Performance of carbon nanomaterials incorporated with concrete exposed to high temperature

: In recent decades, there have been initiatives to incorporate carbon nanomaterials (CNMs) into cement composites, particularly graphene oxide (GO), carbon nano-tubes, graphite (GP), and mild carbon (MC). Nevertheless, little is known about how these CNMs interact with the cement matrix itself. In this research, the impact of CNM incorporation at high temperatures (250, 500, 750, and 1,000°C) on cement ’ s mechanical characteristics and microstructure was investigated. Nine mixes were created with the CNM content (0.1 and 0.3%) being taken into consideration. The microstructure of the CNM composites was further investigated using X-ray di ﬀ ractometry, thermogravimetry, derivative thermogra-vimetry, digital microscopy, and micro-computed tomography (micro-CT). Based on research observations, the study demonstrated that the mechanical properties of most specimens could be enhanced through the introduction of CNMs. The recommended proportions of GP-0.1, GO-0.1, and MC-0.1, in accordance with the weight of the binder, and the impact of the CNMs on the elastic modulus were also assessed. As a consequence, the CNM ’ s porous structure and apparent crack pattern were identi ﬁ ed using microstructure analysis.


Introduction
In addition to the progress made in multidimensional construction, the widespread use of cementitious materials has led to global growth in construction production [1].Cement and concrete have been extensively utilized in the construction sector due to their accessibility, superior mechanical strength, and durability [2].In addition to the recent expansion of concrete usage in many construction sectors, auxiliary thermal cracks have received widespread attention.In general, cement hydration is an exothermic response to volume changes [3,4].The inability to adequately distribute the heat released is a result of the inadequate thermal stimulation of hydration production and the presence of air pores.The temperature rises gradually from the surface to the intramural region with quick heat dissipation, bringing the surface temperature to the same level as the ambient temperature.As a consequence, there is a temperature difference between the interior and the exterior [5,6].
The mechanical, microstructural, and durability characteristics of concrete materials are negatively impacted by temperature increases [7,8].Constituents of concrete, cement, and aggregates are not secure at high excessive temperatures; as a result, certain changes in their properties occur.Numerous research studies have been carried out that explain how temperature affects the characteristics of cementitious composites and how to stop concrete from deteriorating thermally [9][10][11][12][13].Moreover, cementbased materials have been compared with conventional cement compositions [14,15].In the phase of concrete exposure to extremely high temperatures, severe cracking and degradation [16][17][18][19][20] have been shown to occur.Consequently, a thorough understanding of the properties of concrete at high temperatures is required owing to the wide range of concrete applications.Cement paste, or the concrete matrix, is the most important component of concrete when exposed to high temperatures.The primary causes of the deterioration of cementitious composite materials are believed to be the degradation as well as dehydration of calcium silicate hydrate (CSH) and calcium hydroxide (CH) [21][22][23].Furthermore, when exposed to elevated temperatures, porosity enhances and cracks form, resulting in the degradation of their mechanical properties [24].However, a multitude of studies [14] demonstrates that nanoparticles can significantly enhance the characteristics of cementitious materials.At temperatures of 105, 200, 300, and 450°C, the elevated-temperature characteristics of cement paste containing graphene oxide (GO) agglomerates have been investigated [24].In particular, introducing GO might make CH's pore architectures and crystal sizes clearer at room temperature, increasing the cement paste's high thermal stability.
Numerous studies have demonstrated that nanoparticles, including nano silica (NS), nano alumina, and nano clays, can prevent cementitious composites from degrading in the presence of extreme temperatures [25][26][27][28][29].The application of NS, which has been demonstrated to significantly improve the thermal endurance of composite materials through strength maintenance and crack prevention, has received a significant amount of study attention [30][31][32].However, researchers have looked at the possibilities of adding carbon nanomaterials (CNMs) such graphene sulfonate nanosheets (GSNSs), carbon nanotubes (CNTs), and GO to reduce the thermal decomposition of cement-based materials [33][34][35].
This study examines the synergistic reinforcing impact of cement paste whenever hybrid GO/CNTs with varying blending ratios are introduced.UV-Vis spectroscopy, zeta potential, and dynamic light scattering are all techniques used.Because of the increased electrostatic repulsion and smaller particle sizes, the aqueous dispersion efficiency of hybrid GO/CNTs improved much more than that of CNTs.The hybrid GO/CNTs, with a dosage of 0.05% by weight of cement and a GO:CNTs mass ratio of 3:2, increased compressive strength by 45.20%, outperforming the single additions of GO (28.00%) and CNTs (32.58%) with the same dosage [36].For instance, adding 0.1 wt% of MWCNTs to concrete cement pastes that were heated up to as much as 800°C significantly increased the compressive strength of the pastes.Zhang et al.'s [35] study investigated how cement pastes containing hydroxylated MWCNTs at concentrations of 0.1 and 0.2 wt% responded to temperature (up to 600°C).Based on results from scanning electron microscopy (SEM), the addition of MWCNTs to cement pastes revealed a bridging effect of pores and cracks in the cement matrix, which was demonstrated in both the compressive and flexural strengths of the cement pastes up to 400°C.Since plain OPC showed considerable strength reduction with exposure to a temperature of 400°C, MWCNT-incorporated samples showed equal compressive strengths after 200 and 400°C exposure.This research looks into how CNTs and CNT-silica core-shell nanostructures affect cement paste behavior at elevated heat (300, 450, and 600°C).
The addition of GO to cement-based composites decreases their workability because a group of GO nanosheets has a significant surface area that facilitates simple absorption of water molecules during hydration and a bulky longitudinal size that provides a high capacity for water retention [37,38].Despite the drawbacks of GO, adding just 1% by weight of cement (BWOC) raises the compressive strength of hardened cement paste by 63% [39].Pan et al. [15] and Li et al. [40] claim that by adding 0.05% GO BWOC, the cured cement paste's compressive strength and flexural strength were both enhanced by 15-33% and 41-58%, respectively.The compressive and tensile strength increased by 40% after 28 days of curing [41].When compared to normal cement paste, Shang et al. [42] found that cement paste containing 0.04% GO BWOC had a compressive strength improvement of up to 15.1%.When 0.03% GO BWOC was added, the compressive and tensile strength of OPC paste after 28 days of curing increased by 40% [43].According to Pan et al. [15], adding 0.05 wt% GO to cement mortar boosted its tensile strength by 41-59% and compressive strength by 15-33%.Utilizing GO in cement paste and mortar increased their mechanical qualities, according to Lv et al. [44,45].
According to the aforementioned review, research on carbon nanofibers (CNFs) and GNPs has primarily concentrated on two concerns: (1) how to make nanomaterials evenly dispersed in a cementitious matrix [46,47]; and (2) whether using nanomaterials significantly improves the performance of cementitious matrixes and the dosage of nanomaterials needed [48,49].The incorporation of nanomaterials has been demonstrated to improve the compressive, tensile, and flexural properties of ultra-high-performance concrete (UHPC) [50].Specifically, the compressive, tensile, and flexural properties were all significantly increased as the content of CNFs or GNPs was increased from 0 to 0.30%; the bond strength and post-debonding performance of the interface between steel fibers and the cementitious material also improved the cracking resistance and fracture toughness of cement-based materials, CNFs and GNPs have been employed as microscale reinforcement [401, [51][52][53][54].CNFs commonly have dimensions of tens of nanometers in diameter, tens to hundreds of micrometers in length, and a few nanometers in thickness.The compressive strength of a cementitious composite containing 0.16% CNFs rose by 40% in comparison to a plain cementitious composite having no CNFs, according to Gao et al. [52].According to Peyvandi et al. [53], the flexural strength of a cement paste with a mixing ratio (w/b) of 0.2 was enhanced by 70% by adding 0.13% GNPs.When compared to cementitious composites without any CNFs, Tyson et al.'s [54] research showed that adding CNFs boosted flexural strength by 80% and fracture toughness by 270.Huang [48] used % GNPs in the paste with a 0.6 mixing ratio to boost flexural strength by 80%.Porosity reduction and nucleation processes may be responsible for these cement-based nanocomposites' improved mechanical characteristics [48,49,55,56].Furthermore, hydration products like CSH grow more quickly on the surfaces of nanoscale materials [57,58].Large-aspect-ratio nanomaterials are capable of preventing the emergence and spread of microcracks due to their high specific surface areas and nanoscale spacing [59,60].
Numerous studies have been conducted on the mechanical and electrical characteristics of CNT-reinforced cementitious composites [61].As reported by numerous other studies [62,63], Chan and Andrawes [64] provided experimental and theoretical evidence that CNT might enhance the mechanical properties of cement pastes.Eftekhari and Mohammadi [65] suggested that CNT orientation was significant for CNT improvement of cement paste using molecular dynamics simulations.In the absence of a high-temperature-resistant cementitious material, Cwirzen et al.'s [66] research examined the compressive and flexural strengths of CNT-reinforced cementitious composites.Additionally, it was said that the elastic modulus was very nearly 1 TPa [67].Alkhateb et al. [68] found that the CSH gel's interfacial strengths were 1.2, 13.5, 6.1, and 11.8 GPa for the pristine, hydroxyl-functionalized, amine-functionalized, and carboxyl-functionalized GNPs and CSH, respectively.Han et al. [67] assert that the addition of graphene and GO to cement paste increased its mechanical properties as well as Young's modulus.
To examine the impacts of all CNM additions in OPC, three sets of GO, CNT, graphite (GP), and mild carbon (MC)reinforced composites (0.1 and 0.3 wt%) were constructed.Using X-ray diffractometry (XRD), thermogravimetry (TG), and micro-computed tomography, the impacts of CNMs on the microscopic composition, structure, pore size, and mechanical characteristics of CNM-reinforced cement composites were investigated (micro-CT).Additionally, after being exposed to a range of high temperatures (250, 500, 750, and 1,000°C), the thermal behavior of the specimens was examined.

Materials
Table 1 summarizes the OPC (Sungshin Portland Cement, Korea) utilized in this study's chemical characteristics and physical features.CNMs, such as GO, CNTs, GP, and MC, were obtained from the Korean Company with fundamental properties of the CNMs.

Sample preparation
Cement paste samples were established to determine the microstructure of something like the mixtures.The mortar was created using OPC and OPC composites with 0.1 and 0.3% GO, CNT, GP, and MC BWOC in order to determine its mechanical properties.Table 2 contains a list of the combinations' ingredients.For all of the combinations, the cement-to-sand ratio (c/s) was 1:3.The mortar had a 0.5 water-to-cement ratio (w/c).After mixing, samples were taken for microstructural examination.The 50 mm × 50 mm molds were created for compressive strength and thermal heating measurements.Micro-CT analysis was carried out on specimens with dimensions of 30 mm × 30 mm since low-energy X-rays cannot penetrate thicker materials and high-energy X-rays cannot provide low-contrast information.The features and microstructure were examined using micro-CT.Young's modulus was calculated using cylindrical molds.The isopropanol-swapped XRD and TG specimens for each age were vacuum-stored for a day at room temperature (20 ± 2°C).

Micro-structural equipment
Using Cu K target radiation and an X'Pert PRO MPD X-ray diffractometer, the test specimens' XRD patterns were acquired.The samples were continuously scanned at a scanning speed of 2°/min with a step size of 0.002°over a diffraction angle (2) range of 5-70°.Field-emission SEM was used to examine the microstructures of the CNM cement-paste samples for the SEM investigation (Hitachi SU8000, Japan).
Nikon Metrology (XT H 225) created the micro-CT apparatus, which has the following features: a maximum (max) potential of 225 kV, an X-ray spot size of 3 m, a maximum sample size of 100*100 mm, and a maximum sample weight of 15 kg.The cement pastes' thermal resistance was evaluated using micro-CT.

Compressive strength at elevated temperatures
After 1, 7, 28, and 56 days of curing, the compressive strengths of the samples treated at room temperature (20 ± 2°C) were evaluated.To remove water for the high-temperature tests, the samples were removed from the control room and oven-dried for 24 h.The samples were subsequently thermally treated for 28 days in a high-temperature furnace.The samples were then heated to temperatures of 250, 500, 750, and 1,000°C for an hour at rates of one degree per minute before being cooled to room temperature (20 ± 2°C) at one degree per minute.Two cubic specimens were used at each temperature to test the cement paste's compressive strength.

Dynamic equipment for evaluating Young's modulus
The instruments used to measure the dynamic Young's modulus are shown in Figure 1.

Results and discussion
3.1 Microstructural analysis of specimens

XRD analysis of the specimens
The XRD patterns shown in Figure 2 were used to characterize the crystal structures in order to assess the      hydration processes that occurred in addition to the degree of hydration.XRD analysis can often be used to characterize the evolution of crystals in composites, with diffraction peaks ranging from 2θ = 5°to 2θ = 45°.The microstructure and mechanical characteristics of OPC and CNMs (0.1 and 0.3%) after 1, 7, 28, and 56 days of curing are directly influenced by the crystallization process.The main crystalline phases of anhydrous OPC include alite, belite, AFt, monosulfoaluminate, AFm, hemicarboaluminate (HC), and monocarboaluminate (MC).In OPC, calcite (CaCO 3 ) is also present.As a result of GP oxidation, oxygenated functional classification are apparently incorporated into the carbon atomic surfaces, as seen by the increase in interplanar distance.
In CNTs, no major difference can be found in the crystallization peak of the hydration product based on this study, the crystallographic peaks of H C and M C at the early stages of hydration (after 7 days) being lower than those of OPC.Peaks corresponding to H C are evident in the range of 2θ = 10°-12°.
As the mixing volume of GP increases, the formation rate of H C increases after 7 days, and it exhibits characteristics different from those of other carbon-based admixtures.In addition, after 7 days, the CH crystal facet peak is the highest in the GP 0.3 variable, indicating that GP affects the growth of CH together with H C during the early stages of hydration.In addition, for the GP incorporation variable, the transformation of the M C phase of H C proceeds faster than that of the OPC variable after 28 and 56 days.The incorporation of MC and GO results in the formation of M C crystal plane peaks more rapidly than those of the OPC parameters; the incorporation of GO results in the formation of H C crystal planes after 1 day.By contrast, the incorporation of MC contributes to a slightly faster phase change, as there is no crystal plane peak of H C after 28 and 56 days.This difference can be attributed to the structural differences in the particles for each admixture.Additionally, because AFt changes into monosulfoaluminate before transitioning back to the MC or HC phase as we age, the strength of the AFt peak (2θ = 15-25°) is less intense than it was on day 1.The clinker peaks (2 = 25-35°) of C3S and C2S diminish with age in all samples, while the peaks of AFt and AFm rise.For CH, a general hydration product, there are no observable peak differences.No peaks corresponding to the additional CNMs are visible in the ensuing XRD patterns due to the low content of the CNMs.The lack of significant variances in the samples' diffraction patterns suggests that their mineralogical compositions are similar.
Before conducting the XRD studies, all experimental samples were heated and then kept in a dry state for longer than 7 days.The OPC sample, the GO and CNT-0.3 sample, and the crystal plane peaks of the hydration products at 0 and 250°C did not significantly differ from one another.
The GP-and MC-0.3 sample also increased the peak of C3S between 0 and 250°C.This showed how clinker hydration developed, which would have improved compressive strength.After heating to 750°C, portlandite dehydration also caused the crystal plane peak in each sample to disappear, but the calcium silicate crystal plane peak that was produced by the breakdown of the C-S-H gel expanded.Following the temperature increase, all specimens were kept dry for 7 days while an X-ray diffractogram (XRD) experiment was conducted to recover calcite from the initial temperature product.The calcite crystal plane peak of the GP and MC integrated sample was higher than that of the OPC incorporated sample, but that of the GO and CNT incorporated sample was lower than that of OPC.The healing process was impacted by this.

Thermogravimetric analysis
The TG results are shown in Figure 3 for specimens that were maintained for 1, 7, 28, and 56 days.The weight loss of the samples with additional graphene is larger than that of the OPC samples after curing for 28 days, demonstrating that including GO as an additive causes the generation of more hydration products.The weight loss of the OPC and graphene-contained samples is identical after 56 days of cure.The quantity of hydration products for the OPC and GO mixes are comparable over long-term aging, which shows that the addition of graphene causes fast hydration with reference to the early nucleation.The weight losses below 200°C shown in all of the samples' derivative thermogravimetry (DTG) curves (Figure 3) are due to the dehydration of CSH and AFt as well as the evaporation of free and physically bound water.The carbonated AFm phases, including MC and HC, dehydrate at 150°C.Weight loss at temperatures below 360°C has been attributed to the dehydration of siliceous hydrogarnet (C 3 A S H 4 ) [67].It is possible to link portlandite dehydroxylation and calcite carbonation to the peaks with centers at 420 and 740°C, respectively.
After 7 days, all variables except for the variable incorporating CNTs show higher hydration compared to OPC, the AFm phases also being higher.In addition, the peak of the dehydroxylation of portlandite is the highest among the variables containing GP, which is stable with the XRD consequences.The strength enhancement of GP milled carbon and GO in the 7-day strength data is considered to be an increase in the AFm phase.
After 28 days, the AFm phase in the CNT-incorporated variable has a smaller peak compared to other variables, including OPC, which is consistent with the XRD data.Based on the GP and MC mixing parameters after 28 days, hydration is considerably higher than that in OPC, and both the CSH gel and calcium hydrate in the DTG are improved.In the case of the GP and MC mixing parameters, the disintegration of the CSH gel produces a peak of calcium silicate that is relatively high, indicating that there was previously a lot of CSH gel generated.In addition, the peak of the calcite crystal plane at 750°C for GP and MC is relatively high.

Compressive strength of samples at different temperatures
The compressive strengths of the cement pastes are shown in Figure 4    Performance of carbon nanomaterials incorporated with concrete  9 composites can be attributed to the ability to bridge microcracks and the reduction of porosity, resulting in a confined microstructure.According to Devi and Khan [69], GO integration increased compressive strength by 21-55%.Depending on whether paste, mortar, or concrete was being assessed, other research has indicated compressive strength improvements ranging from 10 to 60% [70,71].The ability of CNTs to bridge microcracks at the nanoscale level and the refinement of porositywhich results in a denser microstructureare generally responsible for the strength increases in CNT-incorporated cement-based composites [72].The cement matrix's mechanical characteristics are significantly influenced by the kind, amount, and degree of CNT dispersion [73][74][75].The compressive strength of CNTs decreased by 0.1-0.3% in the majority of specimens in this investigation based on OPC and other CNMs.CNMs have a significant impact on the mechanical properties of cement composites, depending on the type, amount, and degree of CNM dispersion in the cement matrix [76].
In our investigation, we discovered that the addition of GO significantly changed the MC specimens while only marginally enhancing the compressive strength of the cement composite.The CSH of the cement matrix and the MC nanosheets were made more cohesive by the functionalization of the MC surface, which enhanced stress percolation and, ultimately, the mechanical performance of the samples [77].This demonstrated that MC and GP were successfully incorporated into the cement matrix, resulting in an increase in MC and GP strength.Even when there is an acceptable amount of MC and GP present, MC and GP clustering can weaken the bond between MC, GP, and the cement matrix, which lowers the improvement in compressive strength that comes from MC and GP inclusion.
Figure 5 shows the compressive strengths of the heated forms of GO, CNT, GP, and MC (0.1 and 0.3%).The strength gains and losses caused by exposure to high temperatures are nonlinear.The results of estimating the relative residual compressive strength accurately determine how temperature affects cement paste performance.These figures represent the percentage change in compressive strength of cement paste with time relative to the unheated specimen's compressive strength (20 ± 3°C).At 20 ± 3°C, the 28day MC-0.1 and MC-0.3 specimens' compressive strengths increase by 16.75 and 19.82%, respectively.As shown in Figure 5, the corresponding values for MC-0.1 and CNT-0.1 decrease by 11.43 and 16.87%, respectively, upon exposure to a temperature of 250°C.Plain cement paste (OPC) loses the least strength.However, the CNM samples show a notable increase in compressive strength for the unheated samples when compared to the OPC specimens (19.82% enhancement for MC-0.3, although CNT-0.1 and CNT-0.3 decreased by 7.81 and 4.61%, respectively).Additionally, after being exposed to high temperatures, GO-0.1 and GO-0.3 demonstrate exceptional compressive strengths.Because an internal autoclaving reaction causes anhydrous cement grains to hydrate more at higher temperatures, the strength rises, with the increased hydration products filling the pores and boosting the compressive strength [78,79].It is reasonable to infer that the 0.1 wt% GO inclusion aids in the microstructure's densification and compaction, and that the creation of a less permeable microstructure allows the effects of the autoclaving reaction to be amplified.This impact would be felt if GO was used effectively.However, as compared to OPC, the compressive strengths of GO-0.1 and GO-0.3 improve by 12.4 and 0.51%, respectively.Even after being exposed to 500°C, compressive strength is equivalent.Additionally, the concentrations of CNT-0.1 and MC-0.1 are 17.36 and 23.89%, respectively, lower than those of OPC.All the other samples, excluding GP-0.1, exhibit declining behavior.500°C is commonly thought to be harmful to cementitious composites because the decomposition of the CSH gel and CH results in visible cracking in the specimens.
This outcome appears not to be critical for the strength of the specimens when they are not exposed to moisture for an extended period of time since the rehydration of lime to CH is limited [80].Heikal [81] and Kang et al. [82] reported considerable improvements in the compressive strength of cement paste after exposure to temperatures of 400 and 450°C.Hachemi [83] reported similar results for concrete.The anhydrous cement particles were further hydrated as a result of self-autoclaving when exposed to 400°C [81].Only GO-0.1, GO-0.3, and GP-0.1 show an increase in strength when compared to OPC after exposure to 500°C, and at 750°C, CNT-0.1 (28.75%) and MC-0.1 (19.39%) specimens show a sharp decline in strength.Additionally, all specimens (except GO-0.1) show a reduction in strength (an increase of 8.09%).The indicated increase in strength for GO-0.1 is caused by better cement matrices and GO nanosheet adhesion.Furthermore, the GO and GP calcination occurs at 450°C, but the samples are protected from calcination by the GO and GP.These results are in line with the TG analysis, showing that CNM's presence slowed thermal degradation from 450 to 600°C (Section 3.1.2).The cement paste samples retain 80-92% of their initial strength at 600°C, but their strength is dramatically reduced.The strength of the cement pastes significantly increases at a temperature of 1,000°C, with samples of GP and MC specimens maintaining 40-60% more of their initial strength than OPC samples.Additionally, in each sample, GO-0.1 and CNT-0.1 fall by 3.49%.This sort of behavior can be produced by the cohesion between the nanotubes and the CSH   of the cement matrix.After 500°C, the CSH gel completely decomposes, causing a sharp increase in disruptive cracks in the samples, and the mechanical properties of the cementbased composites begin to deteriorate.This conclusion is valid given that CNMs burn around 600°C, as demonstrated by the TG data in Section 3.1.2.Furthermore, the ability to increase the compressive strength of the samples at high temperatures decreases as the volume of nanomaterials increases.After being exposed to a temperature of 500°C, it is clear that the strength of the CNT, GP, and MC specimens has significantly decreased.Therefore, 0.1 wt% of GO is the optimal GO concentration for refining the microstructure after 28 days of curing, with a higher GO content having a lessening effect.The cement paste microstructure was examined using micro-CT at particular test temperatures in order to better comprehend this phenomenon.

Micro-CT test analysis before and after heating
In Figure 6 and Table 3, the micro-CT images of the samples at 20, 250, and 750°C are shown (above the ambient temperature).Quantitative and qualitative data on material volumes and defect volumes were also collected using micro-CT scans.Pore sizes greater than 10 m (the pixel size of the CT images) were taken into account for the micro-CT evaluation.These pore diameters were utilized to calculate the defect and the total volume of the material (Vc).The material volume is the volume of the hydration product and aggregate volume that is left after heating the specimens, whereas the defect volume is the volume of the dehydration and dehydroxylation products that escape after the specimens are heated.Figure 6 shows the defect and material volume changes of the CNM at elevated temperatures.It is evident that the volume decreases with an increase in the heating temperature for the CNMsthat is, the GO, CNT, GP, and MC samples.Below 450°C, the material volume of the cement paste exposed to high temperatures is due primarily to dehydration.The pore structure and crack content of specimens were determined after heat treatment of 250°C.Furthermore, the dehydration of three different forms of waternamely, capillary pore water at 180°C, interlayer water at 350°C, and absorbed water at 450°Ccan be distinguished from the dehydration of the CSH at temperatures lower than 450°C [35].Figure 6 illustrates how the number of specimen deficits changes as temperature rises.Following exposure to a temperature of 250°C, all specimens display a minor coarsening of the porous structure compared to the unheated specimens.At this temperature, changes in the microstructure of cement paste are mostly brought about by the release of water and the onset of hydration products' breakdown [83,85].The pore structure becomes rougher as a result, as previously documented [80].As shown by our investigation, the porosity changes at this temperature are slight.The OPC specimen and the CNM-0.1 and CNM-0.3 (GO, CNT, GP, and MC) specimens, however, clearly differ from one another.Cracks are evident in the plain OPC specimen; however, no visible cracks can be seen in the CNM-0.1 and CNM-0.3 specimens with coarsened pores.The total material volume (Vc) of the OPC sample at 0°C is 23,180.52mm 3 , while the GO-0.1 and GO-0.3 specimens' total volumes are 23,995.16and 25,004.97mm 3 , respectively.Moreover, the total volumes of the CNT-0.1 and CNT-0.3 specimens are 23,930.97and 24,570.73mm 3 , respectively.The total volumes of the GP, MC-0.1, and MC-0.3 specimens are shown in Table 3.Furthermore, for the OPC, GO-0.1, and GO-0.3 specimens, the defect volumes are 273.56,189.13, and 241.13 mm 3 , respectively.Table 3 also displays the defective volumes for CNT, GP, MC-0.1, and MC-0.3.In comparison to OPC, GO and MC include more hydration products, with a higher retention of CH.Additionally, as shown in Table 3, the volume change ratios for the OPC, CNM-0.1, and CNM-0.3 samples at 20 ± 3°C can be represented in percentage terms.
When exposed to temperatures of 250°C, cement pastes' microstructure dramatically deteriorates, with observable fracture patterns present in every specimen.Dehydration of the CSH is the main cause of these fissures and volume alterations.Based on the OPC specimen's material volume (23,018.18mm 3 ) and defect volume, the volume change ratio is 1.13%.(260.14 mm 3 ).The GO-0.1 specimen, however, exhibits a volume change ratio of 1.35% (having a material volume of 24,348.31mm 3 and a defective volume of 329.4 mm 3 ).In terms of cracking, it is likewise the least noticeable, trailed by GO-0.3 (volume change ratio: 0.63%).Additionally, the volume change ratios for the CNT, GP, MC-0.1, and MC-0.3 specimens are 0.63, 0.56, 0.70, 0.66, 0.99, and 1.23%, respectively.The addition of CNMs decreases cement paste cracking at this temperature, as seen in Figure 6.However, the reverse effect is shown as the amount of CNM in the specimens rises.The CNM-0.3 specimen shows more, larger cracks than the CNM-0.1 specimen, as seen in Figure 6.This shows that the CNM-0.3specimen's material volume is less stable than the CNM-0.1 specimen's.This effect may be caused by the CSH connection being stronger when the right amount of CNM is used.Both the mechanical properties of specimens and the microstructure of cementitious materials degrade following exposure to a temperature of 750°C, as illustrated in Figure 6.Increased CSH phase dissolution and the beginning of CH decomposition (400-550°C) are both caused by exposure to temperatures above 450°C [85,86].The micro-CT analysis found that these events significantly increase the cement matrix's porosity and cause microcracking.After being exposed to a temperature of 750°C, the OPC specimen has a total material volume of 20,263.54mm 3 .As seen in Figure 6, the cracking process is significantly reduced when 0.1-0.3wt% of CNMs are added.The solitary microcracks are also bigger and longer in CNM-0.1.The OPC specimen or the specimens containing the ideal amount of CNMs are less susceptible to cracking than specimens containing 0.3 wt% of CNMs.With the exception of the GO-0.3 and GP-0.3 samples, the total material volume increases significantly for the majority of CNMs.These results agree with the results for compressive strength shown in the previous section.
The potential advantages of CNM (GO, CNT, GP, and MC) incorporationidentified by micro-CTcan also be assessed on a large scale, according to the optical microscope study.However, due to the limited changes in the microstructure following exposure to a temperature of 250°C, only tiny flaws can be seen in the specimens.As a result, only the samples that were subjected to temperatures between 250 and 750°C were included in the visual inspection.All specimens show a similar amount of surface cracks; however, the OPC specimen shows the highest rate of deterioration after being exposed to a temperature of 750°C.The mechanical properties of the specimens as well as the microstructure of the cementitious materials degrade following exposure to a temperature of 750°C.The results of the micro-CT analysis are substantiated by visual inspection.
Cluster formation happens when the quantity of CNM specimens is greater than what is ideal, which causes the cement matrix to become more porous and lose strength as a result.The strength and microstructure of specimens containing 0.3 wt% of CNMs are degraded as the temperature rises as a result of this effect, which is less noticeable in these specimens.
In order to provide sufficient thermal tolerance, it is essential to incorporate the right number of CNMs.When a suitable amount of CNM is added, it is possible to anticipate an improvement in the specimens' microstructural and mechanical effects.Because of CNM's improved ability to connect with cement matrices and nanoparticles and stronger thermal resistance, which results in greater strength in both the unheated condition and when exposed to temperatures of up to 750°C, cement composites have better mechanical qualities (compared to that of OPC).

Dynamic modulus of elasticity
The dynamic modulus of elasticity measured by the resonance vibration equipment yielded the elastic modulus, which is summarized in Table 4.The elastic modulus of composites is generally significantly influenced by the stiffness and volume of the components.The elastic modulus of the CNMs cement composite is equivalent to that of cement paste, given that the weight percentage of CNMs in cement paste is only 0.05%.Due to the CNMs effect, there was a little rise in elastic modulus (from 3.48 to 3.70 GPa), which could be attributed to fewer initial shrinkage cracks.The dynamic modulus of the elasticity range is calculated using the exposure temperature values (250, 500, 750, and 1,000°C) and is in the range of 22.08-33.68GPa.The decarbonation of CNMs at 700°C, as is depicted in Table 4, may have contributed to the decrease in the dynamic modulus of elasticity of the CNMs specimen at 750°C.The dynamic modulus of elasticity of the specimens at different temperatures is compared in Figure 7.A greater dynamic elastic modulus relative (percent) suggests that the concrete can tolerate larger loads based on its strength and brittleness characteristics.

Conclusions
The impact of CNMs on the characteristics of cement pastes subjected to elevated temperatures was investigated in this study.The water content, pore structure (materials and defect volume), chemical properties, and compressive strength were analyzed.The findings show that: 1) The compressive strength of cement pastes containing 0.1 and 0.3 wt% of CNMs was greater than that of the OPC paste at a fixed water-to-cement (w/c) ratio.The porosity of the material was examined to understand the effect of nanoscale microcracking on its mechanical properties.In terms of improving the compressive strength among all the samples examined after 28 and 56 days after curing, the GP-0.3 and MC-0.3 samples performed the best.2) Anhydrous cement grains were more hydrated as a result of thermal exposure, which raised the GO-0.1 and GP-0.1 samples' compressive strength at a temperature of 250°C by comparison to the unheated sample.
The compressive strengths of all CNM samples, in contrast to the OPC samples, decreased when exposed to temperatures above 500°C, although the GO-0.1 sample's compressive strength was higher than that of the OPC sample due to the impact of its chemical makeup.Additionally, because of its considerably higher compressive strength, the 1,000°C GP and MC specimens (0.1 and 0.3) exhibited distinct behavior.
3) The reduction in specimen cracking at tested temperatures of up to 500°C was caused by the introduction of an ideal quantity of CNMs.However, substantial carbon nanosheet agglomeration decreased the thermal resistance of the cement paste, leading to lower strength values and more microcracking.4) The enhanced mechanical properties of the CNM cement composite were confirmed using the observed dynamic modulus of elasticity and XRD analyses following thermal exposure.The findings of this study could be used to determine the ideal CNM content for the required mechanical properties of cement nanocomposites.5) Based on the XRD analysis, the addition of CNMs may result in the finer tuning of the CH hydrated effect, which is advantageous for enhancing the mechanical characteristics of cement paste.6) The inclusion of CNMs refines the pore structure of the cement paste at room temperature according to the micro-CT data.Moreover, it prevents the cement paste pores from becoming coarser when exposed to high temperatures.7) The use of GO and GP agglomerates improved the residual compressive strengths of cement paste that had been exposed to high temperatures up to 450°C.

Figure 1 :
Figure 1: Equipment with a dynamic Young's modulus.

Figure 3 :
Figure 3: TG results for the samples.

Figure 4 :
Figure 4: Compressive strengths of all samples.

Figure 5 :
Figure 5: Compressive strengths of all samples after thermal exposure.

Table 1 :
Chemical composition and physical properties of OPC

Table 3 :
Micro-CT data for material volume

Table 4 :
Dynamic modulus of elasticity of the specimens