Influence of nano zirconia on the mechanical and durability properties of high-performance concrete containing nano-silica

High Performance Concrete (HPC) was prepared by reinforcing with hybrid nanoparticles such as nano-SiO2 and nano-ZrO2 and the flowability, strength and durability of the concrete was investigated. Slump study showed that the prepared HPC displayed a slump loss with the addition of hybrid nanoparticles due to the absorption of water by hybrid nanoparticles. Experimental results showed that the inclusion of 10% nano-SiO2 and 5% nano-ZrO2 hybrid nanoparticles (mix code M5) enhanced the compressive strength of concrete and exhibited a maximum of 50.12 MPa and 76.4 MPa at 7 days and 28 days of curing due to the reduction of pore volume. Split tensile strength followed the same trend of variation as compressive strength and the mix code M5 showed the maximum split tensile strength of 5.33 MPa and 7.08 MPa cured at 7 days and 28 days respectively. Flexural study resulted that the mix code M5 exhibited a maximum flexural strength of 5.73 MPa and 7.38 MPa cured at 7 days and 28 days respectively owing to the restriction of crack ignition by reinforced hybrid nanoparticles. Rheological study reported that all the prepared concretes lost the weight by less than 1% and the chloride attack displayed maximum weight loss of 4% whereas alkaline attack showed less weight of concrete.


Introduction
High Performance Concrete is a special type of concrete designed to achieve superior strength and longer durability compared to conventional concrete.It has many applications in construction works such as bridges, high rise buildings, tunnels and pavements.In HPC, the required properties are attained by varying the constituents of concrete, mix proportions and curing time.The performance of the HPC is mainly affected by the ingredients present in it.The advent of nanotechnology has developed the construction industries reasonably to the remarkable extent [1,2].Nowadays, several investigators have applied the usage of nanotechnology in the field of construction industries to improve the performance of the building structures [3][4][5].
Nanoparticles are the particles of size ranging from 1-100 nanometer made from metal oxides, graphenes, carbon nanotubes and ceramic materials.Many researchers have used nanoparticles in the casting of concrete and achieved greater strength and durability compared to conventional concrete [6][7][8][9].For example, Harilal et al developed a high performance green concrete by partially replacing the cement with nanoparticles and fly ash and reported that the optimum mix containing 2 wt% TiO 2 , 2 wt% nano-CaCO 3 and 40 wt% fly ash enhanced the compressive strength, split tensile strength and flexural strength [10].Jayaseelan et al investigated the performance of concrete reinforced with nanoparticles and resulted that the combination of nano-SiO 2 and nano-Fe 2 O 3 improved the compressive strength of the concrete.Further, they pointed out that the high content of nano-Fe 2 O 3 and nano-SiO 2 decreased the water absorption by 54% and 36% respectively, due to refined pores and densified microstructure [11].
Faried et al performed an investigation on ultra high performance concrete, studied the mechanical and durability properties.Results showed that the compressive strength of concrete was improved by 24%, 17%, 14% and 13% while added with nanoparticles such as rice husk, waste glass, metakaolin and silica respectively [9].Hakeem et al studied the impact of silica nanoparticles and micro steel wire on the performance of ultra-high performance concrete.Results exhibited that the permeability of concrete was decreased by 39% while added with 6t% silica.Further, tensile, flexural and compressive strength of concrete was improved by 18%, 20% and 21% respectively [12].Chu et al developed fiber reinforced extrudable concrete by including calcium carbonate and studied its performance.Report showed that the rheological properties of concrete were improved due to the inclusion of nanoparticles and fiber.Further, the nanoparticles assisted to improve the inter layer bond and strength of concrete [13].
Guler et al examined the performance of concrete at ambient and elevated temperatures reinforced with single and hybrid nanoparticles and resulted that the nano-SiO 2 and nano-Al 2 O 3 showed more enhancement in the strength of the concrete compared to that of nano-Fe 2 O 3 and nano-TiO 2 .Moreover, the authors found that the hybrid nanoparticles i.e. 1.5% of nano-SiO 2 and nano-Al 2 O 3 improved the compressive, split tensile and flexural strength of concrete by 14%, 19% and 22% respectively.In addition, the residual compressive strength of control concrete and hybrid nano-added concrete was reduced by 58% and 42% at an elevated temperature of 800 °C [14].
Based on the literature reports, it was concluded that the addition of nanoparticles improved the performance of the concrete.Further, the hybrid nanoparticles showed much more improvement on the strength and durability of concrete compared to the mono nanoparticles [15].Hence, the research curiosity is increasing on the hybrid nanoparticles reinforcement concept in concrete technology.
Zirconia or zirconium oxide is the nanoparticles made from metal oxides used in refractoriness, foundry sand and ceramics owing to its high mechanical strength.Previous researchers have used zirconia nanoparticles in concrete preparation and achieved good results [16,17].Hence, the zirconia nanoparticles have the potential to improve the performance of the concrete.Silica or Silicon dioxide is another well known nanoparticle.It is strong and possesses good abrasive characteristics.Hence, it is expected that the combination of zirconia and silica nanoparticles could produce interesting results in the concrete performance due to the hybrid effect of nanoparticles.
Hence, in this research, an attempt has been made to make high performance concrete using zirconia and silica nanoparticles.Further, the effect of zirconia on the performance of high performance concrete containing silica is to be studied.This work could be a significant work to explore the new scientific information in concrete technology.

Experimental work 2.1. Materials
Ordinary Portland Cement of 53 grade satisfying the IS standards was taken for the present study for experimental investigation.The chemical composition of cement is shown in table 1.The normally available river sand was used as fine aggregate.The coarse aggregate of nominal size 20 mm with relative density of 2.7 was used.The concrete was prepared according to M50 grade mix ratio.Since M50 grade have low tide cement ratio, the super plasticizer admixture was added (1%) with the cement to extend the workability of concrete.The concrete mix ratio is detailed in table 2. The hybrid nanoparticles such as nano-silica and nano-zirconia were used as reinforcing agents in the concrete mix by replacing the cement quantity.The nano-silica content was fixed as constant (10 wt%) as optimum level reported by previous investigators [18] and the nano-zirconia content was varied in the range 1-5 wt%.The properties of nanoparticles are shown in table 3.

Preparation and casting of specimens
Concrete mix was prepared in the concrete mixture and the concrete cast was prepared in steel moulds.The various mix code prepared in this investigation is shown in table 2. Initially, the fine aggregate and coarse aggregate were mixed in the concrete mixture by adding half of the total water requirement and the mix was continued.Secondly, the cement and the 10% by weight SiO 2 were added to the mixture and the mix was continued.Finally, the nano-ZrO 2 ranging from 1%-5% by weight and the remaining water were added in the concrete mix and the mix was continued.The specimens were prepared by casting them in steel moulds.The inside of the steel mould was applied with a liberal coat of grease to stop concrete from adhering to the mould.The designed concrete mix was filled into the moulds in layers.Adequate compaction was administered using table vibrator to avoid honey combing.Finally, the concrete was de-molded and immersed in water for curing.

Testing
Slump flow test was conducted on fresh concrete to understand the workability of concrete according to the recommendations instructed by IS: 1199-1959 (Reaffirmed 2004).Compressive strength of concrete was calculated according to standard ASTM C39/C39-18 using the cylindrical specimen size of 150 mm diameter and 300 mm length.The split tensile strength was measured as per the standard ASTM C496/C496-17 using the cylindrical specimen (Size: 150 mm diameter and 300 mm length).Flexural strength was calculated according to standard ASTM C1609 using concrete prism (Size: 100 mm × 100 mm × 500 mm).Three specimens were tested for each test and the average value was taken for the analysis.Durability test was performed on the prepared concrete cylinder (size: 150 mm diameter and 300 mm length) by soaking in different soaking media such as 2% alkaline, 2% sulphate, 2% acid and 2% chloride.The 28 days cured concrete cylinder was soaked in respective soaking media for 30 days.The weight loss of cylinder was measured by subtracting the weight of soaked cube from the weight of 28 days cured dry cylinder.Further, the compressive strength of soaked cylinder was measured.The loss in compressive strength was measured by subtracting the compressive strength of soaked cylinder from the compressive strength of 28 days cured dry cylinder.

Slump test
Figure 1 shows the slump value of concrete of various mix code.As can be seen from the figure 1, the slump value of concrete decreases with the inclusion of hybrid nanoparticles.For example, the slump value of mix code M5 was observed as 58 mm that is 28% lower than the slump value of M0 mix code.The reduction in slump value was attributed by the inclusion of hybrid nanoparticles with high specific surface area that absorbed more water from the concrete mix and restricted the flow of concrete which in turn decreased the slump value [19].aggregates of the concrete and reduces the volume of pores.Further, the inclusion nanoparticles assisted to decrease the micro-pores by increasing hydration in between cement and aggregates [20].Hence, it is clear that the compressive strength of concrete improves when compared to the plain concrete without nanoparticles.The variation of compressive strength showed the approximate linear relationship with the volume fractions of nano-ZrO 2 .The mix code M5 displayed the superior compressive strength of 50.12 MPa and 76.4 MPa at 7 days and 28 days curing respectively compared to that of M0.The mix code M5 exhibited 14% and 17% enhancement in the compressive strength compared to the compressive strength of M0.The compressive strength result of present study was compared with the similar study carried by the previous researchers [21] and the results are shown in figure 2(d).It is concluded from the figure 2(d) that the nano-ZrO2 enhanced the compressive strength greatly and could be a better substitute to enhance the compressive strength of the concrete for civil work applications.

Split tensile strength analysis
The split tensile strength of concrete is shown in figure 3(a) and the percentage increase in the split tensile strength of the concrete is shown in figure 3(b).The split tensile strength increased with the increase of nano-ZrO 2 content and followed the similar trend of variation as variation of compressive strength with nano-ZrO 2 content.For example, the mix code M5 that contained 5% ZrO 2 and 10% SiO 2 showed the superior split tensile strength of 5.33 MPa and 7.08 MPa cured at 7 days and 28 days respectively compared to the split tensile strength of M0 mix code that contained 0% ZrO 2 and 10% SiO 2 .The split tensile strength of mix code M5 was improved by 11% and 21% when cured at 7 days and 28 days respectively compared to the split tensile strength of mix code M0.Further, the effect of nano-ZrO 2 on the split tensile strength is shown in figure 3(c).As can be seen from figure 3(c), the split tensile strength varied slowly up to the 4% nano-ZrO 2 content and beyond that the split tensile strength varied quickly.Moreover, the shape of the trend line fitting curve plays an important role in the analysis of variation of split tensile strength.The fitting curve of the split tensile strength was observed as concave in shape whereas the fitting curve of the compressive strength (figure 2(c)) was observed as convex in shape.Concave curve indicates the increase in slope and convex curve indicates the decrease in slope.Hence, the split tensile strength varies more with the addition of same amount of nano-ZrO 2 in the concrete compared to the variation of compressive strength.This tendency of variation was well in accordance with the previous study carried by Faez et al [18].

Flexural strength analysis
Figure 4(a) shows the flexural strength of the concrete for various mix code and the percentage increase in the flexural strength is shown in the figure 4(b).It is obviously seen from the plot that the flexural strength of the concrete was increased while increasing the nano-ZrO 2 content and the mix code M5 displayed a maximum value of 5.73 MPa and 7.38 MPa cured at 7 days and 28 days respectively.As the inclusion of nano-ZrO 2 filled the gap present in the concrete and prevented the crack initiation, the flexural strength of the concrete was improved compared to the conventional concrete.Further, the nano fillers have more potential to absorb energies [22].immersion medium due to the reduction of pore volume of the concrete that restricted the penetration of immersion medium and prevented the crack formation.Further, the immersion medium showed a less impact on the weight loss of concrete while volume fractions of the nano-ZrO 2 increased.For example, while including 5% nano-ZrO 2 with concrete, the alkaline, sulphate and acid attack displayed the same results and the chloride attack exhibited some more weight loss of concrete compared to other immersion medium.In addition, the mix code M5 exhibited 9% less in weight loss compared to the weight loss of M0 in chloride attack.Figure 5(b) shows the loss in compressive strength of the concrete measured under various immersions medium.The plot shows that the loss in compressive strength decreases as the nano-ZrO 2 content increases due to the increase in strength of concrete.Further, the concrete becomes more durable, stronger and multifunctional due to the inclusion of nano fillers [23].The concrete displayed the maximum loss in compressive strength while involving into chloride attack and showed minimum loss in compressive strength in the alkaline medium.Further, all the concrete mix code showed a maximum loss in compressive strength of 5% that cannot be considered as major reduction in the strength.

Conclusion
Experimental investigation was performed on the high performance concrete reinforced with hybrid nanoparticles such as nano-silica and nano-zirconia.The following conclusions were derived from the study.• The slump value of all the prepared concretes varied in the range of 58 mm-81 mm and showed a decrement in slump value while increasing the weight faction of hybrid nanoparticles.The high specific surface area of nanoparticles absorbed more water from the concrete and reduced the slump value.
• Compressive strength analysis revealed that the mix code M5 showed the superior compressive strength and exhibited 14% and 17% improvement in the compressive strength at 7 days and 28 days curing respectively compared to that of M0 mix code.The compressive strength of the concrete was improved due to the reduction of pore volume owing to the reinforcement of hybrid nanoparticles.
• Split tensile strength followed the similar trend of variation as compressive strength and the mix code M5 displayed a maximum split tensile strengths of 5.33 MPa and 7.08 MPa during 7 days and 28 days curing respectively compared to all other mix code due to the effect of nanoparticles.
• Flexural strength analysis resulted that the mix code M5 exhibited a maximum flexural strengths of 5.73 MPa and 7.38 MPa at 7 days and 28 days curing respectively compared to other mix code.The enhancement in flexural strength was achieved due to the restriction of crack initiation by the reinforcement of hybrid nanoparticles.
• Rheological study showed that all the prepared concrete displayed more loss in compressive strength in chloride attack and showed less loss in compressive strength in alkaline medium.Further, a maximum loss in compressive strength of 5% was observed for M0 mix code and the minimum loss in compressive strength of 4% was observed for M5 mix code in chloride attack.From the experimental investigations, it is clear that high performance concrete using hybrid nano-silica and nano-zirconia can be effectively used for the civil construction works.

Figure 2 (
a) shows the compressive strength of the concrete and figure 2(b) shows the percentage increase in the compressive strength of the concrete.As obviously seen from figure 2(a), the compressive strength of concrete increases as the volume fractions of the nano-ZrO 2 increased.The effect of volume fractions of nano-ZrO 2 on the compressive strength of the concrete is shown in figure2(c).On increasing the volume fractions of nano-ZrO 2 in the concrete, it can be observed that the nanoparticles occupies the space in between the

Figure 2 .
Figure 2. (a) Compressive strength of concrete.(b) Percentage increase in compressive strength of concrete.(c) Effect of nano-ZrO2 on the compressive strength of concrete at 28 days curing.(d) Comparison of compressive strength of present study with other studies at 28 days curing.

Figure 3 .
Figure 3. (a) Split tensile strength of concrete.(b) Percentage increase in split tensile strength of concrete.(c) Effect of nano-ZrO 2 on the split tensile strength of concrete at 28 days curing.

Figure 4 .
Figure 4. (a) Flexural strength of concrete.(b) Percentage increase in flexural strength of concrete.(c) Effect of nano-ZrO2 on the flexural strength of concrete at 28 days curing.

Figure 5 .
Figure 5. (a) Weight loss of the concrete.(b) Loss in compressive strength.

Table 1 .
Chemical composition of ordinary Portland cement.