Development of Geopolymeric Mortar from Metakaolin and Ignimbrite from the Añashuayco Quarries, Peru, for Civil Construction

Abstract

The construction industry generates large amounts of waste and high CO₂ emissions, especially from cement production. Sustainable alternatives, such as geopolymers, help reduce these impacts by promoting eco-friendly materials. This study aimed to develop geopolymer mortar using ignimbrite (IG) residues from the Arequipa region, Peru, combined with metakaolin (MK). The raw materials were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to assess the chemical composition, structure, and morphology. Geopolymeric mortars were synthesized with varying MK/IG ratios while maintaining a fixed fine sand proportion. An activating solution of 9 mol/L NaOH was used with different liquid-to-solid ratios. Geopolymers cured at room temperature for 28 days exhibited lower compressive strength than those dried at 50 °C for 48 h or sequentially at 50 °C for 48 h followed by 90 °C for 12 h. The highest IG-content mixture achieved a compressive strength of 18 MPa, while the MK-based geopolymer reached 12 MPa, both under high-temperature curing. An increase in the SiO₂/Al₂O₃ molar ratio was also associated with improved mechanical performance, reinforcing the influence of precursor composition on geopolymerization. These results highlight the potential of regional ignimbrite for the production of geopolymer mortar, promoting sustainable and innovative building materials.

1. Introduction

1.1. Research History

Urbanization, population growth, and building renovations have driven the continuous increase in construction and demolition activities, generating large volumes of waste and causing serious environmental impacts [1]. In the traditional linear economy, construction and demolition (C&D) waste is considered as worthless, and is therefore often disposed of in landfills. However, with the growing awareness of sustainability and resource management, the circular economy (CE) has emerged as a model for minimizing the environmental impact of C&D waste. Although fully transitioning to CE in the construction and demolition sectors presents challenges, this approach recognizes the value of waste and reintegrates it into the production cycle, contributing to more sustainable management [2].

The construction industry, in particular, is responsible for a significant increase in natural resource consumption [2]. Additionally, data indicate that construction and demolition (C&D) waste accounts for 30% of the total global waste, with more than 35% of this waste being disposed of in landfills annually [2]. Managing the large volumes of waste generated by human activities, such as manufacturing, construction, mining, dredging, and quarrying, is therefore a significant environmental challenge in many countries today.

Globally, over 10 billion tons of construction and demolition (C&D) waste are generated annually from construction, demolition, and excavation activities. Developed economies, such as China, India, the European Union (EU), and the U.S., produce large annual volumes of C&D waste, while developing economies, such as Brazil and Turkey, also contribute significantly. Approximately 35% of this waste is sent to landfills, with the UK, Brazil, Australia, and the U.S. leading in disposal rates. These figures highlight the relationship between C&D waste production, population growth, and economic development [3]. It is crucial to understand that in Peru, construction and demolition waste (CDW) is classified as non-urban waste and includes materials generated during the construction, restoration, renovation, remodeling, and demolition of buildings as well as infrastructure-related activities. Effective CDW management is essential for environmental sustainability, therefore, regulations vary depending on the volume generated or the size of the project and are overseen by different entities [4]. Nevertheless, Peru still faces environmental challenges directly linked to the construction industry. Construction activities can result in large accumulations of inorganic solid waste, commonly generated from concrete, bricks, mortar, quarries, and other materials.

The city of Arequipa, Peru, is renowned for its colonial architecture and numerous historical monuments including mansions, cathedrals, churches, and bridges [5]. It is commonly referred to as the “White City” due to the extensive use of sillar, a white volcanic stone derived from ignimbrite, in its buildings [5,6]. Sillar is distinguished by its unique physical and mechanical properties, such as high porosity and excellent compressive strength, making it an exceptionally suitable material for construction [7]. This stone is extracted from quarries around the city, particularly from the Canteras de Añashuayco [7]. Due to its composition, it is also classified as ignimbrite, a type of volcanic rock formed from pyroclastic flows [8,9], known for its durability and resistance [7]. Consequently, since many buildings and houses in the city incorporate sillar in their structures, renovations and demolitions often generate sillar residues. Therefore, studying the potential reuse of this material is essential.

Another significant environmental impact of the construction industry is the large amount of carbon dioxide (CO₂) emitted during the production of Portland cement [10]. This material, essential for infrastructure development, undergoes industrial processes that release substantial amounts of CO₂ [11]. Studies indicate that Portland cement manufacturing accounts for approximately 8% of the global carbon dioxide emissions [12]. This percentage is alarming, as it represents a significant share of the total CO₂ emissions, intensifying the greenhouse effect and contributing to climate change [12]. Therefore, seeking more sustainable and efficient alternatives is crucial to reduce the environmental impact of the construction industry.

In the search for sustainable materials, geopolymers have emerged as a promising alternative. These can be synthesized from construction waste, serving as a substitute for traditional cement and significantly reducing CO₂ emissions. This innovative approach not only mitigates the environmental impact, but also promotes resource reuse, enhancing the sustainability and efficiency of the construction industry. The term “geopolymer” was first coined in 1978 by the French scientist Joseph Davidovits [13,14]. As a concept, a geopolymer is an inorganic polymer formed through the polycondensation reaction of aluminosilicate-containing waste materials with alkali activators [13,15] This process leads to the formation of a three-dimensional aluminosilicate structure, which can either be amorphous or semicrystalline, consisting of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra [13]. Over the years, various aluminosilicate sources have been explored for geopolymer synthesis, including natural pozzolans, industrial by-products, and thermally treated clays, with metakaolin remaining one of the most widely studied due to its high purity and reactivity. This diversity of raw materials has encouraged extensive research aimed at tailoring geopolymer compositions for different applications, particularly in the construction sector.

1.2. Literature Review

To properly understand the mechanisms involved in the formation and performance of the materials analyzed in this study, it is essential to provide a clear overview of the geopolymer concept. This section presents the definitions, chemical and structural principles, and main steps of the geopolymerization reaction, together with bibliographic studies that highlight the wide variety of materials used in the synthesis of geopolymers.

Since the late 20th century, geopolymers have attracted increasing interest as sustainable alternatives to traditional cementitious materials [16]. These inorganic binders are produced through the alkaline activation of silica- and alumina-rich sources including industrial by-products, calcined clays, and natural minerals. They exhibit remarkable mechanical and chemical properties and can be synthesized at ambient or elevated temperatures [16].

The versatility of geopolymers enables their application in a wide range of fields including structural concrete, soil stabilization, high-temperature-resistant coatings, hazardous waste encapsulation, and advanced technical ceramics [17]. Moreover, incorporating agro-industrial and construction waste into geopolymer formulations supports an approach aligned with circular economy principles while significantly reducing the carbon footprint compared with conventional Portland cement [17].

Geopolymers are distinguished by their rapid hardening, high durability, resistance to chemical and thermal degradation, and low thermal conductivity. These characteristics classify them as high-performance materials with significant potential for civil engineering and other industries seeking more sustainable and technically advanced solutions [18].

Geopolymers are inorganic cementitious materials formed by the alkaline activation of aluminosilicate-rich precursors, such as metakaolin, using concentrated solutions of sodium or potassium hydroxides and/or silicates [16,19,20]. The geopolymerization process involves three main stages: first, the dissolution of amorphous or partially crystalline phases in a highly alkaline medium, releasing reactive silica and alumina species; second, the reorganization of these species into aluminosilicate gels composed of AlO₄ and SiO₄ tetrahedra interconnected by oxygen atoms; and finally, the polycondensation stage, which results in the formation of a rigid three-dimensional network. This structure is stabilized by alkaline cations (e.g., Na⁺, K⁺), which balance the negative charges generated by the substitution of Si⁴⁺ with Al³⁺ in the tetrahedral framework [19,20]. The final network can be organized into different poly(sialate) structures, such as poly(sialate), poly(sialate-siloxo), and poly(sialate-disiloxo), depending on the Si/Al molar ratio [20]. Geopolymers generally exhibit an amorphous structure with similarities to zeolites, although without long-range crystalline order [16]. The chemical composition, activator concentration, and curing conditions play a critical role in determining the mechanical performance and microstructure of the geopolymer matrix. Elevated curing temperatures accelerate early-age strength development but may increase the porosity and reduce structural compactness. Conversely, curing at lower temperatures promotes the formation of denser and more homogeneous gels, thereby enhancing long-term performance [16].

Several studies in the literature have explored the properties of different types of geopolymers for civil construction applications including pastes, mortars, concretes, and composites [21]. The synthesis of these materials can incorporate a wide variety of aluminosilicate-rich precursors, which are generally classified as natural, calcined natural, industrial waste, construction and demolition waste, and synthetic or modified materials [20]. Among the natural materials, zeolites [22] and natural pozzolans [23], such as volcanic ash [24], stand out. The study conducted by Özen and Uzal [22] investigated the effects of the chemical, physical, and mineralogical characteristics of natural zeolites—specifically clinoptilolite tuffs from the Anatolian region of Turkey, namely Gördes (denoted as G1 and G2) and Bayburt (denoted as BY)—on the geopolymerization reaction and the final properties of the resulting geopolymers. Their study highlighted that the chemical composition (particularly the contents of SiO₂, Al₂O₃, and K₂O), the BET specific surface area, and the presence of potassium feldspar were key factors influencing the efficiency of the geopolymer reaction. As a result, the geopolymers formulated with Bayburt tuff achieved the highest mechanical performance, reaching compressive strengths of approximately 45 MPa after 56 days of curing, outperforming the other formulations.

The category of industrial waste includes fly ash [25], rice husk ash [26], sugarcane bagasse ash [27,28] and blast furnace slag [29]. Among these industrial by-products, fly ash has been widely studied due to its abundance and reactivity. Zeynep Iyigundogdu et al. [25] evaluated geopolymer mortars produced with low-lime fly ash (FA), activated with NaOH and Na₂SiO₃, and cured at temperatures ranging from 60 °C to 120 °C. The best performance was achieved at 100 °C, with a compressive strength of 48.41 MPa and the largest inhibition zones—up to 49.24 mm—against bacteria and fungi. The antimicrobial activity was attributed to the residual alkalinity of the activators retained in the pores of the matrix. The study concluded that such geopolymers are promising for sustainable and hygienic applications in the construction industry.

Ceramic waste, such as brick powder [30], and construction and demolition waste (CDW) [31,32] have also been explored. Among the studies addressing this topic, Neves et al. [31] investigated the performance of geopolymer concretes produced with CDW materials, including bricks (BW), ceramic tiles (TW), and concrete (CW), used as total or partial replacements for fly ash (FA). They employed different concentrations of alkaline activators (10% and 13% Na₂O by mass) and SiO₂/Na₂O ratios (1.0 and 0.5 by mass). The authors also used aggregates such as fine sand (0.125/1 mm), coarse sand (0.125/4 mm), “rice grain” gravel (1/8 mm), gravel 1 (4/16 mm), and gravel 2 (8/31.5 mm). In the curing process, the specimens were kept at 70 °C for 24 h and then stored in a dry chamber for 7, 28, and 91 days prior to testing. The concrete made with 100% FA achieved the highest compressive strength at 28 days, reaching 25.3 MPa. Although partial replacements of FA with 50% BW, TW, and CW resulted in lower strength values compared with the reference concrete, the authors emphasized that these results were promising and highlighted the potential for reusing such materials in geopolymer concrete production [31].

Synthetic or modified materials, such as silica fume [33] and nano-silica [34], have been used to enhance the properties of geopolymers. Finally, calcined pozzolanic materials include thermally treated clays, with metakaolin [35] standing out as a widely used precursor due to its high reactivity and favorable silica and alumina content.

Metakaolin (MK) is a pozzolanic material obtained by calcining kaolinitic clay at temperatures ranging from 500 °C to 800 °C. During this process, the clay structure collapses, forming a highly reactive amorphous aluminosilicate known as metakaolin [36,37], which is characterized by its high reactivity, enhanced compressive and flexural strength, and reduced permeability. Its applications include high-performance concrete, precast concrete, fiber cement and ferrocement products, glass fiber-reinforced concrete, mortars, and stuccoes, providing greater durability and improved finishing [36,37].

Among the volcanic-origin materials used as geopolymer precursors, ignimbrite has attracted growing interest within the scientific community. Although the number of studies is still limited, this material has shown promise due to its high Si/Al ratio—a key characteristic for the development of geopolymeric structures with good performance [38]. In the study conducted by Toprak et al. [38], the authors experimentally evaluated the feasibility of using Ahlat stone powder—a volcanic ignimbrite originating from the Nemrut volcano and locally known as ‘volcanic rock’—as a geopolymer precursor. The material was used as a binder in the production of geopolymer concrete, with aggregates classified into two granulometric ranges: 0–4 mm (comprising 68% of particles between 0 and 2 mm and 13% between 2 and 4 mm) and 4–8 mm (19%). The formulations employed alkaline activator solutions of NaOH and KOH at 10 mol/L, combined with sodium silicate (Na₂SiO₃). After casting, the samples were thermally cured at 90 °C for 48 h in heat-resistant bags and subsequently stored under laboratory conditions until testing. The compressive strength was evaluated at 3, 7, 28, and 90 days, reaching values around 25 MPa at both 28 and 90 days, demonstrating the potential of ignimbrite as a precursor material for the production of geopolymer concretes with good mechanical performance.

Recent studies, including those conducted by our research group, have focused on the use of construction waste—particularly from demolition activities—as well as on the reuse of residual ignimbrite from the Arequipa region. One such study, conducted by Huamán-Mamani et al. [39], evaluated the potential of three residual materials from Arequipa—ground ignimbrite, calcined clay, and demolition mortar waste—as alternative precursors for the production of Portland cement-free geopolymer mortars. The materials, along with fine sand (FS), were processed by grinding and sieving (ASTM No. 140 mesh) under standardized conditions and activated with NaOH solutions at concentrations of 12, 15, and 18 mol/L. Overall, the best performances were observed with the 12 mol/L solution. However, the highest compressive strength was obtained for ignimbrite activated with the 15 mol/L solution, without aggregates, reaching values up to 42 MPa.

In the study by Morais et al. [40], geopolymers were developed by combining metakaolin—both commercial and produced by calcining kaolinite from the Arequipa region—with ignimbrite waste from the Añashuayco quarry in Arequipa, Peru. The formulations were prepared with different MK/IG ratios (100/0 and 60/40), using a 9 mol/L NaOH activating solution, molded into cylindrical form, and thermally cured at 50 °C for 72 h. The results showed that the addition of ignimbrite significantly improved the compressive strength, with the best performance observed in the formulation containing 60% MK and 40% IG, which reached 52.72 ± 1.02 MPa—surpassing the strength of formulations made with metakaolin alone. The study highlights the potential of reusing ignimbrite waste as an alternative geopolymer precursor.

Thus, in this context, the reuse of ignimbrite (white sillar) waste represents a promising strategy for sustainable construction and the promotion of a circular economy in regions such as Arequipa, Peru. Ignimbrite is a pyroclastic volcanic rock widely used in local architecture, particularly in the production of masonry blocks, decorative elements, and sculptures. However, the processes of extraction, cutting, and processing generate a significant amount of waste, often discarded without any form of reuse. Additionally, the renovation or demolition of old buildings also contributes to the generation of ignimbrite waste. These by-products, which still exhibit high concentrations of silica and alumina, can be valorized as precursors in the production of geopolymeric materials. This approach not only reduces the need for extracting new natural resources, but also contributes to mitigating environmental impacts such as the waste accumulation and CO₂ emissions associated with conventional Portland cement production [41].

Building on previous studies that demonstrated the potential of ignimbrite as a geopolymer precursor, this study aimed to develop an eco-friendly geopolymer mortar focused on the reuse of local waste, particularly sillar (ignimbrite) from the Arequipa region, Peru. In this work, different proportions of metakaolin and ignimbrite were investigated, along with the incorporation of fine aggregate (sand), which remains a scarcely explored approach in the literature. The formulations were activated with sodium hydroxide, and their physical and mechanical properties evaluated.

2. Materials and Methods

2.1. Materials

The materials and reagents used included Chinese-origin metakaolin (MK) supplied by Shijiazhuang Chuimou Trade Co., Ltd. (Shijiazhuang, China), sillar or ignimbrite (IG) collected from the Añashuayco sillar mines in Arequipa, Peru, and 99% sodium hydroxide, in addition to tap water.

2.2. Collection and Treatment of Ignimbrite

The ignimbrite (IG) material was collected from the Añashuayco sillar quarries, located in the region of the city of Arequipa, Peru (Figure 1a). These quarries are known for a large quantity of white sillar, a volcanic stone widely used in local construction. After collection, the raw IG material was subjected to a grinding process in a ball mill (Xiamen, Model RBM-20, China), ensuring adequate particle size reduction. Then, the ground material was sieved using a 200 mesh screen to ensure a fine and uniform particle size (Figure 1b,c), suitable for the applications envisaged in subsequent studies.

2.3. Characterization

The samples were analyzed using various characterization techniques to ensure a detailed assessment of their physicochemical and structural properties. X-ray diffraction (XRD) was performed using a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Cu-Kα radiation (λ = 1540 Å) and Bragg–Brentano geometry in the 2θ range (2°–80°), thus allowing for the identification of the amorphous and/or crystalline composition of the samples, providing essential information about their structure. In addition, the samples were characterized by Fourier transform infrared spectroscopy (FTIR) using a Shimadzu IRXross Spectrum equipment with an ATR module (Shimadzu Corporation, Kyoto, Japan), 45 scans, and 4 cm⁻¹ resolution, in the 4000–600 cm⁻¹ range. This technique was crucial to identify the functional groups present, providing a detailed spectrum of the chemical functionalities. Finally, scanning electron microscopy (SEM) analysis was performed using a TESCAN MIRA3 scanning electron microscope equipped with energy dispersive spectroscopy (EDS) (TESCAN ORSAY HOLDING a.s., Brno, Czech Republic). The samples were previously metallized with a thin layer of gold, which allowed for the visualization of the morphology in high resolution. The elemental analysis of the materials used in the synthesis of the geopolymers was performed by X-ray fluorescence (XRF) using a PANalytical ZETIUM Minerals instrument and the WROXI PRO method (Malvern Panalytical B.V., Almelo, The Netherlands).

2.4. Geopolymer Manufacturing Methodology

The methodology followed the Peruvian Technical Standard NTP 334.051:2022 [42], which specifies the use of cube-shaped molds measuring 50 mm × 50 mm × 50 mm. Geopolymer mortars with different proportions were prepared, as summarized in

Table 1. Proportions used in the manufacture of geopolymer mortars, with mass values for MK, IG and fine sand materials, volume of activating solution, and drying processes.

Group	Mixture ID	Proportion %	MK (g)	IG (g)	Fine Sand (Sieved 2 mm)	Liquid to Solid Ratio (L/S)	NaOH 9.0 mol/L (mL)	Drying
1	MI-1-1	100–0	500	0	1375	0.343	171.64	28 days *
	MI-1-2	80–20	400	100	1375	0.343	171.64	28 days *
	MI-1-3	50–50	250	250	1375	0.343	171.64	28 days *
	MI-1-4	20–80	100	400	1375	0.343	171.64	28 days *
	MI-1-5	0–100	0	500	1375	0.343	171.64	28 days *
2	MI-2-1	100–0	500	0	1375	0.484	242	28 days *
	MI-2-2	80–20	400	100	1375	0.484	242	28 days *
	MI-2-3	50–50	250	250	1375	0.484	242	28 days *
	MI-2-4	20–80	100	400	1375	0.484	242	28 days *
	MI-2-5	0–100	0	500	1375	0.484	242	28 days *
3	MI-3-1	100–0	500	0	1375	0.484	242	T = 25 °C—24 h; T = 50 °C—48 h
	MI-3-2	80–20	400	100	1375	0.484	242	T = 25 °C—24 h; T = 50 °C—48 h
	MI-3-3	50–50	250	250	1375	0.484	242	T = 25 °C—24 h; T = 50 °C—48 h
	MI-3-4	20–80	100	400	1375	0.484	242	T = 25 °C—24 h; T = 50 °C—48 h
	MI-3-5	0–100	0	500	1375	0.484	242	T = 25 °C—24 h; T = 50 °C—48 h
4	MI-4-1	100–0	500	0	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h
	MI-4-1-2	100–0	500	0	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h, and 90 °C—12 h
	MI-4-2	80–20	400	100	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h
	MI-4-2-2	80–20	400	100	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h, and 90 °C—12 h
	MI-4-3	50–50	250	250	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h
	MI-4-3-2	50–50	250	250	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h, and 90 °C—12 h
	MI-4-4	20–80	100	400	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h
	MI-4-4-2	20–80	100	400	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h, and 90 °C—12 h
	MI-4-5	0–100	0	500	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h
	MI-4-5-2	0–100	0	500	1375	0.6	300	T = 25 °C—24 h; T = 50 °C—48 h, and 90 °C—12 h

* 28 days without temperature control.

. The synthesis methodology was based on specific factors, such as the concentration of the alkaline activator and the drying temperatures, which were selected according to approaches reported in previous studies [39,40,43].

Initially, the MK and/or IG powders were mixed with fine sand in a hermetically sealed plastic bag for 10 min. After this step, the mixture was transferred to a mixer (Solids mixer, brand: YF, model: JJ-5, serial number: 5001-H; Nanjing T-Bota Scietech Instruments & Equipment Co., Ltd., Nanjing, China), where sodium hydroxide was added according to the proportions in Table 1. The material was then stirred for 5 min and subsequently poured into cube-shaped molds measuring 50 mm × 50 mm × 50 mm, following the specifications of NTP 334.051 [42]. Thus, the molds were covered with plastic bags and placed in a forced-air circulation oven (Universal Stove, brand: MEMMERT, model: UF450 Plus, serial number: B7210368, Memmert GmbH + Co. KG, Schwabach, Germany) for 24 h at 25 °C. It is worth noting that after this initial curing phase inside the molds, two types of drying were applied: one under ambient conditions (without temperature control) for 28 days, and another at 50 °C for 2 days (48 h). Accordingly, the geopolymer mortars were divided into groups based on the liquid-to-solid ratio and the drying process (Table 1).

In Groups 1 and 2, the geopolymer cubes were demolded after 24 h and left to dry under ambient conditions for 28 days. In Groups 3 and 4, the samples were demolded and then placed back into the oven for an additional 48 h at 50 °C. Specifically, for the geopolymers in Group 4, an additional drying stage at 90 °C for 12 h was also performed. It is important to mention that the equipment was configured to reach 50 °C with an approximate heating rate of 3.125 °C/min. After the predetermined period, the heating system was deactivated, initiating the natural cooling stage, which occurred at an approximate rate of 0.06 °C/min. After the curing process, the geopolymers produced using this methodology did not exhibit any cracking. The names of the geopolymer mortars are summarized in Table 1, and the process diagram is shown in Figure 2.

It is important to clarify that the term ‘without temperature control’ refers to curing under natural environmental conditions, without the use of ovens or controlled climate chambers. To provide greater accuracy and address any potential variability, the curing environment was specified as follows: the experiments were conducted in Arequipa, Peru, where the average ambient temperatures during the curing period ranged from approximately 23 °C during the day to 16 °C at night. This clarification is essential, as the ambient temperature has a significant influence on the geopolymerization process and the final properties of the material, as noted in the literature [44,45]. Providing this information helps contextualize the results and reinforces the local character of the study, which aimed to develop geopolymer mortars adapted to the regional climatic conditions.

Compressive strength tests were performed using the Microtest equipment (Microtest S.A., Madrid, Spain) with a capacity of 50 kN, which was configured for a compression speed of 5 mm/min. The statistical analysis of the elastic modulus (Young’s modulus) results was performed using two-way analysis of variance (ANOVA), one-way ANOVA, and Tukey’s post hoc test, with a significance level of 0.05 [46,47].

2.5. Porosity Test

To investigate the porosity of the geopolymer mortars, a methodology was used that used the apparent and real density. Thus, to determine the porosity, it was determined by the simple measurement of masses and volumes, while the real density was determined using the helium pycnometry technique using a Micromeritics AccuPyc II 1345 (Micromeritics Instrument Corporation, Norcross, GA, USA) that considered five purge cycles and five analysis cycles, operating at a pressure of 19.5 psig.

3. Results and Discussion

3.1. X-Ray Diffraction Analysis (XRD)

X-ray diffraction (XRD) analysis was performed to investigate the crystalline or amorphous structure of the materials and determine their crystallographic compositions. The results for the IG and MK materials are shown in Figure 3.

For the IG material, it was possible to observe several peaks (Figure 3a) that are characteristic of this magmatic formation rock, showing the presence of plagioclase (anorthite and albite), as this material originates from pyroclastic flows resulting from volcanic explosions on the Earth’s surface [7]. These substances were crystallized under the influence of pressure and temperature, forming certain crystallographic phases (albite, anorthite, cristobalite and sanidine, among others) [7], with 2θ = 13.73°, 15.14°, 23.5°, 27.73°, and 28.03°, according to the crystallographic data sheets JCPDS-96-100-0035 and JCPDS-96-900-9664 (Committee for Powder Diffraction Standards) [48,49], for anorthite and albite, respectively [39]. Planes were also observed for cristobalite, with 2θ = 21.94°, 28.40° 31.37°, 36.08°, 42.6°, and 56.82°, according to the crystallographic data sheet JCPDS-96-900-8111. The presence of sanidine was observed, with 2θ = 21.08°, 25.81°, 26.92°, and 29.82°, according to the crystallographic record JCPDS 96-900-8220. It was also possible to observe a possible presence of ferrosilite magnesian, with 2θ = 24.42° and 30.80°, according to the crystallographic record JCPDS 01-073-0245. The result observed in the diffractogram for the MK material purchased from China (Figure 3b) verified that the material was quite amorphous, with a halo in the region of 2θ = 18° to 30°, which is characteristic of metakaolin [50,51,52,53].

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR plays a crucial role in the characterization of materials, allowing for the identification of functional groups present in the samples. Figure 4 shows the infrared spectra for the materials IG (Figure 4a) and MK (Figure 4b).

First, for the IG material, it was possible to observe characteristic bands of SiO₂ bonds in the region between 1100 and 600 cm⁻¹, which were attributed to the stretching and bending vibration of O–Si–O bonds [54]. For the MK material, the infrared spectrum is shown in Figure 4b. Analyzing this spectrum, it was possible to observe a small broad band in the region of 3401 cm⁻¹, which may be related to some moisture hydroxyls. The region of 1250 to 950 cm⁻¹ was associated with the asymmetric stretching mode of the Si–O–(Si,Al) tetrahedral bond of the MK structure [16,55]. The band in the region of 1066 cm⁻¹ can be attributed to Si–O stretching vibration, and the band at 804 cm⁻¹ to the O–Al–O bending vibrations of the AlO₄ tetrahedra [16,55].

3.3. Scanning Electron Microscopy (SEM)

Morphological analysis of the materials was performed using scanning electron microscopy (SEM), where Figure 5 shows the SEM image and elemental analysis by EDS of MK and IG. It is possible to observe that the materials presented distinct morphologies, with the presence of disorganized grains and lamellae [56]. The red arrows in Figure 5a,b visually highlight the presence of lamellae in the structure of the materials. The EDS results revealed the presence of the chemical elements Si, Al, and O in the IG, among others, as illustrated in Figure 5c. These data are consistent with the XRD results, which indicated that the IG was composed of anorthite (CaAl₂Si₂O₈), albite (NaAlSi₃O₈), anorthite (CaAl₂Si₂O₈), cristobalite (SiO₂), and sanidine ((K,Na)(Si,Al)₄O₈). The EDS results for MK (Figure 5d) showed the presence of the main constituent elements, which would be Si, Al and O, and it is also common to find the presence of Fe and Ti in these materials [56].

This demonstrates that both IG and MK have excellent geopolymer formation capabilities. One of the essential requirements is that these materials are sources of aluminosilicate, enabling the reaction with an alkaline activator and thus favoring the hydrolysis of Si and Al into soluble species. Another important fact is that due to the characteristics of materials composed of aluminosilicates, IG has great potential for use in geopolymerization processes. However, there are still only a few studies being carried out in this field, highlighting the need for further research to fully explore its capabilities.

3.4. Chemical Composition of Raw Materials

The chemical compositions of the materials used as sources of aluminosilicates, namely metakaolin (MK) and ignimbrite (IG) as well as the fine aggregate (fine sand, FS), are presented in

Table 2. Chemical composition (wt%) of ignimbrite (IG), metakaolin (MK), and fine sand.

Element	IG	MK	Fine Sand
	%	%	%
SiO2	73.56	47.68	57.78
Al2O3	13.31	43.02	17.3
Na2O	4.27	0.07	4.17
K2O	4.1	0.26	1.9
Fe2O3	1.54	3.29	6.89
CaO	1.02	0.56	6.08
TiO2	0.2	1.91	0.85
MgO	0.25	0.2	3.32
Mn3O4	0.08	<L.C.	0.11
P2O5	0.05	0.06	0.21
SO3	0.02	0.27	0.07
ZrO2	0.02	0.09	0.03
V2O5	<L.C.	0.04	0.03
Cr2O3	<L.C.	<L.C.	<L.C.
SrO	<L.C.	<L.C.	0.09
P.C.	1.42	2.49	1.05
TOTAL	100	100	100

<L.C. = Detection limit.

. As expected, FS exhibited a predominance of silicon dioxide (SiO₂ ≈ 57.78%), which is typical of quartz-rich materials. Metakaolin (MK), in turn, was mainly composed of silicon oxide (SiO₂ = 47.68%) and aluminum oxide (Al₂O₃ = 43.02%), consistent with its well-established use as a highly reactive geopolymer precursor.

Ignimbrite (IG) exhibited a high SiO₂ content (73.56%) and a lower Al₂O₃ content (13.31%), along with significant concentrations of alkali oxides (Na₂O = 4.27%, K₂O = 4.10%) and alkaline earth oxide (CaO = 1.02%). This composition is consistent with the X-ray diffraction (XRD) results shown in Figure 3a, which indicated the presence of crystalline phases such as albite (NaAlSi₃O₈), sanidine (KAlSi₃O₈), anorthite (CaAl₂Si₂O₈), and cristobalite (SiO₂)—minerals from the feldspar group, consistent with the elements detected by X-ray fluorescence (XRF). In addition, energy-dispersive spectroscopy (EDS) (Figure 5) data confirmed the predominance of Si, Al, and O as well as the presence of Na, K, and Ca, reinforcing the potential of IG as a reactive precursor material.

Based on the chemical compositions and the masses used in the formulations (MK, IG, and fine sand), the SiO₂/Al₂O₃ molar ratio of the precursor mixtures was calculated, as shown in

Table 3. SiO₂/Al₂O₃ molar ratios calculated based on the chemical composition of the precursors (metakaolin, ignimbrite, and fine sand) used in the geopolymer mortar formulations.

Proportion (MK/IG) %	SiO2/Al2O3
100–0	3.87
80–20	4.25
50–50	4.92
20–80	5.77
0–100	6.48

. It was observed that as the proportion of IG increased (replacing MK), there was a progressive rise in this ratio: from 3.87 in the 100/0 formulation to 6.48 in the 0/100 formulation. This increase was attributed to the higher SiO₂ content and the lower Al₂O₃ content in IG compared with MK, along with the constant contribution of the fine sand, which was rich in silica and low in aluminum.

The molar ratio between silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) in precursor materials significantly influences the degree of geopolymerization of reactive species in geopolymeric systems [57]. Studies have shown that increasing this ratio tends to promote the formation of more extensive and densely cross-linked three-dimensional networks, with a higher proportion of Si–O–Si and Si–O–Al bonds [58], resulting in structurally more stable geopolymeric gels [58]. In the literature, the SiO₂/Al₂O₃ ratio is widely recognized as a key parameter in the behavior of these systems, influencing the degree of polymerization, and potentially the final properties of the material.

3.5. Compressive Strength Result

The compressive strength of the geopolymer mortars was evaluated for different proportions of MK and IG under varying curing conditions and liquid-to-solid (L/S) ratios, as presented in Figure 6 and

Table 4. Values of the compressive strength results for the geopolymeric mortars.

Group	Mixture ID	Proportion %	Compression Strength
1	MI-1-1	100–0	1.47 ± 0.33
	MI-1-2	80–20	2.82 ± 0.26
	MI-1-3	50–50	4.66 ± 0.48
	MI-1-4	20–80	0.59 ± 0.13
	MI-1-5	0–100	2.09 ± 0.34
2	MI-2-1	100–0	5.90 ± 1.92
	MI-2-2	80–20	8.66 ± 1.12
	MI-2-3	50–50	11.73 ± 1.41
	MI-2-4	20–80	6.73 ± 1.37
	MI-2-5	0–100	4.80 ± 0.33
3	MI-3-1	100–0	5.14 ± 0.78
	MI-3-2	80–20	8.03 ± 0.29
	MI-3-3	50–50	9.76 ± 0.16
	MI-3-4	20–80	10.78 ± 0.12
	MI-3-5	0–100	17.67 ± 0.66
4	MI-4-1	100–0	12.13 ± 0.56
	MI-4-1-2	100–0	13.91 ± 0.42
	MI-4-2	80–20	11.83 ± 0.04
	MI-4-2-2	80–20	12.11 ± 0.37
	MI-4-3	50–50	12.43 ± 0.57
	MI-4-3-2	50–50	10.83 ± 0.39
	MI-4-4	20–80	11.89 ±1.29
	MI-4-4-2	20–80	14.62 ± 1.29
	MI-4-5	0–100	6.46 ± 0.88
	MI-4-5-2	0–100	18.68 ± 0.46

. In the following subsections, the results are discussed separately for Groups 1 through 4. Each group was analyzed considering the specific parameters applied during synthesis such as the amount of alkaline activator used, the type and duration of the curing process, and the influence of temperature. These factors were examined in order to better understand their individual and combined effects on the development of compressive strength. It is important to note that the discussion regarding the SiO₂/Al₂O₃ molar ratio will be addressed later, in the section dedicated to the analysis of the elastic modulus (Young’s modulus).

3.5.1. Geopolymer Mortar—Group 1

The geopolymers mortars prepared with an L/S ratio of 0.343 and cured for 28 days at ambient temperature (without temperature control) (Group 1) exhibited a compressive strength of less than 4.7 MPa. This result was attributed to the insufficient amount of NaOH for efficient geopolymerization. As demonstrated in the study by Feng Bowen et al. [59], which investigated different proportions of alkaline activator in MK-based geopolymers, the findings indicated that variations in the amount of alkaline activator solution significantly influenced the geopolymer performance compared with other factors [59]. This may also be related to the temperature conditions to which Group 1 drying was subjected. Both the temperature and concentration of the activator solution are key factors in activating and dissolving MK [60]. Studies indicate that ambient temperature has a significant impact on the dissolution of the aluminosilicate oxides used [60]. It is worth noting that during the geopolymer manufacturing period, in Arequipa, the temperature varied from 22 °C during the day to 16 °C at night. Bharat Bhushan Jindal et al. [61] discussed that the studied geopolymers had a lower compressive strength at low temperatures, thus suggesting a drying system at a temperature of 50 °C [61]. This also explains why materials MI-1-4 and MI-1-5 (Figure 6a) presented a low compressive strength of 0.59 ± 0.13, and 2.09 ± 0.34 MPa, respectively. It is suggested that IG may also be influenced by the low dissolution of Si and Al, with this amount of activator solution that was used, which may lead to an inadequate geopolymerization reaction. It was also observed that as the proportion of GI increased in relation to MK in Group 1 (Figure 6a), the geopolymer mortars exhibited an increase in compressive strength, possibly due to the geopolymerization reaction of IG within the geopolymer. In contrast, the MI-1-4 and MI-1-5 blend (Figure 6a, and Table 4) revealed that the amount of NaOH solution and water was not adequate under these proportions and drying conditions for 28 days, contributing to a weak bond or poor formation between the molecules involved [62]. Another relationship of low compressive strength is related to the amount of water available in the system, where studies have already shown that water is an important part of the geopolymerization process, being one of the factors responsible for the process of destruction of solid particles and the hydrolysis of Al³⁺ and Si⁴⁺ ions dissolved together with the alkaline activator [53]. Another fact observed was the workability of Group 1, which was reduced in relation to the other groups studied [53].

3.5.2. Geopolymer Mortar—Group 2

For the geopolymer mortars in Group 2, prepared with an L/S ratio of 0.484 and cured for 28 days at ambient temperature (without temperature control), the compressive strength results are shown in Figure 6b. The compressive strength of Group 2 was higher compared with the geopolymers in Group 1. This indicates that the greater availability of the activating solution as well as the presence of water enhanced the geopolymerization reaction. Thus, it was possible to observe an increase in compressive strength with the increase in the IG proportion in the samples MI-2-1, MI-2-3, and MI-2-3 of 5.90 ± 1.92, 8.66 ± 1.12, and 11.73 ± 1.41 MPa, respectively. However, for samples with proportions of 20–80 and 0–100 percentage in m/m (MI-2-4 and MI-2-5, respectively), the same geopolymerization reaction problem was observed as in Group 1. In this case, there was an improvement in compressive strength for higher values (Table 4). These results show that for the materials involved, such as MK and IG, the increase in the proportion of the activating solution to 0.484 L/S favored the geopolymerization process. These results are in agreement with the study by Dihaji et al. [63], who investigated the effect of the liquid-to-solid (L/S) ratio on alkali-activated geopolymers synthesized from metakaolin. The authors observed that increasing the L/S ratio up to 0.6 significantly enhanced the compressive strength, which was attributed to improved precursor dissolution and the formation of a denser and more cohesive matrix.

3.5.3. Geopolymer Mortar—Group 3

To further investigate the criteria for optimizing the relationship between materials and the influence of the drying process, a study was conducted in which the drying process was carried out at higher temperatures. Several studies on geopolymers have indicated that performance can vary significantly depending on the type of material used and the variation in the activator. This includes both an increase in compressive strength at higher temperatures and a possible decrease under the same conditions [16,64]. Thus, the results for Group 3 and Group 4 are presented in Figure 6c and Figure 6d, respectively.

In Group 3, the same MK/IG variations were applied, using a ratio of L/S = 0.484. The drying process was carried out at 25 °C for 24 h before demolding. Then, after removing the geopolymer mortar cubes from the molds, they were dried at 50 °C for 48 h. Next, they were cooled to room temperature to perform the compressive strength test. It was possible to observe that under these conditions, the geopolymers followed the same behavior of increasing compressive strength when the IG proportion increased in relation to MK. This drying process favored the geopolymerization of materials with MK/IG proportions of 20/80 and 0/100. This demonstrates that the IG material, under these conditions, required temperature control for the geopolymerization process. Some studies carried out by our research group, which investigated the manufacture of IG and MK geopolymers with another type of geopolymer manufacturing methodology, demonstrated that the greater amount of IG favored the increase in compressive strength [40]. This represents a significant advance in the research criteria related to IG-based geopolymers, as studies on this material remain scarce. Notably, this is the first time that MK/IG-based geopolymer mortars have been investigated with a variety of proportions, together with the incorporation of an aggregate (fine sand), to contribute to the formation of geopolymer mortars.

3.5.4. Geopolymer Mortar—Group 4

The compressive strength results for the geopolymer mortars in Group 4 can be seen in Figure 6d. In Group 4, the geopolymers were synthesized with a 0.6 L/S ratio, using a drying process similar to that of Group 3. Drying was initially carried out at 50 °C for 48 h (after demolding), followed by an additional drying at 90 °C for 12 h. It was observed that increasing the amount of activator solution favored the compressive strength in some cases. Specifically, the materials in Group 4, which were dried only at 50 °C (MI-4-1, MI-4-2, MI-4-3, MI-4-4, and MI-4-5), showed an improvement in compressive strength (Table 4). However, the materials MI-4-4 and MI-4-5 were not completely dry. Therefore, a second drying step was included. Additional drying at 90 °C for 12 h contributed to more complete drying, promoting the geopolymerization reaction, as observed in MI-4-4-2 and MI-4-5-2. Another observation is that for materials with a 50/50 proportion, there was no benefit in the process of a new drying step.

The study by Dihaji et al. [63] reported that for L/S ratios above 0.6, a decrease in compressive strength was observed, associated with oversaturation of the liquid phase and excess sodium silicate, which hindered network condensation and increased porosity. This behavior was also observed in the samples from Group 4. However, in the present study, the implementation of an additional curing step at 90 °C was crucial to overcoming this limitation. The more intense thermal treatment promoted the continuation of the geopolymerization process, contributing to increased compressive strength even in mixtures with a high proportion of activating solution.

This approach shows that for materials with a higher proportion of IG and a greater amount of activating solution, a drying process at a higher temperature is required to favor the dissolution of aluminosilicate ions, thus influencing the geopolymerization process. Another factor that may be related to the differences in the compressive strength of these materials is their porosity.

3.6. Porosity Results

Figure 7 shows the results of the percentage of porosity of the geopolymers of Groups 2, 3, and 4 (Group 4, dried at 90 °C). These materials were selected due to their better geopolymerization results such as compressive strength.

In Group 2, it was observed that as the percentage of pores decreased, there was a corresponding increase in compressive strength. The materials MI-2-1, MI-2-2, MI-2-3, MI-2-4, and MI-2-5 presented porosity values of 34.13 ± 0.05, 33.32 ± 0.11, 30.24 ± 0.08, 29.30 ± 0.15, and 39.96 ± 1.02%, respectively. With these results, it is suggested that there was a variation in porosity between the materials due to the amount of MK/IG used in the mixture. Furthermore, in Group 2, it was observed that MI-2-5 presented a lower compressive strength, which may be related to its more porous structure and low geopolymerization [65].

In Group 3, where a higher temperature was applied in the drying process, an increase in the porosity of the materials MI-3-1, MI-3-2, MI-3-3, and MI-3-4 was observed, which presented porosity values of 23.90 ± 0.18, 25.77 ± 0.30, 28.42 ± 0.21, and 31.50 ± 0.38%, respectively. However, MI-3-5 presented a lower porosity (35.17 ± 0.07%) compared with MI-2-5 (39.96 ± 1.02%) in Group 2. Although the geopolymers of Group 3, dried at 50 °C, presented greater porosity, they demonstrated a higher compressive strength than that of Group 2 [53,65].

For Group 4, the porosity result (Figure 7c) showed a significant modification in the porosity profile. It can be observed that for this group, as the porosity increased, the compressive strength also increased. This effect was especially notable for the materials MI-4-4-2 and MI-4-5-2. This correlation can be explained by the amount of activator solution used for Group 4, where the liquid to solid ratio (L/S) was 0.6. A higher amount of activator solution implies a greater availability of water in the system, which may have increased the spacing between the geopolymer particles, leading to higher porosity. However, it is noteworthy that in this case, the increase in porosity did not compromise the compressive strength of the materials. This characteristic makes these materials promising for applications in pervious mortars as they combine good mechanical strength with a porous structure suitable for permeability.

3.7. Young’s Modulus and the Influence of the SiO₂/Al₂O₃ Molar Ratio

Several experimental factors have been previously discussed throughout this work, such as the MK/IG ratio, curing temperature and time, the amount of activating solution, and porosity levels, with the aim of elucidating the criteria that led to the mechanical performance differences observed among the formulations. In this section, these factors are further examined from an additional complementary perspective, with a particular focus on the stiffness of the geopolymeric mortars and how it correlates with the SiO₂/Al₂O₃ molar ratio, a key structural variable in the development and consolidation of the geopolymeric matrix.

The stiffness results (Young’s modulus) are presented in Figure 8a,b. Figure 8a shows the values obtained for each individual formulation (Mixture ID), while Figure 8b displays the distribution grouped by curing condition (Groups 1, 2, 3, and 4). Overall, it is evident that the Young’s modulus values were strongly correlated with the compressive strength results (Figure 6), suggesting a concurrent evolution of stiffness and mechanical capacity in the geopolymer formulations.

The one-way ANOVA statistical analysis revealed statistically significant differences among the experimental groups (F = 51; p < 0.0001), indicating that variations in the curing conditions had a direct impact on the stiffness of the samples. The Tukey HSD post hoc test confirmed that Group 1 exhibited significantly different behavior compared with the others, while Groups 2 and 3 did not show statistically significant differences between them. In contrast, Group 4, which underwent thermal curing at 50 °C followed by an additional stage at 90 °C, stood out with the highest average Young’s modulus values among all formulations, suggesting that increasing the curing temperature significantly enhanced the consolidation of the geopolymeric matrix. Figure 8b reinforces these observations, showing the progressive increase in Young’s modulus as both the curing temperature and liquid-to-solid ratio were intensified.

These effects can be directly associated with the variation in the SiO₂/Al₂O₃ molar ratio, which has been widely recognized as one of the most critical factors in the synthesis and mechanical performance of geopolymers. In the present study, a progressive increase in this molar ratio—achieved by gradually replacing metakaolin (MK) with ignimbrite (IG)—resulted in higher values of both compressive strength and Young’s modulus. This trend was particularly evident in formulations with a higher IG content, where the SiO₂/Al₂O₃ ratio ranged from 3.87 (100/0) to 6.48 (0/100).

This observation is consistent with several studies in the literature. According to De Silva et al. [57], increasing the SiO₂/Al₂O₃ ratio from 2.5 to 5.01 led to significant strength gains in metakaolin-based geopolymers, especially at later curing stages.

However, in the context of this study, even formulations with relatively high SiO₂/Al₂O₃ ratios (such as 6.48 in the 0/100 formulation) showed high values of Young’s modulus and compressive strength. This suggests that the presence of IG—rich not only in silica but also in alkali ions (Na⁺ and K⁺) and especially calcium (Ca²⁺)—promoted the formation of hybrid gels of the N-A-S-H/C-A-S-H type, which contributed to matrix densification and structural cohesion. These structural effects may have extended the effective range of the SiO₂/Al₂O₃ ratio beyond the limits typically reported in the literature, aligning with behaviors observed in systems formulated with heterogeneous natural precursors [66]. To complement this analysis and gain a better understanding of the formation mechanisms and structural evolution of the phases involved, FTIR spectroscopy was performed on the formulations from Groups 1 to 4, and the results are presented in the next section.

3.8. Fourier Transform Infrared Spectroscopy (FTIR) of Geopolymeric Mortars

These findings are consistent with the results obtained from the FTIR analysis of the geopolymers (Figure 9). This figure presents the infrared spectra of the formulations from Groups 1 to 4. All spectra displayed a broad band around 3400 cm⁻¹, which was associated with the vibration of adsorbed water molecules or hydroxyl groups within the geopolymer structure [16,67]. A band near 1650 cm⁻¹ was also observed, attributed to the angular deformation of water molecules (H–O–H) [16,67].

Notably, an absorption band around 1450 cm⁻¹ showed increasing intensity in formulations with a higher IG content and mechanical strength. This band has been widely associated with the presence of carbonate species, particularly calcium carbonate (CaCO₃) in matrices with higher Ca²⁺ content, and also sodium carbonate (Na₂CO₃), especially in systems with excess alkalis and greater porosity. The formation of these carbonates may result from the reaction between residual alkalis (such as NaOH or Na₂SiO₃) and atmospheric CO₂, or from internal carbonation processes driven by the incorporation of alkaline-earth ions like Ca²⁺ [68,69].

This spectral pattern suggests a possible link between the calcium and sodium incorporated into the matrix and the development of phases associated with carbonation or the formation of hybrid gels such as (C,N)-A-S-H. This observation is consistent with the findings of Petlitckaia et al. [69], who attributed the band centered at 1450 cm⁻¹ to the presence of partially decomposed dolomite and the subsequent formation of carbonates in calcium-rich alkaline systems. Likewise, Aboulayt et al. [70] observed bands between 1410 and 1570 cm⁻¹ in samples containing CaCO₃, associating these contributions with increased matrix densification and mechanical strength, even without changes to the setting mechanism.

Adding to this evidence, Azevedo et al. [67] reported that in metakaolin and red ceramic-based geopolymers, bands around 1460 cm⁻¹ were attributed to the formation of sodium carbonate (Na₂CO₃) resulting from the reaction between excess alkalis and atmospheric CO₂. These bands were more intense in systems with higher porosity, which facilitated CO₂ diffusion and surface carbonation. This condition is also relevant in the present study, as the IG-rich formulations showed increased calcium content, and consequently, greater potential for the formation of alkaline-earth carbonates that contribute to the geopolymerization process.

Therefore, the intensification of the 1450 cm⁻¹ band observed in the FTIR spectra of formulations with higher IG content and mechanical performance reinforces the hypothesis that Ca²⁺ and Na⁺ ions, originating from the ignimbrite and the activating solution, play structurally relevant roles in the formation of C-A-S-H and N-A-S-H gels, and significantly contribute to the improvement of compressive strength in geopolymeric mortars.

4. Conclusions

The characterization results obtained by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray fluorescence (XRF) provided a comprehensive understanding of the chemical, structural, and microstructural properties of the materials used in the production of geopolymers, specifically ignimbrite (IG) and metakaolin (MK). This study revealed important findings regarding the influence of different IG/MK proportions, liquid-to-solid (L/S) ratios of the sodium hydroxide-based activating solution, and the drying conditions applied, all of which significantly impacted the mechanical strength and porosity of the geopolymeric mortars.

The gradual replacement of metakaolin by ignimbrite led to a systematic increase in compressive strength, with the maximum value of 18 MPa observed in the formulation composed entirely of IG, compared with only 12 MPa in the formulation made with 100% MK. This behavior can be attributed to the specific characteristics of ignimbrite, including its chemical composition as identified by XRF, with notable contents of calcium (Ca) and sodium (Na), which may have contributed to the formation of a denser and more cohesive geopolymer matrix. FTIR analysis further suggests the potential role of these species in the structural development of the matrix, reinforcing the importance of understanding their interaction in the geopolymerization process.

Additionally, the analysis of SiO₂/Al₂O₃ molar ratios indicated that values around 3 to 4 favored mechanical strength development, especially when combined with thermal curing conditions. In Group 4, the increased porosity—resulting from a higher L/S ratio—did not compromise the compressive strength, supporting the feasibility of these materials for use in pervious mortar applications. This is particularly relevant for the development of sustainable pavement solutions.

The results of the Young’s modulus also showed a strong correlation with the compressive strength, confirming the mechanical consistency of the formulations. The overall performance of the developed mixtures, even under conditions of higher porosity, highlights the effectiveness of optimizing the L/S ratio as a strategy to balance the mechanical and hydraulic performance.

Future perspectives include exploring different types and concentrations of alkaline activators (such as sodium silicate or potassium hydroxide) as well as alternative aggregates with varying particle sizes and compositions, aiming to expand the functional applications of geopolymer mortars. These studies could contribute to enhancing the durability, permeability, and mechanical performance in real-world scenarios.

This study confirms that ignimbrite is a promising precursor for producing geopolymer mortars, especially for use in permeable pavements. The replacement of MK by IG, combined with the ability to adjust key formulation parameters, represents an important advancement in the search for sustainable and innovative construction materials, with significant potential to reduce the environmental impact of the construction sector.