Physico-chemical properties and durability of a fly-ash-based geopolymer

Hanane Boutkhil; Somia Fellak; Saliha Alehyen; Ahmed Bari; Hafize Fidan

doi:10.1515/chem-2024-0048

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Physico-chemical properties and durability of a fly-ash-based geopolymer

Hanane Boutkhil , Somia Fellak , Saliha Alehyen , Ahmed Bari and Hafize Fidan

Published/Copyright: June 14, 2024

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From the journal Open Chemistry Volume 22 Issue 1

Abstract

Due to the environmental problems linked to the production of portland cement, the search for new more economic and non-polluting construction materials has become a current issue of interest. Geopolymers represent new types of “polymeric” materials of mineral nature. The aim of this work is to study the influence of the Na₂SiO₃/NaOH mass ratio and curing time on the mechanical and microstructural properties of fly-ash-based geopolymers GP-Fs. The samples were synthesized with different Na₂SiO₃/NaOH mass ratios (2.5, 4, and 6) and curing times (2, 12, and 24 h) at 60°C. The GP-F pastes elaborated were analyzed by scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). The sample with the highest compressive strength and density and the lowest water absorption was produced with a Na₂SiO₃/NaOH mass ratio of 2.5 and a curing time of 24 h, which is GP1, as confirmed by FTIR and XRD analyses; in addition, it had a compact structure attributed to a higher pozzolanic reactivity. These optimized geopolymer pastes were studied to assess the durability test, evaluating the resistance to fire and acid attack. Fire resistance was assessed by heating the GP-F pastes to 800°C for 2 h, and acid resistance was studied by immersing specimens in a solution of 6% acetic acid (GP-F-CH₃COOH) and 6% hydrochloric acid (GP-F-HCl) for 2 months. Physico-chemical and microstructural changes before and after heat and acid exposure were studied using several analyses. Samples exposed to an acidic environment display a completely porous shape with some micro-cracks, inducing a decrease of the residual compressive energy. FTIR analysis showed that the geopolymer gel deformed after heating to 800°C, and pores were formed in their structure due to evaporation of water.

Keywords: geopolymer; mechanical properties; durability properties; acid attack; fire resistance

1 Introduction

Geopolymers are important composite materials, which have emerged as promising green alternatives to ordinary Portland cement (OPC) due to their many problems linked to the production of cement; they are generally manufactured by alkaline activation of aluminosilicate raw materials with strongly basic alkaline solutions, which can be natural or from industrial wastes with strongly basic alkaline solutions at room temperature or slightly elevated temperature [1].

Geopolymers are aluminosilicates with an amorphous or a semi-crystalline frame. The geopolymer structure consists of three repeating basic units: poly(sialate) (PS, –Si–O–Al–O–), poly(sialate-siloxo) (PSS, –Si–O–Al–O–Si–O–), and poly(sialate-disiloxo) (PSDS, –Si–O–Al–O–Si–O–Si–O–) [2], which are formed through the polymerization of silicate and aluminate tetrahedra.

Geopolymers have exhibited many advanced properties, namely, high compressive strength, fast curing, and good fire and acid resistance. These properties allow them to be used in various fields such as civil engineering and waste control [3]. This material exhibits better properties than OPC.

Fly-ash (FA) is an industrial waste material produced by coal power plant, which is used as a raw material in the production of geopolymers. The combination of alkaline liquids sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) is used to activate the FA to take part in the polymerization process. The properties and applications of geopolymer mortars have received increasing attention from researchers. Many factors affect the properties of geopolymers, such as the type of starting material, content, concentration of alkaline activators, temperature, and curing time. The durability of alkali-activated geopolymer pastes is investigated as they are reported to exhibit superior durability in aggressive environments [4,5].

The Na₂SiO₃/NaOH ratio have a great impact on the performance of FA based geopolymers and the fire resistance is amoung the parameters of construction materials be considered to assure that the building can be used [6], and the acid resistance is a proper asset for structural materials used in the aggressive surroundings of chemical, mining, mineral processing, and other industries. Being simple in nature, pastes made with OPC and alkali-activated slag get worse in the acid environment [7]. Bakharev [8] investigated the resistance of geopolymer materials to acid attack; he observed that the residual strength following acid and heat treatments is influenced by the quantity of Na₂SiO₃/NaOH mass ratio, and Škvára et al. [9] studied the durability of FA-based geopolymers in aggressive media. The acid solutions used contained MgSO₄, Na₂SO₄, and NaCl. The results showed that FA-based geopolymers have excellent resistance to aggressive media than OPC [9].

In our study, the Si–O–Si and Si–O–Al bonds were broken by aluminosilicate degeopolymerization, so the degradation mechanism depended on the acid solution used [8,9,10].

The purpose of this article was to study the influence of Na₂SiO₃/NaOH mass ratio and the curing time on the mechanical and microstructural properties of FA-based geopolymers.

The key technical properties like compressive strength, durability, bulk density, and water absorption were tested and performed.

2 Methods

2.1 Characteristics of the raw material and synthesis procedure

FA used as a starting material was sourced from a Jerada coal-fired power plant in Morocco. The chemical composition of FA is given in Table 1. The composition of FA was obtained by X-ray fluorescence analysis.

Table 1

Chemical composition (wt%) of used FA

Constituent	SiO₂	Al₂O₃	Fe₂O₃	CaO	K₂O	MgO	TiO₂	SO₃	Others
FA%	48.03	24.63	4.25	9.8	2.08	2.3	2.05	2.66	2.11

The X-ray diffraction (XRD) pattern of FA displayed in Figure 1 was obtained with an X’pert-Pro diffractometer. The data were collected in a 2θ range from 10 to 70° with a step size of 0.02° using Cu Kα1 radiation (λ = 1.54056 Å). The patterns illustrate the crystalline phases present in the FA, namely, quartz and mullite. A large “hump” in the region between 14 and 30° (2θ) represents the amorphous phase present in the FA.

Figure 1

XRD patterns of class C FA (Q: quartz; M: mullite).

The synthesized sample compositions are presented in Table 2.

Table 2

Chemical composition of synthesized geopolymer samples

Mix	FA (g)	Alkaline solution	Na₂SiO₃ (ml)	NaOH (g)	Water (ml)	Curing time (h)	Temperature (°C)
GP1	100	40.00	28.57	11.43	23.82	24	60
GP2	100	40.00	28.57	11.43	23.82	12	60
GP3	100	40.00	28.57	11.43	23.82	2	60
GP4	100	90.00	72	18	37.5	24	60
GP5	100	90.00	72	18	37.5	12	60
GP6	100	90.00	72	18	37.5	2	60
GP7	100	249.9	214.2	35.7	74.3	24	60
GP8	100	249.9	214.2	35.7	74.3	12	60
GP9	100	249.9	214.2	35.7	74.3	2	60

Figure 2 displays the particle size distribution curve of FA class C obtained by laser diffraction particle size analysis, with d (0.5) equal to 29.456 µm and d (0.9) equal to 109.657 µm.

Figure 2

Particle size distribution curve of FA.

The scanning electron microscopy (SEM)/energy-dispersive X-ray spectrometry (EDX) analysis of FA class C was performed by using an FEI Quanta 450 FEG scanning electron microscope. Figure 3 shows the microstructures of FA which are composed of a series of spherical glassy particles of different shapes which contain fine solid spherical particles (microspheres) containing large cenosphere particles (>50 μm) formed during the combustion process.

Figure 3

SEM and EDX analyses of FA.

Thermogravimetric analysis (TGA) is shown graphically in Figure 4, illustrating the relationship between the sample weight change, temperature, and its first derivative (DTG – derivative thermogravimetry).

Figure 4

TGA and DTG of FA.

The effect of temperature on the FA content was studied using a NETZSCH STA 449 F3 Jupiter thermal analyzer to determine the weight loss recorded in a temperature range of 30–1,000°C at 60 ml/min.

FA was heated from 30–1,000°C at atmospheric pressure and in an inert atmosphere at a heating rate of 10°C/min. The temperature range was 90–1,200 K. The sample weight was 6 mg. Study results displayed as thermograms show three weight losses for ash samples. Figure 4 shows significant weight loss. This is caused by the evaporation of water, dehydration of Ca(OH)², and decomposition of CaCO₃ [11]. The total mass loss of the sample was 0.56 mg.

2.2 Sample preparation

The geopolymer specimens were obtained by mixing FA powder class C with an alkaline activator. The alkaline activator solution is a mixture of Na₂SiO₃ solution (18% Na₂O and 63% SiO₂) and NaOH pellets (purity >97%). The alkaline solutions were prepared with different mass ratios of Na₂SiO₃/NaOH (2.5–4–6). The process of geopolymerization is displayed in Figure 5. The pastes were cast in cylindrical plastic molds (35 × 70 mm³) and cured at 60°C for different curing times (2–12–24 h). After demolding, the samples were allowed to air-dry in the laboratory until the day of testing (28 days).

Figure 5

Geopolymerization process and specimens’ fabrication.

The synthesized sample compositions are reported in Table 2.

2.3 Material characterization

The density of GP-F geopolymer was calculated by measuring the dimension and the mass of the samples according to Archimedes’ principle.

Water absorption of samples was obtained by measuring the mass before and after immersing in water during 24 h, calculated using equation (1) according to ASTM C642.

(1) Water absorption = M w – M d M d × 100 ,

where M _w is the saturated weight of samples and M _d is the dry mass.

Compressive strength tests were measured in Caduco laboratory 200 MK2. Fourier transform infrared (FTIR) spectra were collected in MIR transmission mode on a VERTEX 70 instrument, with 8 scans per sample, in the range 4,000 to 400 cm⁻¹, at a resolution of 4 cm⁻¹.

3 Durability tests

Acid resistance was studied by immersing both specimens in 6% acetic acid (GP-F-CH₃COOH) and hydrochloric acid (GP-F-HCl) for 2 months.

The control and test geopolymer pastes were synthesized with a Na₂SiO₃/NaOH ratio of 2.5 to perform the durability test. The samples were cast in plastic cylinders and left for 24 h at 60°C temperature. After demolding, the samples were stored to air-dry in the laboratory, and then they were exposed to fire and acid attack resistances. Fire resistance was evaluated by heating the GP-F pastes at 800°C for 2 h.

4 Results and discussion

4.1 Mechanical and microstructural characterizations

4.1.1 Effect of Na₂SiO₃/NaOH ratio and curing time on bulk density and water absorption

The test results of mixes are summarized in Table 3 (with standard deviation error bars). As shown in Figure 6, the specimens GP1, GP2, and GP3 prepared with a Na₂SiO₃/NaOH ratio of 2.5 showed a higher density (3.25, 2.58, and 2.14 g/m³, respectively) than those prepared at ratios of 4 and 6 of Na₂SiO₃/NaOH solution; the improvement of density when the Na₂SiO₃/NaOH ratio was equal to 2.5 is related to the structure and workability of the samples [12,13]. Lee and Lee [14] proposed that increasing the amount of NaOH would give enough Na⁺ and OH^– ions for complete ionization, and the samples would become stronger when they react with water, which make them denser. The higher concentration of NaOH reduced the flow of the geopolymer and thus reduced the curing time, and the hard ness of the geopolymer paste might make it hard to work with. Therefore, the density was lower.

Table 3

Properties and standard deviation of geopolymer samples

Fresh properties					Hardened properties
					28 days
Mixes	Density (g/m³)	SD	Water absorption (%)	SD	Compressive strength (MPa)	SD
GP1	3.25	0.084	2.1	0.069	21.65	2.65
GP2	2.58	0.048	2.3	0.074	20.45	1.25
GP3	2.14	0.038	3.45	0.044	18.31	1.54
GP4	2.07	0.019	4.65	0.017	13.31	0.22
GP5	1.08	0.024	6.66	0.01	15.41	1.12
GP6	1.17	0.045	8.02	0.05	13.72	1.04
GP7	1.24	0.078	12.33	0.08	11.32	0.54
GP8	1.16	0.016	12.76	0.1	10.45	0.21
GP9	1.02	0.012	13.96	0.12	8.23	0.14

Figure 6

Density and water absorption % of geopolymer specimens.

When the amount of Na₂SiO₃ in these samples was adequate, it led to a reduction of the geopolymer flow; this reduction due to its viscous nature resulted in a more compact structure of the geopolymer.

However, GP1 presents the lowest water absorption of 2.1% at 24 h, while the highest water absorption is observed in GP7, GP8, and GP9 with 12.33, 12.76, and 13.96%, respectively.

The water absorption of FA-based geopolymers decreased with decreasing alkaline solution content. It is worth noting that water absorption is linked to the capillary action. The larger number of pores is due to the connection between them, creating a path of least resistance and continuity. Interconnected pores create a stronger capillary effect, allowing water to pass through easily [15].

4.1.2 Effect of Na₂SiO₃/NaOH ratio and curing time on the compressive strength of geopolymers

The compressive strengths of GP-FA are displayed in Figure 7.

Figure 7

Effect of Na₂SiO₃/NaOH ratio on the compressive strength.

Three different ratios were investigated. With a Na₂SiO₃/NaOH ratio of 2.5, the residual compressive strength after 28 days was higher for GP1 and GP2 than for GP3, which was due to less cracking and continuity of the geopolymerization reaction, confirming the SEM results.

In this case, the increase in the concentration of silicate species allowed the formation of networks and thus the development of compressive strength. The specimens became more viscous, which led to a reduced workability. This indicates that the one-component geopolymer required proper mixing conditions, or else it cannot be used [16].

The strength decreased in all other samples due to the increase in the ratio of Na₂ SiO₃ to NaOH from 4 to 6 and the curing time, which was due to unreacted particles of FA in the geopolymerization process, which can be explained by the intense presence of zeolite crystals [16]. It was observed that an increase in the NaOH concentration resulted in an increase in water volume when water was added, which explains the reduction of compressive strength values. The strength of geopolymers is related to their structure and density [17]. The best strength is achieved when NaOH is enough to balance the charge when replacing the tetrahedral Si with Al.

4.1.2.1 FT-IR spectroscopy

FTIR spectroscopy was used to study the effect of Na₂SiO₃/NaOH mass ratio and curing condition on the structure of FA-based geopolymer. Figure 8 shows the FTIR spectra of nine samples. The main absorption bands were detected at 3,342, 2,359, 2,318, 1,641, 1,477, 1,387, 968, 872, 775, 709, and 668 cm⁻¹. The broad band near 3,342 cm⁻¹ can be ascribed to the OH stretching vibration ( ν (OH)), which originated from water absorption. The prominent peak at 1,641 cm⁻¹ was associated with the bending vibration of the H–O–H bond ( δ (OH)) of absorbed water [17,18]. These two bands present higher intensities in the spectrum of GP9 sample, suggesting higher water absorption. It can be seen that the intensities of these bands increased with time and increase in Na₂SiO₃/NaOH mass ratio. The low water absorption amount was observed for the sample GP1, which indicates clearly the compact structure of this sample. This result is in total accordance with SEM results.

Figure 8

FTIR spectra of FA-based geopolymers prepared with different Na₂SiO₃/NaOH mass ratios and curing times.

Two absorption bands observed for all samples in the 2,400–2,240 cm⁻¹ range belonged to the cracking and re-forming of the functional groups of carboxyl (C═O) and carbonyl (C–O–C), as well as the decomposition of carbonate, represented as CO₂ [19]. The carbonate peak was observed for all samples, suggesting atmospheric carbonation, except for GP7, GP8, and GP9 samples, for which this peak disappeared. As shown in Figure 9, the main analyzed bands in FTIR spectra of FA-based geopolymers were in the region of 1,800–500 cm⁻¹. The peak at 1,477 cm⁻¹ indicative of C–O stretching resulting from the carbonation was observed [20,21]. The presence of nitrate is evident because of the detection of the band at 1,387 cm⁻¹ [22]. FTIR spectra for all sediment samples showed peaks associated with feldspar, quartz, montmorillonite, nitrate, and organic matter. Comparing the FTIR spectra between the nine FA-based geopolymers, the intensities of bands observed at 3,342, 1,641, 1,387, 968, and 872 cm⁻¹ are substantially different.

Figure 9

Zoomed FTIR spectra (2,500–500 cm⁻¹).

In general, the 1,200–950 cm⁻¹ wavenumber range is linked to the Si–O–Si stretching vibration band [20]. Usually, the band is detected at 1,020 cm⁻¹, but in our spectra it was found to be shifted to less than 1,000 cm⁻¹ (968 cm⁻¹), suggesting the penetration of Al⁴⁺ atoms into the original functional group [23]. According to Chindaprasirt et al. [21], the Si–O–Si stretching vibration was more prominent than the O–Si–O bending mode. Consequently, the Si–O–Si vibration was used to indicate the degree of geopolymerization [20,24]. In addition, this band shifted to a higher wavenumber as the Na₂SiO₃/NaOH mass ratio and temperature increased. This can be linked to the Si–O tetrahedra containing the substituted part of Al–O groups in the Si–O–Al chains, leading to changes in the reticular structures, which affected the peak position [24]. The bands related to the bending of the Si–O–Si and O–Si–O bonds were detected in the wavenumber range under 500 cm⁻¹ [24]. The presence of montmorillonite was confirmed by the intense peak detected at 872 cm⁻¹ in the spectra of samples GP2 and GP3 [22]. The weak bands observed at 775, 709, and 668 cm⁻¹ were assigned to Si–O symmetric stretching (quartz) and symmetric stretching of Al–O and Si–O–Al (or) Fe₂O₃ (hematite), respectively [25]. In accordance with previous research [23,26], FTIR spectra for all samples showed peaks associated with feldspar, montmorillonite, quartz, nitrate, and organic matter.

4.1.2.2 XRD analysis

XRD patterns of nine geopolymers synthesized after 28 days of curing and FA patterns are shown in Figure 10.

Figure 10

XRD patterns of GP1 to GP9.

The XRD pattern of the prepared GP-F sample showed a large amorphous bump between 20 and 45° (2θ), which is a characteristic peak of geopolymers [27,28], and these peaks were attributed to quartz and mullite derived from unreacted FA particles. This confirmed that the crystalline phases were not reactive or involved in the geopolymerization process, but simply present as inactive fillers in the geopolymer network. The change in the hump was attributed to the amorphous FA phase from 4–30° to 20–40° (2θ). The conversion of the FA glassy phase to an amorphous alkali aluminosilicate reaction product with zeolites as the secondary reaction product [29] reflects the changes in bonding at the local order level during the geopolymerization process and indicates the formation of an N-A-S-H hydrated alkali aluminosilicate gel, which has been identified as the primary product of geopolymerization in diffraction patterns of geopolymeric materials [26,27,29].

Sharp peaks at 2θ = 12, 23, and 29° were observed in the XRD patterns corresponding to GP1, GP2, and GP3 geopolymers (Na₂SiO₃/NaOH = 2.5), respectively. These peaks are attributed to hydroxysodalite, which is a zeolitic phase produced by the geopolymerization process.

The higher the NaOH content of the limiting reaction mixture, the more intense the peaks from hydroxysodalite will be. Furthermore, the generation of hydroxysodalite was favored by a decrease in the Si/Al ratio and an increase in the NaOH content. These findings are in agreement with numerous publications in the field [30,31,32].

It was noted that the higher the concentration of NaOH, the higher the maximum position of the halo. This increase in the maximum position can be explained by the improved solubility of FA particles in high concentrations of NaOH. After geopolymerization, the crystalline phases present in FA remain after geopolymerization.

Therefore, it is possible to assume that the geopolymerization process is associated with the amorphous and glassy FA phases. Geopolymer binders are often formed by the hardening of an amorphous aluminosilicate gel. Hanjitsuwan et al. [33] examined the NaOH effect on the properties of FA-based geopolymer paste. The researchers employed an alkaline solution to activate FA, and they used different NaOH concentrations. They demonstrated that the glassy component of FA was the initial component to undergo dissolution, resulting in the formation of a novel phase of alkaline aluminosilicate gel with apparent broadness.

During the shift of the hump, they concluded that higher NaOH concentrations had better ability to dissolve FA particles, which resulted in a higher geopolymerization degree and improved properties [34].

4.1.2.3 SEM analysis

The micrographs of four important samples synthesized and the effect of ratio of alkali-cation and curing conditions on the morphology of samples are presented in Figure 11.

Figure 11

SEM micrographs of GP1, GP2, GP8, and GP9.

By comparing the microstructures of the mixes, it can be observed that GP1 and GP2 had a compact and denser structure and well-reacted matrices with a dense gel phase and a uniform distribution. This is obviously attributed to a higher pozzolanic reactivity. However, a lower quantity of gel is formed, and the presence of microcracks, voids, fissures, and unreacted FA particles can be attributed to the high Na₂SiO₃/NaOH ratio added and the curing condition which caused the incomplete geopolymerization reaction for the samples GP8, and GP9, especially the GP9 sample with a mass ratio of 6 and 24 h of curing time; the FTIR analysis confirmed this.

Pores and cracks were observed because of the excessive shrinkage of the geopolymer. A quite porous shape prompted extra strain awareness spots to shape inside the matrix, which caused the structural failure, resulting in the lowest compressive strength and highest water absorption.

4.2 Durability performance

After 60 curing days, GP-F was subject to compressive strength tests. The results are shown in Table 4 and Figure 12.

Table 4

Compressive strength results

Samples	GP-F	GP-F-CH₃COOH	GP-F-HCL	GP-F800°C
Compressive strength (MPa)	32	13.11	7.22	9
SD	2.34	1.52	2.14	1.78

Figure 12

Compressive strength of GP-F before and after high temperature and acid exposure.

The height of bars in Figure 12 remarks that GP-F has a compressive strength achieving approximately 32 MPa, and the values of GP-F decrease after heat treatment to 800°C from 32 to 9 MPa. This will be explained via the materials converted and the deformation of the geopolymer gel [35].

The compressive strength values of GP-F-HCL and GP-F-CH₃COOH after 2 months are approximately performed well at 7.22 and 13.11 MPa, respectively. In HCl solution, the strength increase was small, then in CH₃COOH this reduction of samples could be assigned to the formation of zeolite levels and depolymerization of the geopolymerized product [36].

Figure 13 FT-IR spectra of GP-F before and after immersion tests and heating.

Figure 13

FT-IR spectra of GP-F, GP-F-CH₃COOH, GP-F-HCl, and GP-F800°C.

Before exposure to high temperature and acid, the GP-F has three main peaks at 957, 1,644, and 3,349 cm¹. The bathochromic value of 3,349 cm⁻¹ is attributed to the stretching vibration of the OH band present in the silanol–Si–OH groups and the hydrogen bonding between the adsorbed water [37].

The intense band at 1,644 cm⁻¹ shifted to a lower wavenumber 957 cm⁻¹. This transition, observed during geopolymerization, is due to bond breaking and formation of new bands [38], confirming that geopolymerization did occur [37,38,39].

IR spectra of the sample subject to 800°C heating (GP-800°C) show that the main characteristic band of the geopolymer gel (N-A-S-H) located at 1,632 cm⁻¹ shifted to a lower wavenumber of 972 cm⁻¹ with increasing temperature.

The explanation for this transition is that the geopolymerization reaction proceeds at high temperature between the unreacted FA and the residual alkaline solution present in the geopolymer samples.

In both acidic solutions, the wavenumbers of the bands detected at 1,638 and 3,335 cm⁻¹ increased, which is attributed to the bending vibrations (H–O–H) and stretching vibrations of weakly bond HO molecules adsorbed on surfaces or trapped in large cavities (–OH) [33].

Figure 14 depicts the micrographs of GP-F, GP-F-800°C, GP-F-CH₃COOH, and GP-F-HCL specimens. It can be seen that they are specially composed of round-fashioned debris with exclusive sizes, and the smallest ones are referred to as microspheres. GP-F is a heterogeneous porous mixture consisting of partly or completely unreacted FA particles; numerous white residues on the floor of the geopolymer gel are also noted. The presence of these residues can be because of the excess of alkaline solution that has triggered the surface of the geopolymer matrix [40,41].

Figure 14

Visual changes in GP-F before and after acid exposure and heat.

According to the visual observation of specimens, in GP-F-800°C the sample had a change in colour. It can also be noticed that there are macrocracks on the surface. This changes are defined ny the excessive temperature between the unreacted FA debris and the residual alkaline precipitates to form the aluminosilicate gel, which condenses on the surface, and consequently fills the voids present in the geopolymer and alsoto the combustion of the unburned carbon and the oxidation of the iron of FA [42,43].

Geopolymer samples GP-F-HCL and GP-F-CH₃COOH had a small change in appearance after 2 months of immersion in the acidic solution. They were characterized by a few microcracks, and their color became darker than that before immersion. A porous structure induced deterioration of GP-F upon acid exposure, which would cause damage to the material by forming microcracks. The results showed that new crystals have formed in the matrix.

5 Conclusions

In this present study, the following conclusions can be drawn:

Geopolymer specimens prepared with Na₂SiO₃/NaOH at a ratio equal to 2.5 at 24 h of curing are the best formulation, exhibiting higher compressive strength, dry density, and lower water absorption, compared to other samples; this is due to the compact microstructure, higher amount of hydration products, and the increase in the rate of geopolymerization reaction by activating the dissolution and condensation steps; so, according to statistical analysis, the curing time and mass ratio affect the mechanical properties of geopolymers.
The study of the durability of an acidic environment showed us that the physico-chemical properties of samples were slightly influenced by the attack of acetic acid and especially hydrochloric acid, and the reduction of compressive strength was about 22.56% MPa.
The observed microstructural changes and weight loss explain the effects on the mechanical strength of GP-F samples after exposure to elevated temperature. Due to evaporation of free water, pores were formed in the structure. FTIR analysis demonstrates that the geopolymer gel was distorted after heating, and a reduction in strength was observed.
In the acidic environment, the visual observations showed that the specimens remained nicely, which demonstrated that the structure seems highly homogeneous and denser with a few microcracks, which brought on a lowering of their compressive strength value.
The high-performance of GP-F pastes declines with the formation of fissures in the amorphous polymer matrix; at the same time, the overall low performance of geopolymers becomes worse upon crystallization of zeolites and the formation of fragile grainy structures.

Hanane_boutkhil@um5.ac.ma

Acknowledgements

The authors wish to thank the Researchers Supporting Project number (RSP2024R346) at King Saud University Riyadh Saudi Arabia for financial support. The authors express their sincere thanks to the Faculty of Science Mohammed V, Rabat, Morocco, for providing the necessary research facilities. They acknowledge the support of Pr. Mhamed TAIBI from CSM-LPCMIO/ENS for performing FT-IR analyses.

Funding information: This study was supported by the Researchers Supporting Project number (RSP2024R346) at King Saud University Riyadh Saudi Arabia.
Author contributions: B.H. – methodology, investigation, validation, formal analysis, and writing – original draft, writing, review, and editing; F.S. – data curation, formal analysis, writing, investigation, and reviewing; S.A. – reviewing and editing; B.A. and F.H. – funding acquisition. All the authors agreed on the final version of the manuscript.
Conflict of interest: Authors state no conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2023-11-06

Revised: 2024-05-08

Accepted: 2024-05-21

Published Online: 2024-06-14

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/chem-2024-0048

Keywords for this article

geopolymer; mechanical properties; durability properties; acid attack; fire resistance

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BY 4.0

Physico-chemical properties and durability of a fly-ash-based geopolymer

Article

Abstract

1 Introduction

2 Methods

2.1 Characteristics of the raw material and synthesis procedure

2.2 Sample preparation

2.3 Material characterization

3 Durability tests

4 Results and discussion

4.1 Mechanical and microstructural characterizations

4.1.1 Effect of Na2SiO3/NaOH ratio and curing time on bulk density and water absorption

4.1.2 Effect of Na2SiO3/NaOH ratio and curing time on the compressive strength of geopolymers

4.1.2.1 FT-IR spectroscopy

4.1.2.2 XRD analysis

4.1.2.3 SEM analysis

4.2 Durability performance

5 Conclusions

Acknowledgements

References

Articles in the same Issue

Articles in the same Issue

Articles in the same Issue

4.1.1 Effect of Na₂SiO₃/NaOH ratio and curing time on bulk density and water absorption

4.1.2 Effect of Na₂SiO₃/NaOH ratio and curing time on the compressive strength of geopolymers