3.1. Flowability test of CNT-LWAC
The flowability of CNT added cementitious composites depends on several parameters like ultrasonication energy , treatment of CNTs , the water-cement ratio , the concentration of CNTs , and type of fine fillers . Mostly studied literature [10,34,36,37] illustrates that the incorporation of CNTs reduced the flowability of cementitious composites. While several studies also indicate the increase in flowability of CNT incorporated cementitious composites [38,35,32]. Besides, the incorporation of silica aerogel also reduces the flowability of cementitious composites . However, this study results revealed that the flowability of CNT-LWAC was influenced by the quantity of CNT. The flowability of CNT-LWAC decreases with the escalating doses of CNTs. Almost a 17% reduction in flowability was measured for lightweight concrete specimen prepared with 0.6 wt% CNT. The flowability of CNT-LWAC specimens was shown in Figure 1. The possible reason for the reduction in flowability can be associated with agglomeration and strong van der Waals forces of CNTs. A higher duration of ultrasonication energy can improve the dispersion of CNTs within the concrete and reduce the agglomeration and strong van der Waals forces. Higher doses of superplasticizer can adjust the flowability of the concrete. Moreover, due to the high specific gravity of CNTs, a large amount of superplasticizer is required to overcome the intermolecular forces. Besides, nanoparticles enhance the packing density of concrete by filling up the micro and mesopores which significantly influence the demand of superplasticizer .
3.2. Semi -adiabatic calorimetry test of CNT- LWAC
Figure 2 demonstrates the semi-adiabatic temperature rise of CNT incorporated cement mortar. Study results revealed that by incorporation of silica aerogel setting time of the cement mortar was slightly retarded and produced lower heat of hydration than the plain cement mortar. Study results confirm the reactivity of silica aerogel with cementitious materials. Similar phenomena were noticed in the previous study . While the addition of CNTs to the aerogel adding cement paste SA-2 and SA-3 shows slightly higher produced heat and shortened setting time than the sample SA-2. Besides, by increasing the concentration of CNTs slightly higher produced heat of the exothermic reaction was noticed.
3.3. Strength of CNT-LWAC
Figure 3 shows the compressive CNT-LWAC specimens. The average compressive strength of the control sample was measured at 5.4 MPa. Study results show that the addition of CNT to the lightweight concrete CNT significantly influences the mechanical. By the increase in the concentration of CNTs the compressive strength was gradually increasing. However, the sonicated LWAC specimens showed better improvement in compressive strength against the control concrete sample. The compressive strength of the sonicated concrete specimens was increasing by the increasing doses of CNTs. The highest improvement in compressive strength was measured at 41.48% for AS-6 containing 0.60 wt% CNT. The relative increase in compressive strength of CNT-LWAC portrait in Figure 4. Aerogel is well known lightweight fragile material, and literature confirms the reduction in the mechanical performance of aerogel incorporated cementitious composites [40,41,42]. Incorporation of small doses of CNTs to the aerogel based lightweight concrete significantly increase in compressive strength was noticed. Similarly, an increase in compressive strength phenomena was identified by xu, shilang et al. . The increase in compressive strength can be attributed to the nucleating effects and improvement of the microstructure of CNT-LWAC. The structure of C-S-H was identified surrounding to the CNTs. Moreover, hydration products and CNTs were filling the micro crack/gaps of the lightweight concrete that can provide additional support and helps to increase compressive strength. Detailed descriptions were discussed in 3.5 paragraphs. Besides, literature studies confirm that conforms incorporation of nanoparticles improves the microstructure and provide a denser concrete structure. A decrease in water absorption of the CNT-LWAC confirms the denser structure of CNT-LWAC.
3.4. Water absorption
Figure 5 illustrates the water absorption kinetics of the CNT-LWAC specimens containing up to 0.60 wt% CNTs. Water absorption rate of control sample was measured 14.98%, 22.74%, 32.33% and 34.33% at 15 min, 1 hr, 24 hr and 48 hr that decreased to 13.20%, 16.31%, 28.86% and 30.42% respectively. Madhavi, T. Ch, et al. , and Leonavičius, Dainius, et al.  reported that at a lower concentration of CNTs total pore volume reduced by filling the voids and leads to a reduction in water absorption. Moreover, a high concentration of CNTs in concrete leads to a more porous structure and reduces the mechanical performance of concrete [45,46]. In our study, all nanocomposite concrete specimens show a small decrease in water absorption rate by an increase in the concentration of CNTs. The reduction in water absorption can be attributed to the association of CNTs to the lightweight concrete that helps decrease the micropores and produce the denser concrete structure. Besides, a comparatively higher water abortion rate was observed than Leonavičius et al.  and Kordkheili et al. due to the uses of expanded glass aggregates in the study. Expanded glass aggregates are porous in structure and contain several pores that can absorb water up to 20 to 25% which attributes the increases in water absorption.
3.5. Scanning electronic microscopy (SEM) of CNT-LWAC
The plain image of hydrophobic silica aerogel in Figure 6 (a) suggested that it is a very brittle material having cracks on the surface. Which can easily break into pieces during the mixing process and leads to achieving lower mechanical properties. Moreover, in few aerogel particles, the cracked surface of aerogel in the hydrated concrete specimens was noticed (Figure 6 b). Literatures [25,47, 48] also indicate the brittleness of aerogel, and the incorporation of aerogel decreases the mechanical performances.
Due to the hydrophobic nature of silica aerogel, it has no chemical bonds with the hydrophilic cement matrix. Figure 7 (a) clearly shows the separation gaps between aerogel and surrounding cementitious material in the transition zone (control sample). This phenomenon indicates the lower adhesion properties of hydrophobic silica aerogel. Similar separation gaps were identified by [24,25]. While better adhesion was observed for expanded glass aggregates with cementitious materials (Figure 7 b). Through the separation gaps, air and/or water can easily transport and makes concrete weaker. Interestingly the separation gaps were reduced by utilizing carbon nanotubes (Figure 8). Separation gaps between hydrophobic silica aerogel and surrounding cement-based materials were filled by hydration products and CNTs.
SEM image Figure 9 of CNT-LWAC also shows the CNTs were almost dispersed uniformly within the concrete structure. At the high concentration of CNTs, a network-like distribution within the composite structure was noticed. Zou, Bo, et al., Vesmawala, Gaurang R., et al. and Collins, Frank, et al. also reported that CNTs can be effectively dispersed within the concrete structure by ultrasonication energy and polycarboxylate superplasticizer may be due to this fact high agglomeration of CNTs wasn’t noticed. Unlikely, needle-like structure of ettringite was observed along with the agglomeration of CNTs in the transition zone of aerogel (Figure 8). However, increasing the duration of ultrasonication and concentration of superplasticizer can effectively help to improve the dispersion of CNTs without agglomeration. Moreover, CNTs were found to reinforcing the micropores of the concrete structure Figure 10.
Pinghua Zhu et al.  suggested that due to the hydration process of cementitious materials aerogel particles can slightly react with the pore solution and partially dissolved in an alkaline environment and form C–S–H with a low Ca/ Si ratio. de Fátima Júlio, Maria, et al.  and Hai-li, Cheng, et al. reported that aerogel particles can promote hydration due to high surface activity and ASR can take place and Si-O-Si might form C-S-H. Incorporation of CNTs to the cement composite also provides sites for the formation of calcium silicate hydrate (C-S-H) by acting as a nucleating agent [51,43]. The honeycomb structure of C-S-H and presence of C-S-H nearly to the CNTs can easily be observed in Figure 8 (a), Figure 9 (b), that illustrates the nucleating effects of CNTs.
3.6. X-ray diffraction analysis of CNT-LWAC
Figure 11, Figure 12, Figure 13, and Figure 14 show the X-ray diffraction analysis of expanded glass aggregates, silica aerogel, Portland cement, and LWAC specimens (control, AS-1, AS-3, and AS-6). X-ray diffraction pattern of silica aerogel reveals the amorphous nature of silica aerogel. A broad peak without any narrow peak in the range of 20°–30° were identified. X-ray diffraction pattern of CNT-LWAC concrete specimen illustrates that the intensity of portlandite near about 34° and 47° increases with the increasing concentration of CNT. The increasing peak of calcium silicate hydrate near 29°, 32°, and 50° explained the nucleation effects of carbon nanotubes. Moreover, CNT-LWAC shows a slightly higher amount of calcite and ettringite than the control specimen. However, A higher amount of hydration products was observed for CNT incorporated LWAC specimens. A higher concentration of CNTs within the concrete structure leads to higher growth of hydration products. The similar increasing peak of hydration products by incorporating CNT in the cementitious composite was identified by El-Gamal et al. .