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Effects of nano-SiO2 additives on carbon fiber-reinforced fly ash–slag geopolymer composites performance: Workability, mechanical properties, and microstructure

  • Dongsheng Zhang , Qiuning Yang EMAIL logo , Yafan Wang and Jiabin Li EMAIL logo
Published/Copyright: December 7, 2023
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Nanotechnology Reviews
From the journal Nanotechnology Reviews Volume 12 Issue 1

Abstract

Fly ash and slag are commonly used precursors in alkali-activated concrete. However, they suffer from high brittleness, poor toughness, and susceptibility to cracking. To address these limitations, this experimental study investigates the effects of different contents of nano-silica (SiO2) additives on the workability, mechanical properties, and microstructure of carbon fiber-reinforced fly ash–slag geopolymer composites (CFSGs). The results indicate that owing to its large specific surface area, nano-SiO2 significantly increases the water demand of the geopolymer, thereby considerably decreasing the fluidity and shortening the setting time of the geopolymer. However, nano-SiO2 improves the porosity, water absorption, and mechanical properties of the CFSG. The optimal mechanical strength is obtained by using 2% nano-SiO2. In addition, appropriate nanodoping can relatively improve the bearing capacity and fracture toughness of the specimen. Compared with that of undoped CFSG, the peak load, fracture toughness, unstable fracture toughness, and elastic modulus of the 2%-SiO2-doped CFSG increased by 8.78, 5.0, 9.6, and 9.8%, respectively. The incorporation of nano-SiO2 increases the shrinkage of the geopolymer, with a more significant impact on early shrinkage. Moreover, nano-SiO2 improves the microstructure of the cement matrix and interface through the filling, volcanic ash, and crystal nucleus effects as well as interface regulation. This increases the bonding force between the matrix and carbon fibers (CFs), which results in good bonding between the CFs and geopolymer matrix, accelerated geopolymerization reaction, and denser geopolymer paste, thus improving the mechanical strength of the CFSG.

Keywords: nano-SiO2 ; fracture toughness; geopolymer composites; uniaxial tensile; modification mechanism

1 Introduction

Economic globalization has led to the growth of production, particularly in the construction industry, where cement is one of the most important building materials [1,2]. However, the production of cement is associated with significant environmental issues. In 2020, the cement production in China reached 2.2 billion tons, out of a global production of 4.1 billion tons [3]. Correspondingly, each ton of clinker produced emits approximately 0.86 tons of carbon dioxide (CO2), in addition to releasing harmful gases, such as sulfur oxides (SOx) and nitrogen oxides (NOx), which seriously pollute the environment. Furthermore, the CO2 generated by cement is projected to account for 8% of artificial CO2 emissions [4]. One of the direct consequences of global warming caused by greenhouse gases such as CO2 is the urgent need to seek new green, low-carbon, and environmentally friendly materials to replace cement.

Geopolymers are new green, low-carbon, and environmentally friendly cementitious materials made of industrial waste residue, metakaolin, fly ash (FA), and other raw silicon (Si), and aluminum (Al) materials [5,6]. The addition of an alkaline activator causes dissolution, monomer reconstruction, and polycondensation reactions, resulting in the formation of three-dimensional network structures interlaced by SiO4 and AlO4 tetrahedral units [7]. The CO2 emissions during the process of geopolymer preparation are only 10–20% of those of ordinary Portland cement [8]. Because geopolymers are inorganic polymer materials with excellent properties, such as high early strength, high temperature resistance, chemical corrosion resistance, and good durability [9], they have become a research hotspot and frontier in the field of green building materials [7]. However, they also have limitations, such as high porosity, low tensile strength, and high brittleness, which hinder their widespread application [10,11]. Therefore, the mechanical and other properties of geopolymers should be continuously improved using new regulatory technologies.

To improve the brittleness of cement-based materials, some studies have proposed various methods, such as adding high-performance fibers to modify concrete. Among them, carbon fibers (CFs) have outstanding advantages and are widely used in construction engineering. In particular, CFs exhibit low specific gravity, high elastic modulus, high temperature resistance, and low corrosion resistance; however, they also have disadvantages such as poor toughness and insufficient impact resistance [12,13,14,15,16]. For example, Liu et al. [12] investigated the effect of CFs on the axial stress–strain behavior of concrete and found that the addition of CFs increased the peak stress, residual stress, and ultimate strain of concrete. Wang et al. [14] evaluated the flexural behavior of CF-reinforced concrete (CFRC) beams under impact loading and found that the beams with 0.35% fiber content absorbed most of the energy. Guo et al. [16] studied the effects of different lengths and contents of CFs on the mechanical properties of concrete after high temperature treatment, and the results showed that the optimal content and length of CFs were 1.0 wt% and 10 mm, respectively. The carbon-fiber content has a significant impact on the mechanical properties of CFRC after high temperature treatment, and the effect of the CF length can be ignored. Tang et al. [17] reported that CFs can improve the compressive strength and ultimate strain of recycled aggregate concrete. Currently, most existing studies have focused on CF-reinforced ordinary Portland cement composite materials, whereas studies on the application of CFs to geopolymer composite materials have just begun. Studies have reported that within the geopolymer matrix, CFs can have a bridging effect, which changes the internal stress direction in the geopolymer and prevents inelastic volume expansion during compression [18]. Meng [19] found that when the content of cut CFs is 0.5%, the compressive strength of geopolymer concrete with high-calcium FA is highest, and when the CF content is higher than 0.2%, the generation and development of cracks can be effectively inhibited. Shen [20] investigated the effect of the CF volume fraction on the mechanical properties of geopolymer concrete. The results indicate that the addition of CFs can simultaneously enhance and toughen geopolymer concrete. In addition, with an increase in the CF content, the axial compressive strength, elastic modulus, flexural strength, and flexural compression ratio of geopolymer concrete increased first and then decreased, and the optimal CF content was within the range of 0.8–1.0%. Farhan et al. [21] further showed that the failure modes of CFs in geopolymer can be classified into “fiber pulling out” and “fiber breaking.” During a blending test, Ma et al. [22] found that the bending strength of the geopolymer can be improved by enhancing the interface contact between the CFs and geopolymer matrix and aligning the CFs in the same direction within the geopolymer.

Nanotechnology, through the addition of nanocomponents, has become a new method for material design and performance regulation [23,24]. Nano-SiO2 has received widespread attention owing to its significant impact on the microstructures of geopolymers. By leveraging the size, surface, and quantum effects, among other characteristics of nano-SiO2 nanocomposites, traditional materials can be improved in performance and endowed with special functions to become new materials [25]. The surface energy of ultrafine nanoparticles is extremely high, and numerous broken suspended bonds are formed. Therefore, they exhibit chemical activity, such as small particles formation, strong filling ability, and a physical filling effect. In addition, nano-SiO2 can provide nucleation sites for the precipitation of gel and accelerate the hydration of geopolymers [26,27]. For example, Gao et al. [28,29] added 1.0–3.0% nano-SiO2 to an alkali-activated cementitious material prepared from metakaolin. The results showed that the strength of samples with different water–cement ratios at each age improved because nano-SiO2 accelerated the Si–Al polymerization reaction, generated more gel, and induced a denser structure. In this system, the optimal dosage of nano-SiO2 was 1.0%, and an increase in dosage decreased the enhancement effect. Shahrajabian and Behfarnia [30] found that by adding 1.0, 2.0, and 3.0% (by weight) nano-SiO2 to an alkali-activated cementitious material prepared using slag, the strength of geopolymer increased after 7, 28, 90, and 120 days. In particular, the strength of the sample with 3% nano-SiO2 increased the most significantly, with an increase in strength of approximately 10.0 MPa at all ages. The alkali-activated concrete mixed with nano-SiO2 had a denser structure and improved freeze-thaw resistance. After 300 cycles, the strength loss in the 3.0% nano-SiO2-modified alkali-activated slag sample was approximately 2.0% lower than that of the blank sample. Adak et al. [31] found that when 6.0% nano-SiO2 replaced the FA in equal amounts, the electric flux of the alkali-activated FA sample decreased owing to the improvement in the pore structure, and water absorption also decreased. Therefore, high-performance cement-based materials can be prepared by adding nanocomponents such as ultrafine particles. Fibers and nano-SiO2 have proven to be potential application technologies for improving the performance of geopolymers. In recent years, some researchers have begun to use nanoparticles and trace fibers, such as SiO2 and polyvinyl alcohol fibers [32,33,34], SiO2 and steel fibers [35,36], and nano-calcium carbonate (CaCO3) and basalt fibers [37,38], to modify geopolymers. By producing a denser matrix and more refined interface area, the use of nanoparticles significantly improved the mechanical characteristics and microstructure of fiber-reinforced geopolymer coagulation [35]. To date, limited investigations have been conducted on the mechanical characteristics of geopolymer composites combining CFs with nano-SiO2. Therefore, this study investigated the effects of different contents of nano-SiO2 on the macroscopic properties and microstructure of CF-reinforced FA–slag geopolymer composites (CFSGs).

In the experimental scheme, a geopolymer paste was prepared by adding 0.6% CF and different contents of nano-SiO2 (0, 1, 2, and 3% by weight). The effects of the different contents of nano-SiO2 on the macroscopic properties of the geopolymer composites were systematically investigated. These properties include workability (fluidity, setting time, porosity, and water absorption), mechanical properties (compressive strength, flexural strength, uniaxial tensile performance, and fracture performance), and drying shrinkage. In addition, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to analyze the microstructure, and the modification mechanisms of nano-SiO2 and CFs on the CFSG were explored. This study will clarify the reinforcing mechanism of nano-SiO2 and CF composite on geopolymer materials and promote the use of nanomaterials and fibers in conjunction to realize well-developed FA–slag geopolymer composites.

2 Materials and methods

2.1 Materials

The FA was Class C, with a density of 2.264 g/cm3, water requirement of 86%, and loss on ignition of 2.99%. The slag was S95 grade, with a density of 2,650 kg/m3 and specific surface area of 525 kg/m3. The chemical compositions of FA and slag are shown in Table 1, and their SEM images are shown in Figure 1(a) and (b), respectively. The SEM analysis of nano-SiO2, which was used as an additive in the geopolymer composite material, is shown in Figure 1(c), its physical characteristics are listed in Table 2, and its XRD pattern is shown in Figure 2, revealing that nano-silica was composed mostly of amorphous quartz.

Table 1

Chemical compositions of FA and slag (wt%)

Materials SiO2 CaO Al2O3 Fe2O3 MgO Na2O SO3 TiO2 K2O MnO
Fly ash 47.7 14.3 24 4.52 2.78 1.81 1.33 0.88 2.2 0.206
Slag 28.7 38.1 14.8 0.425 10.6 1.78 2.18 1.14 0.661 1.41
Figure 1 SEM micrographs of (a) FA, (b) slag, and, (c) nano-SiO2.
Figure 1

SEM micrographs of (a) FA, (b) slag, and, (c) nano-SiO2.

Table 2

Properties of nano-SiO2

Property Value
Specific surface area (m2/g) 170–220
Purity (%) ≥99.9
Density (g/cm3) 0.09–0.12
Surface properties Hydrophilicity
Appearance White powder
Figure 2 XRD pattern of nano-SiO2.
Figure 2

XRD pattern of nano-SiO2.

The fine aggregate was natural river sand with a fineness modulus of 2.76, and its particle size distribution, plotted in Figure 3, complied with the particle size grading of building sand specified in GB/T 14684-2011 test method for building sand [39]. Long CFs with a length of 6 mm and carbon content of 95% were used at a volume fraction of 0.6%, and the physical properties of the CFs are listed in Table 3.

Figure 3 Particle size distribution of river sand.
Figure 3

Particle size distribution of river sand.

Table 3

Various mechanical and physical properties of CFs

Properties Value Properties Value
Diameter (μm) 7 Tensile strength (MPa) 3,500
Density (g/cm3) 1.75 Elastic modulus (GPa) 228
Bulk density (g/cm3) 0.4 Resistivity (Ω‧cm) 1.0–1.6

Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution composed the alkaline activator solution. Industrial-grade flake NaOH was produced by Ningxia Jinyuyuan Chemical Group Co., Ltd with a purity of ≥98%. Na2SiO3 was a water glass produced by the Shanghai Building Materials Processing Factory in Xiaodian District, Taiyuan City, with an initial modulus of 3.3 and composition of 8.26% Na2O, 26.49% SiO2, and 65.25% H2O.

2.2 Geopolymer synthesis and fabrication of specimens

Table 4 lists the compositions of CFSGs with different amounts of nano-SiO2 used in this study. The fluidity, setting time, porosity, water absorption, compressive strength, flexural strength, uniaxial tensile performance, fracture toughness, and drying shrinkage of each CFSG specimen were evaluated. The ratios of NaOH and Na2SiO3 to the cementitious material were maintained at 0.35, the cementitious sand ratio was 0.38, and the water–cement ratio was 0.42. The water glass modulus of all CFSGs was 1.4. Our preliminary experimental results revealed that using 0.6% 6 mm CFs can produce geopolymer concrete with better comprehensive performance. At this time, the fiber dispersion is relatively uniform. Therefore, in this study, all CFSGs were prepared with 0.6% 6 mm CFs and nano-SiO2 contents of 1, 2, and 3%.

Table 4

Mix proportion of the CFSGs

Mix label FA (kg/m3) Slag (kg/m3) River sand (kg/m3) Sodium hydroxide (kg/m3) Sodium silicate (kg/m3) Water (kg/m3) CFs (vol%) Nano- SiO2 (kg/m3)
CFSG-0% SiO2 385 165 1,447 24.3 168.2 121.2 0.6 0
CFSG-1% SiO2 381.1 163.4 1,447 24.3 168.2 121.2 0.6 5.5
CFSG-2% SiO2 377.3 161.7 1,447 24.3 168.2 121.2 0.6 11
CFSG-3% SiO2 373.4 160.1 1,447 24.3 168.2 121.2 0.6 16.5

To prepare nano-SiO2 and CF-modified geopolymers, NaOH was mixed with Na2SiO3 solution for 1 day in advance, with NaOH particles dissolved in the Na2SiO3 solution. The alkalinity modulus was adjusted to 1.4, and the solution was stirred until the solid NaOH was completely dissolved, that is, until the solution was clear, and then it was cooled to room temperature to complete the preparation of the alkali activator.

As shown in Figure 4, during the formal mixing process, FA, mineral powder, nano-silica, and river sand were first poured into a mixing pot and stirred dry for 1 min. Then, a mixed solution of water and water glass was added and stirred slowly for 2 min, followed by rapid stirring for 2 min. Finally, CFs were added and stirred rapidly for 3 min to be evenly distributed. The CFSG was poured into the mold to form the desired sample shape. Finally, the sample was placed and molded in a standard curing room (temperature of 20 ± 3°C and relative humidity of 95%) for 1 day. Subsequently, it was demolded and cured continuously until the measured age.

Figure 4 Fabrication process of the CFSG with nano-SiO2.
Figure 4

Fabrication process of the CFSG with nano-SiO2.

2.3 Experimental methods

2.3.1 Workability and setting time

The flowability of freshly ground polymers was tested according to the GB/T 2419-2005 test method for fluidity of cement mortar [40]. Freshly mixed geopolymer slurry was poured into a truncated cone mold with base diameter, top diameter, and height of 100, 70, and 60 mm, respectively. Then, the truncated cone mold was lifted upward to enable the geopolymer slurry to flow and open the vibration table to withstand 25 vibrations of the slurry. Further, GB/T 17671-1999 (“Method of testing cements – Determination of strength”) [41] was confirmed using a Vicat needle apparatus to test the initial and final setting times of the CFSG.

2.3.2 Water absorption and porosity

Based on the specification of ASTMC 642-13 (“Standard Test Method for Density, Absorption, and Voids in Hardened Concrete”) [42], the specimen was 40 mm × 40 mm × 40 mm. The water absorption and porosity of the 160 mm prism mortar specimens were measured. First, each cured specimen was removed and dried in an oven at 110 ± 5°C for at least 24 h. The sample was then cooled to room temperature to determine its mass and measure its porosity and water absorption.

2.3.3 Mechanical properties

Compressive and flexural strength tests were performed on a 300 kN electrohydraulic servo universal test machine. According to JGJ/T 70-2009 (“Standard for test method of performance on building mortar”) [43] and JTG E30-2005 (“Test methods of cement and concrete for highway engineering”) [44], 40 mm × 40 mm × 160 mm standard specimens were tested to determine the compressive and flexural strengths of the geopolymers at 3, 7, and 28 days. The loading rates were 2,400 and 50 N/s, and the strength was calculated as the average of three test results.

2.3.4 Dry shrinkage

Based on JGJ/T 70-2009 (“Standard for test method of performance on building mortar”) [43] and ASTM C596-18 (“Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement ASTM International”) [45], after the molding was completed, the mold was immediately placed in a standard curing room and removed 1 day later. After dismantling the formwork, the specimens were placed in a standard curing room for water curing. After 2 days, they were moved to a chamber at constant temperature and relative humidity of 20 ± 2°C and 60 ± 5%, respectively. The measurement was performed after 1, 3, 7, 14, 28, 42, 60, and 90 days according to the specified time interval. A digital micrometer was used to measure the shrinkage deformation of the concrete with an accuracy of 0.001 mm. Before each shrinkage measurement, the zero point of the instrument was calibrated using a standard rod and rechecked at least one or two times during the measurement process. The drying shrinkage was calculated using equation (1).

(1) ε = L 0 − L t L − L d ,

where ε is the shrinkage value for a certain age period; L 0 is the initial length of the specimen; L t is the length of the corresponding test specimen at the age of testing; L is the preparation length of the specimen; and L d is the length of the shrinkage embedded specimen.

2.3.5 Uniaxial tensile test

Based on JC/T2461-2018 (“Standard test method for the mechanical properties of the ductile fiber reinforced cementitious composites. Beijing: China Building Material Industry Publishing House”) [46], the dumbbell-type specimen used for uniaxial tensile testing was 330 mm × 60 mm × 13 mm. The testing was conducted using the SHT4106-100t electrohydraulic servo universal material testing apparatus at a loading rate of 0.1 mm/min, as shown in Figure 5(a). The strain was measured using a clamp extensometer within a 50 mm equidistant measuring area, as shown in Figure 5(b). During the loading process, the computer automatically collected the load, and the extensometer collected the displacement at a collection frequency of 30 times/s to obtain the stress–strain curve.

Figure 5 Uniaxial tensile test. (a) Test setup and (b) geometric shape of the sample.
Figure 5

Uniaxial tensile test. (a) Test setup and (b) geometric shape of the sample.

2.3.6 Fracture toughness test

Based on the Chinese standard DL/T 5332–2005 (“Norm for fracture test of hydraulic concrete”) [47], using a three-point bending test to investigate the double-K fracture parameters of the geopolymer, 4 sets of 12 specimens with dimensions (B × H × L) of 40 mm × 40 mm × 160 mm were tested. The seam width, length, and height ratio were 2 mm, 16 mm, and 0.4, respectively. As shown in Figure 6, the fracture test parameters were loaded at a rate of 0.1 mm/min using a microcomputer-controlled electrohydraulic servo universal testing machine (100 KN), and the entire fracture test was conducted stably.

Figure 6 Three-point bending loading.
Figure 6

Three-point bending loading.

According to the double-K fracture rule [28], the initial fracture toughness ( K Ic ini ) was calculated using equations (2) and (3).

(2) K Ic ini = 1.5 P Q + mg 2 × 10 − 2 × 10 − 3 × S × a 0 t h 2 × f a 0 h ,

(3) f a 0 h = 1.99 − α ( 1 − α ) ( 2.15 − 3.93 α + 2.7 α 2 ) ( 1 + 2 α ) ( 1 − α ) 3 / 2 ,

where K Ic ini represents the initial fracture toughness (MPa m0.5), P Q is the initial load (N), m is the sample quality (kg), g is the gravitational acceleration (m/s2), S is the span between supports (m), a 0 is the initial crack length (m), h is the sample height (m), and t is the sample thickness (m).

Using K Ic un , the peak load (P max), and the critical equivalent fracture length (a c), the unstable fracture toughness ( K Ic un ) was calculated using equation (4).

(4) K Ic un = 1.5 P max + mg 2 × 10 − 2 × 10 − 3 × S × a c t h 2 × f a 0 h ,

where K Ic un is the unstable fracture toughness (MPa m0.5), P max is the peak load (N), and a c represents the effective crack width (m).

2.3.7 Microstructure analysis

Microscopic testing was conducted on the 28 days polymer slurry, mortar samples were formed, and the mineral composition of the solidified slurry was analyzed using the X’Pert powder type XRD. A Nicolet 5DXC FTIR was used to analyze the functional groups of the slurry, and a SDTQ600 synchronous thermal analyzer was used to test the TGA change characteristics of the geopolymer slurry. A Zeiss SEM was used to observe the micromorphology of the geopolymer mortar samples.

3 Results and discussion

3.1 Fluidity and setting time

As shown in Figure 7, as the substitution rate of nano-SiO2 increases, the fluidity of the geopolymer gradually decreases; however, the amplitude is not very significant. When the content of nano-SiO2 was 3% (CFSG-3% SiO2), the fluidity of the geopolymer was 141 mm, which is 10.2% lower than that of CFSG-0% SiO2 (157 mm). This is because nano-SiO2 has a small particle size and high surface energy, enabling it to absorb a large amount of free water from the mixture. Moreover, it is surrounded by a large number of unsaturated bonds, which have a strong attraction to water molecules, resulting in the formation of relatively strong chemical bonds between water molecules. This further reduces the free water in the geopolymer slurry, leading to a reduction in the mobility of the geopolymer. Evidently, nano-SiO2 can reduce the workability of geopolymer mortar during construction. In practical engineering, attention should be paid to selecting the appropriate amount of nano-SiO2 to maintain good fluidity of geopolymer mortar.

Figure 7 Fluidity of the CFSG with different amounts of nano-SiO2.
Figure 7

Fluidity of the CFSG with different amounts of nano-SiO2.

As shown in Figure 8, the addition of nano-SiO2 shortens the setting time of the geopolymer. Further, as the SiO2 content increases, the matrix setting time decreases. The initial setting time of the geopolymer mortar mixed with 3.0% nano-SiO2 (CFSG-3% SiO2) was 29.2% shorter than that of CFSG-0% SiO2, and the final setting time was 26.8% shorter. This is primarily because the “surface effect” of nano-SiO2 with a large surface area, which has high activity and surface energy, enriches the free phase near the surface of the nanomaterial, forming the “seed nucleation” effect, and accelerating the polymerization reaction. Simultaneously, increasing the concentrations of monomers such as –OSi(OH)3–, –OSi(OH)2O–, and Si in the system promotes the formation of polymers in the system, accelerates the hydration and coagulation hardening of FA/slag, and reduces the coagulation time [14,32].

Figure 8 Setting time of CFSGs with different amounts of nano-SiO2.
Figure 8

Setting time of CFSGs with different amounts of nano-SiO2.

In addition, with the incorporation of nanometers, the final setting time of CFSG ranged from 117 to 148 min, which is about 19.5–24.7% of the time required for the final setting of the ordinary Portland cement (around 600 min) [48]. Such a feature is considered as an advantage in repairing works to replace the high-cost repair binders and in other conditions in engineering applications where fast setting is required.

3.2 Porosity and water absorption

The effect of the nano-SiO2 content on the porosity and water absorption of the geopolymers is shown in Figure 9. Herein, with an increase in the nano-SiO2 content, the porosity and water absorption first decrease and then increase, and the trends of both changes are similar. When the nano-SiO2 content was 2.0% (CFSG-2% SiO2), the porosity and water absorption reached their lowest values of 9.012 and 18.327% and decreased by 0.086 and 0.484%, respectively, compared with the CFSG-0% SiO2 group. This is because after adding an appropriate amount of nano-SiO2, part of it can react with Ca(OH)2 generated from the hydration of FA and slag to generate a C–S–H gel, which renders the internal bonding of the system more compact, thus reducing the porosity. Another part of the nano-SiO2 that do not participate in the reaction play a filling role. Owing to its extremely small particle size, it can fill the internal pores of the system, improve the transition zone structure, optimize the pore structure, and reduce porosity. When the dosage of nano-SiO2 reaches 3.0%, it becomes prone to agglomeration and cannot be well dispersed during stirring, resulting in large defects and an increase in pores inside the geopolymer. Meanwhile, the large specific surface area of nano-SiO2 causes an increase in the amount of water adsorbed by the surface of the mixture.

Figure 9 Water absorption (a) and porosity (b) of CFSGs with different amounts of nano-SiO2.
Figure 9

Water absorption (a) and porosity (b) of CFSGs with different amounts of nano-SiO2.

3.3 Compressive strength

The effect of various nano-SiO2 contents on the compressive strength of CFSGs is shown in Figure 10(a). An increase in the amount of nano-SiO2 led to an increase in the compressive strength of up to 2%, whereas a 3% increase in the amount of nano-SiO2 led to a decrease in the compressive strength of CFSG but not below that of CFSG-0% SiO2. When 2.0% nano-SiO2 was added, the compressive strength of CFSG-2% SiO2 at 3, 7, and 28 days were 21.3, 31.2, and 38.2 MPa, respectively, which were 22.4, 16.9, and 7% higher than that of CFSG-0% SiO2. This is because of the chemical (high volcanic ash reactivity) and physical (filling effect) properties of nano-SiO2. This result is consistent with previous studies on the effect of the addition of nano-SiO2 on the mechanical properties of alkali-activated concrete [33,49–52]. However, when the nano-SiO2 content exceeded 2.0%, the strength of CFSG-3% SiO2 decreased. The van der Waals force between nanomaterials is relatively large, and the specific surface area is large, causing the nano-SiO2 particles to attract each other and agglomerate, thus rendering the dispersion of nano-sized particles challenging. Geopolymer mortar cannot be easily mixed, rendering the internal reaction of the geopolymer mortar insufficient. This results in a loose structure of the test block, thus adversely affecting the strength.

Figure 10 Compressive strength (a) and relative variations (b) in CFSGs with different amounts of nano-SiO2.
Figure 10

Compressive strength (a) and relative variations (b) in CFSGs with different amounts of nano-SiO2.

Moreover, the addition of nano-SiO2 primarily improves the early strength (3 and 7 days) of the geopolymer. For example, the 3-days strength of CFSG-1% SiO2 and CFSG-2% SiO2 increased by 14.4 and 22.4%, respectively, compared with the reference group, whereas the improvement in later strength (28 days) was not significant (Figure 10b), being only 4.8 and 7%, respectively. This is primarily because of the early nucleation effect of nanoparticles, promoting the early crystallization and nucleation of cement hydration. This further enhances the early strength. For later strength, most nanocrystalline nuclei cannot continue to grow, resulting in slow strength growth of the geopolymer mortar. Compared with geopolymer with no added microfibers, CFSGs with added micro CF and different nano-SiO2 contents (0, 1, 2, and 3%) exhibit higher compressive strengths [34,53]. This strongly demonstrates the advantages of adding an appropriate amount (0.6%) of CF and different amounts of nano-SiO2 to the CFSG.

3.4 Flexural strength

Figure 11 shows that the flexural strength of all nano-SiO2 composites were higher than that of CFSG-0% SiO2. With an increase in the nano-SiO2 content, the flexural strength of the concrete-filled steel tube first increased and then decreased. In particular, the flexural strength of CFSG-2% SiO2 was highest when the nano-SiO2 content was 2.0%. At this time, the flexural strengths of the polymer at 3, 7, and 28 days were 4.2, 5.3, and 6.2 MPa, respectively, increasing by 23.5, 15.2, and 5.1% compared with that of CFSG-0% SiO2. With the increase in curing age, the growth rate of the flexural strength slowed down (Figure 11b), which is consistent with the variation pattern of the compressive strength. The improvement in the flexural strength of the CFSG may be attributable to the enhancement of the CFSG matrix by nano-SiO2 and the enhancement of the adhesion between the micro CFs and the geopolymer matrix in the composite.

Figure 11 Flexural strength (a) and relative variations (b) in CFSGs with different contents of nano-SiO2.
Figure 11

Flexural strength (a) and relative variations (b) in CFSGs with different contents of nano-SiO2.

The conventional relationship between compressive strength (f t) and flexural strength (f cu) is an important aspect of concrete strength conversion. The empirical formulas for f t and f cu of ordinary concrete given in the American Concrete Institute (ACI) [54] code was calculated using equation (5).

(5) f t = 0.54 f cu .

The formula for the relationship between the compressive and flexural strengths of ordinary concrete cubes is no longer applicable to geopolymer concrete and must be revised. Based on the experimental results, the relationship between f t and f cu of the CFSG in this study is as shown in Figure 12. Through regression, the following formula can be obtained:

(6) f t = 1.3 f cu − 1.987 .

Figure 12 Relationship between the compressive and flexural strengths of the CFSG.
Figure 12

Relationship between the compressive and flexural strengths of the CFSG.

According to the data in the table, the average value of the ratio between the actual value of the experimental flexural strength and the value calculated using the formula was 1.093, indicating that the proposed equations with high accuracy (R 2 > 0.98) are consistent with the experimental outcomes.

In this study, the f t values reported for all CFSG samples were greater than those predicted by ACI 318 [54]. These results are consistent with those of Sofi et al. [55], who reported that the f t values of inorganic composites are generally greater than those of conventional Portland cement mixtures, owing to the presence of strong and dense interfacial transition zones in the CFSG matrix [35]. Compared with ACI 318, both f c and f t were significantly improved. Therefore, the addition of nano-SiO2 improved the mechanical properties of the CFSG.

3.5 Uniaxial tensile

The uniaxial tensile stress–strain curve of the geopolymer is shown in Figure 13. With the addition of CFs, even after the cracking of CFSG-0% SiO2, the stress decreased relatively slowly. An appropriate amount of nanodoping can improve the bearing capacity of the specimens. At 1 and 2% content, the area of the stress–strain curve of the CFSG-1% SiO2 and CFSG-2% SiO2 geopolymer specimens was larger than that of CFSG-0% SiO2, and it slightly increased with the increase in nanodoping. However, as the nanodosage reached 3%, the curve of CFSG-3% SiO2 decreased over a shorter period.

Figure 13 Stress–strain curves of CFSGs with different amounts of nano-SiO2.
Figure 13

Stress–strain curves of CFSGs with different amounts of nano-SiO2.

The uniaxial tensile test parameters calculated using the stress–strain curve are listed in Table 5, which show that the tensile strength, peak strain, and deformation modulus of the geopolymer first increase and then decrease with an increase in the nano-SiO2 content. Overall, the addition of nano-SiO2 did not significantly improve the tensile performance of the geopolymers. The highest uniaxial tensile performance was reached when the nano-SiO2 content was 2%. At this time, the tensile strength was 5.112 MPa, the peak strain was 0.446%, and the deformation modulus was 24.07 GPa. This was because nano-SiO2 utilized the filling and crystal nucleus effects, increasing the bonding force between the matrix and fibers, as well as the tensile strength. When the content of nano-SiO2 is 3%, owing to the relatively high van der Waals force between nanomaterials and the large specific surface area of nano-SiO2, nano-SiO2 particles attract each other and aggregate, adsorbing surface water during mixing, reducing the amount of water involved in hydration, and reducing the degree of hydration to a certain extent, thereby resulting in incomplete hydration, which affects the improvement of mortar fracture performance. In addition, excessive nano-SiO2 is difficult to disperse uniformly in geopolymer mortar, leading to agglomeration and inability to exert the filling and crystal nucleus effects of nano-SiO2, which results in internal defects in geopolymer mortar and renders it more susceptible to damage under tensile loading, resulting in some decrease in its tensile performance. Evidently, compared with the significant improvement effect of some single doped fibers on the uniaxial tensile properties of cement-based materials [56–58], the improvement effect of nano-SiO2-modified CF-reinforced geopolymer is not very significant; however, it remains better than that on the tensile properties of geopolymer composites (CFSG-0% SiO2) without single doped fibers.

Table 5

Key parameters of CFSGs with different amounts of nano-SiO2 in uniaxial direct tensile test

Mix ID σ fc / MPa σ ut / MPa ε ut / % E t / GPa
CFSG-0% SiO2 4.346 4.881 0.407 22.87
CFSG-1% SiO2 4.457 4.957 0.415 23.46
CFSG-2% SiO2 4.814 5.112 0.446 24.07
CFSG-3% SiO2 4.654 4.988 0.433 23.27

3.6 Fracture behavior

3.6.1 Load–crack mouth opening displacement (CMOD) curves

The load–CMOD curves of CFSGs with varying nano-SiO2 contents are depicted in Figure 14, showing that nano-SiO2 and CFs affect the fracture process of the CFSG. At the beginning of the experiment, the sample underwent linear deformation, followed by nonlinear formation owing to the formation of microcracks. When the content of nano-SiO2 was below 2%, as the content of nano-SiO2 increased, the load–CMOD curve of the specimen gradually became full. At 2%, the load–CMOD curve of CFSG-2% SiO2 was the highest. However, when the content of nano-SiO2 exceeded 2%, the fullness of the load–CMOD curve of CFSG-3% SiO2 decreased.

Figure 14 Load–CMOD curves of CFSGs with different amounts of nano-SiO2.
Figure 14

Load–CMOD curves of CFSGs with different amounts of nano-SiO2.

Figure 15(a) shows that the addition of nano-SiO2 can relatively improve the peak load of the sample. When the nano-contents were 1, 2, and 3%, the peak loads were 3.38, 8.78, and 1.35% higher than those of the reference group, respectively. This indicates that the nano-reinforced sample could withstand larger peak loads during nonlinear deformation. Particularly, when the dosage was 2%, the effect was the most significant. This was primarily because the incorporation of nanomaterials reduced the growth rate of cracks and hindered their development. As shown in Figure 15(b), when the nano-SiO2 contents were 1, 2, and 3% (by weight), the critical effective crack (a c) lengths were 0.31, 0.5, and 0.29 μm higher than the reference group, respectively.

Figure 15 Peak loads (a) and critical effective crack (b) of CFSGs with different contents of nano-SiO2.
Figure 15

Peak loads (a) and critical effective crack (b) of CFSGs with different contents of nano-SiO2.

3.6.2 Fracture toughness

After adding nano-SiO2 to the geopolymer, the initial fracture toughness and unstable fracture toughness of the specimen improved, and both first increased and then decreased with the increase in the nano-SiO2 content (Figure 16). The initial fracture and unstable fracture toughness of CFSG-2% SiO2 increased by 5 and 9.6%, respectively, compared with those of CFSG-0% SiO2. This is because when the content of nano-SiO2 is below 2%, nano-SiO2 is more evenly dispersed in the matrix. This can increase the chain length of C–S–H gel, give full play to the filling effect of nanoparticles, be closely combined with the matrix, render the interior of the test piece relatively dense, and improve the microstructure and performance of geopolymer mortar. Moreover, the CFs in the geopolymer plays a crack resistance role, rendering the development of the geopolymer from an unstable to a destructive state time-consuming. This leads to an improvement in the fracture performance. However, when the content of nano-SiO2 is extremely high (CFSG-3% SiO2), its initial fracture and unstable fracture toughness slightly decrease. However, a slight increase is observed compared with that of CFSG-0% SiO2 because an increase in the specific surface area of nano-SiO2 may result in the absorption of a large amount of surface water after mixing, reducing the amount of water involved in hydration, which results in a certain degree of reduction in hydration, incomplete hydration, and improvement in mortar fracture performance. In addition, excessive nano-SiO2 cannot easily be uniformly dispersed in geopolymer sand slurry, is prone to agglomeration, and is unable to exert the filling and crystal nucleation of nano-SiO2. Consequently, defects are generated inside the geopolymer mortar, significantly reducing the fracture performance of the composite mortar [13,14,25]. However, the fracture toughness of CFSG-3% SiO2 remains slightly better than that of CFSG-0% SiO2, indicating that the combined use of nano-SiO2 and CFs has a certain impact on the fracture toughness of the geopolymer composite material.

Figure 16 Fracture toughness of CFSGs with different contents of nano-SiO2. (a) initial fracture toughness ( K Ic ini {K}_{{\rm{Ic}}}^{{\rm{ini}}} ) and (b) unstable fracture toughness ( K Ic un {K}_{{\rm{Ic}}}^{{\rm{un}}} ).
Figure 16

Fracture toughness of CFSGs with different contents of nano-SiO2. (a) initial fracture toughness ( K Ic ini ) and (b) unstable fracture toughness ( K Ic un ).

3.6.3 Elastic modulus

Figure 17 shows that the elastic modulus of the CFSG with nano-SiO2 is greater than that of the reference group. The elastic modulus of CFSG-2% SiO2 is 9.8% higher than that of the control group (CFSG-0% SiO2) containing 0.6% CFs and no nano-SiO2. Similarly, the elastic modulus of CFSG-0% SiO2 is 5.2% higher than that of the control mixture (CFSG-0% SiO2). The elastic modulus of sample CFSG-3% SiO2 is 3.6% higher than that of the control group (CFSG-0% SiO2) containing 0.6% CFs and no nano-SiO2. Therefore, the sample with 2% nano-SiO2 has the highest axial stiffness and elastic modulus of 25.31 GPa owing to the effective combination of 2% nano-SiO2 and 0.6% CFs in the geopolymer composites.

Figure 17 Elastic modulus of CFSGs with different contents of nano-SiO2.
Figure 17

Elastic modulus of CFSGs with different contents of nano-SiO2.

3.7 Drying shrinkage

The effect of the nano-SiO2 content on the drying shrinkage performance of the geopolymers is depicted in Figure 18. Although CFs have an inhibitory effect on shrinkage, the incorporation of nanoparticles increases the dry shrinkage of geopolymers. The results showed that as the nano-SiO2 content increased from 1 to 3%, the geopolymers shrank rapidly. In addition, nanoparticles have a significant impact on the early shrinkage of the geopolymers. For nano-SiO2 contents of 1.0, 2.0, and 3.0%, the 3-days drying shrinkage rates of the geopolymers were 2.710 × 10–4, 3.049 × 10–4, and 3.369 × 10–4, respectively. Compared with that of CFSG-0% SiO2 (2.417 × 10–4), the growth rates were 12.1, 26.1, and 39.4%, respectively. This is because, at this point, the water in the internal micropores begins to gradually evaporate, and the nano-SiO2 promotes further hydration of the FA and slag. As the curing period was extended to 28 days, the drying shrinkage values of the specimens with 1.0, 2.0, and 3.0% nano-SiO2 increased by 11.6, 18.8, and 20.5% compared with that of CFSG-0% SiO2, respectively. When the curing period reached 90 days, the shrinkage values increased by 10.8, 15.8, and 17.5%, respectively. The drying shrinkage rate of the specimens tended to stabilize after 28 days of drying. Moreover, with an increase in content, the increasing trend of drying shrinkage gradually weakened. This is because the addition of a small amount of nano-SiO2 enhances hydration and stimulates the activity of the FA/slag while stimulating crystal nucleation effect, thereby providing an attachment point for the gel and accelerating the reaction process. When the dosage is considerable, part of nano-SiO2 does not react and functions as a pore filler. Some may experience agglomeration, inhibiting the drying shrinkage of the geopolymers.

Figure 18 Effect of nano-SiO2 contents on the drying shrinkage of CFSGs.
Figure 18

Effect of nano-SiO2 contents on the drying shrinkage of CFSGs.

3.8 XRD analysis

The XRD patterns of the geopolymers with different nano-SiO2 contents are shown in Figure 19. For nano-SiO2, the primary chemical composition is SiO2. Therefore, the crystal phases products of the geopolymer with nano-SiO2 were primarily the diffraction peaks of gismondine, mullite, calcite, quartz, hatrurite, and Portlandite. Notably, for the primary hydrated calcium silicate C–S–H, with an increase in the nano-SiO2 content, the height of the characteristic diffraction peak of the C–S–H gel and zeolite minerals first increased and then decreased. When the nano-SiO2 content was 2%, the peak value was highest, and the amount of product generated in the mortar matrix increased. This increase was conducive to an increase in the slurry strength, and the microstructure of the matrix was dense. Assaedi et al. [59] calculated the content of crystalline compounds (primarily quartz and mullite) and amorphous solid compounds in combination with quantitative X-ray analysis and showed that the presence of amorphous solid components in FA geopolymers was enhanced by the addition of nano-SiO2. The influence of the nano-SiO2 content on the content of amorphous solid compounds in the FA geopolymer was consistent with that on the strength. When the dosage was 3% (CFSG-3% SiO2), the peak value of the C–S–H phase decreased, implying that an excessive dosage of nano-SiO2 led to agglomeration and affected the matrix reaction.

Figure 19 XRD patterns of the CFSGs with different nano-SiO2 contents.
Figure 19

XRD patterns of the CFSGs with different nano-SiO2 contents.

3.9 FTIR analysis

The FTIR spectra of the geopolymer samples with different nano-SiO2 contents are shown in Figure 20. In the CFSG spectrum, a strong signal appeared between 3,300 and 3,650 cm–1 owing to the asymmetric and symmetrical stretching vibrations of OH molecules in the Si-OH group (3,600 cm–1) [60]. The absorption band splitting observed in the CFSG samples in the 1,350–1,530 cm–1 range is related to the C–O stretching of carbonate groups produced by the reaction of unreacted Na+ with atmospheric CO2 during geopolymerization, known as the weathering phenomenon [61], which indicates the elimination of attenuation due to distortion [62]. Similar reactions were observed by Djobo et al. [63] and Kaze et al. [64] for the synthesis of volcanic ash- and laterite-based polymers, respectively.

Figure 20 FTIR of the CFSGs with different nano-SiO2 contents.
Figure 20

FTIR of the CFSGs with different nano-SiO2 contents.

In the geopolymer samples, strong absorption bands that are consistent with the symmetric and asymmetric vibrations of the SI–O–T (T = Al or Si) bonds associated with the geopolymer network appeared at 1,006 and 1,012 cm–1 [65]. This indicated that a depolymerization condensation reaction occurred and a continuous three-dimensional network structure was formed inside the test block. With the addition of nanoparticles, the absorption band strengths at 1,006 and 1,012 cm–1 were enhanced, with the highest absorption band strength observed at a nanoparticle content of 2%. This indicates that nanoparticles promoted the degree of the depolymerization condensation reaction inside the CFSG, leading to an increasingly dense three-dimensional network structure formed inside the CFSG and thereby increasing its strength. With the addition of nano-SiO2, the strength of the Si–O–T bond increased, indicating that the amount of C(N)–(A)–S–H gel increased [66]. When the nanoparticle content increased from 2 to 3%, the vibration band strength at 1,012 cm–1 decreased because the aggregation rate of the nanoparticles increased, decreasing the connectivity. The symmetric tensile vibration modes of the Si–O–Si bond at 694 and 777 cm–1 indicated that quartz did not participate in the alkali activation. The spectral shoulder between 875 and 877 cm–1 was attributed to the presence of M−O vibrations in the gel [67].

3.10 TGA analysis

The thermal properties of the materials were evaluated using TGA. Herein, the thermal stability was investigated using the percentage of weight loss as a function of temperature [68]. The mass loss could be divided into two zones: from 20 to 300°C and from 300 to 800°C. The mass damage in the first region is usually related to damage to capillary (free), adsorbed, and chemically bound waters [69]. The capillary water evaporates at approximately 105°C. As shown in Figure 21, the incorporation of nano-Si reduces the mass damage in the first zone. Compared with that of the reference group CFSG-0% SiO2 (5.15%), the mass losses in CFSG-1% SiO2, CFSG-2% SiO2, and CFSG-3% SiO2 were 4.95, 4.85, and 4.91%, respectively. This may be because nano-SiO2 promoted the formation of geopolymer gels, leading to a reduction in the availability of unbound water in the system.

Figure 21 TGA of the CFSGs with different nano-SiO2 contents. (a) CFSG-0% SiO2, (b) CFSG-1% SiO2, (c) CFSG-2% SiO2, (d) CFSG-3% SiO2.
Figure 21

TGA of the CFSGs with different nano-SiO2 contents. (a) CFSG-0% SiO2, (b) CFSG-1% SiO2, (c) CFSG-2% SiO2, (d) CFSG-3% SiO2.

Within the range of 300–800°C, the mass loss occurs primarily because of the decomposition of CaCO3 and the dehydration of C–A–S–H gel structure water [70]. The decomposition of the reaction products in geopolymers involves a loss in chemically bound water (for example, structural water in the form of an –OH site) and the degradation of the gel structure [71]. The addition of nano-SiO2 increases the mass loss in the second stage, indicating a higher content of reaction products. The significant mass loss at approximately 620°C can be related to the decomposition of calcite [72]. From the overall mass damage perspective, the maximum mass damage occurs at 2%, because the nano-SiO2 in CFSG-2% SiO2 is involved in the polymerization reaction, promoting the generation of additional products and improving the mechanical properties. This observation is consistent with the XRD results (Figure 18). However, the amount of nano-SiO2 that does not react is small, resulting in the greatest mass damage. As the dosage further increases (CFSG-3% SiO2), the nanomaterials aggregate, leading to incomplete participation of nano-SiO2 in the reaction, which results in a decrease in mechanical properties. This indicates that the improvement in the strength of the geopolymers using nano-SiO2 is related to the filling and compaction effects, as well as the content of nano-SiO2 involved in the polymerization reaction.

3.11 SEM analysis

In geological polymer slurries, CFs can be used as microaggregates, and the fibers work closely with the geological polymer matrix to provide good crack resistance (Figure 22a). Figure 22b shows that few unhydrated FA particles are spherical with smooth surfaces. Most FA surfaces are attached to fluffy hydration and granular gel products. Unhydrated FA was embedded in a gel grid. Simultaneously, many blocky and angular crystal structures, which were speculated to be granular SiO2 crystals and Ca(OH)2, appeared around the FA, and a clustered gel was observed. With an increase in nano-SiO2, the microcracks of the geopolymer decreased, the voids in the matrix decreased, and the matrix became dense (Figure 22c). Figure 22c shows that the needle structure increased, indicating the coexistence of the amorphous gel and nano-SiO2, and even the filling of pores by nano-SiO2, which significantly reduces the number of pores and cracks and renders the structure sufficiently compact. In addition, several needle-shaped crystals (Aft) appeared on the surface of the hydrated gel because of the small particle size of nano-SiO2 as the crystal nucleus of the hydration product accelerates the hydration of geopolymer.

Figure 22 SEM of the CFSGs with different nano-SiO2 contents. (a) CFSG-0% SiO2. (b) CFSG-1% SiO2. (c) CFSG-2% SiO2. (d) CFSG-3% SiO2.
Figure 22

SEM of the CFSGs with different nano-SiO2 contents. (a) CFSG-0% SiO2. (b) CFSG-1% SiO2. (c) CFSG-2% SiO2. (d) CFSG-3% SiO2.

When the dosage exceeded 3% (Figure 22d), a certain number of pores appeared owing to the large specific surface area of nano-SiO2. Agglomeration occurred when the dosage was high, causing negative effects. According to the literature [49], nano-SiO2 agglomeration may be because of the small diameter of nano-SiO2 particles, preventing the dispersion of nano-SiO2 and leading to agglomeration during the re-stirring process. This confirms the corresponding mechanical test results.

4 Conclusion

This study investigated the effect of nano-SiO2 on the workability, mechanical properties, and microstructure of CFSGs. CF-reinforced geopolymer mortar without nano-silica was prepared as reference group, and geopolymer mortars with 1, 2, and 3% nano-SiO2 were prepared. The fluidity, setting time, water absorption, porosity, compressive strength, flexural strength, uniaxial tensile properties, fracture toughness, and microstructure were evaluated. The results can be summarized as follows:

  1. Owing to its large specific surface area, the incorporation of nano-SiO2 led to a decrease in the fluidity of the geopolymer, with the degree of decrease increasing with dosage. Moreover, the addition of nano-SiO2 shortened the setting time of the geopolymer. With an increase in the nano-SiO2 content, the porosity and water absorption of geopolymers first decreased and then increased.

  2. The optimal mechanical strength of the CF-reinforced geopolymer paste was obtained using 2% nano-SiO2. This is because nano-SiO2 was involved in the chemical reaction, particle filling, and seed nucleation in FA-based polymers. By increasing the monomer concentration, the degree of polymerization of the geopolymers increased, and the compactness of the geopolymer was improved, thereby improving its mechanical properties. However, as the amount of nano-SiO2 continued to increase (3%), the nanomaterials aggregated, which was detrimental to the compressive strength of the geopolymers.

  3. An appropriate amount of nano-SiO2 doping could relatively improve the bearing capacity of the specimens. With the increase in nano-SiO2 content, the tensile strength, peak strain, and deformation modulus of the geopolymer first increase and then decrease. The highest uniaxial tensile performance was exhibited when the nano-SiO2 content was 2%. At this time, the tensile strength was 5.112 MPa, the peak strain was 0.446%, and the deformation modulus was 24.07 GPa.

  4. The fracture parameters first increased and then decreased with increasing nano-SiO2 content, reaching their maximum values at 2% content. This enhancement might be attributed to the nanoparticle filling effect and the presence of CFs, which imply a bridging phenomenon that transfers the internal stress in the matrix, boosting the capacity to resist fracture initiation and propagation.

  5. Although the incorporation of CFs has a certain inhibitory effect on shrinkage, nano-incorporation increases the shrinkage of the geopolymer, and with increasing nanodosage, the shrinkage rate increases. The effect of nano-addition on the early shrinkage was significant, and the drying shrinkage rate of the sample tended to stabilize after 28 days of curing.

  6. The XRD, FTIR, and TGA analyses showed that the nanoparticles participated in the polymerization reaction, promoting the generation of hydration products. The SEM micrographs show that the nanomaterials transformed the structure of hardened geopolymer concretes into a compact form and served as fillers to these gaps in concrete structures. The CFs and geopolymer matrix exhibited good bonding properties, which may be attributed to the ability of nano-SiO2 to refine and provide a denser geopolymer composite microstructure.

  1. Funding information: This research was funded by the key research and development projects of Ningxia Province of China (Project No. 2022BDE93016 and 2021BEG02014), First Class Discipline Construction in Ningxia Colleges and Universities (discipline of water conservancy engineering) (NXYLXK 2021A03).

  2. Author contributions: Dongsheng Zhang: conceptualization, formal analysis, writing – original draft, writing – review and editing, and project administration. Qiuning Yang: writing – original draft, funding acquisition, and supervision. Yafan Wang: methodology, investigation, formal analysis, and writing – original draft. Jiabin Li: investigation, visualization, and supervision. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2023-08-28
Revised: 2023-10-18
Accepted: 2023-10-31
Published Online: 2023-12-07

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/ntrev-2023-0157
Keywords for this article
nano-SiO2; fracture toughness; geopolymer composites; uniaxial tensile; modification mechanism
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. Preparation of CdS–Ag2S nanocomposites by ultrasound-assisted UV photolysis treatment and its visible light photocatalysis activity
  3. Significance of nanoparticle radius and inter-particle spacing toward the radiative water-based alumina nanofluid flow over a rotating disk
  4. Aptamer-based detection of serotonin based on the rapid in situ synthesis of colorimetric gold nanoparticles
  5. Investigation of the nucleation and growth behavior of Ti2AlC and Ti3AlC nano-precipitates in TiAl alloys
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  9. Study on the chronic toxicity and carcinogenicity of iron-based bioabsorbable stents
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  15. Design of functional vancomycin-embedded bio-derived extracellular matrix hydrogels for repairing infectious bone defects
  16. Study on nanocrystalline coating prepared by electro-spraying 316L metal wire and its corrosion performance
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  18. Tungsten trioxide nanocomposite for conventional soliton and noise-like pulse generation in anomalous dispersion laser cavity
  19. Microstructure and electrical contact behavior of the nano-yttria-modified Cu-Al2O3/30Mo/3SiC composite
  20. Melting rheology in thermally stratified graphene-mineral oil reservoir (third-grade nanofluid) with slip condition
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  31. Brownian and thermal diffusivity impact due to the Maxwell nanofluid (graphene/engine oil) flow with motile microorganisms and Joule heating
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  34. Low-cost and large-scale preparation of ultrafine TiO2@C hybrids for high-performance degradation of methyl orange and formaldehyde under visible light
  35. Utilization of waste glass with natural pozzolan in the production of self-glazed glass-ceramic materials
  36. Mechanical performance of date palm fiber-reinforced concrete modified with nano-activated carbon
  37. Melting point of dried gold nanoparticles prepared with ultrasonic spray pyrolysis and lyophilisation
  38. Graphene nanofibers: A modern approach towards tailored gypsum composites
  39. Role of localized magnetic field in vortex generation in tri-hybrid nanofluid flow: A numerical approach
  40. Intelligent computing for the double-diffusive peristaltic rheology of magneto couple stress nanomaterials
  41. Bioconvection transport of upper convected Maxwell nanoliquid with gyrotactic microorganism, nonlinear thermal radiation, and chemical reaction
  42. 3D printing of porous Ti6Al4V bone tissue engineering scaffold and surface anodization preparation of nanotubes to enhance its biological property
  43. Bioinspired ferromagnetic CoFe2O4 nanoparticles: Potential pharmaceutical and medical applications
  44. Significance of gyrotactic microorganisms on the MHD tangent hyperbolic nanofluid flow across an elastic slender surface: Numerical analysis
  45. Performance of polycarboxylate superplasticisers in seawater-blended cement: Effect from chemical structure and nano modification
  46. Entropy minimization of GO–Ag/KO cross-hybrid nanofluid over a convectively heated surface
  47. Oxygen plasma assisted room temperature bonding for manufacturing SU-8 polymer micro/nanoscale nozzle
  48. Performance and mechanism of CO2 reduction by DBD-coupled mesoporous SiO2
  49. Polyarylene ether nitrile dielectric films modified by HNTs@PDA hybrids for high-temperature resistant organic electronics field
  50. Exploration of generalized two-phase free convection magnetohydrodynamic flow of dusty tetra-hybrid Casson nanofluid between parallel microplates
  51. Hygrothermal bending analysis of sandwich nanoplates with FG porous core and piezomagnetic faces via nonlocal strain gradient theory
  52. Design and optimization of a TiO2/RGO-supported epoxy multilayer microwave absorber by the modified local best particle swarm optimization algorithm
  53. Mechanical properties and frost resistance of recycled brick aggregate concrete modified by nano-SiO2
  54. Self-template synthesis of hollow flower-like NiCo2O4 nanoparticles as an efficient bifunctional catalyst for oxygen reduction and oxygen evolution in alkaline media
  55. High-performance wearable flexible strain sensors based on an AgNWs/rGO/TPU electrospun nanofiber film for monitoring human activities
  56. High-performance lithium–selenium batteries enabled by nitrogen-doped porous carbon from peanut meal
  57. Investigating effects of Lorentz forces and convective heating on ternary hybrid nanofluid flow over a curved surface using homotopy analysis method
  58. Exploring the potential of biogenic magnesium oxide nanoparticles for cytotoxicity: In vitro and in silico studies on HCT116 and HT29 cells and DPPH radical scavenging
  59. Enhanced visible-light-driven photocatalytic degradation of azo dyes by heteroatom-doped nickel tungstate nanoparticles
  60. A facile method to synthesize nZVI-doped polypyrrole-based carbon nanotube for Ag(i) removal
  61. Improved osseointegration of dental titanium implants by TiO2 nanotube arrays with self-assembled recombinant IGF-1 in type 2 diabetes mellitus rat model
  62. Functionalized SWCNTs@Ag–TiO2 nanocomposites induce ROS-mediated apoptosis and autophagy in liver cancer cells
  63. Triboelectric nanogenerator based on a water droplet spring with a concave spherical surface for harvesting wave energy and detecting pressure
  64. A mathematical approach for modeling the blood flow containing nanoparticles by employing the Buongiorno’s model
  65. Molecular dynamics study on dynamic interlayer friction of graphene and its strain effect
  66. Induction of apoptosis and autophagy via regulation of AKT and JNK mitogen-activated protein kinase pathways in breast cancer cell lines exposed to gold nanoparticles loaded with TNF-α and combined with doxorubicin
  67. Effect of PVA fibers on durability of nano-SiO2-reinforced cement-based composites subjected to wet-thermal and chloride salt-coupled environment
  68. Effect of polyvinyl alcohol fibers on mechanical properties of nano-SiO2-reinforced geopolymer composites under a complex environment
  69. In vitro studies of titanium dioxide nanoparticles modified with glutathione as a potential drug delivery system
  70. Comparative investigations of Ag/H2O nanofluid and Ag-CuO/H2O hybrid nanofluid with Darcy-Forchheimer flow over a curved surface
  71. Study on deformation characteristics of multi-pass continuous drawing of micro copper wire based on crystal plasticity finite element method
  72. Properties of ultra-high-performance self-compacting fiber-reinforced concrete modified with nanomaterials
  73. Prediction of lap shear strength of GNP and TiO2/epoxy nanocomposite adhesives
  74. A novel exploration of how localized magnetic field affects vortex generation of trihybrid nanofluids
  75. Fabrication and physicochemical characterization of copper oxide–pyrrhotite nanocomposites for the cytotoxic effects on HepG2 cells and the mechanism
  76. Thermal radiative flow of cross nanofluid due to a stretched cylinder containing microorganisms
  77. In vitro study of the biphasic calcium phosphate/chitosan hybrid biomaterial scaffold fabricated via solvent casting and evaporation technique for bone regeneration
  78. Insights into the thermal characteristics and dynamics of stagnant blood conveying titanium oxide, alumina, and silver nanoparticles subject to Lorentz force and internal heating over a curved surface
  79. Effects of nano-SiO2 additives on carbon fiber-reinforced fly ash–slag geopolymer composites performance: Workability, mechanical properties, and microstructure
  80. Energy bandgap and thermal characteristics of non-Darcian MHD rotating hybridity nanofluid thin film flow: Nanotechnology application
  81. Green synthesis and characterization of ginger-extract-based oxali-palladium nanoparticles for colorectal cancer: Downregulation of REG4 and apoptosis induction
  82. Abnormal evolution of resistivity and microstructure of annealed Ag nanoparticles/Ag–Mo films
  83. Preparation of water-based dextran-coated Fe3O4 magnetic fluid for magnetic hyperthermia
  84. Statistical investigations and morphological aspects of cross-rheological material suspended in transportation of alumina, silica, titanium, and ethylene glycol via the Galerkin algorithm
  85. Effect of CNT film interleaves on the flexural properties and strength after impact of CFRP composites
  86. Self-assembled nanoscale entities: Preparative process optimization, payload release, and enhanced bioavailability of thymoquinone natural product
  87. Structure–mechanical property relationships of 3D-printed porous polydimethylsiloxane films
  88. Nonlinear thermal radiation and the slip effect on a 3D bioconvection flow of the Casson nanofluid in a rotating frame via a homotopy analysis mechanism
  89. Residual mechanical properties of concrete incorporated with nano supplementary cementitious materials exposed to elevated temperature
  90. Time-independent three-dimensional flow of a water-based hybrid nanofluid past a Riga plate with slips and convective conditions: A homotopic solution
  91. Lightweight and high-strength polyarylene ether nitrile-based composites for efficient electromagnetic interference shielding
  92. Review Articles
  93. Recycling waste sources into nanocomposites of graphene materials: Overview from an energy-focused perspective
  94. Hybrid nanofiller reinforcement in thermoset and biothermoset applications: A review
  95. Current state-of-the-art review of nanotechnology-based therapeutics for viral pandemics: Special attention to COVID-19
  96. Solid lipid nanoparticles for targeted natural and synthetic drugs delivery in high-incidence cancers, and other diseases: Roles of preparation methods, lipid composition, transitional stability, and release profiles in nanocarriers’ development
  97. Critical review on experimental and theoretical studies of elastic properties of wurtzite-structured ZnO nanowires
  98. Polyurea micro-/nano-capsule applications in construction industry: A review
  99. A comprehensive review and clinical guide to molecular and serological diagnostic tests and future development: In vitro diagnostic testing for COVID-19
  100. Recent advances in electrocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid: Mechanism, catalyst, coupling system
  101. Research progress and prospect of silica-based polymer nanofluids in enhanced oil recovery
  102. Review of the pharmacokinetics of nanodrugs
  103. Engineered nanoflowers, nanotrees, nanostars, nanodendrites, and nanoleaves for biomedical applications
  104. Research progress of biopolymers combined with stem cells in the repair of intrauterine adhesions
  105. Progress in FEM modeling on mechanical and electromechanical properties of carbon nanotube cement-based composites
  106. Antifouling induced by surface wettability of poly(dimethyl siloxane) and its nanocomposites
  107. TiO2 aerogel composite high-efficiency photocatalysts for environmental treatment and hydrogen energy production
  108. Structural properties of alumina surfaces and their roles in the synthesis of environmentally persistent free radicals (EPFRs)
  109. Nanoparticles for the potential treatment of Alzheimer’s disease: A physiopathological approach
  110. Current status of synthesis and consolidation strategies for thermo-resistant nanoalloys and their general applications
  111. Recent research progress on the stimuli-responsive smart membrane: A review
  112. Dispersion of carbon nanotubes in aqueous cementitious materials: A review
  113. Applications of DNA tetrahedron nanostructure in cancer diagnosis and anticancer drugs delivery
  114. Magnetic nanoparticles in 3D-printed scaffolds for biomedical applications
  115. An overview of the synthesis of silicon carbide–boron carbide composite powders
  116. Organolead halide perovskites: Synthetic routes, structural features, and their potential in the development of photovoltaic
  117. Recent advancements in nanotechnology application on wood and bamboo materials: A review
  118. Application of aptamer-functionalized nanomaterials in molecular imaging of tumors
  119. Recent progress on corrosion mechanisms of graphene-reinforced metal matrix composites
  120. Research progress on preparation, modification, and application of phenolic aerogel
  121. Application of nanomaterials in early diagnosis of cancer
  122. Plant mediated-green synthesis of zinc oxide nanoparticles: An insight into biomedical applications
  123. Recent developments in terahertz quantum cascade lasers for practical applications
  124. Recent progress in dielectric/metal/dielectric electrodes for foldable light-emitting devices
  125. Nanocoatings for ballistic applications: A review
  126. A mini-review on MoS2 membrane for water desalination: Recent development and challenges
  127. Recent updates in nanotechnological advances for wound healing: A narrative review
  128. Recent advances in DNA nanomaterials for cancer diagnosis and treatment
  129. Electrochemical micro- and nanobiosensors for in vivo reactive oxygen/nitrogen species measurement in the brain
  130. Advances in organic–inorganic nanocomposites for cancer imaging and therapy
  131. Advancements in aluminum matrix composites reinforced with carbides and graphene: A comprehensive review
  132. Modification effects of nanosilica on asphalt binders: A review
  133. Decellularized extracellular matrix as a promising biomaterial for musculoskeletal tissue regeneration
  134. Review of the sol–gel method in preparing nano TiO2 for advanced oxidation process
  135. Micro/nano manufacturing aircraft surface with anti-icing and deicing performances: An overview
  136. Cell type-targeting nanoparticles in treating central nervous system diseases: Challenges and hopes
  137. An overview of hydrogen production from Al-based materials
  138. A review of application, modification, and prospect of melamine foam
  139. A review of the performance of fibre-reinforced composite laminates with carbon nanotubes
  140. Research on AFM tip-related nanofabrication of two-dimensional materials
  141. Advances in phase change building materials: An overview
  142. Development of graphene and graphene quantum dots toward biomedical engineering applications: A review
  143. Nanoremediation approaches for the mitigation of heavy metal contamination in vegetables: An overview
  144. Photodynamic therapy empowered by nanotechnology for oral and dental science: Progress and perspectives
  145. Biosynthesis of metal nanoparticles: Bioreduction and biomineralization
  146. Current diagnostic and therapeutic approaches for severe acute respiratory syndrome coronavirus-2 (SARS-COV-2) and the role of nanomaterial-based theragnosis in combating the pandemic
  147. Application of two-dimensional black phosphorus material in wound healing
  148. Special Issue on Advanced Nanomaterials and Composites for Energy Conversion and Storage - Part I
  149. Helical fluorinated carbon nanotubes/iron(iii) fluoride hybrid with multilevel transportation channels and rich active sites for lithium/fluorinated carbon primary battery
  150. The progress of cathode materials in aqueous zinc-ion batteries
  151. Special Issue on Advanced Nanomaterials for Carbon Capture, Environment and Utilization for Energy Sustainability - Part I
  152. Effect of polypropylene fiber and nano-silica on the compressive strength and frost resistance of recycled brick aggregate concrete
  153. Mechanochemical design of nanomaterials for catalytic applications with a benign-by-design focus
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