# Extrudability of geopolymers and control of the formed networks by zeta potential and NMR spectroscopy

Wilfried Cyrille N'Cho, Ameni Gharzouni, Isabel Sobrados, Jenny Jouin, Anne Aimable, Sylvie Rossignol

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Wilfried Cyrille N'Cho, Ameni Gharzouni, Isabel Sobrados, Jenny Jouin, Anne Aimable, et al. Ex-trudability of geopolymers and control of the formed networks by zeta potential and NMR spec-troscopy. International Journal of Ceramic Engineering & Science, 2025, 7 (1), 10.1002/ces2.10244. hal-05404983

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RESEARCH ARTICLE

# Extrudability of geopolymers and control of the formed networks by zeta potential and NMR spectroscopy

Wilfried Cyrille N'Cho<fnref n="1" />

Ameni Gharzouni<fnref n="1" />

Isabel Sobrados<fnref n="2" />

Jenny Jouin<fnref n="1" />

Anne Aimable<fnref n="1" />

Sylvie Rossignol<fnref n="1" />

Correspondence

Sylvie Rossignol, IRCER: Institute for
Research on Ceramics (UMR CNRS 7315),
European Center for Ceramics, 12 rue
Atlantis, 87068 Limoges Cedex, France.
Email: sylvie.rossignol@unilim.fr

Abstract

In this study, the local-scale structure of geopolymers shaped by extrusion or 3D printing was investigated and correlated to the reactivity of the raw materials. The reactivity of the different metakaolin mixtures was evaluated using zeta potential measurements with different alkali silicate solutions, followed by shaping via 3D printing; additionally, the different networks formed were identified via NMR spectroscopy. The results showed that in the presence of weakly reactive basic solutions, the Al(OH)₄⁻ species were released in low amounts due to an increase in the polymerized silicate species in the solution, resulting in a low zeta potential in absolute value. Conversely, for more reactive solutions, the silicate species were more depolymerized, and the siliceous and aluminous metakaolin species were more easily released in solution, resulting in increasingly low zeta potential values. Some samples were 3D printed with metakaolin mixtures using different printers and silicate solutions (K, KNa) with alkaline cation concentrations of [M] > 2.8 M in the mixtures; here, the metakaolin zeta potential values were greater than -50 mV, and the local-scale structure consisted of more than 50% geopolymer networks (Q⁴(3Al) + Q⁴(2Al)).

KEYWORDS

3D printing, local-scale, network, NMR spectroscopy, shaping, zeta potential

1 | INTRODUCTION

Geopolymers are inorganic materials synthesized through
the reaction of an aluminosilicate in an alkaline solution.<fnref n="1" />
Geopolymer materials can develop interesting properties
in the fresh or consolidated state, and these properties
depend on the reactivity of the raw materials that leads
to the formation of the geopolymer networks or differ-
ent networks. These results are generally caused by the
aluminosilicate sources that induce different reactivities

depending on their geological origins or the different treat-
ments, thus highlighting the complexity linked to their
reactivity.<fnref n="2" />,³

Among all raw materials that can be used as alumi-
nosilicate sources to make geopolymers, metakaolin is one
of the most commonly used due to its high reactivity,⁴
as shown by several techniques that measured its wet-
tability and amorphousness using reactive Al in a study
from Gharzouni et al.⁵,⁶ The reactivity of metakaolins can
be determined by zeta potential measurements. The zeta

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2024 The Author(s). *International Journal of Ceramic Engineering & Science* published by Wiley Periodicals LLC on behalf of American Ceramic Society.

<footnote n="1">¹IRCER: Institute for Research on
Ceramics (UMR CNRS 7315), European
Center for Ceramics, Limoges, France</footnote>

<footnote n="2">²Instituto de ciencia de materiales de
Madrid, Consejo superior de
investigaciones científicas (CSIC),
Madrid, Spain</footnote>
potential represents the surface charge of a particle in sus-
pension due to the electrostatic interactions between the
particles in solution.⁷,⁸ The characteristics of the surface
charges are essential for understanding the behavior and
reactivity of clay minerals in acidic or basic media. In basic
media, the AlOH hydroxyl sites are deprotonated in favor
of the surface groups of AlO surface with negative surface
charges.⁹ At pH > 11, Al(OH)₄⁻ predominates.¹⁰,¹¹ In the
case of silica, SiO⁻ surface complexes are formed due to
the deprotonation of the SiOH group,¹² and the siliceous
species present in a highly basic medium are commonly
[SiO₂(OH)₂]²⁻ and [SiO(OH)₃]⁻. Furthermore, the densi-
ties of the ionized groups increase with the amplitude of
the zeta potential.⁹,¹³

Several studies of clay mineral dispersion, particularly kaolinite dispersion, using zeta potential measurements as a function of pH have shown that kaolinite zeta potential values are approximately -50 mV for pH values close to 11.14⁻¹7 Yukselen and Kaya¹⁵ showed that the zeta potential of kaolinite was -42 mV at a pH of 11 and became more negative with increasing pH. They also showed that the zeta potential was sensitive to ion valency in solution, with higher values in the presence of Ca²⁺ or Mg²⁺ and lower values in the presence of Na⁺ and Li⁺. Furthermore, Vane and Zang¹⁷ reported that the zeta potential of kaolinite ranged from +0.7 mV at a pH of 2 to -54 mV at a pH of 10 in a KCl solution, while the zeta potential of bentonite changed by only 5 mV (-31 to -36 mV) over the same pH range. For both clays, the ionic strength was found to have a weak effect, while divalent cations markedly increased the zeta potential. Furthermore, the zeta potential was likely sensitive to the alkali cation size. Kosmulski and Dahlstena¹⁸ showed that for the same electrolyte concentration and pH, the absolute ζ potential decreased according to the series Li ≥ Na > K > Cs. An increase in electrolyte concentration resulted in a decrease in the absolute value of the ζ potential of the clay minerals. Moreover, the absolute ζ potentials at high ionic strengths were high.¹⁸ These results showed that the zeta potential of kaolinites changed according to the chemical species present in the solution. Other researchers have focused on the zeta potential of metakaolin in water suspensions. Geng et al.¹⁹ highlighted that the zeta potential of an MK-water dispersion significantly decreased with increasing solution pH, and resulted in a stable dispersion at pH 8 with a zeta potential of -40 mV. Thus, pre-dispersing MK in water improved the compressive strength compared to that of the MK powder; these results showed the passage of reactive metakaolin species into the solution. Recently, our studies on the reactivity of different metakaolin mixtures have shown that less reactive metakaolin (containing impurities) have zeta potentials less than -50 mV at a pH of 11, while more reactive metakaolin have zeta potentials

between –70 and –80 mV²⁰; thus, these results indi-
cate the passage of more reactive metakaolin species in
solution, showing that the zeta potential is important for
evaluating metakaolin reactivity.

NMR spectroscopy using ²⁷Al and ²⁹Si is commonly employed to identify the Al and Si atomic environment, respectively, in the geopolymers and the connectivity of the silicon atom with the aluminum that forms the geopolymer network Si–O–Al following the procedure by Engelhardt notation Q⁴(nAl).²¹ The formation of networks within the geopolymer matrix is based on the analysis of the NMR spectra and relies on the contributions of the geopolymer network (Q⁴(3Al) + Q⁴(2Al)), the contribution of the aluminum Q⁴(4Al) and the contribution of the silicon-rich phase (Q⁴(1Al) + Q⁴(0Al)). Many studies²²,²³ have revealed that more reactive metakaolins are characterized by the formation of geopolymer networks with greater contributions from the species Q⁴(3Al) and Q⁴(2Al). However, metakaolins with low reactivity or impurities, such as silica, lead to inhomogeneous solutions and favor the formation of different networks, especially the coexistence of silica-rich networks Q⁴(1Al), Q⁴(0Al) and aluminum networks Q⁴(4Al). Lyu et al.²⁴ highlighted that the species Q⁴(4Al) and Q⁴(3Al) are the main contributors to the geopolymer networks and govern the mechanical characteristics via ²⁹Si and ²⁷Al NMR spectroscopy. Prasanphan et al.²⁵ showed that increasing the geopolymer Si/Al molar ratio to 1.58 led to the transformation of Q⁴(4Al) and Q⁴(3Al) species to the Q⁴(2Al) structure and decreased the AlIV content. This increased the mechanical strength up to 79% due to the silica-rich Si–O–Si networks of the three-dimensional aluminosilicate structures. However, the content of the Q⁴(4Al) and AlIV structures increased, with a Si/Al ratio of 1.62, and this led to a 15% decrease in the mechanical strength due to the predominance of the Si–O–Al bonds and Al–O–Al and the formation of the crystalline zeolite phase. Moreover, the structural analysis of the geopolymers using ²⁷Al and ²⁹Si NMR spectroscopy performed by Barbosa and Mackenzie²⁶ revealed that geopolymer networks with high Al/Si contents composed of oxo-aluminate (–Si–O–Al–O–) or silicon sialate (–Si–O–Al–O–Si–O–) units exhibited thermal stability up to 1400°C, whereas silica-rich networks such as sialate-disiloxo (–Si–O–Al–O–Si–O–Si–O–) units became friable and porous at temperatures above 1200°C. These results showed the stability of the geopolymer working properties in the presence of Si–O–Al networks in the structure.

The results from this study show that knowledge of
the Si and Al atomic environment in the geopolymers in
relation to the reactivity of the raw materials is essen-
tial for understanding their working properties, especially
their mechanical strength and thermal stability. However,
<table><caption>TABLE 1 Nomenclature and composition of the different precursors.</caption><thead><tr><th rowspan="2">Name</th><th rowspan="2">Supplier</th><th colspan="4">Weight composition(%)</th><th rowspan="2">Density (g/L)</th><th rowspan="2">SiO<sub>2</sub>nominal/ Al<sub>2</sub>O<sub>3</sub></th><th rowspan="2">SiO<sub>2</sub> reactive/ Al<sub>2</sub>O<sub>3</sub></th><th rowspan="2">H<sub>2</sub>O/ K<sub>2</sub>O</th><th rowspan="2">H<sub>2</sub>O/ Na<sub>2</sub>O</th><th rowspan="2">Heating process</th></tr><tr><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>H<sub>2</sub>O</th><th>M<sub>2</sub>O M = K or Na</th></tr></thead><tbody><tr><td>M5</td><td>ARGECO</td><td>59.9</td><td>35.3</td><td></td><td></td><td>1.44</td><td></td><td>1.00</td><td></td><td></td><td>Flash</td></tr><tr><td>M2</td><td>IMERYS</td><td>54.0</td><td>39.0</td><td></td><td></td><td>1.19</td><td></td><td>1.00</td><td></td><td></td><td></td></tr><tr><td>M1</td><td></td><td>55.0</td><td>40.0</td><td></td><td></td><td>1.17</td><td></td><td>1.00</td><td></td><td></td><td>Rotary furnace</td></tr><tr><td>KI</td><td></td><td>54</td><td>46</td><td></td><td></td><td>1.00</td><td></td><td>1.00</td><td></td><td></td><td></td></tr><tr><td>S1</td><td>WOELLNER</td><td>16</td><td></td><td>79</td><td>6.41</td><td>1182.00</td><td></td><td></td><td></td><td>12.32</td><td></td></tr><tr><td>S3</td><td></td><td>19</td><td></td><td>59</td><td>21.92</td><td>1506.00</td><td></td><td></td><td></td><td>2.69</td><td></td></tr><tr><td>K</td><td></td><td>18</td><td></td><td>66</td><td>17.02</td><td>1386.04</td><td></td><td></td><td></td><td>3.88</td><td></td></tr><tr><td>SNa</td><td></td><td>28</td><td></td><td>64</td><td>8.30</td><td>1360.00</td><td></td><td></td><td></td><td>7.71</td><td></td></tr><tr><td>KNa</td><td></td><td>21</td><td></td><td>60</td><td>19.34</td><td>1475.95</td><td></td><td></td><td></td><td>3.10</td><td></td></tr></tbody></table>

the behavior of the reactive metakaolin species in the presence of alkali silicate solutions of different reactivities and the impact on the local network and shaping by 3D printing are not fully understood. Therefore, the aim of this work was to evaluate the behavior of different metakaolin mixtures in the presence of different silicate alkaline solutions and their impact on shaping by 3D printing and investigate the local scale structure of the geopolymer materials formed. First, the reactivity of different raw materials was determined by zeta potential measurements using several alkali silicate solutions. Subsequently, geopolymers were prepared with alkaline silicate solutions, and their initial viscosities were determined, as well as the printability of some samples with two different printers. Finally, the local-scale structure of the geopolymers was investigated via magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy.

## 2 | EXPERIMENTAL SECTION

### 2.1 | Raw materials

The aluminosilicate source was a mixture of different metakaolins, M1, M2 (supplied by Imerys), M5 (supplied by Argeco), and a kaolin KI (Imerys) and was calcined at 750°C as described in previous work.²⁷ Different commercial alkaline solutions were used as alkali activators: KOH, NaOH at 0.1 mol/L, and silicate alkaline solutions with the following Si/M (with M = K or Na) ratios: S3 (0.67), S1 (1.7), K (0.8), SNa (1.7), and KNa (0.78), all provided by Woellner. Table 1 gives the different precursors compositions. The samples studied within the ternaries are denoted by letters with l/s (water-solid) values between 0.9 and 1.2. For example, A₁ and A'₁ are used for the K and KNa solutions, respectively.

### 2.2 | Methods of characterization

To be “extrudable,” a paste must fulfill the following requirements: the paste must exit the nozzle without clogging the nozzle tip, its shape must be retained once extruded, and the paste cannot flow when the extrusion force is released. Therefore, the paste needs to have a specific rheology as a Bingham fluid, with a non-Newtonian behavior and an adequate initial yield stress. To examine the overall printability of the paste, the geopolymer paste was extruded by hand using a 30 mL plastic syringe (BD Plastik) with a tip diameter of 2.0 mm on Teflon supports. Printing tests were then carried out using two printers, a Delta WASP 2040 and a Potterbot 7. In the WASP 2040 printer, geopolymer paste was introduced into a syringe and then extruded through a nozzle with a diameter ranging from 0.4 to 2 mm equipped with a piston. This piston was activated by pressurized air at a speed of 30 mm/s. With the Potterbot 7 printer, the geopolymer paste was introduced into a chamber and then passed through the nozzle (diameter between 2 and 6 mm) with the help of a piston at an extrusion speed of 10 mm/s.

Viscosity tests were carried out on fresh pastes with a rotational Brookfield Viscometer DV2T coupled with a low shear, low viscosity cylindrical spindle LV-04 (64), and the rotational speed was varied, started at 100 rpm (for viscosity up to 6 Pa·s) and ended at 1 rpm (up to 600 Pa·s). Cylindrical polystyrene vessels (Ø = 28 mm) were used. For each test, the pastes were kept sealed in cylindrical vessels and continuously stirred with a roller mixer to keep them homogenized. Viscosity analysis enabled the determination of two major parameters of geopolymer reactions: the initial viscosity η₀ and the time of setting t_c, from which the paste began to quickly react and then consolidate.

The zeta potential is the electric potential developed at the shear plane of a particle dispersed in a liquid medium.
<figure><img src="image_2.png" /><figcaption>FIGURE 1 Evolutions of M1 and M5 metakaolin zeta potentials as a function of pH for (♦) KOH, (*) NaOH and silicate alkaline solutions (■) S3 (0.67), (●) S1(1.7), (□) K (0.8), (▲) SNa (1.7), and (Δ) KNa (0.78).</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>FIGURE 2 Metakaolin zeta potentials of (●) M1, (▲) M1, (♦) M2, and (■) M5, and the mixtures of (○) MIM1, (Δ) MIM5, (□) M1M5, and (◇) MIM1M5 as a function of the (A) silica concentration for the different alkaline solutions and (B) conductivities.</figcaption></figure>

The evolution of the zeta potential was monitored as a function of the pH. First, a suspension of 0.25 g of the sample was dispersed in 25 mL of distilled water by using a sonotrode for 90 s with 3 s intermittence. Then, the zeta potential was measured with a Colloid Metrix Stabino II zetameter (Meerbusch). The pH was adjusted using (0.1 M) solutions of K and KNa by adding a 15 µL volume of solution every 20 s.

MASNMR measurements were carried out at room temperature using a high-resolution MAS-NMR spectrometer (Bruker AVANCE-400 spectrometer), with the MAS probe rotating at 10 kHz. The resonance frequencies were 104.26 MHz for the ²⁷Al signal and 79.49 MHz for the ²⁹Si signal. MAS-NMR spectra of ²⁹Si (*I* = 1/2) were obtained with a π/2 pulse (4 µs) using a 500 kHz filter. To record the central transitions of aluminum (²⁷Al) (*I* = 5/2), a
$\pi/8$ pulse (1.5 µs) was applied along with a 1 MHz filter. The spectra of the samples obtained via silicon or aluminum NMR were processed and decomposed using Dmfit software (CNRS).²⁸

# 3 | RESULTS AND DISCUSSION

## 3.1 | Metakaolin zeta potential measurements

To understand the characteristics of the different metakaolins in different alkali silicate solutions (KOH, NaOH, S1, S3, SNa, K, and KNa), zeta potential measurements were carried out with these solutions at decimolar concentrations. Figure 1 shows the evolution of the zeta potential for MI and M5 metakaolins with different potassium and sodium silicate solutions (Figure 1) as a function of pH. The plots for the other samples are shown in the Supporting Information.

Regardless of the alkaline silicate solution used, the zeta potential curves increased with the addition of base. To classify the different metakaolins, the zeta potential at a pH of 11 was used to standardize the results.

Whatever the metakaolin or alkaline solutions, the zeta potential values are dependent on the solution. These different values show that the zeta potential is highly dependent on the counterions in the solution.²⁹ Indeed, these differences are likely due to the characteristics of the silicate solutions, such as the concentrations of silicon counterions or the alkali present. To understand the differences in the zeta potential between the alkali silicate solutions, the zeta potential values were expressed as a function of the Si cation concentration [Si] (Figure 2A). Regardless of the metakaolin used with the different alkali silicate solutions, the zeta potentials tend to increase with increasing Si concentration (Figure 2A). Specifically, a more reactive solution correlates to a lower amount of silicon. Consequently, for low Si concentrations and a high M concentration, the zeta potential is high (in absolute value). In this case, the attack of the metakaolin surface is favored, and the species can easily pass into solution. For high Si concentrations, the presence of the species with low depolymerization limits the release of the metakaolin species into solution, leading to the low surface charges of the aluminosilicate sources. To validate this deduction, the zeta potential values were correlated with the conductivity values. The conductivities are listed in Table 2. Figure 2B shows the variation of the zeta potential as a function of the solution conductivity. Low zeta potentials (in absolute value) are obtained at high conductivities for the alkaline solutions with low reactivity (2.00–3.60 mS/cm); however, high zeta potentials (in absolute values) are obtained at low conductivities (0.40–0.90 mS/cm) for the alkaline solutions with high reactivity. The very low zeta potential values are associated with low conductivity owing to ion mobility. Due to the alkaline solutions with low reactivity, the Al(OH)₄⁻ anions are released in limited quantities because of the weak attack forces of the polymerized silicate solutions, leading to a high conductivity. For the depolymerized silicate solutions, the attack forces are stronger, and siliceous and aluminous species can pass through the solution.

<table><caption>TABLE 2 Conductivities of various solutions (pH = 11) in the presence of metakaolin MI, M1, M5, and M2 and mixtures of M1MI, M1M5, M1M5, and M1MIM5.</caption><thead><tr><th rowspan="2">Metakaolins</th><th colspan="6">Conductivity (mS/cm)<br>(0.05 mS/cm)</th></tr><tr><th>S1</th><th>S3</th><th>NaOH</th><th>SNa</th><th>KNa</th><th>K</th></tr></thead><tbody><tr><td>MI</td><td>3.50</td><td>0.49</td><td>0.41</td><td>2.60</td><td>1.94</td><td>0.58</td></tr><tr><td>M1</td><td>2.92</td><td>0.63</td><td>0.43</td><td>3.23</td><td>2.49</td><td>0.72</td></tr><tr><td>M5</td><td>3.59</td><td>0.67</td><td>0.54</td><td>3.51</td><td>2.27</td><td>0.78</td></tr><tr><td>M2</td><td>3.46</td><td>0.69</td><td>0.48</td><td>3.10</td><td>2.47</td><td>0.79</td></tr><tr><td>MIM1</td><td></td><td></td><td></td><td></td><td>2.00</td><td>0.55</td></tr><tr><td>MIM5</td><td></td><td></td><td></td><td></td><td>2.31</td><td>0.75</td></tr><tr><td>M1M5</td><td></td><td></td><td></td><td></td><td>2.44</td><td>0.87</td></tr><tr><td>M1MIM5</td><td></td><td></td><td></td><td></td><td>2.32</td><td>0.72</td></tr></tbody></table>

## 3.2 | Casting: Viscosity measurements

Geopolymers were prepared from alkali silicate solutions K and KNa based on the metakaolin ternary diagrams M1MIM5 and M1M2M5. Figure 3 presents some points selected to examine the fresh properties, mainly the viscosity, of the geopolymers. The other compositions have been tested and are included in our previous article.²⁷ The initial viscosities were measured. The different viscosities are plotted in Figure 4A as a function of the alkali cation concentration in the mixtures. The viscosities can be classified based on the used solutions: the viscosities are greater for the KNa solution and lower for the K solution. In both cases, the viscosity zones of the mixtures, extrudable or not, are defined by a limiting alkaline cation concentration of 2.80 mol/L.²⁷ Consequently, two zones can be defined. At low alkali cation concentrations, the K-based solution leads to partial dissolution of the metakaolins due to the presence of impurities in the metakaolins, and this restricts the passage of reactive species into solution in agreement with the metakaolins zetas potential values (Figure 4B) and prevents extrusion. In the presence of the KNa solution, due to the intrinsic characteristics of the sodium cation, such as the presence of a hydration sphere, the
<figure><img src="image_4.png" /><figcaption>FIGURE 3 Extrudability domain of ternary diagrams of (A) M1MIM5 and (B) M1M2M5 as a function of the alkaline silicate solutions K and KNa.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>FIGURE 4 Viscosities of formulations containing (◇, □, ○, △) extrudable and (◆, ■, ●, ▲) non-extrudable compounds as a function of the (A) alkali cation concentration [M] and (B) metakaolin zeta potentials.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>FIGURE 5 Printed samples (size 2.5 cm) in the viscosity range as a function of the alkali cation concentration [M] with the two printers of (—) Delta WASP 2040 and (—) Potterbot 7.</figcaption></figure>

viscosity reduces the diffusion of the cations and further
leads to poor dissolution of the metakaolins, thus limiting

the extrudability of the paste. When the alkaline concen-
tration increases above this limit, the dissolution of the
metakaolin is favored regardless of the solution. Thus, the
reactive species can easily pass into the solution, and poly-
condensation reactions are facilitated, causing an easier
extrusion of the pastes.

These data show that a solution with an optimal con-
centration of alkaline cations in the mixtures leads to
better dissolution of the metakaolins and therefore favors
extrusion.

3.3 | 3D printing

To validate these hypotheses, some samples need to be
printed in 3D. To do this, printability tests were carried
out on eight samples using two different 3D printers, a
“Delta WASP 2040” with nozzle sizes ranging from 0.4 to
1.6 mm and a “Potterbot 7” printer for nozzle sizes 2 and
6 mm. Figure 5 shows all results according to viscosity.
Regardless of the different formulations, most formula-
tions could be printed by selecting different nozzle sizes
<figure><img src="image_7.png" /><figcaption>FIGURE 6 $^{27}$Al NMR spectra and the contributions and deconvolutions of the geopolymers of (A) GMI, (B) GM1, (C) GM5, and (D) GMIM1M5.</figcaption></figure>

and printer types. Only one formulation, with a low vis-
cosity (7.28 Pa·s), could not be printed as it was at the
extrudable limit of C (GM1<sub>0.5</sub>M2<sub>0.25</sub>M5<sub>0.25</sub>) due to a fluid
paste. In fact, the sample C is a mixture containing highly
reactive metakaolin M2. Indeed, the high reactivity of M2
metakaolin associated to the potassium solution with a
K-cation of high diffusion induces fast passage of alu-
minous species in solution, leading to fluid pastes after
extrusion. Experimentally, samples A (GM5<sub>0.7</sub>MI<sub>0.3</sub>) and D
(GM1<sub>0.5</sub>MI<sub>0.25</sub>M5<sub>0.25</sub>) were the formulations based on the
potassium solution with a concentration of 5 mol/L con-
taining metakaolin M5 and were printed with a 1.6 mm
nozzle; however, sample B (GM1<sub>0.75</sub>M2<sub>0.25</sub>) was printed
with a 0.8 mm nozzle. This was caused by the high silica
content of metakaolin M5, which modified the mixture and
required an increase in the nozzle size. Notably, these for-

mulations were too fluid to be printed with the Potterbot 7
printer, even if the nozzle size was changed.

Changing from solution K to sodium solution KNa led
to an increase in the formulation viscosity. The formu-
lations obtained with metakaolin M5 containing silica
were printed with 0.4–2 mm nozzles for A'(GM1<sub>0.5</sub>M5<sub>0.5</sub>),
B'(GM1<sub>0.5</sub>M2<sub>0.25</sub>M5<sub>0.25</sub>), and C'(GM1<sub>0.33</sub>MI<sub>0.33</sub>M5<sub>0.33</sub>). For
the D'(GM1<sub>0.5</sub>M2<sub>0.5</sub>) sample, a 0.8 mm nozzle was used. All
these formulations were easily printable due to their high
viscosities (A', B', C', and D'). Other M5-based formula-
tions could also be printed with a Potterbot 7 printer using
a 6 mm (B') nozzle, enabling modification of the final size
of the pieces.

These first tests showed that different samples could be
printed with different mixtures by changing the size of the
nozzle and the printer type.
<table><caption>TABLE 3 Positions (ppm), FWHM, and relative area (%) of the geopolymers of GMI, GM1, GM5, GM1M5, GIM5, GMIM1, and GMIM1M5 and the 3D-printed samples of A, B, C, D, A', B', C', and D' deduced from ²⁷Al NMR spectra.</caption><thead><tr><th>Contributions</th><th colspan="8">Samples</th></tr><tr><th></th><th>GMI</th><th>GM1</th><th>GM5</th><th>GM1M5</th><th>GMIM5</th><th>GMIM1</th><th>GMIM1M5</th></tr></thead><tbody><tr><td rowspan="4">Al<sup>IV</sup></td><td>57.24</td><td>57.70</td><td>58.00</td><td>57.70</td><td>57.40</td><td>57.50</td><td>57.70</td></tr><tr><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td></tr><tr><td><strong>98.28</strong></td><td><strong>74.41</strong></td><td><strong>79.29</strong></td><td><strong>76.25</strong></td><td><strong>86.13</strong></td><td><strong>81.53</strong></td><td><strong>81.48</strong></td></tr><tr><td>-</td><td>54.00</td><td>54.00</td><td>56.60</td><td>54.90</td><td>54.80</td><td>56.00</td></tr><tr><td rowspan="4">Al<sup>VI</sup></td><td>-</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td></tr><tr><td>-</td><td><strong>11.73</strong></td><td><strong>13.86</strong></td><td><strong>11.85</strong></td><td><strong>8.74</strong></td><td><strong>9.91</strong></td><td><strong>10.38</strong></td></tr><tr><td>3.50</td><td>1.70</td><td>1.90</td><td>1.50</td><td>1.70</td><td>1.70</td><td>1.70</td></tr><tr><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td></tr><tr><td></td><td><strong>1.72</strong></td><td><strong>13.85</strong></td><td><strong>6.86</strong></td><td><strong>11.90</strong></td><td><strong>5.13</strong></td><td><strong>8.55</strong></td><td><strong>8.14</strong></td></tr><tr><th>Contributions</th><th colspan="8">Samples</th></tr><tr><th></th><th>A</th><th>B</th><th>C</th><th>D</th><th>A'</th><th>B'</th><th>C'</th><th>D'</th></tr><tr><td rowspan="4">Al<sup>IV</sup></td><td>57.60</td><td>57.60</td><td>57.60</td><td>57.70</td><td>57.60</td><td>57.60</td><td>57.70</td><td>57.50</td></tr><tr><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td><td>(11.00)</td></tr><tr><td><strong>98.28</strong></td><td><strong>72.37</strong></td><td><strong>72.80</strong></td><td><strong>75.31</strong></td><td><strong>76.01</strong></td><td><strong>74.11</strong></td><td><strong>82.55</strong></td><td><strong>77.00</strong></td></tr><tr><td>54.90</td><td>54.10</td><td>54.10</td><td>54.30</td><td>54.90</td><td>53.00</td><td>54.90</td><td>54.30</td></tr><tr><td rowspan="4">Al<sup>VI</sup></td><td>-</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td></tr><tr><td>-</td><td><strong>10.66</strong></td><td><strong>13.33</strong></td><td><strong>12.41</strong></td><td><strong>12.03</strong></td><td><strong>9.76</strong></td><td><strong>10.40</strong></td><td><strong>7.83</strong></td><td><strong>10.18</strong></td></tr><tr><td>1.85</td><td>1.50</td><td>1.50</td><td>1.50</td><td>1.85</td><td>1.85</td><td>1.85</td><td>1.85</td></tr><tr><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td><td>(12.00)</td></tr><tr><td></td><td><strong>12.51</strong></td><td><strong>14.30</strong></td><td><strong>14.79</strong></td><td><strong>12.66</strong></td><td><strong>14.23</strong></td><td><strong>15.49</strong></td><td><strong>9.63</strong></td><td><strong>12.82</strong></td></tr></tbody></table>

## 3.4 | Local-scale structure: NMR spectroscopy

Structural analysis using as ²⁷Al and ²⁹Si NMR spectroscopy was initially carried out on the seven geopolymer samples using K solution (GMI, GM1, GM5, GM1₀.₅M5₀.₅ GM1₀.₅MI₀.₅, GMI₀.₅M5₀.₅, and GMI₀.₃₃M1₀.₃₃M5₀.₃₃) and on the 3D-printed samples of A, B, C, D, A', B', C', and D' to verify the formation of the local networks. As an example, only the spectra of the GMI, GM1, GM5, and GMI₀.₃₃M1₀.₃₃M5₀.₃₃ samples are shown in the figures. Other sample spectra are shown in the Supporting Information.

### 3.4.1 | ²⁷Al MASNMR spectroscopy

Figure 6 shows the ²⁷Al MASNMR spectra of the GMI, GM1, GM5, and GM1₀.₃₃MI₀.₃₃M5₀.₃₃ geopolymers. Regardless of the formulation, the different spectra show two main contributions: aluminum with a coordination of 4 (δ = 60 ppm), indicating the formation of a geopolymer network. In the consolidated samples, no pentacoordinate aluminum is observed. The contribution of the six-coordinate aluminum (δ = 0 ppm) is related to unreacted aluminosilicate species such as kaolin or the presence

of impurities.²³ For all spectra, aluminum in the four-coordinate state is predominant over aluminum in the six-coordinate state. Table 3 provides the proportions of the different contributions obtained after deconvolution. The GMI geopolymer has the highest AlIV content at 98% due to the purity of the metakaolin. The percentages of AlIV and AlVI for samples GM1 and GM5 are 85% and 93% and 15% and 7%, respectively; these values are in agreement with the reactivity of the metakaolin used.²⁰ Finally, the proportions of the GM1₀.₃₃MI₀.₃₃M5₀.₃₃ sample are intermediate to those of geopolymers prepared with pure metakaolins. This same observation can be applied to all other geopolymers obtained from the metakaolin mixtures.

In summary, the percentage of AlIV in geopolymer binders is governed by the purity of the aluminosilicate sources. AlIV conversion in the mixtures can be predicted from the properties of pure metakaolins.

### 3.4.2 | ²⁹Si MASNMR spectroscopy

Figure 7 shows the deconvoluted ²⁹Si silicon MASNMR spectra of the GMI, GM1, GM5, and GM1₀.₃₃MI₀.₃₃M5₀.₃₃ geopolymers. The deconvolutions were obtained using a combined Gaussian-Lorentzian fitting function.³⁰ The
<figure><img src="image_8.png" /><figcaption>FIGURE 7 $^{29}$Si NMR spectra and the contributions and deconvolutions of the geopolymers of (A) GMI, (B) GM1, (C) GM5, and (D) GMIM1M5.</figcaption></figure>

offset of the chemical shift between the Q⁴ bands of the
geopolymer network was fixed at 5 ppm; this offset is com-
monly used in these materials. The chemical shifts, full
widths at half-maximum (FWHMs), peak intensities, and
area percentages of the $^{29}$Si curve are listed in Table 4. The
connectivity of the silicon atoms described by Engelhardt²¹
was applied. The entity Q4(nAl) represents the connectiv-
ity of silicon atoms with n aluminum atoms (n ranges from
0 to 4); this is used to describe the rate of silicon atom
polymerization.²¹

The GMI spectra show a small contribution at approx-
imately –79 ppm due to the activator solution used. This
is caused by an excess of the unreacted solution due to
its low reactivity.³¹ Other contributions at –84, –90, –94,
–99, and –104 ppm are attributed to the entities of Q⁴(4Al),
Q⁴(3Al), Q⁴(2Al), Q⁴(1Al), and Q⁴(0Al), respectively.³²,³³
The contribution at –112.2 ppm (16.08%) is related to the
presence of silicic acid and can be explained by an excess
of unreacted siliceous species.³⁴ For sample GM1, the same
contributions were identified. In addition, a slight con-
tribution was observed from quartz (6.68%) at –107 ppm
due to the presence of impurities and a small quantity of
silicic acid (3.48%). The same analysis applies to sample

GM5, which has a fairly high quartz content (14%). Finally,
GM1<sub>0.33</sub>MI<sub>0.33</sub>M5<sub>0.33</sub> is a superposition of the three con-
tributions of the geopolymers GMI, GM1, and GM5. The
NMR spectra of the other samples from the metakaolin
mixtures are also composed of superpositions of the ref-
erence samples. These data are provided in *Supporting
Information*.

3.4.3 | Summary

The formation of the different networks within the for-
mulations is analyzed from NMR data, and relies both on
the contributions of the geopolymer networks (Q⁴(3Al)
+ Q⁴(2Al)), the aluminum-rich contribution Q⁴(4Al) and
the silicon-rich phase (Q⁴(1Al) + Q⁴(0Al)).²² The percent-
ages of the Q⁴(3Al), Q⁴(2Al), (Q⁴(1Al) + Q⁴(0Al)), and
Q⁴(4Al) species are plotted as a function of the metakaolin
and mixture zeta potential values and the alkali cation
concentration [M] in the geopolymers to establish a cor-
relation with the reactivity of the raw materials, as shown
in Figure 8. Regardless of the sample, the contribution of
aluminum (Q⁴(4Al)) slightly changes as a function of zeta
<table><caption>TABLE 4 Positions (ppm). FWHM. and relative area (%) of the geopolymers of GMI, GM1, GM5, GM1M5, GMIM5, GMIM1, and GMIM1M5 and the 3D-printed samples of A, B, C, D, A', B', C' and D' deduced from ²⁹Si NMR spectra.</caption><thead><tr><th>Contributions</th><th>Samples</th><th>GMI</th><th>GM1</th><th>GM5</th><th>GM1M5</th><th>GMIM5</th><th>GMIM1</th><th>GMIM1M5</th></tr></thead><tbody><tr><td>Solution</td><td></td><td>-79.48<br>(2.35)</td><td>-78.60<br>(3.00)</td><td>-80.00<br>(3.00)</td><td>-</td><td>-80.50<br>(3.50)</td><td>-79.30<br>(3.50)</td><td>-</td></tr><tr><td></td><td></td><td><strong>1.33</strong></td><td><strong>2.45</strong></td><td><strong>1.12</strong></td><td>-</td><td><strong>2.68</strong></td><td><strong>2.51</strong></td><td>-</td></tr><tr><td>Q4 (4Al)</td><td></td><td>-84.83<br>(5.70)</td><td>-84.00<br>(5.40)</td><td>-85.10<br>(7.50)</td><td>-83.70<br>(5.70)</td><td>-84.70<br>(6.30)</td><td>-84.20<br>(5.60)</td><td>-83.00<br>(5.80)</td></tr><tr><td></td><td></td><td><strong>12.187</strong></td><td><strong>12.93</strong></td><td><strong>13.99</strong></td><td><strong>12.23</strong></td><td><strong>12.87</strong></td><td><strong>12.69</strong></td><td><strong>11.71</strong></td></tr><tr><td>Q4 (3Al)</td><td></td><td>-90.20<br>(5.75)</td><td>-88.40<br>(5.70)</td><td>-89.80<br>(7.00)</td><td>-88.10<br>(5.60)</td><td>-88.80<br>(6.30)</td><td>-88.80<br>(5.50)</td><td>-88.00<br>(5.70)</td></tr><tr><td></td><td></td><td><strong>22.96</strong></td><td><strong>17.67</strong></td><td><strong>22.85</strong></td><td><strong>18.26</strong></td><td><strong>15.04</strong></td><td><strong>18.37</strong></td><td><strong>21.57</strong></td></tr><tr><td>Q4 (2Al)</td><td></td><td>-94.95<br>(5.80)</td><td>-92.50<br>(5.40)</td><td>-94.30<br>(6.00)</td><td>92.60<br>(5.50)</td><td>93.10<br>(6.40)</td><td>93.10<br>(5.50)</td><td>-93.00<br>(5.70)</td></tr><tr><td></td><td></td><td><strong>22.8</strong></td><td><strong>28.68</strong></td><td><strong>21.92</strong></td><td><strong>29.02</strong></td><td><strong>28.68</strong></td><td><strong>25.59</strong></td><td><strong>26.97</strong></td></tr><tr><td>Q4 (1Al)</td><td></td><td>-99.80<br>(5.70)</td><td>-97.00<br>(5.50)</td><td>-98.60<br>(7.00)</td><td>-98.00<br>(5.60)</td><td>-98.50<br>(6.30)</td><td>-98.10<br>(5.60)</td><td>-98.20<br>(5.70)</td></tr><tr><td></td><td></td><td><strong>16.73</strong></td><td><strong>18.6</strong></td><td><strong>15.24</strong></td><td><strong>20.82</strong></td><td><strong>20.43</strong></td><td><strong>18.57</strong></td><td><strong>17.98</strong></td></tr><tr><td>Q4 (0Al)</td><td></td><td>-104.90<br>(5.50)</td><td>-102.10<br>(5.40)</td><td>-103.40<br>(7.00)</td><td>103.40<br>(5.60)</td><td>-103.80<br>(6.00)</td><td>-103.80<br>(5.60)</td><td>-103.20<br>(5.60)</td></tr><tr><td></td><td></td><td><strong>7.72</strong></td><td><strong>8.75</strong></td><td><strong>10.88</strong></td><td><strong>10.17</strong></td><td><strong>8.81</strong></td><td><strong>11.36</strong></td><td><strong>9.18</strong></td></tr><tr><td>Quartz</td><td></td><td>-</td><td>-107.50<br>(5.00)</td><td>-110.50<br>(9.00)</td><td>-108.00<br>(7.00)</td><td>-108.50<br>(6.00)</td><td>-109.00<br>(5.00)</td><td>-107.51<br>(5.80)</td></tr><tr><td></td><td></td><td><strong>7.40</strong></td><td><strong>13.99</strong></td><td><strong>9.51</strong></td><td><strong>7.66</strong></td><td><strong>4.47</strong></td><td><strong>8.05</strong></td><td><strong>8.05</strong></td></tr><tr><td>Silicic acid</td><td></td><td>-111.20<br>(11.00)</td><td>-112.00<br>(5.00)</td><td>-</td><td>-</td><td>-112.50<br>(5.00)</td><td>-112.30<br>(6.00)</td><td>-112.20<br>(6.00)</td></tr><tr><td></td><td></td><td><strong>16.08</strong></td><td><strong>3.52</strong></td><td>-</td><td>-</td><td><strong>3.83</strong></td><td><strong>6.44</strong></td><td><strong>4.54</strong></td></tr><tr><th>Contributions</th><th>Samples</th><th>A</th><th>B</th><th>C</th><th>D</th><th>A'</th><th>B'</th><th>C'</th><th>D'</th></tr><tr><td>Solution</td><td></td><td>-</td><td>-</td><td>-79.60</td><td>-79.60</td><td>-</td><td>-79.60</td><td>-</td><td>-79.60</td></tr><tr><td></td><td></td><td>-</td><td>-</td><td>(5.60)</td><td>(5.20)</td><td>-</td><td>(4.00)</td><td>-</td><td>(4.00)</td></tr><tr><td></td><td></td><td>-</td><td>-</td><td><strong>4.39</strong></td><td><strong>4.11</strong></td><td>-</td><td><strong>1.55</strong></td><td>-</td><td><strong>1.57</strong></td></tr><tr><td>Q4 (4Al)</td><td></td><td>-83<br>(5.90)</td><td>-83.90<br>(5.40)</td><td>-83.70<br>(5.60)</td><td>-83.90<br>(5.80)</td><td>-83.00<br>(5.90)</td><td>-83.30<br>(5.80)</td><td>-84.70<br>(6.30)</td><td>-83.00<br>(5.80)</td></tr><tr><td></td><td></td><td><strong>9.65</strong></td><td><strong>9.08</strong></td><td><strong>10.09</strong></td><td><strong>10.30</strong></td><td><strong>10.26</strong></td><td><strong>8.92</strong></td><td><strong>9.86</strong></td><td><strong>8.91</strong></td></tr><tr><td>Q4 (3Al)</td><td></td><td>-88.1<br>(5.50)</td><td>-88.50<br>(5.50)</td><td>-88.10<br>(5.60)</td><td>-88.50<br>(5.90)</td><td>-88.80<br>(5.50)</td><td>-88.40<br>(5.80)</td><td>-88.80<br>(6.30)</td><td>-88.10<br>(5.80)</td></tr><tr><td></td><td></td><td><strong>19.59</strong></td><td><strong>17.38</strong></td><td><strong>16.84</strong></td><td><strong>19.70</strong></td><td><strong>17.42</strong></td><td><strong>18.77</strong></td><td><strong>18.68</strong></td><td><strong>18.12</strong></td></tr><tr><td>Q4 (2Al)</td><td></td><td>-92.85<br>(5.40)</td><td>-93.00<br>(5.40)</td><td>-92.80<br>(5.60)</td><td>-92.90<br>(5.90)</td><td>-93.10<br>(5.50)</td><td>-93.10<br>(5.80)</td><td>-93.10<br>(6.40)</td><td>-93.00<br>(5.60)</td></tr><tr><td></td><td></td><td><strong>24.21</strong></td><td><strong>27.82</strong></td><td><strong>26.17</strong></td><td><strong>26.80</strong></td><td><strong>26.87</strong></td><td><strong>25.88</strong></td><td><strong>26.46</strong></td><td><strong>26.98</strong></td></tr><tr><td>Q4 (1Al)</td><td></td><td>-97.70<br>(5.60)</td><td>-98.20<br>(5.60)</td><td>-97.70<br>(5.60)</td><td>-97.90<br>(5.50)</td><td>-98.10<br>(5.60)</td><td>-98.00<br>(5.80)</td><td>-98.50<br>(6.30)</td><td>-98.30<br>(5.80)</td></tr><tr><td></td><td></td><td><strong>18.86</strong></td><td><strong>19.97</strong></td><td><strong>17.53</strong></td><td><strong>17.70</strong></td><td><strong>18.68</strong></td><td><strong>17.33</strong></td><td><strong>17.13</strong></td><td><strong>17.50</strong></td></tr><tr><td>Q4 (0Al)</td><td></td><td>-102.60<br>(5.60)</td><td>-102.90<br>(5.60)</td><td>-102.80<br>(5.40)</td><td>-103.00<br>(5.50)</td><td>-103.80<br>(5.60)</td><td>-103.00<br>(5.80)</td><td>-103.80<br>(6.00)</td><td>-103.00<br>(5.60)</td></tr><tr><td></td><td></td><td><strong>12.76</strong></td><td><strong>9.85</strong></td><td><strong>10.47</strong></td><td><strong>10.34</strong></td><td><strong>12.03</strong></td><td><strong>11.37</strong></td><td><strong>11.52</strong></td><td><strong>10.92</strong></td></tr></tbody></table>

(Continues)
<table><caption>TABLE 4 (Continued)</caption><thead><tr><th>Contributions</th><th colspan="8">Samples</th></tr><tr><th></th><th>A</th><th>B</th><th>C</th><th>D</th><th>A'</th><th>B'</th><th>C'</th><th>D'</th></tr></thead><tbody><tr><td rowspan="3">Quartz</td><td>-107.80</td><td>-107.60</td><td>-107.70</td><td></td><td></td><td>-108.10</td><td></td><td>-108.10</td></tr><tr><td>(5.70)</td><td>(5.60)</td><td>(5.60)</td><td>(5.60)</td><td>(5.00)</td><td>(5.40)</td><td>(6.00)</td><td>(5.40)</td></tr><tr><td><strong>10.01</strong></td><td><strong>8.54</strong></td><td><strong>8.32</strong></td><td><strong>7.61</strong></td><td><strong>8.40</strong></td><td><strong>9.16</strong></td><td><strong>8.70</strong></td><td><strong>9.28</strong></td></tr><tr><td rowspan="3">Silicic acid</td><td>-111.74</td><td>-112.00</td><td>-112.00</td><td></td><td></td><td>-112.30</td><td>-112.50</td><td>-112.20</td></tr><tr><td>(5.80)</td><td>(5.70)</td><td>(5.70)</td><td>(6.00)</td><td>(6.00)</td><td>(5.80)</td><td>(5.00)</td><td>(5.80)</td></tr><tr><td><strong>4.92</strong></td><td><strong>7.36</strong></td><td><strong>6.42</strong></td><td><strong>6.83</strong></td><td><strong>6.33</strong></td><td><strong>6.83</strong></td><td><strong>7.65</strong></td><td><strong>6.80</strong></td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>FIGURE 8 Evolution of the contributions ($Q^4(3Al) + Q^4(2Al)$), ($Q^4(1Al) + Q^4(0Al)$), and $Q^4(4Al)$ of the geopolymers for the (■, ●, ▲) K solution and (□, ○, △) KNa solution as a function of (A) the zeta potential of the mixture and (B) the alkali cation concentration [M].</figcaption></figure>

potential and alkali cation concentration. This is caused by the very fast formation of these species from highly depolymerized siliceous species. For the geopolymer network ($Q^4(3Al) + Q^4(2Al)$), the values slightly decrease as the zeta potential increases, reaching a plateau at $\zeta = -50$ mV. On the other hand, the contributions of the more silicon-rich network ($Q^4(1Al) + Q^4(0Al)$) increase. Again, an alkali cation concentration [M] > 2.8 M appears to be the intersection point between these two variations. For values above 2.8 M, the geopolymer network predominantly has a less siliceous network. Therefore, the use of metakaolin or a mixture with a low zeta potential needs to produce a suitable geopolymer network, with a minor proportion of secondary networks. In the presence of metakaolin M5 ([M] < 2.8 and $\zeta > -50$ mV), the geopolymer network is shared with secondary networks, reflecting the heterogeneous nature of the material at the local scale.

## 3.5 | Discussion

The connectivity of the silicon atoms determined from the NMR spectra of different samples can be used to determine the Si/Al ratio; this Si/Al ratio can be compared to the ratio obtained from the geopolymer formulations

((Si/Al)<sub>exp</sub>/(Si/Al)<sub>NMR</sub>) and expressed as a function of the contributions of the geopolymer network ($Q^4(3Al) + Q^4(2Al)$) (Figure 9A). Regardless of the formulation, the amount of ((Si/Al)<sub>exp</sub>/(Si/Al)<sub>NMR</sub>) decreased with increasing geopolymer network size. This result was caused by the amount of siliceous species in the solution used. Finally, the type of network formed from the zeta potential values of the metakaolin mixtures can be predicted, and the geopolymer network ($Q^4(3Al) + Q^4(2Al)$) is reduced with increasing zeta potential. Moreover, an alkali cation concentration [M] of greater than 2.8 M also controlled the formation of the geopolymer network in the mixtures as along with the viscosity of the various alkaline silicate solutions. To correlate these data, Figure 9B shows the contributions of the geopolymer network ($Q^4(3Al) + Q^4(2Al)$) as a function of these parameters. Regardless of the samples tested with alkali silicate K or KNa solutions, the contribution of the geopolymer network ($Q^4(3Al) + Q^4(2Al)$) is greater than 50% and is associated with increasingly low metakaolin zeta potential values (less than -50 mV) and an alkali cation concentration in the mixtures [M] > 2.8 M. Clearly, the zeta potential values of metakaolins (less than -50 mV) show a high availability of aluminous species in solution likely to participate in the geopolymerization reactions. Thus, for [M] > 2.8 M, the
<figure><img src="image_10.png" /><figcaption>FIGURE 9 Evolution of the (A) $(Si/Al)_{exp}$/($(Si/Al)_{NMR}$) ratio and the (B) metakaolin zeta potentials with different alkali cation concentrations [M] as a function of the geopolymer network contributions.</figcaption></figure>

metakaolins are sufficiently dissolved for the formation of $Si-O-Al$ oligomers of the same nature, with polycondensation reactions favoring the formation of a geopolymer network ($Q^4(3Al) + Q^4(2Al)$). These results are in agreement with the results from a previous study²⁷ on the extrudability of different metakaolin blends defined by [M] > 2.8 M and 3D-printed samples using different printers. Overall, the reactivity of the raw materials strongly influences the shape and local structure of the geopolymer. These results show that shaping by extrusion or 3D printing is governed by the metakaolin reactivity and, consequently, the formation of a geopolymer network.

## 4 | CONCLUSION

In this study, the local-scale structure of geopolymers shaped by extrusion or 3D printing in relation to the reactivity of the raw materials was investigated. The reactivity of different metakaolin mixtures was evaluated using zeta potential measurements with different alkali silicate solutions (KOH, S1, S3, SNa, K, and KNa), followed by shaping via 3D printing, and the different networks were identified via NMR spectroscopy analysis. The results revealed the following:

■ In the presence of weakly reactive basic solutions, $Al(OH)4^-$ species were released in low amounts due to more polymerized silicate species in the solution, resulting in low absolute zeta potential values and high conductivity. Conversely, for more reactive solutions, the silicate species were more depolymerized, and the siliceous and aluminous metakaolin species could more easily pass through the solution, resulting in increasingly low zeta potential values (high in absolute value) associated with a low conductivity.

■ 3D samples could be printed from the different metakaolin mixtures using different printers, with the nozzle size containing K and KNa silicate solutions and an alkaline cation concentration in the mixtures of [M] > 2.8 M.

■ The contributions of the geopolymer networks ($Q4(3Al) + Q4(2Al)$) with K and KNa solutions were greater than 50% for metakaolin zeta potentials less than -50 mV and alkali cation concentrations greater than 2.8 M.

All results showed the impact of the reactivity of the raw materials on the geopolymer network formation and 3D printing.

## ORCID

Jenny Jouin ID https://orcid.org/0000-0002-0141-3048

Sylvie Rossignol ID https://orcid.org/0000-0001-7559-5764

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** N'Cho WC, Gharzouni A, Sobrados I, Jouin J, Aimable A, Rossignol S. Extrudability of geopolymers and control of the formed networks by zeta potential and NMR spectroscopy. Int J Ceramic Eng Sci. 2025;7:e10244. https://doi.org/10.1002/ces2.10244