1. Introduction
Various pollutants cause damage to the environment and, therefore, dangerous consequences for humans, animals, and plants due to their accumulation in the environment. These pollutants are of great concern worldwide, so several studies are being conducted to develop new ways to reduce them [
1]. In recent years, different industries have sought to reduce their environmental impact by innovating products that do not cause environmental problems. The construction sector has been no exception. Nevertheless, cement continues to be one of the materials that generates the most pollution in its manufacturing process. However, it is still one of the most frequently used materials in the construction industry.
It is indisputable that the cement industry is currently facing a significant identity crisis, with key figures (architects, engineers, and even a portion of the scientific community) being fully aware of this reality [
2]. On one hand, the economic progress of developing countries, an inconceivable process without the rapid construction of modern and extensive infrastructures, clearly foresees a substantial increase in cement demand for the next 40–50 years, potentially doubling or even tripling current production. However, on the other hand, the existing technology in Portland cement, a legacy of the 20th century characterized by high energy consumption, the use of non-renewable resources, substantial carbon dioxide emissions, and questionable durability, among other aspects, is unequivocally incompatible with a rational notion of ‘sustainable’ development [
2].
The literature and research suggesting an immediate change in construction binder technology argue that a new product cannot be launched into the market without undergoing exhaustive scientific-technical analysis. The scientific history of these materials presents inherent complexity and a considerable level of difficulty. Firstly, a single physicochemical concept has given rise to diverse terminology: terms such as alkaline cements, geocements, geopolymers, and inorganic polymers are used to describe the alkaline activation of materials such as kaolin, limestone, or dolomite [
2].
Concrete accounts for 5–8% of total anthropogenic CO
2 emissions, with 95% of CO
2 produced during cement manufacturing. Half of it is released due to limestone decarbonization during cement production. However, due to consumption growth, especially in developing countries, it is challenging to envision a reduction in CO
2. Hence, alternative cements are urgently needed. They are commonly referred to as geopolymers. Nevertheless, it is important to note that not all geopolymers exhibit a low carbon footprint; only a specific subset of geopolymers, termed ‘one-part geopolymers’, has demonstrated significantly lower carbon footprint levels than Portland cement-based mixtures. Alkali-activated cements and concretes are another promising alternative, formed by the alkali activation of aluminosilicate sources, which can be natural materials like lateritic clays, synthetics, or industrial wastes [
3].
Although the use of Portland cement is frequent, it is recognized that its production leads to many environmental problems, such as the depletion of natural resources, environmental degradation, high greenhouse gas emissions, and atmospheric pollution. The significant emission of greenhouse gases in cement manufacture is considered a significant contribution to global warming [
4]. In this regard, in order to find a solution that does not have a negative environmental impact, an alternative binder with low CO
2 emissions was implemented, such as a geopolymer [
5]. This material is an alternative to Portland cement and has several advantages from an environmental perspective, as well as high compressive strength and durability against chemical attack [
6,
7]. Generally, conventional geopolymer comprises precursor materials with a high proportion of aluminosilicate, such as fly ash, and aluminosilicate [
8], bottom ash, slag [
9,
10], and ashes from the incineration of municipal solid wastes [
11]. The alkaline activator for conventional geopolymers is usually a combination of two chemical compounds: sodium silicate (NS) and sodium hydroxide (NH). A wet amalgamation process usually prepares the mixture.
Although fly ash-based geopolymers have been studied as a possible alternative in recent years, they have yet to become a viable option due to the complexity of obtaining the material and the cost involved [
12]. In this sense, lateritic clays form in humid tropical regions characterized by warm, rainy climates where sedimentary materials resulting from rock decomposition are deposited. These soils exhibit specific properties rendering them valuable for both construction and agricultural purposes. In Bolivia, the eastern regions host the development of lateritic soils owing to a range of favorable conditions during different phases of their formation and alteration.
However, this specific Bolivian region lacks materials meeting the stringent specifications mandated by regulations. Additionally, transportation expenses associated with these materials significantly escalate the construction costs. Consequently, exists a necessity to conduct studies for lateritic soils. This examination aims to elucidate technical concepts ensuring their optimal utilization, not solely restricted to road constructions, but extending to a diverse application in civil constructions.
Laterites are reddish and yellowish soils formed in tropical and subtropical areas. They are formed due to kaolinite alteration by iron minerals through the induration phenomenon. Lateritic clays are a material that comes from the lateritic soil typical of tropical and subtropical regions, typical of the Bolivian pre-Cambrian, ranging from the province of San Ramon, which is located in the city of Santa Cruz and continues through the department of Beni until reaching Brazil [
13].
Rodrigue et al. compiles all the research carried out on laterite and its use as a geopolymer, but the performance of the binder was evaluated in terms of resistance only on compressive strength [
5]. Kaze et al. also examined how the silica modulus in the activating solution affects the performance of iron-rich laterite-based geopolymer binders subjected to calcination at 600 °C [
14]. This research indicated that a silica modulus 0.75 was the optimum for achieving the highest compressive strength in calcined laterite; however, the study focuses solely on the alkaline activation of iron-rich aluminosilicates (laterites), without conducting additional strength or flexural tests to evaluate the geopolymer. On the other hand, Poudeu et al. combined crude and calcined laterite (at temperatures between 500–600 °C) to produce geopolymeric materials based on Cameroonian lateritic soil and found that the synthesized products could be a low-cost alternative for housing construction [
15]. Similarly, Kamseu et al. used iron-rich laterite in its raw state and reactive silica from rice husk ash as starting materials for geopolymer production. The results demonstrated the formation of new bonding phases, such as hinsingerite. This additional formation enhanced the geopolymer binder, resulting in improved cohesion between particles and increased densification of the interconnected pores in the matrix [
16].
These studies collectively highlight the importance of lateritic clay-based geopolymer. However, mineralogical, chemical, and mechanical property studies have not been carried out in conjunction, in order to study the effectiveness of the geopolymer and its application in construction, beyond only applying them to roads. Therefore, by reviewing the existing materials that can be used in Bolivia, the present research proposes to evaluate a geopolymer based on laterite clays, from its mechanical properties of compression and bending, its mineralogical and chemical composition, as an alternative to hydraulic conglomerate that would significantly reduce the environmental impact of its manufacturing process, taking into account that in Bolivia the environmental impact of the cement manufacturing process is high. To this end, the evaluation of the geopolymer based on lateritic clays was carried out through the elaboration of cubes and prisms that were subjected to mechanical tests of compression, bending, chemical, and mineralogical studies.
Regarding the results of the present investigation, the values of the average resistance of the compression tests of the samples of Portland cement IP, a hydraulic cement in which the constituent pozzolan is present up to 40% by mass, and the geopolymer indicate that the geopolymer based on laterite clay shows a significant difference of lower resistance with the Portland cement mortar. Likewise, the average flexural strength tests of the most representative samples of the geopolymer indicate a slight increase concerning the prismatic cement mortar specimens, which generates expectations regarding the different uses that can be given to the evaluated geopolymer, thus opening new lines of research.
2. Experimental Program
2.1. Materials
Lateritic soils are typical of tropical areas and cover an extensive strip from the province of San Ramón located in the department of Santa Cruz, Bolivia, which continues through the department of Beni—Bolivia—until reaching an extensive area of Brazil. In this sense, the material object of this study is extracted from the RN 10 route on the way to San Javier located in the department of Santa Cruz, whose coordinates of the extraction area are: Latitude 16°17′39.86″ S; Longitude 62°31′4.38″ W. The exploration of the deposit was carried out to obtain the required mineral raw material as indicated by ASTMC 75 [
17].
For the testing process, sand was used to make the cement mortar, and laterite clay was used for the geopolymer. The granulometric curve of the laterite clay sample analyzed is shown in
Figure 1. The high content of fines in the samples collected in this study (%) is related to the fact that, during the exploration with machines, the laterite sand layers are mixed, unintentionally, with a portion of the lower layer that is mainly composed of fine soil. The resulting material is representative of the field exploration process.
Table 1 shows the physical properties of the laterite soil studied and the standards used for characterization tests. This material is classified as low plasticity clay (CL) by the Unified Soil Classification System [
18,
19] and classified by AASHTO [
20] as clay soil (A-6 (4)).
As for the fine sieved soil, it is observed that the particles have an orange hue. When this fine material is exposed to water, it takes on a very particular reddish color, which allows to observe that this material has a high content of iron and aluminum, as well as high silica content whose contribution is of a whitish color, and the combination of both gives a characteristic orange color.
The chemical composition of the fine fraction, analyzed by photometry and X-ray fluorescence assay, determined that the sample is composed of aluminum (19.84%), iron (5.42%), sodium (0.10%), silica (40.71%), aluminum oxide (0.95), and iron oxide (20.13%). These chemical and mineralogical characteristics are similar to other studies in Brazil, India, Australia, and Africa [
22,
23].
Portland cement (IP type) was used for the mortar manufacturing process [
24]. This cement has between 20% and 40% pozzolana in its composition. Portland cement type IP is the binder widely produced and used for construction in Bolivia. The cement has a specific gravity of 2.90.
2.2. Specimens Molding and Preparation
The molding and preparation of specimens followed the detailed flowchart presented in
Figure 2, designed for the comparison between two types of mortars: one based on Portland cement and the other on geopolymers, focusing on their compression and flexural strength. Material characterization, including Portland cement and sand, preceded the determination of proportions for the Portland cement-based mortar, following ASTM C109 regulations [
25].
For the compression test, a mixture of 82 g of Portland cement, 226 g of sand, and 60 mL of water was used in a mold with dimensions of 50 mm × 50 mm × 50 mm. In the flexural test, the mixture consisted of 166 g of Portland cement, 458 g of sand, and 60 mL of water, in a mold with dimensions of 40 mm × 40 mm × 160 mm. Verification of the geopolymers based on lateritic clay and sodium hydroxide involved three processes. First, the material was obtained and characterized through physical tests, revealing low plasticity clay with a plasticity index of 15.96%. Subsequently, it was confirmed that the material corresponded to lateritic soil through chemical and mineralogical tests, validating the sodium content at 10%, lower than the required 15% for it to be considered saprolitic material.
Molar ratios were verified through fluorescence and X-ray tests. The final dosing for the compression test was 1 L of distilled water, 480 g of NaOH, 190 g of lateritic clay, and 118 g of sand, in a mold of dimensions 50 mm × 50 mm × 50 mm. For the flexural test, the dosing consisted of 1 L of distilled water, 480 g of NaOH, 530 g of lateritic clay, and 168 g of sand, in a mold of dimensions 40 mm × 40 mm × 160 mm. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted to determine the crystalline structure of geopolymers, the X-ray diffraction technique was employed using equipment from Panalytical XperPro (Malvern Panalytical, Malvern, Worcestershire, UK), with Cu-Kα radiation (wavelength λ = 1.54060 Å; voltage of 45 kV; current of 40 mA). Measurements were conducted in the upper region of the sample, covering a 2θ range from 20° to 90°. Additionally, the scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDX) technique was utilized, with Tescan Vega3 model (TESCAN, Brno, Czech Republic), using secondary electrons (SE). This technique was applied to visualize and analyze the chemical composition, as well as to identify the minerals present in the geopolymers formation process. Tests were performed on samples baked at 30 °C and 60 °C. After mixing and pouring, the specimens underwent a curing process in an oven for 24 h at temperatures of 30 °C and 60 °C for the geopolymers-based mortar, while the Portland cement-based mortar cured submerged in water for the same period.
2.2.1. Physical Tests
Initially, the lateritic soil was classified as clay of low plasticity (CL) by the unified soil classification system. Next, the material added to make the mortar corresponds to sand, classified as poorly graded sand (SP), with the following characteristics: curvature coefficient of 1.47, uniformity coefficient, and specific gravity of 2.69. The characterization was carried out under ASTM D2487 criteria [
19]. The results of the lateritic soil and sand particle size are shown in
Figure 3. The physical properties of the lateritic soil are shown in
Table 1. The lateritic sample tests were performed according to ASTM standard specifications; the standards used were raw material extraction according to the ASTM D75 method [
17], determination of liquid limit, plastic limit, and plasticity index ASTM D4318 [
21], total water content of aggregates by drying ASTM C566 [
26], and a method for determining the true density, net density, and water absorption of fine aggregates ASTM C128 [
27].
2.2.2. Chemical and Mineralogical Tests
The material characterization was conducted to verify its compliance with the required chemical and mineralogical characteristics. In this process, a sieved sample of lateritic material was used, with particles passing through sieve number 200. A total of 8 kg was selected. From this quantity, 200 g was specifically extracted to carry out the initial analysis through flame photometry and chemical decomposition.
Table 2 shows the result of the values presented by the material from the chemical decomposition. According to Culshaw (2001), where sodium is found to the point that it forms 10% of sodium or more of the total cation exchange capacity, the deposits are dispersible. They are prone to rapid erosion and lubrification of lateritic tropical soil. It is concluded that the material presents an amount of sodium and, therefore, is lateritic because the percentage of sodium is 0.10 and does not exceed 0.15 to consider the material a saprolitic soil [
28].
Consecutively, fluorescence assay was performed, Quiniou et. al. (2014) specify that it is an assay that is used in different disciplines for chemical composition determination of a wide variety of sample types. It is nondestructive, multi-elemental, fast, and cost-effective and can be applied directly to certain types of samples without any preparation, and it is ideal for many applications [
29]. As this technique combines high precision and accuracy with easy and fast sample preparation, it is possible to automate it. Likewise, it can provide both qualitative and quantitative information, making it possible to conduct a rapid detection analysis.
Thus, two samples of 200 g of the finest material passing through the sieve No. 200 were used for the test. Therefore, the test showed that these materials contain only silicon, aluminum, and iron, minerals necessary to form geopolymers. Next, with the analysis of minerals obtained through the X-ray fluorescence test, we proceeded to perform the molar ratios; for this, we worked with the atomic weights and with the weights obtained in the fluorescence and flame photometry tests. Davidovits (2013) presents molar ratio intervals and explains that a solid solution can be given starting from Si/Al = 1 (see
Table 3). The same was used to validate the molar ratios to validate the geopolymer [
30].
2.2.3. Mechanical Testing
In the execution of mechanical tests, experiments were conducted on samples composed of lateritic clay and sand with lateritic clay. Initially, the consistency of the lateritic clay-based hydraulic binder was determined, following the normal consistency procedure established by ASTM C187 [
31] for cement. The Vicat needle was allowed to drop for 30 s, permitting a penetration of at least 1 cm into the geopolymers, aiming to measure the initial setting time of the pastes.
The water-to-binder ratio was measured to assess the influence of aluminosilicate precursors used in the process, which included lateritic clay and sodium hydroxide in the formation of the hydraulic binder. A variation in the consistency of the binder was observed, attributed to the temperature increase in the mixtures of geopolymers with sodium hydroxide or clay with sand. The results of the water-to-binder ratios for pure clay and the mixture with sand are detailed in
Table 4 and
Table 5, respectively. This analysis enabled the identification of differences in the mechanical properties of the binder based on the composition of the mixture, contributing to a deeper understanding of its behavior and performance.
2.2.4. Compression Test of 50 × 50 × 50 mm Cubes of Cement and Geopolymer Mortar
Tests were conducted on cement mortars with sand and mortars with lateritic clay-based geopolymers in accordance with ASTM C1157 [
32], which adhere to the requirements outlined in ASTM C778 [
31]. Initially, compressive strength tests were carried out on samples of cement and sand mortar, followed by tests on lateritic clay-based geopolymers. The cubes had dimensions of 50 mm x 50 mm, with curing times of 7, 14, and 28 days, followed by a 24 h drying period in an oven at 30 °C. The cement mortar and geopolymers were mixed in a ratio of 1:2.75, determined through molar relationships. Three samples were tested for each type of mortar (geopolymer and cement), with the average compressive strength recorded. Water–cement and water–geopolymer ratios were adjusted to achieve a fresh mix flow of 110 ± 5%, according to ASTM C1437 [
32], resulting in ratios ranging between 0.40 and 0.50 for both cases.
The initial setting time of the pastes was measured using the Vicat needle apparatus. The amount of water in both mixtures, with geopolymers and cement, was measured to determine the setting time required to achieve normal consistency, following ASTM C187 [
33]. Following the volume dosing relationship, it was weighted to obtain precise quantities for the preparation of cement mortar cubes. Initial quantities were as follows: cement = 82 g, sand = 226 g, and water = 60 mL for geopolymers; water and NaOH = 70 mL (alkaline solution), lateritic clay = 190 g, and sand = 118 g.
For the experimental procedure, materials (geopolymer and sand) were weighed, water was added for cement mortar, and sodium hydroxide (NaOH) was added for geopolymers. They were mixed until a homogeneous consistency was achieved, observing that the water/cement ratio was suitable for compaction. Once the mixture was prepared, the molds were filled in four layers of eight strokes, followed by smoothing the surface, and finally, the curing process was carried out.
2.2.5. Bending Test for 40 × 40 × 160 mm Prisms of Cement and Geopolymer Mortars
For the prism bending test, cement mortars with sand and geopolymers based on lateritic clays were prepared. The prisms, measuring 40 mm by 40 mm by 160 mm, were fabricated with a ratio of 1:2.75, validated through molar relationships.
Tests were conducted on three samples of cement mortars with sand and geopolymers based on lateritic clays, recording the average flexural strength values. To prepare the samples of Portland cement paste and water, aimed at obtaining a mortar paste with normal consistency, and for the geopolymers, sodium hydroxide (NaOH) was added as per ASTM C187 [
34]. The fresh paste was used to create prismatic samples measuring 40 mm by 40 mm by 160 mm, which were stored in sealed conditions. After 24 h, the specimens were demolded, and their initial lengths were immediately measured. Subsequently, the samples were subjected to a vapor pressure of approximately 2 ± 0.07 MPa at a temperature of 216 °C for 3 h.
The preparation of the three samples, both for cement mortar and geopolymers, followed ASTM C348 [
35]. A volume dosage with a water–cement ratio of 0.485 was used, equivalent to a dosage of 1 part cement and 2.75 parts sand. It was weighted to obtain precise amounts for prism fabrication, with the following quantities: cement = 166 g, sand = 458 g, and water = 60 mL for cement mortar; for geopolymers, water and NaOH = 220 mL (alkaline solution), lateritic clay = 530 g, and sand = 168 g. These materials were carefully mixed until a homogeneous blend was achieved, followed by water addition and the continuation of the mixing process. The molds were filled starting with an initial layer of approximately 2.5 cm thickness, followed by compaction with 32 strokes on the surface in 4 stages of 8 strokes each, and finally, surface smoothing. The curing procedure was the same as previously described in the compression test section.
After the cement prisms were made, the preparation of geopolymers was initiated. In the initial phase, sodium hydroxide was mixed with distilled water in precise proportions (1 L of distilled water per 480 g of sodium hydroxide) to generate an alkaline solution. This process was conducted cautiously due to the exothermic reaction produced. The alkaline solution was poured onto the carefully combined mixture of sand and clay. Using paddles, the mixture was meticulously stirred for approximately 5 min, eliminating lumps and obtaining a homogeneous blend. Subsequently, the mixture was transferred to the molds, following the same compaction and leveling steps previously described for the cement prisms. Throughout the process, the temperature of the geopolymer was monitored. Initially, a temperature rise was observed, reaching 61 °C due to the NaOH. This thermal elevation modified the clay, raising it to 41 °C, a temperature that gradually decreased over a period of 80 min until reaching ambient temperature. It is emphasized that maintaining this material in tropical environments with temperatures above 30 degrees is crucial, as the cooling time is essential for successful geopolymerization. Hence, it is imperative to keep the material at an appropriate temperature to achieve successful geopolymerization.
It is noteworthy that, in contrast to cement, which experiences a temperature rise over time, the geopolymer underwent a curing process for 24 h at a constant temperature of 30 °C, followed by exposure to a temperate environment of no less than 30 °C.
Figure 4a,b, showing the cubes and prisms of cement and geopolymer mortar, are attached for visual reference.
4. Conclusions
The overall objective of this study is to evaluate the compressive strength and microstructural characteristics of a geopolymer based on laterite clays. The study has among its objectives to complete the knowledge gaps on alternative binder materials with low emissions of CO2. Therefore, it is necessary to highlight that, in this first phase, mechanical, chemical, and mineralogical aspects were evaluated with the geopolymer, which gave positive results of the geopolymer as an alternative to be used, allowing this to be deepened in other studies.
Through the results of mechanical strength and microstructure, it is possible to determine that the mechanical behavior of the geopolymer, in terms of compressive strength, is 64% compared to the Portland cement IP. Thus, the geopolymer based on lateritic clays does not meet the compressive strength to match cement. However, using scanning electron microscopy and the X-ray scattering test, it can be determined that the decrease in compressive strength is because constant temperatures from 30 °C are required in a controlled environment to avoid the generation of voids and generate better bonds between minerals due to the presence of plagioclases and feldspars that are also found in Portland cement and that with a better alloy, a higher compressive strength will be generated. On the other hand, the results of the flexural strength tests determine that the geopolymer is outperforming in flexural strength by 29.3% compared to Portland cement IP.
The scanning electron microscopy indicates the presence of albite. This material belongs to the sodium plagioclase family, which is an aluminosilicate; quartz, a smooth material of irregular structure with colloidal edges; bytownite, a material that presents an irregular structure similar to clay lumps and the presence of amblygonite, all of them provide hardness to the material. However, it is essential to highlight that the fractures are because, although a geopolymerization has been performed, the degree of alloy between minerals is not high enough to reduce the fracture and that there is no union between the different minerals in their entirety.
It is possible to establish that the higher the degree of geopolymerization, the greater the mechanical strength, as evidenced by the results of X-ray diffraction and scanning electron microscopy, and that the higher the amount of heat, the better the bonds are generated. However, due to continuous uncontrolled exposure, it expands with the heat, generating air voids that turn the material into one with lower resistance, so it is recommended to have the material under constant control at the time of curing.