Development of a Highly Conductive Nano-Engineered Geopolymer for Geothermal Applications

A geothermal energy extraction system has been identified as an eco-friendly solution to increasing greenhouse gas emissions because of its stability and relatively low carbon footprint. One method involves circulating a fluid (usually water) through underground reservoirs to capture the heat and then bringing the heated fluid to the surface for generating energy. Geothermal energy harnesses the natural heat from the interior of the Earth. A critical aspect of extracting geothermal energy involves the establishment of efficient heat exchange systems deep within the Earth's crust. These systems rely on the creation of deep wells, wherein pipes are inserted to facilitate the extraction and circulation of the Earth's latent thermal energy. The surrounding material in these deep wells is not only required to provide structural stability but also needs to exhibit exceptional thermal conductivity to optimize energy transfer (Fig. 1).

Traditionally, concrete, typically Portland cement, has been the material of choice for lining deep wells due to its wide availability and general durability. The material is commonly applied to the oil and gas sector, working as a barrier to prevent the wellbore from unintended fluid flows. However, due to obvious changes in the components (i.e., the reaction of carbon dioxide with the calcium hydroxide in hydrated cement under CO₂-rich brine in geothermal wells) and microstructure of cement hydration productions (i.e., calcium silicate hydrate phase change when temperature is above 110 °C or 230 °F), the strength of oil well cement would be damaged by high temperature. The degradation limits the adaptability of common oil well cement for cementing under high temperatures such as geothermal wells [1–4]. The thermal performance of the cement is majorly determined by the water-cement ratio, curing time, and cement content [5–8]. However, the production of cement emits a large amount of CO₂. In 2021, the United States produced 69 million metric tons of carbon dioxide equivalent from cement production facilities, which is nearly 5% of total industrial greenhouse gas emissions [9]. As the quest for improved thermal performance and sustainable construction methods intensifies, alternative materials have come into focus. Enter geopolymers—a class of inorganic polymers derived from natural sources, offering a unique blend of structural integrity and thermal conductivity. Additionally, since the utilization of industrial waste (i.e., fly ash and slag) during the manufacture, the greenhouse gas emissions of geopolymer are 62–66% lower than emissions from cement [10].

Zhang et al. [11] evaluated the performance of geopolymer-based materials with silicon carbide additives under shallow U-shape borehole heat exchangers. The maximum compressive strength could be as high as 135 MPa, while thermal conductivity was 5.35 W/m · K when the temperature was 35 °C. As temperature increases, the compressive strength of the geopolymer significantly decreases. Ogienagbon et al. [12] compared the mechanical performance of geopolymer and industrial expansive cement under elevated temperature conditions at 90 °C using triaxial compressive tests. Compared to the expansive cement, geopolymer had a lower Young's modulus and a higher confined compressive strength [13,14]. Numerical simulations showed that stress in geopolymer is lower than that of expansive cement due to its elasticity. The feature could help the material perform better in downhole dynamic conditions when it experiences stress. Denduluri et al. [15] introduced a novel geopolymer consisting of potassium activators, fly ash, metakaolin, and slag under the environments with 315 °C of bottom hole temperature and 3000 psi of bottom hole pressure. The uniaxial compressive strength could be up to 6000 psi, while it is 325 psi of the tensile strength. The bond strength was three–seven times higher than that of ordinary Portland cement. Paiva et al. [16] investigated three kinds of geopolymer—a base geopolymer, a geopolymer with microsilica, and a geopolymer with microsilica and mineral fiber. The fiber significantly increased the compressive strength, Young's modulus, and tensile strength while the shrinkage was reduced. Moreover, many previous investigations proved that geopolymers perform a degradation-resistance under CO₂-rich conditions [17–21]. Blended cement and geopolymer were also employed in the applications of shallow wells. Table 1 presents the recent studies.

Carbon nanotubes (CNT) and graphite (GRT) are two common materials used to enhance the mechanical strength of cementitious materials [27]. Maho et al. [28] added 0.2% CNT to the high calcium fly ash geopolymer. The compressive strength increased to 34.3 MPa. As a comparison, it was 25.4 MPa for the geopolymer with no CNT. Continuing to increase the CNT to 0.6%, the strength decreased to 32.2 MPa, which still improved the mechanical performance. Similar to the compressive strength, the flexural strength increased by more than 25%. Nadi et al. [20] employed the GRT to enhance the strength and setting performance of metakaolin-based geopolymers. The results showed that when 30% wt% of GRT was added, the compressive strength increased by 49% and the setting time was 663 min. Compared to other amount of added GRT, 30% wt% is the optimized design. MacKenzie and Bolton [29] focused on the tensile strength of clay-based geopolymer with CNT. The additives had a negligible effect. Besides mechanical strength, CNT and GRT are proven to improve thermal conductivity. Samuel et al. [30] evaluated the effect of graphite flakes on the thermal conductivity of geopolymer composites. When 44 vol% of graphite flakes were added, the thermal conductivity had the highest value—8 W/m · K. Zhu et al. [31] reported that the thermal conductivity increased by more than 70% using silica-coated CNT. The enhancement proved the feasibility of the application of geopolymer in areas where thermal conductivity was desired. However, most previous applications were under relatively low-temperature conditions, geothermal environments are not considered. It should be noted that due to the difference in thermal expansion coefficient between CNT/GRT, the integrity of geopolymer composites may be damaged after thermal treatment, resulting in cracks and other types of failures [32,33]. Therefore, investigations of the performance of nano-engineered geopolymers for geothermal applications under high temperatures were important. In this study, highly conductive nano-engineered geopolymer cement tailored for geothermal applications using prototype laboratory samples is demonstrated. The research encompasses an evaluation of thermal conductivity, shear bond strength, and compressive strength of newly designed geopolymer mixtures as well as the thermal conductivity of large-scale geopolymer samples, with a specific emphasis on their performance in the context of fluid flow for enhanced geothermal systems. The objective includes (i) evaluating the thermal conductivity of control geopolymer mixtures as well as the ones modified with CNT and GRT to establish baseline heat transfer properties; (ii) quantifying and comparing the mechanical characteristic of geopolymer mixtures with and without nanomaterials including compressive strength, shear bond, elastic modulus, and Poison's ratio to ascertain their load-bearing capacity and structural integrity; (iii) performing a large-scale prototype to evaluate the borehole exchanger system under geothermal conditions; and (iv) extending the experimental results using numerical simulations.

To comprehensively understand the performance of the highly conductive nano-engineered geopolymer, small-scale CNT and GRT-modified geopolymer samples are compared with the conventional form. A series of experiments are performed to measure the mechanical and thermal properties, such as thermal conductivity, compressive strength, shear bond, elastic modulus, and Poisons ratio. A large-scale prototype, including a pipe loop and cementitious geopolymer, is designed to evaluate the thermal conductivity of the composite incorporating CNT and GRT.

CNT and GRT are selected as optimizer additives for the geopolymer. Tables 2 and 3 summarize the components of the control and modified geopolymers. ADVA 190 is added to increase the workability of the mixture.

A novel mixing method is developed to evaluate the mechanical properties of the geopolymer pastes. The control and modified geopolymer are mixed using a bench shear mixer (Fig. 2). For the control geopolymer, four of seven parts of 10 M NaOH and 3/7 parts of sodium silicate are mixed in a beaker, allowing it to cool for at least 30 min. Next, the sand and fly ash are hand-mixed in a pan using a stainless steel spoon. 30% of the NaOH and sodium silicate solution is then slowly added to the sand and fly ash mixture in a mixing bowl and mixed with a spoon. An additional 30% of the solution is added, and the mixture is mixed for 2 min using a bench shear mixer or drill at a maximum speed of 3000 rpm. The remaining 20% of the solution is gradually added, and the mixing continues for another 2 min. Finally, the required plasticizer is mixed into the last 20% of the solution, which is then added to the geopolymer matrix and mixed for 1–2 min. The geopolymer matrix is then poured into a cylinder with 4 in. of height and 2 in. of diameter, taps to remove trap air, seals, and placed in a water bath set at 49 °C for the required curing time (i.e., 3, 7, 28, and 96 days).

For the modified geopolymer, similar to the control geopolymer, the mixing process begins by thoroughly mixing 4/7 parts of 10 M NaOH and 3/7 parts of sodium silicate in a beaker and allowing it to cool for at least 30 min. Next, the sand and fly ash are hand-mixed in a pan using a stainless steel spoon. 30% of the NaOH and sodium silicate solution is then slowly added to the sand and fly ash mixture in a mixing bowl and mixed with a spoon. An additional 30% of the solution is added, and the mixture is mixed for 2 min using a bench shear mixer or drill at a maximum speed of 3000 rpm. The remaining 20% of the solution is gradually added, and the mixing continues for another 2 min. Then, 25% of the required CNT and GRT are added to the mixture and mixed for 2 min. 20% of the plasticizer is added to the remaining 20% of the solution, which is then added to the geopolymer, CNT, and graphite matrix and mixed for 1–2 min. Finally, the geopolymer matrix is poured into a cylinder, tapped to remove trapped air, sealed, and placed in a water bath set at 49 °C for the required curing time (i.e., 3, 7, 28, and 96 days).

Both control and modified geopolymers are tested under uniaxial compression using an 810 hydraulic servo loop materials testing system. The real-time load and deformation are recorded to determine compressive strength, elastic modulus, and poison ratio. The loading rate is kept at 0.02 in/min (Fig. 3).

The mixing design and procedure for controlled geopolymer and geopolymer with carbon nanotubes are the same. However, there are differences in the preparation of the shear bond sample. A cylindrical mold with a height of 8 in. and a diameter of 4 in. is used for sample preparation. To stabilize the test specimen, two circular wooden plates are fabricated. The first plate is fixed with a 0.75-in. segment of high-density polyethylene (HDPE) pipe via adhesion, while the second top plate serves as a guide to maintain the straightness of the HDPE pipe, as shown in Fig. 4. The HDPE pipe is adhered to prevent the pipe from floating and being lifted due to buoyancy, ensuring the integrity of the measurements taken during the test and allowing for an accurate determination of the sample's characteristics. A shear bond testing apparatus is employed to displace the inner material within the HDPE pipe. A controlled and gradual application of mechanical force from the top is exerted, resulting in the displacement of the material within the HDPE pipe. The real-time load and deformation are recorded to determine the shear bond using the 810 Material Testing System-Servo Hydraulic Loop. The loading rate is kept at 0.02 in./min (Fig. 4).

Following the established mix design procedure, the material is prepared for density measurement using a pycnometer. The process involves accurately weighing an empty pycnometer, filling it with water to determine its volume, draining the water and drying the pycnometer, placing the prepared material into the pycnometer, and weighing the final assembly. The density of the material can then be determined by the ratio of the mass of the material to the volume.

The Conductivity test is performed with a Tempo Thermal Analyzer. The first hole is drilled in the sample and a small layer of Arctic Silver 5 thermal paste is applied onto the sensor. The sensor is then inserted into the hole, and then, readings can be recorded every 2 mins.

The large-scale prototype experiments aim to evaluate the thermal conductivity of a geopolymer composite incorporating carbon nanotubes and graphite in pipe-geopolymer systems. Before experimenting, developing appropriate testing methodologies and protocols to ensure validity and accuracy is crucial. The large-sized Geopolymer sample has a height of 32 in. and a diameter of 12 in. and is molded within a construction mold. The construction mold is fixed onto a base plate, which has a thickness of 0.75 in. and a diameter of 12 in.. The sample is adhered internally and externally to the construction mold using high-strength construction glue to prevent any leaks. Additionally, two HDPE pipes connected at the bottom (U-shaped) are positioned inside the construction mold and secured onto a wire spacer, which is located 2 in. above the base plate. This placement was done to prevent uplift pressure and ensure the pipe was positioned straight and upright (Fig. 5).

In accordance with the mix design proportions (see in Sec. 1), the three materials (sand, fly ash, and alkali) are divided into nine separate containers and are poured in three stages. One-third of the total sand is added to an automatic mixer bucket, followed by one-third of the total fly ash material. The dry materials are thoroughly blended for 1 min at a low RPM of 50 utilizing a motorized large mixer. The process is repeated three times. Subsequently, half of the one-third alkali is added to the mixture and blended for 1 min at low RPM before increasing the RPM to medium (150 rpm) for an additional minute. The remaining alkali is then added to the mixture, and the RPM is maintained at a medium speed setting for 5 min. Later, the plasticizer is added to the mixture during thorough agitation of the mixture for another 5 min using a heavy-duty hand mixer that operates at a high speed of 3000 rpm. Finally, the mixture is placed into the construction mold and consolidated using a hand-tamping rod to eliminate any air pockets. The process was repeated two more times.

To ensure a uniform temperature distribution across the entire sample, an 8-in.-thick heat blanket is wrapped around the mold, with a 1-in. gap between the insulation and the blanket. The heat blanket serves as a source of heat, and its temperature is regulated by a temperature controller, which is set to maintain a temperature of 49 °C. A construction mold that measures 14 in. in thickness and is encased with 1-in.-thick ceramic insulation is designed. The insulation is strategically positioned to prevent any heat dissipation during the curing process, which necessitates a constant temperature of 49 °C. The ceramic insulation effectively seals the sides of the sample, while the bottom and top of the mold are hermetically sealed using an expanded polystyrene (EPS) insulation foam board. To continuously monitor the temperature of the sample, a real-time data acquisition system consisting of three thermocouples is installed at the top, middle, and bottom of the sample to monitor the temperature changes. The geopolymer sample is cured for 7 days at 49 °C and rested for 2 days at room temperature before testing (Fig. 6).

After the curing, the insulation mold is removed. A water bath is utilized to provide consistent bottom heat. Embedded thermos couples (Fig. 5(a)) and data acquisition systems are used to collect data. A fluid circulation system is set to pump the water from the tank to the inlet pipe, circulate through the geopolymer system, and return (Fig. 7).

A 3D numerical simulation is developed to verify thermal conductivity tests of the large prototype samples using the finite element software package ANSYS. The dimensions of the transient thermal model are the same as the experiments shown in Fig. 8. Fine meshes are applied with the average mesh size of 0.08 in. for the pipe as well as inside water, and 0.2 in. for the outside geopolymer. The prototype is heated from the bottom at 49 °C. The ambient temperature is set to 23 °C. The properties of the geopolymer are based on the experimental tests. Table 4 presents the properties used in the simulation.

Figure 9 illustrates the compressive strength, elastic modulus, and Poisons ratio of the control geopolymer matrix. The compressive strength and the elastic modulus increase with increasing curing days. However, no significant difference is observed in the strength and modulus at 28 and 96 days of curing. The Poison's ratio also slightly decreases with the curing time.

Figure 10 presents the strength and modulus measurements of geopolymer with 0.2% CNT and 1% GRT. The compressive strength is about 2527 psi and the elastic modulus is about 1205 Ksi at 3-day curing time, showing lower than the control geopolymer matrix. The modified geopolymers are not completely cured with some moist pockets within the samples. Poison's ratio is 0.32, higher than the control geopolymer. Nevertheless, the strength and modulus of the modified mixture keep increasing with the increase in curing days. At 96 days of curing, the compressive strength of modified mixtures is slightly higher than the control mixtures. On the other hand, the elastic modulus of the modified mixture is about 22% lower than the modulus values of the control ones.

Shear bond tests are conducted for geopolymer mixtures at 3, 7, and 28 days. Figure 11 shows the shear bond results of the control samples and modified geopolymer cured at 49 °C. For the controlled geopolymer, the shear bond first increases for 7-day curing period and then decreases significantly at 28-day curing period. This is because at 28 days of the curing period, the majority of the alkaline reacts with fly ash and most of the water in the solution evaporates, creating micro-level shrinkage. Thus, there is a loss of bond between the pipe and the geopolymer mixture. An interestingly similar phenomenon is observed in modified mixtures but with lower shear bond values.

The density of the control geopolymer is 1.908 kg/m^3, while it is 1.930 kg/m³ of the modified mixture. The identification of varying densities between the conventional geopolymer and its counterpart modified with CNT and GRT reflects the intricate interplay of material composition and nanostructural enhancements. The disparity in densities can be attributed to the distinct physicochemical characteristics introduced by the incorporation of CNT and GRT. Carbon nanotubes and graphite, owing to their unique nanoscale morphology, exhibit altered volumetric interactions within the geopolymer matrix. The nanostructures, when introduced, have the potential to occupy additional spaces within the material structure, leading to an increase in overall density. Conversely, the conventional geopolymer, devoid of such nanostructural inclusions, maintains a comparatively lower density.

The summary of thermal conductivity test results is shown in Fig. 12. The thermal conductivity of control and modified mixtures at 3-, 28-, and 96-day curing is similar. At 7 days, the control geopolymer is more than 45% higher than the modified mixture. This is because most of the alkaline is reacted, and the sample becomes dried. Thus, there is not much impact on thermal conductivity values. Unexpectedly, the thermal conductivity values for modified mixtures are slightly lower than the control ones. This is attributed to an increase in air void contents in the modified mixtures. The addition of nanomaterials created more micro and macro voids, causing hindrances in heat flow. This aspect is further discussed below.

As shown in Figs. 13(a)–13(c), the Portland cement has almost negligible voids compared to control and modified geopolymers. Voids, also known as pores or air pockets, within a material can significantly impact the thermal conductivity. When it comes to heat transfer, voids can act as barriers that impede the movement of heat due to poor conductivity compared to solid materials. When a material contains voids, especially if they are interconnected or numerous, they can act as insulating pockets. Heat energy attempting to move through the material encounters these voids, and because of the lower thermal conductivity of air compared to the solid material, the heat transfer process is slowed down.

The discovery of air pockets in the geopolymer when using CNT and GRT mixtures, which resulted in thermal conductivity, calls for a closer look. The increased occurrence of air pockets in this mixture is likely due to factors that need careful consideration. Incorporating nanostructured materials like carbon nanotubes and graphite into a matrix poses challenges due to the inherent characteristics of these nanomaterials. Ensuring dispersion at the nanoscale level is quite complex because of agglomeration and inadequate wetting behaviors.

This can result in the creation of localized regions with elevated porosity, acting as air pockets upon curing. This phenomenon can be further exacerbated by limitations in current mixing technologies. Some advanced mixing technologies, such as vacuum-assisted degassing, may mitigate the formation of voids.

The thermal conductivity test of large geopolymer samples cured for 7 days at 49 °C is shown in Figs. 14 and 15, respectively. The tests are conducted for around 3 days. The temperature observed in the bottom sensor increases rapidly since the samples are immersed in the water bath, which serves as the source. The modified geopolymer performs better than the control sample. For example, the bottom sensor, which is closest to the heat source, takes 230 mins to reach 40 °C for the control geopolymer sample. In contrast, the geopolymer with CNT and GRT samples achieves the same temperature in 84 mins. Similarly, vertical heat transfer to the middle sensor is also better for modified geopolymers. The middle sensor's temperature increases to 30 °C in 560 mins, while the control geopolymer takes 973 mins. Numerical models verify the observations (Fig. 16). The model can be used for extended studies on the effects of geopolymer properties on the performance of geothermal systems.

Figures 17–19 illustrate the conductivity behavior of different materials for the three discharge rates, 2 gallon/min (Q1), 4 gallon/min (Q2), and 6 gallon/min (Q3). The samples are subjected to an additional heating phase lasting 2 h. This step aims to simulate real-world conditions where the materials are exposed to prolonged heating scenarios. The elevated temperature allows for the observation of any distinct thermal behavior and the influence of the earlier water circulation. By methodically executing this experimental procedure, the study aims to capture the intricate interplay between water circulation, temperature changes, and material properties. The controlled variations in discharge rates and subsequent heating afford a comprehensive understanding of the thermal dynamics exhibited by the samples. Ultimately, this approach furnishes insights into the materials' viability for geothermal applications and their performance under conditions that emulate real-world scenarios.

The control geopolymer performs better than the modified geopolymer. For example, the control geopolymer mixtures exhibit a higher rise in temperature at 7000 s due to fewer air voids in the matrix, especially at Q2 (Table 5). Additionally, in geopolymer mixtures, only the solid particle in an alkaline solution reacts with fly ash, and water is a medium for workability and flow of particles. Water does not play much part in the reaction process. Hence, for long-term curing at high temperatures, the water eventually dries, causing the formation of micro- and macro-level air voids and resulting in lower thermal conductivity at longer thermal curing.

In the relentless pursuit of enhancing geothermal energy systems through innovative materials and rigorous experimentation, this study embarks on an exhaustive exploration of geopolymer composites and their performance within simulated geothermal conditions. By holistically integrating material synthesis, thermal conductivity assessments, water flow analyses, shear bond strength measurements, and compression strength evaluations, a comprehensive understanding of material behavior unfolds, contributing to the evolution of sustainable energy solutions. Experiments with pipes reveal how effectively heat is transferred from the geothermal source to the working fluid, guiding the optimization of pipe materials, diameters, and flowrates for maximum thermal efficiency. Scaling up requires ensuring efficient heat transfer across larger volumes of fluid and longer distances. Additionally, the experiments underscore the importance of insulation in maintaining heat within the system; insights gained from scaled-down experiments inform the type and thickness of insulation needed for larger-scale operations to minimize heat loss and maximize energy output. Based on the results and analysis, the following conclusions are drawn:

1. The compressive strength and elastic modulus of geopolymer mixtures increase as an increasing curing period. Compared to the control geopolymer, the addition of CNT and GRT degrades the strength and elastic modulus. The nanomaterials modification of geopolymer mixtures retards the geopolymer reaction causing a decrease in strength and modulus during the early curing period. Overall, the strength and modulus of all mixtures meet the standard requirements for subsurface applications.

2. Poison's ratio of geopolymer mixtures varies with the different curing periods. The CNT and GRT-modified mixtures exhibit slightly higher values as compared to the control mixtures, mainly due to the retention of moisture during the earlier stages of curing. This will give better ductility for the modified geopolymer mixture.

3. The shear bond of the geopolymer mixtures with HDPE pipe initially increases until the 7-day curing period but decreases at the 28-day curing period. This is attributed to the shrinkage of the geopolymers at higher curing periods.

4. The thermal conductivity of geopolymer mixtures decreases with the increase in the curing period due to the loss of entrapped water in the voids. The modified mixtures exhibited on average 15% lower thermal conductivity values. This is attributed to higher micro and macro levels of void formations due to conventional mixing, as observed by optical image analysis.

5. Thermal conductivity assessments unveil the formidable impacts of nanostructured additives on heat transfer capabilities. While traditional geopolymer materials demonstrate commendable thermal performance at water flow, the introduction of CNT and GRT prompts a better static heat transfer within the mixture.

6. The numerical simulations evaluate the performance of the geopolymer under elevated temperatures. The models are able to verify the experimental observations and can be used for further studies on the effect of geopolymer properties on the performance of geothermal systems.

This material is based upon work supported by the U.S. Department of Energy, Office of Science under Award Number# DE-SC0021682.

This paper was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent not applicable. This article does not include any research in which animal participants were involved.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.