷ 1.5% has a higher bending strength than control concrete. If the NS particle is participation is near to 2%, however, the concrete strength reduces. The explanation for this phenomenon is that the primary causes for the increased concrete strength were the pozzolanic reaction and the nano-filling effect [56]. NS particles, in particular, may interact with water molecules in the concrete mixture and produce silanol groups because of their enormous specific surface areas and ultra-high reactivity (Si-OH). Si-OH then interacts with Ca 2+ in calcium hydroxide (Ca(OH) 2 crystals to produce a C-S-H gel [57]. Furthermore, the unreacted NS particles scatter in smaller areas and fill the vacancy, refining the pore structure and increasing the concrete's compactness. Excess NS absorbs the water originally required for cement hydration, resulting in insufficient cement hydration and decreasing the strength of the concrete [58]. NS particles have extremely large specific surface areas, and when the added NS exceeds the nanoparticles, they have strong water absorption [57]. The results obtained from this indicated that the appropriate amount of NS content was 1.5%, which significantly improves the compressive and flexural strengths of HSC. Similarly, the characteristics of NS were also obtained from the findings of the previous studies [48,49]. A crucial aspect of the NS material made from the RHA is its excellent absorption properties based on the characteristics mentioned earlier. This facilitates the mineralization process when used as an additive for concrete.   areas and fill the vacancy, refining the pore structure and increasing the concrete's compactness. Excess NS absorbs the water originally required for cement hydration, resulting in insufficient cement hydration and decreasing the strength of the concrete [58]. NS particles have extremely large specific surface areas, and when the added NS exceeds the nanoparticles, they have strong water absorption [57]. The results obtained from this indicated that the appropriate amount of NS content was 1.5%, which significantly improves the compressive and flexural strengths of HSC.   Figures 14 and 15 present the development of the compressive strength with a curing age in a range of from 3 to 56 days for various NS ratios of 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%. In general, a longer HSC curing age offers a higher compressive strength due to its formation through chemical condensation before the final step of gel hardening. Figure 14   pactness. Excess NS absorbs the water originally required for cement hydration, resulting in insufficient cement hydration and decreasing the strength of the concrete [58]. NS particles have extremely large specific surface areas, and when the added NS exceeds the nanoparticles, they have strong water absorption [57]. The results obtained from this indicated that the appropriate amount of NS content was 1.5%, which significantly improves the compressive and flexural strengths of HSC.   Figures 14 and 15 present the development of the compressive strength with a curing age in a range of from 3 to 56 days for various NS ratios of 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%. In general, a longer HSC curing age offers a higher compressive strength due to its formation through chemical condensation before the final step of gel hardening. Figure 14 Figures 14 and 15 present the development of the compressive strength with a curing age in a range of from 3 to 56 days for various NS ratios of 0, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%. In general, a longer HSC curing age offers a higher compressive strength due to its formation through chemical condensation before the final step of gel hardening. Figure 14 shows that the 28-day specimens containing 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% NS have mean compressive strengths of 71. 25 Figure 15 shows that the 28-day specimens containing 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% NS have mean flexural strengths of 7.05 MPa, 7.21 MPa, 7.35 MPa, 7.67 MPa, 7.46 MPa, 7.26 MPa, respectively. In general, looking at the data plotted in Figures 14 and 15, the development, compressive and flexural strengths with an age curing of 3-28 days can be analytically described by a logarithmic equation, Equation (1), as follows: where a, and b are the experimental coefficients; t is curing age, days; f ci is the compressive strength at the curing age of t-day, MPa.
where a, and b are the experimental coefficients; t is curing age, days; is the compressive strength at the curing age of t-day, MPa.
In addition, the coefficients of determination, R 2 , are found to be greater than 0.95 for compressive strength, which confirms that the logarithmic equations are highly reliable when predicting the compressive strength development of HSC with a curing age of 7-28 days. However, the coefficients of determination, R 2 , are greater than 0.74 for flexural strength, which should be considered with further specimens.  Figures 14 and 15 also indicated that adding NS to HSC significantly enhanced the compressive strength of concrete at an early age, from 3 to 7 days. An NS content of 1.5% yields the best compressive strength value, irrespective of the curing period. At a 1.5% content of NS, the increase in compressive strength at 3, 7, 14, and 28 days of curing was 20.8%, 19.5%, 14.04%, and 12.06%, compared to the control sample. Similarly, the flexural strength of specimen containing 1.5% NS at a curing time in the range of 3-56 days increased by 19.19%, 16.43%, 8.79%, and 8.12% compared to that of control concrete.
The 28-day compressive strength was preferential for use in practical civil engineering. Based on the obtained data, this study used ACI [59], Equation [2], to determine the compressive strength at t-day based on 28 days:  In addition, the coefficients of determination, R 2 , are found to be greater than 0.95 for compressive strength, which confirms that the logarithmic equations are highly reliable when predicting the compressive strength development of HSC with a curing age of 7-28 days. However, the coefficients of determination, R 2 , are greater than 0.74 for flexural strength, which should be considered with further specimens. Figures 14 and 15 also indicated that adding NS to HSC significantly enhanced the compressive strength of concrete at an early age, from 3 to 7 days. An NS content of 1.5% yields the best compressive strength value, irrespective of the curing period. At a 1.5% content of NS, the increase in compressive strength at 3, 7, 14, and 28 days of curing was 20.8%, 19.5%, 14.04%, and 12.06%, compared to the control sample. Similarly, the flexural strength of specimen containing 1.5% NS at a curing time in the range of 3-56 days increased by 19.19%, 16.43%, 8.79%, and 8.12% compared to that of control concrete.
The 28-day compressive strength was preferential for use in practical civil engineering. Based on the obtained data, this study used ACI [59], Equation (2), to determine the compressive strength at t-day based on 28 days: where α and β are the experimental coefficients; t is curing age, days; f ci is the compressive strength at the curing age of t-day, MPa; f c is the compressive strength at 28-day. The non-linear regression method was analyzed based on the experimental data. The values of α, β, and R 2 were shown in Table 11.

Effect of NS Content on Strength Development
The compressive and flexural strengths, both without and with NS contents, are presented in Figures 16 and 17. In general, with an NS content in a range of 0-2.5%, the developed compressive and flexural strengths follow with a parabolic function. At 28 days of curing, the compressive and flexural strengths were in the range of 71.25-81.25 MPa and 7.05-7.67 MPa, respectively. As seen in these figures, the strengths significantly increased when NS content increased from 0 to 1.5%; however, the strength decreased with a further increase in NS content. Thus, the appropriate NS content was to be found 1.5%. These results were similar to the previous finding reported by Zhang et al. [26], who investigated polyvinyl alcohol fiber content and NS particles in terms of flexural strength; however, the optimum NS content was revealed to be 1.2%. As with the current results, Zhang et al. [28] indicated that the NS replacement was in the range of 2-3%; the compressive strength, splitting tensile strength, and flexural strength obtained the highest values in coal fly ash concrete. There are two reasons for the improvement in HSC strength, including the pozzolanic reaction and nano-filling [56]. In this study, the SF and NS react with Ca(OH) 2 to form CaSiO 3 , which noticeably improves the microstructure of the mixtures [26]. Additionally, NS particles are smaller than cement particles, and can effectively enhance the interfacial structure properties. When NS content is greater than 1.5%, the sizeable molecular force was beyond the optimum content for NS addition in cementitious composites, which tend to flock and reduce strength, as explained by Abbasi et al. [60].

Correlation between Compressive Strength and Flexural Strength
The flexural strengths of concrete were established based on the compressive strengths obtained for different countries. Table 12 presents some empirical formulations obtained for plain concrete [61].
Based on the data obtained in this study, an empirical formulation has been established between flexural strength and compressive strength, in Equation (3), as follows: where k is the values presented in Figure 18, which depends on the NS content and curing age.
Zhang et al. [28] indicated that the NS replacement was in the range of 2-3%; the pressive strength, splitting tensile strength, and flexural strength obtained the highes ues in coal fly ash concrete. There are two reasons for the improvement in HSC stre including the pozzolanic reaction and nano-filling [56]. In this study, the SF and NS with Ca(OH)2 to form CaSiO3, which noticeably improves the microstructure of the tures [26]. Additionally, NS particles are smaller than cement particles, and can effec enhance the interfacial structure properties. When NS content is greater than 1.5% sizeable molecular force was beyond the optimum content for NS addition in cement composites, which tend to flock and reduce strength, as explained by Abbasi et al. [  Zhang et al. [28] indicated that the NS replacement was in the range of 2-3%; th pressive strength, splitting tensile strength, and flexural strength obtained the highe ues in coal fly ash concrete. There are two reasons for the improvement in HSC str including the pozzolanic reaction and nano-filling [56]. In this study, the SF and N with Ca(OH)2 to form CaSiO3, which noticeably improves the microstructure of th tures [26]. Additionally, NS particles are smaller than cement particles, and can effe enhance the interfacial structure properties. When NS content is greater than 1.5 sizeable molecular force was beyond the optimum content for NS addition in cemen composites, which tend to flock and reduce strength, as explained by Abbasi et al.     Figure 19 presents the results of the permeability of chloride ions with various NS contents. It indicated that when the number of NS particles in the concrete increases, the chloride penetration resistance of the concrete gradually improves. When the percent substitution of NS increases, the chloride ion permeability of the concrete first drops, then increases. The data presented in Figure 19 indicated that the permeability values of chloride ions are in the range of 361-882 C and 264-678 C for 28 and 56 days of testing, respectively, irrespective of the NS content used. The chloride ion permeability was at its lowest when the NS replacement amount was 1.5%. The HSC containing 1.5% NS had the best anti-chloride ion penetration capabilities, of 59% and 61%, respectively, for 28 and 56 days, compared to the control sample. The chloride ion permeability of concrete increased gradually as the replacement level of NS climbed to 2.0% and 2.5%, respectively, but was still reduced by 38% and 25%, respectively, compared to the control concrete at 28 days. The tendency of reducing the chloride ion permeability was also reported in many previous  Figure 19 presents the results of the permeability of chloride ions with various NS contents. It indicated that when the number of NS particles in the concrete increases, the chloride penetration resistance of the concrete gradually improves. When the percent substitution of NS increases, the chloride ion permeability of the concrete first drops, then increases. The data presented in Figure 19 indicated that the permeability values of chloride ions are in the range of 361-882 C and 264-678 C for 28 and 56 days of testing, respectively, irrespective of the NS content used. The chloride ion permeability was at its lowest when the NS replacement amount was 1.5%. The HSC containing 1.5% NS had the best anti-chloride ion penetration capabilities, of 59% and 61%, respectively, for 28 and 56 days, compared to the control sample. The chloride ion permeability of concrete increased gradually as the replacement level of NS climbed to 2.0% and 2.5%, respectively, but was still reduced by 38% and 25%, respectively, compared to the control concrete at 28 days. The tendency of reducing the chloride ion permeability was also reported in many previous studies [56,62]. This phenomenon can be explained by NS being a nanoscale substance that may change hazardous pores into safe ones and lower the porosity of concrete. When added to the concrete as a nanoscale material, with an average particle size smaller than that of cement in concrete, it can prevent the formation of concrete pores, refine pore size, and make concrete microstructures denser and more homogeneous. Furthermore, the NS in the cement matrix can effectively block or cut off capillaries in the concrete, resulting in tortuosity and more disconnected transport channels, which improve the concrete sample's chloride permeability resistance [18,28]. As previously stated, the NS particles will agglomerate and become unable to disperse evenly in the cement matrix after being introduced to the mixture at larger replacement ratios of NS, meaning that the chloride ion permeability will be lower than that of the concrete with the optimal threshold NS.

Conclusions
This study presents experimental research on Nano-SiO 2 modulation from the waste RHA, mechanical engineering properties, and chloride ion permeability of HSC containing various Nano-SiO 2 contents. Based on the obtained data, the following conclusions can be drawn.
The Nano SiO 2 was produced from RHA in southern Vietnam using the sol-gel method. Specific techniques have evaluated the physical and chemical properties of NS. EDX spectroscopy data indicated that SiO 2 has a primary atomic composition of Si (28.78%) and O (57.92%), with a Si:O atom ratio of about 1:2. The XRD results revealed that NS material was mainly crystalline, and amorphous SiO 2 was mixed in with the crystalline SiO 2 phase in the sample.
BET methods found the specific surface area of nano SiO 2 to be about 258.3 m 2 /g. SEM and TEM techniques showed that the NS material made from rice husk ash includes tiny particles, ranging from approximately 10 to 15 nm; SiO 2 is in the form of crystals, made up of numerous microscopic particles that cluster together to create porous SiO 2 blocks. Based on the obtained results, it can be concluded that the NS from RHA could be applied as a suitable application as a binder for building materials, enriching the source of building materials and contributing to protection of the environment.
HSC containing NS contents ranging from 0.5% to 2.5% had good performances in terms of its compressive and flexural strengths. The addition of NS to mix significantly improved the strength of concrete at the short-term curing age of 3 days compared to longer curing ages. The compressive and flexural strengths reached their highest values with the NS replacement level of 1.5%. HSC containing 1.5% NS yielded the compressive and flexural strengths of 81.25 MPa and 7.67 MPa, respectively, at 28 days of curing. There are two possible reasons for the increase in strength: the interaction of NS particles with Ca(OH) 2 generating more C-S-H gel, and the densification of the microstructure significantly improving the mechanical properties of concrete specimens.
Compared to control concrete, a small dose of NS can significantly improve the chloride penetration resistance of the HSC. A homogeneous dispersion of NS particles was obtained at the 1.5% replacement level. The reduction in the chloride ion permeability was because the NS could cut off capillaries in the concrete, resulting in tortuosity and more disconnected transport channels. The chloride ion permeability reached its lowest when the NS replacement amount was around 1.5%, and the concretes had the best anti-chloride ion penetration capability at 59% and 61%, respectively, for 28 and 56 days.
The compressive and flexural strengths developed with an age curing of 3-28 days by a logarithmic rule, with R 2 ≥ 0.74. Based on ACI, the compressive strength at 28 days was employed to determine the compressive strength at t-day, with R 2 ≥ 0.95. The flexuralcompressive correlations were established from the obtained data; however, the results had limited accuracy. Further studies should include more results.