Geopolymer Materials Based on Natural Pozzolans from the Moroccan Middle Atlas

Abstract

The pozzolans of the Moroccan Middle Atlas are derived from a low explosive volcanism, mostly strombolian. They are mainly composed of olivine and pyroxene, presenting a less homogeneous structure (irregular vesicles). The main target of this project is to study the use of natural pozzolans (NP) and metakaolin as precursors for the production of geopolymeric binders. The characterization of raw materials and elaborated geopolymers was carried out to study their mineralogical, chemical, microstructural, and mechanical properties. The studied pozzolans and kaolin were crushed, grinded, and sifted to get a fine grain size diameter of less than 100 µm. Then, they were calcined at 750 °C for 2 h to achieve an amorphous structure, increasing of their reactivity. Geopolymer production consists of mixing pozzolans and metakaolin with different amounts with an alkaline solution of sodium hydroxide and sodium silicates. The mass proportion of metakaolin (MK) used in this study was 10%, 20%, and 30%. In the present work, the amount of metakaolin was added as a source of alumina. The elaborated geopolymers were characterized using XRD, FTIR, TGA, and SEM analyses. The compressive strength was measured at 7, 14, and 28 days. The results showed interesting mechanical proprieties at about 18 MPa at 28 days with the mixture containing 20% MK. The addition of MK showed a significant increase in mechanical properties of the elaborated geopolymer. Meanwhile, the other results confirmed the training of new phases in addition to N-A-S-H gel. All these results indicate that the use of pozzolans in the production of geopolymers could be a great solution for the sustainable management of this mineral resource.

1. Introduction

(NP) is a volcanic aluminosilicate; its name originates from the city of Pozzuoli in Italy, where it was discovered. NPs are pyroclastic rocks formed by explosive volcanic eruptions. During explosive volcanic eruptions, the pressure decreases, leading to the formation of a microporous structure caused by the release of gases dissolved in magma elements. Simultaneously, the rapid cooling process results in their glassy structure [1,2].

The literature has proved that NP has cementing properties, and therefore, it largely employed as a supplementary cementitious material (SCM) in the Ordinary Portland cement (OPC) industry [1]. Pozzolanic properties are the main properties reflecting the cementitious activity of NPs, which generally depends on the content of reactive SiO₂ and Al₂O₃ [3]. NPs are constituted of sodic-calcic plagioclase feldspars, volcanic glass, amphiboles, pyroxenes, and quartz [4]. NP’s mineralogical composition and pozzolanic reactivity allow their use in the formulation of geopolymers to replace or substitute OPC.

Geopolymers are a category of semi-crystalline aluminosilicate products manufactured at room or moderately elevated temperature by a chemical reaction between an aluminosilicate powder and a concentrated alkaline solution. They are recognized by the following general Formula (1):

M + n {(SiO₂)z, AlO₂} n, wH₂O

(1)

where:

• M+: monovalent cation,
• z: Si/Al molar ratio,
• n: degree of polymerization,
• w: the amount of water.

Geopolymers are differentiated by the Si/Al molar ratio. The synthesis of geopolymers can be summarized in four steps [5,6,7,8,9]. The first step is the dissolution of the “reactive” glass of the source material, rich in Si⁴⁺ and Al³⁺, when in contact with the alkaline solution [10,11,12]. This chemical attack causes a rupture of the Si–O–Si, Si–O–Al and Al–O–Al bonds, characterizing the structure of the source material. This “destruction–dissolution” leads to the formation of monomers, which are tetrahedrons of SiO₄ [13]. The second step consists in the “coagulation” and reorganization of these monomers to form a chain comprising tetrahedrons. A cation located in the cavity of the chain allows the neutralization of the ionic charge due to the replacement of a silicon tetrahedron by an aluminum tetrahedron. As the reaction progresses, increasingly more Si–O and Al–O bonds are broken, promoting the formation of 3D-structure gel. The chemical composition of this gel is similar to zeolites, but it presents an amorphous structure [5]. A wide variety of aluminosilicates, such as metakaolin, slags, ashes, clays, and NPs are used as sources for the formulation of geopolymers.

NPs are proved to have pozzolanic properties in their natural state or after calcination. However, the calcination of NPs could increase their reactivity and then their mechanical properties [14]. Thus, there are many studies on geopolymers obtained from NPs, but few on calcined pozzolans (CPs). Geopolymers synthesized from NPs [15] attained cementitious material from pozzolanic soil mixed with NaOH and Na₂SiO₃ activators [16]. They mentioned that the geopolymerization reaction takes place when dissolving the alumino-silicate phases and reorganizing the newly formed crystalline phases. Research in [17] and [18,19,20] investigated the impact of the activator ratio and polymerization temperature on the UCS of geopolymers obtained from sodium-solution-activated zeolites. These two studies proved the possibility of carrying out the geopolymerization of pozzolanic material and that the increase in the activator concentration improves the mechanical strength of the geopolymers. The two main characteristics of raw material that affect the geopolymerization are the content of amorphous constituents and the SiO₂/Al₂O₃ ratio [5]. The SiO₂/Al₂O₃ ratio should be between 2 and 4, and the reactive amorphous phase should be at least 50% by weight. NPs that fulfill these characteristics could be a good option for obtaining geopolymers [5,21,22,23]. However, the use of several methods for improving the reactivity of NPs were carried out, such as prolonged grinding, elevated curing temperatures, and calcination [14,24]. These methods have shown an important improvement in the reactivity of NPs. Bondar et al. [14] calcined NPs at 700 °C, 800 °C, and 900 °C to enhance their mechanical properties, especially their compressive strength. Their results revealed calcination leads to moderate to high strength in geopolymers based on calcined pozzolans. The calcination process allows breaking Si–O–Si, Si–O–Si and Al–O–Al bonds to provide reactive spices able to dissolve easily in the alkaline solutions.

The present paper aims to use the NPS as precursors in alkali-activation technology and investigate the enhancement of their reactivity through the calcination process, as this method is not sufficiently studied in the literature. On the other hand, Morocco disposes of huge volcanic deposits that are not exploited. To the authors’ knowledge, no previous studies have been interested in the alkali-activation of pozzolans derived from volcanic Moroccan deposits.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Moroccan Volcanic Deposits NPs

The volcanic cone of Jbel Hebri is located in the M.A.M and presents a strombolian dynamism that gives a global illustration of the activity of the majority of the volcanoes of the quaternary volcanic plateau of the Middle Atlas Causse. It is characterized by basaltic flows with an alkaline character and stratified projection products (pyroclastic) as well as the enclaves of various origins (mantle, crustal, gabbroic accumulates) [25,26].

The volcanoes of the Middle Atlas have displayed a large amount of volcanic activity. These phenomena highlight the formation of large quantities of natural volcanic pozzolans. Pozzolanic rocks are dense and loose pyroclastic materials with honeycomb structures and are mainly composed of volcanic glass present in variable sizes (blocks, ash, lapilli) and different colors (red, black), as shown in Figure 1. The pozzolan rocks used in the present work were sampled from the Middle Atlas, and they were dehydrated in a furnace at 105 °C for 24 h. Then, they were crushed, ground, and sieved to obtain a particle-size diameter of less than 100 µm. Figure 2 shows the distribution size curves of natural pozzolans and kaolin. The grinding was performed using a rotary ball mill for 10 min with 600 rpm. Finally, the acquired powder was calcined at 750 °C for 2 h at 10 degrees Celsius/minute. The calcination temperature was chosen founded on the TGA results and literature [14].

2.1.2. Kaolin and Metakaolin

The kaolin (K) powder was supplied from Spain. Both kaolin and pozzolans were calcined at 750 °C for 2 h at a throughput of 10 °C/min in order to reach the maximum reactivity that allows a rapid dissolution of aluminum and silicium. The choice of the temperature was based on the TGA results. The chemical and physical composition of (K) and pozzolans are summed up in

Table 1. Chemical and physical properties of NP and K.

Components	LOI	$\mathbf{\%}\mathbf{S}\mathbf{i}{\mathbf{O}}_2$	$\mathbf{\%}\mathbf{A}{\mathbf{l}}_2{\mathbf{O}}_3$	$\mathbf{\%}\mathbf{F}{\mathbf{e}}_2{\mathbf{O}}_3$	%CaO	%MgO	$\%\mathbf{Ti}{\mathbf{O}}_2$	$\mathbf{\%}\mathbf{N}{\mathbf{a}}_2\mathbf{O}$	$\mathbf{\%}{\mathbf{P}}_2{\mathbf{O}}_5$	$\mathbf{\%}{\mathbf{K}}_2\mathbf{O}$	%MnO
Pozzolans	18.89	35.42	11.04	11.25	8.66	8.00	2.24	2.00	1.28	0.89	0.18
Metakaolin	10.15	51.4	36.43	0.29	0.25	0.29	0.15	0.12	0.08	0.4	0.01
Physical properties			D10 (µm)	D50 (µm)	Density (g/cm3)			Specific surface (m2/g)
	NP		3.70	19.53	1.83			0.42
	K		1.02	3.11	2.61			17.27

LOI = Loss of Ignition.

.

2.2. Geopolymers Synthesis

The elaboration of the geopolymer pastes consists of mixing the alkaline solution, constituted by NaOH (10 M) and Na₂SiO₂ (SiO₂/Na₂O = 3.2), with the calcined pozzolans and metakaolin at different amounts. The addition of MK provides more alumina to the mixtures, leading to a lower Si/Al, about 2, as required by the literature [27]. The percentage of metakaolin did not succeed 30% because the paste does not harden at more than this amount at an ambient temperature. The alkali hydroxide is believed to be an element that allows the dissolution of Al/Si, and the alkali silica functions as a binder and dispersant [28,29].

The geopolymers were developed by the following specific steps:

• Mix the raw material (powder: CP + MK) with alkaline solution until element is well mixed (5 to 10 min).
• Put the mixture into cylindrical plastic molds with a diameter of 2 mm and height of 40 mm, and then close them to avoid humidity.
• Demolding of the samples after curing at ambient temperature for t = 24 h.
• The geopolymers are then tested for UCS (7, 14, and 28) days.

Figure 3 shows the experimental procedure for producing geopolymers based on pozzolans and metakaolin.

2.3. Analysis Methods

This research is based on different analyses that allow the characterization of the materials. The mineralogical composition was determined based on X-ray diffraction (XRD), using a Bruker D8-advanced diffractometer with CuKa radiation, scanning step size 0.02, and time/step 0.6 copper anode with Kα = 1.5415 Å.

Fourier-transform infrared spectroscopy (FTIR) is a technique based on radiation-matter interactions valence, vibrations, or elongation (stretching), for which the distance between 2 atoms increases or decreases, and the axis of the bond remaining unchanged. Vibrations of angular deformation (bending) noted δ, for which the position of the atoms varies compared to the axis of the original bond. The spectrometer used was a Bruker Optics Vertex 70.

A laser analyzer, Mastersizer 2000, was used to determine the particle size distribution of raw materials. The surface area was measured with BET equipment using N₂ adsorption with sample degassed at 150 °C for 24 h.

The thermogravimetric analysis (TGA) was performed at 800 °C for 2 h, and the temperature values corresponding to the mass losses provided complementary information that was obtained using differential thermal analysis (DTA) [30]. The apparatus used in this study was named “TA TGA 55”, which had a maximum temperature of 1000 °C and a speed of 10 °C/min. Scanning electron microscopy (SEM) consists of using the electrons emitted by the surface of a solid when an electron beam, whose energy is a few KeV, bombards it. At the point of impact, several rays are emitted, but only the secondary and backscattered electrons contribute to the formation of the topographic image of the sample. This technique provides information on the morphology of raw materials or finished products [30]. SEM observations were performed using a Tescan Vega Microscope equipped with a tungsten filament electron gun.

3. Results and Discussion

3.1. Characterization of Raw Materials

3.1.1. XRD

The results of the XRD study (Table 1) show a high silicon oxide content of 35.42%, which is explained by the presence of augite (pyroxene) and forsterite (olivine). The aluminum oxide content of 11.04% comes from the augite shown in the XRD spectrum. The iron oxide in the sample reaches 11.25% from hematite. The presence of forsterite and pyroxenes explains the content of manganese oxide, which is 8%. The pozzolan sample has other less abundant elements found in the pyroxenes. The calcination of NPs promotes their reactivity [24]. Figure 4 shows the diffractograms of NPs and CPs; it can be observed that peaks corresponding to augite and diopside phases decreased with the calcination, which is in accordance with the literature [31]. However, the nepheline phase is the newly formed phase, which could result from the recrystallization of the sample. Nepheline is known to have taken part in geopolymerization [32]. In addition, the amorphous aspect was observed for the CPs, as shown in Figure 4.

Regarding kaolin, the amount of silicon oxide is about 51.4%, justified by the abundance of quartz. The calcination of kaolin at 750 °C for 2 h leads to the dehydroxylation reaction and the destruction of clay minerals, producing an amorphous structure presented by the hump observed in Figure 2 [33,34]. The quartz phase still appeared after calcination; as mentioned by several works, quartz is not affected by a temperature around 800 °C [35].

3.1.2. FTIR

The infrared spectrum of the pozzolans present the typical olivine–pyroxene band. From 1150 and 700 cm⁻¹, there is a strong band with a peak of 1024 cm⁻¹, which concordant to the (Si–Al–O, Si–O) elongation vibrations. The band at 609 cm⁻¹ is attributed to form (SiO₄), and the little band at 526 cm⁻¹ resembles to the deformation vibrations of the (O–Si–Al–O) junctions. The band observed at 902 cm⁻¹ could be attributed to the elongation vibrations of the (Al–OH) and (Si–OH) junctions. The other band located at 808 cm⁻¹ appeared only in CPs and indicates the elongation vibrations of the (Si–O) junction. This fact indicates the dehydroxylation of the powder. Finally, the strips observed at 3431 cm⁻¹ and 1647 cm⁻¹ resemble the (-OH) groups. After the calcination of the pozzolans at 800 °C for 2 h, (O–H) bond disappears, which is in agreement with the escape of water, and the little strips nearby the 526 and 609 cm⁻¹ resemble the vibrations of the deformation of (O–Si–Al–O) junctions [15].

The FTIR spectrum shows the usual main bands of metakaolin (Figure 5), outlined as such: below 900${\mathrm{cm}}^{-1}$, the 790${\mathrm{cm}}^{-1}$ and 468${ \mathrm{cm}}^{-1}$ strips are related to the elongation vibrations of Si–O–(Si, Al) bonds. Around 1000 cm⁻¹, namely the 1011${ \mathrm{cm}}^{-1}$ band, it is associated to (Si–O–Si) strips. The strips around 3600 cm⁻¹ and 1600 cm⁻¹ are due to the tare elongation and deformation vibrations of the (O–H) bond (the molecular water absorbed in the metakaolin structure) [36]. These bands completely disappeared after calcination due to water loss.

3.1.3. TGA-DTG

The TGA and DTG analysis of NP and kaolin (K) from the ambient temperature to T = 900 °C at a heating rate of 10 degrees Celsius/min showed three weight losses corresponding to the total weights of 16.7 and 9.0% for NP and K, respectively. For the NPs, the first weight deficit from T = 25 °C to T = 300 °C corresponds to the elimination of free and bound water from the surface [37]. The second weight deficit from 300 °C to 550 °C is due to the dehydroxylation of structural hydroxyl groups and the dehydroxylation of NP [38,39]. The third weight deficit is not significant, and this could be attributed to the variation of the thermos balance during the measurements (Figure 6).

On the other hand, the TGA and DTG curves of kaolin showed a small mass deficit observed in the interval of 50–150 °C, which is related to the deficit of acquired water. The second deficit between 550 and 750 °C is a major mass deficit of about 9%. This deficit is correlated to the dehydroxylation of (K) and its transformation to (MK); this treatment is carried out in a temperature interval of 500–750 °C [40,41].

3.1.4. SEM

The microstructure (SEM) image (Figure 7) indicates that pozzolans have irregularly shaped aggregates with varying sizes, ranging from approximately 0.8 µm to 14 µm. The SEM photographs also show that the pozzolans have a very porous structure. The SEM images indicate a laminated structure in the form of a stack of small mille-feuille platelets. This structure is characteristic of kaolinite and is in agreement with the XRD analysis, which shows the abundance of kaolinite [14].

3.2. Characterization of the Elaborated Geopolymers

3.2.1. XRD

Moreover, the effect of these peaks is considerable, which is in line with the results of the following authors.

The XRD spectra of the formulations developed after 28 days of hardening are indicated in (Figure 8). Quartz (SiO₂) is a mineral originating from metakaolin. Enstatite (Mg₂Si₂O₆), diopside (CaMgSi₂O₆), and augite ((Ca,Na)(Mg,Fe,Al)(AL,Si)₂O₆) are minerals derived from CP. Faujasite ((Na₂,Ca,Mg)_3.5, 32(H₂O)) is a new phase formed after alkaline activation. The diffractograms allow us to observe the appearance and/or the decrease or the disappearance of some peaks. The nepheline phase that exists in CP no longer appears in the elaborated geopolymer. This fact explains that this mineral participates in geopolymerization [32]. However, the other crystalline phases, such as diopside, augite, and quartz, observed in the diffractograms of raw materials (Figure 7) are not affected by geopolymerization. The faujasite phase is generally synthesized under the conditions of extreme temperature and pressure resulting from the degradation of the silicate and aluminum [42]. Furthermore, the addition of metakaolin to the CPs makes the effect of these peaks considerable, which is in line with the results of [43,44]. The peaks observed between 22° and 58° in CP shift slightly to the values 18° to 48° in elaborated geopolymers (P1, P2, P3, and P4). This could be due to the degradation of the raw material and the formation of a more recent phase that are present in the geopolymer materials due to the SS/SH ratio of the alkaline solution [45,46,47]. This ratio is also related to the degree of polycondensation during geopolymerization [48].

3.2.2. FTIR

Figure 9 indicates the IR bands of the elaborated geopolymers. The O–H stretching strips situated at 3466 cm⁻¹ are granted to surface-adsorbed and chemical-bound water [49]. The bands at 1650 cm⁻¹, attributed to H–O–H flexural wave propagation bands, contribute to the water molecules related to the products of the reaction [50]. The major band at 1000 cm¹ corresponds to the Si–O–Al/Si elongation vibration.

After the alkali reaction, this spectrum shifted from 1024 cm⁻¹ to a lower value, showing that non-connecting oxygen particles became connecting oxygen particles on the surface of the aluminosilicate [51]. The bands at 758 cm⁻¹ and 739 cm⁻¹ are attributed to the Si–O vibration due to the existence of quartz. The absorption bands at 1445 cm⁻¹ correspond to the C=O stretching vibration resulting from the atmospheric carbonation of the pastes [4]. Finally, the bands at 567 cm⁻¹ and 460 cm⁻¹ conform to the O–Si–Al–O bond deformation vibrations, indicating the dissolution and participation in geopolymerization. This result correlates with those obtained by DRX [3]. Based on these results, as found in IR, we determined that the increase in metakaolin leads to the rise of the Si/Al ratio, causing the substitution of Al to form the Si–O–Si bond; more substituted Al allows for the formation of the Si–O–Al junction [39].

3.2.3. Compressive Strength (UCS) and Physical Properties

Figure 10 indicates the (UCS) of P 1, P 2, P 3, and P 4 samples at 7, 14, and 28 days of hardening. It can be seen that the progress of the resistance does not indicate a clean trend for the limited hardening period (7 and 14 days); it varies from 2.9 MPa to 18.7 MPa for all samples.

This last point is consistent with the work of Tchakoute et al. Sample P1 made with 100% CP showed a (UCS) of 15.8 MPa at 28 days. With the addition of metakaolin, the strength of the geopolymers produced increased to 18.7 MPa and then decreased to 10 MPa for 30% MK. This last point is consistent with the work of Tchakoute et al. [52]. They revealed that an excess of alumina resulting from the use of MK reduces the stickiness of the geopolymers and does not allow a rapid dissolution of the CP particles, influencing the quality of the mechanical strength by a decrease that reaches 10 MPa, which is remarkable for the P4 sample [4]. The presence of metakaolin provides the required amorphous aluminosilicates, promoting the geopolymerization reaction [53,54]. Furthermore, Na₂O/Al₂O₃ and SiO₂/Al₂O₃ ratios varied from one mixture to another. Both decreased with the addition of MK. The maximum compressive strength was obtained with 20% of metakaolin, a SiO₂/Al₂O₃ ratio of 2.04 and a Na₂O/Al₂O₃ of 0.44. These values are approximately the same as those proposed by Duxson [55]. The decrease in resistance noticed for P4 and attributed to the excess of alumina, which hinders the dissolution of CP particles. On the other hand, this decrease could result from the addition of water, as shown in

Table 2. Experimental parameters of elaborated geopolymers.

Formulation	% Metakaolin	% Pozzolans	L/S	NaOH/Na2SiO3	SiO2/Al2O3	Na2O/Al2O3
P 1	0	100	1	1	2.73	0.65
P 2	10	90	1	1	2.28	0.50
P 3	20	80	1.10	1	2.04	0.44
P 4	30	70	1.25	1	1.84	0.36

; the increase in the percentage of MK demands more water to make the paste workable. Finally, the compressive strength in most CP- and CP+MK-based geopolymers increases with increasing curing age from 7 to 28 days of hardening.

The density and water absorption are physical characteristics that are essential to evaluate the physical character of the CP-MK mixture. Density and water absorption measurements of the processed geopolymers were measured and added in

Table 3. UCS at 28 days and physical properties of elaborated geopolymers.

Samples	UCS at 28 Days (MPa)	Density (g·cm−3)	Water Absorption (%)
P1	15.8	2.15	18.25
P2	11.2	1.89	22.76
P3	18.7	2.39	15.97
P4	10.0	1.64	24.54

. The measurements of the two parameters are in agreement with the UCS values, as it can be seen that P3, which has the higher density of 2.39 g·cm⁻³, has a minimum water absorption of 15.97%, which confirms that the UCS of P3 reached 18.7 MPa.

3.2.4. SEM

The scanning electron microscope elaborated geopolymers are presented in Figure 11. The samples P1, P2, P3, and P4 present a dense structure, but sample P1 shows some cracks, contrary to the other samples. The absence of cracks in pastes P2, P3, and P4 can be explained by the addition of metakaolin, which is not the case for paste P1 [56,57].

According to [4,58], the absence of cracks in geopolymers indicates an increase in their compaction and a high quality of strength and endurance. This is in agreement with the value of compressive strength found in our study.

The presence of cracks in P1 influences the strength of geopolymeric materials. The addition of metakaolin allowed a more compact and uniform texture than geopolymer made only of pozzolan. P4 showed the presence of significant small pores, cracks, and fissures, explaining the mechanical resistance to aging at 28 days. In addition, the image of P4 showed unreacted CP particles immersed inside the geopolymeric binder, confirming that an excess of alumina affects the dissolution of CP particles.

4. Conclusions

Moroccan NPs are produced during volcanic emissions in the Middle Atlas, which formed a chain of volcanoes in the Ifrane–Azrou region as well as the Timhdite plateau and Jbel Habri. Pozzolans are volcanic ash materials with a trachytic composition and are characterized by their friability and black or red color. They are used in the manufacturing of cement, geopolymers, and lightweight concrete.

NP was calcined at 750 °C in order to gain better reactivity to the powder. Metakaolin was added to the mixtures P1, P2, P3, and P4 in different amounts of up to 30% to increase the alumina content and improve the UCS of geopolymers. Maximum UCS was about 18.7 MPa and was obtained for 20% metakaolin, which is explained by the presence of more alumina, which promotes the dissolution, condensation, and reorganization reaction. In addition, the physical properties showed that P3 presents a higher density and minimum water absorption. The addition of 30% MK decreased the mechanical strength of the geopolymer to 10 MPa with a maturation period of 28 days, showing that an excess of alumina stopped the dissolution of CP particles. In parallel, it was observed that the presence of MK necessitates an increase in a liquid-to-solid ratio, negatively influencing the strength and texture of the mixture.

The alkaline activation of the CP material utilizing the blend of sodium hydroxide (SH) and sodium silicate (SS) allows the formation of sodium aluminum silicate hydrate and faujasite, which is a zeolite resulting from the dissolved reactive powder and the activator. However, the XRD and FTIR results confirm that some phases are not affected by the alkali-activation reaction, such as augite, forsterite, and diopside. In addition, the natrite was formed by the carbonation of the materials. All these results show that the utilization of pozzolans to produce a binder could be an effective method for the use of NPs presenting the reduction in CO₂ emissions.