The Mechanical and Thermal Properties of Cement CAST Mortar/Graphene Oxide Composites Materials

In recent years, the construction industry has tried to improve the properties of cement mortar using nanotechnology system composites (Naganathan et al., 2014; Raki et al., 2010; Sanchez & Sobolev, 2010). Although a high performance was observed in cement composites, the crack in structure was observed in those materials. The disadvantages of cement-based materials should be improved, e.g., being prone to cracking, low toughness, low tensile strength, and poor insulator (Dissanayake et al., 2017; Ferrándiz-Mas et al., 2014). A method to improve cement-based materials is to use composites with fillers which have excellent properties. Generally, the properties of cement composites are dependent on the fillers. For example, the thermal insulator properties of cement were reduced when the polyacrylamide (PAM) and fly ash cenospheres (FAC) fillers increased due to the low resistance of fillers (Kaya & Kar, 2016). Moreover, the high tensile strength and good stress properties were also improved by cement composites nanomaterial fillers (Cao et al., 2013; Jiang et al., 2018; Wang et al., 2021).

Over the last several years, the conventional nanomaterials including carbon-based nanomaterials such as carbon nanotubes (CNTs) (Li & Leung, 1992; Sagar et al., 2012), carbon black (Goracci & Dolado, 2020), carbon nanofibers (CNFs) (Popov, 2004; Roychand et al., 2016), MECNT (Lojka et al., 2021), Graphene Oxide (GO) (De Jong & Geus, 2000; De Volder et al., 2013; Llobet, 2013), and, reduced graphene oxide (rGO) (Geim & Novoselov, 2007; Wang et al., 2021; Zhou et al., 2016) have widely been used as fillers to improve the cement-based properties, including cracking, toughness, tensile strength, and thermal insulator. Less than 0.50 wt.% of the carbon nanomaterials were added to the cement-based materials to produce sustainable composites and to improve the mechanical properties and durability (Marcano et al., 2010; Zhu et al., 2010). GO is one of the good candidates because of its outstanding tensile strength, large surface area properties, and better dispersibility in aqueous media such as water (Compton & Nguyen, 2010; Gholampour et al., 2017). According to Wang et al. (2021) study, the effect of GO on the mechanical properties of cement composites is compared to rGOs. The tensile strength in GO–cement composites is higher than rGO–cement composites due to higher interactions of GO and CSH. Cao et al. (2013) reported that the increase in the tensile strength and compressive strength in 3 days to about 78.6 and 38.9% of cement–GO (0.03%) nanosheet composites was observed due to the formation of flower-like crystals. Namsone et al. (2017) reported that the specific mixtures of the GO solution and ordinary Portland cement with a curing age of 28 days showed maximum compressive strength at a GO content of 0.05 wt% and maximum tensile strength at a GO content of 0.03 wt%. The influence of GO composites on cement mortar composites was reported by Wang et al. (2017). The compressive strength was increased to 63% in 1% of nano GO–cement mortar composites in 28 days. Moreover, the compressive and tensile strengths of cement mortars GO composites at 0.03% of GO were observed at about 21.37 and 53.77%, respectively (Kang et al., 2017). According to Phrompet et al. (2019), the mechanical properties of cement-based composites were improved by the composites between rGO and man-sized C3AH6 cement. Although mechanical properties of cement composites are important in the infrastructure of buildings, the thermal insulation property is helpful in reducing the energy for cooling and heating in buildings (Du et al., 2020). Therefore, the GO has a good candidate material due to low thermal conductivity properties (Xu & Gao, 2011). Moreover, the improvement of the thermal insulation properties of cement-based composites by adding low GO concentrations has been rarely studied. The cements used in the study to fabricate cement composites were the cement CAST 11 LW mortar as insulating castable ASTM C 401 class 0. The chemical composition of cement was CaO (29.0%), Al₂O₃ (14.5%), SiO₂ (54.5%), Fe₂O₃ (0.5%) and SO₃ (0.5%). Therefore, in this work, the influence of the content of GO sheets on cement CAST 11 LW mortar composites in terms of compressive, tensile strengths and thermal properties was investigated for heat insulating castable brick at high temperature. The synthesized GO/CAST 11 LW mortar (GMT) composites with different contents of GO using the conditions of 0, 0.01, 0.03, 0.05, and 0.1 wt% were prepared and abbreviated as the GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively. The investigation of morphology and structure of (GMT) composites were observed by TEM, SEM, XRD, and Raman spectroscopy, respectively.

The morphology and structure of GO sheets, GMT_0 and GMT_0.05 composite, were analyzed by TEM images as shown in Fig. 2a–c. Fig. 2a reveals the close-fitting presence of a large thin plate-like shape of GO sheets with translucent and crease surface as seen in the inset of Fig. 2a-1. Moreover, the SAED pattern in the inset of Fig. 2a-2 indicates the halo diffraction rings, signifying the amorphous layer with random orientations of a polycrystalline structure (Compton & Nguyen, 2010). Furthermore, the TEM images of GMT_0 and GMT_0.05 composites, as shown in Fig. 2b–c, respectively, discloses the fractions of particles with tiny particles of irregular shapes and micrometers in sizes of CSH phase in the cement mortar. The small incomplete grain growth of CH phase and the CA tube was found on CSH surface. The CA tube appeared as rod-like crystals in the early stages of reaction as massive growths filling pores or cracks in mature concrete or mortar. It can be seen in the high-magnification image of Fig. 2c-1 that the surface of CSH phases is seized by GO sheets which increases the tightness of both the hardened cement paste and mortar. The SAED pattern in the inset of Fig. 2b-2 displays the diffraction spots of GMT_0 sample, while GMT_0.05 composite reveals the concentric diffraction rings of spots as shown in inset of Fig. 2c-2, indicating a polycrystalline nature (Liu et al., 2014). This result confirms the existence of GO sheets embedded in the GMT_0.05 composite. Fig. 2c-3 indicates the EDS spectra of GMT_0.05 composite which has the C, Al, Ca, Si, and O elements (Liu et al., 2014).

Fig. 3a shows XRD pattern of GO sheets, GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively. The XRD pattern of GO sheets displays a broad diffraction peak appeared at 2° of 11.16° which corresponded to the crystalline planes of (002) with a determined d-spacing of 0.810 nm (Phrompet et al., 2019). Next, XRD pattern of GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites can detect the phases of the usual cement hydrates as calcium silicate hydrate (CaH₂O₄Si; CSH), calcium hydroxide (Ca(OH)₂; CH) and Ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃·26H₂O; CA) and silicon dioxide (SiO₂; Si) (Wang et al., 2017). The XRD results can confirm the pure phase of cement mortar.

The Raman spectra of GO sheets, GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites are shown in Fig. 3b. Generally, the Raman spectra of GO sheets displays two main peaks at ~ 1338 cm⁻¹ and ~ 1586 cm⁻¹, known to be the D and G-bands in the structure of carbon atom, respectively (Phrompet et al., 2019; Suk et al., 2010). In principle, the D-band defects and disorders the first hexagonal graphitic layers. In addition, the G-band is related to the sp²-bonded carbon atom vibration in 2D hexagonal lattice (Phrompet et al., 2019; Suk et al., 2010). Next, Raman spectra of GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites are observed as well as pure mortar (Carrasco et al., 2017). The band at ~ 714 cm⁻¹ is determined by asymmetric stretching of Si–O–Al and the band at 1088 cm⁻¹ may be attributed to lattice mode. Next, the bands at ~ 953 cm⁻¹ are assigned to υ₁(SiO₄) symmetric stretching vibration of C–H–S phase, which is the main phase formed during cement hydration (Carrasco et al., 2017). The band at ~ 670 cm⁻¹ is determined by asymmetric stretching of Si–O–Si bending modes involving tetrahedra. The bands at ~ 483 cm⁻¹ are symmetric bending υ₂(Si–O) and small band at ~ 365 cm⁻¹ is ascribed to residual Ca(OH)₂ (Carrasco et al., 2017; Suk et al., 2010). The band at ~ 299 cm⁻¹ is assigned to the deformation of the silicate network along with Ca–O stretching (Phrompet et al., 2019; Suk et al., 2010). These results indicated that all samples are at the pure phase of cement mortar. At high composite concentrations (> GMT_0.03), the two peaks were obviously observed indicating the influence of GO composites which increases the tightness of both the hardened cement paste and mortar.

TGA data are demonstrated in Fig. 3c. The weight loss and DTG peaks of GMT composites gradually decreased with increasing temperature. This result indicated a specific phase of compounds decomposition. The three steps of decomposition in all samples were observed. At a low temperature ~ 30–100 °C, the decreasing weight loss of about 4% should be related to the decomposition of the absorbed water molecules (Singh et al., 2015). At temperature ~ 100–500 °C, the decreasing weight loss should be the decomposition of CSH, mono-carbonate. Moreover, at the high temperature range of 600–750 °C, the major weight loss was observed because of the decomposition of the calcined as CO₂ (Singh et al., 2015). Interestingly, the DTG peak of GMT composites gradually increased when the concentrations of GO increased. This result indicated that the concentrations of GO can improve the thermal stability in mortar composites as shown in the inset of Fig. 3d.

The relationship between mechanical properties and the microstructure of the composites was characterized by SEM images. However, the GO sheets are not detected due to small sheets. The surface of the mortar particle should be deposited by the GO sheet. According to Fig. 4a–d, the SEM images of the GMT_0, GMT_0.03, GMT_0.05 and GMT_0.1 composites reveal random sizes and the content of CH, CA, and CSH as ordinary cement mortar. Obviously, the influence of GO on cement mortar size can improve the micro-compactness, micro-cracks and low percentage porosity (Lv et al., 2013), as shown in Table 1. Therefore, adding GO to low contents of GMT composites can increase the mechanical properties because the GO sheet plays the role of a bridge through covalent bonding with CSH (Kang et al., 2017).

Fig. 5a shows the comparison of compressive strengths at the curing age of 3, 7, 21, 28 and 77 days for GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively. The compressive strengths of the GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites were greater than GMT_0 (without GO loading) sample for all the curing ages at 3, 7, 21, 28 and 77 days, respectively, as summarized in Table 2. The excellent mechanical properties of GMT composites compared with GMT_0 explained by a lot of functional groups in GO, such as carboxylic acid that reacted with CSH, CH and CA in the mortar, should be forming a strong covalent bond (Qureshi & Panesar, 2019). A curing age at 7 days had the highest percentage compressive strength value of 11.084 ± 0.620 MPa when the GO content was 0.05 wt% (GMT_0.05) which is higher than the percentage value of GMT_0 approximately 36.71%, as shown in Fig. 5b. Namsone et al. (2017) reported that the 28-day compressive strengths were observed about 6.68–12.49 MPa for a foamed matrix using expanded glass aggregates in cement mortar. Moreover, Rafiee et al. (2013) reported that the compressive strength of the mixture increased by approximately 15% in comparison to ordinary mortar when the GO concentration was 1.0 wt% as GO reinforces the crevices in cement. However, at the high GO concentrations > 0.05 wt%, the compressive strengths gradually decreased because of the self-accumulation of GO sheets. Furthermore, when the curing age under tap water is higher than 7 days, the percentage compressive strength showed a low value by approximately 26.83, 17.00, 32.71 and 34.70% for the curing age at 3, 21, 28 and 77 days, respectively, measured by a GMT_0.05 sample. Fig. 5c shows a comparison between the tensile strengths of GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively, at the curing age of 3, 7, 21, 28 and 77 days to analyze the effect of GO on tensile strength. The tensile strength of GMT_0.05 sample was demonstrated to be the highest by up to 61.45% at 7 days of a curing age as shown in Fig. 5d. As Qian et al. (2015) reported that the compressive strength of cement composite specimens with low GO concentrations of 0.05 wt% increased by up to and 18.7% and 13.7% at a curing age of 7 and 28 days, respectively. The tensile strength values of all sample are summarized in Table 2. The strengths of the GMT composites increased in the compressive and tensile strength tests because of the addition of GO which promotes hydration, decreases pore volume, accelerates crystallite formation, and causes the crystallites to align. This increases the tightness of both the hardened cement paste and mortar (Liu et al., 2012). Also, the mechanical property of cement mortar was improved by the GO composites.

Fig. 6a–b demonstrates the volumetric heat capacity (Cv), the thermal conductivity (κ), and thermal diffusivity (a) of GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively at the curing age of 7 days. In Fig. 6a, the volumetric heat capacity values were 1.489 × 10⁶, 1.471 × 10⁶, 1.449 × 10⁶, 1.515 × 10⁶ and 1.408 × 10⁶ J/m³ K for GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively.The results showed that the GMT_0.05 had the lowest Cv value compared to other samples. Furthermore, the thermal conductivity and the thermal diffusivity are shown in Fig. 6b. The thermal conductivity was found to be 0.578 ± 0.008, 0.568 ± 0.023, 0.549 ± 0.025, 0.513 ± 0.028 and 0.490 ± 0.026 W/m K for GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively, whereas the values of thermal diffusivity were found to be at the range of 0.38 × 10^–6 to 0.33 × 10^–6 m²/s. The thermal conductivity value of mortar about 0.264 W/m K is similar to what was observed in Jiang et al. (2018) study. The thermal conductivity is decreased with increasing GO contents due to the low thermal conductivity or the exellent insulator of GO sheets and good interactions between CSH, CH and CCA phases in the cement mortar and GO sheets. The low thermal conductivity was observed at 0.05 wt% (GMT_0.05) and 0.1 wt% (GMT_0.1) of GO content similar to what was observed in the low thermal insulation property of GMT composites. Generally, the material with thermal conductivity values lower than 0.250 W/m K is known as a heat insulator material (Zhou et al., 2010). Therefore, the heat insulator of cement mortar was enhanced by the GO composites.

The heat insulator behavior of GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites at the curing age of 7 days was investigated. The temperature distribution on the surface of GMT composites was observed at the experimantal time for 0, 10, 20 and 30 min by using the infrared thermal imaging as shown in Fig. 7. The temperature of the hot surface was 350 °C. According to Fig. 7, both sides of temperatures on high temperature and low temperature for all GMT composites are increased with increasing time until the temperature becomes constant. The results showed that all GMT composites displayed temperature stability on the hot plate at the same time about 30 min. The heat insulator behavior of mortar is similar to what was reported in another study conducted by Yousefi et al. (2020). In all GMT composites, the increase of low temperature was observed as well as pure mortar (GMT_0). The temperature differences (ΔT) of each surface side are particularly important because the materials, which can block the heat transfer, should be applied to insulating material between the hot and cold surface of thermoelectric devices. The insulating materials can increase the output efficiency in energy conversion devices such as thermoelectric devices. For example, Lv et al. (2020) reported that the output efficiency of thermoelectric device increased with the decrease of the thermal properties of fillers in space between legs. Moreover, the output efficiency of thermoelectric properties decreased about ~ 1% because the thermal insulation materials are not added in the space between legs according to Li et al. (2018). Therefore, the heat insulation materials should be a factor causing the increase in the output efficiency of thermoelectric properties because of the reduction or elimination of the heat transfer (Li et al., 2018; Lv et al., 2020). The ΔT values of GMT composites are summarized in Table 3 and found to be at the range of 276–279 °C and 269–271 °C for 20 and 30 min, respectively. Interestingly, for GMT_0.05 and 0.1 composites, the temperature difference values increased due to the low thermal conductivity of GO sheets compared with the rGO or Graphene (Mahanta & Abramson, 2012). Moreover, the reaction with CSH and CH phases in the cement mortar on the surface of GO should reduce the thermal insulation property of GMT composites.

Furthermore, the thermal resistance (R), which is the resistance to heat flow as heat insular of the material of GMT composites, was defined by R = t/κ, where t is the insulation thickness and κ is the thermal conductivity of sample (Mahanta & Abramson, 2012). The thermal resistance values of GMT composites in the area of 5 × 5 cm² and thickness of 5 cm are summarized in Table 3. The thermal resistance value was 0.087, 0.115, 0.091, 0.098 and 0.102 m² K/W for samples of the GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively. The sample of GMT_0.05 and GMT_0.1 composites displayed the high thermal resistance value. This result indicated that those samples provided the high thermal resistance values.

The investigation of the surface by overall heat transfer coefficient (U) as inversion of thermal resistance value was done through U = 1/R (Mahanta & Abramson, 2012; Zhou et al., 2010). The overall heat transfer coefficient value was 11.556, 11.369, 10.972, 10.252 and 9.792 W/m² K for samples of the GMT_0, GMT_0.01, GMT_0.03, GMT_0.05, and GMT_0.1 composites, respectively.

Regarding the heat transfer rate (Q), it was calculated by Q = κA(ΔT/t), where κ is the thermal conductivity of samples; t is the insulation thickness; ΔT is the temperature differences; and, A is the arear of sample. The Q of GMT composites remained at the cold side for 20 and 30 min. The results demonstrated a decrease in heat transfer rate with increasing GO contents, as displayed in Table 3. These results indicated the effect of GO nanosheet connected with the CSH, CH and CCA phases in the cement mortar on the thermal insulation property of GMT composites (Zhou et al., 2010). The low heat transfer rate of GMT_0.05 and GMT_0.1 composites for 30 min was 0.465 and 0.443 W/min, respectively, indicating lower rate than the samples without GMT_0 composite. Therefore, the GMT composites can be a new pathway to significantly improve the heat insulation of mortar composites.

The influence of GO sheets loading contents on the mechanical and thermal properties of the mortar composites was investigated. Three crystalline phases of CaH₂O₄Si, Ca(OH)₂, Ca₆[Al(OH)₆]₂(SO₄)₃·26H₂O and SiO₂ were observed in all conditions, indicating the mortar material. The size of cement mortar deposited on the surface of GO is slightly decreased with increasing the GO contents. The small particle sizes of mortar, which created the CSH, CH, and CA phases on the surface of GO in the cement mortar, should be explained by increasing the compressive and tensile strength values. Interestingly, the thermal conductivity is decreased with increasing GO contents. The thermal conductivity values of 0.513 ± 0.028 and 0.490 ± 0.026 W/m K for GMT_0.05 and GTO_0.1 composites, respectively, indicate a heat insulator material. Moreover, the low heat transfer rate of GMT_0.05 composite for 30 min was 0.465 W/min, demonstrating a new pathway to significantly improve the heat insulation of mortar composites. The interactions between CSH, CH and CA phases in the cement mortar and GO sheets can affect the heat insulation of GMT composites. Therefore, the mechanical and thermal properties of mortar nanocomposite materials were improved by GO nanosheets.