Mechanical Properties and Performance of CNT–Reinforced Mortars (CEM II/B–L and CEM I) for Crack Bridging and Protective Coating Applications

Abstract

Cement–based mortars are essential in both modern construction and heritage conservation, where balancing mechanical strength with material compatibility is crucial. Mortars containing ––binders with low hydraulic activity, such as CEM II/B–L, often exhibit increased porosity and diminished strength, limiting their suitability for structurally demanding applications. This study investigates the potential of multiwalled carbon nanotubes (MWCNTs) to enhance the mechanical and microstructural properties of mortars formulated with both CEM II/B–L and CEM I binders. The influence of CNT incorporation was systematically assessed through compressive and flexural strength tests, vacuum saturation tests, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential thermal analysis (DTA). The results demonstrate significant mechanical improvements attributable to nanoscale mechanisms including crack bridging, pore–filling, and stress redistribution. Microstructural characterization revealed a refined pore network, increased densification of the matrix, and morphological modifications of hydration products. These findings underscore the effectiveness of CNT reinforcement in cementitious matrices and highlight the critical role of binder composition in influencing these effects. This work advances the development of high–performance mortar systems, optimized for enhanced structural integrity and long–term durability.

1. Introduction

Cement–based mortars remain fundamental to both modern construction and repair practices due to their proven structural reliability, workability, and versatility across a wide range of applications [1,2,3]. From load–bearing masonry to the delicate restoration of heritage substrates, these materials must satisfy demanding performance criteria, balancing mechanical strength, durability, and compatibility with existing structures [4]. This balance is especially vital in architectural conservation, where mortars are required not only to provide structural support but also to safeguard the integrity and authenticity of historical fabric and original construction materials [5]. Accordingly, the selection of binder type, mixture design, and admixtures plays a decisive role in determining the long–term success and compatibility of such interventions.

Traditional Portland cement (CEM I) mortars are widely utilized for general construction, owing to their rapid strength development and robust mechanical performance [6]. Nonetheless, their application in heritage conservation is often constrained by inherent drawbacks such as excessive stiffness, cracking, and deformation due to significant shrinkage [7]. These characteristics can promote microcracking within the mortar. Nevertheless, degradation of the original substrate material typically occurs only in cases of significant mechanical incompatibility or inadequate bonding between the mortar and the substrate. On the other hand, blended cements such as CEM II/B–L, which incorporate ground limestone as a partial clinker replacement, have gained traction in conservation practices due to their lower heat of hydration, reduced chemical aggressiveness, and improved compatibility with historic carbonate substrates [8,9,10]. However, these benefits may be offset in some cases by increased porosity, reduced compressive strength, and elevated permeability, factors that can potentially limit their suitability for load–bearing applications and affect long–term durability under adverse environmental conditions [11,12,13].

To address the limitations of conventional cementitious systems, recent research has increasingly focused on micro– and nanomodifications that can enhance mortar performance without undermining material compatibility or sustainability [14,15,16]. Among these innovations, the incorporation of carbon–based nanomaterials—particularly MWCNTs—has emerged as a promising enhancement technique [17]. MWCNTs possess extraordinary intrinsic properties, including ultrahigh tensile strength, remarkable stiffness, thermal stability, and an exceptionally high aspect ratio [18,19,20]. These characteristics allow for efficient interaction with cementitious matrices at multiple scales, potentially improving stress transfer, microcrack resistance, and the overall integrity of the hardened system [21].

Numerous investigations have demonstrated that MWCNTs can significantly enhance both the mechanical performance and durability of cement–based composites [22,23]. Two primary reinforcement mechanisms are well–documented mechanisms: (i) crack bridging, whereby CNTs span across microcracks and redistribute localized stresses, thereby delaying crack propagation and improving fracture resistance [24]; and (ii) pore structure refinement, through which CNTs contribute to matrix densification and reduce pore connectivity, ultimately lowering permeability [25]. The crack bridging mechanism has been substantiated through scanning and transmission electron microscopy (SEM and TEM), which have shown CNTs embedded within hydration products and effectively firmly anchored within the matrix, where they resist pull–out forces. Notably, even at low dosages (<0.1% by cement weight), MWCNTs have been associated with considerable increases in flexural strength depending on CNT dispersion quality, interactions with chemical admixtures, and specific mix design parameters [26,27,28,29].

Equally critical is the role of carbon nanotubes (CNTs) in influencing the evolution of pore structure within cementitious matrices. Owing to their nanoscale dimensions and high specific surface area, CNTs are capable of penetrating capillary voids and disrupting their continuity, thereby promoting the development of a more tortuous and refined pore network [30,31,32].This effect has been consistently verified through techniques such as mercury intrusion porosimetry (MIP) and vacuum saturation, which reveal a notable reduction in the volume of capillary pores, particularly those within the 50–200 nm range, and a corresponding decrease in overall permeability. These microstructural modifications significantly enhance resistance to deterioration mechanisms, including carbonation, chloride penetration, and freeze–thaw cycling, all of which are critical concerns for both exposed structural components and repair mortars in heritage and modern construction applications.

Additionally, CNTs have been observed to act as nucleation sites for hydration products, influencing both the kinetics and morphology of phases such as calcium silicate hydrate (C–S–H), ettringite, and portlandite (C–H) [33,34]. This leads to a denser microstructure with more cohesive interparticle linkages, which in turn enhances load transfer capacity. The effectiveness of these interactions, however, is highly sensitive to CNT dispersion quality, surfactant chemistry, and cement type [35,36]. In blended systems like CEM II/B–L, the presence of limestone filler affects the pH, ionic concentration, and hydration kinetics, all of which may affect CNT dispersion and their ability to integrate within the matrix [37,38].

Despite the considerable potential of CNTs in enhancing cementitious composites, most research efforts have been concentrated on ordinary Portland cement (CEM I) mortars [39]. The combined benefits of CNT reinforcement and the environmental or compatibility advantages offered by blended cements remain underexplored. Blended binders, while more sustainable and conservation–friendly, require careful optimization due to slower hydration rates and increased porosity, factors that can influence the degree and mechanisms of CNT–induced reinforcement.

In recent years, the interest in carbon nanotube (CNT)–reinforced composites has grown substantially, with numerous experimental and analytical studies examining their mechanical behavior in different contexts. Novel approaches have incorporated sustainable resources, such as biochar derived from spent coffee grounds, into cementitious matrices, offering both performance benefits and environmental advantages [40]. In parallel, the integration of CNTs into advanced structural elements—such as sandwich beams with pultruded GFRP cores—has demonstrated improved flexural performance depending on the CNT placement and concentration [41]. Furthermore, the static behavior of nanocomposite plates and shells reinforced with agglomerated CNTs has been analytically modeled, emphasizing the influence of CNT dispersion and volume fraction gradients on stiffness and load–bearing capacity [42]. At the structural level, the dynamic response and snap–through instability of CNT–reinforced shallow arches resting on nonlinear elastic foundations have also been rigorously investigated, revealing the importance of CNT gradation and thermomechanical effects on the stability domain [43]. These diverse investigations highlight the broadening scope of CNT applications in composites, while also underscoring the need for targeted studies in traditional binder–based matrices such as lime mortars.

This study addresses a critical research gap by systematically investigating the effects of MWCNT incorporation in mortars formulated with both CEM I and CEM II/B–L binders. The experimental program involves mechanical testing—compressive and flexural strength—alongside porosity evaluation using mercury intrusion porosimetry (MIP) and vacuum saturation techniques. Microstructural analysis is also conducted using scanning electron microscopy (SEM) and differential thermal analysis.

By correlating mechanical performance with microstructural characteristics, the study aims to clarify how CNTs interact with different cement types. The findings are expected to inform the development of advanced mortar formulations that achieve a balance between structural performance, long–term durability, and material compatibility. These properties are essential not only for modern infrastructure but also for heritage restoration, where compatibility and durability are especially critical.

2. Materials and Preparation of Specimens

To assess the effects of multiwalled carbon nanotube (MWCNT) incorporation and binder type on the performance of cement–based mortars, four distinct mixtures were prepared: two reference mixtures using different cements (CEM II/B–L and CEM I) and their corresponding MWCNT–modified counterparts, with 0.10% CNTs by weight of cement each. The mixtures were designated as follows: (a) AREF (CEM II/B–L − reference), (b) ACNT (CEM II/B–L + CNTs), (c) HREF (CEM I − reference), and (d) HCNT (CEM I + CNTs). The MWCNTs, supplied as a dry powder, possessed diameters ranging from 2 to 10 nm, lengths between 10 and 30 μm, and a specific surface area exceeding 500 m²/g. Although CNTs with ~10 nm diameters possess a higher specific surface area, their dispersion potential and interfacial adhesion are influenced not only by size but also by factors such as surface chemistry, functionalization methods, and dispersion techniques. Such properties facilitate microstructural densification and nanoscale reinforcement within the cementitious matrix. The compositions of raw materials used in this study are detailed in

Table 1. Composition of cement–based mortar mixtures with and without CNTs.

	AREF	ACNT	HREF	HCNT
Aggregates
Calcareous sand (0–4 mm)	5.25	5.25	5.25	5.25
Binders
CEM II/B/L	1.75	1.75	0	0
CEM I	0	0	1.75	1.75
Nanomaterials
Carbon nanotubes (CNTs)	0	0.00175	0	0.00175
Water
Volume (L)	0.945	1.138	1.085	1.208
Water/cement ratio	0.54	0.65	0.62	0.69
Workability (mm)	167.31	161.12	162.5	160.87

.

Both binders complied with the EN 197–1 [44] and were employed at identical mass fractions across all mixtures to ensure comparability. CEM II/B–L 32.5N, a limestone–blended Portland cement characterized by moderate early strength development, and CEM I 32.5N, a conventional Portland cement with higher clinker content and accelerated hydration kinetics, were selected to represent contrasting hydration behaviors and compositional profiles. The fine aggregate consisted of well–graded, crushed calcareous sand, chosen for its chemical compatibility and granulometric suitability. A fixed cement–to–sand mass ratio of 1:3 was adopted to balance workability and mechanical performance while promoting optimal packing density and minimizing intergranular porosity. All dry constituents were oven–dried prior to mixing to eliminate moisture–induced variability. The water–to–binder (w/b) ratio was adjusted to account for binder–specific water demand and the rheological effects of CNT incorporation, resulting in final values ranging from 0.54 to 0.69.

The mixing procedure involved initial dry homogenization of cement and sand for one minute, followed by gradual addition of the mixing water (with or without CNTs) and continued mixing for five minutes. Fresh mortars were cast into standardized steel molds and compacted using a vibrating table to reduce air entrapment and enhance density uniformity. Workability was assessed via flow table testing. Although the presence of MWCNTs marginally reduced flowability—attributed to increased water adsorption and internal friction—the recorded flow diameters (160–168 mm) remained within acceptable limits for practical application. These results confirm that the adopted dispersion and mixing protocols effectively integrated the MWCNTs without compromising fresh–state performance.

MWCNTs were dispersed in 750 mL of tap water using a sonication bath (Elma P300H Ultrasonic Cleaner, Elma Schmidbauer GmbH, Singen, Germany) operating at 37 kHz for 30 min to prevent agglomeration and achieve uniform dispersion. In addition, a suitable surfactant, which contained carboxyl functional groups, was added to the solution to aid dispersion. Carboxyl–containing surfactants enhance CNT dispersion in aqueous media primarily through hydrogen bonding and electrostatic stabilization. While π–π stacking interactions are significant for aromatic surfactants, they are less relevant for carboxyl functional groups. The stabilization mechanisms thus involve distinct chemical interactions that should be clearly differentiated. The resulting adsorption forms a stabilizing shell around the nanotubes, increasing electrostatic repulsion among CNTs and preventing aggregation caused by van der Waals forces. Consequently, a stable colloidal suspension is achieved, promoting improved dispersion quality prior to incorporation into the binder matrix. However, it is recognized that the chemical environment in CEM II/B–L systems may compromise long–term dispersion stability due to carbonate–related ionic interactions that diminish the effectiveness of surfactant stabilization.

Indeed, previous studies have demonstrated that carboxyl–functionalized CNTs require optimized ultrasonication energy to prevent agglomeration. For instance, Zou et al. [45] reported that dispersion efficiency plateaus at approximately 150 J/mL in aqueous suspensions of carboxyl–functionalized CNTs. Additionally, other researchers [46,47] have shown that the combination of sonication and surfactant use consistently improves the microstructure of cementitious matrices. These findings support our interpretation that, within the high ionic strength environment of CEM II/B–L, divalent ions may interfere with surfactant adsorption, thereby contributing to the late–stage pore coarsening observed in our study.

3. Methods and Characterization Techniques

All mortar specimens (Figure 1) were constructed according to a standardized mixing and casting procedure to ensure reproducibility and consistency across the experimental matrix. The mixtures were prepared using a planetary mixer with a 30 L capacity, incorporating high–purity calcitic limestone and municipal tap water. All specimens were cured in a temperature and humidity chamber at 20 ± 2 °C and at a relative humidity of 95 ± 2%. These conditions were maintained continuously for the entire curing duration to ensure consistent hydration and strength development. The mortars remained in the chamber for 28 days up to testing.

Mechanical characterization was carried out in accordance with the EN 12390–3 [48] and EN 12390–5 [49] international standards. Compressive strength was determined for cubic specimens, while flexural strength was assessed on prisms using a three–point bending setup with a span–to–depth ratio of 4:1. The loading rate was set at 0.5 MPa/s for compressive tests and 0.05 MPa/s for flexural tests, in compliance with the relevant standard specifications. To ensure statistical validity, a minimum of three specimens per mixture and curing age were tested (n = 3), and the reported values represent the arithmetic mean of the respective measurements. Figure 1 illustrates the casting of prisms within steel molds (left) and the hardened prismatic mortars (right) after 28 days of water–curing up to testing.

The static modulus of elasticity in compression (E_c) was determined in accordance with ASTM C469–14 [50]. Cylindrical specimens measuring 50 mm in diameter and 100 mm in height were subjected to axial loading using a servo–hydraulic universal testing machine equipped with a certified load cell and high–resolution linear variable differential transformers (LVDTs). To accurately capture longitudinal deformation, strain gauges with a gauge length of 100 mm were mounted on opposite sides of the specimen’s surface. The compressive load was applied incrementally up to 40% of the ultimate compressive strength to ensure that the material response remained within the linear elastic domain. Axial strain was continuously recorded via data acquisition software throughout the test. The static modulus of elasticity was calculated as the slope of the linear portion of the recorded stress–strain curve, in accordance with the ASTM standard procedure.

The static modulus of elasticity was calculated using the following equation, in line with [51]:

$${\mathrm{E}}_{\mathrm{c}}={\displaystyle \frac{\left({\mathsf{\sigma }}_{\mathsf{\alpha }}-{\mathsf{\sigma }}_{\mathrm{b}}\right)}{({\mathsf{\epsilon }}_{\mathsf{\alpha }}-{\mathsf{\epsilon }}_{\mathrm{b}})}}$$

(1)

where σₐ and σ_b represent the upper and lower stress levels, respectively, typically selected within the linear elastic region of the material response. These values are taken as σₐ = f_c/3 and σ_b = 0.15·f_c, where f_c denotes the compressive strength of the cement mortar. Correspondingly, εₐ and ε_b are the axial strains measured at these stress levels during the compression test.

Porosity and apparent density were assessed via the vacuum saturation and MIP methods. For the capillary porosity, the samples were subjected to vacuum saturation in a desiccator at a reduced pressure of approximately 10⁻¹ bar for a period of 3 ± 0.5 h. This step was essential to evacuate entrapped air from the pore structure, facilitating the subsequent water saturation phase. The specimens were then immersed and maintained under atmospheric pressure in distilled water for 24 h to achieve full saturation of the pore network. This meticulous preparation protocol ensured consistent and reproducible conditions for subsequent porosity and permeability analyses. From the liquid–saturated mass (w_sat), immersed liquid–saturated mass (w_a), dry mass (w_d), bulk density (ρ_b), bulk volume (V_b), and porosity (P_o) of each sample were calculated using the following equations:

$${\mathsf{\rho }}_{\mathrm{b}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{d}}{\mathsf{\rho }}_{\mathrm{w}}}{{\mathrm{w}}_{\mathrm{s}\mathrm{a}\mathrm{t}}-{\mathrm{w}}_{\mathrm{a}}}}$$

(2)

$${\mathrm{V}}_{\mathrm{b}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{s}\mathrm{a}\mathrm{t}}-{\mathrm{w}}_{\mathrm{a}}}{{\mathsf{\rho }}_{\mathrm{w}}}}$$

(3)

$${\mathrm{P}}_{\mathrm{o}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{s}\mathrm{a}\mathrm{t}}-{\mathrm{w}}_{\mathrm{d}}}{{\mathrm{w}}_{\mathrm{s}\mathrm{a}\mathrm{t}}-{\mathrm{w}}_{\mathrm{a}}}}$$

(4)

where ρ_w is the density of water (998 kg/m³).

To investigate the internal pore structure, mercury intrusion porosimetry (MIP) was performed on ~5 g samples extracted from the specimens using a high–pressure porosimeter (0.1–400 MPa), and intrusion data were collected to evaluate total porosity, pore size distribution, critical diameters, and estimated permeability. Differential and cumulative intrusion curves were analyzed to identify threshold pore sizes and to detect shifts in pore population due to binder type or CNT incorporation. Total porosity was measured using a Carlo Erba 2000 mercury porosimeter (Carlo Erba Instruments, Milan, Italy) and the total porosity of mortars was estimated at 28 and 90 days. The total porosity of the sample was calculated using the Washburn equation [52].

$${\mathrm{P}}_{\mathrm{c}}={\displaystyle \frac{2\mathsf{\gamma }\mathrm{c}\mathrm{o}\mathrm{s}\left(\mathsf{\theta }\right)}{\mathrm{r}}}$$

(5)

where,

P_c is the applied pressure (Pa),

r is the radius of the pore into which mercury intrudes (m),

γ is the surface tension of mercury (N/m),

θ is the contact angle between mercury and the solid sample surface (°).

Thermal characterization was carried out via simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using a SHIMADZU DTG–60H system (Shimadzu Corporation, Kyoto, Japan). Approximately 70 mg of finely powdered material was heated from ambient temperature to 780 °C at a rate of 5 °C/min under static air conditions. The technique enabled the quantification of portlandite (Ca(OH)₂) and carbonate (CaCO₃) phases, providing insight into hydration evolution and carbonation resistance. Mass loss intervals were associated with specific thermal events, and quantitative interpretations were made using the derivative thermogravimetric (DTG) curves.

Microstructural investigations were conducted on fractured surfaces of prismatic mortar specimens using scanning electron microscopy (SEM); for the analyses, a JEOL JSM–6610LV SEM (JEOL Ltd., Tokyo, Japan) was used operating at an accelerating voltage of 30 kV under high vacuum conditions with resolutions up to 3.0 nm. Prior to analysis, samples were sputter–coated with a thin layer of gold to ensure surface conductivity. Backscattered electron (BSE) imaging was utilized to distinguish hydration products, pore voids, and MWCNTs, while secondary electron imaging (SEI) provided detailed observations of surface morphology and crystal structure. The microstructural assessment focused on evaluating the dispersion quality of CNTs and the interfacial bonding between CNTs and the mortar matrix as well as identifying crack bridging mechanisms and densification effects induced by CNT incorporation. Furthermore, the size, shape, and morphology of crystalline phases within the mortar matrix were thoroughly examined to provide comprehensive insight into the material’s microstructural characteristics.

4. Experimental Results

4.1. Mechanical Performance

Figure 2 presents a detailed analysis of the mechanical performance of cementitious mortars containing CEM II/B–L, focusing on the effect of incorporating 0.1% by weight CNTs on their flexural and compressive strengths after 28 and 90 days of curing.

As illustrated in Figure 2, the mechanical properties of the mortars demonstrate a characteristic evolution over time, influenced by both the binder type and the presence of CNTs. The AREF mixture, based on the composite cement CEM II/B–L, exhibited moderate strength development. Flexural strength increased slightly from 1.73 MPa at 28 days to 1.86 MPa at 90 days, while compressive strength rose from 8.30 MPa to 9.07 MPa over the same period. This limited enhancement reflects the slower hydration kinetics and microstructural densification commonly associated with CEM II/B–L binders, which tend to result in a more porous matrix compared to pure Portland cement systems.

In contrast, the ACNT mixture—comprising the same cement type with the addition of CNTs—exhibited significantly improved mechanical performance. At 28 days, the flexural strength increased to 2.14 MPa, corresponding to a 23.7% improvement compared to AREF_28, while compressive strength rose to 12.05 MPa, representing a 45.2% increase. After 90 days, the flexural strength further improved to 2.25 MPa, an enhancement of 21.0% relative to AREF_90, while compressive strength reached 11.40 MPa, signifying a 25.7% increase over the control. These results highlight the beneficial influence of CNTs on the mechanical performance, likely due to their bridging action, crack–arresting capabilities, and their ability to promote a denser and more cohesive microstructure through enhanced hydration.

In conclusion, the integration of CNTs within both CEM II/B–L and CEM I cementitious mortars substantially enhances their mechanical properties through synergistic mechanisms of crack bridging, pore refinement, and hydration promotion. These findings underscore the potential of CNTs as a nanotechnological tool for the development of high–performance, durable cement–based materials.

Figure 3 provides a comprehensive and rigorous analysis of the mechanical strength of cementitious mortars based on hydraulic cement (CEM I), focusing on the effects of incorporating CNTs at a concentration of 0.1% by weight. The mechanical performance of these cement–based mortars was evaluated through flexural and compressive strength tests conducted at curing ages of 28 and 90 days under water. The aim was to determine the effectiveness of CNTs in enhancing the mechanical properties of hydraulic cement mortars, which, despite higher inherent strength compared to mortars, can benefit from nanoscale reinforcement for improved durability and performance.

At 28 days, the reference cement mortar exhibited a flexural strength of 3.99 MPa and a compressive strength of 9.96 MPa. Upon addition of CNTs, the flexural strength increased to 5.36 MPa, corresponding to a 34.09% enhancement, while compressive strength rose to 13.55 MPa, reflecting a 35.94% improvement relative to the unmodified reference.

At 90 days, continued hydration and matrix densification resulted in increased mechanical strengths for all mixtures. The flexural and compressive strengths of the reference mortar increased to 5.56 MPa and 15.20 MPa, respectively. Correspondingly, the CNT – reinforced mortar achieved 6.48 MPa in flexural strength and 19.88 MPa in compressive strength. These values represent further enhancements of 16.55% in flexural strength and 30.66% in compressive strength over the respective reference values at 90 days.

In conclusion, the integration of CNTs into hydraulic cement mortars formulated with CEM I enables a marked optimization of their mechanical properties. This nano–reinforcement strategy effectively addresses durability and performance limitations inherent to traditional cementitious materials, offering promising prospects for advanced construction applications requiring enhanced mechanical resilience and long–term stability.

4.2. Young’s Moduli Through Compression Testing

The modulus of elasticity (Figure 4) reflects the capacity of a cementitious composite to resist deformation under applied stress and is intrinsically linked to the material’s internal structure and degree of densification. In the present study, the temporal evolution and absolute values of MoE at 28 and 90 days exhibit strong agreement with both mechanical strength performance and porosity characteristics of the investigated mortars.

Across all compositions, the incorporation of MWCNTs led to measurable increases in stiffness, indicating a nanoscale enhancement of the cement matrix. For mortars prepared with a CEM II/B–L binder—typically associated with lower early–age strength and higher open porosity—the addition of CNTs resulted in a significant increase in MoE, rising from 20.90 GPa in AREF to 26.06 GPa in ACNT at 28 days (+24.7%), and from 21.88 GPa to 28.37 GPa at 90 days (+29.6%). This substantial improvement corresponds closely with the densification effect observed in MIP analyses, which revealed a marked reduction in total porosity and a refinement of the capillary pore network. The physical filling of voids by well-dispersed CNTs, combined with their ability to bridge microcracks, reduces local deformations and contributes to a more homogeneous and stiffer cementitious matrix. This effect proves particularly beneficial in mortars with inherently weaker mechanical frameworks, where nano–reinforcement offsets the limitations imposed by low clinker content and high limestone filler proportions.

In contrast, mortars incorporating CEM I—characterized by inherently higher mechanical strength and relatively low porosity—exhibited more moderate improvements upon CNT incorporation. Specifically, the MoE increased from 29.94 GPa in HREF to 32.56 GPa in HCNT at 28 days (+8.8%), and from 34.46 GPa to 35.57 GPa at 90 days (+3.2%). These more restrained gains suggest that in systems with already optimized hydration kinetics and dense microstructures, the marginal benefit of CNTs is comparatively limited. Nevertheless, the slight enhancement in stiffness still aligns with a concurrent increase in flexural strength, reflecting the synergistic role of CNTs in improving tensile load transfer and inhibiting crack propagation at the nanoscale.

The strong correlation observed between the modulus of elasticity and total porosity across all mortar types underscores the critical role of the internal pore network in stiffness development. Mortars exhibiting reduced porosity consistently demonstrated higher MoE values, indicative of improved load–bearing continuity and enhanced elastic response. Furthermore, the parallel trends noted in compressive and flexural strength reinforce the interpretation that MoE serves not only as an indicator of the immediate mechanical response but also as a reliable descriptor of the underlying microstructural quality. Collectively, these findings highlight the potential of CNTs to tailor the elastic and mechanical performance of cementitious mortars, particularly in formulations where inherent matrix weaknesses constrain structural performance.

4.3. Porosity Measurements

4.3.1. Vacuum Saturation

Figure 5 presents the evolution of capillary porosity over time, as measured by the vacuum saturation method, for all cementitious mortar mixtures at curing ages of 28 and 90 days. The four mixtures exhibit distinct porosity trends that closely interact with the presence of CNTs and the binder type, directly influencing the mechanical performance of the mortars.

Between 28 and 90 days, all mixtures demonstrated a decrease in open porosity, reflecting ongoing hydration and pozzolanic reactions, which promote matrix densification. The reference Portland cement mortar (HREF) exhibited a reduction in open porosity from 32.71% to 31.14% (4.80% decrease), while the CNT–reinforced HCNT mixture showed a smaller relative decrease from 33.72% to 32.52% (3.56%). Although the porosity reduction in HCNT was less pronounced, this mixture consistently exhibited the highest compressive and flexural strengths (19.88 MPa and 6.48 MPa, respectively, at 90 days), indicating that factors beyond mere densification contribute substantially to mechanical enhancement. The CEM II/B–L based mortars (AREF and ACNT) showed greater porosity decreases, from 33.48% to 30.39% (9.25%) for AREF, and from 34.00% to 30.71% (9.70%) for ACNT. The larger reduction in porosity corresponds with the slower hydration kinetics of CEM II/B–L, which result in a more gradual microstructural consolidation. However, despite this higher densification, ACNT’s mechanical performance surpasses that of AREF by a significant margin, demonstrating the pivotal role of CNTs in reinforcing the matrix at the nanoscale.

This apparent divergence between porosity reduction and mechanical strength emphasizes the multifaceted impact of CNTs on mortar performance. While porosity reduction generally correlates with improved mechanical properties by decreasing void volume and enhancing matrix continuity, CNTs provide an additional nano–reinforcement effect through microcrack bridging, pore refinement, and improved load transfer. These mechanisms effectively enhance the mortar’s resistance to crack initiation and propagation, contributing to increased flexural and compressive strengths independently of bulk porosity. Moreover, the CNTs facilitate accelerated hydration by acting as nucleation sites for C–S–H formation, which further consolidates the microstructure and strengthens the cementitious matrix. This synergy results in mortars that combine moderate porosity reduction with superior mechanical resilience.

In conclusion, while reductions in open porosity reflect fundamental hydration and microstructural development processes essential to mechanical improvement, the presence of CNTs enhances mortar performance beyond these effects. CNT–modified mortars exhibit a unique reinforcement mechanism that decouples porosity from strength, delivering significantly enhanced mechanical performance, which is essential for advanced construction applications requiring long-term durability and reliability

Figure 6 presents the evolution of the bulk density of the four studied cementitious mortar mixtures at 28 and 90 days. The quantitative assessment of these data, when analyzed in conjunction with porosity and mechanical strength results, reveals crucial insights into the microstructural modifications induced by CNT addition and binder type, thereby elucidating the reinforcement mechanisms at play.

The reference Portland cement mortar (HREF) exhibited a noticeable increase in bulk density from 1656.2 kg/m³ to 1671.6 kg/m³ over the curing period, corresponding to a +0.93% change. This increase aligns with the progressive hydration reactions and the consequent formation of calcium silicate hydrate (C–S–H) phases, which enhance matrix consolidation. Conversely, the CNT–modified HCNT mortar showed a markedly smaller density increment from 1645.9 to 1647.8 kg/m³ (+0.12%), maintaining the lowest absolute density among the mixtures at both curing ages. The blended cement–based mortars (AREF and ACNT) revealed relatively minor bulk density increases: AREF rose from 1699.1 to 1705.9 kg/m³ (+0.40%), while ACNT increased from 1688.0 to 1691.1 kg/m³ (+0.18%). These modest density variations reflect the slower and more gradual hydration kinetics typical of blended cements, as well as the pozzolanic activity of the CNT.

When these bulk density changes are correlated with the open porosity measurements, a complex microstructural behavior emerges. Despite limited bulk densification, both AREF and ACNT mixtures achieved substantial reductions in open porosity (−9.25% and −9.70%, respectively), indicative of a significant pore network reorganization and refinement rather than simple mass compaction. Similarly, HCNT, despite the smallest density increase and a moderate porosity decrease (−3.56%), attained superior mechanical strength values. This disparity underlines that the mechanical enhancements conferred by CNTs are primarily attributed to nanoscale reinforcement mechanisms—such as microcrack bridging, pore size refinement, and improved interfacial bonding—rather than increases in bulk density. Moreover, although HREF demonstrated the highest relative increase in bulk density among groups, it did not outperform the CNT–enhanced mortars in terms of compressive or flexural strength. This observation underscores a critical decoupling between density gain and mechanical performance, highlighting that densification alone is not a sufficient predictor of mortar robustness.

In conclusion, the incorporation of CNTs promotes the optimization of the cementitious microstructure by enhancing structural integrity and load transfer capabilities at the nanoscale, independently of significant bulk density changes. The weak correlation between density evolution and strength development further confirms that CNTs act through refined nano–reinforcement pathways that transcend conventional hydration–induced densification. These findings establish CNTs as pivotal additives for engineering high-performance, durable cement mortars suitable for modern construction challenges.

4.3.2. Mercury Intrusion Porosimeter (MIP)

The MIP results presented in Figure 7 reveal significant trends in total porosity and bulk density for the four batches under examination at both 28 and 90 days of curing. The reference cement mortar (AREF) exhibited a total porosity of 33.68% at 28 days, which decreased to 31.45% at 90 days, corresponding to a 6.62% reduction. The incorporation of CNTs into the cement matrix (ACNT) resulted in a lower initial porosity of 31.26%, which further decreased to 28.98% at 90 days, representing a slightly greater relative reduction of 7.29%. These findings indicate that CNTs contribute to a modest densification of the cement matrix during hydration.

In the hydraulic cement mortars, the reference mix (HREF) showed a porosity decrease from 32.47% to 30.02% between 28 and 90 days, reflecting a 7.55% reduction. The addition of CNTs (HCNT) yielded the lowest overall porosity values among all mixtures, declining from 29.65% at 28 days to 27.76% at 90 days, corresponding to a 6.37% reduction. This consistent decrease in total porosity across all mixtures highlights ongoing microstructural refinement driven by hydration and pozzolanic reactions, with CNT–containing mortars exhibiting slightly more compact pore structures, particularly within the cement matrices [53,54].

Regarding bulk density, all mortars demonstrated a general decreasing trend with curing time. The AREF mixture decreased from 1.641 g/cm³ at 28 days to 1.603 g/cm³ at 90 days, a reduction of approximately 2.31%. The ACNT mortar showed a more pronounced density decline of 6.76%, from 1.522 to 1.419 g/cm³, suggesting possible rearrangement of the pore network or less efficient particle packing due to the presence of CNTs. For the hydraulic cement mortars, HREF decreased from 1.566 to 1.489 g/cm³ (4.91%), while HCNT experienced the largest drop of 7.58%, from 1.489 to 1.376 g/cm³.

Comparing the effect of CNT addition relative to their respective reference mortars at 28 days, ACNT exhibited an initial porosity 7.18% lower than AREF and a density reduction of 7.25%, indicating overall microstructural refinement but also potential heterogeneity at early curing stages. Similarly, HCNT showed an 8.70% lower porosity and a 4.91% lower density compared to HREF at 28 days. At 90 days, ACNT maintained a porosity of 7.83% lower than AREF, while HCNT preserved a 7.55% reduction compared to HREF. However, differences in density became more pronounced, with ACNT and HCNT registering 11.48% and 7.58% lower densities, respectively, than their reference counterparts.

These results suggest that CNTs contribute to a denser and more refined pore structure by reducing total porosity, especially during early curing stages. However, this densification is accompanied by a decrease in bulk density, likely attributable to microstructural phenomena such as increased microporosity or altered packing efficiency. The combined porosity–density profiles correlate closely with the enhanced mechanical performance observed in CNT–reinforced mortars, supporting the hypothesis that CNTs improve strength primarily through microstructural densification, pore bridging, and potential modifications of hydration kinetics. The relatively lower porosity and superior mechanical strength of HCNT, in particular, underscore the beneficial role of CNTs in hydraulic cement matrices where pozzolanic reactions and pore refinement are more pronounced.

Although both methods demonstrate a reduction in porosity with curing time, total porosity values measured by mercury intrusion porosimetry are generally higher than the open porosity values obtained through vacuum saturation. This discrepancy arises because vacuum saturation exclusively measures the interconnected, open porosity accessible to water infiltration, whereas MIP captures both open and closed porosity, including micropores and isolated voids not detectable by vacuum saturation. Consequently, this difference highlights the CNTs’ influence not only on reducing open pores but also on modifying the closed pore network—an effect that vacuum saturation cannot reveal. The ability of CNTs to refine both open and closed porosity components is critical for enhancing the mechanical properties and durability of cement–based mortars.

As illustrated in Figure 8, both curing age and CNT incorporation significantly influence the porosity (P) and apparent density (ρ) of cement mortars. Prolonged curing from 28 to 90 days led to a notable reduction in porosity and a concomitant increase in density, highlighting the progressive refinement of the pore structure over time. Moreover, the inclusion of CNTs resulted in additional improvements in these parameters, suggesting their beneficial role in densifying the microstructure and enhancing the material’s compactness. To reconcile these findings with the observed mechanical performance, it is imperative to integrate the complementary insights derived from vacuum saturation tests. Although mercury intrusion porosimetry (MIP) measurements indicated a reduction in total porosity for ACNT specimens, vacuum saturation tests revealed an increase in open porosity. This apparent discrepancy is attributed to the enhancement of pore connectivity induced by CNTs. The increased connectivity suggests the development of more continuous capillary networks, which, despite a decrease in overall pore volume, may facilitate the ingress of fluids and aggressive agents or promote the coalescence of microcracks under mechanical loading. Consequently, these interconnected pathways can serve as stress concentrators, undermining the internal cohesion of the cementitious matrix. Therefore, the observed reduction in compressive strength is not contradictory to the decreased total porosity; rather, it reflects the complex structural implications of enhanced pore connectivity, particularly in nano–reinforced blends, which can offset the advantages conferred by a denser microstructure.

As shown in Figure 9, the evolution of average pore diameter as a function of curing time and binder composition provides valuable insights into the microstructural development of the studied cement–based mortars and their implications for mechanical performance. In the reference cement mortar (AREF), the average pore diameter remained relatively stable between 28 and 90 days (328.5 nm and 327.2 nm, respectively), indicating limited microstructural refinement during curing. The addition of CNTs in the ACNT mixture resulted in a significant reduction in pore diameter at 28 days (281.5 nm), which can be attributed to the physical filling and bridging actions of the nanomaterials that promote early densification of the cement matrix. However, the subsequent increase to 331.3 nm at 90 days may suggest a disruption of matrix integrity, possibly due to localized CNT agglomeration or interference with hydration and cementitious interactions, ultimately leading to a coarser pore structure at later curing ages.

In contrast, the HREF mortars exhibited markedly finer pore structures. This mixture showed a progressive reduction in average pore diameter from 83.9 nm to 67.5 nm over time, consistent with ongoing hydration and pozzolanic activity that refine the capillary pore network. This trend was further enhanced in the HCNT formulation, where pore diameters decreased to 74.5 nm at 28 days and 72.3 nm at 90 days. The consistently smaller pore sizes in HCNT compared to HREF reflect the synergistic effect of hydraulic reactions and CNT reinforcement in developing a denser and more homogeneous microstructure. The early stabilization of the pore system in HCNT also suggests that CNTs accelerate matrix densification, leading to reduced porosity and more favorable conditions for strength development.

A clear inverse correlation is observed between average pore diameter and total porosity, supporting the understanding that finer pores contribute to reduced overall void content. More importantly, the combination of smaller pore size and lower total porosity in the HCNT mix aligns with its superior mechanical behavior, as demonstrated by the previously reported increases in compressive and flexural strength. These findings underscore the role of CNTs not only as passive fillers but as active agents in microstructural refinement, directly supporting enhanced mechanical performance. Therefore, the incorporation of CNTs, particularly within hydraulic binder systems, emerges as a promising strategy to enhance both durability–related and structural properties of cementitious mortars through refined porosity and reduced pore diameter.

4.4. Differential Thermal Analysis (DTA)

This study investigates the thermal behavior and compositional evolution of four mortar mixtures through differential thermal analysis (DTA) combined with quantitative phase analysis, as illustrated in Figure 10. The mortar formulations were prepared using two cement types, CEM I and CEM II/B–L, and were either unmodified or modified with CNTs. The analysis focused on characterizing phase transitions and decomposition phenomena occurring over the temperature range of 30 to 800 °C, with particular emphasis on the key hydration products calcium silicate hydrate (C–S–H), portlandite (C–H), and calcium carbonate (CaCO₃) [55,56,57].

The DTA results revealed distinct differences in phase transformation behavior among the mortar mixtures, particularly within the thermal decomposition intervals associated with C–S–H gel decomposition and chemically bound water loss (30–400 °C), C–H decomposition (400–500 °C), and calcium carbonate decarbonation (600–800 °C). In the lower temperature range, which corresponds to the evaporation of physically bound water and the dehydration of hydration products such as ettringite and C–S–H gel, the CNT–modified mortars exhibited evidence of a more consolidated microstructure. Specifically, the HCNT mixture showed the highest mass loss of 3.11%, followed closely by the HREF mixture at 3.00%, whereas the ACNT and AREF mixtures recorded lower values of 2.06% and 2.05%, respectively. These findings suggest that HCNT and HREF contain a greater quantity of retained hydration products, indicative of a denser microstructure arising from more complete hydration.

Within the intermediate temperature range of 400 to 500 °C, corresponding to the decomposition of C–H, the CNT–containing mixtures demonstrated higher C–H contents. The HCNT mixture exhibited the highest C–H content at 3.24%, followed by ACNT at 2.76%, HREF at 2.25%, and AREF at 1.83%. The elevated Ca(OH)₂ content in CNT–modified mortars implies either a retardation of carbonation processes or a higher degree of ongoing hydration, reflecting the stabilizing influence of CNTs on hydration products.

At higher temperatures between 600 and 800 °C, associated with the decarbonation of calcium carbonate, the incorporation of CNTs resulted in a slight reduction in CaCO₃ formation compared to the unmodified mortars. Both HCNT and ACNT mixtures exhibited the lowest carbonate contents, approximately 22.16%, while HREF and AREF demonstrated higher values of approximately 23.33% and 27.16%, respectively. The reduced carbonate content in CNT–containing mixtures suggests diminished carbonation, likely attributable to matrix densification and pore refinement that restrict CO₂ ingress. Notably, ACNT displayed the lowest overall CaCO₃ content among the four mixtures, indicating that the synergistic interaction between CNTs and CEM II/B–L cement enhances thermal resilience and mitigates deleterious carbonation processes.

Overall, the HCNT mixture exhibited the most favorable thermal profile, characterized by the highest C–H content (3.24%), the greatest water-related mass loss (3.11%), and the lowest calcium carbonate quantity (~22.16%). These values correspond to an improved Ca(OH)₂ to CaCO₃ ratio of approximately 0.146, which is significantly higher than those observed in ACNT (0.125), HREF (0.096), and AREF (0.067). This ratio reflects superior preservation of hydration products and minimized carbonation, thereby confirming the beneficial role of CNTs in stabilizing the cementitious microstructure. The combination of enhanced hydration, reduced porosity, and restricted carbonation in HCNT correlates well with its superior mechanical performance observed at advanced curing stages.

In contrast, the AREF and ACNT mixtures exhibited lower C–H retention and more pronounced carbonate formation, consistent with their comparatively diminished strength values. These observations reinforce the conclusion that CNTs not only influence the hydration process but also contribute significantly to the durability and thermal stability of the cementitious matrix by promoting a refined microstructure and limiting the formation of thermally unstable phases.

In summary, the thermal analysis of the four mortar mixtures underscores the pivotal role of CNTs in enhancing the thermal stability, hydration efficiency, and overall durability of cementitious matrices. CNT–modified mortars, particularly HCNT, consistently demonstrated superior retention of hydration products (Ca(OH)₂), reduced carbonation (CaCO₃), and higher bound water content, indicative of a denser and chemically more stable microstructure. These features translate into enhanced mechanical performance, confirming that CNTs act not only as physical reinforcements but also as modifiers of hydration and carbonation pathways. Among all mixtures, HCNT emerged as the most thermally and structurally resilient formulation, providing a compelling rationale for the inclusion of CNTs in advanced cementitious materials aimed at achieving long-term durability and performance.

4.5. Microscopy Analyses (SEM)

The microstructural analysis of cement–based mortars (Figure 11), as revealed by scanning electron microscopy (SEM), provides essential insights into the mechanisms governing their mechanical performance and durability. The reference mortar (AREF) formulated without carbon nanotubes presents a loosely packed and heterogeneous matrix, which is characterized by a high frequency of interparticle voids and limited continuity within the binder phase. SEM imaging of AREF distinctly shows large, angular grains interspersed with sparse, gel-like hydration products, a morphology that is indicative of slow reaction kinetics associated with the CEM II/B–L binder system. This binder relies predominantly on carbonation processes, which further contributes to the observed microstructural features. The open and porous framework of AREF is quantitatively confirmed by mercury intrusion porosimetry (MIP) and vacuum saturation measurements, which indicate both high total porosity and relatively large average pore diameters. Such a microstructure inherently promotes increased water absorption due to the extensive connectivity of macro-voids. This, in turn, negatively impacts the mortar’s durability by facilitating the ingress of deleterious agents and reducing resistance to mechanical stresses. Mechanically, the AREF mortar exhibits low compressive and flexural strengths, which can be attributed to the absence of cohesive, crack bridging phases and the insufficient internal cohesion of the matrix.

In contrast, the hydraulic binder reference mortar (HREF) displays a markedly denser and more homogeneous microstructure. SEM analysis reveals finely interlocked particles embedded within a matrix rich in calcium silicate hydrate (C–S–H). The images show a notable reduction in capillary voids and a lower prevalence of unreacted grains, which is consistent with the observed decrease in both total porosity and pore size, as well as with lower water absorption values measured experimentally. This microstructural densification is reflected in the improved mechanical properties of HREF. Nevertheless, localized microcracks are still apparent within the matrix, which may serve as weak zones under flexural loading conditions. Despite the presence of these microcracks, HREF demonstrates superior structural integrity and strength compared to AREF, underscoring the beneficial effect of hydraulic binder chemistry on matrix development and performance.

The incorporation of carbon nanotubes into the mortar matrix induces significant transformations in both microstructure and associated performance characteristics. SEM observations of the ACNT formulation, that contains carbon nanotubes, reveal a more cohesive binder phase wherein thread–like CNT structures bridge adjacent particles and form fibrous networks. These networks facilitate efficient stress transfer across microcracks and inhibit their propagation, thereby enhancing the overall toughness of the matrix. The presence of CNTs also promotes the alignment and densification of hydration products, effectively refining the pore network by filling voids and reducing macroporosity. As a result, the ACNT mortar exhibits decreased total porosity and smaller average pore diameters at 28 days of curing, findings that are corroborated by lower water absorption measurements and enhanced flexural strength relative to AREF. However, at later curing stages, partial agglomeration of CNTs is observed, which can locally disrupt the continuity of the matrix and contribute to an increase in porosity and water absorption, as evidenced by MIP data at 90 days. This heterogeneity highlights the critical importance of achieving uniform CNT dispersion to ensure sustained improvements in microstructure and performance.

Among all the studied samples, the HCNT mortar, which combines a hydraulic binder with carbon nanotubes, exhibits the most refined and compact microstructure. SEM images demonstrate that CNTs are uniformly dispersed and integrated within a densely packed C–S–H gel network. The nanotubes visibly bridge pores at both the micro- and nanoscale, thereby contributing to the formation of a continuous and resilient matrix with minimal capillary voids. This optimized microstructure is reflected in the lowest total porosity and smallest pore diameters measured among all formulations, as well as in significantly reduced water absorption, confirming the development of a highly impermeable internal framework. The microstructural enhancements observed in HCNT are directly responsible for its superior mechanical performance, as evidenced by the highest compressive and flexural strengths recorded in this study. The synergy between accelerated hydration kinetics, efficient CNT dispersion, and improved matrix cohesion not only enhances durability and resistance to crack initiation and propagation but also extends the service life of the material under both mechanical and environmental stressors.

The mechanisms underlying the reinforcing effects of CNTs, as elucidated by SEM imaging, encompass both physical and chemical phenomena. Physically, the fibrous CNT networks provide critical pathways for stress transfer, effectively arresting crack growth and enhancing fracture toughness. Chemically, CNTs serve as nucleation sites for the formation of C–S–H, thereby promoting a more continuous and densely packed matrix. The effectiveness of these mechanisms is highly dependent on the quality of CNT dispersion within the matrix. Uniform distribution, achieved through sonication bath methods, minimizes the occurrence of agglomerations, which can otherwise compromise both microstructural integrity and mechanical properties.

A clear interrelation is established between microstructure, porosity, water absorption, and mechanical behavior across all mortar samples. Denser microstructures with well-dispersed CNTs consistently exhibit lower porosity, reduced pore sizes, and diminished water uptake, all of which contribute to improved mechanical strength and durability. Conversely, matrices that are porous and heterogeneous demonstrate inferior performance and are more susceptible to environmental deterioration. This comprehensive analysis confirms that the strategic incorporation of CNTs into hydraulic binder systems represents a promising approach for the development of high-performance cementitious materials. Such enhancement is achieved through microstructural refinement, improved internal cohesion, and crack bridging reinforcement, provided that uniform dispersion of the nanotubes is maintained throughout the matrix.

5. Discussion

The experimental findings of this study clearly demonstrate that the incorporation of CNTs enhances the performance of cement–based mortars across multiple dimensions, mechanical, physical, and microstructural. These enhancements, however, are strongly influenced by the type of cement employed, with mortars incorporating CEM I (ordinary Portland cement) exhibiting more pronounced improvements compared to those based on CEM II/B–L (limestone cement). The discussion that follows synthesizes data from mechanical strength tests, vacuum saturation, and mercury intrusion porosimetry (MIP), pore size distribution, and SEM micrographs, to elucidate the mechanisms underlying CNT–induced enhancements and their dependency on the cement matrix.

Compressive and flexural strength results consistently indicate a performance increase following CNT incorporation in both cement systems. The CEM–I–based CNT–reinforced mortar (HCNT) exhibited the most significant strength gains, especially at 90 days, underscoring the synergistic interaction between CNTs and the hydration processes characteristic of Portland cement. CNTs function as nanoscale crack bridging agents that distribute tensile stresses and inhibit crack propagation [58]. Their high specific surface area also facilitates robust interfacial bonding with hydration products, particularly calcium silicate hydrate (C–S–H), thereby contributing to a more cohesive and mechanically resilient microstructure [59,60]. While the CEM II–based CNT–reinforced mortar (ACNT) also displayed strength improvements, these were less pronounced and showed some decline over time. This performance attenuation may result from the inherently slower and less complete hydration associated with limestone–containing cements as well as potential CNT agglomeration [61,62,63,64]. The reduced clinker content in CEM II/B–L diminishes the formation of reactive hydration products, thereby limiting the long–term reinforcing potential of CNTs.

Porosity measurements corroborate the mechanical findings, revealing a reduction in total pore volume following CNT incorporation. In ACNT, porosity obtained by MIP was significantly lower than in the corresponding reference mortar (AREF) at 28 and 90 days, suggesting that CNTs contribute to initial matrix densification via pore filling, enhanced packing density, and promotion of hydration nucleation. In contrast, HCNT exhibited a progressive and sustained reduction in porosity from 28 to 90 days, consistent with the high reactivity of CEM I. The favorable interaction between CNTs and clinker phases promotes extended hydration and densification, with CNTs potentially serving as nucleation sites for C–S–H, further improving the matrix compactness. This porosity reduction correlates directly with the observed mechanical performance and implies improved durability, as decreased porosity typically leads to lower permeability and enhanced resistance to environmental exposure.

It is worth mentioning the contradictory results between vacuum saturation and MIP methods. Cement mortars exhibited slightly increased porosity values when measured by vacuum saturation compared to mercury intrusion porosimetry (MIP). This discrepancy can be attributed to the fundamental differences in the measurement principles of the two methods. MIP assesses the total accessible pore volume of water–permeable pores, whereas vacuum saturation fills primarily the capillary pores within cementitious composites, enabling the measurement of open porosity accessible to water. Additionally, the presence of CNTs can alter the pore structure by creating more tortuous and complex pore networks, enhancing connectivity and thus water accessibility during vacuum saturation, but limiting mercury intrusion due to constricted pore throats. Therefore, vacuum saturation captures a broader range of pore sizes and interconnectivity, leading to higher measured porosity in CNT–modified mortars compared to MIP results.

The evolution of average pore diameter further illuminates a microstructural development. In AREF, the average pore size remained nearly unchanged over time, indicating limited refinement of the pore structure in CEM II/B–L mortars. Although ACNT exhibited a notable pore size reduction at 28 days, this trend reversed by 90 days, possibly reflecting structural disintegration or nonuniform CNT dispersion. Such instability could compromise long–term durability and highlights the need for improved dispersion techniques when CNTs are used with blended binders. Conversely, HCNT maintained a consistently fine pore structure, with a lower average pore diameter than its reference counterpart (HREF) compared to ACNT analogues at 28 and 90 days. This persistent pore refinement suggests that CNTs not only densify the initial matrix but also contribute to preserving its integrity over time. These results indicate that CNTs may exhibit enhanced compatibility with the hydration dynamics and microstructural evolution of CEM I relative to CEM II/B–L under the specific conditions studied. However, compatibility is influenced by factors such as mix design, CNT functionalization, and dispersion methods, and successful integration of CNTs in blended cements has been reported in other studies when these parameters are optimized.

SEM micrographs provide direct visual evidence of the structural alterations induced by CNT addition. The AREF sample displayed a discontinuous and porous matrix, with loosely bonded hydration products and substantial intergranular voids—features typical of cements with reduced clinker content. ACNT showed a relatively denser structure, with CNTs visible as fibrous bridges between hydration products, suggesting improved cohesion. Nonetheless, the presence of inhomogeneities and poorly packed zones indicates incomplete CNT dispersion and possible limited compatibility with slower hydration products. In contrast, HREF exhibited a more continuous matrix, indicative of robust C–S–H formation. The HCNT sample showed the most compact and homogeneous microstructure, with CNTs effectively embedded in the hydrated matrix, visibly bridging microcracks and enhancing the particle network. This dense morphology aligns well with the mechanical and porosity results, reinforcing the superior performance of HCNT mortars.

Overall, the results indicate that the reinforcing efficacy of CNTs is significantly influenced by the cement type. In CEM–I–based mortars, the high clinker content and rapid hydration kinetics facilitate uniform CNT dispersion and integration within the evolving C–S–H matrix. This leads to a highly refined and cohesive structure that benefits from both mechanical and chemical reinforcement provided by the CNTs. In contrast, the performance of CNTs in CEM–II–based mortars appears more susceptible to dispersion challenges and long–term microstructural instability. The lower availability of reactive phases in CEM II/B–L hampers the full development of binding products, potentially limiting CNT efficacy. Furthermore, the altered hydration pathways of blended cements may not support optimal CNT–matrix interactions, reducing their ability to bridge microcracks and refine the pore network consistently over time.

These insights underscore the potential of CNTs to significantly enhance the mechanical and durability–related properties of cement–based mortars—particularly when paired with CEM I. Their incorporation improves strength, decreases porosity, and supports microstructural refinement. For practical applications, CNT–reinforced mortars based on ordinary Portland cement could offer marked advantages in structural performance, service life, and environmental resistance. However, broader implementation in CEM II/B–L systems will require refinement of dispersion protocols and possibly chemical surface modification of CNTs to enhance their compatibility with blended binders.

While the present investigation focused on the laboratory–scale performance evaluation of CNT–reinforced mortars, the assessment of their durability under actual field conditions remains paramount for their long–term implementation in restoration applications and exposed masonry structures [65]. CNTs have shown considerable potential in enhancing resistance to various environmental degradation mechanisms that compromise the longevity of cementitious materials [66,67]. Extensive research has demonstrated that CNT incorporation can significantly reduce water permeability, limit chloride ion ingress, and delay microcrack propagation under thermal and hygric cycling [68]. These enhancements may contribute to improved freeze–thaw resistance and carbonation durability relative to conventional mortars [69]; however, the underlying mechanisms have been well–documented, including nanoscale crack bridging, refinement of the pore structure, and improved interfacial bonding between the matrix and the nanomaterial [70]. Specifically, studies have reported reductions in chloride diffusion coefficients ranging from 19% to 63%, depending on CNT content and dispersion efficiency [68]. Furthermore, CNTs have been shown to impede crack formation under cyclic environmental stress through mechanisms such as nanoscale crack bridging, enhanced load transfer, and increased toughness at the matrix–CNT interface [71,72]. In lime–based composites, additional benefits have been observed in terms of moisture buffering capacity and dimensional stability under ambient environmental fluctuations [73]. These effects may be partially influenced by the inherently low thermal expansion coefficient of CNTs; however, at typical low dosages, their impact on the overall thermal expansion of cementitious composites is often limited. Consequently, improvements in structural compatibility with historic masonry substrates are likely the result of a combination of factors beyond solely the thermal expansion characteristics of CNTs.

These findings form a robust foundation for the development of advanced cementitious materials and restoration mortars where high performance, durability, and sustainability are concurrently pursued.

6. Limitations and Future Work

Despite the notable improvements observed in the mechanical performance and microstructural characteristics of the investigated mortars, several inherent limitations of the present study warrant careful consideration. Foremost among these is that the temporal scope was limited to a maximum curing period of 90 days. While this duration suffices to assess early– to mid–term developments in strength and pore structure, it does not capture potential long–term phenomena such as matrix degradation, pore coarsening, or durability–related deterioration. Accordingly, this temporal constraint represents a significant limitation that restricts the generalizability of the findings to extended service life encountered in practical applications.

Moreover, the experimental investigations, conducted at a laboratory scale used small prismatic specimens under strictly controlled curing conditions. Although such standardization enhances reproducibility, it may not fully replicate the complexity of field–scale environments, where factors such as spatial humidity gradients, mechanical restraints, and variable loading conditions exert considerable influence on material performance over time. The uniform dispersion of MWCNTs within the cementitious matrix remains a persistent technical challenge. Despite the application of ultrasonic dispersion and surfactant stabilization methods, localized agglomeration of nanotubes may occur, potentially inducing heterogeneity in both mechanical response and microstructural development.

An additional limitation pertains to the interaction between CNTs and the binder chemistry. This discrepancy suggests that the long–term dispersion stability and synergistic effects of CNTs may be adversely affected by the carbonate–rich environment characteristic of blended cement matrices, thereby necessitating further chemical and physical optimization. However, the improvements were more pronounced in CEM–I–based systems. This discrepancy suggests that the long–term dispersion stability and synergistic effects of CNTs may be adversely affected by the carbonate–rich environment characteristic of blended cement matrices, thereby necessitating further chemical and physical optimization.

To thoroughly evaluate the long–term stability and practical applicability of CNT–modified mortars, future research should extend the curing period to six months or longer, thereby enabling the investigation of time–dependent changes in pore structure, hydration kinetics, and strength development. It is also essential to incorporate durability testing protocols under simulated environmental conditions, including accelerated carbonation exposure (EN 12390–12 [74] or RILEM CPC–18 [75]), freeze–thaw cycling (ASTM C666 [76]) and sulfate resistance (ASTM C1012 [77] or EN 12390–11 [78]) [79,80]. Investigations should encompass both laboratory–controlled and field–representative exposure scenarios to elucidate coupled degradation mechanisms arising from temperature and moisture fluctuations, deicing salts, and atmospheric pollutants.

Furthermore, the evolution of CNT dispersion and interfacial bonding under prolonged chemical and physical stressors merits detailed examination. This should be conducted through advanced characterization techniques, including scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) at extended time intervals. Such comprehensive studies are imperative to validate the long–term durability of CNT–enhanced mortars and to substantiate their integration into infrastructure applications that demand sustained service life performance.

7. Sustainability Considerations and Practical Applications

The incorporation of CNTs into cement–based mortars present novel opportunities for performance–driven material design. However, it simultaneously raises significant sustainability and cost considerations. The current production of CNTs is both energy–intensive and expensive, potentially limiting its widespread adoption in large–scale applications and its economic feasibility. Nevertheless, if the enhancements in mechanical strength and microstructural refinement enable reductions in binder content or cross–sectional dimensions, the overall environmental footprint of CNT–based cementitious systems could be substantially decreased. These trade–offs merit comprehensive evaluation through life cycle assessment (LCA) methodologies to quantify their net sustainability impact.

From an application perspective, CNT–reinforced mortars are suitable for high-performance construction components and specialized repair applications. Their improved flexural strength and refined pore structure render them ideal for structural overlays, crack–resistant plasters, prefabricated panels, and restoration mortars for heritage structures, where mechanical robustness and dimensional stability are essential. Conversely, in CEM–II–based mixtures, characterized by slower hydration kinetics and more variable microstructural development, the use of CNTs may be more appropriate for non–load–bearing or structural applications, unless CNT dispersion techniques are significantly improved.

Overall, the findings indicate that, with careful formulation strategies, CNT–modified mortars offer a structural performance, durability, and long–term stability, particularly in applications demanding long–term mechanical reliability and environmental resilience.

8. Conclusions

This study presents a thorough experimental investigation into the effects of carbon nanotube (CNT) incorporation on the performance of cement–based mortars formulated with either CEM I (ordinary Portland cement) or CEM II/B–L (limestone cement). The analysis encompassed multiple performance domains, including mechanical strength development, thermal behavior, microstructural morphology, and pore structure characteristics. The results demonstrate that both CNT addition and cement type significantly influence the physical and chemical evolution of the mortar matrices.

• Mechanical performance: The inclusion of CNTs markedly enhanced both compressive and flexural strengths in mortars formulated with both types of cement. The HCNT group exhibited the most pronounced mechanical improvements, particularly at 90 days, indicating a synergistic interaction between CNTs and the hydration kinetics characteristic of Portland cement. Conversely, the ACNT mixture showed early strength gains followed by a moderate decline at later ages; this behavior is likely attributable to its lower clinker content and less effective CNT dispersion.
• Microstructural morphology: Scanning electron microscopy (SEM) revealed that CNT addition improved microstructural compactness and internal cohesion. In HCNT, CNTs were well integrated within the binder matrix, frequently bridging microcracks and enhancing continuity among hydration products. ACNT also displayed improved morphology relative to its reference counterpart, although certain microstructural inconsistencies were observed probably due to incomplete CNT dispersion. Reference mixes exhibited more porous and discontinuous matrices, particularly in AREF.
• Thermal stability: CNT–modified mortars demonstrated enhanced thermal stability, as evidenced by higher retained C–H content and lower degrees of carbonation, as inferred from thermogravimetric analysis. The C–H contents were notably elevated in both HCNT and ACNT, indicating better preservation of hydration products. These effects were more pronounced in HCNT, suggesting that the higher clinker reactivity facilitates more effective interactions between CNTs and the cement matrix under thermal exposure.
• Pore structure and densification: CNTs contributed to matrix densification; this effect was especially evident during the early stages of curing, as reflected by reductions in total porosity. The HCNT mixture exhibited a continuous decline in porosity from 28 to 90 days, whereas ACNT experienced a slight increase in porosity over time, potentially linked to CNT clustering or reduced hydration activity. Furthermore, the average pore diameter remained consistently lower in HCNT, indicating a more refined and stable pore network compared to other samples.

Overall, the findings provide compelling evidence that the incorporation of CNTs substantially enhances the mechanical performance, microstructural integrity, thermal stability, and pore structure of cement–based mortars. These benefits were more pronounced in mixtures containing CEM I binder, owing to their higher clinker content and more favorable hydration kinetics, which promote improved CNT dispersion and interaction with hydration products. Although improvements were also observed in CEM–II–based mortars, their long–term performance may require further optimization of mix design and CNT incorporation techniques. This study underscores the importance of tailoring nanomodification strategies to the specific characteristics of the cementitious matrix to maximize performance gains.

It is important to note that this investigation examined a single CNT dosage (0.1% by weight of cement), selected based on prior literature as a representative concentration for initial evaluation. While this approach enabled a focused analysis of CNT effects, exploring a broader range of CNT dosages—such as 0.05%, 0.2%, or higher—represents an essential research direction for understanding dose–dependent effects and optimizing composite performance.