A new type of geopolymer with an iron–oxygen–silicon linkage is synthesized and reported for the first time. The aim was to enable the iron-rich clay material (laterite) as a raw material for the geopolymerization. Iron was used in different ratios ranging 1–3 wt. % in the raw mix designing geopolymer followed by activation with concentrated alkali solutions of NaOH and KOH in different concentrations. The bonding of Fe–O–Si was confirmed from the FTIR peaks in NaOH- and KOH-based geopolymers. X-ray diffraction studies confirmed the formation of zeolitic, sodalite, and almandite phases. The final product has shown a compressive strength of 2371.8 and 1503 kN/m2 and can be used as a construction material.

Geopolymeric materials are emerging polymers, synthesized in 1982 for the first time by Davidovits. Geopolymer is a three-dimensional polymer having various types of amorphous and semi-crystalline phases with a linkage of Si–O–Al. The polymers have tetrahedral coordinated links of Si4+ and Al3+ ions with oxygen linkage as bridges. Alkali ions Na+ (sodium ion)/K+ (potassium ion) balance the negative charge of AlO4− groups.1,2 In general, the empirical formula of geopolymers can be given as
Mn[(SiO2)zAlO2]nwH2O27,28,
where M represents alkali ions (Na+ and K+), n is the degree of poly-condensation, and z is the molar ratio of silica to alumina.

Geopolymer consists of mineral molecular network/chains which are bonded by covalent linkage containing various kinds of molecular linkages, including sialate–polysialate (–Si–O–Al–O–), siloxo–polysiloxo (–Si–O–Si–O–), –disiloxo–sialate/disiloxo–polysialate–polysialate (–O–Si–O–Al–O–Si–O–Si–), siloxo–sialate, siloxo–polysialate (–Al–O–Si–O–Al–O–Si–), phosphate/polyphosphate (–P–O–P–O–), phospho–siloxo/polyphospho–siloxo (–O–Si–O–P–O–P–), organo–siloxo/poly–silicone [–(R)–Si–O–Si–O–(R)], and phospho–sialate/polyphospho–sialate (–O–Si–O–Al–O–P–O–P–O–).3,4

Geopolymerization reaction is a process in which alumino-silicates from its minerals containing metakaolinite polymerize in a strong basic/alkaline medium.5 This reaction is a complex process, which still needs deep study and research is going on currently. A very simple and general mechanism includes the condensation type of polymerization among orthosialiate ions, being taken as assumed monomers of geopolymer as proposed by Davidovits in 1982 and is given as
(Si2O5Al2O2)n+H2nOKOH/NaOHn(OH)3SiOAl(OH)3(OH)3SiOAl(OH)3KOH/NaOH(Na,K)(OSiOOOAlOO)n.
(1)

Equation (1) shows the geopolymer synthesis having the ratio of silica and alumina as 1. The ratio is not necessarily be one and may vary depending on the elemental composition of the starting material like aluminosilicates and alkaline solution activators. The usual geopolymerization process includes the dissolution of silica and alumina from the precursor and poly-condensation resulting in the geopolymer.6 

In the present time, Portland cement is the most applicable binder in construction industry, but its manufacturing is not environment friendly because a very huge quantity of greenhouse gas (carbon dioxide) emission occurs from the kiln;2,7,8 however, it is highly expensive as it requires as high temperature as 1450 °C in cement kiln during clinkerization. This high temperature is achieved from the burning of fuel, including coal, natural gas, or oil, which makes its production cost reasonably high due to the highest rates of fuel. According to a rough calculation, about one ton of carbon dioxide is produced with the production of one-ton cement during the calcination of limestone and combustion of fuel, such as furnace oil and coal. Furthermore, the cement strength drops at a higher temperature, which may be due to some physical and chemical changes.3,9 Spalling is another problem associated with concrete structure containing Portland cement, which may result in a layer by layer corrosion of concrete layer and the exposure of steel present in the concrete to fire.10 

Addressing all of the above problems associated with Portland cement, geopolymer cement is the best option being a more environment friendly substitute for Portland cement in many applications11,12 as its synthesis does not involve the limestone calcination and, hence, the release of carbon dioxide. Moreover, the fuel consumption during geopolymerization is comparatively very low as it does not involve as high temperature as in the Portland cement. Geopolymer cement is also assumed to have excellent fire-resisting quality13 with ceramic-like characteristics.

Geopolymer has many advantages as compared to Portland cement with respect to compressive strength development, excellent resistance to different aggressive environments, including acids and basis, thermal resistivity in high-temperature applications, relatively low discharge of greenhouse gases, good setting and hardening, and so on. The extraordinary interest in graphene is a result of its outstanding mechanical and chemical properties, such as its enormous surface area (2630 m2 g−1), good heat conductivity, high Young’s modulus, very high light transmittance of 98%, excellent gas impermeability, chemical stability, and exceptional quantum hall properties.14 The characteristic resistance of geopolymer against high temperature is of pronounced attention3,15 as the organic polymers are very much affected by the high temperature and most of them burn above 400 °C.11 Similarly, the Portland cement also deteriorates above 500 °C,16 while geopolymer remains stable and un-deteriorated up to 1000 °C;17 hence, it serves as a promising material for a diversity of applications. Previous research18 has proved that geopolymer synthesized from fly ashes, slags, and thermally activated clay (meta-kaolinite) shows comparatively higher strength compared to that of an un-calcined material, such as kaolinite, proving that a calcined material leads to excellent geopolymerization. This is why natural aluminosilicate materials need to be thermally activated prior to geopolymerization at different temperatures depending on the nature of the materials.19 Industrial wastes including ashes are calcined to a greater extent and have been extensively studied as starting materials for geopolymers synthesis not only because of the widespread characteristics of products but also due to several substantial ecofriendly benefits, such as comparatively low discharge of greenhouse gases and fuel requirements in comparison to the Portland cement.20 

The purpose of this research includes the synthesis of iron-based geopolymer that is another class of geopolymer containing iron in the main chain instead of some aluminum. The main objective is to utilize the normal clay containing iron, instead of kaolinite. With the help of this work, the clay found abundantly in Khyber Pakhtunkhwa will have the potential of better properties as compared to the existing geopolymer.

For the synthesis of iron-based geopolymers, laterite clay samples were collected from the District of Nowshera and sodium silicate, sodium hydroxide, potassium hydroxide, and iron hydroxide (Merk) from the market Peshawar, Pakistan. Double-deionized water was used from our inorganic laboratory of Abdul Wali Khan University Mardan. The chemical composition of raw materials used is given in Table I.

TABLE I.

Chemical composition of laterite clay.

wt. %
SiO2Al2O3Fe2O3CaOMgOK2ONa2OMoisture/H2OLOI
63.92 17.06 5.88 3.91 2.3 3.13 3.1 0.099 0.64 
wt. %
SiO2Al2O3Fe2O3CaOMgOK2ONa2OMoisture/H2OLOI
63.92 17.06 5.88 3.91 2.3 3.13 3.1 0.099 0.64 

Two types of geopolymers using sodium hydroxide and potassium hydroxide, respectively, were prepared to have SiO2/Al2O3 of 2.5 and different amounts of iron and were named GPN and GPK, respectively; details of which are given in Table II. For making the raw mixes, a thermally treated clay sample at a temperature of 900 °C was blended with sodium silicate and different amounts of iron hydroxide at an appropriate ratio as given in Table II. The mixes containing iron were blended using a blender and then a slurry was made by adding NaOH and KOH having a concentration of 3M each. The thoroughly blended slurry was molded into a cubic mold having 50 mm of internal diameter as per ASTM C109. The cubes after drying were placed in the oven at 60 °C for 24 h. The samples were then de-molded and treated in the oven at 60 °C for 7 days.

TABLE II.

Raw mix designing for geopolymers with different ratios of iron.

Raw material taken% weight taken
Clay 74.3 61.9 50.5 36.3 25.4 
Aluminum hydroxide 10.4 11.9 13.8 15.8 17.2 
Sodium silicate 15.3 26.1 37.7 47.9 57.4 
Fe (wt. %) 3.00 2.50 2.00 1.50 1.00 
Si/Al 2.5 2.5 2.5 2.5 2.5 
Raw material taken% weight taken
Clay 74.3 61.9 50.5 36.3 25.4 
Aluminum hydroxide 10.4 11.9 13.8 15.8 17.2 
Sodium silicate 15.3 26.1 37.7 47.9 57.4 
Fe (wt. %) 3.00 2.50 2.00 1.50 1.00 
Si/Al 2.5 2.5 2.5 2.5 2.5 

Laterite clay found in Khyber Pakhtunkhwa has been used as a starting material, and the selection of which was based on its natural abundance containing iron as well. In order to get the required composition and ratio of iron in the mixes, a small amount of iron has also been blended along with sodium silicate. The material was studied using X-ray fluorescence spectroscopy (XRF), XRD, and FTIR for different parameters. Laterite clay was studied for loss on ignition (LOI) using a laboratory muffle furnace at 1000 °C and for elemental composition, including silica, iron, alumina, magnesium, and calcium using XRF as clear from Table I.

The resulting geopolymer composites were studied for different physical and chemical parameters using SEM, FTIR, and XRD techniques. The compressive strength of the geopolymer composites produced after being subjected to hydrothermal treatment for a week was evaluated using a Universal Testing Machine (UTM).

The elemental composition of the laterite clay used in this work is given in Table I. Clay contains a high quantity of silica which is the major component of clay. Other elements, such as MgO, Al2O3, CaO, and Fe2O3, are also present in smaller amounts as clear from Table I. Elemental composition of laterite clay shows that it constitutes silica and alumina as 63.92% and 17.06%, respectively, as the major constituents along with some minor constituents, including Fe2O3, CaO, and MgO, as given in Table I.

According to ASTM, this clay may also demonstrate natural pozzolana due to silica, iron, and alumina in sufficient amounts. On ignition, the kaolinite present in the meta-kaolinite loses water in the de-hydroxylation and, therefore, shows a 14% loss of ignition.

Figure 1 shows the IR spectra of clay as such and thermally activated clay, respectively. The peak at 3600–3700 cm−1 corresponds to the OH group present in the kaolinite (Fig. 1). Qualitative and quantitative features regarding the order and disorder of clay can be assumed from the intensities of these peaks (Slavik et al.). The presence of external and internal surface OH groups can be seen from the peaks at 3620 cm−1.21,22 The quantitative extent of orderness/disorderness of kaolinite may be obtained from the disappearance peaks at 3620 cm−1.23 The clay used in this study as per the above ratios of intensities has a disordered structure. The spectrum of the thermally treated clay at 900 °C [Fig. 1(b)] shows that the peaks assigned to –OH disappear completely, which exists in the peak of untreated samples of clay. The stretching frequency at 979 cm−1 corresponds to the presence of –Si–O bonds in SiO4. The two peaks in untreated clay, which are consigned to –Si–O, transform into a single peak in the spectra of thermally treated clay, representing the existence of amorphous silica in thermally treated clay. The peak at 794 cm−1 in the spectrum of untreated clay moved to 775 cm−1 in the spectra of thermally activated clay, which shows that clay has been transformed into an amorphous form. The whole discussion shows that the transformation of kaolinite into meta-kaolinite and an amorphous form may be confirmed by the IF studies.

FIG. 1.

FT–IR spectrum of clay sample: un-calcined (a) and calcined at 900 °C (b).

FIG. 1.

FT–IR spectrum of clay sample: un-calcined (a) and calcined at 900 °C (b).

Close modal

Figure 2 represents the XRD patterns of as received (untreated) and thermally activated samples, respectively. Comparing the peaks of both the clay samples, it can be observed that all the peaks assigned to the kaolinite clay in the untreated clay reduce to a greater extent and some disappear in the pattern of thermally treated clay showing that, on thermal activation, clay becomes amorphous.24 It can be concluded that the thermal treatment of clay is required before submitting it to geopolymerization.

FIG. 2.

XRD pattern of the clay sample: un-calcined (a) and calcined at 900 °C (b).

FIG. 2.

XRD pattern of the clay sample: un-calcined (a) and calcined at 900 °C (b).

Close modal

1. FT–IR studies

Figure 3 represents the FT–IR spectra of geopolymers modified with iron in the main chain having potassium hydroxide and sodium hydroxide as alkaline activators, respectively. From Fig. 3(a) with KOH as an activator, it is clear that the main peaks appear at 3452, 3526, 1021, 891, 945, 751, 670, 617, 559, 450, 514, 421, and 409 cm−1. Similarly, in the peak of geopolymer with NaOH as an activator, the peaks appear at 1431, 979, 882, 797, 695, 640, 603, 556, 458, and 433 cm−1 [Fig. 3(b)]. Comparing the two spectra, it can be observed that the spectrum of KOH-based geopolymers has a greater number of peaks as compared to that of NaOH-based geopolymers. This confirms the effectiveness of KOH as an alkaline activator as compared to that of NaOH in geopolymerization reactions and bringing structural changes.

FIG. 3.

FT–IR spectrum of iron-modified geopolymer obtained by using: KOH solution (a) and NaOH solution (b).

FIG. 3.

FT–IR spectrum of iron-modified geopolymer obtained by using: KOH solution (a) and NaOH solution (b).

Close modal

The peaks at 3526 cm−1 with 93% transmittance, may be due to O–H stretching, confirm the growth of the silanol (Si–OH) group, which may also be confirmed by the absence of the peak due to H–O–H bonds at 1500–1600 cm−1 range. Observing the peak of NaOH-based geopolymers, no such peak can be seen, evidencing no growth of the silanol group. The peak at 1431 cm−1 in NaOH alkaline activator-based geopolymer may be assigned to the O–C–O stretching in the carbonate (CO3−2) group. It is believed that sodium carbonate is formed by the reaction of atmospheric carbon dioxide with surplus NaOH present in the geopolymer. In the spectrum of KOH-based geopolymers, there is no such peak appeared, showing that KOH takes part only in geopolymerization reactions while no side-reactions occur.

Curious observance of Fig. 3(a) shows that two strong peaks at 1021 and 945 cm−1, while one strong peak in Fig. 3(b), i.e., with sodium hydroxide as an alkaline activator at 979 cm−1, show the stretching vibration of Si–O–Si and Al–O–Si. Similarly, the peaks at 891 and 882 cm−1 both in potassium hydroxide and sodium hydroxide-based geopolymers, respectively, may be ascribed to the symmetric-stretching of Si–OH. The peak in the spectrum of clay of Si–O–Si and Al–O–Si at 575 cm−1 shown in Fig. 1(b) is observed to shift to 559 cm−1, i.e., low frequency in KOH-based geopolymers while to 556 cm−1 in sodium hydroxide-based geopolymer. The shifting peak from one position in clay to another position in the geopolymer confirms not only the changes in microstructure because of the development of new products but also gives information on the substitution of SiO4 by tetrahedral AlO4, which leads to the change of bonding setting of Si–O. The greater shifting of peaks in potassium hydroxide geopolymer as compared to that of sodium hydroxide-based geopolymers may be because KOH causes greater changes compared to that of NaOH in the microstructure of clay to form geopolymer.25 

The peak at 670 and 514 cm−1 in the spectrum of KOH-based geopolymer [Fig. 3(a)], while that at 695 and 640 cm−1 in NaOH-based geopolymer [Fig. 3(b)], may be assigned to Si–O–Fe linkage.26 Observing the intensity of Fe–O–Si in both spectra, it can be concluded that the bond in KOH-based geopolymers is much stronger than that of NaOH-containing geopolymers. Similarly, the band appearing in the range of 620–600 cm−1 may be assigned to the Al–O bond (bending vibration) in either of the spectra, as clearly shown in Figs. 3(a) and 3(b), respectively. The sharp peaks appeared at 421 and 409 cm−1 in KOH-based geopolymers, assigned to Al–O and Si–O, respectively, while in NaOH geopolymers, the same peaks appear at 458 and 433 cm−1 suggesting the creation of geopolymer. From the intensities of peaks in each spectrum, it can be concluded that, in KOH-based geopolymer, the degree of geopolymerization is better than that of NaOH-based geopolymer.

2. XRD analysis

Figure 4 shows the XRD patterns of iron-modified geopolymer prepared in this work. At the initial stages, the geopolymer synthesized seems to be amorphous. Thermally activated clay on alkali activation results in the crystalline phase development including sodium chabazite and hydroxyl sodalite. XRD diffractogram of iron-modified geopolymer using 6M potassium hydroxide and 8M sodium hydroxide solutions as alkaline activators is given in Figs. 4(a) and 4(b), respectively. Major peaks appear at 2θ of 18° to 50° for potassium hydroxide-based iron-modified geopolymers, whereas for sodium hydroxide containing iron-modified geopolymer, the major peaks appear at 2θ of 18° to 60°, which may be due to the presence of zeolite, quartz, sodalite, hematite, and almandite phases. From these peaks, it is obvious that both the geopolymers seem to be semi-crystalline in nature for having diffraction peaks as well as hump at 2θ value of 8°–18°. On close examination of the humps in the pattern of potassium hydroxide-based geopolymers is relatively wider and sharper as compared to the pattern of sodium hydroxide-based geopolymer samples. Both the patterns reveal some peaks at 2θ values of 21°, 37°, 50°, and 60° from starting raw material but relatively with condensed intensity, which are assigned to some unreacted quartz contents. Both the patterns, i.e., KOH and NaOH-based geopolymers, may also be described with peaks at 29° and 33°, associated with the almandite phase. In both the patterns, the peaks at 18° correspond to the zeolite phase. The peak at 46° in the XRD pattern of KOH-activated geopolymers is assigned to the sodalite phase, whereas the pattern of NaOH-based geopolymers does not consist of such peak. The larger peak intensity of almandite and condensed peak intensities due to quartz and hematite in potassium hydroxide-containing geopolymer in comparison to sodium hydroxide-containing geopolymers noticeably show that iron has participated in the geo-polymeric reaction to give the Fe–O–Si linkage. The intense peak and wider hump of almandite with reduced intensities of quartz and hematite peaks in geopolymer with potassium hydroxide as an alkaline activator than that with sodium hydroxide-based geopolymers means that the former shows better activity than latter alkaline solution with respect to the synthesis of iron-modified geopolymer.

FIG. 4.

XRD pattern of iron-modified geopolymer using: KOH solution (a) and NaOH solution (b).

FIG. 4.

XRD pattern of iron-modified geopolymer using: KOH solution (a) and NaOH solution (b).

Close modal

3. Compressive strength

Figure 5 shows the compressive strength of iron-modified clay-based geopolymers containing iron in the range of 1, 1.5, 2, 2.5, and 3 wt. % both with KOH and NaOH solutions prepared under optimum conditions. Results reveal that the strength of potassium hydroxide-based geopolymers is greater than that of sodium hydroxide-based geopolymers. Moreover, a regular decrease in the strength of KOH-based geopolymers with the increase in the content in the geopolymer starting material and an increase in the compressive strength in the case of NaOH-based geopolymers are observed with the increase in the iron content. When the percentage of iron increases from 2.5–3, the compressive strength decreases again. The variation in the compressive strength may be due to the precipitation of iron ions with hydroxyl ions, affecting the dissolution of aluminates and silicates, which results in the decline of polymerization. Geopolymer with 1 wt. % of iron with potassium hydroxide showed the highest compressive strength, i.e., 2371.8 kN/m2, while in the case of sodium hydroxide-based geopolymers, the highest compressive strength was found to be 1503 kN/m2 with 2.5 wt. % of iron. Some of the irons present in the clay also contributed to the geopolymerization process to give Fe–O–Si linkage imparting a complimenting role in the enhanced compressive strength.

FIG. 5.

Compressive strength of iron-modified geopolymers using KOH and NaOH solutions.

FIG. 5.

Compressive strength of iron-modified geopolymers using KOH and NaOH solutions.

Close modal

4. SEM analysis

SEM micrograph of KOH and NaOH-containing iron-modified geopolymers is shown in Fig. 6. It is clear from the figure that potassium hydroxide-based geopolymers have layered structure morphology with some roughly established constructions in the shape of mesas on the surface. The SEM micrographs disclose the smooth and unchanging geopolymer gel with some unreacted particles of clay embedded on the surface. The SEM micrograph image of sodium hydroxide-based geopolymers is shown in Fig. 6(b) that demonstrates some consistently dispersed, distinct, and distinguishable arrangements, typically in the shape of particles and perpendicular developments with various dimensions. The micrographs illustrated that potassium hydroxide-based geopolymers are comparatively more consistent as compared to that of sodium hydroxide-based iron geopolymers. The micrographs also have some minor pits and claps, which may be due to the testing of geopolymer for different parameters.

FIG. 6.

SEM images of iron-modified geopolymers using KOH (a) and NaOH (b) as alkaline activators.

FIG. 6.

SEM images of iron-modified geopolymers using KOH (a) and NaOH (b) as alkaline activators.

Close modal

This research is a concrete step toward the possible solution of the present cement with respect to the production of greenhouse gas emissions and the great demand for fuel during clinkerization by introducing a new type of cement. Moreover, most of the locally obtainable clay containing iron content can be used as a starting material for the preparation of geopolymer. The newly synthesized geopolymer can compete with the existing Portland cement with a compressive strength of 2040.85 kN/m2, which shows that it can be used as a good and green construction material.

Researchers support Project number (RSPD2024R1060), King Saud University, Riyadh, Saudi Arabia.

The authors have no conflicts to disclose.

Akbar Ali: Conceptualization (equal); Investigation (equal); Writing – original draft (equal). Noor-ul-Amin: Data curation (equal); Investigation (equal); Supervision (equal). Hamza Ahmad: Project administration (equal); Resources (equal); Software (equal). Sana Noor: Methodology (equal); Project administration (equal); Resources (equal). Sabiha Sultana: Conceptualization (equal); Methodology (equal); Validation (equal). Huzaifa Umar: Conceptualization (equal); Supervision (equal); Visualization (equal). Hijaz Ahmad: Investigation (equal); Writing – review & editing (equal). Fuad A. Awwa: Formal analysis (equal); Resources (equal); Validation (equal). Emad A. A. Ismail: Conceptualization (equal); Project administration (equal); Writing – original draft (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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