Geopolymers from Olive Stone Bottom Ashes for Sustainable Construction: Influence of the Molding Method

Abstract

The forming methodology influences the physicochemical, mechanical, and microstructural properties. In this study, which aims to develop a geopolymeric material for potential insulation applications in buildings such as vertical walls, geopolymers were developed using industrial wastes from different industries: slate stone cutting sludge (SSCS) and chamotte (CH) were used as precursors, and olive stone bottom ash (OSBA) and sodium silicate (Na₂SiO₃) were used as alkaline activators. Two forming methods were evaluated: uniaxial pressing and casting of the material, varying the forming method and the liquid/solid ratio. The results showed that the pressed geopolymers achieved higher bulk densities (up to 2.13 g/cm³) and significantly higher compressive strength (28.04 MPa at 28 days), attributable to a higher compactness and degree of geopolymer reaction. In contrast, the casting geopolymers exhibited surface efflorescence, related to slower curing and higher porosity, which reduced their compressive strength (17.88 MPa). In addition, the pressed geopolymers showed better thermal stability and fire performance. These results demonstrate that the variation of the forming method has a direct influence on the material properties of geopolymers, and that the pressing process allows for a reduction of the alkaline activator content, thus reducing its environmental footprint.

1. Introduction

The challenge of developing sustainable building materials has become a key challenge for the construction industry during the transition to a circular economy. The construction industry is a major consumer of natural resources, produces considerable waste, and contributes significantly to atmospheric carbon dioxide (CO₂) emissions [1,2,3]. Therefore, there is a growing need to incorporate strategies that favor the efficient use of raw materials, the revalorization of industrial by-products, and the reduction of the environmental impact associated with manufacturing processes. In this context, research applied to the valorization of industrial waste has advanced notably, with promising results in both cementitious matrix materials and ceramics. In both cases, the incorporation of waste has contributed to improving technological and environmental properties, reducing the use of virgin resources and optimizing processes such as ceramic firing [4,5].

Geopolymers have appeared as a viable solution to traditional building materials due to their lower energy requirements, ability to incorporate industrial residues, and reduced carbon footprint [6,7,8]. A key advantage of geopolymer technology is its capacity to transform waste and mineral by-products into useful construction inputs via alkaline activation. This approach not only conserves natural raw materials but also addresses challenges related to industrial waste disposal.

One of the most promising lines of innovation is the application of alkaline-activated materials in the manufacture of roofing products such as roof tiles. The traditional production of ceramic tiles demands high energy input due to firing temperatures above 1000 °C, and their final characteristics depend on factors such as raw material quality, conforming methods, and thermal treatment [9,10].

In contrast, geopolymer-based tiles can be produced at ambient temperature [11,12,13,14] or with moderate thermal curing [15,16,17], offering both high strength and lower production costs.

Geopolymers represent a technologically and environmentally efficient solution capable of gradually replacing conventional materials by integrating industrial by-products into their formulation, thus promoting circular and more responsible construction practices. These materials are synthesized using aluminosilicate precursors, for example, fly ash, metakaolin, slag, or slate stone cutting sludge (SSCS) and chamotte (CH)—combined with alkaline solutions like sodium hydroxide (NaOH), potassium hydroxide (KOH), or sodium silicate (Na₂SiO₃), which form three-dimensional polymeric networks during the reaction [18,19,20,21,22,23]. Additionally, researchers have explored the partial replacement of commercial activators with waste-derived alkaline sources such as biomass fly ash (BFA) and bottom ash (BA) [24,25,26,27].

However, the production of geopolymers still faces significant challenges regarding their processability and sustainability. Geopolymers can be cured over a wide range of temperatures, although conventional methods are often limited to ambient temperature curing, which involves slower processes, or oven curing at moderate temperatures [28]. Increasing the temperature during curing favors the faster dissolution of aluminosilicate precursors and the increased evaporation of unbound water, which, in turn, accelerates the development of N-A-S-H-type gels, thus facilitating hardening of the material and early development of its mechanical strength [29]. However, excessive setting speed caused by elevated temperatures can increase internal porosity, making it difficult to form a compact microstructure and decreasing long-term mechanical strength [30]. In parallel, recent research has indicated that applying pressure during the molding of geopolymer pastes contributes significantly to the removal of entrapped air, decreasing porosity and improving the density of the material, which also favors reaction kinetics and improves the structural performance of the final product.

The most commonly used traditional forming method is the casting or pouring method [31,32,33], in which mixtures are prepared with a high liquid/solid ratio to ensure workability. The methodology is based on casting the material in a fluid state into a mold and demold when it has reached a solid consistency after a certain curing time. However, this technique requires large amounts of alkaline solution, which significantly increases both energy consumption and CO₂ emissions related with the development of chemical activators [34]. On the other hand, excess water and alkaline reagents can lead to a porous microstructure, negatively affecting the mechanical properties and the longevity of the final material [35]. In previous research, adequate mechanical strength values were observed by casting forming [36,37]; however, the curing time was long.

To address these challenges, the uniaxial pressing or compaction method is used to mitigate the problems mentioned above, although it is conventionally applied in the production of ceramic geopolymers [38]. This method is carried out by introducing the material in a less fluid state than in the casting into a mold on which a force is applied. When the force is released, the compacted material is demolded. Such a methodology makes it possible to reduce the amount of water and alkaline activator required as demonstrated in previous research [34,39], favoring the formation of geopolymers with a denser microstructure, lower porosity and, in many cases, higher mechanical strength. On the other hand, it has been shown that pressing can improve hardening acceleration and reduce efflorescence by limiting the mobility of soluble ions within the system [40]. The pressing process in geopolymer manufacture can be carried out either cold or hot, depending on performance requirements and process conditions. Cold pressing allows efficient compaction of the geopolymer matrix, reducing porosity and improving mechanical properties, all with low energy consumption, as it does not require the use of high temperatures [41].

In contrast, hot pressing combines high pressures with moderate to high temperatures, which favors a higher densification of the material and can lead to significantly higher compressive strengths, in some cases exceeding 160 MPa [28,42]. However, the main disadvantage of this technique is its high energy cost, as well as a lower feasibility for large-scale industrial applications due to the complexity and resource consumption of the process [43].

Despite these advances, comparative knowledge between cast and press formed geopolymers, especially when using unconventional raw materials such as SSCS remains limited. Existing literature has mainly focused on traditional systems based on metakaolin, slags or FA [2,34,44,45], without thoroughly exploring the impact that the forming method may have on the physical, mechanical, structural and environmental behavior of geopolymers with more complex or heterogeneous compositions.

Within this framework, the purpose of this work is to evaluate the properties of geopolymers produced by casting and cold-pressing methods, using, as precursors, a combination of SSCS and CH, and as alkaline activators, OSBA and Na₂SiO₃ that have already been studied in previous research [46]. This research centered in the development of geopolymers formed by casting the material, obtaining promising results of compressive strength (24.12 MPa). In order to improve the properties of the geopolymers and reducing the curing time, the purpose of this investigation is to study the effect of the forming method on the properties of geopolymers. (mass loss, bulk density, linear shrinkage, porosity, capillary and immersion water absorption, thermal conductivity, freeze–thaw effect, and fire resistance), chemical (crystalline phases and degree of reaction), mechanical (compressive strength), and microstructural (scanning electron microscopy (SEM) and X-ray dispersive spectroscopy (EDS)) properties of the resulting geopolymers will be studied. The tendency to efflorescence will also be evaluated, as this phenomenon can compromise both the durability and aesthetics of the material in real solutions.

2. Materials and Methods

2.1. Raw Materials

Slate stone cutting sludge (SSCS), used as the primary aluminosilicate source for the geopolymer formulation, was supplied by the companies Perforaciones Noroeste (Oviedo, Spain) and Pizarras Ortegal (Ortigueira, Spain). This waste is generated during mining and processing operations in the extractive industry of natural non-carbonated dark grey tectonic compression slate with a slightly rough texture and inclusions of very small and dispersed metallic minerals, which is accumulated in settling ponds. Its composition consists mainly of the water used in the cutting process and fine slate particles in suspension. As a secondary precursor, grog or chamotte (CH), provided by Cerámica San Francisco (Bailén, Spain) from ceramic bricks that did not comply with the technical regulations for their application in construction, was used. As an alkaline activator, a mixture was prepared in varying proportions of olive stone bottom ash (OSBA) provided by Garzón Green Energy (Bailén, Spain) from a kiln fed with olive stones for heat generation, and sodium silicate (Na₂SiO₃) (29.20% SiO₂, 8.90% Na₂SO₃, 61.90% H₂O) (Panreac, Barcelona, Spain).

Elemental analysis using the Truspec Micro (LECO Corporation, St. Joseph, MI, USA) was conducted to quantify the content of carbon (C), nitrogen (N), and hydrogen (H). The results presented in

Table 1. CHN determination of SSCS, OSBA, and CH.

Raw Material	SSCS	OSBA	CH
C (%)	0.569	3.855	0.420
N (%)	0.028	0.010	0.001
H (%)	0.376	3.036	0.001

show that SSCS, OSBA, and CH contained low carbonate content, which favors the geopolymerization reaction. A high carbonate concentration leads to a reduction in alkalinity, which hinders the complete dissolution of the precursors [47]. This limitation leads to the formation of incomplete geopolymeric networks that negatively affect the compressive strength and chemical characteristics of the geopolymer.

The granulometric of the precursor materials (SSCS and CH) and the alkaline solution (OSBA) were obtained with the Malvern Mastersizer 2000 equipment (Malvern Panalytical, Westborough, MA, USA). Since the granulometric influences the reaction of the precursors with the alkaline solution, the distribution of particles was reduced to a size below 100 µm to achieve compatibility of the mixture components [48]. The granulometric distribution of SSCS is influenced by the machining technique employed to produce it, showing a median distribution (D₅₀) of 10.266 µm and a specific surface area of 1.31 m²/g. In this study, the OSBA was processed by grinding in a Retsch PM200 planetary mill (RETSCH GmbH, Haan, Germany) and then sieved to obtain particles smaller than 400 µm. Prior to this, the OSBA was calcined at 950 °C for one hour to eliminate any residual organic material and enhance its reactivity [49]. After calcination, the OSBA was ground and sieved to 300 µm to minimize particle size, lower the specific surface area, and enhance its reactivity with Na₂SiO₃ [50]. This waste material is rich in alkaline oxides, mainly K₂O, which promotes the formation of ionic species in an alkaline environment. These species facilitate the solution of partially crystalline or amorphous phases found in aluminosilicate precursors (SSCS and CH), allowing the liberation of Si and Al monomers. These monomers participate in polycondensation reactions that create a three-dimensional aluminosilicate network characteristic of geopolymer gel. The measured D50 and specific surface area for OSBA were 30.053 µm and 0.972 m²/g, respectively. Similarly, CH fragments underwent the same physical treatment, resulting in a D90 below 81.556 µm and a specific surface area of 1.11 m²/g. Figure 1 displays the particle size distributions of SSCS, OSBA, and CH.

The reactivity of geopolymer precursors is strongly influenced by parameters such as the degree of crystalline order and the chemical composition of the precursor–activator system, as well as by the density (

Table 2. Density of OSBA, CH, and SSCS.

Raw Materials	Density (Kg/m3)
SSCS	2.45 ± 0.11
CH	2.50 ± 0.18
OSBA	2.71 ± 0.15

). Materials with lower densities tend to exhibit a greater specific surface area, which is linked to a less compact structural arrangement. This favors dissolution kinetics in alkaline media, promoting the release of the ionic species of the aluminosilicates (Si⁴⁺ and Al³⁺), fundamental in gel polymerization [51]. The density of SSCS (2.45 kg/m³) is related to the high fines content, as corroborated in Figure 1. CH showed a similar density (2.50 kg/m³) due to the compact structure of the material. The density of OSBA was higher (2.71 Kg/m³), related to the presence of glassy phases and partially molten structures as a consequence of the heat treatment at high temperatures, which produces denser particles [52].

The morphology of the particles influences the reactivity of the precursor–activator systems and the development of the microstructure of the final material. On the other hand, morphology also influences the behavior of the particles during the forming of the specimens. Figure 2 shows the SEM images of the three materials used: (a) SSCS, (b) CH, and (c) OSBA.

The SEM image of SSCS showed a lamellar and flaky morphology, characteristic of the phyllosilicate-rich minerals presents in the shale. This morphology favors good dispersion in the matrix but may hinder mechanical compaction because of their tendency to align parallel to each other, generating zones of weakness [53]. CH showed an irregular particle structure. Its rough surface also favors chemical adhesion with geopolymer gels during reaction [54]. OSBA is formed by particles with predominantly spherical shape and porous surface. These spheres present a morphology characteristic of high temperature biomass combustion processes [55].

The chemical composition of the raw materials, obtained using X-ray fluorescence spectrometry (XRF) with a Bruker Pioner S4 Explorer equipment (Bruker AXS GmbH, Karlsruhe, Germany), revealed that both SSCS and CH possessed compositions appropriate for serving as precursors in geopolymerization. In particular, these materials presented high contents of silicon (SiO₂) and aluminum (Al₂O₃) oxides, reaching values of 65.68% and 13.77% in the case of SSCS, and 55.61% and 17.60% in CH, respectively. The SiO₂/Al₂O₃ molar ratios obtained (4.76 for SSCS and 3.16 for CH) were within the optimal range (1–5) to favor the aluminosilicate gel formation required for the formation of a stable geopolymeric matrix [56].

On the other hand, OSBA presented a composition rich in alkali oxides, with an elevated concentration of K₂O (27.50%) and CaO (34.20%). This richness in basic elements makes it an effective material as an alkaline solution, promoting the solution of the precursors and facilitating the generation of N-A-S-H and C-A-S-H types of gels, which are fundamental for the development of solid three-dimensional networks characteristic of geopolymeric materials [57].

Table 3. Chemical composition results of SSCS, CH, and OSBA.

Raw Material	SSCS	CH	OSBA
K2O	2.82	4.67	27.50
CaO	1.43	7.24	34.20
Al2O3	13.77	17.60	0.779
SiO2	65.68	55.61	4.12
P2O5	0.169	0.159	1.85
MgO	1.95	3.46	3.82
Fe2O3	6.87	8.03	0.94
Cl	0.0147	-	0.092
Na2O	1.55	0.371	0.761
SO3	0.389	0.675	0.361
TiO2	0.921	0.717	0.060
MnO	0.0883	0.118	0.125
V2O5	0.0174	0.0126	-
CuO	0.385	-	0.055
Cr2O3	0.0161	0.0305	0.048
ZnO	0.0194	0.0163	-
NiO	0.0262	-	0.041

lists the detailed chemical composition values of SSCS, CH, and OSBA.

X-ray diffraction (XRD) analyses of the raw materials SSCS, OSBA, and CH were obtained with a PANalytical X’Pert Pro diffractometer (Malvern Panalytical) (see Figure 3), scanning over an angular range from 4° to 70° (2θ) with a size of 0.0168°. The instrument operated at 45 kV and 40 mA, while the samples were rotated at 10 rpm throughout the test. The results reveal a complex and heterogeneous mineralogy, in which crystalline phases associated with the previously determined chemical composition predominate.

In the case of SSCS, the diffractograms reveal a predominant presence of silicon dioxide (SiO₂), mainly as quartz (Q) and muscovite (M) (KAl₂(AlSi₃O₁₀)(OH)₂). Additionally, phases containing iron (Fe) and magnesium (Mg) were detected, including clinochlore (Cl) ((AlSi₃)O₁₀(OH)₈) and chamosite (Ch) ((Fe₅Al)(AlSi₃)(OH)₈), along with minor amounts of titanium dioxide (TiO₂) in the form of rutile (R).

Regarding CH, diffractograms reveal a mineralogy dominated by silica- and alumina-rich phases, mainly as quartz (Q), similar to SSCS. In addition, hematite (H) was detected as an Fe₂O₃-bearing phase, as well as akermanite (Ak) (Ca₂MgSiO₇), which contributes to the mineralogical complexity of the material and may influence its behavior during the geopolymerization reaction.

OSBA presented a mineralogy dominated by carbonate phases and alkaline oxides, with the presence of free lime (silt, L) (CaO), together with minor amounts of quartz (Q) and periclase (P) (MgO). This component is consistent with its residual nature as a by-product of biomass combustion [58].

The bands groups in SSCS, CH, and OSBA were obtained using Fourier Transform Infrared Spectroscopy (FTIR) with a Jasco Analytical 6800FV spectrophotometer (JASCO Corporation, Tokyo, Japan). Spectra were determined in the range of 4000 to 400 cm⁻¹ (Figure 4), enabling the identification of characteristic chemical bonds and functional groups for each material.

Both SSCS and OSBA displayed broad absorption bands within the 3626-2983 cm⁻¹ range, which corresponded to the O–H stretching vibrations, indicative of physically adsorbed water on the surface of the particles [59]. Additionally, a distinct band observed between 1455 and 1416 cm⁻¹ was linked with the C–O vibrational asymmetric stretching, suggesting the presence of carbonate phases [57].

In the FTIR spectra of SSCS and CH, bands appeared between 976 and 972 cm⁻¹, related to Si–O–T vibrational asymmetric stretching (T=Si or Al), a feature typical of silica-rich materials and reflected in the chemical composition determined by XRF (Table 3) [60,61]. For OSBA, the absorption peak detected at 885 cm⁻¹ was linked to C–O bond vibrations in carbonate groups, likely resulting from surface carbonation caused by exposure to atmospheric CO₂ or secondary reactions [61].

In the 800-700 cm⁻¹ region, bands characteristic of bending vibrations from symmetric Si–O–Si stretching were clearly defined in SSCS and CH samples [60,62]. Meanwhile, absorption features between 700 and 600 cm⁻¹ were associated with vibrations attributed to the existence of quartz in all three raw materials, aligning with the mineralogical data obtained through XRD analysis (Figure 3) [21,63]. Finally, bands within the 523-418 cm⁻¹ range were associated with bending vibrations of Si–O bonds, representing the silicate tetrahedra found within the material matrices [60].

Table 4. Characteristic FTIR absorption bands of SSCS, CH, and OSBA.

Functional Group	Wavenumber Range (cm−1)	FTIR Bands (cm−1)			References
Raw Materials		SSCS	CH	OSBA
O–H vibrational stretching	3626–2983	3326, 3311	-	2983	[59]
CO vibrational asymmetric stretching	1455–1416	1433	1455	1416	[57]
Si–O–T vibrational asymmetric stretching	972–976	976	972	-	[60,61]
Vibration in carbonate groups of CO bonds	885	-	-	879	[61]
Si–O–Si bending symmetric stretching	777–775	777	775	-	[60,62]
Quartz vibrational bending	648	648	-	-	[21,63]
Si–O vibrational bending	523–418	523, 462, 418	448, 418	512, 419	[60]

shows the summary of the characteristic spectral bands identified for each material and their corresponding vibrational attribution.

2.2. Mix Design and Methodology

The formulation of the geopolymers was based on the adaptation of a previously validated experimental design [46]. The liquid/solid (L/S) and Na₂SiO₃/OSBA ratios were selected considering the porosity, morphology and specific surface of the precursor materials (SSCS and CH), factors that directly influence the water and activator demand to ensure adequate dispersion and interaction between the components. Thus, these proportions made it possible to obtain mixtures with good workability and optimum mechanical properties, minimizing undesirable phenomena such as efflorescence or excessive porosity. However, in order to optimize rheological conditions and curing time during casting, the liquid/solid (L/S) ratio was reduced. During the experimental phase, eight series of blends were developed: four corresponding to geopolymers formed by pressing (series P1, P2, P3, and P4) and four by casting (series C1, C2, C3, and C4). Each mixture was composed using varying proportions of SSCS, CH, OSBA, Na₂SiO₃, and distilled water (W_d).

The L/S ratios varied significantly depending on the forming method. In pressed geopolymers, the values ranged from 0.37 to 0.39, while in casting ones they ranged from 0.53 to 0.57. Similarly, the proportion of Na₂SiO₃ to OSBA remained in the range of 2.2 to 3.9. The amount of distilled water added was adjusted incrementally with increasing precursor and activator content, in order to maintain an adequate rheology of the system. This adjustment is caused by the porosity and specific surface characteristics of the solids used, factors that increase the water demand to achieve a homogeneous dispersion and facilitate the geopolymerization reaction [57]. It should be noted that a common L/S ratio was not used for both forming methods, as each technique has different rheological requirements: the casting process needs more fluid mixtures to fill the molds efficiently, while pressing allows denser mixtures that allow adequate consolidation under pressure. For this reason, it was not feasible to maintain a constant L/S ratio between the two methods without compromising the feasibility of processing.

Table 5. Geopolymer mix composition with SSCS, CH, OSBA, and Na₂SiO₃.

Method	Specimen	SSCS (g)	CH (g)	OSBA (g)	Na2SiO3 (g)	Wd (g)	Na2SiO3/OSBA	L/S Ratio	pH Alkaline Solution
Pressing	P1	160	90	75	70	168	2.2	0.37	12.15
	P2	160	90	55	70	168	3.1	0.39	12.21
	P3	128	122	75	70	168	2.2	0.37	12.16
	P4	128	122	55	70	168	3.1	0.39	12.26
Casting	C1	160	68	75	90	216	2.9	0.53	12.14
	C2	160	68	55	90	216	3.9	0.57	12.22
	C3	128	100	75	90	216	2.9	0.53	12.17
	C4	128	100	55	90	216	3.9	0.57	12.24

shows the detailed proportions of the raw materials, as well as the Na₂SiO₃/OSBA, liquid/solid ratios, and the pH values obtained for each mixture. On the other hand,

Table 6. Geopolymer molar proportion of Si/Al, Na/Si, K/Si, and Ca/Si.

Method	Specimen	Si/Al	Ca/Si	K/Si	Na/Si
Pressing	P1	4.39	0.17	0.19	0.89
	P2	4.37	0.14	0.17	0.89
	P3	4.64	0.18	0.19	0.87
	P4	4.62	0.15	0.16	0.88
Casting	C1	5.01	0.15	0.17	1.06
	C2	4.99	0.12	0.15	1.08
	C3	5.31	0.16	0.17	1.05
	C4	5.29	0.13	0.15	1.06

presents the Si/Al, K/Si, Na/Si, and Ca/Si molar proportions, fundamental parameters to understand the evolution of the microstructure and the formation of the geopolymeric matrix in both forming modes.

The alkaline activator was formulated from OSBA previously calcined at 950 °C, as described above, combined with Na₂SiO₃ and Wd, in varying proportions according to the experimental design. The preparation procedure consisted of the initial dissolution of OSBA in Wd, using a magnetic stirrer (model 690-1, Nahita, Auxilab S.L., Beriáin, Spain) for 10 min. Subsequently, Na₂SiO₃ was added in a controlled manner by dropwise addition, and stirring was maintained for an additional 10 min to ensure adequate homogenization of the activating solution.

The solid precursors (SSCS and CH) were mechanically homogenized in a planetary mixer (Proeti model, Proetisa S.A., Madrid, Spain) for 5 min. After that, the alkaline solution was slowly added on top of the precursor mixture, prolonging the stirring for 10 min until a uniform geopolymeric mass free of dry aggregates was obtained.

The consolidation of the geopolymeric material was carried out by two different methodologies: uniaxial press-molding and casting.

In the pressing method, the geopolymer paste was placed into a steel mold measuring 60 × 30 × 15 mm and compressed under a uniaxial pressure of 15 MPa for 10 s to densify the matrix and minimize initial porosity. The samples were demolded immediately after pressing and then cured at room temperature (21 ± 5 °C) for 28 days.

As for the casting method, the geopolymer mortar was casted into silicone molds with cubic geometry (35 × 35 × 35 mm). To minimize the formation of air occlusions and to favor a homogeneous distribution of the material, the molds were vibrated for 20 s on a vibrating table. The samples were demolded after 7 days of initial curing and then kept at room temperature (21 ± 5 °C) until 28 days of total curing were completed. For both methods, 7 specimens per family were produced.

Figure 5 shows schematically the methodology followed for the preparation and forming of the geopolymers.

Figure 6 presents the microstructure evolution mechanisms of the geopolymers prepared by the two methodologies. The ease of handling the geopolymer mixture was not a key factor in the press-forming technique, as having too much liquid phase might cause the material to spill out of the mold during compaction. For this reason, a semi-dry consistency mix was used, which is more suitable for uniaxial pressing. During the pressing process, the application of pressure generated a progressive compaction of the mass, accompanied by a slight loss of pressure due to internal redistribution of particles. To counteract this effect and ensure adequate densification, it was necessary to keep the pressure constant for 10 s.

This consolidation process facilitated the reduction of porosity, promoting closer contact between the solid particles and improving the precursors’ contact with the alkaline solution. The high applied pressure favored the initial dissolution of the reactive phases, increasing the liberation of ionic species as Si⁴⁺ and Al³⁺ [36], which are crucial for the forming of three-dimensional aluminosilicate networks characteristic of geopolymer gels. In addition, the moderate proportion of activators in the mixture prevented cracking phenomena related to premature evaporation of water during curing, improving the structural integrity of the gels.

In the forming process by casting the geopolymer mortar, the formability of the mixtures was increased by increasing the L/S ratio to ensure adequate flowability and to allow for the complete filling of the molds. To this end, the alkaline activator content of the C mixes was increased. However, excess alkaline activator can cause the formation of micro-cracks caused by the fast evaporation of free water during the setting and curing stages. This loss of water generates internal stresses that compromise the integrity of the hardened matrix. In addition, the absence of external pressure during casting-in-mold forming results in weaker packing of solid particles, which limits the proximity between the precursors and the alkaline activator. This mechanism decreases the efficiency of the initial dissolution and consequently results in a lower degree of geopolymeric conversion. Consequently, a higher proportion of unreacted particles and a more porous microstructure appear, which negatively affects the density, mechanical strength, and durability of the material compared to the properties obtained in press-formed geopolymers.

Both pressing and casting geopolymers were tested under the standards and equipment listed in

Table 7. Parameters, standards, and equipment used.

Parameter	Standard	Equipment
Weight loss	-	Balance RB-30KG Cobos (Cobos, Barcelona, Spain)
Linear shrinkage	UNE-EN 772-16 [64]	Digital gauge
Capillary water absorption	UNE-EN 772-11 [65]	Stopwatch and balance RB-30KG Cobos (Cobos)
Cold water absorption	UNE-EN 772-21 [66]	Thermostatic bath Bunsen and balance Balance RB-30KG Cobos (Cobos)
Boiling water absorption	UNE-EN 772-7 [67]	Thermostatic bath Bunsen and balance RB-30KG Cobos (Cobos)
Bulk density and open porosity	UNE-EN 772-4 [68]	Hydrostatic balance
Freeze–thaw resistance	UNE-EN 15304 [69]	Freezer
Thermal conductivity	UNE-EN 12667:2002 [70]	HFM 446 Lambda Eco-Line Netzsch (Netzsch, Selb, Germany)
Compressive strength	UNE-EN 772-1:2001+A1:2016 [71]	Shimadzu AG-300KNX (Shimadzu, Kyoto, Japan)
DRX	-	X’Pert Pro PANalytical (Malvern Panalytical)
FTIR	-	FT-IR Vertex 70 Bruker (Bruker AXS GmbH)
SEM–EDX	-	Microscope Carl Zeiss Merlin (Carl Zeiss AG, Oberkochen, Germany)
Efflorescence	UNE 136029 [72]	Humidity chamber
Fire resistance	UNE-EN 13238:2011 [73] UNE-EN ISO 11925-2 [74]	Forge
Thermal stability	-	Combustion chamber

to evaluate their properties.

The test plan was divided into physical, chemical, mechanical, and microscopic characterization. The physical characterization of the geopolymer mortar consisted of tests to determine the mass loss and linear shrinkage (UNE-EN 772-16 [64]) experienced during curing of the specimens, percentage of water absorbed by capillary action (UNE-EN 772-11 [65]) and immersion (UNE-EN 772-21 [66]), bulk density (UNE-EN 772-4 [68]), porosity (UNE-EN 772-4 [68]), thermal conductivity (UNE-EN 12667: 2002 [70]) and evaluations of the condition of the specimens after freeze–thaw episodes (UNE-EN 15304 [69]), and efflorescence produced in the presence of moisture (UNE 136029 [72]). Fire resistance was studied under the UNE-ES 13238:2011 [73] and UNE-EN ISO 11925-2 [74] standards. The chemical characterization focused on FTIR and XRD analysis of the geopolymer mortar, while the mechanical characterization consisted of the identification of compressive strength (UNE-EN 772-1:2001+A1:2016 [71]). Finally, the microscopic characterization focused on a microstructural analysis by microscopy.

2.2.1. Mass Loss and Linear Shrinkage

For the physical characterization of the geopolymer, 6 specimens per family were tested. The mass variation observed in the specimens is mainly related with the loss of free water after the curing process [75]. The mass loss was quantified on the RB-30KG Cobos scale by the difference between the beginning and ending weight of the specimens, and the dimensional variation was carried out, using a digital caliper, from the difference between the longitudinal measurements before and after curing under controlled conditions.

2.2.2. Capillary and Immersion Water Absorption, Bulk Density, Open and Closed Porosity

The hygroscopic behavior of the materials was evaluated by means of water absorption tests by capillarity and total immersion. For this purpose, the RB-30KG Cobos precision balance (resolution 0.001 g) was used, and measurements were taken after specific periods of exposure to water in a thermostatic bath, following the standardized methodologies indicated in Table 7.

Capillary absorption provides critical information on the performance of the material in environments exposed to intermittent humidity or variable climatic conditions. On the other hand, the identification of the immersion absorption makes it possible to evaluate the maximum degree of water saturation achievable by the material. Both parameters are influenced by the microstructural properties of the material [76].

Finally, the open porosity and density of the material were obtained by the hydrostatic method, based on the submerged mass and the mass saturated in water, which allows an accurate estimation of the accessible porosity and the density at the real volume of the system.

On the other hand, the closed porosity was determined by tomography on Bruker’s Skyscan 2214 equipment (Bruker AXS GmbH) with a voxel size of 5.6 μm. The exposure time was 1.3 s; the rotation size was 0.4°, and the number of projections taken was 901.

2.2.3. Freeze–Thaw Cycles

To evaluate the mechanical strength and durability of geopolymeric materials against external climatic conditions, an accelerated test of resistance to damage by freeze–thaw thermal cycles was carried out. For this purpose, a specimen from each of the conformed families was used and tested under the UNE-EN 15304 standard [69] performing a total of 50 freeze–thaw cycles. This experimental protocol allows the identification of possible microstructural degradation mechanisms. During freezing, the water retained in the pores undergoes an increase in volume, generating internal pressures that can induce cracking, the loss of structural integrity, and a decrease in mechanical properties [77].

The procedure consisted of subjecting the samples to 50 consecutive freeze–thaw cycles. Each cycle included a freezing phase at a controlled temperature of −15 ± 2 °C during 8 h, followed by a thawing stage in a climatic chamber at 20 ± 2 °C and 95% relative humidity for another 8 h. The specimens were placed on a grid to allow uniform air circulation and simulate real exposure conditions.

Figure 7 shows the freeze–thaw mechanism of geopolymers formed by pressing and casting the geopolymer mortar.

2.2.4. Thermal Conductivity

The thermal conductivity of the geopolymeric materials was realized by the standardized procedure UNE-EN 12667:2002 [70] using a heat flow meter. For this purpose, 3 specimens from each of the families were tested to ensure structural homogeneity.

The specimens were each individually analyzed on the Netzsch HFM 446 Lambda Eco-Line device (NETZSCH-Gerätebau GmbH, Selb, Germany). This device employs dual heat flow transducers placed between two isothermal plates to generate a controlled temperature gradient across the specimen thickness. During the test, the top of the device was automatically adjusted to ensure uniform contact with the specimen surface, minimizing thermal interface resistances.

Prior to measurement, the system was calibrated using standard materials with certified thermal conductivities, to ensure traceability and accuracy of the results obtained. The thermal conductivity (λ) was determined from the stabilized heat flow and the imposed thermal gradient, allowing to evaluate the insulating capacity of each geopolymer formulation.

2.2.5. Fire Resistance and Thermal Stability

The fire resistance test was developed to evaluate the thermal behavior and physicochemical stability of the geopolymer mortar under extreme conditions of direct exposure to flame. The experimental methodology was designed in accordance with UNE-EN 13238:2011 [73] and UNE-EN ISO 11925-2 [74].

For this purpose, a specimen was selected from each of the formulations developed, which were subjected to a flame generated by a standard Bunsen burner. The orientation of the specimen during the test was 45° with respect to the burner axis, ensuring a uniform exposure of the surface to the flame front. Each specimen was subjected to a continuous exposure for 15 s, keeping the test conditions controlled in terms of the gas flow rate and the distance between the flame and the surface.

At the end of the exposure, a detailed visual inspection of the affected surface was carried out. The evaluation focused on the analysis of physical behavior after exposure, recording parameters such as the presence and extent of thermal cracking, surface carbonization, material detachment, and bubble formation due to internal decomposition or partial structural collapse.

In addition, the specimens were tested to a thermal stability test. To achieve this objective, the specimens were exposed to thermal exposure at 200, 400, and 800 °C during 2 h, with a controlled heating rate of 10 °C/min. After the exposure time, the specimens were cooled by natural convection to room temperature.

2.2.6. Efflorescence

The specimens were subjected to a standardized procedure (UNE 136029 [72]) of exposure in a humid environment in order to evaluate the tendency to the formation of salt efflorescence, a phenomenon that can compromise both the aesthetics and durability of cementitious and geopolymeric materials. The methodology employed was based on controlled conditions that simulate an environment conducive to the transport and deposition of soluble salts to the surface of the material.

The test was performed in a humidity-controlled climatic chamber, maintaining a relative humidity of 70 ± 5% and a constant temperature of 20 ± 5 °C, under conditions of zero convection to avoid disturbances in capillary transport. The specimens were placed in a vertical position on a tray with a sheet of distilled water, ensuring an immersion of 2.5 cm. This partial contact with the water allowed the capillary rise of ionic solutions, favoring the conditions for the development of efflorescence in the area exposed to the air.

The exposure period under these conditions was 7 days. After this time, the samples were removed from the water and placed on a dry surface at controlled room temperature (21 ± 5 °C) for 24 h, to allow surface drying and facilitate crystallization of the migrated salts. Subsequently, the specimens were stored in a desiccator for 24 h.

The evaluation of efflorescence was carried out by visual inspection of the surface of the specimens tested and mineralogical analysis by XRD of the specimens affected by the efflorescence phenomenon.

2.2.7. Compressive Strength

The compressive strength of geopolymeric materials was determined by standardized mechanical tests. The tests were evaluated using a Shimadzu AG-300KNX universal testing machine (Shimadzu) (UNE-EN 772-1:2001+A1:2016 [71]). The specimens were submitted to axial compression at 7, 14, and 28 days of curing, in order to study the evolution of the mechanical characteristics along the geopolymerization process. During the test, an increasing load was applied at a constant strain rate, according to the parameters established in the standard, recording the maximum stress supported before the structural collapse of the specimen.

3. Results

3.1. Physical Characterization of the Geopolymer Mortar

Table 8. Results of physical properties of geopolymers.

Specimen	Mass Loss (%)	Linear Shrinkage (%)	Capillarity Water Absorption (g/m2 min)	Immersion Absorption (%)	Bulk Density (g/cm3)	Open Porosity (%)	Thermal Conductivity (W/mK)
P1	2.13 ± 0.31	0.36 ± 0.09	1789.41 ± 48	6.21 ± 0.84	2.13 ± 0.12	4.48 ± 0.94	0.697 ± 0.024
P2	2.24 ± 0.36	0.44 ± 0.12	1864.21 ± 51	7.45 ± 0.75	1.77 ± 0.08	5.43 ± 0.57	0.604 ± 0.057
P3	1.95 ± 0.41	0.30 ± 0.10	1836.70 ± 36	7.11 ± 0.64	1.96 ± 0.15	4.96 ± 0.81	0.649 ± 0.061
P4	1.77 ± 0.38	0.28 ± 0.08	1849.67 ± 44	7.26 ± 0.71	1.89 ± 0.16	5.11 ± 0.44	0.612 ± 0.073
C1	3.24 ± 0.41	0.51 ± 0.21	1891.74 ± 74	7.87 ± 0.91	1.72 ± 0.20	5.70 ± 0.36	0.590 ± 0.095
C2	3.47 ± 0.45	0.60 ± 0.16	1945.63 ± 63	8.49 ± 0.81	1.55 ± 0.14	6.37 ± 0.92	0.559 ± 0.056
C3	3.31 ± 0.50	0.48 ± 0.18	1903.54 ± 72	7.99 ± 0.57	1.68 ± 0.13	5.93 ± 0.98	0.578 ± 0.091
C4	3.04 ± 0.47	0.43 ± 0.13	1938.11 ± 59	8.23 ± 0.72	1.62 ± 0.09	6.12 ± 1.71	0.571 ± 0.089

presents the results of the physical properties determination test performed on the four material families produced by pressing and the four families obtained through casting, following 28 days of curing at room temperature (21 ± 5 °C).

3.1.1. Determination of Mass Loss

Mass loss in geopolymeric materials is closely associated with the evaporation of free water that does not directly participate in the geopolymerization reactions during the curing process. This phenomenon occurs primarily during the curing process when the structurally unbound water is removed from the cured matrix.

Figure 8 presents the results of mass loss observed in both press-formed and casting-formed geopolymeric specimens after a 28-day curing period at room temperature (21 ± 2 °C). The formulations made by the dry-pressing technique showed significantly lower mass loss, with values ranging from 1.77% to 2.24%, compared to the samples made by the casting process, whose losses ranged from 3.04% to 3.47%.

This difference is mainly attributed to the W_d content employed in each forming method. In the case of pressing, the initial mix was designed with a lower liquid/solid ratio, thus reducing the quantity of free water available for subsequent evaporation. This resulted in a more compact structure that limits water evaporation from the interior to the surface during curing. Within the group of pressed geopolymers, the P4 family showed the lowest mass loss value (1.77%), related to its lower liquid/solid ratio (0.39) and better Na₂SiO₃/OSBA ratio (0.6).

3.1.2. Determination of Linear Shrinkage

The linear shrinkage observed in geopolymeric samples is principally linked to the evaporation of free water during the curing process, as well as to the reorganization of the particles that make up the matrix. This phenomenon causes a reduction in the overall volume of the system, a common phenomenon in geopolymeric-type cementitious materials [78], and is strongly associated with the evolution of the densification of the microstructure and the progressive closure of the capillary porosity.

The results obtained (Figure 9) show that the volume changes produced in the shaped specimens were negligible since the curing was performed at room temperature. Several studies have highlighted that curing at temperature produces greater volumetric variations [79]. Other authors have reported that the incorporation of CH into geopolymers reduced shrinkage values [80,81]. Specimens with higher CH content showed slightly lower linear shrinkage values related to the higher stability of the geopolymer mortar. P4 exhibited the lowest value (0.28%). In addition, the OSBA content favored granular packing that provided higher volumetric stability. Within each group, P2 and C2 showed the highest linear shrinkage values (0.39 and 0.60%, respectively) related to the lower Na₂SiO₃/OSBA ratio (0.6) and the lower CH content that generated a less reactive matrix, with lower dimensional stability.

3.1.3. Determination of Capillary and Immersion Water Absorption of Geopolymers

The water absorption capacity of geopolymers was used to evaluate their performance under water exposure conditions. Figure 10 and Figure 11 show the results obtained for water absorption tests by capillarity (g/m² min) and by immersion (%).

The results show a clear trend in which geopolymers formed by pressing present lower values in both capillary and immersion absorption compared to those obtained by casting. This behavior is mainly attributed to the higher densification produced by the pressure applied during forming, which significantly reduces open porosity and limits capillary pore connectivity. The incorporation of OSBA favored granular packing due to the spherical morphology (Figure 2), which contributed to porosity reduction. This fact was more noticeable in pressed formulations, where the combination of low L/S ratio and compaction pressure maximized packing efficiency.

P1 showed the lowest capillary absorption (1798.41 g/m² min) and the lowest immersion absorption (6.21%), which indicates a more compact microstructure related to the granulometry of the precursors.

The casting formulations presented higher adsorption capacity. C2 showed the maximum values both by capillarity (1945.63 g/m² min) and by immersion (8.49%). This behavior is associated with the absence of mechanical compaction during forming, which favors porosity retention. In addition, the lower amount of OSBA present in the mixture led to less geopolymer gel formation.

3.1.4. Determination of Bulk Density

The densification of the geopolymeric matrix is directly associated with the alkaline polymerization process, which implies the dissolution of precursors rich in aluminosilicates in a highly alkaline environment, which is immediately followed by the release of ionic species [82]. These species form an amorphous three-dimensional network through polycondensation mechanisms, giving rise to a hardened matrix.

In this context, the higher addition of OSBA in formulations P1, P3, C1, and C3 showed a significantly impact on the bulk density of the geopolymers (2.13, 1.96, 1.72, and 1.62 g/cm³, respectively). This behavior is associated with the higher availability of calcium, in the form of CaO, present in OSBA, which facilitates the dissolution of aluminosilicates and pro-moves a more efficient polymerization [83]. Calcium acts as a catalyst that accelerates the formation of C-A-S-H type reaction products, contributing to a denser microstructure with less residual porosity.

Figure 12 presents the bulk density values determined experimentally.

3.1.5. Determination of Porosity

The total porosity of the geopolymeric samples was closely associated with the degree of densification of their matrix and, consequently, a lower number of accessible pores.

Figure 13 evidence the variation in porosity percentages between the families elaborated by pressing and casting the material, a fact that is corroborated in the tomography in Figure 14 of P1 and C1. This trend was associated with the granular packing of the particles after the application of mechanical force and to the L/S ratio of geopolymers. For the C series, the higher L/S ratio (0.53–0.57) implies a higher amount of free water producing interconnected pores during the curing process. Moreover, the increased porosity is also related to an excess of Na₂SiO₃ that can produce incomplete dissolution of the aluminosilicates present in the mortar [84].

The increase of the Na/Si proportion at P1, together with the raise of the Ca/Si proportion, improved the initial solubilization of the precursors, since the presence of Na⁺ cations stabilized the dissolved species in the alkaline medium producing a structure with lower porosity (4.48%). This phenomenon favors the development of a more continuous and uniform geopolymeric network, which also has an impact on a less porous microstructure [85].

The tomography in Figure 14 shows how the forming method influenced the porosity of the material. P1 showed a dense and compact microstructure with a low pore volume. On the contrary, C1 presented external cracks, corroborating the lower cohesion of the geopolymer during the forming method, and a high internal porosity with different pore sizes.

Table 9. Results of porosity analysis of P1 and C1.

	P1	C1
Total VOI volume (mm3)	717.81	738.08
Object volume (mm3)	713.91	711.50
Percent object volume (%)	99.45	96.40
Number of closed pores	192,311	520,383
Volume of closed pores (mm3)	3.16	17.19
Closed porosity (%)	9.44	2.36
Total volume of pore space (mm3)	3.90	26.58
Total porosity (%)	0.54	3.60

presents the porosity analysis of the geopolymers. The volume of closed porosity in P1 (9.44%) was significantly higher than that presented in C1 (2.36%), while both samples presented low open porosity (0.10 and 1.27%, respectively).

3.1.6. Determination of Thermal Conductivity

The thermal conductivity of geopolymeric materials was linked to the open porosity, pore size and distribution, and bulk density, parameters that control the efficiency of thermal transport by modifying the effective path for heat flow within the matrix.

The P series exhibited a thermal conductivity range between 0.604 and 0.697 W/mK. These specimens showed thermal behavior superior to those obtained in the C series due to their higher degree of structural compaction. Higher densification provides more continuous thermal paths, facilitating heat transfer through the solid network of the material. Moreover, the decrease in the volume of interconnected pores decreased the amount of trapped air, whose low thermal conductivity acted as a barrier to heat flow. On the contrary, the C series, with valors between 0.559 and 0.590 W/mK, showed a lower thermal conductivity, correlated with a higher porosity. The significant presence of occluded air in the open pores caused the interruption of thermal flow propagation. The increase in Ca/Si, associated with the increase in the amount of OSBA in the mortars, reinforced the densification of the matrix through the development of C-A-S-H-type gels.

It is remarkable that the values determined for both series were considerably lower than the values for the original raw materials: mineral slate had a typical thermal conductivity of approximately 1.43 W/mK [86], and chamotte had a typical thermal conductivity of around 1.0 W/mK [87]. This reduction in the thermal conductivity of the final material is directly related to the geopolymerization process and the resulting microstructure. In addition, OSBA, which contained oxides with low thermal conductivity, and sodium silicate also influenced the reduction of thermal transport. Together, these factors contributed to a better insulation of the final material, which reinforced its potential application as a sustainable building material.

Figure 15 shows the thermal conductivity results obtained from the experimentation.

3.1.7. Assessment of Freeze–Thaw Cycles

After the completion of the 50 freeze–thaw cycles, the geopolymer were evaluated a physically and optically to determine the impact on the material under adverse conditions. The procedure included detailed visual inspection and quantification of mass loss as an indicator parameter for the degree of deterioration.

As shown in Figure 16, series P specimens showed no significant structural alterations, retaining their surface integrity with no visible cracking, delamination, or spalling. This behavior is attributed to a lower initial porosity and a denser matrix, factors that limit water penetration and, consequently, minimize the internal stresses generated during freezing of the absorbed water.

Series C specimens showed more evident surface damage, with localized flaking phenomena observed in certain areas. This deterioration is directly associated to the higher porosity and capillary permeability of these formulations, which favor water retention in the matrix that expands by freezing, promoting surface degradation.

Additionally, a change in colorimetry was observed in all the test specimens. This color change, attributed to the formation of efflorescence, was not accompanied by textural alterations.

The evolution of mass loss associated with freeze–thaw cycles is presented in Figure 17. The results indicate an upward trend in mass loss as the cycle number increases. This is attributed to the progressive accumulation of microstructural damage caused by the expansion of the frozen water inside the porous matrix of the geopolymer.

Series C specimens showed significantly higher mass losses compared to the series P specimens. This higher mass loss was closely linked to the visually observed surface flaking phenomenon. On the other hand, series P samples presented lower mass losses. This behavior suggests a higher dimensional stability and residual mechanical strength in the pressed formulations under extreme durability conditions.

Within each series of specimens, no significant differences in mass loss values were identified between the different formulations, suggesting that the forming method had a more decisive influence on the behavior against cyclic thermal attack than variations in the chemical composition of the samples analyzed.

3.1.8. Determination of Fire Resistance and Dimensional Stability

Direct flame exposure of the geopolymers resulted in the appearance of surface cracks that were apparent in the series C geopolymers associated with the lower compaction of the material and thus higher porosity (Figure 18). This thermal response to flame showed the influence of the forming method on the behavior under extreme thermal loads.

Regarding the material’s thermal stability (Figure 19), surface micro-cracking and flaking were observed in the C-series geopolymers following exposure to 200 °C, which became more visible with increasing temperature at 400 and 800 °C. This increased susceptibility to cracking suggests that the higher porosity and lower densification reduce the material’s resistance to thermal gradients. No significant differences were identified between formulations within each series, suggesting that the thermal response is a function of the forming method.

3.2. Chemical Characterization of the Geopolymer Mortar

3.2.1. XRD Analysis

The X-ray diffraction (XRD) determination of the geopolymers (Figure 20) indicates that several crystalline phases originally present in the raw materials persisted in the final product, suggesting that the geopolymerization process did not lead to a complete transformation of all mineral constituents.

Quartz (Q) (SiO₂) and muscovite (M) (KAl₂(AlSi₃O₁₀)(OH)₂) were among the dominant phases identified. Additionally, albite (A) (NaAlSi₃O₈) was detected, with its diffractometric peak intensity diminishing as the Na₂SiO₃/OSBA molar ratio decreased. This reduction implies an enhanced degree of geopolymerization and a higher level of amorphous N-A-S-H-type gels. Lower levels of soluble sodium appear to limit the accumulation of unreacted ionic species, thus promoting more efficient polymer network development [88].

These findings demonstrate that while a substantial portion of the matrix transitions to an amorphous structure during geopolymerization, certain original crystalline phases remain. These residual minerals may become embedded within the matrix, potentially influencing its microstructure, durability, and mechanical behavior. The observed decrease in peak intensity—particularly for albite and feldspar-related phases—was closely linked to improved reactivity within the alkaline activation process.

3.2.2. Efflorescence Evaluation

The efflorescence analysis revealed the appearance of primary efflorescence on the surface of geopolymer C. This was mainly associated with the liquid/solid proportion of the geopolymer mortars. The fluid presence in the initial mixture of geopolymers played a determining factor in the mobility of unreacted aqueous species. An excess of alkaline solution in the formulation can facilitate the migration of water along with soluble ionic species not embedded in the geopolymer gel structure to the surface of the material. During this process, alkali ions not integrated into the geopolymeric network (mainly Na⁺ and K⁺) can be transported along with water toward the outer layers of the geopolymer. Once in contact with the atmosphere, such ions can react with ambient CO₂, generating alkaline carbonates (Na₂CO₃, K₂CO₃) through a surface carbonation process. This reaction is visibly manifested as a salt crystallization on the surface of the material [86]. The pressed geopolymers did not show soluble deposits and no significant differences between the formulations of each series were appreciated. Figure 21 shows the state of the casting geopolymers after the efflorescence test.

Efflorescence analysis (Figure 22) revealed the presence of cronstedtite (Cr), an iron-rich phyllosilicate belonging to the serpentine group. This phase, absent in the initial sample, has probably formed as a transformation product of residual gelled species in the presence of moisture and atmospheric CO₂. On the other hand, the presence of chlorite (Chl) was identified. Its formation in the geopolymeric system can be linked to a structural reorganization of the N-A-S-H or C-A-S-H gel in conditions of prolonged saturation, low temperature, and a high concentration of metal ions, particularly iron and magnesium [89].

3.2.3. FTIR Analysis

Figure 23 presents the FTIR determination with the identification of functional groups present in the different geopolymer formulations. The increase of OSBA content in the blends produced a slight shift of the wavenumbers in the main band related with Si–O–T vibrational asymmetric stretching bonds located in the interval 975–972 cm⁻¹. This spectral variation was interpreted as an indicator of increased matrix polymerization, due to the progressive substitution of silicon atoms by aluminum atoms in the network structure, which reduced the vibrational energy of the system. This transition was associated with increased matrix densification and a favorable evolution of the N-A-S-H-type gel, consistent with a more efficient geopolymerization process [90]. In the 3351–3332 cm⁻¹ region, bands attributed to hydroxyl group (O–H) stretching vibrations were identified, associated with structural water and hydroxylated groups not completely removed during curing [84,90]. On the other hand, the absorption signals detected in the range of 1650 to 1644 cm⁻¹ were associated with the bending vibrations of the H–O–H bond, which indicate the content of physically adsorbed or retained water within the capillary pores of the material [21,91]. Meanwhile, the bands that appeared between 1416 and 1411 cm⁻¹ corresponded to the asymmetric stretching of carbonate ions (CO₃)²⁻, suggesting the formation or presence of carbonate compounds [27,92]. In the range of 775–771 cm⁻¹, the bands were associated with Si–O–Si bending symmetric stretching bonds, characteristic of not fully reacted siliceous structures, which evidences the coexistence of partially transformed phases within the geopolymeric matrix [91]. On the other hand, the bands located between 526–524 cm⁻¹ were associated with quartz vibrational modes, confirming the presence of undissolved inert crystalline phases, while between 418 and 417 cm⁻¹, deformation vibrations of Si–O bonds appeared [21,27].

Table 10. FTIR characteristic absorption bands.

Functional Group	Wavenumber Range (cm−1)	FTIR Bands (cm−1)								References
Raw Materials		P1	P2	P3	P4	C1	C2	C3	C4
O–H vibrational stretching	3354–3332	3351	3351	3345	3347	3349	3332	3348	3350	[84,90]
H–O–H bending vibration	1650–1644	1650	1650	1650	1644	1650	1650	1650	1650	[21,91]
CO vibrational asymmetric stretching	1485–1411	1411	1412	1412	1414	1410	1414	1416	1412	[27,92]
Si–O–T vibrational asymmetric stretching	975–972	973	972	975	974	970	972	972	972	[90]
Si–O–Si bending symmetric stretching	755–751	754	754	755	755	751	752	753	752	[91]
Quartz vibrational bending	526–524	526	525	526	526	524	525	525	525	[21,27]
Si–O vibrational bending	418–417	418	418	418	417	417	417	417	417	[21,27]

presents the characteristic peaks identified for each of the analyzed geopolymeric families.

3.3. Mechanical Characterization of the Geopolymer Mortar

The mechanical performance of the geopolymer specimens was determined after 7, 14, and 28 days of curing under defined temperature and humidity conditions (

Table 11. Compressive strength at 7, 14, and 28 days.

Specimen	Compressive Strength (MPa)
	7 days	14 days	28 days
P1	17.31 ± 1.05	22.31 ± 2.94	28.04 ± 0.89
P2	11.21 ± 0.98	16.40 ± 2.71	22.13 ± 1.24
P3	14.40 ± 1.36	19.69 ± 1.86	25.31 ± 1.03
P4	15.34 ± 2.11	19.01 ± 2.84	24.98 ± 1.33
C1	5.34 ± 0.54	11.20 ± 1.26	17.88 ± 1.55
C2	3.36 ± 0.23	8.19 ± 1.38	16.84 ± 2.09
C3	6.41 ± 0.89	10.97 ± 1.08	14.92 ± 1.62
C4	3.36 ± 0.47	8.79 ± 0.94	22.03 ± 1.24

). As these curing conditions remained consistent throughout the tests, the differences observed in compressive strength are primarily associated with the composition of the mixtures and the shaping technique, both of which affect the resulting bulk density [93].

Compressive strength is directly related to bulk density. The 28-day compressive strength values for the pressed geopolymers were established in a range between 22.13 and 28.04 MPa, while for the casting geopolymers, values between 14.56 and 17.88 MPa were obtained. The P1 blend with the highest bulk density (2.13 g/cm³) showed the best compressive strength value (28.04 MPa) related to the lower pore volume present in the matrix. The increase in porosity produces voids in the matrices where stresses are concentrated leading to structural failures [94].

Within each series, it was found that the mechanical strength increased with decreasing Si/Al molar proportions due to the equilibrium of Si⁴⁺ and Al³⁺ ionic ratios and the supply of higher amount of soluble silica [95]. The influence of OSBA on the geopolymerization reaction was evidenced by improvements in compressive strength values between P1 and P2 (28.04 and 22.13 MPa, respectively), between P3 and P4 (25.31 and 24.98 MPa, respectively), between C1 and C2 (17.88 and 14.56 MPa, respectively), and between C3 and C4 (16.84 and 14.92 MPa). These decreases were due to the reduction of the Na₂SiO₃/OSBA ratio that produced a balanced three-dimensional network with a higher number of Si–O–T bonds and to the high Ca/Si proportion that caused improvements in the stability of the geopolymer gel. Figure 24 graphically shows the compressive strength values determined at 7, 14, and 28 days of the curing process.

3.4. Microscopic Characterization of the Geopolymer Mortar

SEM–EDX was carried out on the eight families of geopolymers formed by pressing and casting after 28 days of curing (Figure 25, Figure 26, Figure 27, Figure 28, Figure 29, Figure 30, Figure 31 and Figure 32). The EDX spectra showed the presence of the main elements associated with the reaction products: calcium (Ca), aluminum (Al), silicon (Si), sodium (Na), and potassium (K), with the additional existence of magnesium (Mg), attributed to the contribution of CH included in the formulations [96]. These elements evidence the existence of C-A-S-H and N-A-S-H gels.

From a microstructural point of view, a marked difference was observed between the forming series. The C series samples, such as C1 and C4, evidenced a more heterogeneous microstructure, characterized by a greater presence of cracks and porosity. These defects are attributable to the lower densification achieved by the casting method, as well as to the higher L/S ratio, which favors the initial mobility of the particles but generates a higher degree of shrinkage and voids during curing. The P series geopolymers showed a more compact microstructure with fewer visible defects, which is directly related to the higher efficiency of the forming process in the consolidation of the matrices. These samples showed Si/Al molar ratios between three and five, a range that has been identified in the literature as optimal for the development of a dense and stable three-dimensional network [82].

4. Conclusions

This research carried out a physical, chemical, mechanical, and structural comparison of geopolymers formed by uniaxial pressing and casting. From the results obtained, the influence of the forming method was evaluated. This analysis provides knowledge to improve strength and durability and eliminate microstructural defects.

• The pressed geopolymers showed a more compact microstructure. It is corroborated that the pressure applied during forming improves particle packing and reduces the amount of alkaline activator.
• The pressed geopolymers showed a more compact and less porous microstructure than those manufactured by casting. It is corroborated that the pressure applied during forming improves particle packing and reduces the amount of alkaline activators.
• A compressive strength of the pressed geopolymers of 28.04 MPa was obtained, an increase of 36%. This fact shows the influence of the L/S proportion on the curing kinetics of the geopolymers and limits the application of the cast-forming method to applications that do not require a short demolding time. However, the long-term behavior of the material has not been addressed, which is proposed as a future line of research to validate its applicability in buildings.
• SEM–EDX showed an improvement in the development of C-A-S-H and N-A-S-H gels in the series of pressed geopolymers, associated with improvements in the interconnection of the aluminosilicate precursors particles and the alkaline solutions.
• The efflorescence of the C series geopolymers is conditioned by the higher liquid content and an excess of alkaline activator that favors the mobility of aqueous species and the migration of soluble salts to the surface.
• The casting geopolymers showed flaking and cracking upon temperature exposure as a consequence of the lower compaction of the material.
• The process of manufacturing geopolymers by pressing would be easily scalable to the industrial scale, as it does not require high curing temperatures and can be implemented with conventional pressing technology. This technical simplicity, together with lower energy consumption, facilitates its integration into existing industrial processes.

Based on these findings, the pressing method presented significant advantages over casting the geopolymer mortar. Therefore, the manufacture of geopolymers with the by-products studied constitutes a viable alternative for the manufacture of alternative construction materials.