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Effects of nano-SiO2 modification on rubberised mortar and concrete with recycled coarse aggregates

  • Jianbai Zhao , Baifa Zhang , Jianhe Xie EMAIL logo , Yanhai Wu , Zhihao Wang and Peng Liu
Published/Copyright: January 18, 2022
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Abstract

With the recent acceleration of industrialisation and urbanisation, increasing quantities of demolished construction waste and waste tyres are being produced. The production of recycled coarse aggregate (RCA) and rubber particles from this waste, for use as partial or full replacements of normal aggregate in cement concrete, is attracting attention as a solution to the problem of solid waste management. However, the greater incidence of defects in RCA and rubber particles than in normal aggregate limits their application in construction industries. This study evaluated an economic and environmental approach to optimise the performance of rubberised concrete with RCA. Two types of nanomaterials, nano-SiO2 (NS) solution and NS sol–gel, were used to pretreat RCA and rubber. The effect of the treatment time on the physical properties of the RCA was tested, and the mechanical properties of the rubberised mortar prepared with pretreated rubber were investigated. In addition, a compression test for the rubberised recycled aggregate concrete (RRAC) was designed using the Taguchi method. The effects of four factors (water–cement ratio, rubber content, rubber size, and aggregate treatment) on the stress–strain curve, compressive strength, elastic modulus, specific toughness, and failure patterns of RRAC were also analysed. The results showed that the NS-treated RCA exhibited lower water absorption rate and better mechanical properties. Moreover, the NS-modified rubber enhanced the compressive and flexural strengths of the rubberised mortar by 35 and 17%, respectively. Interestingly, it was found that simultaneous treatment of both RCA and rubber could negatively affect RRAC. Scanning electron microscopy indicated that NS improved the interfacial transition zone separating RCA and rubber from the cement matrix, whereas the pretreated RCA tended to bond with the pretreated rubber in RRAC.

Graphical abstract

1 Introduction

With the recent acceleration of urbanisation, more buildings are being demolished, producing large amounts of demolished construction waste (DCW). It is reported that DCW will reach 27 billion tonnes per year worldwide by 2050 [1]. China produces 2.4 billion tonnes of DCW annually [2]. However, the utilisation rate of DCW is less than 10% in China, implying that the country needs to improve its recycling and reutilisation of DCW. In addition, it is reported that more than one billion tyres are scrapped every year around the world, which further increases the quantity of solid waste [3]. Landfills and burning remain the main methods of dealing with waste tyres [4]. DCW and waste tyres are serious ecological threats as they cause soil and water pollution. It is vital to recycle DCW and waste rubber, not only to protect the environment but also to conserve resources.

On the other hand, the consumption of raw materials for concrete production has increased rapidly with the rapid development of construction projects in developing countries, and the demand for more natural aggregates has resulted in a heavy burden on the environment [5]. Using crushed DCW as recycled coarse aggregate (RCA) to produce new environmentally friendly recycled concrete composites and members is of particular interest to maintain natural aggregate and realize waste resources recycling [6,7,8]. However, due to RCA’s high porosity and the presence of aged mortar on its surface, RCA addition degrades the mechanical properties of the concrete [9]. As for waste tyres, they can be crushed into rubber particles and added to concrete as fine aggregates [4,10]. Researchers have suggested that the addition of rubber to concrete improves its energy absorption capacity [11], sound absorption performance [12], and freeze–thaw durability [13]. However, the compressive strength, splitting tensile strength, and bending strength of rubberised concrete decrease, which is attributed to the low strength and high hydrophobicity of the rubber particles [14,15,16]. To investigate the synergistic effect of RCA and rubber particles in concrete materials and improve the utilisation rate of recycled materials in concrete, rubberised recycled aggregate concrete (RRAC) was developed [17,18]. RRAC is produced by replacing normal coarse aggregates with RCA and partly replacing fine aggregates with rubber. However, RRAC, which contains RCA and rubber, has been reported to exhibit poor static mechanical performance [17,18], which needs to be improved.

It is believed that pretreatment of aggregates plays an important role in improving the properties of RRAC. Many researchers have reported that the presence of aged mortar on the surface of RCA significantly influences the bonding between the RCA and new mortar [19,20]. Some methods, such as heating [21], acid treatment [22], and water washing [23], have been used to remove the attached aged mortar, whereas other methods, such as cement slurry coating [24], carbonation [25], and chemical solution soaking [26], have focused on strengthening the attached mortar. Moreover, chemical treatment has been shown to modify the surface of rubber to improve the interfacial adhesion between the rubber particles and the cement matrix. The most commonly used chemical agents are NaOH [27], silane coupling agent [28], and acid [29]. Although the abovementioned methods improve RCA and rubber, they suffer from some disadvantages. For instance, some methods involve complicated procedures [30], some introduce harmful ions (Cl and SO 4 2 ) [31], while others produce chemical waste [32], which make them difficult to apply widely in construction engineering.

As green and efficient materials, nanomaterials are widely used in various fields, and their application to cement-based materials has become a popular research topic [33,34]. A number of investigations have been conducted on concrete treated with nanomaterials, among which nano-SiO2 (NS) is considered the most effective additive [35,36]. Due to its high specific surface area and high reactivity, NS can decrease the porosity of concrete and promote the hydration reaction [37,38]. It has been established that NS addition is also suitable for concrete containing RCA or rubber, as it strengthens the interfacial transition zone (ITZ) between the paste and the aggregates [39,40]. In previous studies, researchers had tended to focus on the effect of NS on the cement matrix, and NS was added to concrete as an additive. It has been reported that RCA and rubber are the weakest parts of RRAC [41]. Effective pretreatment of the aggregates is critical for improving the properties of RRAC, which points to the need for further investigation on direct modification of the aggregates by NS.

The aim of this study is to develop a better understanding of the role of NS in aggregate modification. First, RCA and rubber particles were pretreated with two types of nanomaterial, NS solution and NS sol–gel. The effect of the treatment time on the physical properties of the RCA was tested, and the mechanical properties of the rubberised mortar prepared with the pretreated rubber were investigated using flexural and compression tests. In addition, a cylinder axial compression test for RRAC was designed using the Taguchi method with four factors and three levels. The stress–strain curve, compressive strength, elastic modulus, specific toughness and failure modes of RRAC were analysed. Finally, the microstructures of RCA and the rubber particles were observed using scanning electron microscopy (SEM), and the NS-strengthening mechanism was revealed.

2 Materials

2.1 Cementitious material

Ordinary Portland cement (OPC; P.O.42.5) with a fineness modulus of 1.1 was used as the cementitious material. Its physical properties and chemical composition are shown in Table 1.

Table 1

Properties and chemical composition of OPC

Content Fineness Loss on ignition
CaO (%) SiO2 (%) Fe2O3 (%) Al2O3 (%) SO3 (%) MgO (%)
62–67 20–24 5–6 4–7 2.12 0.9 1.1 1.7

2.2 Aggregates

The RCA used was produced by crushed waste concrete, which was obtained from demolished concrete pavement with a compressive strength of 20–30 MPa. It was washed with water to remove impurities and improve its properties. The physical properties of the RCA were tested using the Chinese standard GB/T 14685-2011 [42], and the results are tabulated in Table 2. River sand and rubber particles were used as fine aggregates. Well-graded river sand with a fineness modulus of 2.74 was dried before use. Rubber particles were produced from waste tyres by a factory in Foshan, China. Three rubber sizes – 6 mesh, 60 mesh, and a mixed size ranging from 6 mesh to 60 mesh – similar to the size of river sand were used. The particle size distribution of the aggregates is shown in Figure 1. Figure 2 shows the fine aggregates and their micromorphology.

Table 2

Physical properties of aggregates

Aggregates Size (mm) Apparent density (kg/m3) Bulk density (kg/m3) Water absorption rate (%) Crushing index (%)
RCA 5–20 2,596 1,354 4.82 10.47
River sand 0.075–4.75 2,500 1,780 0.61
Rubber 0.25–3.35 1,050 608
Figure 1 
                  Sieve analysis results of aggregates.
Figure 1

Sieve analysis results of aggregates.

Figure 2 
                  Aggregates used and their micromorphology as revealed by SEM. (a) RCA, (b) river sand (c) 6 mesh rubber, (d) 60 mesh rubber, (e) SEM of sand, (f) SEM of 60 mesh rubber, and (g) SEM of 6 mesh rubber.
Figure 2

Aggregates used and their micromorphology as revealed by SEM. (a) RCA, (b) river sand (c) 6 mesh rubber, (d) 60 mesh rubber, (e) SEM of sand, (f) SEM of 60 mesh rubber, and (g) SEM of 6 mesh rubber.

2.3 Additives

A polycarboxylate superplasticiser (SP) solution was used to adjust the workability of the mixes. The SP solution was diluted to a concentration of 20% from the mother liquor, which had a solid content of 30.1%. The volume percentage of air in the SP solution was 5%. The properties of the SP solution are shown in Table 3.

Table 3

Properties of the superplasticiser provided by the manufacturer

Colour Concentration (%) Water reduction (%) Air content (%) Solid content (%) Contraction ratio (%)
Light yellow 20 20 3.2 30.1 ≥97

2.4 Nanomaterials for modifying aggregates

Two types of nanomaterials, NS solution and NS sol–gel, were used to modify the rubber and RCA. NS solution with 1% NS powder (Nanjing Tansail Co., China) by weight was prepared. The mother liquor of the commercial NS sol–gel (Nanjing Tansail Co., China) had 20% NS by weight. The nanomaterials used are shown in Figure 3, and their properties are shown in Tables 4 and 5.

Figure 3 
                  Nanomaterials used and SEM of NS powder. (a) NS powder, (b) SEM of NS powder, (c) NS solution, and (d) NS sol–gel.
Figure 3

Nanomaterials used and SEM of NS powder. (a) NS powder, (b) SEM of NS powder, (c) NS solution, and (d) NS sol–gel.

Table 4

Properties of NS powder

Size (nm) Purity (%) Specific surface area (m2/g) Bulk density (g/cm3)
20 99 200 0.1
Table 5

Properties of NS sol–gel

SiO2 (wt%) Size (nm) pH Viscosity (mPa⋅s) Density (g/cm3)
20 100 6–9 <100 1.21

3 Experimental regime

3.1 Aggregate treatment

Owing to the low solubility and easy agglomeration of NS powder, NS solution with a low concentration of 1% NS powder was manually prepared in this study. In addition, considering the particle size of NS in sol–gel is 5 times bigger than that in the NS powder solution (Tables 4 and 5), the well-dispersed NS sol–gel with a concentration of 20% NS was diluted to a concentration of 5% prior to use.

Figure 4 shows the treatment processes of RCA and rubber particles. Both the RCA and rubber particles were first immersed in NS solution or NS sol–gel, respectively, for 24, 48, and 72 h. Then, they were dried at room temperature before using.

Figure 4 
                  Treatment methods: (a) Treatment methods of RCA; (b) treatment methods of rubber for preparing rubberised mortar; and (c) treatment methods of RCA and rubber for preparing RRAC. *Combined method denotes simultaneous treatment of rubber and RCA.
Figure 4

Treatment methods: (a) Treatment methods of RCA; (b) treatment methods of rubber for preparing rubberised mortar; and (c) treatment methods of RCA and rubber for preparing RRAC. *Combined method denotes simultaneous treatment of rubber and RCA.

To investigate the modification effect of RCA by NS treatment, the water absorption rate and crushing index of pretreated RCA were tested and the optimal NS type and treatment time were evaluated, as shown in Figure 4(a).

To investigate the effect of the modified rubber by NS treatment, rubberised mortar was prepared and the mechanical properties of the rubberised mortar were tested, as shown in Figure 4(b).

Based on the above results of the physical properties of the modified RCA and mechanical properties of the mortar with modified rubber, three aggregate treatment methods, including rubber modification (RM), RCA modification (AM), and simultaneous modification of rubber and RCA (RM + AM), were designed for preparing RRAC (Figure 4c).

3.2 Mix proportions

3.2.1 Rubberised mortar

To meet the flowability requirements, seven groups of mortar mixtures with a water–cement (w/c) ratio of 0.5 were designed, as shown in Table 6. The 60 mesh rubber particles were used and treated with the NS solution or NS sol–gel for 24, 48, and 72 h. Following the suggestion of previous study [18], the rubber was used to replace 10% of the river sand by volume.

Table 6

Mix proportions of the mortar mixtures

Mix proportions (unit weight: kg/m3)
Group Water Cement Sand Rubber NS type Treatment time
Reference 225 450 1,305 32
S24 225 450 1,305 32 NS solution 24 h
S48 225 450 1,305 32 NS solution 48 h
S72 225 450 1,305 32 NS solution 72 h
G24 225 450 1,305 32 NS sol–gel 24 h
G48 225 450 1,305 32 NS sol–gel 48 h
G72 225 450 1,305 32 NS sol–gel 72 h

Note: S24 and G24 denote rubber treated by NS solution and NS sol–gel for 24 h, respectively.

3.2.2 RRAC

The Taguchi method was used to design the mix proportions of RRAC. Four factors – the w/c ratio, rubber content, rubber size, and processing method for the aggregates – were considered, and each factor comprised three levels. Therefore, an orthogonal array L9 (34) developed by Taguchi was used to represent a full factorial experiment. The factors and levels used are shown in Table 7. Three w/c ratios were investigated – 0.3, 0.4, and 0.5 – and rubber was used to replace 5, 10, and 15% of the river sand by volume. Three rubber sizes were used: 6 mesh, 60 mesh, and 6–60 mesh (mixed size). As shown in Figure 4, three aggregate treatment methods were used for preparing RRAC, including RM, AM, and RM + AM. Both RCA and rubber were treated by NS sol–gel for 48 h, which was in accordance with the synthetical consideration of the performance improvement and economic cost (immersion time). Nine groups of mix proportions were designed, as shown in Table 8.

Table 7

Factors and levels used in the experiment

Levels Factor A Factor B Factor C Factor D
w/c ratio Rubber content (%) Rubber size Aggregate treatment
1 0.3 5 6 Mesh RM
2 0.4 10 60 Mesh AM
3 0.5 15 Mix RM + AM
Table 8

Mix proportions of RRAC used in the experiment

Mix proportions (unit weight: kg/m3)
Group Orthogonal combination Water Cement Sand RCA Rubber SP
0.3–5%-6-RM A1B1C1D1 185 617 607.2 958.8 12.9 1.9
0.3–10%-60-AM A1B2C2D2 185 617 575.3 958.8 25.8 1.9
0.3–15%-M-RM + AM A1B3C3D3 185 617 543.3 958.8 38.8 1.9
0.4–5%-60-RM + AM A2B1C2D3 185 463 665.8 1051.2 14.2 1.4
0.4–10%-M-RM A2B2C3D1 185 463 630.7 1051.2 28.4 1.4
0.4–15%-6-AM A2B3C1D2 185 463 595.7 1051.2 42.5 1.4
0.5–5%-M-AM A3B1C3D2 185 370 701.1 1,107 14.9 1.2
0.5–10%-6-RM + AM A3B2C1D3 185 370 664.2 1,107 29.8 1.2
0.5–15%-60-RM A3B3C2D1 185 370 627.3 1,107 44.8 1.2

3.3 Specimen preparation

3.3.1 Rubberised mortar specimen

To examine the compressive and flexural properties of the rubberised mortar, prismatic specimens with length of 40 mm, width of 40 mm, and height of 160 mm were prepared and divided into seven groups. Each group consisted of a set of three specimens. The specimens were prepared using a 5 L cement mortar mixer by following the steps shown in Figure 5(a): first, the rubber particles were pretreated with NS solution or NS sol–gel and dried at room temperature; then, the cement, river sand, and rubber particles were mixed under dry conditions for 5 min, after which water was added and mixed with the powder for 5 min. Next the mixture was moulded and vibrated for 1 min. After 24 h, it was demoulded and cured at room temperature for 28 days.

Figure 5 
                     Specimen preparation methods: (a) rubberised mortar and (b) RRAC.
Figure 5

Specimen preparation methods: (a) rubberised mortar and (b) RRAC.

3.3.2 RRAC specimen

A 30 L horizontal mixer was used to prepare 27 RRAC cylinder specimens with diameter of 100 mm and height of 200 mm. Figure 5(b) shows the preparation procedure: first, the rubber particles and RCA were pretreated with NS sol–gel for 48 h, and dried at room temperature. Next cement, river sand, and the rubber particles were mixed under dry conditions for 5 min, after which SP and half of total water were added to the mixture and mixed for 3 min. Finally, RCA and the remaining water were added, and the mixing was continued for 5 min. After mixing, the mixture was moulded and vibrated for 2 min. After 24 h, it was demoulded and cured at room temperature for 28 days.

3.4 Experimental methods

3.4.1 Tests for RCA properties

The water absorption rate and crushing index of RCA after treatment were tested in accordance with the Chinese standard GB/T 14865-2011 [42]. The water absorption rate of RCA was calculated using equation (1):

(1) W = G 1 G 2 G 2 ,

where W is the water absorption rate of RCA; G 1 is the mass of the aggregate sample under the saturated surface dry condition; and G 2 is the mass of the dried RCA.

The crushing index of RCA was calculated using equation (2):

(2) Q e = M 1 M 2 M 1 ,

where Q e is the crushing index of RCA; M 1 is the mass of the aggregate sample; and M 2 is the residual mass of the RCA after crushing.

3.4.2 Prism flexural and compression tests

The prism flexural test of the mortar specimens was conducted in accordance with the Chinese standard GB/T 17671-1999 [43], using a YAW-300C machine with the loading speed set to 50 N/s, as shown in Figure 6. After the flexural test, the compression test was conducted on a lateral section of the broken prism with a compression area of 40 mm × 40 mm. The compression test was also conducted using the YAW-300C machine with the loading speed set to 2,400 N/s.

Figure 6 
                     Prism flexural and compression test machine.
Figure 6

Prism flexural and compression test machine.

3.4.3 Cylinder axial compression test

The cylinder axial compression test was conducted by the Chinese standard GB/T 50081-2019 [44] using a Matest material testing machine, which has a capacity of 4,000 kN. The loading speed was set to 0.12 mm/min. Loading was stopped when the stress decreased to 10% of the peak stress. Two linear variable displacement transducers were used to measure the axial strain of the specimen. In addition, two strain gauges were attached to the middle of the cylinder to measure the hoop strains of the specimen, as shown in Figure 7. Before testing, plaster with a compressive strength of 800 MPa was used to level the surface of the cylinder to eliminate load eccentricity. Both the load data and strain data were collected by a high-speed acquisition system at a frequency of 1 Hz.

Figure 7 
                     Cylinder compression test machine.
Figure 7

Cylinder compression test machine.

3.4.4 SEM test

To observe the microstructure of the specimen after the compression test, SEM test was performed using an S3400N machine with a working distance of 4 mm and an accelerating voltage of 15 kV. The samples, which were obtained from the fracture surfaces of the specimens, were less than 10 mm × 10 mm × 10 mm in size. The samples were gold coated and tested under high vacuum.

4 Result and discussion

4.1 Effect of treatment on water absorption rate and crushing index of RCA

Figure 8 shows the effect of the treatment time on the water absorption rate of RCA. Increasing the treatment time significantly reduced the water absorption rate of RCA. The water absorption rate of RCA treated with NS solution decreased linearly as the treatment time increased. With a treatment time of 72 h, the water absorption rate of RCA decreased by over 30%. This result can be explained by the pozzolanic reaction between the NS particles and Ca(OH)2 and the filling effect of the NS particles, which densified the microstructure of RCA. The water absorption rate of RCA treated with NS sol–gel was much lower than that treated with NS solution. For instance, when RCA was treated with NS sol–gel for 24, 48 and 72 h, the water absorption rate was decreased by 28, 32, and 56%, respectively. This is because a layer of silica gel from the well-dispersed NS sol–gel was formed homogeneously on the surface of RCA, leading to the attachment of considerable number of agglomerated NS particles [39,45].

Figure 8 
                  Effect of treatment time on the water absorption rate of RCA.
Figure 8

Effect of treatment time on the water absorption rate of RCA.

As shown in Figure 9, NS also greatly influenced the crushing index of RCA. After treatment with either NS solution or NS sol–gel, the crushing index decreased approximately linearly with increasing treatment time. After treatment with NS sol–gel, the crushing index of RCA decreased by approximately 10% more than that after treatment with NS solution at same treatment time on average. The improvement in the crushing index of RCA treated with NS sol–gel is attributed to the reaction of NS particles with aged mortar to form C–S–H gel, which increased the strength of the RCA. Previous studies also reported that the average microhardness of the aged mortar at the surface layer of RCA can be enhanced after NS treatment [39,45].

Figure 9 
                  Effect of treatment time on the crushing index of RCA.
Figure 9

Effect of treatment time on the crushing index of RCA.

4.2 Effect of treatment on compressive and flexural strength of rubberised mortar

The effect of NS treatment on the compressive strength of the rubberised mortar is shown in Figure 10. The compressive strength of the rubberised mortar increased with the treatment time. For instance, after treatment with NS solution for 72 h, the compressive strength of the rubberised mortar was 23.93 MPa, an increase of 35% compared with that of untreated mortar. This can be attributed to the consumption and refinement of Ca(OH)2 crystals by NS, making the rubber particles and cement paste connect closely with the C–S–H crystal to form a more compact concrete structure [46,47].

Figure 10 
                  Effect of treatment time on compressive strength of rubberised mortar.
Figure 10

Effect of treatment time on compressive strength of rubberised mortar.

A similar improvement was also found in the rubberised mortar treated with NS sol–gel. Moreover, treatment of rubber with NS sol–gel produced a slight increase in the compressive strength of the rubberised mortar compared with that in the NS solution treatment. A possible reason is that NS sol–gel attaches more evenly on the rubber surface. Another interesting finding is that when the treatment time was changed from 48 to 72 h, the increase in the compressive strength slowed down, demonstrating that 48 h treatment is sufficient to improve its compressive strength if time cost needs to be considered.

Figure 11 demonstrates the effect of NS treatment on the flexural strength of the rubberised mortar. With treatment times of 24, 48, and 72 h, the flexural strength of the rubberised mortar increased by 12, 14, and 17% after treatment with NS solution, respectively. It has been reported that the addition of rubber can improve the toughness of the mortar. However, due to the hydrophobicity of the rubber particles, a substantial amount of air enters the mortar. As a result, the number of pores increases, and the bonding between rubber and the cement matrix is weakened [46,47]. The NS attached to the surface of the rubber significantly improved the hydrophilic property.

Figure 11 
                  Effect of treatment time on flexural strength of rubberised mortar.
Figure 11

Effect of treatment time on flexural strength of rubberised mortar.

The type of NS material showed only a slight influence on the flexural strength of the rubberised mortar: NS sol–gel performed a little better than NS solution. For instance, with a treatment time of 24 h, there was no difference between the flexural strengths of rubberised mortar treated with NS sol–gel and with NS solution. A possible reason for the indistinguishable effects of the two treatments at 24 h is that treatment time is too short for NS sol–gel to adhere to the rubber particles effectively. In addition, for the treatment time of 48 and 72 h, there is no change in the flexural strength of NS sol–gel-treated rubberized mortar. This could be attributed to the reason that the rubber was covered by a layer of silica gel, which prevented nanoparticles from continuing to adhere to the rubber particles, thus limiting the improvement in flexural strength. In general, the mechanical properties of the rubberised mortar improved dramatically after rubber was treated with NS sol–gel. However, a turning point was reached (48 h for NS sol–gel) beyond which the improvement in the properties of rubberised mortar with increasing treatment time slowed down. Therefore, considering time cost, 48 h was regarded as the optimum time, and it was the treatment time used with RRAC. However, for NS solution, the turning point of the treatment time occurred at 24 h, which is mainly attributed to the lower concentration and poorer dispersion of NS solution compared with NS sol–gel.

4.3 Effect of orthogonal factors on RRAC

The compressive properties of the tested RRAC specimens are listed in Table 9, including compressive strength, elastic modulus, and specific toughness. It is noted that these results in Table 9 are the average values of three same specimens.

Table 9

Compression test results of RRAC

Group w/c ratio Rubber content (%) Rubber size (mesh) Aggregate treatment Compressive strength (MPa) Elastic modulus (GPa) Specific toughness (%)
0.3–5%-6-RM 0.3 5 6 RM 51.91 49.10 0.37
0.3–10%-60-AM 0.3 10 60 AM 46.76 40.03 0.45
0.3–15%-M-RM + AM 0.3 15 Mix RM + AM 32.21 30.38 0.43
0.4–5%-60-RM + AM 0.4 5 60 RM + AM 31.88 30.00 0.44
0.4–10%-M-RM 0.4 10 Mix RM 18.68 30.42 0.70
0.4–15%-6-AM 0.4 15 6 AM 31.32 46.35 0.38
0.5–5%-M-AM 0.5 5 Mix AM 23.19 24.61 0.82
0.5–10%-6-RM + AM 0.5 10 6 RM + AM 17.75 29.06 0.68
0.5–15%-60-RM 0.5 15 60 RM 18.38 20.45 0.65

4.3.1 Stress–strain curve

The axial compressive stress–strain relations of the RRAC specimens are shown in Figure 12. It is observed that similar to the stress–strain curve of OPC concrete, the curve of RRAC mainly consists of an elastic ascending section and an inelastic descending section. The incorporation of rubber led to a flatter curve in the ascending section compared with the curve of recycled concrete in a previous study [48].

Figure 12 
                     Stress–strain curve of RRAC.
Figure 12

Stress–strain curve of RRAC.

Figure 13 shows that the w/c ratio greatly influences the stress–strain curve of RRAC. Expectedly, as the w/c ratio decreased, the curve of the ascending section became steeper, and the peak stress increased, indicating that a lower w/c ratio can significantly improve the compressive strength and stiffness of RRAC, but at the expense of greater brittleness. The descending section of the curve showed a flatter trend with increasing w/c ratio. A possible explanation for this is that a softer cement matrix makes it easier for the toughness of rubber to manifest itself.

Figure 13 
                     Stress–strain curve of RRAC with different w/c ratios of: (a) 0.3, (b) 0.4, and (c) 0.5.
Figure 13

Stress–strain curve of RRAC with different w/c ratios of: (a) 0.3, (b) 0.4, and (c) 0.5.

Figure 14 shows the effect of the rubber content on the stress–strain response of RRAC. In general, in the ascending section of the curve, which corresponds to greater rubber content, RRAC exhibited a lower peak stress, implying that the addition of rubber decreased the strength of RRAC. Comparing Figure 14(a) and (c), it is clear that with increasing rubber content, the descending section of the stress–strain curve of RRAC became flatter. This result is consistent with the characteristics of rubberised concrete with and without the recycled aggregate [49,50], which are caused by the greater toughness of rubber compared with river sand. The figure also shows that the hoop strain of RRAC increased with higher rubber content, proving that the weaker aggregate tended to produce more ductile concrete than the stronger aggregate.

Figure 14 
                     Stress–strain curve of RRAC with different rubber contents of: (a) 5%, (b) 10%, and (c) 15%.
Figure 14

Stress–strain curve of RRAC with different rubber contents of: (a) 5%, (b) 10%, and (c) 15%.

Figure 15 shows the stress–strain curve of RRAC with different rubber sizes. In the ascending part of the curve, with the same unit stress increase, the strain developed in RRAC with 60 mesh rubber was higher than that developed in RRAC with 6 mesh rubber. That is, smaller rubber particles decreased the elastic modulus of RRAC. However, no significant change was found in the descending stage of the stress–strain curves of RRAC with 6 mesh and 60 mesh rubber. In addition, the inclusion of mixed-size rubber particles lowered the peak stress and flattened the descending section of the stress–strain curve. These results indicate that well-graded rubber particles decreased the strength of RRAC, but absorbed energy more effectively after the peak stress.

Figure 15 
                     Stress–strain curve of RRAC with different rubber sizes: (a) 6 mesh rubber, (b) 60 mesh rubber, and (c) mixed-size rubber.
Figure 15

Stress–strain curve of RRAC with different rubber sizes: (a) 6 mesh rubber, (b) 60 mesh rubber, and (c) mixed-size rubber.

The type of aggregate treatment method is also an important factor for the stress–strain curve of RRAC, as shown in Figure 16. A comparison of Figure 16(a) and (b) indicates that RM improved the toughness of RRAC and AM improved its strength, which is consistent with the results of earlier studies on RCA concrete and rubberised concrete [51,52]. However, the curves of RRAC with RM and AM simultaneously display neither of the abovementioned features, as shown in Figure 16(c).

Figure 16 
                     Stress–strain curves of RRAC with different aggregate treatment methods: (a) RM, (b) AM, and (c) RM and AM.
Figure 16

Stress–strain curves of RRAC with different aggregate treatment methods: (a) RM, (b) AM, and (c) RM and AM.

4.3.2 Compressive strength

To investigate the effect of orthogonal factors on the compressive strength of RRAC, Figure 17 presents the average compressive strength of the specimens with the same level of orthogonal factor. Similar analysis is used hereinafter for the elastic modulus and specific toughness of the RRAC.

Figure 17 
                     Effects of orthogonal factors on compressive strength of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber particle size, and (d) treatment method.
Figure 17

Effects of orthogonal factors on compressive strength of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber particle size, and (d) treatment method.

Figure 17(a) shows that the compressive strength of RRAC decreased rapidly as the w/c ratio was increased, which has also been reported for rubberised concrete without RCA [53]. The compressive strength of RRAC decreased by 37 and 55%, respectively, as the w/c ratio was increased from 0.3 to 0.4 and 0.5. It is believed that RRAC is sensitive to the influence of the w/c ratio, which is attributed to the high water absorption rate of RCA [54].

Figure 17(b) shows that the rubber content decreased the compressive strength of RRAC. This reduction was mainly caused by the low strength and stiffness of the rubber particles, and the weak bonding between rubber and the cement paste. However, the compressive strength of RRAC did not decrease linearly with the increasing rubber content. Changing the rubber content from 10% to 15% had little effect on the compressive strength. This is in line with the results obtained from the rubberised concrete without RCA [14,55].

Figure 17(c) shows the influence of the rubber size on the compressive strength of RRAC. For single-size rubber particles, the compressive strength of RRAC decreased slightly as the rubber size was decreased. This is mainly because the rubber particles act as a solid hydrophobic air-entraining agent, and smaller rubber particles with a larger specific surface area have better gas adsorption capacity [56]. However, the compressive strength of RRAC with mixed-size rubber particles was significantly lower than that of RRAC with single-size rubber particles, possibly because well-graded rubber particles tend to aggregate and form defects in concrete.

Figure 17(d) shows that among the three aggregate treatment methods, AM has the best effect on compressive strength. However, simultaneous treatments with RM and AM decreased the compressive strength. This could be because the NS molecules remaining on the surface of the rubber and RCA tended to aggregate, which made it easier for rubber particles to adhere to the surface of RCA. Thus, weak areas were produced, which reduced the bonding between the aggregates and mortar.

4.3.3 Elastic modulus

The effects of the orthogonal factors on the elastic modulus of RRAC are shown in Figure 18. According to the Chinese standard GB/T 50081-2019 [44], the elastic modulus of the tested specimen was defined as the secant modulus of its stress–strain curve at the point of the 1/3 peak stress. As shown in Figure 18(a), expectedly, the elastic modulus of RRAC decreased by 10 and 38% as the w/c ratio was changed from 0.3 to 0.4 and 0.5, respectively, which is consistent with a previous investigation on the rubberised concrete [53]. The mortar matrix became softer because it contained more water, resulting in a relative incompact system.

Figure 18 
                     Effect of orthogonal factors on elastic modulus of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber size, and (d) aggregate treatment.
Figure 18

Effect of orthogonal factors on elastic modulus of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber size, and (d) aggregate treatment.

Figure 18(b) shows the effect of rubber content on the elastic modulus of RRAC. The addition of rubber slightly decreased the elastic modulus of RRAC, as rubber particles have a lower elastic modulus than sand. This makes the elastic modulus of rubber particles and mortar different, which facilitates crack formation [57].

Figure 18(c) shows that, as with the compressive strength, the elastic modulus of RRAC decreased with smaller rubber size. However, rubber size had a greater influence on the elastic modulus than on the compressive strength. Moreover, as with the compressive strength, the elastic modulus of RRAC with mixed-size rubber particles was lower than that with single-size rubber particles. This can be explained by the aggregation of rubber particles, which was also the main reason for the decreased compressive strength with mixed-size rubber particles described earlier.

Figure 18(d) shows the influence of aggregate treatment methods on the elastic modulus of RRAC. RRAC with AM exhibited the highest elastic modulus, indicating that RCA could play an important role in determining the elastic modulus of RRAC. However, simultaneous treatment of RRAC with RM and AM decreased the elastic modulus. This decrease is attributed to the formation of weak zones, as mentioned earlier when discussing the compressive strength.

4.3.4 Specific toughness

Toughness is generally used to characterise the energy dissipation capacity of concrete under uniaxial compression, which is calculated as the area surrounding the stress–strain curve of a concrete specimen [58]. Furthermore, the specific toughness is defined as the ratio of the area under the stress–strain curve to the compressive strength. Table 9 and Figure 19 show the calculation results for the specific toughness of RRAC.

Figure 19 
                     Effect of orthogonal factors on specific toughness of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber size, and (d) aggregate treatment.
Figure 19

Effect of orthogonal factors on specific toughness of RRAC. (a) Water–cement ratio, (b) rubber content, (c) rubber size, and (d) aggregate treatment.

As shown in Figure 19(a), the w/c ratio clearly influences the specific toughness of RRAC. RRAC specimens with w/c ratios of 0.4 and 0.5 had 21% and 72% higher specific toughness values, respectively, than RRAC with a w/c ratio of 0.3. These observations are explained by the increased brittleness of RRAC at lower w/c ratios, which weakens its deformation capacity.

Figure 19(b) shows that as the rubber content was increased, the specific toughness of RRAC first increased and then decreased, which agrees with results obtained from previous studies [49]. The main reason for the increase in the specific toughness is the excellent energy absorption capacity of the rubber particles. However, high rubber content reduces the density of RRAC and leads to hole formation, which decreases its energy absorption capacity [57].

Figure 19(c) shows the effect of rubber size on the specific toughness of RRAC. Rubber size had only a slight influence on the specific toughness of RRAC. However, RRAC with mixed-size rubber had a higher specific toughness than RRAC with single-size rubber, possibly because mixed-size rubber particles have greater potential to absorb energy.

Figure 19(d) shows that among the three aggregate treatment methods, RM produced the highest specific toughness, indicating that rubber plays an important role in the toughness of RRAC. However, simultaneous treatment of RRAC with RM and AM resulted in a lower specific toughness.

4.3.5 Comparison of orthogonal factors on RRAC

The standard error of three levels was used to characterise the influence of the orthogonal factors on the properties of RRAC. The calculated standard errors of three levels of each factor are shown in Table 10. The w/c ratio is the most influential factor for the mechanical properties of RRAC, followed by rubber size. Rubber content has greater influence on the compressive strength and specific toughness of RRAC than aggregate treatment; however, aggregate treatment has greater influence on the elastic modulus.

Table 10

Standard error of three levels of each factor

Factors w/c ratio Rubber content Rubber size Aggregate treatment
Compressive strength 11.63 4.72 4.85 3.59
Rank 1 3 2 4
Elastic modulus 7.80 1.10 7.09 3.59
Rank 1 4 2 3
Specific toughness 0.15 0.06 0.09 0.03
Rank 1 3 2 4

The optimal level of the factors is shown in Table 11. It can be seen that the orthogonal combination of 0.3–5%-6-AM is the optimal combination for the compressive strength and elastic modulus of RRAC. In addition, RRAC with a w/c of 0.5 and a rubber content of 10% has the highest specific toughness, and mixed-size rubber particles and RM can also improve the specific toughness of RRAC.

Table 11

Optimal level of the orthogonal factors

Factors w/c ratio Rubber content (%) Rubber size Aggregate treatment
Compressive strength 0.3 5 6 Mesh AM
Elastic modulus 0.3 5 6 Mesh AM
Specific toughness 0.5 10 Mixed size RM

4.3.6 Failure modes

The failure patterns of the specimens are shown in Figure 20. With low w/c ratios, RRAC tended to break into large pieces, whereas under high w/c ratios, a number of small pieces peeled off from RRAC. The greater stiffness of RRAC at lower w/c ratios caused more brittle failures. In addition, it is believed that RCA content is responsible for an important effect on the failure pattern of RRAC. As the rubber content was increased, multiple thin cracks were observed on the surfaces of the specimens, which were mainly caused by the low elastic modulus of rubber. Rubber particles enhance the capacity for deformation before cracking and inhibit the propagation of micro-cracks [49]. No obvious differences in RRAC behaviour were observed with single-size rubber. However, RRAC with mixed-size rubber tended to produce long, wide cracks that penetrated the specimen. Another interesting observation was that some rubber particles connected closely with the matrix on the failure surface of RRAC with RM treatment, whereas rubber particles were found to peel off the failure surface of RRAC without RM treatment. This finding has evidenced that rubber treatment by NS can effectively improve the ITZ zone of rubber and the cement matrix.

Figure 20 
                     Failure patterns of RRAC. (a) 0.3–5%-6-RM, (b) 0.3–10%-60-AM, (c) 0.3–15%-M-RM + AM, (d) 0.4–5%-60-RM + AM, (e) 0.4–10%-M-RM, (f) 0.4–15%-6-AM, (g) 0.5–5%-M-AM, (h) 0.5–10%-6-RM + AM, and (i) 0.5–15%-60-RM.
Figure 20

Failure patterns of RRAC. (a) 0.3–5%-6-RM, (b) 0.3–10%-60-AM, (c) 0.3–15%-M-RM + AM, (d) 0.4–5%-60-RM + AM, (e) 0.4–10%-M-RM, (f) 0.4–15%-6-AM, (g) 0.5–5%-M-AM, (h) 0.5–10%-6-RM + AM, and (i) 0.5–15%-60-RM.

4.3.7 SEM test

Figure 21(a)–(d) shows the microstructures of RRAC with and without RM treatment. A comparison of these figures shows that rubber particles without treatment bonded poorly with the cement matrix, and partially or completely peeled off. Cracks extended along the ITZ, which was the main reason for failure of the specimen. After RM treatment, the rubber particles connected closely with the cement matrix, resulting in a denser RRAC structure. The improvement in the ITZ between the rubber and mortar improved the mechanical properties of RRAC.

Figure 21 
                     Comparison of microstructures of RRAC with and without rubber modification treatment: (a and b) with treatment and (c and d) without treatment.
Figure 21

Comparison of microstructures of RRAC with and without rubber modification treatment: (a and b) with treatment and (c and d) without treatment.

The microstructures of RRAC with and without AM treatment are shown in Figure 22(a)–(d). The images show that without AM treatment, the structure of RRAC is loose. In addition, numerous wide cracks appeared around the ITZ. NS treatment improved the ITZ of RCA, and the cracks tended to occur in the cement matrix. Further, the surface of RCA became smoother with fewer pores, indicating that NS filled the holes of RCA effectively.

Figure 22 
                     Comparison of microstructures of RRAC with and without RCA modification treatment: (a and b) with AM treatment and (c and d) without AM treatment.
Figure 22

Comparison of microstructures of RRAC with and without RCA modification treatment: (a and b) with AM treatment and (c and d) without AM treatment.

Figure 23(a) and (b) shows the microstructures of RRAC treated simultaneously with AM and RM. Rubber particles and RCA tended to aggregate, which significantly reduced the strength of RRAC. Cracks extended along the interface between RCA and rubber, finally penetrating the cement matrix and connecting them, which is attributed to the degraded mechanical properties of RRAC due to the simultaneous AM and RM.

Figure 23 
                     Microstructures of RRAC with simultaneous RCA modification and rubber modification treatments (a and b).
Figure 23

Microstructures of RRAC with simultaneous RCA modification and rubber modification treatments (a and b).

5 Conclusion

An experimental study was conducted to investigate the effects of nano-SiO2 modification on RCA and rubber, aiming to improve the performance of the rubberised concrete with RCA. Two types of nanomaterials, NS solution and NS sol–gel, were used to pretreat RCA and rubber by immersion. The physical properties of RCA were tested. Mechanical and microstructural tests were conducted on rubberised mortar and RRAC modified with three methods, and the following are the conclusions from the test results:

  1. NS treatment significantly improved the physical properties of RCA. The water absorption rate and crushing index of RCA decreased after treatment with NS solution and NS sol–gel, with NS sol–gel exhibiting a stronger effect than NS solution. With 48 h treatment, water absorption rate decreased by 32 and 23%, and the crushing index decreased by 39 and 29% for NS sol–gel and NS solution, respectively.

  2. NS treatment of rubber improved the compressive strength and flexural strength of the rubberised mortar. This is attributed to the consumption and refinement of Ca(OH)2 crystals by NS attached to the surface of the rubber, leading to a close connection between the rubber and the matrix. The NS treatment time of rubber did not linearly contribute to the mechanical properties of the rubberised mortar. It is suggested that based on a synthetical consideration of the performance improvement and economic cost, rubber treated with NS sol–gel for 48 h is the optimal method for improving the compressive strength and flexural strength of the rubberised mortar.

  3. Treatment with NS modified the ITZ between RCA, rubber, and the matrix effectively. The rubber particles and RCA combined closely with the cement matrix, leading to a compact RRAC microstructure. However, simultaneous treatment of rubber and RCA created a weak zone, where rubber and RCA tended to aggregate. Cracks extended along the interface of RCA and rubber, penetrating the cement matrix.

  4. The addition of rubber might significantly reduce the mechanical properties of RRAC. The results indicate that RRAC with a modified rubber content no more than 10% could display good compressive behaviour. In addition, single-size rubber particles rather than mixed-size rubber particles are suggested to be used for maintaining the compressive strength of RRAC, and bigger rubber size is beneficial to the mechanical properties of RRAC.

  5. Among the three aggregate treatment methods of RRAC, RM increased the toughness of RRAC, whereas AM increased its compressive strength and elastic modulus. However, simultaneous application of RM and AM treatments degraded the mechanical properties of RRAC, mainly because the NS molecules remaining on the surface of rubber and RCA tended to aggregate. Thus, NS modification with simultaneous application of RM and AM cannot be recommended.

  6. It is suggested that for the rubberised concrete without RCA, rubber modification with NS sol–gel can effectively improve the mechanical properties of the concrete. As for the rubberised concrete with RCA, only RCA modification with NS sol–gel was the recommended treatment method for increasing the mechanical properties of the concrete.

As the cost of nanomaterials continue to drop with the progress of nanotechnology, NS modification has great potential for use in aggregate modification, as it is an efficient and environment-friendly method. However, the effect on the aggregate depends on the type of nanomaterial used, and more investigations on various nanomaterials and aggregate modification methods will be conducted in the future.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China and the Guangdong Basic and Applied Basic Research Foundation.

  1. Funding information: This study was funded by the National Natural Science Foundation of China (Nos. 12072078 and 11672076) and the Guangdong Basic and Applied Basic Research Foundation (No. 2019B151502004).

  2. Author contributions: 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: 2021-11-10
Revised: 2021-11-30
Accepted: 2021-12-06
Published Online: 2022-01-18

© 2022 Jianbai Zhao et al., published by De Gruyter

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

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  5. Glutathione-loaded non-ionic surfactant niosomes: A new approach to improve oral bioavailability and hepatoprotective efficacy of glutathione
  6. Relationship between mechano-bactericidal activity and nanoblades density on chemically strengthened glass
  7. In situ regulation of microstructure and microwave-absorbing properties of FeSiAl through HNO3 oxidation
  8. Research on a mechanical model of magnetorheological fluid different diameter particles
  9. Nanomechanical and dynamic mechanical properties of rubber–wood–plastic composites
  10. Investigative properties of CeO2 doped with niobium: A combined characterization and DFT studies
  11. Miniaturized peptidomimetics and nano-vesiculation in endothelin types through probable nano-disk formation and structure property relationships of endothelins’ fragments
  12. N/S co-doped CoSe/C nanocubes as anode materials for Li-ion batteries
  13. Synergistic effects of halloysite nanotubes with metal and phosphorus additives on the optimal design of eco-friendly sandwich panels with maximum flame resistance and minimum weight
  14. Octreotide-conjugated silver nanoparticles for active targeting of somatostatin receptors and their application in a nebulized rat model
  15. Controllable morphology of Bi2S3 nanostructures formed via hydrothermal vulcanization of Bi2O3 thin-film layer and their photoelectrocatalytic performances
  16. Development of (−)-epigallocatechin-3-gallate-loaded folate receptor-targeted nanoparticles for prostate cancer treatment
  17. Enhancement of the mechanical properties of HDPE mineral nanocomposites by filler particles modulation of the matrix plastic/elastic behavior
  18. Effect of plasticizers on the properties of sugar palm nanocellulose/cinnamon essential oil reinforced starch bionanocomposite films
  19. Optimization of nano coating to reduce the thermal deformation of ball screws
  20. Preparation of efficient piezoelectric PVDF–HFP/Ni composite films by high electric field poling
  21. MHD dissipative Casson nanofluid liquid film flow due to an unsteady stretching sheet with radiation influence and slip velocity phenomenon
  22. Effects of nano-SiO2 modification on rubberised mortar and concrete with recycled coarse aggregates
  23. Mechanical and microscopic properties of fiber-reinforced coal gangue-based geopolymer concrete
  24. Effect of morphology and size on the thermodynamic stability of cerium oxide nanoparticles: Experiment and molecular dynamics calculation
  25. Mechanical performance of a CFRP composite reinforced via gelatin-CNTs: A study on fiber interfacial enhancement and matrix enhancement
  26. A practical review over surface modification, nanopatterns, emerging materials, drug delivery systems, and their biophysiochemical properties for dental implants: Recent progresses and advances
  27. HTR: An ultra-high speed algorithm for cage recognition of clathrate hydrates
  28. Effects of microalloying elements added by in situ synthesis on the microstructure of WCu composites
  29. A highly sensitive nanobiosensor based on aptamer-conjugated graphene-decorated rhodium nanoparticles for detection of HER2-positive circulating tumor cells
  30. Progressive collapse performance of shear strengthened RC frames by nano CFRP
  31. Core–shell heterostructured composites of carbon nanotubes and imine-linked hyperbranched polymers as metal-free Li-ion anodes
  32. A Galerkin strategy for tri-hybridized mixture in ethylene glycol comprising variable diffusion and thermal conductivity using non-Fourier’s theory
  33. Simple models for tensile modulus of shape memory polymer nanocomposites at ambient temperature
  34. Preparation and morphological studies of tin sulfide nanoparticles and use as efficient photocatalysts for the degradation of rhodamine B and phenol
  35. Polyethyleneimine-impregnated activated carbon nanofiber composited graphene-derived rice husk char for efficient post-combustion CO2 capture
  36. Electrospun nanofibers of Co3O4 nanocrystals encapsulated in cyclized-polyacrylonitrile for lithium storage
  37. Pitting corrosion induced on high-strength high carbon steel wire in high alkaline deaerated chloride electrolyte
  38. Formulation of polymeric nanoparticles loaded sorafenib; evaluation of cytotoxicity, molecular evaluation, and gene expression studies in lung and breast cancer cell lines
  39. Engineered nanocomposites in asphalt binders
  40. Influence of loading voltage, domain ratio, and additional load on the actuation of dielectric elastomer
  41. Thermally induced hex-graphene transitions in 2D carbon crystals
  42. The surface modification effect on the interfacial properties of glass fiber-reinforced epoxy: A molecular dynamics study
  43. Molecular dynamics study of deformation mechanism of interfacial microzone of Cu/Al2Cu/Al composites under tension
  44. Nanocolloid simulators of luminescent solar concentrator photovoltaic windows
  45. Compressive strength and anti-chloride ion penetration assessment of geopolymer mortar merging PVA fiber and nano-SiO2 using RBF–BP composite neural network
  46. Effect of 3-mercapto-1-propane sulfonate sulfonic acid and polyvinylpyrrolidone on the growth of cobalt pillar by electrodeposition
  47. Dynamics of convective slippery constraints on hybrid radiative Sutterby nanofluid flow by Galerkin finite element simulation
  48. Preparation of vanadium by the magnesiothermic self-propagating reduction and process control
  49. Microstructure-dependent photoelectrocatalytic activity of heterogeneous ZnO–ZnS nanosheets
  50. Cytotoxic and pro-inflammatory effects of molybdenum and tungsten disulphide on human bronchial cells
  51. Improving recycled aggregate concrete by compression casting and nano-silica
  52. Chemically reactive Maxwell nanoliquid flow by a stretching surface in the frames of Newtonian heating, nonlinear convection and radiative flux: Nanopolymer flow processing simulation
  53. Nonlinear dynamic and crack behaviors of carbon nanotubes-reinforced composites with various geometries
  54. Biosynthesis of copper oxide nanoparticles and its therapeutic efficacy against colon cancer
  55. Synthesis and characterization of smart stimuli-responsive herbal drug-encapsulated nanoniosome particles for efficient treatment of breast cancer
  56. Homotopic simulation for heat transport phenomenon of the Burgers nanofluids flow over a stretching cylinder with thermal convective and zero mass flux conditions
  57. Incorporation of copper and strontium ions in TiO2 nanotubes via dopamine to enhance hemocompatibility and cytocompatibility
  58. Mechanical, thermal, and barrier properties of starch films incorporated with chitosan nanoparticles
  59. Mechanical properties and microstructure of nano-strengthened recycled aggregate concrete
  60. Glucose-responsive nanogels efficiently maintain the stability and activity of therapeutic enzymes
  61. Tunning matrix rheology and mechanical performance of ultra-high performance concrete using cellulose nanofibers
  62. Flexible MXene/copper/cellulose nanofiber heat spreader films with enhanced thermal conductivity
  63. Promoted charge separation and specific surface area via interlacing of N-doped titanium dioxide nanotubes on carbon nitride nanosheets for photocatalytic degradation of Rhodamine B
  64. Elucidating the role of silicon dioxide and titanium dioxide nanoparticles in mitigating the disease of the eggplant caused by Phomopsis vexans, Ralstonia solanacearum, and root-knot nematode Meloidogyne incognita
  65. An implication of magnetic dipole in Carreau Yasuda liquid influenced by engine oil using ternary hybrid nanomaterial
  66. Robust synthesis of a composite phase of copper vanadium oxide with enhanced performance for durable aqueous Zn-ion batteries
  67. Tunning self-assembled phases of bovine serum albumin via hydrothermal process to synthesize novel functional hydrogel for skin protection against UVB
  68. A comparative experimental study on damping properties of epoxy nanocomposite beams reinforced with carbon nanotubes and graphene nanoplatelets
  69. Lightweight and hydrophobic Ni/GO/PVA composite aerogels for ultrahigh performance electromagnetic interference shielding
  70. Research on the auxetic behavior and mechanical properties of periodically rotating graphene nanostructures
  71. Repairing performances of novel cement mortar modified with graphene oxide and polyacrylate polymer
  72. Closed-loop recycling and fabrication of hydrophilic CNT films with high performance
  73. Design of thin-film configuration of SnO2–Ag2O composites for NO2 gas-sensing applications
  74. Study on stress distribution of SiC/Al composites based on microstructure models with microns and nanoparticles
  75. PVDF green nanofibers as potential carriers for improving self-healing and mechanical properties of carbon fiber/epoxy prepregs
  76. Osteogenesis capability of three-dimensionally printed poly(lactic acid)-halloysite nanotube scaffolds containing strontium ranelate
  77. Silver nanoparticles induce mitochondria-dependent apoptosis and late non-canonical autophagy in HT-29 colon cancer cells
  78. Preparation and bonding mechanisms of polymer/metal hybrid composite by nano molding technology
  79. Damage self-sensing and strain monitoring of glass-reinforced epoxy composite impregnated with graphene nanoplatelet and multiwalled carbon nanotubes
  80. Thermal analysis characterisation of solar-powered ship using Oldroyd hybrid nanofluids in parabolic trough solar collector: An optimal thermal application
  81. Pyrene-functionalized halloysite nanotubes for simultaneously detecting and separating Hg(ii) in aqueous media: A comprehensive comparison on interparticle and intraparticle excimers
  82. Fabrication of self-assembly CNT flexible film and its piezoresistive sensing behaviors
  83. Thermal valuation and entropy inspection of second-grade nanoscale fluid flow over a stretching surface by applying Koo–Kleinstreuer–Li relation
  84. Mechanical properties and microstructure of nano-SiO2 and basalt-fiber-reinforced recycled aggregate concrete
  85. Characterization and tribology performance of polyaniline-coated nanodiamond lubricant additives
  86. Combined impact of Marangoni convection and thermophoretic particle deposition on chemically reactive transport of nanofluid flow over a stretching surface
  87. Spark plasma extrusion of binder free hydroxyapatite powder
  88. An investigation on thermo-mechanical performance of graphene-oxide-reinforced shape memory polymer
  89. Effect of nanoadditives on the novel leather fiber/recycled poly(ethylene-vinyl-acetate) polymer composites for multifunctional applications: Fabrication, characterizations, and multiobjective optimization using central composite design
  90. Design selection for a hemispherical dimple core sandwich panel using hybrid multi-criteria decision-making methods
  91. Improving tensile strength and impact toughness of plasticized poly(lactic acid) biocomposites by incorporating nanofibrillated cellulose
  92. Green synthesis of spinel copper ferrite (CuFe2O4) nanoparticles and their toxicity
  93. The effect of TaC and NbC hybrid and mono-nanoparticles on AA2024 nanocomposites: Microstructure, strengthening, and artificial aging
  94. Excited-state geometry relaxation of pyrene-modified cellulose nanocrystals under UV-light excitation for detecting Fe3+
  95. Effect of CNTs and MEA on the creep of face-slab concrete at an early age
  96. Effect of deformation conditions on compression phase transformation of AZ31
  97. Application of MXene as a new generation of highly conductive coating materials for electromembrane-surrounded solid-phase microextraction
  98. A comparative study of the elasto-plastic properties for ceramic nanocomposites filled by graphene or graphene oxide nanoplates
  99. Encapsulation strategies for improving the biological behavior of CdS@ZIF-8 nanocomposites
  100. Biosynthesis of ZnO NPs from pumpkin seeds’ extract and elucidation of its anticancer potential against breast cancer
  101. Preliminary trials of the gold nanoparticles conjugated chrysin: An assessment of anti-oxidant, anti-microbial, and in vitro cytotoxic activities of a nanoformulated flavonoid
  102. Effect of micron-scale pores increased by nano-SiO2 sol modification on the strength of cement mortar
  103. Fractional simulations for thermal flow of hybrid nanofluid with aluminum oxide and titanium oxide nanoparticles with water and blood base fluids
  104. The effect of graphene nano-powder on the viscosity of water: An experimental study and artificial neural network modeling
  105. Development of a novel heat- and shear-resistant nano-silica gelling agent
  106. Characterization, biocompatibility and in vivo of nominal MnO2-containing wollastonite glass-ceramic
  107. Entropy production simulation of second-grade magnetic nanomaterials flowing across an expanding surface with viscidness dissipative flux
  108. Enhancement in structural, morphological, and optical properties of copper oxide for optoelectronic device applications
  109. Aptamer-functionalized chitosan-coated gold nanoparticle complex as a suitable targeted drug carrier for improved breast cancer treatment
  110. Performance and overall evaluation of nano-alumina-modified asphalt mixture
  111. Analysis of pure nanofluid (GO/engine oil) and hybrid nanofluid (GO–Fe3O4/engine oil): Novel thermal and magnetic features
  112. Synthesis of Ag@AgCl modified anatase/rutile/brookite mixed phase TiO2 and their photocatalytic property
  113. Mechanisms and influential variables on the abrasion resistance hydraulic concrete
  114. Synergistic reinforcement mechanism of basalt fiber/cellulose nanocrystals/polypropylene composites
  115. Achieving excellent oxidation resistance and mechanical properties of TiB2–B4C/carbon aerogel composites by quick-gelation and mechanical mixing
  116. Microwave-assisted sol–gel template-free synthesis and characterization of silica nanoparticles obtained from South African coal fly ash
  117. Pulsed laser-assisted synthesis of nano nickel(ii) oxide-anchored graphitic carbon nitride: Characterizations and their potential antibacterial/anti-biofilm applications
  118. Effects of nano-ZrSi2 on thermal stability of phenolic resin and thermal reusability of quartz–phenolic composites
  119. Benzaldehyde derivatives on tin electroplating as corrosion resistance for fabricating copper circuit
  120. Mechanical and heat transfer properties of 4D-printed shape memory graphene oxide/epoxy acrylate composites
  121. Coupling the vanadium-induced amorphous/crystalline NiFe2O4 with phosphide heterojunction toward active oxygen evolution reaction catalysts
  122. Graphene-oxide-reinforced cement composites mechanical and microstructural characteristics at elevated temperatures
  123. Gray correlation analysis of factors influencing compressive strength and durability of nano-SiO2 and PVA fiber reinforced geopolymer mortar
  124. Preparation of layered gradient Cu–Cr–Ti alloy with excellent mechanical properties, thermal stability, and electrical conductivity
  125. Recovery of Cr from chrome-containing leather wastes to develop aluminum-based composite material along with Al2O3 ceramic particles: An ingenious approach
  126. Mechanisms of the improved stiffness of flexible polymers under impact loading
  127. Anticancer potential of gold nanoparticles (AuNPs) using a battery of in vitro tests
  128. Review Articles
  129. Proposed approaches for coronaviruses elimination from wastewater: Membrane techniques and nanotechnology solutions
  130. Application of Pickering emulsion in oil drilling and production
  131. The contribution of microfluidics to the fight against tuberculosis
  132. Graphene-based biosensors for disease theranostics: Development, applications, and recent advancements
  133. Synthesis and encapsulation of iron oxide nanorods for application in magnetic hyperthermia and photothermal therapy
  134. Contemporary nano-architectured drugs and leads for ανβ3 integrin-based chemotherapy: Rationale and retrospect
  135. State-of-the-art review of fabrication, application, and mechanical properties of functionally graded porous nanocomposite materials
  136. Insights on magnetic spinel ferrites for targeted drug delivery and hyperthermia applications
  137. A review on heterogeneous oxidation of acetaminophen based on micro and nanoparticles catalyzed by different activators
  138. Early diagnosis of lung cancer using magnetic nanoparticles-integrated systems
  139. Advances in ZnO: Manipulation of defects for enhancing their technological potentials
  140. Efficacious nanomedicine track toward combating COVID-19
  141. A review of the design, processes, and properties of Mg-based composites
  142. Green synthesis of nanoparticles for varied applications: Green renewable resources and energy-efficient synthetic routes
  143. Two-dimensional nanomaterial-based polymer composites: Fundamentals and applications
  144. Recent progress and challenges in plasmonic nanomaterials
  145. Apoptotic cell-derived micro/nanosized extracellular vesicles in tissue regeneration
  146. Electronic noses based on metal oxide nanowires: A review
  147. Framework materials for supercapacitors
  148. An overview on the reproductive toxicity of graphene derivatives: Highlighting the importance
  149. Antibacterial nanomaterials: Upcoming hope to overcome antibiotic resistance crisis
  150. Research progress of carbon materials in the field of three-dimensional printing polymer nanocomposites
  151. A review of atomic layer deposition modelling and simulation methodologies: Density functional theory and molecular dynamics
  152. Recent advances in the preparation of PVDF-based piezoelectric materials
  153. Recent developments in tensile properties of friction welding of carbon fiber-reinforced composite: A review
  154. Comprehensive review of the properties of fly ash-based geopolymer with additive of nano-SiO2
  155. Perspectives in biopolymer/graphene-based composite application: Advances, challenges, and recommendations
  156. Graphene-based nanocomposite using new modeling molecular dynamic simulations for proposed neutralizing mechanism and real-time sensing of COVID-19
  157. Nanotechnology application on bamboo materials: A review
  158. Recent developments and future perspectives of biorenewable nanocomposites for advanced applications
  159. Nanostructured lipid carrier system: A compendium of their formulation development approaches, optimization strategies by quality by design, and recent applications in drug delivery
  160. 3D printing customized design of human bone tissue implant and its application
  161. Design, preparation, and functionalization of nanobiomaterials for enhanced efficacy in current and future biomedical applications
  162. A brief review of nanoparticles-doped PEDOT:PSS nanocomposite for OLED and OPV
  163. Nanotechnology interventions as a putative tool for the treatment of dental afflictions
  164. Recent advancements in metal–organic frameworks integrating quantum dots (QDs@MOF) and their potential applications
  165. A focused review of short electrospun nanofiber preparation techniques for composite reinforcement
  166. Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review
  167. Latest developments in the upconversion nanotechnology for the rapid detection of food safety: A review
  168. Strategic applications of nano-fertilizers for sustainable agriculture: Benefits and bottlenecks
  169. Molecular dynamics application of cocrystal energetic materials: A review
  170. Synthesis and application of nanometer hydroxyapatite in biomedicine
  171. Cutting-edge development in waste-recycled nanomaterials for energy storage and conversion applications
  172. Biological applications of ternary quantum dots: A review
  173. Nanotherapeutics for hydrogen sulfide-involved treatment: An emerging approach for cancer therapy
  174. Application of antibacterial nanoparticles in orthodontic materials
  175. Effect of natural-based biological hydrogels combined with growth factors on skin wound healing
  176. Nanozymes – A route to overcome microbial resistance: A viewpoint
  177. Recent developments and applications of smart nanoparticles in biomedicine
  178. Contemporary review on carbon nanotube (CNT) composites and their impact on multifarious applications
  179. Interfacial interactions and reinforcing mechanisms of cellulose and chitin nanomaterials and starch derivatives for cement and concrete strength and durability enhancement: A review
  180. Diamond-like carbon films for tribological modification of rubber
  181. Layered double hydroxides (LDHs) modified cement-based materials: A systematic review
  182. Recent research progress and advanced applications of silica/polymer nanocomposites
  183. Modeling of supramolecular biopolymers: Leading the in silico revolution of tissue engineering and nanomedicine
  184. Recent advances in perovskites-based optoelectronics
  185. Biogenic synthesis of palladium nanoparticles: New production methods and applications
  186. A comprehensive review of nanofluids with fractional derivatives: Modeling and application
  187. Electrospinning of marine polysaccharides: Processing and chemical aspects, challenges, and future prospects
  188. Electrohydrodynamic printing for demanding devices: A review of processing and applications
  189. Rapid Communications
  190. Structural material with designed thermal twist for a simple actuation
  191. Recent advances in photothermal materials for solar-driven crude oil adsorption
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