INTRODUCTION
A geopolymer (GP) material can be defined as an inorganic, amorphous, polymeric, 3D structure consisting of alumina, silica, and alkali metal oxides,1 synthesized from the mixture of an aluminosilicate source with a solution based on amorphous silica dissolved in highly alkaline1–3 or highly acidic4 medium. GP shares chemical similarities with zeolites and may exhibit a semicrystalline structure depending on the geopolymerization conditions.5 Si- and Al-based materials, such as metakaolin, from mineral origin, and fly ash, an industrial waste, are commonly used to produce geopolymers. Highly Ca-based products, such as mining wastes, metallurgical slag, and other industrial by-products5,6 are the baseline for manufacturing alkali-activated materials. The diversity of aluminosilicate sources allows for the creation of many products with varied applications. While this diversity expands their use, it also makes it challenging to standardize production and optimize properties for specific uses. Customizing the production process to fit the source materials and the desired outcome necessitates a detailed knowledge of and control over the involved chemical processes.5,7,8
Metakaolin, derived by calcining kaolinitic clay at approximately 750°C,9 serves as a key aluminosilicate source for GP, making it as reactive as pozzolan. However, unlike ordinary Portland cement (OPC) and alkali-activated materials that form hydrated calcium silicates, GPs undergo a polycondensation reaction involving silica and alumina.5,10 This reaction allows geopolymers to develop strength more rapidly than OPC, often achieving a significant portion of their final strength within 1 to 3 days. However, this rapid strength gain is not universal, and geopolymers may continue to gain strength over longer periods, typically requiring less time than the 28 days needed for OPC.4 The geopolymerization process includes: (1) dissolution of aluminosilicates; (2) polycondensation through the reorganization of precursor ions; and (3) polymerization, which involves precipitation at room temperature.5,7,11 Geopolymers are inherently resistant to high temperatures due to their structure of tetrahedral units of (AlO4)‾ and SiO4, creating a stable, rigid solid structure balanced by Group I cations.4,12
The alkali solution, commonly known as “waterglass”, plays a critical role in this process. It typically combines NaOH and/or KOH, with water and amorphous SiO2,1 facilitating the control of the rate at which strength develops by enabling the hydrolysis of siliceous and aluminum species.5 The chemical characteristics of alkalis are crucial in determining the development and properties of geopolymers, particularly when choosing between sodium (Na⁺) and potassium (K⁺) ions. Sodium ions, with their smaller atomic radius and higher charge density, are known for their capacity to expedite the reaction process more efficiently than potassium ions. This efficiency, however, leads to the formation of a paste with increased viscosity, making it more difficult to achieve a thorough mix.13,14 The enhanced workability offered by potassium-based solutions simplifies the processing of GP. Additionally, the lower viscosity associated with these solutions is beneficial for applications demanding high precision or in areas that are challenging to access.
The specific composition of waterglass, along with the GP's own composition, the granularity of the precursor, and synthesis conditions such as temperature, pressure, and humidity,7,13,14 plays a crucial role in determining the materials’ microstructure. Research has highlighted the importance of identifying factors that might optimize the GP material design.15–19 It was found that the chemical composition of the waterglass is the most important parameter for obtaining maximum compressive strength,17,20 with the H2O/K2O molar ratio affecting paste fluidity,9 and K2O/Al2O3 influencing polymerization degree.20 Excess of water increases porosity,1,21 while the SiO2/Al2O3 ratio impacts strength development,9,14 with higher Si:Al ratios leading to more drying shrinkage due to capillary deformations.22 Added to that, the strength of GP is also dependent on the presence and distribution of defects such as cracks and voids.14
Previous works have used isothermal calorimetry to quantify geopolymerization through heat release data.7,9,23 Findings indicated that Na-based waterglasses were roughly three times more reactive than K-based waterglasses over a period of 7 days, though both showed similar strength values after 28 days.2 Still, a major obstacle in implementing the GP technology in similar applications to that of concrete is their tendency to leach alkalis when cured in water,24 resulting in efflorescence issues, especially for Na-based formulations.25,26 This leaching is controlled by (1) diffusion, where a concentration gradient exists between the pore solution and surface; and (2) solubility, requiring either new, less soluble surface phases or similar concentration levels between the surface and pore solution.27
Potassium-based geopolymers offer additional advantages over sodium-based alternatives for cementing applications, including better flowability, thermal stability, and a reduced tendency to leach.28,29 However, limited research has explored their application under similar conditions. Kamali et al.30 and Singh and Subramaniam21 found that both potassium-waterglass (K-WG) geopolymers and Na-WG-based geopolymers synthesized with low-calcium fly ash, respectively, did not achieve significant strength quickly enough for oil well cementing, which demands rapid initial setting and short wait-on cement times. Notably, studies focusing on metakaolin-based geopolymers under similar applications are absent. The scarcity of research on metakaolin-based geopolymers for similar applications highlights a critical gap in the field.
Optimizing the reaction kinetics of potassium-based geopolymers is vital for these applications, particularly given the potential benefits of using highly reactive precursors like metakaolin. Understanding how metakaolin influences the performance of geopolymers under varied ambient conditions could lead to significant advancements in their application, especially in areas requiring rapid strength development and environmental resilience, such as deep-sea drilling, geothermal energy production, and high-temperature industrial processes. Therefore, this research focused on understanding the initial reaction mechanisms of metakaolin-based geopolymers, utilizing isothermal calorimetry to compare four K-WG formulations. An ultrasonic cement analyzer (UCA) was employed to link these calorimetric observations with compressive strength evolution. Moreover, to explore strength development and leaching resistance across different curing environments (dry and saturated), comprehensive tests including destructive compressive strength assessments and pH analyses were performed. This approach provided valuable perspectives on the potential applicability of these materials.
EXPERIMENTAL PROGRAM
Materials and geopolymer design
To investigate the influence of varying H2O/K2O and SiO2/K2O molar ratios on the early-age reaction kinetics of a metakaolin-based GP, four distinct compositions of K-WG were explored. The reference composition was designed to replicate a well-known geopolymer stoichiometric formula (K2O⋅Al2O3⋅4SiO2⋅11H2O),1 effectively equating to a mixture of 2KOH + 2SiO2 + 10H2O. The next two variations modified the water content in the alkaline solution to 9 and 8 moles, respectively, aiming to examine the effects of increased solution alkalinity, which is conducive to enhanced dissolution and polycondensation processes.5,7 The fourth formulation retained the reference water amount (10H2O) but featured a higher concentration of KOH, corresponding to 2.6KOH + 2SiO2 + 10H2O, to boost alkalinity without significantly affecting rheology. The specifics of each formulation, including their weights, are meticulously outlined in Table 1.
TABLE 1 Composition of K-waterglass formulations, detailing the adjustments in water (H2O), potassium hydroxide (KOH), and silicon dioxide (SiO2) contents by weight.
Formulations | Reagents (g) | Waterglass (g) | ||||
KOH | H2O | SiO2 | K2O | H2O | SiO2 | |
WG1 (2KOH + 2SiO2 + 10H2O) |
272.07 | 436.47 | 291.46 | 228.42 | 480.12 | 291.46 |
WG2 (2KOH + 2SiO2 + 9H2O) |
284.48 | 410.75 | 304.77 | 238.84 | 456.39 | 304.77 |
WG3 (2KOH + 2SiO2 + 8H2O) |
298.09 | 382.57 | 319.34 | 250.27 | 430.39 | 319.34 |
WG4 (2.6KOH + 2SiO2 + 10H2O) |
327.00 | 403.53 | 269.47 | 274.54 | 455.99 | 269.47 |
The preparation of waterglass for this study began with the following steps: (1) the potassium hydroxide (PROQUÍMIOS, 88.9% purity, in lentil form) was dissolved in deionized water; (2) hydrophilic silica fume (AEROSIL—Evonik) was gradually added to the solution; (3) the mixture was stirred on a magnetic mixer for a minimum of 24 h. Following this mixing period, the waterglass was allowed to settle for at least another 24 h prior to use, to ensure it reached thermal equilibrium with its surroundings.
For the GP formulations, a highly reactive form of metakaolin (MetaMax—BASF) was used exclusively as the source of aluminosilicate. The composition of all GP was standardized to a molar ratio of SiO2/Al2O3 = 4, a ratio acknowledged for yielding enhanced mechanical properties.14,31,32
The metakaolin powder underwent analysis using X-ray fluorescence on Shimadzu EDX-800 HS equipment, equipped with a Si/Li type detector and an Rh anti-cathode. Each sample used five milligrams of powder, and the elemental analysis covered elements from C to U. The voltage and amperage settings were adjusted based on the sample type; for metakaolin, settings of 50 kV + 66 µA and 15 kV + 557 µA were used, while for silica, the settings were 50 kV + 129 µA and 15 kV + 359 µA. The findings, detailing the chemical composition of the materials, are displayed in Table 2.
TABLE 2 Oxide composition (% wt.) of metakaolin MetaMax from BASF determined by X-ray fluorescence.
Al2O3 | SiO2 | TiO2 | Fe2O3 | K2O | SO3 | CaO | Others |
50.02 | 48.28 | 1.11 | 0.355 | 0.109 | 0.055 | 0.026 | 0.045 |
Given the chemical compositions of the different waterglass formulations outlined in Table 1, alongside the detailed composition of metakaolin from Table 2, the design for each GP mixture was calculated. The results of these calculations, specifying the precise quantities and ratios for each component in the mixtures, are presented in Table 3.
TABLE 3 Detailed geopolymer composition: Breakdown of chemical ratios and component weights.
Nomenclature (Formulation) |
H2O/ K2O | K2O/ Al2O3 | SiO2/ Al2O3 | Composition (g) | |
Metakaolin | Waterglass | ||||
GP1 (WG1+MK: K2O 4SiO2 Al2O3 11H2O) |
11 | 1 | 4 | 1000.00 | 1890.00 |
GP2 (WG2+MK: K2O 4SiO2 Al2O3 10H2O) |
10 | 1 | 1000.00 | 1830.00 | |
GP3 (WG3+MK: K2O 4SiO2 Al2O3 H2O) |
9 | 1 | 1000.00 | 1750.00 | |
GP4 (WG4+MK: 1.3K2O 4SiO2 Al2O3 H2O) |
8.7 | 1.3 | 1000.00 | 1860.00 |
Metakaolin and waterglass were mixed in the specified proportions, adhering to a meticulously designed mixing protocol to ensure optimal results. The procedure was as follows:
The ingredients were combined using an IKA 60 control mixer, operating at a speed of 1500 rpm for a minimum of 10 min. This step was crucial for achieving a homogeneous mixture and maximizing the reactive potential of the components.
To eliminate air bubbles, which could negatively impact the structural integrity of the geopolymer, the resultant paste was subjected to vibration at a frequency of 3 Hz for 10 min.
For evaluating the uniaxial compressive strength of the GP, cubic specimens measuring 50.8 mm on each side were meticulously cast in metal molds. These procedures were carried out at a controlled room temperature of 21°C. The curing of these specimens was executed under two distinct conditions to assess the impact of the environment on the GP's properties. One batch was fully immersed in water to ensure saturation, while the other batch was left to cure under ambient room conditions, referred to as dry conditions for clarity. To prevent premature dehydration and the formation of cracks, the molds were securely wrapped in transparent acetate sheets, a practice recommended by existing literature.31,33–37
Following casting, the specimens were demolded after one day. For immediate compressive strength assessments, three specimens from each curing condition were selected. Additionally, three specimens allocated for water curing were placed in deionized water, with tests planned for them after a 3-day period. The remaining specimens designated for dry curing were stored in a hermetically sealed container. This arrangement allowed for testing at two additional time points: after 3 days and again at 28 days. These intervals provided critical data for comparing the effects of curing conditions on the GP's development over time.
Testing methods
Setting time measurement and chemical properties
To evaluate the setting time and the degree of geopolymerization for each GP formulation, isothermal calorimetry was employed. This technique involved monitoring the heat of geopolymerization and the heating rate using a Calmetrix model I-Cal HPC calorimeter. The measurements were taken continuously over a period of 7 days, with data recorded at 1 min intervals, all at a constant temperature of 21°C. To ensure that the results were directly comparable across different samples, it was imperative to standardize the weight of the paste for each sample. Therefore, a uniform sample weight of 50 g was chosen for this purpose. The initial setting time considered was the time at half the amplitude of the polycondensation peak, with the final setting time corresponding to the peak value.37 To evaluate the reaction kinetics at early ages, the JMAK model9 was used with Equation (1):
The setting time for all GP formulations was measured over a 7-day period using the Vicat apparatus, in accordance with ASTM C191.38
Nondestructive compressive strength evolution
The UCA technique was deployed to track the development of compressive strength across different GP compositions, complementing the insights gained from calorimetry. This combined dataset aims to shed light on the distinct reaction kinetics and project the final mechanical characteristics of the GPs.
Immediately after mixing, the GP paste was transferred to the UCA device (Chandler, model 4265), where it underwent continuous monitoring over a period of 7 days, with data captured at 30 s intervals, all while maintained at a standard room temperature of 21°C. It is pertinent to acknowledge that this apparatus is traditionally calibrated for analyzing Portland cement pastes. Hence, when applied to alternative materials such as GPs, recalibrations of the equations for compressive strength calculations become imperative. This adjustment is vital due to the inherent differences in density and ultrasonic pulse velocity between GPs and Portland cement paste of similar compressive strength levels. The ultrasonic pulse velocity, a critical factor in these measurements, is primarily dictated by the paste's density and specific characteristics, as highlighted in prior studies.39,40
To ensure the accurate interpretation of ultrasonic strength data for potassium-based geopolymers (K-GPs), a specialized fitting relationship devised by Kamali et al.30 was utilized. This approach, detailed in Equation 2, was specifically tailored to accommodate the unique properties of K-GPs, thereby facilitating a more precise assessment of their compressive strength development over time (Equation 2):
pH measurement
The leaching behavior of GPs was assessed through pH measurements to gauge the potential release of alkaline substances into water. The methodology, adapted from Wang et al.,41 involved preparing powdered samples from parts of GPs that had been cured under both dry and saturated conditions. By grinding these samples into a fine powder, 2 g of the resulting material was then dissolved in 20 mL of deionized water, creating a solution with a 1:10 concentration. This solution was stirred daily over a period of 3 days to ensure a consistent mixture. The 3-day period aimed to ensure a homogeneous mixture for a more reliable comparison of pH values between the geopolymer cured under different conditions.
Additionally, to examine the pH levels of the curing medium itself, aliquots were extracted from 250 cm3 of initially deionized water where cubic GP samples, each with a side measuring 50.8 mm, were submerged. These samples were part of an experiment to observe the pH change over time, specifically on days 1 and 3 of immersion, with 20 mL of water being collected for analysis at each time point.
The concentration of alkalis in these solutions was determined using a pH meter. To ensure accuracy, pH readings were taken after a 5 min stabilization period in the solution. The reliability of each measurement was ascertained by a criterion that there should be no fluctuation in pH greater than 0.25 within a 3 min observation window following the initial reading. This methodological approach provided a detailed assessment of the GP materials’ stability and their interaction with water, crucial for understanding environmental implications and the durability of GP-based structures in aqueous environments.
Uniaxial compressive strength
To determine how leaching impacts the compressive strength of GPs, cubic samples measuring 50.8 mm on each side and cured under both dry and saturated conditions were subjected to destructive testing at room temperature (21°C). The compressive strength tests were conducted using an MTS 810/500 machine, equipped with a 250 kN load cell, and executed at a displacement rate of 0.5 mm/min. To prepare the samples for testing and to guarantee the accuracy of the results, their surfaces were sanded with paper to achieve smoothness. These tests were conducted after 1 and 3 days of curing; however, only GP4 could be evaluated at these intervals, as the other formulations required more than 3 days to harden. Additionally, to establish a baseline for comparison, all sample variations that had been stored in sealed compartments at 21°C for 28 days were also tested. This approach aimed to assess the mechanical integrity of the GPs over the short term and provide a reference point for understanding the long-term effects of curing conditions on GP strength.
The GP samples evaluated in the uniaxial compression tests are systematically represented in Table 4. This schematic layout helps to visualize the experimental setup and categorizes the samples according to their specific curing conditions and the corresponding testing timelines.
TABLE 4 Geopolymers tested in uniaxial compression tests as a function of time and curing medium.
Curing days | |||
1 | 3 | 28 | |
GP1 | - | - | X |
GP2 | - | - | X |
GP3 | - | - | X |
GP4 | X | X | X |
RESULTS AND DISCUSSION
Hardening properties
The calorimetry outcomes for each GP formulation are depicted in Figure 1.
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Figure 1A demonstrates that all samples exhibited a peak postmixing, indicative of the heat generated by the wetting and dissolution processes of the aluminosilicate precursor, as supported by references.9,11,23,37,42 This thermal response originates from the rapid absorption of the alkali solution on the metakaolin surface, leading to the disruption of Si–O and Al–O bonds under the attack of hydroxide ions (OH‾).23,37,43 Such interactions culminate in the formation of poly-sialate oligomers (–Si–O–Al –O–).42,43 A detailed view provided by the magnified section of Figure 1A for the initial hour of curing highlights the dissolution peaks across all geopolymer formulations. The observed data suggests that higher concentrations of hydroxide in the WG fabrication enhanced the dissolution of aluminosilicates, as evidenced by the increased magnitude of the initial peaks recorded for GP3 and GP4 formulations.
Observing Figure 1A, it is noteworthy that a second exothermic peak emerges, uniquely associated with GP4. This peak marks the onset of gel formation and polymerization processes.9,11,23,37,43 During this period, the silicate and aluminate monomers, previously formed during the dissolution stage, begin to polymerize into aluminosilicate oligomers. These oligomers act as foundational geopolymer seeds, which play a critical role in the strength development of geopolymers.23,37,44 Concurrently, polymerization-condensation reactions initiate a structural reorganization, culminating in the formation of a robust three-dimensional network characterized by poly-sialate-siloxo linkages (–Si–O–Al–O–Si–O–),11,42,43 which is essential for the material's final properties. This finding indicates a significant correlation between the KOH and H2O concentrations in the WG and their ability to promote the conversion of precursors. Specifically, lower H2O/K2O levels, as observed for GP4, aid in quickly achieving the optimal concentrations of silicate and aluminate monomers necessary for initiating the polycondensation reaction.23,43
The setting time, closely linked to workability, was evaluated using the Vicat apparatus, and the results are presented in Table 5.
TABLE 5 Setting times determined from the curves obtained by the Vicat tests for all GP formulations (GP1, GP2, GP3, and GP4).
Geopolymer | Initial setting | Final setting |
GP1 | 196 h 40 min | 201 h 0 min |
GP2 | 188 h 30 min | 192 h 20 min |
GP3 | 178 h 10 min | 180 h 20 min |
GP4 | 18 h 30 min | 19 h 10 min |
The initial setting time for GP4 aligns with the value reported by Brandvold et al.45 However, the final setting time was approximately 6 h shorter, likely due to the reduced water content in the alkaline solution. For the other GP formulations, the setting time exceeds 7 days (refer to Table 5). This behavior is primarily attributed to the water and alkali content, which significantly influence the reactivity of the mixtures.
To enhance clarity, the GP4 polycondensation peak shown in Figure 1A was replotted over a shorter timeframe in Figure 1B. This revealed an initial setting of 12 h and 30 min, with a final setting time of 19 h and 30 min. The latter corresponds well with the value obtained with Vicat's Apparatus (refer to Table 5).
Since alkali concentration strongly influences the dissolution and polycondensation stages in geopolymers, lower alkali levels can lead to these stages overlapping, creating combined peaks.23 This was less pronounced observed with GP3, and more pronounced with GP1 and GP2, prompting the graph plot in Figure 1C to show the heat flux from 3 to 7 days of curing. The graph reveals that the polycondensation peak for GP1, GP2, and GP3 starts only after 120 h.
Reactivity in GP systems can also be evaluated by measuring the accumulated heat, as shown in Figure 1D. Simultaneously increased concentrations of K2O and H2O can enhance the dissolution of soluble species and the formation of reaction products, as exemplified by the GP4 formulation, evidenced by its accumulated heat of 200 J/g at 7 days, significantly higher than the 10 to 30 J/g observed for GP1–GP3.
To assess the geopolymerization kinetics, a linear adjustment of ln(-ln(1-α)) versus ln(t) was performed, in line with the JMAK model.9 The outcomes for each geopolymer formulation are displayed in Figure 2. The fit to the model demonstrated an excellent correlation (R2 > 0.95) across all geopolymer formulations. Using the results of this fit, the Avrami parameters were calculated and are presented in Table 6.
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TABLE 6 Avrami's parameters for all geopolymer designs.
GP1 | GP2 | GP3 | GP4 | |
n | 0.417 | 0.461 | 0.464 | 0.621 |
k (h−1) |
1.650 × 10⁻⁵ | 1.355 × 10⁻⁵ | 1.336 × 10⁻⁵ | 6.716 × 10⁻⁶ |
The analysis of geopolymerization across different formulations points toward a one-dimensional diffusion-controlled mechanism, reflected by Avrami exponents (n) being less than 1. This suggests that the mobility of soluble species may be hindered by previously formed nuclei, affecting their radial growth rate.9 Notably, the Avrami exponents (n) increase with lower H2O/K2O levels, suggesting more soluble species dissolved in the waterglass, as evidenced by variations in the first exothermic peak. Additionally, the K2O/Al2O3 molar ratio in GP4, which is 1.3, further supports the efficiency of this formulation in promoting geopolymerization, compared with other formulations with a lower ratio. This higher ratio in GP4 suggests a conducive environment for a more extensive network formation, enhancing the material's structural integrity and potentially its mechanical properties.
Regarding the Avrami growth rate (k), which measures nucleation and growth rates during reactions,9 geopolymers prepared with increased H2O content exhibited higher growth rates. This could be due to either a scarcity of nucleation sites caused by an excess of OH⁻ ions relative to the soluble species in highly alkaline solutions, slowing down the growth rates of reaction products, or the alkaline solution enhancing the dissolution of the precursor's soluble species, thus increasing viscosity and reducing their mobility. Although a highly alkaline solution leads to complete geopolymerization, it might negatively affect the growth rate of individual nuclei.9
Transient state properties
Figure 3 shows the results of compressive strength development (obtained from ultrasonic pulse measurements) for all GP formulations.
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Evidently, the GP4 made it possible to achieve the shortest initial setting time, approximately 13 h, which is compatible with the value calculated from the previous calorimetry data. From that dataset, the other formulations (GP1–GP3) showed changes in the gel structure only after 120 h.
This is attributed mostly to their increased H2O/K2O ratios, consistent with previous reports in the literature,37 which may reduce the rate of geopolymerization23 and thus slow the setting.41 Notably, GP4 demonstrates the most significant initial strength gain, which can be attributed to two key factors: an increase in the alkalinity of the waterglass and a higher K2O/Al2O3 molar ratio relative to the other formulations. This elevated K2O/Al2O3 ratio in GP4 enhances the geopolymerization degree, contributing to its superior performance. This intensified reaction process ultimately results in greater compressive strength values at the same curing age, underscoring the impact of chemical composition on the mechanical properties of geopolymers.
As the geopolymerization process produces heat1 and leads to increased strength, it is reasonable to assume a certain correlation between heat, degree of geopolymerization, and strength.7 Figure 4 shows the relationship between sonic strength and accumulated heat release (obtained by calorimetry) for GP4.
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A linear correlation between heat and sonic strength was found, consistent with results reported in the literature.7,43 Figure 4 shows that the more total heat generated, the greater the compressive strength obtained,37 and the excellent fit found (R2 = 0.99) may be related to the high purity of metakaolin used in the present work. One of the main difficulties in using the ultrasonic cement analyzer is that this equipment is calibrated for cement pastes. Thus, the results generated for the geopolymer are only qualitative, used to suggest the setting and strength gain over time, and should not be used quantitatively to assign compressive strength values. However, a published work provided an adjustment for geopolymer pastes,30 based on transit time values. Therefore, a similar adjustment is proposed here and shown in Figure 5, presenting the new sonic strength curve for GP4.
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As demonstrated in Figure 5, the pulse velocity increases with curing time as the transit time is reduced,39 and this is associated with the transition from the gel state to the hardened state. Over time, the transit time reduces due to the geopolymer strength gain. Corrected sonic strength values are shown in Figure 5. However, these values must be compared with measurements of compressive strength obtained by destructive tests to verify the adequacy of the adjustment made by Kamali et al.30 with the geopolymer formulation of the present work. Furthermore, the development of geopolymer strength is complex, as it is not only a function of the degree of reaction but also how many aluminosilicate units are deposited and interconnected.7
Alkali-leaching
This section investigates the potential for alkali metals to leach from geopolymers during the curing phase, particularly under dry and saturated conditions. The pH levels of the geopolymers, influenced by the curing environment, are meticulously cataloged in Table 7. This evaluation is crucial for assessing the long-term durability and environmental sustainability of geopolymer materials, as alkali leaching can significantly affect both.
TABLE 7 pH values for different unary geopolymers, considering the two curing conditions (dry and saturated).
Cure medium | pH | Variation (%) | |
GP1 | Dry (54% RH) | 10.24 | 13.57 |
Saturated (100% RH) | 8.85 | ||
GP2 | Dry (54 % RH) | 9.80 | −0.31 |
Saturated (100% RH) | 9.83 | ||
GP3 | Dry (54% RH) | 8.95 | −0.11 |
Saturated (100% RH) | 8.96 | ||
GP4 | Dry (54% RH) | 9.30 | −11.40 |
Saturated (100% RH) | 10.36 |
For GP1, the reduction in pH after saturated curing is directly linked to its high water content, which increases the separation of solid molecules. This separation allows more alkalis to become free in the pore solution, facilitating their leaching into the surrounding medium.46 Additionally, the slower geopolymerization process in GP1 contributes to the presence of more free alkalis in the pore solution.
In contrast, GP2 and GP3 showed only minor differences in pH values between the curing media, suggesting that the dominant leaching mechanism for these formulations is diffusion.27
For GP4, the pH after saturated curing was higher than that observed after dry curing. This aligns with the observation that saturated curing delays the geopolymerization process (as reflected in the compressive strength shown in Figure 6B), resulting in more free alkalis being retained within the geopolymer structure. This behavior is consistent with formulations containing high K2O content (Table 3), as also reported by Mokhtari et al.26
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Furthermore, differences in stoichiometry may affect the stability of the resulting geopolymer network, leading to different leaching characteristics.25,27,46 It is also important to note that pH measurements are influenced not only by the amount of alkali but also by the presence of other ions. As a result, pH measurements are most effective for qualitative comparisons of the effects of different curing conditions on the same geopolymer formulation.
An additional analysis involved measuring the pH of the water in which the geopolymer was immersed during curing. This analysis was performed specifically for GP4, as it was the only formulation to harden within 7 days). The results are shown in Table 8.
TABLE 8 pH values of the curing water at 1 and 3 days, after GP4 curing stages.
Solution | pH | Variation (%) |
Deionized water (reference) | 6.66 | – |
Water after 1 day of curing | 7.78 | 16.82 |
Water after 3 days of curing | 12.33 | 85.14 |
As shown in Table 8, the pH value of the water increased during the geopolymer curing. So, also considering the data presented in Table 7, it can be said that in addition to leaching, there is surface washing and solubilization.27,46
Hardened properties
The compressive strength values obtained from the destructive tests conducted are shown in Figure 6.
Compressive strength at 28 days (Figure 6A) increased with increasing K2O concentration, as dissolution was favored and, consequently, enabled a greater degree of geopolymerization.19 Furthermore, the SiO2/K2O molar ratio of 1.53 (corresponding to the GP4 formulation) led to maximum compressive strength, because silica and alumina were intensively dissolved, which enhanced polycondensation.47 It is noted in Figure 6A that the compressive strength decreased significantly when the SiO2/K2O molar ratio was increased from 1.53 to 2 (formulations GP1–GP3); likewise, the compressive strength reduces with an increase in the H2O/K2O ratio, given the same SiO2/K2O ratio (GP1–GP3), which can be related to the reduction in alkalinity. The elevated K2O/Al2O3 molar ratio of 1.3 in GP4 likely contributed to an increased degree of geopolymerization, which could account for its superior compressive strength compared with other formulations after 28 days. This higher ratio facilitates a more efficient polymerization process, enhancing the structural integrity and mechanical properties of the GP.
As can be seen in Figure 6B, the compressive strength of GP4 at saturated curing was about 70% lower when compared with dry curing at 1 day. In this case, a possible explanation is that the degree of geopolymerization for the dry-cured sample was higher compared with the saturated curing due to the faster removal of free water from the waterglass, increasing the alkalinity of the paste and accelerating the gelation of the aluminosilicate particles,33,48,49 well supported by the pH values for GP4 presented in Table 7. However, at 3 days of curing, no significant difference was noted in compressive strength after subjecting the GP4 samples to the two-curing media. It may be related to the slower evaporation of water in the saturated condition, reducing the probability of drying cracking.
The progression in compressive strength observed at different curing stages is closely tied to the amplification of the gel phase and its densification, a phenomenon clearly illustrated in Figure 7. Moreover, this figure also reveals the presence of unreacted white metakaolin particles, pointing to the incomplete reaction of some components within the geopolymer matrix. As detailed in Figure 7B, there's a discernible enhancement in the microstructure's density at the 28-day evaluation, indicating a more advanced degree of geopolymerization. This intensified polymerization process directly impacts the material's mechanical characteristics, as evidenced by a significant 402% increase in compressive strength after 28 days. This substantial gain in strength underscores the importance of the geopolymerization process in the development of the material's structural and mechanical properties over time.
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Comparing the results from Figures 6B and 5, a difference of 8.3% for the compressive strength values at 1 day and of 5.8% at 3 days can be verified. So, it can be concluded that the adjustment performed in the sonic strength values can be used to predict up to a small variation in the compressive strength of the GP4. Although GP3 showed a higher dissolution peak than GP4 (Figure 1A), its lower water/solid ratio may have resulted in higher amounts of unreacted MK particles, and thus the geopolymerization had a lower degree,14 as revealed by Figure 1C. This is one of the key factors that supports the low compressive strength of GP3 when compared with that of GP4.
CONCLUSION
This study provides insightful observations on the impact of varying K-WG compositions on the geopolymerization process and early-age strength development. The findings underscore the critical influence of SiO2/K2O and H2O/K2O molar ratios on these parameters. Notably, within the same SiO2/K2O molar ratio, an increase in the H2O/K2O ratio correlates with reduced compressive strength, likely due to decreased alkalinity and a consequent reduction in polycondensation efficiency. Additionally, the K2O/Al2O3 ratio plays a decisive role in geopolymerization, with a ratio of 1.3 in the GP4 formulation achieving the most rapid polycondensation.
Isothermal calorimetry has proven to be an effective tool for monitoring geopolymerization, where the intensity and timing of the polycondensation peak serve as indicators of setting times, leading to a result comparable to that obtained by the Vicat apparatus. pH measurements reveal that, aside from the stoichiometric composition, the formulated geopolymers exhibit commendable water resistance, hinting at a diffusion-limited leaching process. Furthermore, an increase in the K2O/SiO2 molar ratio is found to mitigate efflorescence, although higher humidity levels tend to exacerbate it.
At room temperature (21°C), waterglass formulations WG1 to WG3 exhibited the least effective dissolution of metakaolin, leading to a delayed polycondensation reaction. This delay is attributed to insufficient alkali content for fostering quicker polycondensation.
Adjustments made in analyzing sonic strength values indicated a minor discrepancy in compressive strength values between 1 and 3 days, thereby affirming the utility of the ultrasonic cement analyzer in predicting the compressive strength of GP4. Moreover, a direct correlation between the total heat released during geopolymerization and strength development, as measured by the ultrasonic cement analyzer, suggests a strong relationship between polycondensation and strength gain in metakaolin-based geopolymers, with an excellent linear fit (R2 = 0.99) observed.
Postsaturated curing, geopolymers displayed reduced compressive strength after 1 day compared with dry-cured specimens, attributable to delayed geopolymerization reactions. However, by day 3, strengths in both saturated and dry conditions leveled, possibly due to slower water evaporation under saturated conditions, which diminishes the likelihood of drying-induced cracking. The observed increase in compressive strength over time is linked to the growth in gel phase quantity and densification.
This investigation has successfully demonstrated a method for optimizing geopolymer formulation and predicting strength through adjustments in SiO2/K2O and H2O/K2O molar ratios. The introduction of a higher alkali ion content is pivotal in geopolymer formation, aiding in charge balance. Specifically, the K-WG composition with SiO2/K2O = 1.53 and H2O/K2O = 8.69 showcased rapid strength gain and minimal leachability, making it an ideal candidate for applications in saturated environments requiring rapid strength development.
ACKNOWLEDGMENTS
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001. The authors also thank the laboratory staff of the Rock Mechanics Laboratory at PUC-Rio, the support of the Materials and Chemistry Technology Service at the Nuclear Engineering Institute of the Nuclear Energy Commission, and the researchers at the Institute of Construction Materials at the Technische Universität Dresden.
Kriven WM. Geopolymer‐based composites. Comprehensive Composite Materials II. 2018;5:269–280. [DOI: https://dx.doi.org/10.1016/B978-0-12-803581-8.09995-1]
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Abstract
This study delves into the impact of different potassium‐waterglass (K‐WG) compositions on the early reaction dynamics and strength evolution in metakaolin‐based geopolymers (GP). By maintaining a constant SiO2/Al2O3 ratio of 4, the study explores the influence of varying H2O/K2O and K2O/Al2O3 ratios on GP properties under both dry and saturated curing conditions. Early reaction kinetics are examined using isothermal calorimetry at room temperature (21°C), and pH measurements provide insights into alkali leaching. A strong correlation was found between total heat release and strength gain, as evidenced by ultrasonic cement analyzer (UCA) readings. The study further identifies that increased H2O/K2O ratios prolong setting times and delay the geopolymerization peaks, while a higher K2O/Al2O3 ratio enhances the geopolymerization process. Vicat tests confirmed the results obtained by calorimetry and UCA: only the GP4 formulation (H2O/K2O = 8.7 and K2O/Al2O3 = 1.3) hardened in less than 7 days. Additionally, it was found that saturated curing conditions decelerate strength development, with an initial notable decline in compressive strength at 24 h compared with dry curing. However, this difference diminishes to a negligible 7.6% after 3 days. Optimal ratios of H2O/K2O = 8.7 and K2O/Al2O3 = 1.3 were determined to be critical for achieving reliable strength measurements at 1 day of curing. pH assessments indicated strong water resistance in all GP formulations, with leaching primarily governed by diffusion mechanisms. Specifically, the K‐WG composition with SiO2/K2O = 1.53 and H2O/K2O = 8.69 showcased minimal leachability. These fundamental findings are crucial for the later design of GP materials that require rapid strength development, especially crucial for applications necessitating cementing under extreme conditions, such as deep‐sea drilling, geothermal energy production, and high‐temperature industrial processes.
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Details
1 Department of Civil and Environmental Engineering, Pontificia Universidade Católica do Rio de Janeiro (PUC‐Rio), Rio de Janeiro, RJ, Brazil
2 Department of Biosystems Engineering, Pirassununga, SP, Brazil