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Article

Degradation Potential of Metakaolin-Based Geopolymer Composites Immersed in Real and Simulated Acidic Environments

by
Shriram Marathe
1,2,
Natalia Szemiot-Jankowska
1,
Sanjeev Kumar
3 and
Murugan Muthu
1,*
1
Department of Materials Engineering and Construction Processes, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-372 Wrocław, Poland
2
Department of Civil Engineering, NMAM Institute of Technology (NMAMIT), Nitte (Deemed to be University), Karkala 574110, Karnataka, India
3
Department of Engineering, Norfolk State University, Norfolk, VA 23504, USA
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(2), 468; https://doi.org/10.3390/su17020468
Submission received: 11 December 2024 / Revised: 31 December 2024 / Accepted: 8 January 2025 / Published: 9 January 2025

Abstract

:
This study investigates the degradation potential of metakaolin-based geopolymer (GP) composites when exposed to real and simulated acidic environments. Traditional OPC concrete, commonly used in wastewater treatment facilities, faces considerable deterioration due to the destructive chemical composition of municipal wastewater. This extensive investigation aims to assess the performance of GP composites as a sustainable alternative to such materials. The metakaolin-based GP mortar samples were prepared and subjected to immersion in a primary clarifier unit at a local wastewater treatment plant (real) and a laboratory-made acetic acid solution (simulation) for up to four weeks after curing. The analysis included measurements of % strength and % weight loss, as well as characterization techniques such as isothermal calorimetry, microstructure (SEM), and mercury porosimetry (MIP). The outcomes signified a cumulative heat generation of 534 J/g at three days, with an average compressive strength of 79.4 MPa past 28 days. Exposure to acetic acid led to a 13% decline in compressive strength and a 3.90% loss in sample weight, while exposure to real wastewater resulted in an 18% strength and a 5.60% weight loss. Observations from SEM revealed microstructural changes, including the formation of biofilms and air voids, indicating multifaceted interactions between the GP matrix and its surrounding environment. This research effectively highlights the potential of metakaolin-based geopolymer composites to improve durability against acidic conditions, suggesting future applications in the construction of infrastructure exposed to such harsh chemical environments.

1. Introduction

Portland cement reinforced concrete is often used to build structural units in wastewater treatment facilities [1]. Typical municipal wastewater has a neutral pH and contains aggressive species such as chlorides, sulfates, ammonium, magnesium, and CO2 in aqueous form [2], which are not desirable for cement components. Continuous exposure of concrete units to this wastewater results in degradation, which has a huge economic impact on wastewater treatment industries [3]. Chemical, physical, and biological processes interact during this concrete degradation, which causes the failure of structural units and, thus, disrupts treatment operations [4]. However, the interplay between these different processes is not sufficiently understood. Understanding the nature of these phenomena requires detailed knowledge of the processes that occur and their relationship [5]. There is a great need for research into the development of new long-lasting binders [5], and geopolymers (GPs) and alkali-activated materials are among the most promising options compared to their OPC counterpart [6,7]. The mineral binder of GP is generally formed by the alkali activation of metakaolin or by industrial spin-offs such as fly ash and blast furnace slag. GP has several advantages over conventional OPC binders, such as being more resistant to chemical environments [8], having lower CO2 emissions during the production stage [9], displaying overall sustainability [6], and having built-in recyclability [10].
The manufacture of cement (i.e., OPC) is a significant contributor to global CO2 emissions, releasing approximately 0.73 to 0.85 tons of CO2 into the atmosphere for every ton of OPC produced [11], which accounts for up to about 8% of the overall CO2 emissions from industrial and energy sources. GPs are highly beneficial in terms of energy consumption, since a reduction in energy of 59% is achieved through their production compared to traditional OPC-based concretes [12]. The fire resistance of GP concrete has been reported to be very high, with little structural damage observed at up to 800 °C, as well as no degradation after repeated freeze–thaw cycles [13]. One potential precursor material for the production of a GP is metakaolin [14], which is obtained by the dehydroxylation of pure kaolin [15]. An alkaline solution facilitates the dissolution of metakaolin’s chemical components, subsequently leading to orientation, poly-condensation, and the establishment of a three-dimensional tetrahedral framework composed of AlO4 and SiO4. The connectivity throughout the SiO4 and AlO4 tetrahedral element is achieved through the sharing of oxygen atoms, with the negative charge of the aluminum ions in four-fold coordination being neutralized by alkali metals [15]. The metakaolin-based GP exhibits very high rigidity and has been widely proposed to have high potential as a means of wear protection for the surfaces of industrial concrete floors [16]. The subsequent step would be to focus on the interfaces between geopolymer binders and the biotic components of municipal wastewater, which affect long-term durability due to the presence of bacteria in the form of a biofilm attached to concrete surfaces [17]. The chemical oxidation of hydrogen sulfide (H2S) in contact with the cement hydration matrix of concrete sewer pipes produces sulfur-based compounds with diverse levels of oxidation, which are metabolized by different bacteria from running wastewater, with sulfuric acid being the final product [18]. However, H2S is also found in municipal wastewater, where it is discharged from the façade of the water, enters the ambience, and oxidizes the exposed surfaces of structural units in the wastewater treatment plant, producing sulfuric acid with a degradation effect [19]. Clarifiers and activated sludge storage tanks based on reinforced concrete are generally not susceptible to progressive degradation due to H2S, but, in turn, are mostly affected by the acid environment created at the interface between the nitrifying biofilm and the cement components [20]. The degradation behavior of wastewater treatment facility units based on GP concrete operating in real wastewater environments has not been reported in the literature. Considering this research gap, the following study was planned to fill the gap, considering the practical requirements.
Accordingly, in the present study, metakaolin-based GP mortar samples were prepared and continuously immersed in the primary clarifier unit of a wastewater treatment facility and in a laboratory-made acetic acid solution for up to four weeks. Their weight and strength losses at the end of such exposures were measured, to understand the GP composite degradation in real and simulated acidic environments. The primary clarifier unit representing the closest environment to actual sewers was chosen to immerse the mortar samples. The interaction between the GP binder hydrates and microorganisms was assessed by measuring the total protein concentration of the biofilm that was deposited on the surfaces of the sample due to continuous exposure to real wastewater. Further, scanning electron microscopy (SEM) was employed to study the biofilm formations in the microstructures of the degraded samples, while the results of the isothermal calorimetry and “mercury intrusion porosimetry” (MIP) examinations revealed the hydration and pore characteristics in the moist cured GP mortar samples at the end of 7 days. These findings will help researchers to understand the degradation potential of GP samples due to real wastewater conditions and highlight the budding of this new generation binder material with improved chemical resistance, which could prove to be a reliable alternative to its traditional OPC counterpart, which is typically used in the construction of such storage units exposed to aggressive chemicals.

2. Experimental Methodology

2.1. Sample Preparation

The GP mortar mix used in this study was prepared using highly pure metakaolin (400 g), river sand (1350 g), and commercial potassium-based water glass (425 g). These were obtained from NewChem (Traiskirchen, Austria), a local supplier, and the Wollner Company (Ludwigshafen, Germany), respectively. The specific gravity of the river sand was determined to be 2.65. Table 1 lists the oxide composition of the metakaolin, which was obtained by performing an X-ray fluorescence test.
The metakaolin powder was pressed into a circular disc using a pelletizer and scanned with the help of an X-ray fluorescence instrument (Nex De, Rigaku, Tokyo, Japan). The loss in ignition of metakaolin was found to be low, suggesting that the raw material is free of carbon species and organic matter. This ignition loss was found by heating the metakaolin up to 1000 °C in a hot air oven. The powder sample was ground and sieved to an amount of less than 75 µm through a standard strainer and scanned using an X-ray diffraction (XRD) instrument (D8 model, Bruker, Billerica, MA, USA), operated at a current of 40 mA and a voltage of 40 kV. During this measurement, a Cu Kα source was used, and the sample was scrutinized at a pace of 0.02°/min for a diffraction position array of 5–80°.
Figure 1 demonstrates the metakaolin XRD pattern. Mullite (3Al2O3·2SiO2) and anatase (TiO2) were found to be major constituents of the metakaolin sample. The raw materials were poured into a Hobart high-speed blender and blended for 3 min to cast mortar sections of a standard size, 50 × 50 × 50 mm3. Acrylic molds were used to prepare these samples and were demolded after 24 h of preparing, then cured with moisture in a laboratory environment for up to 28 days. The sections were sealed with an ultra-thin polyethylene sheet to avoid moisture loss due to evaporation. Specimen consolidation was completed using a vibrating worktable.

2.2. Isothermal Calorimetry and Compressive Strength Test

The isothermal calorimetry test was performed according to ASTM C1702 [21]. About 30 g of fresh GP paste was loaded into the conduits of an elevated exactitude isothermal calorimeter (I-Cal 8000, Calmetrix, Arlington, MA, USA) and the development of heat up to 3 days was deliberated in a laboratory area preserved at 25 °C and 65% relative humidity (RH). A servo-managed compression examination (C089, MATEST, Treviolo, Italy) appliance with a total aptitude of 3 × 103 kN was used to examine the mortar samples. The load pace was computed according to EN 12390-3 [22] and the strength was computed by dividing the failure load by the area of a cross-section of the sample. The mean of four replicates was considered and reported.

2.3. Simulated Acid Attack Test

Laboratory-grade acetic acid (CH3COOH) was obtained from Sigma-Aldrich, St. Louis, MO, USA. The mortar samples soaked overnight in tap water were immersed in CH3COOH solution of 0.50 molar concentrations for up to four weeks. This acid solution, which had an initial pH of around 2.81, was replenished every week to accelerate sample degradation. A 0.001 g precision weight balance was employed to track the alteration in the specimen’s weight at the conclusion of each week. Care was taken to eliminate the surface of the free solution from the acid-exposed swabs by employing a dry cloth before the weight was documented. The mass of the wet sample (in grams) after the attack by CH3COOH and the respective mass of the wet sample after curing (in grams) were used to compute the percent of diminished weight in GP mortar specimens resulting from acid attack using Equation (1). The pH of the extraneous acid solution was evaluated at the conclusion of each week using an emulate pH meter (PC-700, Eutech, Singapore) in a laboratory environment that was maintained at 25 °C. In accordance with the NIST regulations, the pH had been calibrated at three distinct levels (pH 4.01, 7, and 9.21) prior to the measurements. The average of three measurements was calculated and reported as the result.
W e i g h t   l o s s   i n   % = W 1 W 2 W 1 × 100

2.4. Wastewater Attack (Actual)

The wet mortar samples were immersed in wastewater flowing through a primary clarifier unit of a local wastewater treatment plant in Poland. Samples were suspended within this unit using a wooden support and rope, and exposure continued for up to four weeks. The degraded sample was collected and stored in 30 mL of 0.20 g/L NaOH solution for 12 h to remove excess biofilm and organic matter attached to the surface of the sample. The protein concentration in the removed biofilm layers was measured with the help of a total protein kit (TP300, Sigma Aldrich, Steinheim, Germany) and the readings were taken according to the manufacturer’s technical guidelines. Bovine serum albumin was used to calibrate protein concentration curves. The average of three repetitions was calculated and reported as the result.

2.5. Characterization Studies

The mortar chunks destroyed (upon testing) in the compression test machine were meticulously sectioned into small pieces with a diamond-tipped exactness cutter. An MIP test was conducted to elucidate the pore size distribution in this matrix of matter and measure its effective porosity, defined as the proportion of accessible pores in a porous solid. To do this, the chopped parts underwent vacuum drying for two days before being analyzed using an MIP (Pascal-140/440, Thermo Scientific, Waltham, MA, USA) and an SEM (Evo LS25, Zeiss, Oberkochen, Germany). Additionally, for the SEM analysis, the completely dry sample was affixed to a steel stub with carbon glue, gold-coated for 60 s using a sputtering coater, and subsequently examined via the secondary electron phase at an accelerated voltage of 20 kV. To obtain a good understanding of the biofilm formation, the degraded sample was freeze-dried, then sectioned, gold-coated, and studied with SEM at a higher voltage.

3. Results and Discussion

3.1. Sample Degradation

Figure 2 shows the isothermal calorimetry result that revealed the heat developments caused by mixing metakaolin with water glass. The collective heat generated at the conclusion of 3 days was 534 J/g of metakaolin used. The ceiling quantum of heat was generated at 3.60 h from assimilation, indicating that the GP binder rapidly sets. Mortar samples based on this composition were revealed to possess an average compressive strength of 79.4 MPa at the end of 28 days.
The MIP test results are shown in Figure 3. The cumulative porosity of the GP mixture was calculated to be 15.60%. The allotments of gel pores (<10 nm), capillary pores (10–10,000 nm), and macro pores (>10,000 nm) within this sample were calculated to be 11%, 16%, and 64%, respectively. Aligizaki [23] has classified the sizes of the gel pores and capillary pores in the cementitious composite matrix as 1–10 nm and 10–10,000 nm, correspondingly. Any pores above these sizes were classified as macropores in the composite. The GP sample was found to be mechanically stronger, which might be due to its dense microstructure and binder composition reactions [24]. Generally, the reaction mechanism of the GP mix is divided into four stages [25], where the initial stage is a process in which metakaolin is dissolved by an alkaline solution to release aluminate and silicate species, thus, turning into four coordinated aluminum atoms, and the second stage is a process in which these species react with silicate from an alkaline solution to produce aluminosilicate oligomers. The next stage is a process in which these oligomers are turned into aluminosilicate gels through polymerization and gelation. The curing process is the final stage of the GP reaction mechanism, and the setting time is usually determined according to the proportion of the mixture, the curing conditions, and the presence of impurities [26].
Figure 4 shows the cut section of these samples that have been degraded due to continuous exposure to laboratory acid and real wastewater attack for up to four weeks. The results clearly indicate that the aggregates remain intact, but the binder matrix underwent antagonistic degradation. Color change and the formation of biofilms were observed in samples exposed to the real wastewater environment. Acid exposure reduced the compressive strength and weight of the samples to 13% and 3.9%, but these losses at the end of the wastewater exposure period were determined to be 18% and 5.6%, respectively, which could be due to the combined erosion and leaching effects caused by moving water in the primary clarifier unit. The immersion of the sample increased the pH of the acid solution to 4.91 at the end of the first week. However, this change was reduced with increasing exposure time, and the pH results are illustrated in Figure 5. The maximum weight loss was observed to be about 4.0% at the end of 4 weeks of exposure. The pH increase represents the neutralization of acidic elements by the discharge of potassium-based elements from GPs. Gao et al. [14] reported the termination of the KOH, KHCO3, and K2CO3 elements from the metakaolin-based GP matrix due to attack by hydrochloric acid. These species are usually soluble in acid and can neutralize hydrogen ions and liberate potassium ions ahead of contact with the acid solution. The average protein concentration in the biofilm extracted from the surfaces of the wastewater-exposed samples was determined to be 0.81 g/m2.

3.2. Microstructural Assessment

The SEM images in Figure 6 reveal the microstructure of the GP samples before and after the attack with acetic acid. The main components observed in this microstructure are rounded sand particles, air voids, and GP gel with a sheet structure (Figure 6A,B). The acid attack mainly leached the alkali of the GP gel and the resulting microstructure was found to be porous and to have many microcracks (Figure 6C,D).
The sand particles were not affected by the acid species, but the binder component was completely degraded. To obtain elemental information, we finely polished the surface of the acid-degraded sample and examined it under SEM using the backscattered electron (BSE) mode. The BSE images shown in Figure 7 reveal the degraded microstructure of the acid-exposed samples. Several cracks were observed in these samples, and the interface between the sand particles and the degraded binder components was weak (Figure 7A–C). The binder component was mainly composed of silica (Si), alumina (Al), and potassium (K), which was evaluated with the help of energy dispersive spectroscopy (EDS) studies (Figure 7D). Multiple EDS points were collected from the samples’ microstructures and a scatter plot was drawn using the Si/Al and K/Al ratios (Figure 7E). The moisture-cured GP sample was found to have a K/Al ratio between 0.17 and 0.83, while the K/Al ratio in the acid-exposed sample was determined to be mainly between 0.05 and 0.23. The K/Al ratio of the GP sample was significantly reduced due to the attack by acetic acid. The binder component was found to be rich in alkalis, but the acid attack primarily leached the alkalis from the GP microstructure, which significantly affected the mechanical performance. The SEM images in Figure 8 reveal the microstructures of the GP samples after storage in real wastewater. A thick layer of biofilm was formed on the exterior of the samples as a result of this exposure. Biofilm with a reticulate structure was seen everywhere in the microstructure and invertebrate eggs, protozoa, and organic fibers, which were at their initial stages of development, were seen to be interlaced within the biofilm.

4. Conclusions

The results of this study afford important insights into the degradation characteristics of metakaolin precursor-based GP composites immersed in both simulated acid and actual wastewater environments. The GP mortar exhibited significant initial strength, showcasing its potential as a robust alternative to cements in wastewater treatment facilities. Specifically, the study revealed that while exposure to acetic acid resulted in notable reductions in both compressive strength and weight, the degradation effects were aggravated under real wastewater conditions, underscoring the complex interplay between chemical attacks and microbial activity. The key outcomes are as follows:
  • The metakaolin GP samples exhibited substantial initial compressive strength, averaging 79.4 MPa after 28 days of curing, indicating their potential as a robust construction material. Conversely, the isothermal calorimetry results indicated a cumulative heat generation of 534 J/g, demonstrating the rapid setting time and energy efficiency of the GP binder compared to its traditional OPC counterparts.
  • The GP samples immersed in a 0.50 molar acetic acid solution demonstrated a strength reduction of 13% and a weight loss of 3.90% after four weeks, highlighting the impact of acidic environments on material integrity. In contrast, the GP samples exposed to actual wastewater conditions experienced an 18% reduction in strength and a weight loss of 5.60%. This suggests that real-world conditions may lead to more severe degradation due to factors such as microbial activity and fluid movement.
  • The SEM results revealed significant microstructural changes in the geopolymer samples after exposure, including the formation of biofilms and air voids, indicating interactions between the GP matrix and its environment that could affect long-term structural resilience.
While the present investigation provides precious insights into the performance of metakaolin-based GPs in aggressive environments, certain limitations must also be acknowledged. The duration of exposure was limited to four weeks in this research, which may not completely capture the long-term degradation potential of these materials in real wastewater treatment applications. Additionally, the study primarily focused on acetic acid and municipal wastewater; further exploration of other aggressive chemicals commonly found in wastewater is essential for a more comprehensive understanding. Further, future research should also focus on enhancing the chemical resistance and overall durability of such geopolymer composites through the incorporation of various additives or modifiers. Additionally, the observed microstructural alterations—including the formation of biofilms and air voids—emphasize the future necessity for extended exploration to understand the interactions between GP binders and the biological entities present in wastewater. Overall, the potential integration of GPs into existing wastewater treatment frameworks could lead to significant advancements in sustainable construction practices, ultimately contributing to improved resilience against environmental challenges.

Author Contributions

Data curation, writing—original draft preparation, writing—review and editing, visualization, project administration: S.M.; data curation, formal analysis, investigation: N.S.-J.; writing—review and editing, resources: S.K.; conceptualization, methodology, software, validation, formal analysis, investigation, resources, writing—review and editing, supervision, funding acquisition: M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This article has been supported by the European Funds for Social Development (FERS) program and the Support for Alliances of European Universities NAWA program numbered BPI/WUE/2024/1/00031/DEC/1.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We appreciate the technical support from Neven Ukrainczyk and Eddie Koenders from the Technische Universität Darmstadt in Germany and Lukasz Sadowski from the Wroclaw University of Science and Technology in Poland.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD pattern of metakaolin.
Figure 1. XRD pattern of metakaolin.
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Figure 2. The heat flow and cumulated heat curves of the calorimetry result.
Figure 2. The heat flow and cumulated heat curves of the calorimetry result.
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Figure 3. The cumulative intrusion and differential intrusion curves indicating the MIP result.
Figure 3. The cumulative intrusion and differential intrusion curves indicating the MIP result.
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Figure 4. The degradation of the geopolymer samples exposed to laboratory acid and real wastewater in the primary clarifier unit of a wastewater treatment plant.
Figure 4. The degradation of the geopolymer samples exposed to laboratory acid and real wastewater in the primary clarifier unit of a wastewater treatment plant.
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Figure 5. The progressive loss in sample weight due to the continuous exposure to the acid solution.
Figure 5. The progressive loss in sample weight due to the continuous exposure to the acid solution.
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Figure 6. SEM images revealing the microstructure of the GP samples (A,B) before and (C,D) after the acid attack.
Figure 6. SEM images revealing the microstructure of the GP samples (A,B) before and (C,D) after the acid attack.
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Figure 7. (AC) SEM-BSE images illustrating the microstructures of the acid-degraded GP samples, and the corresponding (D) EDS spectrum and (E) EDS scatter plot.
Figure 7. (AC) SEM-BSE images illustrating the microstructures of the acid-degraded GP samples, and the corresponding (D) EDS spectrum and (E) EDS scatter plot.
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Figure 8. (AD) SEM images showing the attachment of biofilm and different biological species over the surfaces of the GP specimens exposed to real wastewater.
Figure 8. (AD) SEM images showing the attachment of biofilm and different biological species over the surfaces of the GP specimens exposed to real wastewater.
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Table 1. Oxide composition of metakaolin precursor.
Table 1. Oxide composition of metakaolin precursor.
OxidesCaOAl2O3SiO2MgOFe2O3Na2OSO3TiO2K2OIgnition Loss
Wt%0.0243.8753.110.030.430.230.031.710.190.38
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Marathe, S.; Szemiot-Jankowska, N.; Kumar, S.; Muthu, M. Degradation Potential of Metakaolin-Based Geopolymer Composites Immersed in Real and Simulated Acidic Environments. Sustainability 2025, 17, 468. https://doi.org/10.3390/su17020468

AMA Style

Marathe S, Szemiot-Jankowska N, Kumar S, Muthu M. Degradation Potential of Metakaolin-Based Geopolymer Composites Immersed in Real and Simulated Acidic Environments. Sustainability. 2025; 17(2):468. https://doi.org/10.3390/su17020468

Chicago/Turabian Style

Marathe, Shriram, Natalia Szemiot-Jankowska, Sanjeev Kumar, and Murugan Muthu. 2025. "Degradation Potential of Metakaolin-Based Geopolymer Composites Immersed in Real and Simulated Acidic Environments" Sustainability 17, no. 2: 468. https://doi.org/10.3390/su17020468

APA Style

Marathe, S., Szemiot-Jankowska, N., Kumar, S., & Muthu, M. (2025). Degradation Potential of Metakaolin-Based Geopolymer Composites Immersed in Real and Simulated Acidic Environments. Sustainability, 17(2), 468. https://doi.org/10.3390/su17020468

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