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Article

Combined Effect of Multi-Walled Carbon Nanotubes and Silica Fume on Mechanical, Physicochemical, and Thermal Properties of Concrete Composites

by
Evalyn Joy C. Cea
1,2,
Christine Joy M. Omisol
1,3,
Kent Andrew Q. Tuble
1,4,
Ariel G. Bongabong
1,4,
Blessy Joy M. Aguinid
1,
Dan Michael A. Asequia
1,
Daisy Jane D. Erjeno
1,
Mary Ann N. Ahalajal
2,
Felrose P. Maravillas
2,5,
Applegen I. Cavero
2,
Gerard G. Dumancas
6,7,
Roberto M. Malaluan
1,8 and
Arnold A. Lubguban
1,8,*
1
Center for Sustainable Polymers, Mindanao State University—Iligan Institute of Technology, Iligan City 9200, Philippines
2
Civil Engineering Doctorate Program, Graduate School of Engineering, Mindanao State University—Iligan Institute of Technology, Iligan City 9200, Philippines
3
Chemical Engineering Department, College of Engineering, Mindanao State University—Main Campus, Marawi City 9700, Philippines
4
Materials Science and Engineering Program, Graduate School of Engineering, Mindanao State University—Iligan Institute of Technology, Iligan City 9200, Philippines
5
Department of Civil Engineering, Capitol University, Cagayan de Oro City 9000, Philippines
6
Department of Chemistry, New Science Building, 1601 E. Market Street, Greensboro, NC 27411, USA
7
Honors College, North Carolina Agricultural and Technical State University, 1601 East Market Street, Greensboro, NC 27411, USA
8
Department of Chemical Engineering & Technology, College of Engineering, Mindanao State University—Iligan Institute of Technology, Iligan City 9200, Philippines
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(7), 1087; https://doi.org/10.3390/buildings15071087
Submission received: 19 February 2025 / Revised: 18 March 2025 / Accepted: 24 March 2025 / Published: 27 March 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

This study investigates the combined effects of multi-walled carbon nanotubes (MWCNT) and silica fume on the workability, microstructure, thermal stability, and compressive strength of concrete. The addition of these admixtures slightly reduced slump performance due to their reinforcing effect. However, the concrete remained within the medium workability range. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) confirmed the successful integration of MWCNT and silica fume. The analysis revealed improved matrix densification and stronger interfacial bonding. Thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy provided further insights into the material’s thermal stability and chemical interactions. These microstructural and thermal improvements contributed to a 31.5% increase in compressive strength compared to the control mix. The findings highlight the synergistic role of MWCNT and silica fume in enhancing concrete performance.

1. Introduction

Concrete is a fundamental construction material due to its versatility and structural performance. Continuous advancements in mix design, placement techniques, and curing methods have improved its properties. However, optimizing multiple characteristics—such as workability, durability, shrinkage resistance, and strength—remains challenging due to trade-offs. For instance, reducing the water–cement ratio improves durability but negatively impacts workability [1]. Chemical admixtures can enhance flowability but may alter consistency and compromise strength [2]. Researchers have explored the use of nanoparticles and supplementary cementitious materials to address these challenges, but their effects on setting time, long-term stability, and sustainability require further investigation [3,4].
Among these materials, silica fume has gained attention as a supplementary cementitious material. It is an ultrafine byproduct collected from the production of silicon and ferrosilicon alloys in electric arc furnaces. Without proper utilization, large quantities of silica fume can become an environmental burden, requiring safe disposal. However, its pozzolanic properties make it highly effective in enhancing concrete performance. The fine particles of silica fume react with calcium hydroxide (Ca(OH)2) during cement hydration, forming additional calcium silicate hydrate (C–S–H) gel, which strengthens the microstructure and reduces permeability [5,6,7]. Studies have demonstrated that silica fume can significantly improve the compressive strength and durability of concrete. Research indicates that dosages typically range between 5% and 15% of cement replacement, with compressive strength improvements of 20% to 50%, depending on curing conditions and mix proportions [8,9,10]. Despite these benefits, concerns exist regarding the leaching of heavy metals and potential environmental risks associated with silica fume recycling [11]. Therefore, further investigation into its environmental implications and safe incorporation into concrete is necessary.
Another promising additive for improving concrete performance is multi-walled carbon nanotubes (MWCNTs). Due to their exceptional strength-to-weight ratio, MWCNTs have been explored as reinforcements in cementitious composites. However, their effectiveness depends on achieving uniform dispersion within the cement matrix. The aggregation of MWCNTs can lead to inconsistent mechanical properties and reduced performance. Various dispersion techniques have been studied, including sonication with surfactants and chemical functionalization, to improve their interaction with the cement matrix [12,13,14].
Several studies have examined the impact of MWCNT dosage on concrete performance. Research has shown that MWCNTs incorporated at dosages between 0.02% and 0.1% by weight of cement can improve compressive strength by 10% to 40%, depending on the dispersion methods and curing conditions [15,16,17]. Additionally, MWCNTs have been observed to refine the microstructure, reducing porosity and enhancing load transfer within the matrix. However, challenges such as increased viscosity and reduced workability have been reported, necessitating the use of dispersing agents or supplementary materials to mitigate these effects.
The combination of MWCNTs and silica fume presents an opportunity to address these challenges by leveraging their complementary effects. Silica fume aids in the dispersion of MWCNTs, ensuring more effective reinforcement, while MWCNTs enhance the mechanical performance of the composite. Additionally, silica fume contributes to long-term durability by reducing permeability and refining the microstructure.
This study aims to evaluate the synergistic effects of MWCNTs and silica fume on concrete’s workability, microstructure, thermal stability, and compressive strength. Workability changes will be assessed to determine how the admixtures influence flowability. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) will be used to analyze microstructural interactions, while Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) will provide insights into chemical bonding and thermal stability. Finally, compressive strength will be examined to quantify mechanical performance, with variations explained based on the previous analyses. By exploring these aspects, this research provides a comprehensive understanding of the effects of MWCNT–silica fume admixtures, contributing to the development of high-performance concrete composites.

2. Materials and Methods

2.1. Materials

In this study, Type 1 ordinary Portland cement (OPC) was sourced from Republic Cement Mindanao, Inc. (Iligan City, Philippines). The cement complies with ASTM C150 and has a Blaine fineness of 350 m2/kg, ensuring adequate reactivity. The chemical composition of the cement is detailed in Table 1. Silica fume was obtained from M.M. U-Chem Industries, Inc. (Davao City, Philippines) as a byproduct of the silicon and ferrosilicon industry. The particle size distribution of silica fume is significantly finer than OPC, with an average particle size of 1 µm, contributing to enhanced particle packing and improved compressive strength. The chemical composition of silica fume is presented in Table 1. Silica fume from ferrosilicon and silicon metal production may contain minor amounts of heavy metals, usually in concentrations below regulatory leaching limits [18,19]. Additionally, it has been suggested that well-sourced silica fume typically exhibits low leachability due to the encapsulation of contaminants within the amorphous silica matrix. The multi-walled carbon nanotubes (MWCNTs) used in this study were an exclusive product of US Research Nanomaterials, Inc. (Houston, TX, USA). Their chemical composition and physical specifications, including length, diameter, and purity, are detailed in Table 2 and Table 3. The polycarboxylate-based superplasticizer (PCE-SP) was procured in liquid form from M.M. U-Chem Industries, Inc. (Davao City, Philippines). PCE-SP enhances MWCNT dispersion and improves the rheology of the cementitious mixture. The properties of the superplasticizer are summarized in Table 4.
For the aggregate phase, washed sand was used as the fine aggregate, with a particle size range of 0.15–4.75 mm, a specific gravity of 2.63, and water absorption in a saturated surface-dry (SSD) condition of 3.91%. Gravel (maximum size: 10 mm) was utilized as the coarse aggregate. Lastly, potable water was used in the mix to prevent unwanted chemical and physical reactions.

2.2. Preparation of Samples

Eighteen (18) mixes were prepared and investigated in triplicates. Five varying loadings of MWCNT were added in the mixtures in the range of 0–0.05 wt.% of cement. Silica fume concentration was varied between 0–10 wt.% of cement in each mixture in increments of 5 wt.%. The amounts of water and PCE-SP were kept constant at 50 wt.% and 0.01 wt.% of cement, respectively. A control cement sample was also formulated without the addition of MWCNT, silica fume, and PCE-SP. Table 5 shows the different concrete mix designs.
The preparation of these concrete aggregate samples followed these major steps:
Dry Mixing: Fine and coarse aggregates of ordinary Portland cement were dry-mixed for 2 min using an electric handheld mixer to ensure uniform distribution of the dry components.
MWCNT Dispersion Preparation: MWCNT was first dispersed in a polycarboxylate-based superplasticizer (PCE-SP) solution to minimize agglomeration. The dispersion was sonicated using Branson Digital Sonifier 400 W at 20 kHz for 30 min to achieve stable, uniform dispersion [8].
Liquid Phase Incorporation: The MWCNT-PCE-SP dispersion was combined with water and stirred for 1 min to ensure homogeneity before being added to the dry mix.
Wet Mixing: The water and dispersed MWCNT solution were gradually poured into the dry mixture while continuously stirring at a controlled speed. Subsequently, the blend was mixed while concurrently incorporating silica fume powder to stabilize the dispersion of MWCNT and inhibit their reagglomeration in the fresh mixture. A total mixing time of 5 min was maintained for each specimen.
Final Mixing and Quality Check: The mixture was checked for uniformity, ensuring no visible clusters or dry pockets remained. The mixtures were then poured into 150 mm-diameter by 300 mm-height cylindrical molds and allowed to set for 24 h, after which they were demolded and allowed to undergo a 28-day wet curing process. After this, the cured specimens were allowed to dry at room temperature for another 24 h before being subjected to testing and characterization.
Curing Method: The samples were fully submerged in water for 28 days to maintain uniform hydration.
Temperature Control: The curing water was kept at a constant temperature of 23 ± 2 °C to prevent fluctuations that might affect cement hydration and strength development.
Humidity Conditions: Since water curing was used, humidity was inherently maintained at saturation levels to prevent premature drying and shrinkage.
Post-Curing Process: After 28 days, the specimens were removed from the water and left to air-dry at room temperature (approximately 25 °C and 60% relative humidity) for 24 h before testing.

2.3. Testing and Characterizations

A total of three cylindrical specimens (150 mm × 300 mm) were prepared and tested for each of the 18 different mix designs, following ASTM C39/C39M [23]. In total, 54 specimens were tested for compressive strength. The results were averaged, and standard deviations were calculated to assess variability and ensure statistical reliability. Freshly mixed concrete samples were used for the workability test following the steps detailed in ASTM C143/143M slump test [24]. Moreover, the microstructure and fracture surface of the concrete samples were characterized using HITACHI SU-1510 (Tokyo, Japan) scanning electron microscopy (SEM) at 5kV and 10K magnification to validate the presence of MWCNT. Then, an energy dispersive spectroscopy (EDS) analysis and mapping were performed using JEOL JSM-6510LA (Tokyo, Japan) with an average count rate of 3500 cps to determine and map the atomic composition of the concrete samples.
Further characterization was conducted for the unmodified and modified concrete samples for their thermal behavior and differences in chemical structures. The thermal stability test was done using a PerkinElmer TGA4000 thermogravimetric analyzer (Waltham, MA) with a temperature range of 30 °C to 900 °C at a heating rate of 10 °C/min in an inert environment with a nitrogen flow of 20 mL/min. Pertinent functional groups were examined using a Shimadzu IRTracer-100 Fourier transform infrared (FTIR) spectrometer with a QATR-10 single reflection diamond crystal attenuated total reflectance (ATR) accessory (Kyoto, Japan). The test was done between the wavenumber range 500–4000 cm−1 at a 4 cm−1 resolution.

3. Results and Discussion

3.1. Workability of the Concrete Mixes

The slump test results, illustrated in Figure 1, reveal distinct trends in workability based on the composition of the concrete mixes. For concrete samples without MWCNTs, workability increased with higher silica fume content. This can be attributed to the fine particle size of silica fume, which improves particle packing and acts as a filler, reducing internal friction in the mix. In contrast, a significant reduction in workability was observed as MWCNT content increased. This behavior is due to the high surface area and strong van der Waals forces of MWCNTs, which promote agglomeration in aqueous media. The formation of these micro-aggregates reduces the free water available for cement hydration, increasing viscosity and shear resistance within the mix [10,11].
Additionally, nearly all MWCNT-containing samples exhibited further workability loss as silica fume content increased, except at MWCNT loadings >0.03%, where the effect was less pronounced. This suggests that at very low MWCNT dosages, the influence of silica fume on flowability is more dominant. These findings confirm that rheological properties are highly influenced by the interaction of admixtures, particularly MWCNTs and silica fume.
While reduced workability can sometimes lead to increased porosity due to difficulties in compaction, in this study, slump values remained within the medium workability range (50–100 mm) [25]. This suggests that adequate consolidation could still be achieved during casting. However, to better understand the potential impact of workability loss on porosity and compressive strength, future research should include porosity analysis using methods such as Mercury Intrusion Porosimetry (MIP), water absorption, and scanning electron microscopy (SEM).
These findings align with previous research showing that higher MWCNT dosages increase mix viscosity, making proper dispersion and compaction critical for maintaining mechanical performance [10,14,26]. Despite the challenges posed by reduced workability, the concrete mixes in this study remained within a range suitable for construction applications.

3.2. Microscopy and Morphological Characterization

SEM-EDS characterization was also conducted to gain insight into the morphology and composition of the concrete samples. Figure 2 shows the SEM micro images of the (a) control concrete sample and the (b–d) concrete sample with 0.02% MWCNT and 10% silica fume. An apparent narrowing of concrete microcracks can be seen between Figure 2a,b. The presence of MWCNT throughout the concrete matrix is apparent, including those at the edges of the microcrack, where the admixture appeared to fill in the cracks, leading to a more tapered gap. The reduction in size of the microcracks within the matrix enhances the mechanical characteristics of concrete composites. This aligns with the observations regarding the compressive strength of the concrete samples containing MWCNT. Moreover, Figure 2c shows the entanglement of MWCNT in clumped silica fume, while Figure 2d illustrates the embedded MWCNT in the concrete matrix acting as additional nucleation sites for C–S–H bond formation. These images also depict the likelihood of MWCNT at higher concentrations to agglomerate and not be equally dispersed in the matrix, leading to heightened local stress that weakens the strength of the concrete [27]. These micrographs revealed that MWCNTs are embedded within the concrete matrix, often located at microcrack interfaces, suggesting that they contribute to crack bridging and improved mechanical strength. Additionally, MWCNTs were observed to be entangled with silica fume particles, forming a denser, more cohesive matrix that enhances concrete integrity. The modified samples exhibited fewer and narrower microcracks than the control, indicating improved structural stability due to the presence of MWCNT and silica fume.
A better understanding of the elemental distribution and configuration of the samples can be gained from the EDS mapping results shown in Figure 3. The elemental distribution of the control sample in Figure 3A shows a baseline map typical of standard concrete. On the other hand, the modified concrete sample in Figure 3B reveals the effect of MWCNT and silica fume modification of concrete. A significant increase in the presence of silicon is noted owing to the addition of 10% silica fume in the sample, indicating enhanced pozzolanic activity and additional calcium silicate hydrate (C–S–H) formation. Moreover, the EDS mapping suggests a more homogenous and denser microstructure in the modified sample compared with the control, which is indicative of enhanced binding at the microstructural level of the concrete matrix, thus leading to improvements in mechanical properties [28]. This further supports the findings in the compressive performance of the modified samples, wherein all modified samples exhibited higher compressive strength.
Figure 4A,B show the complete EDS spectra of the control and the concrete sample with 0.02% MWCNT and 10% silica fume, respectively, while the numerical elemental composition of carbon, oxygen, and silicon for both samples is listed in Table 6 to quantify further the effect of the modification done on the sample. It is apparent from Table 6 that the silicon content of 2CN-10SF was increased, coupled with an increase in oxygen attributed to the silica fume admixture. It also showed a more uniform spread of oxygen (O) and calcium (Ca) in modified samples, suggesting improved cement hydration and matrix densification. However, a slight decrease in carbon was detected, which might have been caused by the limited area selected for testing. Nevertheless, these findings verify the existence of well-dispersed MWCNT within the concrete matrix, and due to their highly minute dimensions, finely dispersed MWCNTs (with a diameter of less than 7 nm) have the capability to occupy the tiny pores between the grains of the cement. They collectively demonstrate that silica fume aids in dispersing MWCNT within the cementitious matrix, preventing agglomeration and promoting better interaction with hydration products. This reinforces the hypothesis that the combined addition of MWCNT and silica fume enhances the mechanical properties of concrete by improving its microstructure. Furthermore, their expansive active surfaces can serve as supplementary nucleation sites for the creation of C–S–H bonds.

3.3. Thermal Analysis

The effects on the thermal stability of the modification of concrete with MWCNT and silica fume are probed in this section in comparison with the unmodified concrete. Figure 5a,b illustrate the TG and DTG thermograms of the control and 2CN-10SF, respectively.
The TG thermograms of the samples depict six (6) weight loss stages expressed as peaks on their corresponding DTG curve. The first four DTG peaks in Figure 5a at 53 °C, 85 °C, 144 °C, and 460 °C illustrate the weight losses due to the vaporization of water and degradation of hydration products, including Ca(OH)2 [29]. The same set of peaks is evident on the thermogram of the concrete sample modified with 0.02% MWCNT and 10% silica fume (2CN-10SF) at roughly the same temperatures in Figure 5b. The successive peaks at 736 °C and 782 °C for the control sample in Figure 5a represent the degradation of carbonates based on calcite (CaCO3) and limestone, respectively [28]. The main difference between calcite and limestone is that calcite is a mineral with a chemical formula of CaCO3, while limestone is a rock composed mainly of calcite but also contains other minerals such as silica. In contrast, the degradation peaks of carbonate in the modified sample 2CN-10SF shown in Figure 5b depict significantly different values compared with the control. In this case, the recorded degradation peak for carbonates based on calcite is at 705 °C, 31° lower than the control. On the other hand, the degradation peak of carbonates based on limestone was noted at 802 °C, 20° higher than the control. The increased thermal stability of limestone may be attributed to the added silica fume in the modified concrete sample. However, the decrease in the degradation temperature of calcite may be owed to an accelerated degradation process because of the heat barrier effect of the accumulated heat in the sample [30]. It is also worth noting that the modified concrete sample 2CN-10SF recorded only a total mass loss of 6.05% after being subjected to heating in an inert environment up to 900 °C compared with the 13.81% mass loss of the control sample. This suggests that the reinforcement provided by the MWCNT and silica fume admixtures not only improved the mechanical properties of the modified concrete sample but also its thermal performance.

3.4. Chemical Structure Evaluation

Figure 6 illustrates the comparison between the chemical structures present in the unmodified concrete sample (control) and the modified concrete sample using 0.02% MWCNT and 10% silica fume (2CN-20SF). It can be observed that pertinent vibrational peaks are present in both concrete samples but with a noticeable difference in peak intensity. The broad characteristic bands at 3387 cm−1 and 3379 cm−1 for the control and 2CN-10SF samples, respectively, correspond to the stretching vibrations of O–H groups in absorbed water. The presence of this peak may be an indication of layer hydrogen bonding in the samples [31], which influences the mechanical properties of concrete. Thus, the greater intensity of the O–H band in 2CN-10SF, suggesting a higher likelihood of H-bonding, coincides with the improvement in the compressive performance of the sample. This agrees with the results of a related study, which revealed a linear correlation between compressive strength and the present H-bonds in mortar pastes [32].
Moreover, the C=O and C–O bands of carbonate (CO32−) at 1744 cm−1 and ca. 1400 cm−1 in both unmodified and modified samples appear to show the same trend as the O–H band. These peaks in the spectrum of 2CN-10SF have markedly greater intensity, signifying the presence of more carbonates. The same can be observed for the peaks of 2CN-10SF at 961 cm−1, 872 cm−1, 660 cm−1, and 510 cm−1, which indicate the presence of silica (Si–O) and silicane (Si–H), wherein the corresponding peaks on the spectrum of the control have noticeably lower intensity. These observations are in accordance with the improved thermal stability of 2CN-10SF due to the presence of more calcite and silica that constitute limestone.
These chemical characterization results, in conjunction with the thermal analyses of the samples, confirm that the addition of MWCNT and silica fume enhances the thermal stability, hydration efficiency, and structural integrity of concrete. The formation of additional C–S–H and denser microstructures reduces the likelihood of degradation under high temperatures and extends the lifespan of the material in harsh environmental conditions.

3.5. Compressive Strength of the Concrete Mixes

The 28-day compressive strength test results of the concrete samples are shown in Figure 7. It can be observed that the addition of MWCNT has varying degrees of effect at different levels of silica fume. There is a steady increase in the compressive strength of concrete with the increasing addition of MWCNT until 0.02 wt.%. After this point, the compressive strength of the concrete samples consistently decreased as the concentration of MWCNT was further increased. This finding aligns with results from similar studies, which suggest that lower concentrations of MWCNT have improved the compressive performance of concrete, whereas higher doses have diminished the compressive strength [33,34]. The optimal point of 0.02 wt.% MWCNT in this study also coincides with the results of Morsy et al. [13], wherein the same MWCNT loading was concluded to be the best additive amount to obtain the relatively highest compressive strength of the concrete specimens.
The decline in compressive strength at increased concentrations of MWCNT may be due to the agglomeration of MWCNT within the hardened structure, leading to a rise in local stress, which weakens the strength of the concrete matrix [27]. Moreover, the MWCNT may not be adequately wetted during the mixing process, consequently leading to withdrawal and the subsequent formation of cracks [14].
Despite the discernible decrease in the compressive strength of the samples at higher MWCNT loading, the opposite is evident with silica fume loading. Figure 1 depicts a steady increase in compressive strength as the silica fume loading is increased across all MWCNT loading. This positive influence of silica fume loading on the compressive strength of the concrete samples can be attributed to an improvement in the strength of the cement with the successful anchoring of MWCNT due to additional C–S–H bonds from the reaction between CaO and silica fume from the hydrated cement [12,13,15,27,35,36].
The combined effect of MWCNT and silica fume produced a concrete sample with a maximum compressive strength of 42.2 MPa at 0.02% MWCNT and 10% silica fume loading. This result is 31.5% higher than the control sample, with a compressive strength of only 32.1 MPa.

3.6. Environmental Impacts

The incorporation of MWCNT and silica fume into concrete has potential environmental implications that require further investigation. The use of silica fume in concrete helps reduce landfill waste and minimizes the environmental footprint of industrial processes. Also, incorporating silica fume reduces cement consumption, which lowers CO2 emissions associated with cement production. While silica fume is an industrial byproduct that can reduce cement demand and mitigate CO2 emissions, its composition may include trace heavy metals that could leach under certain environmental conditions. Similarly, MWCNTs, due to their nanostructured nature and chemical stability, have been reported to persist in aquatic and soil environments, raising concerns about long-term accumulation and ecotoxicological effects. Previous studies have suggested that MWCNTs may influence the mobility of contaminants, including heavy metals, and alter microbial activity in soils [18,19]. However, the extent of these interactions when MWCNTs are incorporated into concrete remains largely unexplored. Future research should include standardized leaching assessments, such as the Toxicity Characteristic Leaching Procedure (TCLP) and European Norm EN 12457, to evaluate the environmental compatibility of MWCNT-SF concrete compared to conventional cementitious materials. Nevertheless, the enhanced mechanical properties and thermal stability of concrete containing MWCNT and silica fume contribute to longer service life, reducing the need for frequent repairs and lowering resource consumption over time. This could result in a decrease in raw material demand and reduced environmental impact from replacement cycles.

4. Conclusions

This study examined the mechanical, physicochemical, morphological, and thermal properties of concrete modified with multi-walled carbon nanotubes (MWCNTs) and silica fume, highlighting their synergistic effects on performance. The results demonstrated that MWCNTs at low dosages (≤0.02%) enhanced compressive strength by bridging microcracks, while silica fume improved strength consistently due to its pozzolanic reaction and pore-filling capability. However, increased silica fume content reduced workability, reinforcing the need for an optimal balance between admixture dosage and fresh-state properties. The highest compressive strength gain (31.5% increase) was observed at a medium workability level (50–100 mm slump), with an optimal admixture combination of 0.02% MWCNT and 10% silica fume, resulting in a compressive strength of 42.2 MPa. Beyond this threshold, excessive MWCNT loading led to agglomeration, reducing workability and limiting the effective dispersion of silica fume, which in turn diminished its ability to refine the microstructure. Thermogravimetric analysis (TGA) revealed that the modified concrete sample had higher thermal stability, with a lower overall mass loss (6.05%) compared to the control (13.81%), suggesting improved durability at elevated temperatures. The shifts in carbonate decomposition temperatures indicated changes in cement matrix composition, with silica fume enhancing limestone stability while also accelerating calcite decomposition. The Fouriertransform infrared spectroscopy (FTIR) results confirmed an increase in silicate polymerization, reinforcing the formation of calcium-silicate-hydrate (C–S–H) phases, which contributed to both strength development and long-term durability.
MWCNTs remain relatively expensive compared to conventional concrete additives. However, their cost has been decreasing due to advancements in large-scale production and improved synthesis methods. The optimal dosage (0.02% by weight of cement) identified in this study suggests that only a small amount is required to achieve significant strength enhancements, potentially mitigating cost concerns. Silica fume is a byproduct of the silicon and ferrosilicon industries, making it more affordable than MWCNTs. Its use in concrete not only improves mechanical properties but also reduces landfill waste and minimizes the environmental footprint of industrial processes. Incorporating silica fume also reduces cement consumption, which lowers CO2 emissions associated with cement production. The initial cost increase from adding MWCNT and silica fume may be offset by long-term benefits, including reduced maintenance costs, enhanced durability, and extended service life of concrete structures. Further studies could assess cost-benefit analyses and explore scaling strategies to enhance affordability in practical applications.
This study provides valuable insights into the interplay between MWCNTs and silica fume, demonstrating their potential for enhancing strength, durability, and thermal performance. However, the reduction in workability and its possible impact on porosity must be considered when optimizing mix designs for structural applications. Future research should focus on dispersion techniques, porosity analysis, long-term durability, and environmental sustainability to fully realize the benefits of these advanced concrete formulations.

5. Recommendations

Despite the promising results presented in this study, several limitations should be acknowledged. MWCNT dispersion remains a key challenge due to their tendency to agglomerate, even with the use of a polycarboxylate-based superplasticizer (PCE-SP) and probe ultrasonication. Future research should explore functionalization techniques or alternative dispersants to improve their uniformity in the concrete matrix. Additionally, measurement accuracy and batch variability could introduce slight deviations in compressive strength and slump test results. Employing advanced rheometry techniques would enhance the precision of fresh-state property assessments. Moreover, the observed increase in the compressive strength of the best sample despite an observed decrease in workability should be investigated further. Specifically, the effect of the addition of both MWCNT and silica fume to the porosity of the concrete sample methods such as Mercury Intrusion Porosimetry (MIP) and water absorption.

Author Contributions

Conceptualization, E.J.C.C. and C.J.M.O.; methodology, E.J.C.C. and A.I.C.; validation, B.J.M.A., D.M.A.A. and D.J.D.E.; formal analysis, E.J.C.C., M.A.N.A., K.A.Q.T. and C.J.M.O.; investigation, E.J.C.C., M.A.N.A., F.P.M. and A.I.C.; resources, R.M.M. and A.A.L.; data curation, K.A.Q.T. and C.J.M.O.; writing—original draft preparation, E.J.C.C., F.P.M., A.I.C. and C.J.M.O.; writing—review and editing, C.J.M.O., K.A.Q.T., A.G.B., B.J.M.A., D.J.D.E. and G.G.D.; visualization, K.A.Q.T., C.J.M.O., D.M.A.A. and B.J.M.A.; supervision, R.M.M. and A.A.L.; project administration, R.M.M. and A.A.L.; funding acquisition, G.G.D. and A.A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Department of Science and Technology (DOST) in the Philippines, specifically the Engineering Research Development for Technology (ERDT) program.

Data Availability Statement

Data will be made available upon request.

Acknowledgments

The author would like to acknowledge the Department of Science and Technology (DOST) in the Philippines, specifically the Engineering Research Development for Technology (ERDT) program for the funding and support that they provided. Also, this study was made possible through the state-of-the-art laboratory facility of the Center for Sustainable Polymers, formerly the Bio-Products Research Laboratory, MSU-Iligan Institute of Technology, Philippines.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Slump test of the concrete samples with varying multi-walled carbon nanotubes (MWCNT) and silica fume loading.
Figure 1. Slump test of the concrete samples with varying multi-walled carbon nanotubes (MWCNT) and silica fume loading.
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Figure 2. SEM images of the (a) control sample and (b–d) concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading showing (b) MWCNT bridging the concrete microcracks, (c) MWCNT entangled in clumped silica fume, and (d) MWCNT embedded in the concrete matrix.
Figure 2. SEM images of the (a) control sample and (b–d) concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading showing (b) MWCNT bridging the concrete microcracks, (c) MWCNT entangled in clumped silica fume, and (d) MWCNT embedded in the concrete matrix.
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Figure 3. EDS mapping of (A) unmodified control sample and (B) modified concrete sample using 0.02% MWCNT and 10% silica fume.
Figure 3. EDS mapping of (A) unmodified control sample and (B) modified concrete sample using 0.02% MWCNT and 10% silica fume.
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Figure 4. EDS spectra of (A) the unmodified concrete sample (control) and (B) the concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading.
Figure 4. EDS spectra of (A) the unmodified concrete sample (control) and (B) the concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading.
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Figure 5. TG and DTG thermograms of the (a) unmodified concrete (control) and (b) modified concrete using 0.02% MWCNT and 10% silica fume admixtures (2CN-10SF).
Figure 5. TG and DTG thermograms of the (a) unmodified concrete (control) and (b) modified concrete using 0.02% MWCNT and 10% silica fume admixtures (2CN-10SF).
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Figure 6. Fourier transform infrared (FTIR) spectra of unmodified concrete (control) and concrete sample modified with 0.02% MWCNT and 10% silica fume admixtures (2CN-10SF).
Figure 6. Fourier transform infrared (FTIR) spectra of unmodified concrete (control) and concrete sample modified with 0.02% MWCNT and 10% silica fume admixtures (2CN-10SF).
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Figure 7. Compressive strength of the concrete samples with varying multi-walled carbon nanotubes (MWCNT) and silica fume loading.
Figure 7. Compressive strength of the concrete samples with varying multi-walled carbon nanotubes (MWCNT) and silica fume loading.
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Table 1. Chemical composition of Type 1 ordinary Portland cement (OPC) and silica fume [20].
Table 1. Chemical composition of Type 1 ordinary Portland cement (OPC) and silica fume [20].
ConstituentOPC (Mass%)Silica Fume (Mass%)
SiO221.993.0
Al2O34.90.4
Fe2O33.70.5
CaO62.30.7
MgO2.00.5
K2O0.50.8
Na2O0.3–
SO32.2–
Loss on ignition (LOI)1.781.5
Sum99.597.4
Table 2. Chemical composition of the multi-walled carbon nanotubes (MWCNT) [21].
Table 2. Chemical composition of the multi-walled carbon nanotubes (MWCNT) [21].
ElementComposition (%)
C97.58
Al0.19
Cl0.58
Co1.01
S0.24
Mn0.33
O0.13
Table 3. Specifications of the multi-walled carbon nanotubes (MWCNT) [21].
Table 3. Specifications of the multi-walled carbon nanotubes (MWCNT) [21].
PropertySpecifications
Purity>95% (carbon nanotubes) >97% (carbon content)
Outside diameter<7 nm
Inside diameter2–5 nm
Length10–30 µm
SSA>500 m2/g
Colorblack
Ash<1.5 wt.%
Electrical conductivity>100 s/cm
Tap density0.27 g/cm3
True density~2.1 g/cm3
Table 4. Specifications of polycarboxylate-based superplasticizer (PCE-SP) [22].
Table 4. Specifications of polycarboxylate-based superplasticizer (PCE-SP) [22].
PropertySpecifications
Visual appearancePale brown viscous liquid
Density (23 °C)1.12 ± 0.2 kg/L
pH (23 °C)9.0 ± 0.5
Solid content (%)No crystallization
Stability (0 °C, 24 h)≤0.10
Na2SO4 (%) Na2O + 0.658 K2O (%)≤4.0
Table 5. Mix proportions of concrete samples in terms of mass ratios.
Table 5. Mix proportions of concrete samples in terms of mass ratios.
MixtureSampleCementSandGravelWaterSuperplasticizerMWCNTSilica Fume
1CONTROL1.02.41.50.50.0000.00000.00
21CN-0SF1.02.41.50.50.0010.00010.00
32CN-0SF1.02.41.50.50.0010.00020.00
43CN-0SF1.02.41.50.50.0010.00030.00
54CN-0SF1.02.41.50.50.0010.00040.00
65CN-0SF1.02.41.50.50.0010.00050.00
70CN-5SF1.02.41.50.50.0010.00000.05
81CN-5SF1.02.41.50.50.0010.00010.05
92CN-5SF1.02.41.50.50.0010.00020.05
103CN-5SF1.02.41.50.50.0010.00030.05
114CN-5SF1.02.41.50.50.0010.00040.05
125CN-5SF1.02.41.50.50.0010.00050.05
130CN-10SF1.02.41.50.50.0010.00000.10
141CN-10SF1.02.41.50.50.0010.00010.10
152CN-10SF1.02.41.50.50.0010.00020.10
163CN-10SF1.02.41.50.50.0010.00030.10
174CN-10SF1.02.41.50.50.0010.00040.10
185CN-10SF1.02.41.50.50.0010.00050.10
Table 6. Elemental composition of the unmodified concrete sample (control) and the concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading (2CN-10SF).
Table 6. Elemental composition of the unmodified concrete sample (control) and the concrete sample with 0.02% multi-walled carbon nanotubes (MWCNT) and 10% silica fume loading (2CN-10SF).
ElementLineCONTROL2CN-10SF
Weight (%)Atomic (%)Weight (%)Atomic (%)
C K7.8013.577.0811.48
O K45.3859.3048.6559.23
Si K8.546.3615.0410.43
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Cea, E.J.C.; Omisol, C.J.M.; Tuble, K.A.Q.; Bongabong, A.G.; Aguinid, B.J.M.; Asequia, D.M.A.; Erjeno, D.J.D.; Ahalajal, M.A.N.; Maravillas, F.P.; Cavero, A.I.; et al. Combined Effect of Multi-Walled Carbon Nanotubes and Silica Fume on Mechanical, Physicochemical, and Thermal Properties of Concrete Composites. Buildings 2025, 15, 1087. https://doi.org/10.3390/buildings15071087

AMA Style

Cea EJC, Omisol CJM, Tuble KAQ, Bongabong AG, Aguinid BJM, Asequia DMA, Erjeno DJD, Ahalajal MAN, Maravillas FP, Cavero AI, et al. Combined Effect of Multi-Walled Carbon Nanotubes and Silica Fume on Mechanical, Physicochemical, and Thermal Properties of Concrete Composites. Buildings. 2025; 15(7):1087. https://doi.org/10.3390/buildings15071087

Chicago/Turabian Style

Cea, Evalyn Joy C., Christine Joy M. Omisol, Kent Andrew Q. Tuble, Ariel G. Bongabong, Blessy Joy M. Aguinid, Dan Michael A. Asequia, Daisy Jane D. Erjeno, Mary Ann N. Ahalajal, Felrose P. Maravillas, Applegen I. Cavero, and et al. 2025. "Combined Effect of Multi-Walled Carbon Nanotubes and Silica Fume on Mechanical, Physicochemical, and Thermal Properties of Concrete Composites" Buildings 15, no. 7: 1087. https://doi.org/10.3390/buildings15071087

APA Style

Cea, E. J. C., Omisol, C. J. M., Tuble, K. A. Q., Bongabong, A. G., Aguinid, B. J. M., Asequia, D. M. A., Erjeno, D. J. D., Ahalajal, M. A. N., Maravillas, F. P., Cavero, A. I., Dumancas, G. G., Malaluan, R. M., & Lubguban, A. A. (2025). Combined Effect of Multi-Walled Carbon Nanotubes and Silica Fume on Mechanical, Physicochemical, and Thermal Properties of Concrete Composites. Buildings, 15(7), 1087. https://doi.org/10.3390/buildings15071087

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