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Article

Thermal Properties of MWCNT-rGO-MgO-Incorporated Alkali-Activated Engineered Composites

by
Mohammad A. Hossain
and
Khandaker M. A. Hossain
*
Department of Civil Engineering, Toronto Metropolitan University, Toronto, ON M5B 2K3, Canada
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(3), 117; https://doi.org/10.3390/jcs9030117
Submission received: 5 January 2025 / Revised: 9 February 2025 / Accepted: 19 February 2025 / Published: 3 March 2025
(This article belongs to the Special Issue Recent Progress in Hybrid Composites)
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Abstract

This study evaluates the influence of multiwall carbon nanotubes (MWCNTs), reduced graphene oxide (rGO), and magnesium oxide (MgO) on the thermal conductivity of alkali-activated engineered composites (AAECs). Thirty-two ambient-cured AAECs consisting of two types of powdered-form reagents/activators (type 1—calcium hydroxide: sodium meta silicate = 1:2.5; type 2—calcium hydroxide: sodium sulfate 2.5:1), two dosages of MgO (0 and 0.5%) of MgO, three percentages (0, 0.3%, and 0.6%) of MWCNTs/rGO, and binary (45% ground granulated blast furnace slag ‘GGBFS’ and 55% Class C fly ash ‘FA-C’) and ternary combinations (40% GGBFS, 25% FA-C and 35% class F fly ash ‘FA-F’) of industrial-waste-based source materials, silica sand, and polyvinyl alcohol (PVA) fiber were developed using the ‘one-part dry mix’ technique. Problems associated with the dispersion and agglomeration of nanomaterials during production were avoided through the use of defined ultra-sonication with a high-shear mixing protocol. The impact of the combination of source materials, activators, and MgO/MWCNT/rGO dosages and their combinations on the thermal properties of AAECs is evaluated and discussed based on temperature–time history and thermal conductivity/diffusivity properties along with micro-structural characteristics. It was found that the change in temperature of the AAECs decreased during testing with the addition of MWCNTs/rGO/MgO. The thermal conductivity and diffusivity of AAECs increased with the increase in MWCNT/rGO/MgO contents due to the formation of additional crystalline reaction products, improved matrix connectivity, and high conductivity of nanomaterials. MWCNT AAECs showed the highest thermal conductivity of 0.91–1.26 W/mK with 49% enhancement compared to control AAECs followed by rGO AAECs. The study confirmed the viability of producing MgO/MWCNT/rGO-incorporated AAECs with enhanced thermal properties.

1. Introduction

Alkali-Activated Binders (AABs) or geopolymers with low carbon potential developed through the activation of aluminosilicate-based source materials (e.g., fly ash, slag, and metakaolin) by alkaline reagents (e.g., NaOH, KOH, Na2SiO3, K2SiO3) with satisfactory strength and fire/chemical resistance are low-carbon, sustainable, green alternatives to cement-based concretes [1,2,3,4,5,6,7,8]. Thermal properties are essential for the development of energy-efficient buildings and various construction applications, including pavements and bridge decks, which have used geopolymers [9].
Carbon nanotubes (CNTs) are suitable materials due to their exceptional mechanical and self-sensing characteristics in producing high-performance nanocomposites [10,11,12]. Graphene and CNTs with very high thermal conductivity of 3000–5300 W/mK [13] and 2000~4000 W/mK [14] can significantly improve the thermal properties of concrete. Individual multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs) exhibited high thermal conductivity over 3000 W/mK [15] and 3500 W/mK [16], respectively. However, an entangled network of carbon nanotube sheets only showed thermal conductivity of 20–30 W/mK at room temperature [17]. Thermal conductivity data for reduced graphene oxide (rGO) films showed large variations from 30 to 2600 W/mk [18]. rGO generally has a 2D membrane structure and is thermodynamically stable [19,20]. Previous studies have concentrated on the mechanical, microstructural, and conductive performance of MWCNT-incorporated AAB/geopolymer mix systems [21]. Difficulties associated with dispersion and interfacial MWCNT–inorganic matrix interaction should be resolved to utilize MWCNTs as reinforcement in AAB systems [12].
Thermal properties play an important role in determining the energy efficiency of materials and heat loss or gain in concrete buildings [22]. The thermal diffusivity of most concrete types is low, and due to the direct correlation between thermal diffusivity and thermal conductivity, the latter is lower as well [23]. Thermally conductive geopolymer/AAB with improved energy efficiency performance had been developed by incorporating conductive fillers providing a path for phonon transport [24]. Aggregate, binder, and alkali types/content in the activator and water-cement ratio influenced the thermal expansion characteristics of geopolymers [25]. The addition of 1 wt% of carbon nanofibers (CNFs) and CNTs increased the thermal conductivity of cement-based composites by 9% and 26%, respectively [26]. On the other hand, the addition of 1 vol% of CNTs increased the thermal conductivity by 11.1% in concrete-based composites [27].
The thermal conductivity of conventional concrete ranges between 0.62 and 3.3 W/m/K, depending on coarse aggregate type, moisture condition, and temperature [28,29,30,31]. In contrast, lightweight insulating concrete, such as that containing polystyrene beads or cellular lightweight concrete, exhibits much lower thermal conductivity, ranging from 0.07 to 0.33 W/m/K [32].
The thermal conductivity of porous geopolymer concrete (GPC) incorporating fly ash, water glass, sodium water glass and H2O2 with a density of 240–335 kg/m3 was 0.0744 W/mK [33]. The thermal conductivity of near-dried geopolymer foam was in the range of 0.15 to 0.48 W/mK with density ranging from 720 to 1600 kg/m3 [34]. A study on lightweight foamed geopolymer concrete incorporating low-calcium fly ash, palm oil fuel ash, and oil palm shell was conducted and thermal conductivity value of 0.47 W/mK was observed with a density range of 1300–1700 kg/m3 [35]. Lightweight geopolymer concrete based on recycled expanded polystyrene as an aggregate had thermal conductivity in the range of 0.121 to 0.207 W/mK with a 500 to 800 kg/m3 density [36]. Geopolymer concrete with microencapsulated phase change materials with a density of 1875 kg/m3 showed a thermal conductivity value of was 0.74 W/mK. According to Kamseu et al. [37], the thermal conductivity of metakaolin-based geopolymers improved from 0.30 to 0.59 W/mK with an increase in Si/Al from 1.23 to 2.42 due to enhanced polycondensation, reduction in median pore size and aided heat transfer. The incorporation of conductive nanofillers is a feasible method to develop geopolymer concrete with improved thermal properties.
Metakaolin (MK)-based geopolymers are inherently insulating due to their amorphous and nanoporous structure [37,38]. Most work on metakaolin GP composites has involved the addition of materials or structural features, including glass microspheres [39], polystyrene (PS) cork [40], mineral particles, rubber disks, and induced porosity [41,42,43,44] to improve their insulating ability.
The thermal properties of geopolymer composites were improved with the use of fillers, such as quartz, silicon carbide, graphene nanoplatelets (GNPs), CNTs, diamond, and calcite [24,45,46,47,48]. Crystalline minerals with higher conductivity than amorphous ones can be taken from waste sources to be used as alternatives to quartz to enhance thermal conductivity. Research has shown that adding MWCNTs can improve the thermal conductivity of GPC. For instance, a study by Dong et al. [49] reported a significant increase in thermal conductivity after the addition of 0.5% MWCNTs by weight. The uniform dispersion of MWCNTs is crucial for achieving optimal thermal performance. Saafi et al. [50] demonstrated that incorporating rGO into GPC substantially increased thermal conductivity. The optimal rGO content was around 0.3% by weight, beyond which the thermal conductivity gains diminished, likely due to aggregation and ineffective dispersion.
Zero-cement-based, alkali-activated engineered composites (AAECs) are alternatives to traditional PVA-fibre-reinforced engineered cementitious composite (ECC) with high-strain hardening and multiple micro-cracking characteristics [1]. Novel aspects of these multi-functional AAECs are the incorporation of nanomaterials (MWCNTs and rGO to induce conductive and self-sensing properties) and MgO additives for self-healing and shrinkage compensation as per Sherir et al. [51]. No work has been conducted to develop and assess the thermal conductivity of MWCNT/rGO/MgO-incorporated AAECs with polyvinyl alcohol (PVA) fibers. This research presents the thermal properties of developed AAEC mixes with various dosages (0%, 0.3%, and 0.6% by wt) of MWCNTs/rGO, 5%MgO, and their combinations. Thermal characteristics such as temperature evolution, thermal conductivity, and resistivity are described based on the influence of dosages of MWCNTs/rGO/MgO and other parameters such as binary/ternary combinations of source materials (fly ash class C ‘FA-C’, fly ash class F ‘FA-C’, and ground granulated blast furnace slag ‘GGBFS’) and reagent/activator types. The findings this study inform engineers and designers of the thermal characteristics of developed AAECs for their construction applications.

2. Experimental Program, Materials, and Mix Design

A total of 32 ambient-cured green AAEC mixes were developed using the ‘one-part dry mix’ technique incorporating two types of powder-based activators/reagents (type 1 and type 2), varying contents of MgO (0% and 5 wt%), MWCNTs/rGO (0%, 0.3%, and 0.6 wt%) and their combinations (5%MgO–0.5% or 0.6%MWCNTs and 5%MgO–0.5% or 0.6%rGO), binary (FA-C + GGBFS)/ternary (FA-C + FA-F + GGBFS) combinations of industrial-waste-based source materials, silica sand, and PVA fiber. The effects of activator types and MWCNT/rGO/MgO/MgO-MWCNT/MgO-rGO combinations and dosages on the thermal conductivity properties of AAECs were investigated

2.1. Materials and Properties

The binary (designated as B) AAEC mixes were prepared by mixing high-calcium FA-C and GGBFS, whereas the ternary ones (designated as T) were developed by mixing high-calcium FA-C, low-calcium FA-F and GGBFS [52]. A polycarboxylate, ether-based, high-range, water-reducing admixture (HRWRA) with a solid content of 40% was used to ensure workability. Silica sand with a maximum particle size of 600 µ was used as fine aggregate. The MgO was prepared by burning MgCO3 for two hours at 900 °C [51]. Additionally, 2% oil-coated PVA fibers, 8 mm in length and 39 μm in diameter, were used. MWCNTs with a diameter of 20–30 nm, length of 10–30 μm, Blaine fineness of 110 m2/g, and electrical conductivity of greater than 10−2 s/cm were employed. rGO (black powder form) with a thickness of 3–6 μm, a diameter of 0.5 µm, Blaine fineness of 130 m2/g and an electrical conductivity of around 560 s/cm was used. The physical properties and chemical compositions of FAs, GGBFS, silica sand, and MgO are presented in Table 1 and can be found in [10,52].
Two types of activators were used [1,10,53]—activator type 1 consisted of calcium hydroxide (CaOH2) with sodium meta silicate (Na2SiO3.5H2O) in a ratio of 1:2.5, while activator type 2 was made of Ca(OH)2 and sodium sulfate (Na2SO4) in a ratio of 2.5:1.

2.2. Mix Design and Mixing Procedure

The mix designs of AAECs are presented in Table 2 with their mix designations. Four control mixes (B1, B2, T1, and T2) without MgO, MWCNT, or rGO addition were produced based on a previous study [1] besides the other 28 AAEC mixes containing MgO, MWCNTs, and rGO [10,52]. The binary (B) mixes contained the combinations of FA-C and GGBFS, whereas the ternary (T) mixes were prepared with FA-C, FA-F, and GGBFS. The FA and GGBFS contents were varied from 52% to 60% and 38% to 45%, respectively, while the silica sand and HRWRA were kept constant at 30% and 0.02%, respectively, by the mass of the total binder. The water-to-binder ratio varied from 0.35 to 0.4 to achieve a minimum slump flow diameter of 500 mm. The fundamental chemical ratios in terms of SiO2/Al2O3, Na2O/SiO2, CaO/SiO2, and Na2O/Al2O3 (as presented in Table 2) are within the range detailed in previous research [4,54].
The mixing and production processes of AAECs were as per Hossain and Hossain [52]. Firstly, for proper dispersion, MWCNTs/rGO was added to two-thirds water and 50% HRWRA in a beaker and sonicated inside a sonicator with a probe for 30–40 min by applying a sonication energy of 50 J/mL~75 J/mL. The sonicated mix was then added into the rigorously blended dry mix of source, activator, and MgO materials, while mixing continued in a shear mixer, then silica sand and PVA fibers were added to prepare MWCNT/rGO/MgO-incorporated AAECs with a total mixing time of about 20–25 min.

2.3. Specimens, Test Methods, and Testing Procedures

Two 50 mm × 50 mm × 50 mm cube specimens were produced for each of the 32 AAECs. The specimens were de-moulded after 24 h of casting and were kept in a curing chamber maintained at 23 ± 3 °C and 95 ± 5% relative humidity (RH) until testing at 28 days. The 28-day dry density of specimens was evaluated [6]. A TCKit (C-Therm, 2400 series: C-Therm Technologies Ltd., Fredericton, NB, Canada) equipped with a computer-aided data acquisition system was used to record temperature evolution during testing and thermal conductivity/diffusivity measurements (Figure 1) by placing a thermal sensor between two identical specimens at 23 to 25 °C.
The thermal conductivity (λ) in W/mK is calculated as per Equation (1) [55,56,57].
Q = −λA(dT/dx)
where Q is the heat flow rate of thermal conduction (W), A is the cross-section area that the heat flows through (m2), dT is the differential temperature across the sample (°C), and dx is the differential thickness of the test sample (m).
The thermal diffusivity (α) of a material (m2/s) can mathematically be expressed as per Equation (2):
α = λ/ρcp
where cp is the specific heat capacity, and ρ is the density of the material.
SEM and EDS analyses were conducted on 10 mm × 5 mm × 5 mm chip specimens taken from the core to determine the reaction products in AAECs at 2000× (10 μm) [52]. The XRD analysis was conducted to determine mineral phases on finely ground specimens (collected from the cube core) that had been passed through a 200-mesh sieve [52]. A Bruker D8 Endeavor diffractometer (equipped with a Cu X-ray source and operating at 40 kV and 40 mA; a range of 5–70° θ) was used.

3. Results and Discussion

Table 3 summarizes the dry density, thermal conductivity/diffusivity, and thermal conductivity change (increase/decrease). The effects of MWCNT/rGO dosages with their combinations and reagent/activator types on the temperature evolution and thermal conductivity/diffusivity are described.

3.1. Effect of Reagent Type, MWCNT, rGO, and MgO Contents on Temperature Change of AAECs

The relationship between the change in temperature (ΔT) and time during a thermal conductivity test of AAEC materials depends on several factors, including the material thermal properties and the experimental setup. A typical thermal conductivity test subjected a specimen to a controlled heat flow or temperature gradient. The ΔT within the specimen was monitored over time with a computer-aided data acquisition system to determine its thermal conductivity and resistivity. The ΔT over time in specimens made of control, MgO, MWCNT, and rGO AAECs over the test duration is presented in Figure 2, Figure 3 and Figure 4. During the initial stages of the test, the temperature gradient was established (during the initial 2 to 3 min), and then specimens started to equilibrate with the applied heat. The rate of temperature change with respect to time was relatively high at this point as the sample adjusted to the heat transfer. As time progressed, the temperature gradient across the sample began to stabilize, and the rate of temperature change decreased. This occurred because the AAEC material reached a steady-state condition, where the heat flowing into the specimens was balanced by the heat being conducted through it. At a steady state, the ΔT approached zero with respect to time, indicating a constant temperature distribution within the specimens.
The ΔT over the test duration of binary (B) and ternary (T) MWCNT AAECs compared to their control counterparts (B1, B2, T1, and T2) is presented in Figure 2. The ΔT decreased as MWCNT content increased from 0 to 0.6% during the thermal conductivity test, and MWCNT AAECs showed lower ΔT than control AAECs. Control B1 and T1 with reagent 1 showed a lower steady-state ΔT (29 °C) compared to 34 °C of their counterparts B2 and T2 with reagent 2. MWCNT AAECs showed a lower steady-state ΔT than their control AAEC counterparts, with binary MWCNT AAECs showing higher steady ΔT (21–25 °C) than their ternary counterparts (10–13 °C).
The ΔT over the test duration of binary (B) and ternary (T) MgO AAECs compared to their controls is presented in Figure 3. Control B1 and T1 with reagent 1 showed a lower steady-state ΔT (29 °C) compared to 34 °C of their counterparts B2 and T2 with reagent 2. MgO AAECs showed lower steady-state ΔT during testing than their control AAEC counterparts, with binary MWCNT AAECs showing slightly higher steady ΔT (14–15 °C) than their ternary counterparts (13–14 °C).
The ΔT over the test duration of binary and ternary rGO AAECs compared to their controls is presented in Figure 4. Control B1 and T1 with reagent 1 showed lower steady-state ΔT (29 °C) compared to 34 °C of their counterparts B2 and T2 with reagent 2. rGO AAECs showed a lower steady-state ΔT than their control AAEC counterparts with binary rGO AAECs showing higher steady ΔT (12–14 °C) than their ternary counterparts (10–11 °C). In general, MWCNT/rGO/MgO AAECs showed lower steady-state ΔT than their control AAEC counterparts during testing due to the presence of conductive MWCNT/rGO material and MgO that enhanced thermal conductivity with the formation of a conductive network and denser matrix.

3.2. Effect of Reagent Type and MWCNT/rGO/MgO Dosages on the Thermal Conductivity/Diffusivity of AAECs

3.2.1. Control and MgO AAECs

The thermal conductivity of control AAECs (B1, B2, T1, and T2) varied from 0.74 W/mK to 0.84 W/mK, while that of MgO AAECs varied from 0.75 W/mK to 0.85 W/mK. The thermal diffusivity of control AAECs ranged from 0.70 × 10−6 m2/s to 1.57 × 10−6 m2/s, while that of MgO ranged from 0.54 × 10−6 m2/s to 1.65 × 10−6 m2/s (Table 3). The thermal conductivity of MgO AAECs was 0.44% to 10.1% higher than that of their control counterparts. Binary and reagent 2 control/MgO AAECs produced higher thermal conductivity than their ternary and reagent 2 counterparts.
The better thermal properties of binary AAECs than their ternary counterparts as observed in this study are attributed to the predominant formation of reaction products C-A-S-H/C-S-H compared to the additional development of amorphous N-C-A-S-H/N-A-S-H in ternary composites as evident from the SEM/EDS and XRD analyses in research studies by the authors [1,52]. In addition, reagent 2 generally produced higher thermal conductivity compared to reagent 1 AAECs due to the development of crystalline C-S-H gel (in addition to C-A-S-H/N-C-A-S-H or C-A-S-H/C-S-H in the matrix [1,52].
The higher thermal conductivity of MgO AAECs is due to the development of Mg(OH)2 crystals, magnesium–aluminum hydrotalcite (Mg6Al2(OH)16CO3.4H2O and M-S-H (as evident in T2M5 mix in Figure 5)) in addition to regular C-A-S-H/N-C-A-S-H or C-A-S-H/C-S-H (depending on binary/ternary mix and reagent types). The presence of these reaction products can be confirmed from the elements (Ca = 21.2%, Si = 5.1%, Al = 2.4%, Mg = 1.6%, and O = 47.1%, for example, in T2M5, Figure 5) and detailed XRD analysis presented in Hossain and Hossain [52]. The formation of such compounds is also confirmed from other research studies [58,59,60].

3.2.2. MWCNT AAECs

The effect of reagent type and MWCNT content on the thermal conductivity and diffusivity of AAECs is illustrated in Figure 6. The thermal conductivity of MWCNT AAECs ranged between 0.91 W/mK and 1.26 W/mK (Table 3). Binary and reagent 2 control/MWCNT-AAAECs produced higher thermal conductivity than their ternary and reagent 1 counterparts. The thermal diffusivity of MWCNT AAECs varied from 0.74 × 10−6 m2/s to 2.28 × 10−6 m2/s (Table 3). Irrespective of the reagent type, the thermal conductivity/diffusivity of AAECs increased with the increase in MWCNT content from 0% to 0.6% with respect to control AAECs (Figure 6). However, for thermal diffusivity or conductivity, 0.3%MWCNTs seemed to be the optimum content for binary and ternary MWCNT AAECs with reagent type 2 (B2C3, T1C3, and T2C3). For MWCNT AAECs, thermal conductivity increase ranges of 16.6–41.7% and 23.2–49.4% were found for 0.3%MWCNTs and 0.6%MWCNTs, respectively, with an overall increase range of 16.6–49.4% compared to their control AAEC counterparts (Table 3). Such increases in thermal conductivity of 26% and 11% were observed with the addition of 1%CNTs in cement-based composites [26,27] and with 0.5% MWCNTs by weight [49]. This suggests that MWCNTs are very effective (especially at a high dosage of 0.6%) in enhancing the thermal conductivity of AAECs.
The higher thermal conductivity of MWCNT AAECs was not only due to formation of regular C-A-S-H/N-C-A-S-H or C-A-S-H/C-S-H crystalline/amorphous reaction products as evident from mix B2C3 in Figure 7 (showing an elemental compositions of Ca = 12.4%, Si = 5.5%, Al: 2.8%, Na: 2.4%, Mg = 1.8%, and O = 48.9%) and XRD analysis in Hossain and Hossain [52] but to a greater extent also associated with good MWCNT–matrix bonding, producing better connectivity (as indicated by the MWCNT dispersion in the matrix in Figure 7) [52,61] and the very high (2000~4000 W/mK) thermal conductivity of CNTs [14].

3.2.3. rGO AAECs

The thermal conductivity and diffusivity of rGO AAECs are summarized in Table 3 and compared with control AAECs in Figure 8 to analyze the effect of reagent type and rGO content.
The thermal conductivity of rGO AAECs ranged between 0.88 W/mK and 1.26 W/mK, while the diffusivity ranged between 0.84 × 10−6 m2/s and 1.84 × 10−6 m2/s (Table 3). Binary and reagent 2 control/rGO-AAAECs produced higher thermal conductivity than their ternary and reagent 1 counterparts as observed for control, MgO, and MWCNT AAECs. Irrespective of the reagent type, the thermal conductivity/diffusivity of rGO AAECs increased with an increase in rGO content from 0% to 0.6% with respect to control AAECs (Figure 8). An increase in thermal conductivity with the incorporation of rGO into GPC was also reported in other research studies [50]. For rGO AAECs, thermal conductivity increase ranges of 3.6–27.7% and 2.1–28.6% were found for 0.3%rGO and 0.6%rGO, respectively, which were lower than respective MWCNT AAECs. Also, a higher dosage of rGO (0.6%) was not as effective as that of its MWCNT counterparts. rGO incorporation increased the thermal properties of AAECs but was found to be less effective than MWCNTs.
Additional crystalline zeolite (Na2Al2Si3O8·2H2O) formation (due to rGO incorporation) coupled with regular C-A-S-H/N-C-A-S-H or C-A-S-H/C-S-H can be attributed to thermal conductivity enhancement as can be seen from SEM-EDS analysis (Figure 9, rGO AAEC mix B2R6, for example), confirmed from XRD analysis in Hossain and Hossain [52] and reported in other research studies [62]. The increase in matrix porosity (especially at higher dosages) [63] and less connectivity with the addition of rGO, besides other factors, might be associated with the production of inferior thermal properties compared to their MWCNT counterparts. rGO presence could not be identified due to its ultrafine characteristics, but its presence was confirmed due to the formation of zeolite as shown in Figure 9.

3.2.4. MgO-MWCNT/rGO AAECs

The effects of MWCNT/rGO contents and reagent types on the thermal conductivity/diffusivity of MgO-MWCNTs and MgO-rGO AAECs are presented in Figure 10 and Figure 11, respectively, with results summarized in Table 3. The thermal conductivity and diffusivity of MgO-MWCNT AAECs ranged from 0.88 W/mK to 0.96 W/mK and from 0.65 × 10−6 m2/s to 0.95 × 10−6 m2/s, respectively, while those for MgO-rGO AAECs ranged from 0.84 W/mK to 0.88 W/mK and from 0.72 × 10−6 m2/s to 1.24 × 10−6 m2/s, respectively (Table 3). MgO-MWCNT AAECs showed higher thermal conductivity than MgO-MWCNT AAECs and had lower thermal conductivity than their MWCNT and rGO AAEC counterparts but higher values compared to control and MgO AAECs. Thermal conductivity increases of 3.6–27.7% and 2.1–27.7% were observed for MgO-MWCNTs and MgO-rGO AAECs, respectively. Thermal conductivity increased with the increase in MWCNT (Figure 10) and rGO contents (Figure 11), with binary MgO-MWCNT/rGO AAECs showing higher thermal conductivity. The thermal properties of MgO-rGO and MgO-MWCNT AAECs were affected by the combined effect of these nanomaterials and MgO additive in the matrix as described in previous sections due to formation of similar reaction compounds (as indicated in Figure 12 for MgO-rGO AAEC mix B2M5R6 by the presence of Ca = 10.1%, Si = 7.3%, Mg = 3.8%, Al = 3.4%, O = 52.1%) and conductivity of MWCNT/rGO dispersion forming a conductive network.

3.3. Thermal Conductivity vs. Dry Density of AAECs

Figure 13 shows a plot showing the variation in thermal conductivity with dry density of all 32 mixes. The density of AAECs ranged between 1960 kg/m3 and 2152 kg/m3 (Table 3). Generally, an increase in thermal conductivity is expected with an increase in density for conventional concrete. Figure 13 shows a general increase in thermal conductivity with the increase in density; however, it is not distinctive or well correlated. This was expected as AAECs are distinctively different due to the addition of different nanomaterials as these highly conductive materials and their connectivity to the matrix greatly affected the thermal conductivity.

4. Conclusions

Thirty-two AAECs were developed from source materials (FA-C, FA-F, and GGBFS) in binary (FA-C + GGBFs) and ternary (FA-C + FA-C + GGBFS) combinations as multi-component powder-form reagents (type 1: calcium hydroxide + sodium metasilicate; type 2: calcium hydroxide + sodium sulphate) with 2% v/v PVA fibres, 5% MgO, and MWCNTs/ rGO (0%, 0.3%, and 0.6 wt%). The following conclusions are derived based on the thermal and microstructural characteristics of AAECs:
  • MWCNT/rGO/MgO AAECs showed lower steady-state temperature change (ΔT) than their control AAEC counterparts during the thermal conductivity test, with binary AAECs showing higher steady ΔT than their ternary counterparts.
  • The thermal conductivity and diffusivity of AAECs increased with the increase in MWCNTs/rGO (0–6%) and MgO (0–5%). Binary and reagent 2 AAECs showed higher thermal conductivity than their ternary and reagent 1 counterparts.
  • The thermal conductivity values of MgO AAECs were 0.44–10.1% higher than their control counterparts. Binary and reagent 2 control/MgO AAECs produced higher thermal conductivity than their ternary and reagent 1 counterparts.
  • The thermal conductivity values of MWCNT AAECs were 16.6–41.7%, and 23.2–49.4% higher for 0.3% and 0.6%MWCNT contents, respectively, with respect to control AAECs.
  • For rGO AAECs, increases of 3.6–27.7% and 2.1–28.6% in thermal conductivity were found for 0.3% and 0.6% rGO, respectively, compared to control counterparts. Thermal conductivity increases of 3.6–27.7% and 2.1–27.7% were observed for MgO-MWCNTs and MgO-rGO AAECs, respectively.
  • MWCNT AAECs showed the highest thermal conductivity (0.91–1.26 W/mK) followed by rGO (0.87–1.02 W/mK), MgO-MWCNTs (0.88–0.96 W/mK), MgO-rGO (0.84–0.88 W/mK, MgO (0.75–0.85 W/mK), and control (0.74–0.84 W/mK) AAECs. MWCNTs were found to be more effective in increasing the thermal properties of AAECs.
  • Thermal conductivity enhancement occurred due to the development of additional crystalline reaction products in the case of binary/reagent 2 (C-S-H), MgO (M-S-H and Ht), and rGO (zeolite) in addition to basic C-A-S-H/N-C-A-S-H or C-A-S-H/C-S-H gels. Thermal conductivity enhancement was also related to nanomaterial–matrix bonding, matrix connectivity, and conductivity of nanomaterials.
  • This study suggests that MWCNTs and rGO are effective in increasing the thermal conductivity of AAECs and can be used for producing MWCNT/rGO-incorporated conductive AAECs for construction applications.

Author Contributions

Conceptualization, K.M.A.H. and M.A.H.; methodology, K.M.A.H. and M.A.H.; formal analysis, K.M.A.H. and M.A.H.; investigation, K.M.A.H. and M.A.H.; resources, K.M.A.H.; writing—original draft, K.M.A.H. and M.A.H.; writing—review and editing, K.M.A.H.; supervision, K.M.A.H.; project administration, K.M.A.H.; funding acquisition, K.M.A.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Sciences and Engineering Research Council (NSERC) Canada, grant number RGPIN-2019-5613.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Natural Science and Engineering Research Council (NSERC), Canada. The authors also acknowledge the support provided by the technical staff of the Concrete and Advanced Concrete Material laboratories of Toronto Metropolitan University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Thermal conductivity test setup.
Figure 1. Thermal conductivity test setup.
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Figure 2. Change in temperature vs. time during testing for binary/ternary control and NWCNT AAECs with reagent type 1 or 2.
Figure 2. Change in temperature vs. time during testing for binary/ternary control and NWCNT AAECs with reagent type 1 or 2.
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Figure 3. Change in temperature vs. time during testing—(a) binary, (b) ternary control and MgO AAECs with reagent type 1 or 2.
Figure 3. Change in temperature vs. time during testing—(a) binary, (b) ternary control and MgO AAECs with reagent type 1 or 2.
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Figure 4. Change in temperature vs. time during testing for binary/ternary control and rGO AAECs with reagent type 1 or 2.
Figure 4. Change in temperature vs. time during testing for binary/ternary control and rGO AAECs with reagent type 1 or 2.
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Figure 5. Typical SEM-EDS analysis of MgO AAECs (T2M5).
Figure 5. Typical SEM-EDS analysis of MgO AAECs (T2M5).
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Figure 6. Effect of reagent type and MWCNT content on thermal conductivity/diffusivity of MWCNT AAECs. (a) Binary, (b) Ternary.
Figure 6. Effect of reagent type and MWCNT content on thermal conductivity/diffusivity of MWCNT AAECs. (a) Binary, (b) Ternary.
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Figure 7. Typical SEM-EDS analysis of MWCNT AAECs (B2C3).
Figure 7. Typical SEM-EDS analysis of MWCNT AAECs (B2C3).
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Figure 8. Effect of reagent type and rGO content on thermal conductivity/diffusivity of rGO AAECs. (a) Binary, (b) ternary.
Figure 8. Effect of reagent type and rGO content on thermal conductivity/diffusivity of rGO AAECs. (a) Binary, (b) ternary.
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Figure 9. Typical SEM-EDS analysis of rGO AAECs (B2R6).
Figure 9. Typical SEM-EDS analysis of rGO AAECs (B2R6).
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Figure 10. Effect of reagent type and 5% MgO + MWCNT contents on the thermal conductivity/diffusivity of MgO-MWCNT AAECs.
Figure 10. Effect of reagent type and 5% MgO + MWCNT contents on the thermal conductivity/diffusivity of MgO-MWCNT AAECs.
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Figure 11. Effect of reagent type and 5%MgO + rGO contents on thermal conductivity/diffusivity of MgO-rGO AAECs.
Figure 11. Effect of reagent type and 5%MgO + rGO contents on thermal conductivity/diffusivity of MgO-rGO AAECs.
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Figure 12. Typical SEM-EDS analysis of MgO-rGO AAECs (B2M5R6).
Figure 12. Typical SEM-EDS analysis of MgO-rGO AAECs (B2M5R6).
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Figure 13. Variation in thermal conductivity with dry density of all AAECs.
Figure 13. Variation in thermal conductivity with dry density of all AAECs.
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Table 1. Chemical composition and physical characteristics of materials.
Table 1. Chemical composition and physical characteristics of materials.
Chemical
Composition
(%)		FA-CFA-FGGBFSSilica
Sand		HRWRAMgO
SiO236.5355.6635.9799.70 2.02
Al2O318.2622.099.180.14 6.124
Fe2O35.664.260.500.016 0.94
CaO20.977.9738.610.01 2.40
MgO5.081.1610.990.01 92.26
K2O0.681.490.360.04 -
Na2O4.044.100.280.01 -
MnO0.030.030.250.00 -
TiO21.260.610.390.00 -
P2O50.960.430.010.00 -
L.O.I.2.181.050.740.00 1.14
pH 6.00
Density (g/cm3)2.612.022.872.651.063.58
Retained on 45 µ, %-18.00-3.00
Blaine fineness (m2/kg)315.00306.00489.30-
Table 2. Mix proportions of 32 AAEC mixes.
Table 2. Mix proportions of 32 AAEC mixes.
AAECs
Mix ID.		Total SCMs (Binder *)MgO/MWCNTs/rGOSCMsReagent
Component Ratio		R./BChemical Ratios
(SCMs + Reagent)
FA-CFA-FGGBFSSiO2/
Al2O3		Na2O/
SiO2		CaO/
SiO2		Na2O/
Al2O3
Four basic AAEC mixes (with 0% MgO/MWCNTs/rGO)
B1100.5500.451:2.50.092.620.090.840.23
B20.5500.452.5:10.122.560.141.020.35
T10.250.350.401:2.50.092.750.080.590.22
T20.250.350.402.5:10.122.690.120.730.32
Four AAEC mixes with 5% MgO
B1M510.050.5200.431:2.50.092.580.090.850.23
B2M50.5200.432.5:10.122.510.141.030.35
T1M50.240.330.381:2.50.092.970.050.540.14
T2M50.240.330.382.5:10.122.970.050.540.14
Eight AAEC mixes with 0.3% and 0.6% MWCNTs
B1C310.0030.5500.451:2.50.092.620.090.840.23
B2C30.5500.452.5:10.122.560.141.020.35
T1C30.250.350.401:2.50.092.750.080.590.22
T2C30.250.350.402.5:10.122.690.120.730.32
B1C610.0060.5500.451:2.50.092.620.090.840.23
B2C60.5500.452.5:10.122.560.141.020.35
T1C60.250.350.401:2.50.092.750.080.590.22
T2C60.250.350.402.5:10.122.690.120.730.32
Eight AAEC mixes with 0.3% and 0.6% rGO
B1R310.0030.5500.451:2.50.092.620.090.840.23
B2R30.5500.452.5:10.122.560.141.020.35
T1R30.250.350.401:2.50.092.750.080.590.22
T2R30.250.350.402.5:10.122.690.120.730.32
B1R610.0060.5500.451:2.50.092.620.090.840.23
B2R60.5500.452.5:10.122.560.141.020.35
T1R60.250.350.401:2.50.092.750.080.590.22
T2R60.250.350.402.5:10.122.690.120.730.32
Eight AAEC mixes with 5%MgO and MWCNTs or rGO (0.3% or 0.6%)
B2M5C310.05/0.0030.5200.432.5:10.122.560.141.020.35
T2M5C30.240.330.382.5:10.122.970.050.540.14
B2M5C60.5200.432.5:10.122.560.141.020.35
T2M5C60.240.330.382.5:10.122.970.050.540.14
B2M5R310.05/0.0060.5200.432.5:10.122.560.141.020.35
T2M5R30.240.330.382.5:10.122.970.050.540.14
B2M5R60.5200.432.5:10.122.560.141.020.35
T2M5R60.240.330.382.5:10.122.970.050.540.14
* All numbers are mass/wt ratios of binder; binder represents source materials (SMs): supplementary cementitious materials (SCMs) such as FA-C, FA-F, GGBFS, and activators; B1 and B2: binary AAECs with activator type 1 and type 2, respectively; T1 and T2: ternary AAECs with activator type 1 and type 2, respectively; M5, C3/C6, and R3/R6: AAECs with 5% MgO, 0.3%/0.6% MWCNTs, and 0.3%/0.6% rGO, respectively. All mixes contained 2% PVA fibre.
Table 3. Density, thermal conductivity, and thermal diffusivity of AAEGC mixes.
Table 3. Density, thermal conductivity, and thermal diffusivity of AAEGC mixes.
AAEC
Types		AAEC
Mix ID		28-Day Dry Density
(kg/m3)		28-Day Thermal
Conductivity (λ)
(W/mK)		28-Day Thermal
Diffusivity (α)
(×10−6 m2/s)		Change in Thermal Conductivity
as Compared to Control Specimens
(%)
ControlB120000.761.040
B220320.841.570
T121200.761.420
T220640.740.700
MgO 5%B1M520160.830.8710.10
B2M520720.850.540.44
T1M519840.791.653.18
T2M520560.750.701.68
MWCNT 0.3%B1C320080.912.1520.62
B2C320800.982.2816.62
T1C320160.971.8727.46
T2C320960.970.7431.60
MWCNT 0.6%B1C620441.072.2841.65
B2C620361.262.2449.44
T1C619600.941.6823.16
T2C621680.920.8124.25
rGO 0.3%B1R320320.871.2015.32
B2R321521.021.4420.92
T1R320600.881.7314.65
T2R320960.900.8421.50
rGO 0.6%B1R620600.901.1618.73
B2R620601.051.6924.60
T1R620680.881.8415.33
T2R621760.920.8924.42
Combination of MWCNTs, rGO, and MgOB2M5C320730.880.653.55
T2M5C320580.950.9526.80
B2M5C620740.880.744.49
T2M5C620590.960.8827.69
B2M5R320810.860.722.06
T2M5R320980.841.2411.75
B2M5R620820.870.783.70
T2M5R620990.881.1518.61
Mean value of three measurements (presented) with 1% to 2% deviation from the mean.
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MDPI and ACS Style

Hossain, M.A.; Hossain, K.M.A. Thermal Properties of MWCNT-rGO-MgO-Incorporated Alkali-Activated Engineered Composites. J. Compos. Sci. 2025, 9, 117. https://doi.org/10.3390/jcs9030117

AMA Style

Hossain MA, Hossain KMA. Thermal Properties of MWCNT-rGO-MgO-Incorporated Alkali-Activated Engineered Composites. Journal of Composites Science. 2025; 9(3):117. https://doi.org/10.3390/jcs9030117

Chicago/Turabian Style

Hossain, Mohammad A., and Khandaker M. A. Hossain. 2025. "Thermal Properties of MWCNT-rGO-MgO-Incorporated Alkali-Activated Engineered Composites" Journal of Composites Science 9, no. 3: 117. https://doi.org/10.3390/jcs9030117

APA Style

Hossain, M. A., & Hossain, K. M. A. (2025). Thermal Properties of MWCNT-rGO-MgO-Incorporated Alkali-Activated Engineered Composites. Journal of Composites Science, 9(3), 117. https://doi.org/10.3390/jcs9030117

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