Eco-efficient recycling of engineering muck for manufacturing low-carbon geopolymers assessed through LCA: Exploring the impact of synthesis parameters of the performance

doi:10.21203/rs.3.rs-4591486/v1

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Research Article

Eco-efficient recycling of engineering muck for manufacturing low-carbon geopolymers assessed through LCA: Exploring the impact of synthesis parameters of the performance

https://doi.org/10.21203/rs.3.rs-4591486/v1

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published 05 Sep, 2024

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The construction industry's excessive reliance on cement has led to significant environmental concerns. With the push towards global low-carbon and sustainable development goals, there is an urgent need to find building materials that can replace cement. In this study, engineering muck (EM) produced by foundation pit engineering in subtropical area was used as raw material. The properties of EM were activated by pre-treatment methods to prepare low-carbon geopolymers. This study investigated the effects of synthesis parameters (SiO₂/Na₂O ratio and liquid-solid ratio) on the performance of the alkaline activated EM-based geopolymers. The results showed that the geopolymer with a SiO₂/Na₂O ratio of 1.5 achieved the highest compressive strength of 40 MPa in 7 days, exhibiting the densest structure and fewest cracks. In addition to also having the smallest pore sizes and highest thermal stability, indicating optimal pore structure for minimizing evaporation. This study showed that increasing the liquid-solid ratio refined the pore structure, but increased carbonate formation and mass loss at elevated temperatures. Moreover, a life cycle assessment (LCA) was used to compare the cradle-to-gate environmental impacts of the EM-based geopolymers and cement concretes, including global warming and acidification. The LCA demonstrated the CO₂ and SO₂ emissions of EM-based geopolymers were reduced by 4–26% and 8–19%, respectively, compared to concrete. This study suggests the use of alkaline activation technology to transform the EM into the geopolymers should be expected to become a substitute for concrete, providing a new type of green building material for the geotechnical engineering.

Engineering muck

Geopolymer

Synthesis parameters

Green building materials

Life Cycle Assessment

The acceleration of urbanization has been accompanied by the production of a large amount of construction waste [1]. Construction waste in China experienced a dramatic rise from 0.47 billion tons in 2006 to 3.2 billion tons in 2021, with projections suggesting that the total amount could surpass 4 billion tons by 2026 [2]. Engineering muck (EM), a major component of construction waste, is primarily generated by underground construction activities, such as tunnelling and the construction of underground facilities [3]. EM typically accumulates rapidly over a short period because of limitations in processing technology, which poses challenges to large-scale development and utilization. Currently, the most common treatment method for the EM is direct disposal, which includes landfill, marine landfill and subgrade filling. Among these methods, landfill disposal in waste dumps remains the most common approach due to low cost and convenience [4].

Improper geotechnical risk management at landfill sites can lead to severe safety issues. A typical example is the catastrophic 2015 geotechnical landslide at a landfill site in Shenzhen, China, where the collapse consisted more than 100,000 m³ of the EM, resulting in significant casualties and economic losses [5]. Additionally, landfill disposal of the EM can occupy a large number of valuable land resources, resulting in urban land shortage, especially in large cities. In the practice of geotechnical engineering, the subgrade is usually backfilled with the EM with good performance [6]. However, in 2024, under the influence of heavy rainfall, a highway pavement collapse occurred in Meizhou, China, caused by the landslide of EM, resulting in the death of 48 people. This tragic event caused widespread concern and deep social reflection.

Although the EM has high strength in its natural state, long-term weathering can cause the rock to become loose and the material more porous. Once the water penetrates, the bearing capacity and shear strength would decrease sharply, thus increasing the potential risk of such geotechnical engineering [7]. In view of this, traditional landfill disposal or subgrade filling methods are not suitable for dealing with the EM. Accordingly, it is necessary to optimize the properties of the EM in ways that would provide more scientific and environmentally friendly solutions.

Considering the extensive distribution of the EM in the geotechnical field, most scholars have used EM in combination with simple sieving to make it become an aggregate of concrete materials, which has been used to replace part of the amount of natural sand and improve the performance of concrete [8, 9]. However, few scholars have studied the clay minerals in the EM. The EM in subtropical areas is the product of a combination of climate, rainfall, and temperature factors. The humid environment accelerated the weathering process [10, 11]. This process not only decomposes solid rock and soil but also increases the dissolution and leaching of carbonates, resulting in the formation of clay minerals such as kaolinite, halloysite, and illite [12]. Therefore, the EM can be considered to pre-treated by mechanical and heat treatment mechanisms that activated the chemical activity of clay minerals, making the EM a natural precursor for geopolymers. The process of geopolymerization typically includes the dissolution of silicate and aluminate compounds in a highly alkaline environment, leading to the formation of geopolymer [13]. Accordingly, geopolymerization technology offers the potential to enhance the value of the EM, since the activated EM capable of reacting with the alkaline solutions to produce high-performance geopolymer [14]. Geopolymers are considered as a sustainable building material due to the abundance of raw materials that produce superior durability, mechanical and thermal properties.

Many studies have shown that the geopolymers prepared by clay minerals have excellent mechanical and microscopic properties [15]. These properties of the geopolymers are influenced by a variety of factors, such as the properties of raw materials, synthesis parameters (the type, proportion, concentration of alkaline activators, liquid-solid ratio), and curing conditions, etc [16–18]. As the EM was rich in clay minerals, the EM can be used to prepared geopolymers, which were also affected by these factors. The properties of the raw materials are critical and directly related to the properties of the geopolymers. In addition, the synthesis parameters (proportion of the alkaline activator and the liquid-solid ratio) have also been shown to be key factors affecting the geopolymer, which are the focus of this study.

Based on the study by Duxson et al. (2007) [19], the dissolution of aluminate and silicate minerals as well as the ion type balance, are crucial for alkaline activation materials. The mixture of NaOH solution and Na₂SiO₃ solution is a commonly used alkaline activator in the preparation of the geopolymers, as it can effectively dissolve aluminates and silicate minerals [20, 21]. When exploring the proportions and concentration levels of the alkaline activator, the molar ratio of SiO₂/Na₂O emerges as the most critical factor. Gao et al. (2014) [22] found that when the SiO₂/Na₂O molar ratio was 1.50, the metakaolin-based geopolymer exhibited the largest compressive strength, and the densest microstructure with fewer unreacted particles. Ma et al. (2012) [23] demonstrated that the increase in the Na₂O content can promote geopolymerization, whereas an increase in SiO₂ content can enhance the microstructure of the geopolymers. In addition, water serves not only as the medium for dissolving aluminosilicate precursors and alkaline activators, but also supports the transformation of various ions and the polycondensation of Al and Si ions in geopolymerization. Lizcano et al. (2012) [24] demonstrated that liquid-solid ratio was a major factor affecting the density and porosity of the metakaolin-based geopolymer. These findings highlight the necessity for further research to determine the optimal SiO₂/Na₂O ratio of alkaline activator and liquid-solid ratio design for manufacturing the geopolymers with excellent performance.

From a sustainability perspective, in the production and design of the geopolymers, it is essential not only to consider various properties but also to incorporate environmental factors into considerations. Faced with the contradiction between economic development and ecological carrying capacity, especially in the construction industry and geotechnical engineering, adopting scientific methods to assess environmental impacts and adhering to Life Cycle Assessment (LCA) standards has become a necessary way to achieve emission reduction goals and transition to a resource-efficient society [25]. LCA is used to measure environmental impacts through various metrics, offering researchers an additional perspective for assessing the environmental analysis of preparing geopolymers. LCA has been widely applied to the environmental impacts of the geopolymers to confirm advantages over conventional concrete [26]. Critical to the evaluation of the LCA analysis is the selection of an appropriate mix design [27]. For the geopolymers, the use of the alkaline solutions is an important environmental consideration, and evaluating alkaline levels with LCA can provide effective environmental management strategies. However, the majority of these studies have focused on the fly ash-based or slag-based geopolymers [28–30], and relatively few LCA assessments of the EM-based geopolymers. This gap will limit the applications and development of more sustainable waste management strategies for the EM.

In recent years, the geopolymers have been used in geotechnical engineering fields, such as durable materials, infrastructure construction related to foundation and slope engineering [31], and building materials such as replacing cement [32]. The growth in population and urbanization has led to a significant surge in concrete demand [33]. However, as ordinary Portland cement (OPC) is a primary binder in concrete, the production of concrete is associated with significant energy consumption and dust generation, contributing considerably to carbon dioxide (CO₂) emissions. [33, 34]. Globally, the cement industry accounts for approximately 10% of total CO₂ emissions [35]. With the global drive to reduce carbon emissions, the geopolymers have received increasing attention as a suitable alternative to OPC due to their reduced environmental impact [36, 37]. In particular, the EM as raw materials for the geopolymers makes effective use of these wastes and reduces the environmental burden of waste disposal. The EM-based geopolymers show great potential as building materials, providing a sustainable and low-carbon solution for the building materials industry and geotechnical engineering.

In summary, in the field of geotechnical engineering, the landfill disposal for the EM in the subtropical region must be considered an inappropriate and wasteful of use construction resources. By combining the pre-treated EM and the alkaline solutions to prepare the EM-based geopolymers, it is possible to significantly enhance the utilization efficiency and value of the EM. The properties of the geopolymers are affected by the synthetic parameters, especially the proportion of the alkaline activators and liquid-solid ratio. Although the alkaline solutions used in the geopolymers have less impact on the environment than the traditional cement production process, its production and use process still inevitably causes a certain burden on the environment. Therefore, it is important to use LCA to comprehensively evaluate the environmental impact of the EM-based geopolymers. To develop the geopolymers with better performance and promote their application in geotechnical engineering, the optimal synthesis parameters and the LCA analysis of the alkaline-activated EM-based geopolymers need to be further studied.

Accordingly, in this study, clay minerals-rich EM was obtained through pretreatment. Subsequently, the EM was reacted with the alkaline solutions to prepare the EM-based geopolymers. The effects of different synthesis parameters (SiO₂/Na₂O ratio and liquid-solid ratio) on the development of compressive strength and microstructure in the EM-based geopolymer were investigated. A variety of characterization methods were employed to study the EM-based geopolymers, including X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and thermogravimetric (TG) analysis. Furthermore, the environmental impact of the EM-based geopolymers and conventional concrete was analyzed and compared in terms of global warming (CO₂ emissions) and acidification (SO₂ emissions) through LCA. This study provides better insight into the properties and environmental effects of the EM-based geopolymers in the geotechnical engineering field.

2.1. Materials

The EM used in this study was recycled from a 10 m deep foundation pit in Southern China (lat. 23°, long. 113°).

The EM was dried in a 100°C oven for 24 h, then ground and crushed, and then passed through a 200 mesh (0.075 mm) sieve shaker. According to the results of previous pre-tests with different calcination temperatures, since the calcination of the EM at 650°C had a high activity and the produced geopolymer that preformed well, the calcination temperature of the EM in this study was selected as 650°C. The EM was calcined in a muffle furnace at 650°C, as the precursor, named EM₆₅₀.

The mass percentages of the oxides in the EM₆₅₀, as determined through X-ray fluorescence (XRF) analysis, were detailed as follows: SiO₂ (41.67%), Al₂O₃ (46.61%), Fe₂O₃ (8.40%), K₂O (1.16%), TiO₂ (1.11%), CaO (0.29%) and Others (0.76%). Accounting for 88.28% of the total composition, the main components of the EM₆₅₀ were SiO₂ and Al₂O₃. The surface color of the EM₆₅₀ showed a remarkable brownish red color, which was mainly attributed to the oxidation of iron during the calcination, as shown in Fig. 1a. The laser particle size analysis of the EM₆₅₀ (Fig. 1b) showed that the average volume diameter, average surface area diameter and specific surface area of the EM₆₅₀ were 27.56 µm, 5.36 µm and 414.50 m²/kg, respectively. The EM₆₅₀ exhibited a D₁₀ particle size of 2.12 µm, D₅₀ particle size of 13.45 µm and D₉₀ particle size of 75.54 µm. Based on previous research results, the reactivity of the EM can be improved by reducing the particle size [38].

The SEM image (Fig. 2a) showed that the EM₆₅₀ exhibited particles with plate-like and tubular structures. EDX analysis (Fig. 2b) showed the Si/Al ratios were both 1, identifying the particles as kaolinite and halloysite. Additionally, particles with layered structures were also observed, with EDX results showing a Si/Al ratio of 2 and trace amounts of the element K, which was confirmed as illite. Figure 3 shows the XRD pattern of the EM and the EM₆₅₀, showing the presence of the diffraction of kaolinite, halloysite, illite and quartz. For the EM, the diffraction of kaolinite and halloysite was very strong, whereas for the EM₆₅₀, the diffraction of kaolinite and halloysite was significantly reduced, with a broad reflection observed at approximately 20–30°(2θ), indicating the formation of an amorphous structure.

A commercial sodium silicate (Na₂SiO₃) that consisted of 27.30 wt% SiO₂, 8.54 wt% Na₂O and 64.16 wt% water was provided by Jiashan County Yourui refractory Co, Ltd, China. Chemical-grade hydroxide (NaOH) with a minimum purity of 96% sodium was supplied by Tianjin Fengboat Chemical Reagent Co, Ltd, China. Ultrapure water was added to the NaOH pellets, and the concentration of the NaOH solution was adjusted to 10 mol/L. The alkaline activator solution was prepared by mixing Na₂SiO₃ solutions with NaOH solutions and storing at room temperature for 1 day before use. The mass ratios of Na₂SiO₃ solution to NaOH solution were set at 1/3, 1/2, 1, 2 and 3, with the corresponding molar ratios of SiO₂ to Na₂O being 0.4, 0.5, 0.9, 1.5 and 1.8, respectively.

2.2. Preparation of the geopolymers

The EM-based geopolymers were mixed with the precursor and the alkaline activator in a rotative mixer, with the ratio of the alkaline activator to the precursor set as 0.70, 0.75, 0.80, 0.85 and 0.90. The EM-based geopolymers were cast into 2.0×2.0×2.0 cm silicon molds, vibrated on a vibration table, and then cured at an ambient temperature of 23–25°C for 24 h before demolding. The cured geopolymers were then sealed in plastic bags and cured at ambient temperature for 7 and 28 days, respectively. The obtained products were labelled “GP_M−L”, where “M” stood for the molar ratio of SiO₂/Na₂O ratio, and “L” stood for the mass ratio of alkaline activator to precursor. In this study, the latter was specifically defined as the liquid-solid ratio.

2.3. Characterization methods

Following the with the standard method of ASTM C191, the setting times of geopolymeric pastes with different SiO₂/Na₂O ratios and liquid-solid ratios were performed by a Vicat instrument. An STS100KN electronic universal testing machine was used to test the compressive strength of the EM-based geopolymers at 7 days and 28 days, at a loading rate of 1 mm/min.

The microcosmic characteristics of the EM-based geopolymers cured for 28 days were analyzed. XRD patterns within the 10° to 40°(2θ) range were recorded on a Bruker D8 Advance diffractometer (Mannheim, Germany). The experiment was conducted with the device set to a voltage 40 kV and a current of 40 mA and a scanning speed of 3°per minute, measured in 2θ. FTIR spectra ranging from 4000 to 400 cm^− 1 were collected on a Nicolet iS20 spectrometer (Thermo Scientific, USA). The spectral resolution was set to 4 cm^− 1 and each spectrum was the result of accumulating data from 64 scans. The SEM and EDX analyzes were conducted using a TESCAN MIRA scanning electron microscope (TESCAN, Czech). The accelerating voltage employed was 15 kV, ensuring high-resolution imaging and accurate elemental analysis. MIP analysis was performed with an AutoPore Iv 9600, a device manufactured by Micromeritics, USA. This instrument was used to conduct a comprehensive analysis of the pore size distribution and the cumulative pore volume within the samples. The intrusion pressures ranged from as low as 0.3 to as high as 33,000 psi, with corresponding minimum and maximum pore sizes of 800 µm and 5 nm, respectively. Thermogravimetric (TG) analyzes of the EM-based geopolymers were conducted using a Netzsch TG 209 F3 analyzer (Germany). The temperature for the analysis was set from a low of 30°C to a high of 1000°C. The heating rate was controlled at 10°C per minute under a nitrogen atmosphere, ensuring consistent increase in temperature throughout the experiment.

Life Cycle Assessment (LCA) is an analytical method used by SimaPro 9 software (Netherlands) to assess the environmental impacts, the sustainability and feasibility of a specific process. This assessment provides a comprehensive analysis of the various factors that are critical to the preparation of a life cycle inventory. The scope of this study was from cradle to gate, covering the period from material acquisition to the production stages of the geopolymers. The functional unit was defined as the production of 1t geopolymer. The system boundary is shown in Fig. 4. The list of the raw materials acquisition included the EM, NaOH, Na₂SiO₃ and water. The list of production stages included electrical energy for drying, sieving, calcining, mixing and vibrating. It should be emphasized that the EM was a solid waste and the EM used in this study was collected from a nearby location, both the costs of the raw material and transportation were excluded from the analysis, since the purpose of this study is to promote the recycling and utilization of local EM. In SimaPro 9 software, the Ecoinvent 3 database and the widely adopted CML-lA baseline method (V3.05/EU25) were used for the calculations. In this study, the results were classified into two impact categories: global warming and acidification.

3.1 Physical and mechanical properties of the EM-based geopolymers

3.1.1 Setting times of the EM-based geopolymers

Figure 5 presents the initial and final setting time of the EM-based geopolymers. An increase in the SiO₂/Na₂O ratio resulted in an extension of the initial setting time of the EM-based geopolymer from 205 min to 335 min, and prolonged the final setting time from 326 min to 497 min. However, the setting time of GP_{− 1.5−0.75} was less than that of GP_{− 0.9−0.75} (Fig. 5a). The setting time was partially a reflection of the process of dissolution, recombination and polycondensation of geopolymerization [39]. When the SiO₂/Na₂O ratio was low (that is, the relative content of NaOH was high, and the relative content of Na₂SiO₃ was low), the excessive alkali content led to an intense geopolymerization with the aluminosilicates in the precursor, resulting in a rapid setting time, such as GP_{− 0.4−0.75} and GP_{− 0.5−0.75}. Having the appropriate SiO₂/Na₂O ratio ensured that the dissolution and polycondensation processes occurred in a distinct sequence, which accelerated the reaction and shortened the setting time, such as GP_{− 1.5−0.75}. Conversely, when the SiO₂/Na₂O ratio was high (that is, the relative content of NaOH in the alkaline activator was low), the dissolution, recombination and polycondensation processes tended to overlap, preventing complete dissolution of silicaluminate, thereby inhibiting the reaction and resulting in a prolonged setting time [40], such as GP_{− 1.8−0.75}.

The setting time of the EM-based geopolymers depend on the SiO₂/Na₂O ratio of the alkaline activator and are affected by the liquid-solid ratio to some extent. Accordingly, as the of liquid-solid ratio increased, the initial setting time of the EM-based geopolymer increased from 275 min to 330 min, and the final setting time increased from 406 min to 477 min (Fig. 4b). An increase in the liquid-solid ratio led to an elevation in the fluidity. As shown in Fig. 5a, it should be noted that the GP_{− 1.5−0.70} exhibited significantly low liquidity, making the molding process exceedingly challenging. Furthermore, GP_{− 1.5−0.90} showed a liquid state, and the EM-based geopolymer with a liquid-solid ratio of 1 cannot be molded under ambient temperature. Therefore, the analysis of GP_{− 1.5−1} was not included in this study. In addition, the water was involved in the dissolution, hydrolysis and repolymerization, serving as a medium for transporting ions. However, some water within geopolymerization system did not engage in the reactions and eventually evaporated into the ambient atmosphere. The increase in liquid-solid ratio expanded the gap between the EM₆₅₀ particles, preventing the timely formation of a spatial flocculation structure [41], and subsequently extending the setting time. As discovered by Zu et al. (2009) [42], a high liquid-solid ratio accelerated the dissolution and hydrolysis of silicates, although it might otherwise adversely affect the setting times.

3.1.2 Compressive strength of the EM-based geopolymers

Figure 7 shows the compressive strength of the EM-based geopolymers at 7 days and 28 days. There was an increase in the compressive strength of the EM-based geopolymer after 28 days of curing as compared to 7 days of curing, and even exceeded 50 MPa at 28 days for GP_{− 1.5−0.90}. This phenomenon was attributed to further geopolymerization during this period. This process clearly illustrated the necessity for an appropriate curing period.

For the 7 days compressive strength, Fig. 7a shows that the EM-based geopolymers with SiO₂/Na₂O ratios of 0.4 and 0.5 exhibited poor compressive strength. The underlying cause of this phenomenon was that an exceedingly high alkali content in the alkaline activator led to rapid precipitation of the geopolymer, thereby preventing sufficient condensation, and inhibiting the formation of an effective and complete geopolymer framework [42]. As the water evaporated, the structure of the geopolymer was compromised, adversely affecting its compressive strength [43]. Moreover, an excess NaOH solution may have reacted with CO₂ in the atmosphere, leading to the formation of sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) [44]. As can be seen from the Fig. 6b, the surface of GP_{− 0.4−0.75} and GP_{− 0.5−0.75} had distinctive white substances, characterized by a partially frosted appearance and the presence of large pores, resulting in a decrease in the compressive strength of the GP_{− 0.4−0.75} and GP_{− 0.5−0.75}.

Increasing the SiO₂/Na₂O ratio to 1.5 resulted in the 7 days compressive strength of G-1.5-0.75 reaching 42.69 MPa, suggesting that a higher SiO₂/Na₂O ratio can effectively improve the mechanical properties of the EM-based geopolymer. However, increasing the SiO₂/Na₂O ratio to 1.8 resulted in a reduced compressive strength of 27.28 MPa, due to the reduced alkali content in the alkaline activator reducing the reactivity. This was attributable to a reduced content of alkaline failing to create a sufficiently strong alkaline environment, which complicated the disruption of Al–O and Si–O linkages within the silicate, consequently, affecting the condensation polymerization process, and resulting in the formation of only a few robust macromolecular polymeric formations [45].

Figure 7b shows that as the liquid-solid ratio was increased, the strength of the EM-based geopolymer was increased. There was also a corresponding gradual decrease in the surface porosity of the geopolymers from GP_{− 1.5−0.70} to GP_{− 1.5−0.90}, as shown in Fig. 6b. However, this increase in strength was not as pronounced as that obtained by adjusting the SiO₂/Na₂O ratio. The higher the liquid-solid ratio, the higher the water content and alkali content. Although the increase of liquid-solid ratio prolonged the setting time, it was beneficial to the adequacy of geopolymerization [41], and the development of the compressive strength.

3.2 Analysis of mineralogy of the EM-based geopolymers

3.2.1 XRD results

Figure 8 presents the XRD patterns of the EM-based geopolymers with different SiO₂/Na₂O ratios and liquid-solid ratios. The EM-based geopolymers, characterized by different SiO₂/Na₂O ratios and liquid-solid ratios, showed similar diffraction in the XRD patterns, suggesting a consistency in the structural characteristics of these materials. Compared to the XRD patterns of the EM₆₅₀, the diffractions of kaolinite and halloysite in the EM-based geopolymers were reduced, indicating that only a small portion of these minerals were not fully involved in geopolymerization. Additionally, the broad reflection center of the EM₆₅₀ was around 25° (2θ), as shown in the Fig. 3. But the reflection center of the EM-based geopolymers shifted towards higher angles, near 27° (2θ), indicating that the EM-based geopolymers were formed following the reaction with an alkaline solution [16]. Moreover, no significant changes in the diffraction of illite and quartz were observed, after interacting with the alkaline solutions. This phenomenon can be attributed to the inherent stability of quartz, which was known for its exceptional durability under various chemical conditions [46]. Concurrently, the alkaline solutions primarily dissolved the substance on the surface of the illite, yet it maintained its crystal integrity [47].

3.2.2 FTIR results

FTIR spectra of GP_{− 0.9−0.75}, GP_{− 1.5−0.75}, GP_{− 1.5−0.80}, GP_{− 1.5−0.90} and GP_{− 1.8−0.75} are presented in Fig. 9. From the analysis results, FTIR spectra showed the forms of the kaolinite and halloysite-based geopolymers. The absorption bands observed near 3439 cm^− 1 and 1640 cm^− 1 were attributed to the stretching vibrations of -OH bond and bending vibrations of H-O-H bond, respectively [48]. Both kaolinite and halloysite contain hydroxyl groups, essential for their molecular structure [43]. The wide band near 3439 cm^− 1 indicated the completion of dehydroxylation of kaolinite and halloysite. This observation was consistent with the results of XRD analysis, which showed a significant reduction in the diffraction intensities of kaolinite and halloysite in XRD patterns.

The bands observed around 1383 cm^− 1 and 715 cm^− 1 in the FTIR spectra of the EM-based geopolymers were attributed to asymmetric stretching vibration and bending vibration of the O-C-O bond, respectively. The low intensity of this peak suggested that the interaction between the alkaline solutions and atmospheric CO₂ led to the formation of a small amount of Na₂CO₃ [13].

All EM-based geopolymers exhibited a prominent band ranged from 1200 to 900 cm^− 1, associated with the asymmetric stretching vibration of the Si-O-T (Si or Al) bond, which has been instrumental in analyzing the chemical characteristics of the geopolymers [49]. For GP_{− 0.9−0.75}, the wave number was recorded at 1019 cm^− 1. As the SiO₂/Na₂O ratio increased, the wave number of the Si-O-T bond shifted first to the low wave number, and then to the high wave number. Specifically, the wave numbers for GP_{− 1.5−0.75} and GP_{− 1.8−0.75} reached 1016 cm^− 1 and 1022 cm^− 1, respectively. As the liquid-solid ratio increased, a shift in the wave number of the Si-O-T bond towards the lower wave number was observed, with the wave numbers for GP_{− 1.5−0.80} and GP_{− 1.5−0.90} reaching 1015 cm^− 1 and 1012 cm^− 1, respectively. These changes reflected that a suitable Na₂SiO₃ solution provided sufficient Si-OH necessary for geopolymerization, and the [SiO₄] tetrahedra were replaced by [AlO₄] tetrahedra during the polycondensation process [50], forming a stable geopolymer network structure. Therefore, geopolymerization of GP_{− 1.5−0.90} was the most reactive, which was consistent with the above analysis.

The bands observed in the approximate range of 600–800 cm^− 1 were associated with the stretching vibrations of Si-O-Al and Si-O-Si bonds [49]. The absorption band observed near 539 cm^− 1 was attributed to the stretching vibration of the Si-O-Al bond. Meanwhile, the absorption band near 457 cm^− 1 was linked to the bending vibration of the Si-O-Si bond [51].

3.3 Analysis of microstructures of the EM-based geopolymers

3.3.1 SEM results

Figure 10a-c shows the SEM images and EDX results of the EM-based geopolymers with different SiO₂/Na₂O ratios, taking GP_{− 0.9−0.75}, GP_{− 1.5−0.75} and GP_{− 1.8−0.75} as examples. From SEM observations, GP_{− 0.9−0.75} and GP_{− 1.8−0.75} showed numerous pores structures. For the EM-based geopolymer with a SiO₂/Na₂O ratio of 0.9, the rapid setting time and quick evaporation of moisture might be due to the high alkali content, leading to the formation of larger pores, and thereby reducing the compressive strength [43]. However, the EM-based geopolymer with a SiO₂/Na₂O ratio of 1.8 exhibited some spongy structures, indicative of geopolymerization. Moreover, the spongy structures also suggested that the dissolution of Si and Al was insufficient, possibly due to the presence of unreacted clay minerals, resulting in more pores, which adversely affected the compressive strength [52].

In addition, cracks were more specifically observed in the EM-based geopolymers based on the SEM image at higher magnification. Cracks can significantly affect the uniform structure of the geopolymers [53], the GP_{− 0.9−0.75} and GP_{− 1.8−0.75} exhibited more cracks than GP_{− 1.5−0.75}. This was attributed to the dense structure of GP_{− 1.5−0.75} that required more energy to destruct, which is the reason for its higher compressive strength. It is important to note that the presence of pores and cracks on the surface of the geopolymers may be due to the following reasons: i) Bubbles generated during preparation process; ii) Cracks caused by evaporation and shrinkage of alkaline solutions and water during the curing process. The presence of an increased number of pores and cracks in the geopolymer structure facilitated rapid water loss [54]. The EDX results showed that the Si/Al ratios for GP_{− 0.9−0.75}, GP_{− 1.5−0.75}, and GP_{− 1.8−0.75} were 1.48, 1.54, and 1.43, respectively. With the increase in the SiO₂/Na₂O ratio, the Si/Al ratio first increased and then decreased. For GP_{− 1.8−0.75}, the low Si/Al ratio indicated that the low alkali content led to inadequate geopolymerization, impeding the formation of Al-O-Al bonds between [AlO₄] tetrahedra and preventing more Al from being dissolved in the geopolymer network [55]. A high Si/Al ratio leads to a higher geopolymerization, resulting in a more stable geopolymer network structure [56], which contributed to better development of the compressive strength [53]. This indicated that the dissolution was most complete for GP_{− 1.5−0.75}, hence, GP_{− 1.5−0.75} had relatively compact and dense structure, further confirming the above analysis.

Figure 11 shows the SEM images and EDX results of the EM-based geopolymers with different liquid-solid ratios. GP_{− 1.5−0.90} and GP_{− 1.5−0.80} exhibited comparable microstructural characteristics, as the liquid-solid ratio increased, the structures became increasingly compact. This observation confirmed the variation in pore on the surface of the geopolymers from GP_{− 1.5−0.70} to GP_{− 1.5−0.90}, as shown in Fig. 6b. As observed from the magnified SEM images, the matrix surfaces were covered by many fine particles, among which GP_{− 1.5−0.80} had uneven particle distribution (Fig. 11b), while GP_{− 1.5−0.90} had more uniform particle distribution (Fig. 11a). The EDX results showed that the Si/Al ratio of GP_{− 1.5−0.80} and GP_{− 1.5−0.90} was 1.56 and 1.58, respectively. The increase of liquid-solid ratio led to the increase of the Si/Al ratio, which was because more adequate geopolymerization promoted the dissolution of Al. In this process, more [AlO₄] tetrahedra replaced [SiO₄] tetrahedra, which contributed to the formation of stable geopolymer network structure [56], indicating that the increase of liquid-solid ratio was conducive to gepolymerization, consistent with the above FTIR analysis.

3.3.2 MIP results

The assessment of mechanical properties and microstructure was based on pore structures, which included parameters such as porosity, pore volume, and pore diameter distribution. These parameters have been critical in the study of the geopolymer structures. [57]. The pores of geopolymers can be classified by size into the following categories: gel pores (d₁ < 10 nm), capillary pores (10 ≤ d₂ < 100 nm), transition pores (100 ≤ d₃ < 1000 nm) and large pores (d₄ ≥ 1000 nm) [17]. Figure 12 presents the MIP results of the EM-based geopolymers with different SiO₂/Na₂O ratios and liquid-solid ratios, and Table 3 provides a summary of the corresponding pore parameters. Figure 12a shows that the proportion of transition pores was relatively high in GP_{− 0.4−0.75} and GP_{− 0.5−0.75}, accounting for 55% and 57%, respectively. However, in GP_{− 0.9−0.75}, GP_{− 1.5−0.75}, and GP_{− 1.8−0.75}, the capillary pores accounted for the highest proportion, ranging from 60–72%, with large pores accounting for 16–22%. This indicated that most of the pores in the EM-based geopolymers consisted of transition pores and capillary pores. In terms of the liquid-solid ratio, an increase to 0.9 led to the proportion of capillary pores increasing to 80%, an 8% increase over GP_{− 1.5−0.75}. With an increased proportion of capillary pores, the proportion of transition pores and large pores decreased, suggesting a denser microstructure in GP_{− 1.5−0.90}.

From the pore size distribution, it was observed that the largest pore sizes were found in GP_{− 0.4−0.75} and GP_{− 0.5−0.75} (Fig. 12b), with average diameters of 191.61 nm and 171.30 nm, respectively. For GP_{− 0.9−0.75}, GP_{− 1.5−0.75}, and GP_{− 1.8−0.75}, the average pore diameters were 28.82 nm, 23.21 nm, and 30.59 nm, respectively. It was worth noting that among the three, GP-1.5-0.75 exhibited the smallest average pore diameter, in agreement with SEM observations.

Regarding cumulative pore volume, GP_{− 0.4−0.75} and GP_{− 0.5−0.75} exhibited the largest pore volumes of 0.31–0.32 cm³/g. The cumulative pore volume of GP_{− 0.9−0.75}, GP_{− 1.5−0.75}, and GP_{− 1.8−0.75} showed minimal variation, ranging from 0.25 to 0.27 cm³/g (Fig. 12c). Furthermore, the porosity of the EM-based geopolymers ranged between 33% and 39%. Similarly, with an increase in the SiO₂/Na₂O ratio, the porosity initially increased and then decreased.

The results indicated that for the EM-based geopolymers with SiO₂/Na₂O ratios of 0.4, 0.5, the elevated alkaline conditions may accelerate geopolymerization, leading to premature setting time and rapid moisture evaporation. These combined effects tend to result in larger pore structures, significantly affecting the material's density and strength [58]. Among the EM-based geopolymers with SiO₂/Na₂O ratios of 0.9, 1.5, and 1.8, GP_{− 0.9−0.75} had a larger pore volume and size. Whereas GP_{− 1.8−0.75} had a greater number of pores. The pore structure of GP_{− 1.5−0.75} was particularly compact, contributing to superior compressive strength. Moreover, an increase in the liquid-solid ratio led to correspondingly decreases in pore size distribution, pore volume, and porosity. The increase of liquid-solid ratio could enhance the reactivity of the EM₆₅₀ with the alkaline solutions, while the presence of the unreacted particles (e.g. NaOH) could result in coverage on the surface of the EM-based geopolymer, thereby reducing the pore structures [59].

These findings indicated that the proportion of alkaline activators in the EM-based geopolymer was the primary factor influencing both the microstructure and compressive strength, and that increasing the liquid-solid ratio could further improve these properties.

Table 3

Pore parameters of the EM-based geopolymers based on MIP results.
Geopolymer	Pore volume (cm³/g)	Average pore diameter (nm)	Porosity (%)
GP_{− 0.4−0.75}	0.32	191.61	39.66
GP_{− 0.5−0.75}	0.31	171.30	39.53
GP_{− 0.9−0.75}	0.27	28.82	36.73
GP_{− 1.5−0.75}	0.25	23.21	34.51
GP_{− 1.5−0.90}	0.24	21.01	33.94
GP_{− 1.8−0.75}	0.26	30.59	35.78

3.3.3 TG results

To analyze the thermal stability and thermal decomposition process of geopolymers, TG tests were carried out to obtain information on the mass change of geopolymers during high temperatures. Figure 13 shows the TG results of the EM-based geopolymers with different SiO₂/Na₂O ratios and liquid-solid ratios. Based on the changes in DTG (Fig. 13b), the thermal stability of the EM-based geopolymers in this study could be categorized into four regions: below 120°C, 120–300°C, 300–750°C, and 750–1000°C. During the aging process in ambient environments, most of the excess water was released from the geopolymer structure and subsequently evaporated, referred to as free water. In cured geopolymers, bonded water was present when water became physically entrapped within large pores or intergranular sites [60]. The first and second regions involved the mass loss of the free and bound water with clay minerals (e.g., illite). The mass losses in third and fourth regions were attributed to the dehydroxylation of the clay minerals (kaolinite, halloysite and illite) and the decomposition of carbonate, respectively [54]. However, TG curves for all EM-based geopolymers stabilized after 750°C, indicating an absence of significant mass change in this region.

The total mass loss of GP_{− 0.4−0.75} and GP_{− 0.5−0.75} was the largest, there was little difference between them: both of which were 14.1%. This was attributed to the low SiO₂/Na₂O, as the EM-based geopolymer exhibited large pores. through its pore structures, the excess alkaline solutions were evaporated and transferred to the surface in the geopolymer and subsequently reacted with atmospheric CO₂, forming carbonate. The decomposition of the carbonate during heating resulted in mass loss [54]. In the range of SiO₂/Na₂O ratio 0.4–1.5, the total mass loss of the EM-based geopolymers was inversely proportional to the SiO₂/Na₂O ratios. GP_{− 0.9−0.75} and GP_{− 1.5−0.75} showed the total mass loss of 12.7% and 10.6%, respectively. GP_{− 1.5−0.75} had the lowest mass loss and excellent thermal stability. This was due to its dense pore structure, which significantly reduced the evaporation of the alkaline solutions and water [61, 62]. In addition, it was worth noting that the TG curve of GP_{− 1.5−0.90} was similar to that of GP_{− 0.9−0.75}, which was significantly smaller than that of GP_{− 1.5−7.5}. This was because an increase in the liquid-solid ratio corresponded to an increase in the amount of the alkaline solutions, which tended to absorb CO₂ in the air to produce carbonates. These carbonates were susceptible to decomposition at high temperatures. As a result, the loss of the TG curve of GP_{− 1.5−0.90} was greater than that of GP_{− 1.5−7.5}.

However, the total mass loss of the GP_{− 1.8−0.75} was 11.6%, which was higher than the GP_{− 1.5−0.75} and lower than the GP_{− 0.9−0.75}, respectively. When the SiO₂/Na₂O ratio was 1.8, a decrease in the alkali content reduced the degree of geopolymerization, resulting in larger pores within the EM-based geopolymer and facilitating the evaporation of water. This accounted for the higher total mass loss in GP_{− 1.8−0.75} compared to GP_{− 1.5−0.75}. Additionally, the reduction in NaOH content and the relative increase in Na₂SiO₃ content resulted in decreased water buffering capacity. This led to the alkaline activators becoming viscous, producing a sponge-like geopolymer that inhibited the adsorption and desorption of water vapor [58]. This explained why the total mass loss of GP_{− 1.8−0.75} was lower than that of GP_{− 0.9−0.75}. These results were consistent with the above test analyzes.

3.4 Environmental impact analysis

3.4.1 LCA modeling

Figure 14. shows the checklist of the LCA modeling. In this study, the production stages (Fig. 14b) consisted of (1) Drying in an oven (3 kW) for 24 h; (2) Sieving in a sieve shaker (0.125 kW) for 1 h; (3) Calcinating in a muffle furnace (4 kW, heating rate of 10°C/min), which took 1.08 h to heat up to 650°C, followed by 2 h of calcination; (4) Mixing in a rotative mixer (0.550 kW) for 1 h; and (5) Vibrating in a vibration table (0.550 kW) for 1 h. It was worth noting that the design ratio of concrete in this study referred to the research of Li et al. (2022) [63]. In addition, the preparation of concretes did not require the calcination process, and the rest of the process was the same as for the preparation of the EM-based geopolymers. In this study, the environmental impacts, including global warming and acidification, of these EM-based geopolymers and concretes were analyzed by the LCA from the cradle to the grave.

3.4.2 Global warming for the EM-based geopolymers and ordinary concrete

Global warming, characterized by the long-term increase in Earth's surface temperature, is largely attributed to human activities, primarily the emission of greenhouse gases such as CO₂ into the atmosphere [64]. Figure 15 shows the global warming for the EM-based geopolymers and ordinary concrete. The EM-based geopolymers and ordinary concrete contributed most significantly to global warming during the material acquisition stage, accounting for 50–60% of the impact. Furthermore, the electricity consumed during the production stage also contributed to this impact.

The carbon dioxide (CO₂) produced by the EM-based geopolymers with different SiO₂/Na₂O ratios ranges from 249 to 252 kg CO₂/t, as shown in Fig. 15a. As the ratio of SiO₂/Na₂O increased, there was a corresponding decrease in CO₂ emissions. This reduction can be attributed to the decrease in NaOH usage as the SiO₂/Na₂O ratio rose, coupled with the fact that the emission of NaOH significantly surpass those of Na₂SiO₃, with CO₂ emissions from NaOH being 1900 kg CO₂/t compared to 57.21 kg CO₂/t from Na₂SiO₃ [65]. The CO₂ emissions from the EM-based geopolymers with different liquid-solid ratio ranged from 243 to 265 kg CO₂/t, as shown in Fig. 15b. The increase in liquid-solid ratios resulted in higher CO₂ emissions, which could be attributed to the greater utilization of the alkaline solutions. For every increase of 0.1 in the liquid-solid ratio, there was an increase of 10–11 kg CO₂/t in CO₂ emissions. The CO₂ emissions from the ordinary concretes with different strengths ranged from 217 to 338 kg CO₂/t, as shown in Fig. 15c. As the strength of concrete increased, the CO₂ emissions also increased, mainly due to the higher cement content, which had CO₂ emissions of up to 930 kg CO₂/t. Notably, the CO₂ emissions from the EM-based geopolymers prepared in this study fall within the range of C10-C20. The CO₂ emissions from geopolymers were reduced by 4–26%. Specifically, the compressive strength of GP_{− 1.5−0.75} exceeded that of C40, while its CO₂ emission remain below 88 kg CO₂/t compared to C40.

For geopolymers, the preparation of the alkaline solutions during the material acquisition stage was the primary contributor to the cause of global warming (Fig. 15d). Significant amounts of alkaline activator are required for the geopolymer production, with 0.28 t Na₂SiO₃ solution and 0.14 t NaOH solution being necessary to produce 1t GP_{− 1.5−0.75}. The production of the activators is highly energy-intensive, involving: (1) the production of Na₂SiO₃ requires melting silica sand and soda ash at approximately 1400°C; (2) the production of NaOH through the electrolysis of salt water, which is an energy-intensive process [34]. However, for ordinary concrete, the use of cement during material acquisition is the main contributing factor to global warming (Fig. 15e). Therefore, controlling the usage of the alkaline solutions and cement is essential to effectively reduce CO₂ emissions.

3.4.3 Acidification for the EM-based geopolymer and ordinary concrete

Acidification involves the release of acidic substances into the environment, primarily due to emissions of sulfur and nitrogen oxides. Upon reacting with atmospheric water vapor, these oxides are converted into acidic compounds that settle on the Earth's surface [66]. Figure 16 shows the acidification for the EM-based geopolymers and ordinary concrete. Figure 16a shows that the sulfur dioxide (SO₂) emissions of the EM-based geopolymers with different SiO₂/Na₂O ratios were consistent at 1.76 kg SO₂/t; however, as the SiO₂/Na₂O ratios ratio increased, there is little change in the proportional of the SO₂ emissions during the material acquisition and production stages. The emissions of SO₂ from the EM-based geopolymers with different liquid-solid ratios ranged from 1.73 to 1.84 kg SO₂/t, and the liquid-solid ratios were positively proportional to the SO₂ emissions (Fig. 16b). Figure 16c shows that the SO₂ emissions from the ordinary concretes with different strengths vary from 1.56 to 2.17 kg SO₂/t. With an increase in concrete strength, the SO₂ emissions increased correspondingly, primarily due to increased cement content. Notably, the SO₂ emissions from the EM-based geopolymers prepared in this study were reduced by 8–19%, compared to ordinary concrete.

Unlike global warming, the primary impact of the EM-based geopolymers on acidification occurred during the production stage (Fig. 16d). This was due to the emission of SO₂ resulting from electricity usage. Electricity generation, predominantly from coal combustion, significantly contributes to the SO₂ emissions [67]. However, for ordinary concretes, the material acquisition stage, specifically cement production, also played a major role in acidification (Fig. 16e). Cement production processes emit substantial amounts of CO₂. As atmospheric CO₂ concentrations rise, the oceans become more acidic. The absorption of large amounts of CO₂ by the oceans results in a decrease in seawater pH, which can have detrimental consequences for marine life [68]. Therefore, to reduce SO₂ emissions, it is necessary to decrease the consumption of electricity and cement.

As we all know, cement is a vital building material. The cement industry consumes substantial energy and significantly contributes to CO₂ emissions. The required temperature reaches up to 1450°C for the formation of stable cement clinker [69]. However, this study revealed that calcined EM at only 650°C was sufficient to produce geopolymers with excellent properties. In summary, this EM-based geopolymer production method can significantly reduce electricity and coal consumption, thus reducing SO₂ and CO₂ emissions, which would be in line with the concept of environmental protection.

The above results have demonstrated that the synthesis parameters have a positive effect on the performance of the EM-based geopolymers. Among them, the SiO₂/Na₂O ratio has the most significant effect on the mechanical properties and microstructure, consistent with the studies of Gao et al. (2014) [22] and Mu et al. (2018) [70]. In this study, the SiO₂/Na₂O ratio was adjusted by fixing the amount of sodium silicate solution and adjusting the amount of NaOH solution. The experimental results show the excellent performance of geopolymers prepared from clay minerals-rich EM. When the SiO₂/Na₂O ratio reached 1.5 in this study, the EM-based geopolymers exhibited optimal compressive strength and the most compact microstructure due to the sufficient reaction. Specifically, the strength surpassed 40 MPa after just 7 days, a performance comparable to that of high-strength concrete. Leong et al.(2016) [71] and Zhang et al. (2022) [43] have shown that there is a need for sufficient soluble SiO₂ and Na₂O to fully promote the condensation reaction and the formation of a dense structure. Therefore, the optimum synthesis parameter of the SiO₂/Na₂O ratio was 1.5 in this study. Sandanayake et al. (2022) [72] and Mir et al. (2023) [73] found that LCA can be used to assess the environmental impact of geopolymers and thus better optimize the synthesis parameters of geopolymers. Based on LCA, this study found that the EM-based geopolymers showed lower CO₂ and SO₂ emissions compared to conventional concrete. These findings underscore their environmental benefits and compatibility with sustainable, low-carbon development strategies. It was worth noting that the liquid-solid ratio also affected the performance of the EM-based geopolymers [74, 75]. Although GP_{− 1.5−0.90} had the best compressive strength and microstructure and pore results, GP_{− 1.5−0.75} showed excellent performance when combined with TG test results and LCA analysis. Therefore, based on the above research results, the optimum synthesis parameter of the liquid-solid ratio was 0.75. Therefore, the use of alkaline activation technology to transform the EM into geopolymers is expected to become a substitute for concrete, providing a new type of green building material for the construction industry and for geotechnical engineering.

This study investigated the effect of synthesis parameters (SiO₂/Na₂O ratio and liquid-solid ratio) on the performance of the EM-based geopolymers. Specifically, the geopolymer with a SiO₂/Na₂O ratio of 1.5 (GP_− 1.5) exhibited superior compressive strength, reaching 40 MPa in 7 days. SEM images revealed that the GP_− 1.5 exhibited the densest structure and the fewest cracks, contributing to its higher strength. MIP results showed that the GP_− 1.5 had the smallest average pore size and cumulative pore volume, with capillary pores accounting for 72% of the total pores. TG results showed that the GP_− 1.5 showed the highest thermal stability and lowest mass loss, suggesting that the SiO₂/Na₂O ratio of 1.5 optimized the pore structure of the EM-based geopolymer to minimize the evaporation of the alkaline solutions and water. Furthermore, increasing the liquid-solid ratio was found to refine the pore structure, and reduce both pore volume and size, which in turn improved mechanical properties. However, an increased liquid-solid ratio resulted in higher carbonate formation due to excess alkaline solutions, leading to increased mass loss upon decomposition at elevated temperatures. According to LCA analysis, the EM-based geopolymers in this study had significantly less impact on global warming and acidification than ordinary cement concrete. Specifically, the emissions of CO₂ and SO₂ were reduced by 4–26% and 8–19%, respectively, underscoring the potential of the EM-based geopolymers for sustainable development and reduced carbon footprint.

The findings of this study can serve as an experimental reference for the use of EM-based geopolymers to replace cement concretes as a building material. Future research on the durability properties of the EM-based geopolymers is expected in geotechnical engineering fields.

Author Contribution

Bingxiang Yuan: Funding acquisition, Visualization, Writing-original draft, Writing-review&editing. Jingkang Liang: Validation, Data curation, Formal analysis, Writing-original draft.Xianlun Huang: Validation, Formal analysis, Visualization. Qingyu Huang:Validation, Formal analysis, Visualization. Baifa Zhang: Funding acquisition, Investigation, Formal analysis, Data curation, Writing-review&editing. Guanghua Yang: Validation, Visualization. Yonghong Wang: Validation. Junhong Yuan: Data curation. Hongyu Wang: Data curation. Peng Yuan: Formal analysis.

Acknowledgement

The authors would gratefully like to acknowledge the support provided by the National Natural Science Foundation of China (No. 52278336, 42302032 and 51978177) and Guangdong Basic and Applied Research Foundation (No.2023B1515020061 and 2022A1515240037). The help from Shiyanjia Lab (http://www.shiyanjia.com) in performing SEM experiments is greatly appreciated.

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Eco-efficient recycling of engineering muck for manufacturing low-carbon geopolymers assessed through LCA: Exploring the impact of synthesis parameters of the performance

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Abstract

Figures

1. Introduction

2. Experimental program

2.1. Materials

2.2. Preparation of the geopolymers

2.3. Characterization methods

3. Results and discussion

3.1 Physical and mechanical properties of the EM-based geopolymers

3.1.1 Setting times of the EM-based geopolymers

3.1.2 Compressive strength of the EM-based geopolymers

3.2 Analysis of mineralogy of the EM-based geopolymers

3.2.1 XRD results

3.2.2 FTIR results

3.3 Analysis of microstructures of the EM-based geopolymers

3.3.1 SEM results

3.3.2 MIP results

3.3.3 TG results

3.4 Environmental impact analysis

3.4.1 LCA modeling

3.4.2 Global warming for the EM-based geopolymers and ordinary concrete

3.4.3 Acidification for the EM-based geopolymer and ordinary concrete

4. Conclusion

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

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