Performance Attenuation of Nano-Modified Concrete under High Temperature: Comparison
Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by Deprizon Syamsunur.
The effect of nanoparticles on the hydration of cement depends not only on the type of material but also on the temperature. A certain amount of nanomaterials can effectively improve the mechanical properties of concrete, and when nano concrete is exposed to high temperatures, its internal structure and mechanisms are relatively altered. Under the temperature conditions studied of 40 °C to 70 °C, nano-SiO2 and nano-C-S-H shortened the induction period and the arrival time of the peak heat release rate of hydration, and nano-C-S-H also increased the peak heat release rate. The addition of nanoparticles and high temperatures led to the early production of hydrate layers. Emerging concepts propose inorganic insulation nanomaterials to reduce energy use and consumption and to plan and design green buildings. The science and engineering of nanotechnology have improved the high-temperature performance of concrete. The addition of nano-SiO2 can improve the role of the thermal insulation capacity of concrete, due to the dense internal structure, increasing the specific heat capacity and reducing the thermal diffusivity. Concrete in high-temperature areas is prone to structural damage and these damages seriously affect the service life of buildings. There is a need to conduct research and develop new nanomaterial-modified concrete with good mechanical properties and durability at high temperatures. NC and NS-modified concrete have a good evaluation in terms of economy and performance, because the constant deterioration of extreme high-temperature environments on concrete is a long-term impact, while whether nano concrete has a lot of resistance is based on the effects of fire and internal ambient temperature. 
  • rheological properties
  • nano-silica
  • nano-calcium carbonate

1. The Effect of Nano-SiO2 and Nano-CaCO3on the Properties of Concrete

1.1. Rheological Properties

The rheological properties of concrete have been evaluated accordingly by many scholars in the configuration of concrete. As NS and NC are in the nanoscale (0–100 nm) range, very small nanoparticles fill the pore structure of concrete. Nano concrete exhibits greater water absorption and a larger specific surface area, which reduce compatibility and flowability [63][1]. NS and NC mixed with cementitious materials produce new hydration products that fill the tiny internal voids, thus affecting the workability of the concrete. Yassoub Ahmed et al. [64][2] proposed that increasing the NS admixture reduced the slump results due to the extremely small particle size of NS, significantly affecting the workability of nano concrete, with higher admixture levels causing high internal agglomeration. This author utilised the use of dry mix and wet mix methods’ conclusion consistently. Nooruddin et al. [65][3] found that the initial and final setting times of cement mortars decreased with increasing and decreasing NS; the author used a 0.5% water–cement ratio without any type of water-reducing agent. Experiments were carried out by Mugilvani et al. [66][4] using a water–cement ratio of 0.45, with NS replacing cement up to 20%, 30% and 50% of the mix. To control the flow of the concrete, 20 mL of ultra-high performance water reducing agent was added during the experiments, as did the authors Danielraj et al. [67][5], but in the experiments, 0% to 2% NS and 10% micron-silica were used. Increasing the particle size of NS particles will inevitably change the rheological properties of nano concrete. R. Liu et al. [34][6] investigated the effect of changing the interfacial transition zone for different water–cement ratios on the durability properties of concrete. By reducing the experimental water–cement ratio from 0.5 to 0.35, the interfacial transition zone of the modified cementitious material of NS overlapped the reaction, which is the main reason for the reduced fluidity of the concrete. The NS modification consumed more water and also increased the water retention and cohesion to a certain extent, making the concrete more viscous in appearance. R. Liu et al. [68][7], comparing the durability performance of 0.4 water–cement ratio with 0.3 water–cement, found the filling and water absorption of NC caused the state of concrete to be altered. Atis et al. [69][8] found the reduction in the fluidity of NC concrete was due to the increase in alkali activator (NaOH) and due to the high specific surface area that NC possesses, reducing the concrete’s compatibility. There is some similarity between the filling of NS and NC particles on the alteration of rheological properties, by reducing the workability, but improving the mechanical and durability properties of the concrete with the incorporation of nanomaterials. There is a functional relationship between these properties, and according to scholarly research, it is noted that increasing the admixture of concrete increases its mechanical properties and resistance to chloride ion corrosion [70,71,72][9][10][11].

1.2. Mechanical Properties

The dynamic mechanical properties of concrete were tested by the compressive strength test, flexural strength test and splitting strength test, etc. The test process of casting cubic, rectangular and cylindrical samples can visualize the trend of a strength change of different kinds of nanomaterials in different shapes of concrete after different temperatures [73][12]. The incorporation of certain amounts of NC and NS materials significantly improved the mechanical properties of concrete [74,75][13][14].
As shown in Figure 1 [76][15], the mechanical properties changed at 28 days for different NS admixtures, with a gradual increase in compressive strength for 1%, 2% and 3%. The compressive strength of the NS material replacing 4% cement started to decrease as the nano-doping increased. Yassoub Ahmed et al. [64][2] concluded that NS mixed at 1.5% had the best performance, increasing the compressive strength, flexural strength and modulus of elasticity by 28%, 57% and 62%, respectively, over the normal concrete samples. Nooruddin et al. [65][3] reported that mixing a small amount of NS reduced the initial and final setting time of cement mortar, and that concrete mixed with 3% NS had higher compressive and splitting tensile strengths, with increases of 22.09% and 32.19%, respectively, over the control samples. NS content greater than 3% resulted in a gradual reduction in the mechanical new properties of concrete. AlKhatib, Maslehuddin et al. [77][16] used cement kiln dust and electric arc furnace dust within the two industrial waste-mixed NS to develop high-performance concrete. Using 10%, 15% and 20% cement kiln dust and 10% electric arc furnace dust to replace cement, respectively, the results showed that the compressive and flexural strengths of concrete decreased, while the flexural and compressive strengths of concrete mixed with NS increased.
Figure 1. Compressive strength of different NS content at 28 days [76].
Compressive strength of different NS content at 28 days [15].
T. Wang et al. [78][17] concluded that the 1-day combined strength of UHPC increased significantly with increasing amounts of Li2CO3 and NC, and that NC was effective in mitigating the loss of the 28-day combined strength of UHPC. After modification, the UHPC combined strength and flexural strength increased by about 68% and 38%, respectively, over the control sample. The optimum dosages of 3–4% NC were obtained, and 2% and 3% of NS and NC, respectively, influenced the heat of hydration and compressive strength of the cementitious material. The addition of 2% NS had the most significant effect on the mechanical properties of the concrete [79][18]. The mechanical properties of concrete such as compressive strength, flexural strength and splitting strength were improved by mixing NC alone or by combining NS and NC. Mixing NS alone reduces the compressive strength, flexural strength and splitting tensile strength of concrete and can increase the modulus of elasticity of concrete [49][19]. The combination of NS and NC can improve the static and dynamic mechanical properties of concrete, and the effect is reduced compared to NC alone, confirming that the activity of NS is lower at this point.

1.3. Durability

NS and NC contribute significantly to the improvement of the durability properties of concrete. The addition of 1%, 2%, 3% and 4% of the cement admixture of both NS and NC can reduce the water absorption of concrete. NC makes the cement matrix microstructure denser and produces more hydration products to improve the compressive strength and durability of concrete [91,92,93][20][21][22]. NS has better water absorption than NC, and a more neutral mixture of composite NS and NC is the most effective [94][23]. This reduction is due to the presence of nanoparticle size in the pores, resulting in reduced porosity and permeability, and the optimum admixture of NS and NC mixed with blended concrete at 3% each can minimise water absorption [95][24]. Singh et al. [96][25] used concrete mixes blended with different amounts of fly ash, 3% NS particles and 6% silica fume to investigate the mechanical properties and durability of the new concrete. Compared to conventional concrete, the mixes mixed with NS showed a significant reduction in a carbonation depth of 73% after 180 days of exposure, a reduction of 39% after 180 days of erosion containing NS in sulphate and a combined specimen of fly ash and NS showed a 30% reduction. Mixing NS in concrete improved the durability and service life of the concrete. A. Zhang et al. [97][26] had some research results on the durability performance of NS-modified concrete. The drying process and water absorption process perspectives were investigated to test the chloride ion permeability and resistivity of concrete. Single-doped NS and compound-doped NS+nano-Al2O3 can reduce the weight loss during drying and the capillary permeability coefficient during water absorption. By increasing the nanomaterial content, the amount of charge passing through the sample is increased and the resistivity is subsequently reduced. Nanomaterial contents greater than 1% tend to agglomerate during hydration with the cementitious material, making the concrete less durable. Smaller admixtures allow for more uniform dispersion of nanoparticles and 0.5% nano concrete has better durability. In addition, the addition of NC also reduces the shrinkage of the concrete to a large extent. Figure 2 compares the change in shrinkage values between micron calcium carbonate and NC at 7 days and 28 days.
Figure 2. Changes in shrinkage properties of concrete after incorporation of NC [98].
Changes in shrinkage properties of concrete after incorporation of NC [27].
Based on the durability studies carried out by many scholars, the water absorption, hazardous material permeability and freeze-thaw resistance of nano concrete are demonstrated, and Table 1 below shows the findings for NS and NC-modified concrete. The volcanic ash effect and microfilling effect of a certain amount of NC and NS replacement cement incorporated into the concrete results in a more dense concrete, reduced water absorption and hazardous material permeability, and improves freeze-thaw resistance. Authors Nejad et al. [82][28] used fly ash composite-modified concrete at only 1% admixture, where nanomaterials and fly ash were used in combination, while authors H. Liu et al. [87][29] used NC alone to increase the testing of hydrochloric acid resistance experiments to increase the admixture of NC modification to 3% in order to obtain a better durability performance response. It is advisable to use between 2.0% and 3.0% nano-doping for bulk concrete. The review found that NS has a better ability to modify the durability properties of concrete than NC, which refines the crystalline form and enhances the structure of the interface. The modification of NS depends on the level of activity, and hydration is prominent. In addition, there is a significant difference with the water–cement ratio of the concrete, where a relatively higher ratio facilitates the hydration of NS [68][7].

2. Influence of Nano Concrete Properties under High-Temperature Environment

Experiencing a fire or extreme high-temperature weather, through a series of reflections, the internal properties of concrete may change dramatically, by setting different high-temperature environments and simulating the state of concrete structure analysis, to derive the excellent medium difference of different nanomaterial admixture concrete, so as to develop the analysis [40,107][38][39]. Many experts and scholars have devoted themselves to the study of the fire resistance of nano concrete [108][40], which provides an important contribution to human production and life [109][41]. A comparison of the effects of geopolymers containing 2% NC and 2% NS exposed to temperatures of 60 °C and 90 °C follows. The cumulative heat-indicated reaction levels measured for 80 h at 60 °C and 90 °C show opposite results, as shown in Figure 3 and Figure 4. At a curing temperature of 90 °C, the addition of SiO2 nanoparticles resulted in a lower cumulative heat of reaction than the addition of NC until 40 h into the reaction, with the results showing a higher heat of reaction by the end of 80 h. The relative increase in the cumulative heat of reaction for NS addition can be seen in Figure 4, which shows peaks in the corresponding reaction rate curves between 35 and 45 h [69][8].
Figure 3. Hydration rate curve and cumulative heat of hydration of geopolymer slurry at 60 °C [69][8]. (a) Rate of hydration at 60 °C, (b) Total heat hydration at 60 °C.
Figure 4. Hydration rate curve and cumulative heat of hydration of geopolymer slurry at 90 °C [69][8]. (a) Rate of hydration at 90 °C, (b) Total heat hydration at 90 °C.

2.1. Residual Properties

Nanomaterials mixed with other media, such as fibres, will have different mechanical property degradations in high-temperature environments. L. Wu et al. [110][42] investigated the residual properties of carbon fibre-reinforced concrete after high temperatures with different NS-doping levels. The residual mechanical properties of NS carbon fibre-reinforced concrete after high temperatures were higher than those of ordinary concrete. After heating at 775 °C, the residual compressive strength, splitting strength and flexural strength of 0.25% carbon fibre and 1% NS concrete were 5.2%, 10.9% and 8.9% higher than those of ordinary concrete, respectively. The authors concluded that NS could effectively improve the mechanical properties of concrete after a high temperature and that the synergistic effect of NS and carbon fibre is the main factor for the improved mechanical properties of NS carbon fibre-reinforced concrete after a high temperature. A similar study was carried out by the authors Polat et al. [105][36], here using the singular material NS to improve the high-temperature resistance of the concrete, even when heated to 750 °C. The samples showed a 54% increase in compressive strength over the original concrete: a more striking finding than that of the previous author. The NS concrete specimens showed a faster loss of residual compressive strength of the concrete from 500 °C to 750 °C, with a 24% loss of compressive strength for the 500 °C samples: using a cement-mortar mixture, the temperature loss results were even more pronounced, as also reflected by the authors of Shah et al. [111][43]. In addition, for the different mechanical properties’ experiments, the loss of strength was also highly dependent on the size of the cement mixture, i.e., cubic (150 × 150 × 150), rectangular (10 × 10 × 40) and cylindrical (Ø150 × 300), etc.
Yonggui et al. [38][44] studied the compressive and splitting strengths of basalt fibre and NS in different temperature environments. By testing a large number of samples, it was concluded that the mechanical properties of concrete gradually decreased as the temperature continued to rise, and the authors concluded that the residual compressive strength of concrete between 25 °C and 600 °C was a quadratic function of temperature, with the compressive strength decreasing in the range of 400–600 °C to between 40–60%. The splitting tensile strength decreased significantly up to 400 °C, with decreases ranging from 20% to 60%, and smaller decreases above 400 °C. When the temperature was between 25 °C and 200 °C, the relative residual splitting strength increased linearly with temperature, confirming that the mechanical effect on nano fibre concrete is minimal in the 200 °C range. When the temperature exceeded 200 °C, the relative residual splitting strength was quadratic as a function of temperature. The splitting strength of concrete was less able to resist higher temperatures due to changes in the internal microstructure. Elsayd et al. [112][45] investigated the changes in mechanical and fire resistance properties using different nano-combinations for a room temperature environment of 25 °C and different high-temperature environments (200 °C, 400 °C, 500 °C, 600 °C, 700 °C and 800 °C). In the 200 °C range, both NS and nano-clay improved the mechanical properties of the concrete, gradually decreasing the residual compressive strength as the temperature increased. Exposure to 800 °C for 2 h resulted in a loss of up to 60% of the concrete’s strength. Compared to normal concrete, concrete configured with 3% NS and composite material (1% NS + 4% nano-clay) replacing cement, heated at 800 °C for 2 h after standard curing, increased the strength of the concrete by 19.8% and 14.7%, respectively, which can be used as an optimum percentage for the fire resistance properties of concrete. From a microscopic analysis, NS and NC-modified concrete specimens can still preserve better mechanical properties under different high-temperature environments, even when heated up to 800 °C. In response to the conclusions given by various scholars, the strength loss of nano concrete was not the same when heated to different high temperatures. No major differences were found between NS and NC in terms of the effect of mechanical properties in high-temperature environments. In addition to the mechanical compressive experimental tests and the splitting experimental tests, the review further expands on the flexural properties of nano concrete in a high-temperature environment.
NS and NC can improve the flexural properties of concrete in different temperature environments. By comparing 25 °C and 600 °C high temperatures, mixing NS improved the flexural strength and energy absorption capacity of the material. The flexural strength of NS mixed with 1.5% increased by 27% over the control sample in a room temperature environment, and by 21% after 600 °C high temperature. Unlike the NS modification, the addition of 3.0% NC was better at high temperatures than at room temperature. The flexural strength of concrete at an ambient temperature of 25 °C increased by 9% over the control sample and could be increased by 23% at 600 °C [113][46]. It is worth noting that the NS admixture at this point was only half of NC and the NS-modified concrete was able to provide better flexural performance results than NC. Cao et al. [114][47] compared the effect of NC and mixed micron calcium carbonate (MC) on the high-temperature properties of concrete. NC had the best high-temperature properties of the concrete samples, indicating that NC improved the high-temperature properties of the cement paste more significantly than MC, and NC particle size was beneficial to the development of high-temperature resistant mechanical properties. An overview of many articles found that many scholars are happy to start studies using NS to improve the high-temperature properties of concrete based on the activity and excellent modification ability of nano. From a durability point of view, the use of composites with different degrees of reaction, NC and NS, can result in a more neutral concrete with long-lasting performance excellence to meet the needs of the construction market.

2.2. Deformation Attenuation

The ambient environment of NS and NC makes the interfacial transition zone of concrete denser and holds with the aggregates to form higher mechanical properties [115][48]. In contrast, in high-temperature environments, these voids gradually increase being amplified, creating micro-cracks until the interfacial state of the concrete is destroyed [31,116,117][49][50][51]. When concrete samples mixed with 3% NS and 15% alccofine were heated between 400 and 800 °C, the compressive strength of the concrete generally decreased, with the most significant loss of strength at 800 °C, and the surface colour changing from grey to white and finally to brown. The microscopic voids in the concrete became larger with increasing temperature [118][52]. Due to the fire and the increase in temperature, the residual compressive strength of NS and alccofine concrete at 200 °C lasting 4–8 h was higher than the strength of room-temperature concrete samples, and with no significant change in the surface of the concrete at 200 °C heated for 4 h and extended burning time, the surface of concrete became light grey. At 400 °C at 4 h of heating, the NS and alccofine concrete samples and control samples showed a light grey surface; at 8 h and 12 h of heating, the concrete surface turned dark grey; at 600 °C at high temperatures, the concrete surface also changed colour to brownish red. As the temperature increased to 800 °C, more significant numbers and widths of cracks appeared on the concrete surface. As the heating time continued to increase, the concrete colour changed from grey to white. From 600 °C to 1000 °C, aggregate decomposition was detected and the colour eventually turned red [119][53]. It increased linearly with increasing nano-doping and increasing temperature. Kantarci et al. [120][54] reported no significant change in the surface of concrete heated to 300 °C, which still contained a small amount of gloss, and gradually turned grey when heated from 500 °C to 700 °C, as shown in Figure 5. Cao et al. [114][47] investigated the effect of high-temperature environments from 200 °C to 1000 °C on the appearance of cracking of micron and NC-modified cement pastes. A visual analysis of the concrete surfaces showed that both NC and micron calcium carbonate (MC) affected their high-temperature properties. Within 600 °C, NC concrete samples and MC concrete samples produced less microcracking, while surface microcracking in the control concrete samples developed faster. At 800 °C high temperature, NC concrete samples produced more significant cosmetic microcracking than MC and the control concrete. The change in the appearance of cracks in the concrete exposed to different temperatures for 2 h is shown in Figure 6. Figure 7 shows SEM images after high-temperature heating: comparing the change in appearance for different NS dosages, the concrete shows a decrease in surface breakage after high-temperature heating. Temperatures reached 800 °C and the concrete was generally damaged, producing irregular cracks. NS had a positive effect on improving the high-temperature resistance of the concrete.
Figure 5. Variation in the appearance of concrete containing 2% NS after different temperatures [120].
Variation in the appearance of concrete containing 2% NS after different temperatures [54].
Figure 6. Exposed to different high-temperature environments [114]. ① Control samples, ② NS concrete samples, ③ MC concrete samples.
Exposed to different high-temperature environments [47]. ① Control samples, ② NS concrete samples, ③ MC concrete samples.
Figure 7. Appearance deformation of NS with different dosages after heating at 400 °C to 800 °C [121].
Appearance deformation of NS with different dosages after heating at 400 °C to 800 °C [55].

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