Hexagonal boron nitride reinforced quick-setting multifunctional cement

Abstract

Cement is one of the most widely used building materials due to its strength and durability. However, conventional cement has a very high setting time, which makes it less attractive for applications requiring quick-setting behavior, such as rapid construction, emergency repairs, underwater construction, and 3D printing. The present study proposes hexagonal boron nitride (hBN) as a potential accelerant to impart quick-setting behavior to conventional cement. hBN is a two-dimensional material renowned for its exceptional thermal conductivity, chemical stability, and mechanical strength. Our study investigates the incorporation of hBN nanoparticles into class G Portland cement to enhance its mechanical, thermal, and rheological properties. Our experimental investigation demonstrates that hBN acts as an excellent accelerant in cement by reducing the dormancy period by up to 2 h and enhancing the overall setting kinetics. This makes hBN a promising candidate for quick-setting cement applications. Further thermal analysis reveals an improved heat dissipation capability, with lower surface temperatures and enhanced structural integrity due to reduced porosity and microcrack formation. Mechanical testing demonstrates substantial improvements in compressive strength (up to 29%), compressive modulus (up to 45%), and energy absorption capacity (up to 31%) for 1% hBN-reinforced cement compared to neat cement. Moreover, hBN-reinforced 3D-printed cement structures exhibit a 72% increase in compressive strength. The hBN-reinforced cement ink also demonstrates enhanced printability, characterized by superior flow stability, better structural recovery, and reliable shape retention, making it ideal for 3D printing applications.

Introduction

Cement is one of the most utilized structural materials to date. It is the most common construction material due to its ability to form a strong and durable adhesion when mixed with water and aggregates, and therefore, it has been used for decades to build resilient infrastructure [1]. However, cement has poor endurance, low toughness, and low tensile strength [2, 3]. One potential approach to addressing its limitation is the reinforcement of the cement matrix using fillers such as silica fume, plant-based fibers, glass fibers, fly ash, etc. [4–7]. Such reinforcement also influences the curing process of cement. For instance, fly ash and silica fume react with the calcium hydroxide (Ca(OH)₂) produced during the hydration process and form calcium silicate hydrate (C-S-H), leading to a denser and stronger cement matrix [8]. Silica fume is a fine filler that reduces cement porosity by filling up the micro-voids within the cement matrix. As a result, the microstructure becomes very dense, reducing the crack propagation propensity under stress [9]. Also, plant-based fibers and glass fibers bridge cracks within the structure, ensuring a more uniform stress distribution that can occur during curing [10, 11]. These fibers also play a crucial role in energy absorption and dissipation, reducing crack formation and propagation probabilities during curing and in the long term [12]. Furthermore, fillers like fly ash can improve the moisture retention capability of cement, which reduces material shrinkage during the curing process and lessens internal stress development, preventing early-stage cracking [13, 14]. Conventional fillers have come a long way in promoting an effective curing process by reducing shrinkage, accommodating better hydration, and, more importantly, enhancing the material’s strength to resist crack formation and propagation. Apart from these conventional materials, nanomaterials have opened new possibilities for cement reinforcement. Compared to their bulk counterparts, materials at the nanoscale exhibit exceptional physical characteristics, such as a high Young’s modulus and tensile strength [15–17]. Researchers have shown that adding nanomaterials to cement, such as carbon nanotubes, carbon nanofibers, graphene, graphene-oxide, nano-SiO₂, nano-CaCO₃, and nano-C-S-H, can significantly improve the mechanical properties of the cement-based materials, such as compressive and flexural strength, and toughness, compared to their neat counterparts [18]. These nanomaterials act as accelerants, a chemical additive that speeds up the curing process, leading to a rapid hydration process and a reduced setting time, which results in enhanced initial strength of the material. Moreover, since accelerants facilitate quicker hardening, they allow for the quicker removal of formwork and an earlier transition to later construction stages. Such improved material properties stem from the enhanced interfacial bonding that occurs when nanomaterials are dispersed within the base material matrix.

A noteworthy two-dimensional (2D) nanomaterial is hexagonal boron nitride (hBN), which can function as a nanofiller for cement. The structure of hBN resembles isoelectric forms of graphene, and the 2D crystal structure of hBN demonstrates strong thermodynamic (air-stable up to 1000°C) and chemical stability with a melting temperature of roughly 2600°C [19]. Kim et al. used AFM to evaluate multilayer hBN and found it to have a Young’s modulus of 1.16 ± 0.1 TPa with a strength of 37 GPa and a fracture strain of 0.35% for a 15 nm-thick flake [20]. Since hBN has shown impressive mechanical properties along with exceptional thermodynamic and chemical stability, they have been utilized to influence the mechanical and physical properties of polymers and their composites [21]. As such, hBN could also have a lot of potential as an appropriate additive to improve cement properties. We hypothesized that hBN may improve overall structural performance by strengthening the interfacial bonding in cement and cementitious composites. Moreover, the in-plane thermal conductivity of a single-layer hBN is 751 Wm⁻¹K⁻¹ [22, 23]. Such an excellent thermal characteristic of hBN is especially appealing since it translates into the composite’s ability to transmit and dissipate heat.

Here, we investigated the effect of hBN nanoparticles on the mechanical, thermal, and rheological properties of commercially available class G Portland cement. We explored various loading rates of hBN (1 wt.% and 5 wt.% hBN) and examined its role as a setting accelerant, strength enhancer, and thermal dissipater in both traditional and 3D-printed structures. Interestingly, we observed hBN as an accelerant in cement with its positive effects on structural properties. Our current study suggests that with the addition of hBN, there is a noticeable shift in the heat release rate and a significant reduction in setting time, indicating that hBN functions as an inert accelerator. Mechanical testing of the hBN-reinforced cement specimens revealed that the compressive strength increased by up to 29%, while compressive modulus and toughness improved by up to 45% and 31%, respectively. For 3D-printed structures, hBN further improved compressive strength by 72%, with compressive modulus and toughness increasing by 69% and 88%, respectively. We also investigated the heat dissipation characteristics of hBN-reinforced cement and found that the maximum temperature of the surface decreased as the hBN loading increased. Moreover, the incorporation of hBN reduced the dormancy stage by almost 2 h compared to neat cement. These findings have potential applications in the field of high-performance construction material development and in the 3D printing industry to print strong and durable structures.

Results and discussion

Quick setting behavior of hBN-reinforced cement

The setting behavior of the cement is conceptualized by the heat of hydration and its characteristics over time. Dry cement generally lacks the bond formation capacity with coarse and fine aggregates. When only mixed with water, it obtains the desired chemical bond property. The chemical reactions that transpire between water and cement are called the hydration of cement. The process of hydration is an intricate process that involves several parallel and simultaneous but separate chemical reactions. The reaction begins immediately upon the binder’s interaction with water and is coupled with the release of thermal energy, known as heat of hydration. The key contributors in this respect are dicalcium silicate (C₂S), tricalcium silicate (C₃S), tricalcium aluminate (C₃A), and tricalcium aluminate ferrite phase (C₄AF), whose chemical reactions liberate most of the heat during the hydration process [24]. The complete hydration process takes place in five stages, chronologically named mixing, dormancy, hardening, cooling, and densification [25]. In the mixing stage, aluminates in the mixture react with sulfates and water while releasing heat and forming a calcium-aluminate-sulfate-hydrate (C-A-S-H) gel. The heat of hydration shows its initial peak at this stage, a characteristic that can be attributed to the rapid hydration and consequential accelerated heat generation process. Following the mixing stage comes the dormancy stage, where tricalcium silicates (C₃S) and dicalcium silicates (C₂S) dissolve slowly with the accumulation of hydroxyl ions and calcium ions in the solution. Later, the hardening stage takes charge, which involves the formation of fiber-like Calcium-Silicate-Hydrate gel (C-S-H) and Calcium-Hydrate (C-H) crystals, which are hydration products of C₂S [26]. Such a phenomenon is the direct consequence of the solution being supersaturated with calcium ions. These hydration products of C₂S gradually accumulate on the initial cement grain. The nucleation of these hydration products occurs on the cement grains, followed by the heat release [27]. Similarly, the mixture stiffens as the C-S-H gel meshes with other solids. This marks the beginning (initial setting phase) of the acceleration of hydration and setting, which takes several hours. At this stage, the heat release from the exothermic C₃S reactions peaks, indicating the final setting of the mixture. Right after the final setting with the slowing of the C₃S reactions, the cooling phase is realized by the dropping off of the peaks. Such a drop-off advances the entire cement matrix into the densification stage. It is the last and most critical stage where strength development and permeability reduction continuously occur if water is available for hydration [28].

The focal point of our research is to analyze the hardening phase of cement hydration with the addition of hBN as a setting accelerant and strength enhancer. Figure 1 shows the heat of the hydration plot of the hBN-reinforced cement at various loading rates at well temperature (82.22°C). The first distinction between neat cement and hBN-reinforced cement is the duration of the dormancy stage. For the neat cement, the dormancy stage lasts almost 4 h compared to the hBN-reinforced cement, which does not exceed more than 2 h. hBN-reinforced cement has a relatively short setting acceleration period compared to neat cement, by almost 2 h. This observation highlights the use of hBN as an admixture for the chemical reaction that transpires between the water and the cement particles and helps the cement set faster, also known as an accelerant. As we can see from Fig. 1A, the hBN-reinforced cement exhibits an earlier peak than that of the neat cement. Moreover, the peak is sharper with an increased intensity and a reduced duration of occurrence. The plot also reveals that from the very beginning of the reaction to ∼6.5 h, the heat of hydration is substantially higher for the hBN-reinforced cement than that of the neat cement. After that, the neat cement claimed back the hierarchy of the heat of hydration over the hBN-reinforced cement. Similar qualitative behavior is observed in several other studies [29–31]. Based on the general trend, it can be abstracted that, for hydration products, the hBN nanoparticles act as potential heterogeneous sites for nucleation where CSH gel hydrates can initially form [32].

The grain boundaries were densely populated with nuclei and underwent a full transformation at an early stage of the hydration process [33–35]. Therefore, it influences the rapid release of alkali and Ca2 + ions in the interstitial solution at the very early stage of the hydration process, affecting the formation of hydro-silicates and ettringite according to the following reaction [31].

With an increased rate of reaction, the bonds are being formed, and as a result, an early peak with a reduced dormant period and increased heat release than neat cement is observed. The reduced dormancy period can also be attributed to the improvement in setting kinetics [30]. Moreover, the addition of the hBN nanoparticles increases the hydrates’ production rate, and therefore, the peak of the heat of hydration escalates for the hBN-reinforced cement. Therefore, the reaction mechanism is altered in the presence of hBN, resulting in an early final setting of the mixture at  ∼ 4 h, which compensates for the neat cement by  ∼ 4 h. Moreover, the heat release rate varies with increased loading due to the increased number of available activation sites on the hBN surface. The variation, however, reveals a nonlinear correlation between the loading conditions and the heat of hydration. This is because the maximum distinction is observed between the neat cement and the hBN-reinforced cement with 0.1% loading. However, such a distinction is not substantial when the total heat generation of the neat cement over a period of 20 h, as shown in Fig. 1B, is compared with that of the hBN-reinforced cement. Although the composition of the reaction product is altered because of changes in the CSH structure, this skews only the heat generation rate and does not influence the total heat of hydration. Therefore, it can be abstracted that the compositional changes in the CSH structure do not significantly affect the thermodynamic state of the reaction products. As the total heat generation is a state function and not a path function, it solely depends on the thermodynamic state and composition of the reactants and products of the reaction. Therefore, with a maximum concentration of only 1% hBN, the percentage change in total heat generation is not substantial, although a slight variation in total heat generation can be observed under different loading conditions, which can be attributed to the compositional changes in the reactants and products. Moreover, in the case of increasing the hBN loading percentage from 0.1% to 1%, although we can observe the augmentation of the accelerating effect, it did not result in a proportional increase in the rate of heat production or in the decrease of the setting time. This behavior demonstrates that the addition of hBN loses its effectiveness with increasing concentration.

Heat dissipation behavior of hBN-reinforced cement

To obtain a comprehensive understanding of the heat dissipation characteristics of the cement samples, we synthesized three cylindrical specimens of varying compositions of hBN nanoparticles. The surface temperature curve and intensity of the samples (Neat, 1% hBN, and 5% hBN) are shown in Fig. 2. A careful examination of Fig. 2A reveals that under the same power of illumination, the sample with a higher concentration of hBN has a lower maximum averaged surface temperature. Furthermore, from Fig. 2B, it is evident that the spatial temperature variation has a sharp peak for the neat cement specimen but becomes blunt for the 1% and 5% hBN-reinforced cement. Such behavior can be attributed to the fact that the addition of hBN improves heat dissipation. This is because, with the addition of hBN, an enhanced heat transfer network is formed within the cementitious matrix. The formation of such pathways can be realized from two perspectives. First, the neat cement specimen has a high thermal resistance since its constituents have poor thermal conductivity. Since hBN has a very high thermal conductivity, it improves the overall thermal conductivity of the system. Therefore, heat dissipation occurs at a faster rate, resulting in a lower maximum surface temperature with increased percentages of hBN. Secondly, a lot of pores and microcracks are formed in the neat cement specimen during the release of hydration energy. These pores and micro-cracks are generally occupied by air, water, fly ashes, or other less conductive compounds. Due to the incorporation of these compounds, the total cementitious compound experiences high thermal resistance in the case of heat dissipation. Externally added hBN nanoparticles replace these poorly conducting components, forming a new network that provides a less resistive pathway for thermal energy. Furthermore, hBN nanoparticles promote heat conduction during the hydration process, which prevents the formation of primary microcracks [21]. Also, the addition of hBN reduces the porosity of cementitious structures [36]. Consequently, less space becomes available for the low-conductive materials to occupy and offer low thermal resistance. Thus, the overall heat transfer characteristics of the hBN-reinforced cement improve. Such an improved thermal characteristic prevents crack formation due to a localized temperature gradient and helps transfer thermal energy during friction [37]. Another important aspect of Fig. 2 is the difference between the improvement in thermal characteristics of cement with 1% and 5% hBN. The difference in surface temperature distribution between neat cement and 1% hBN-reinforced cement is more substantial than that of 1% and 5% of hBN-reinforced cement. Such a behavior signifies that the heat dissipation characteristic tries to reach a plateau with increased loading, meaning that beyond a certain threshold, an increased concentration of hBN has no noteworthy contribution to the heat dissipation characteristic.

Mechanical properties of hBN-reinforced cement

The cementitious nanocomposites are multiscale, multiphase, and heterogeneous in nature, and their macro-mechanical properties are inextricably connected to the structure and distribution of the porous network within the cement matrix. The addition of hBN densifies the composite and yields a more compact structure with a refined porous network [38]. Therefore, hBN acts as a reinforcing agent that can substantially modify micro- and nanostructures and improve the composite’s strength. In the present study, the reinforcing effect of hBN nanoparticles in pure cement is studied through uniaxial quasistatic compression tests on solid specimens (5.08 cm × 2.54 cm × 2.54 cm). These tests were carried out at a loading rate of 1.5 mm/min. at room temperature. Figure 3 depicts the plots of stress–strain correlation, compressive strength, compressive moduli, and energy absorption after seven days of hydration for the API cementitious structures.

The addition of hBN nanoparticle increases the compressive strength based on the loading percent of hBN in cement, starting with an ∼8.7% to ∼29% (0.1% hBN to 1.0% hBN) relative to neat cement (Fig. 3A, 3B). Similar modifications can also be observed in nano-BN cement nanocomposite [21]. From a macroscopic viewpoint, the increased strength of a nanocomposite is due to the load transfer effect, in which the external load is shared between the matrix and the nanomaterials, the latter of which typically have greater load bearing capacity, modulus of elasticity, flexural strength, compressive strength, etc. However, to get a complete picture of how the mechanical characteristics of cement are enhanced, we must go beyond the macroscopic processes and understand the microscopic interaction between the hBN nanoparticles and the cement matrix. The micromorphological analysis of the nano-BN-incorporated cementitious composite by Wang et al. [21] can provide great insights in this regard. Based on their analysis, it can be deduced that the presence of hBN in cement augments the Ca/Si ratio both in the interfacial transition zone and the composite matrix. Such an increment occurs as the O–Ca–O bond replaces the Si–O–Si bond in the C–S–H structure. Therefore, the silicate tetrahedron of C–S–H incorporates a high volume of calcium ions. In addition to bond replacement, hBN could also alter the structural unit of the silicate tetrahedron of C–S–H by affecting the polymerization of [SiO4]⁴⁻. Modification of compressive strength is also related to the fact that the addition of hBN improves structural integrity by filling the micropores and reducing porosity. As a result, the stress concentration zones in the cementitious system are also lowered. Moreover, the increased hydration heat transfer rate due to the increased thermal dissipation of hBN results in the densification of the cement [39, 40]. Such a dense structure with a modified porous network delays the crack’s formation. Furthermore, the improved network delocalizes the internal stress and redistributes it, resulting in a substantial decrease in stress concentration in the weak zones of the cementitious structures. Therefore, the physical bridging and chemical modification endow the cementitious compound with high compressive strength. In addition to compressive strength, we calculated the compressive moduli and toughness (total area under the stress-strain curve, which results in energy absorption at failure).

Compressive moduli (Fig. 3C) and energy absorption capacity (Fig. 3D) improve by ∼7% to ∼45% and ∼10% to ∼31%, respectively, compared to neat cement, as the loading percentage increases from 0.1% to 1% of hBN. The hBN itself has a higher modulus than the cement, and therefore, the addition of hBN improves the overall compressive modulus of the composite system. Another factor that affects the compressive modulus of the cement is its porosity. The higher the porosity, the lower the modulus of elasticity. This is because increased porosity promotes less bonding between the cement matrix and the particles. Therefore, there is a negative correlation between the degree of porosity and the compressive modulus. As the addition of hBN to the cement promotes greater bonding by reducing porosity and induces a bridging mechanism within the cement matrix, a substantial improvement in compressive modulus is observed. Moreover, the chemical modification in the C-S-H structure also contributes to the augmentation of the compressive moduli of the hBN-reinforced cement. The increased energy absorption capacity of the hBN-reinforced cement can also be realized from the concept of crack bridging within the matrix. Then, we printed the hBN-reinforced cement samples by direct ink writing technique (Fig. 4A) [41–43] and assessed its mechanical characteristics, as shown in Fig. 4 (B-E). 3D printed structures of hBN-reinforced cement had superior compressive properties than the 3D printed neat sample. For example, the compressive strength of 1% hBN-reinforced printed samples increased by ∼72% over that of neat cement (Fig. 4C). Similarly, compressive moduli (Fig. 4D) and energy absorption (Fig. 4E) have increased by ∼69% and ∼88%, respectively.

While we can see that hBN can improve the mechanical properties of cement significantly, it is very important to compare it with other 2D nanomaterials. Therefore, to understand how different nanomaterials influence the properties of cement, we compared the effects of hBN, graphene oxide, carbon nanotubes, nano-silica, and carbon nanofibers on different mechanical properties [44–48]. Supplementary Table S1 provides a comparative overview of the enhancements observed in cement when modified with various nanomaterials.

Rheological analysis of hBN-reinforced cement ink

The rheological behavior of neat and 1% hBN-reinforced cement ink is shown in Fig. 5. Oscillatory measurements were carried out at different strain rates to determine the viscoelastic properties of the ink. The storage modulus and loss modulus as a function of oscillatory strain (strain%) are shown in Fig. 5A. It is observed that the storage modulus of the hBN-reinforced sample is around 1.1 × 10⁶ Pa at a very low strain rate (∼0.01%), which is 35% higher than the neat sample (∼8 × 10⁵ Pa). The incorporation of hBN significantly increases the storage modulus in ink, which signifies that there can be greater flow recovery for the hBN-reinforced cement ink toward the original state when the shear force is removed. Jiao et al. [49] reported a similar increase in the storage modulus with an increase in the concentration of Fe₃O₄ nanoparticles and theorized that the addition of nanoparticles positively assists the structural buildup of the cementitious paste caused by colloidal interaction at the initial setting stage. A similar explanation also applies here. Regarding the loss modulus, the distinction between hBN-reinforced cement and neat cement ink is not too significant. Although there is some fluctuation between them, they are tightly wrapped around each other, with a variation of 0 to 10%.

Additionally, hBN influences the flow point (intersection of G’ and G’’) of the ink by increasing the shear strain rate from ∼4% for the neat cement to ∼40% for the hBN-reinforced cement ink. The modified cement has enhanced rheological characteristics to be flowable and stable in morphology for 3D printing of any complicated structures. Moreover, the ratio of the loss modulus to the storage modulus for hBN-reinforced cement ink is less than that of the neat cement ink, which clearly indicates that the hBN-reinforced cement has a more prominent solid behavior than that of the neat cement. Hence, hBN-reinforced cement had a better filamentary shape retention quality after being extruded from the nozzle. We also analyzed the thixotropic behavior, which is a time-dependent, reversible breakdown characteristic of the particle network structure due to the application of shear followed by a structural restoration on repose [50]. Figure 5B displays the thixotropic characteristic of the inks through the variation of the complex viscosity with the time using the 3ITT test, demonstrating the quick recovery of the gel structure and viscosity after the application of a shear rate. Initially, neat cement had a higher increment rate for complex viscosity over time than hBN-reinforced cement ink. However, in the case of gel structure recovery, the hBN-reinforced cement ink shows better performance with a higher rate of complex viscosity increment with time than that of neat cement ink.

This work also addresses other rheological characterizations, such as the stress response and viscous behavior of the ink at different strain rates. Figure 6A shows that shear stress reduces with increasing shear strain. This is due to the thixotropic nature, which refers to the negative correlation between shear stress and shear rate. This behavior is best illustrated in Fig. 6B, where the viscosity of both the neat and hBN-reinforced cement inks decrease as the shear rate increases. This is a well-known non-Newtonian fluid phenomenon known as shear thinning behavior. This type of behavior is closely tied to the cement’s structural paradigm. The flow resistance is decreased because of the structural reformation, which also lowers the viscosity. Also, it is clear from Fig. 6B that neat cement and the 1% hBN-reinforced cement induce the same behavior. This demonstrates unequivocally that the additional hBN contributes insignificantly to the structural alterations of the ink when a shear force is applied.

Conclusions

This research sheds light on the effect of the addition of the hBN nanoparticles on the cement from thermomechanical and rheological standpoints. The first drastic change is observed at the initial stage of hydration, where the increased heat release rate with time evidently makes it clear that hBN, when mixed with cement, acts as an inert accelerator that influences the rate of reaction. Acting as potential heterogeneous sites for nucleation where CSH gel hydrates can initially form, the hBN nanoparticles alter the reaction mechanism and reduce the curing time. Moreover, the greater thermal conductivity of the hBN nanoparticles improves the heat dissipation characteristics while reducing the potential for early-stage thermal cracking from the temperature gradient and improving durability. The mechanical behavior is also impacted by the presence of the hBN nanoparticles due to the overall degradation of the porosity. Therefore, the structure is more compact, with improved compressive strength, compressive moduli, and energy absorption capacity. Lastly, the rheological characteristics are also affected by the lamination between the cement matrix and the added hBN nanoparticles. It is observed that the hBN-reinforced cement ink exhibits better gel structure recovery with time than neat cement, although both inks have similar shear thinning behavior under applied shear.

Experimental section

Materials

An API class G cement (Dyckerhoff AG, Germany), hexagonal Boron Nitride nanoparticles (hBN, <150nm, Sigma Aldrich, USA), hydroxyethyl-cellulose (HEC, FSA-3, Fritz Industries, Inc., USA), nanoclay (hydrophilic bentonite, Sigma-Aldrich, USA), 2-Ethyl-1-hexanol (Sigma-Aldrich, USA), and polycarboxylate ether (Ethacryl G, Coatex, USA) were used in this study.

Cement slurry preparation

Dry materials (Cement, HEC, and hBN nanoparticles) were weighed and mixed using a centrifugal mixer to obtain homogenous powder. The powder is then introduced slowly into a blender containing water, aiming for a final slurry volume of 115 ml. API Specification 10-B2 was used for this mixing.

Direct ink writing (DIW) ink preparation

Cement is mixed with appropriate amounts of hydroxyethyl cellulose and nanoclay. Polycarboxylate ether and 2-ethyl-1-hexanol were added to water and mixed with a Thinky Planetary Centrifugal Mixer (AR230, Thinky USA, Inc.) in a 125-ml container at 2000 rpm for 5 min. The dry materials were then added to the container containing the fluids and then mixed using a centrifugal mixer at 1500 rpm for 10 min with 5 14-inch stainless steel balls for better mixing.

Direct ink writing (DIW)

Cement-based ink was 3D printed using a high-resolution 3D printer (Hyrel Engine HR). Using a cold flow syringe head (SDS 150 extruder), the ink was extruded at room temperature on a silicone mat placed on a glass substrate for easy removal post-curing. The ink was then loaded onto a 150 ml Luer-lock syringe with a diameter of the internal nozzle of 2 mm and vibrated, ensuring there were little or no air bubbles. To determine the print path, the slic3r software is used to generate the G-code script based on parameters like geometry, extrusion width, printing speed, and layer height. After 24 h, the parts were immersed in DI water for 7 days to cure.

Uniaxial compression tests—API test specimens

Uniaxial compressive strength (UCS) tests were conducted on the neat and hBN-reinforced cement samples. The specimens (1 × 1 × 2 in) are cast with a water/cement ratio of 0.4 and cured in the water for 7 days before the test. The 3D digital image correlation (DIC) technique is also applied to validate the test results. The speckle pattern required by DIC is sprayed on the same face where the traditional strain gauge is attached. During the test, the loading rate is controlled at 0.002 in/min. The deformation of the specimen is recorded by both a strain gauge and cameras. The postprocessing of the images is done by the Digital Image Correlation Engine. The average strain in the region of interest is compared with the strain gauge reading.

Uniaxial compression tests—3D printed test specimens

At room temperature, uniaxial in-plane compression tests of a solid structure were performed with an Instron 4505 testing machine (Instron, USA) equipped with a 100 kN load cell. The samples were held between two crossheads and compressed at a constant pace of 2.5 mm/min while simultaneously being checked for misalignment and detachment. The load cell of the testing apparatus was used to measure the load and record the movement of the crosshead. The load-displacement data was collected, and at least five specimens were evaluated to ensure the consistency of the data.

Isothermal calorimetry

The influence of hBN on the hydration and setting time of cement slurries was studied using a TAM Air isothermal calorimeter (TA Instruments, USA), which collected data on the heat developed from the cement hydration process at 82.22°C (180°F) over time.

Heat dissipation

In situ surface temperatures were recorded with a thermal IR camera (FLIR, A615) during the measurements. White light from a supercontinuum laser (Fianium, WL-SC-400-8, 400–900 nm, 4 ps, 80 MHz) was applied for light-induced heating without further focusing (gaussian beam diameter D4 = 2.16 mm). A neutral density filter (Thorlabs, NDC-100C-4M) was applied to adjust the total power of illumination to 38 mW. Direct illumination of the thermal camera with the light source did not cause any observable increase in temperature, indicating that the illumination source does not have mid-IR (2–10 m) photons. Therefore, the measured temperature increases during the experiments resulted only from photothermal heating effects rather than the scattering of the incident light. For each sample, a glass Petri dish with a diameter of 93.0 mm was flipped on the optical table to hold the sample, which was placed in the center of the dish. The incident light is aligned to be perpendicular to and centered on the top surface of the sample, and the thermal image was taken at 25° away from the incident light. The focus of the thermal camera was optimized before measuring each sample and kept consistent throughout the measurement with respect to that sample. However, for different samples, the focus may be different depending on the shape and geometry of the sample. The thermal images of the illuminated samples were then collected, and the temperature line profile along different directions on the sample surface was extracted for comparison. In the heat dissipation measurements, the laser illumination was on for 60 s and then off for another 60 s to finish one round of measurement for each sample, during which the maximum surface temperature on the sample was monitored as a function of time.

Rheological characterizations

The rheological properties of the cement ink were measured using a parallel plate measurement system mounted on a stress and strain-controlled rheometer (MCR 302, Anton Parr, Austria). Oscillatory amplitude sweeps were performed at an angular frequency of 1 Hz with strain rates from 0.01% to 100%. To study the thixotropic behavior of the inks, we performed a three-interval thixotropy test (3ITT). The first interval was a 5-min small-amplitude oscillatory shear (SAOS) experiment with the frequency set to 1 Hz and the amplitude of the strain set to 0.1%. The second interval was 1 min of rotational shear at a rate of 20/s, while the third interval was 5 min.

Supplementary data

Supplementary data is available at OXFMAT Journal online.

Author contributions

Vijay Vedhan Jayanthi Harikrishnan (Formal analysis [lead], Investigation [lead]), Maruf Md Ikram (Formal analysis [supporting], Writing – Original Draft [lead]), Sudheendhra Herkel (Formal analysis [supporting], Investigation [supporting], Methodology [supporting]), Wei Meng (Investigation [supporting], Methodology [supporting]), Ali Khater (Investigation [supporting]), Kenneth Johnson (Investigation [supporting]), Peter Boul (Resources [supporting], Supervision [supporting]), Minghe Lou (Investigation [supporting]), Satish Nagarajaiah (Supervision [supporting], Writing – Review & Editing [lead]), and Muhammad M. Rahman (Conceptualization [lead], Supervision [supporting], Writing – Review & Editing [lead]), Pulickel Ajayan (Funding acquisition [lead], Resources [lead], Supervision [supporting])

Funding

The authors gratefully acknowledge financial support from the Aramco Services Company, United States.

Conflict of interest: The authors declare no conflicts of interest.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

References