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Article

Pozzolanic Potential of Calcined Clays at Medium Temperature as Supplementary Cementitious Material

by
Valber Domingos Pinheiro
1,
Ramon Fernandes de Abreu
1,
Jonas Alexandre
2,*,
Gustavo de Castro Xavier
2,
Markssuel Teixeira Marvila
3 and
Afonso Rangel Garcez de Azevedo
2
1
Laboratory of Advanced Materials, State University of the Northern Rio de Janeiro, Campos dos Goytacazes 28013-602, Brazil
2
Civil Engineering Laboratory, State University of the Northern Rio de Janeiro, Campos dos Goytacazes 28013-602, Brazil
3
CRP-Rio Paranaíba Campus, UFV-Federal University of Viçosa, Rio Paranaíba 38810-000, Brazil
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(17), 7508; https://doi.org/10.3390/su16177508
Submission received: 23 July 2024 / Revised: 21 August 2024 / Accepted: 26 August 2024 / Published: 30 August 2024
(This article belongs to the Section Sustainable Materials)
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Abstract

:
Global warming is intensified by substantial greenhouse gas emissions, with the cement industry contributing significantly by releasing around 0.8 tons of CO2 per ton of cement produced. To mitigate these impacts, in this study, we investigated the pozzolanic potential of calcined clays, assessing their influence on the properties of Portland cement as sustainable alternatives for partial replacement. Three clays from Campos dos Goytacazes, RJ, were analyzed. After drying and calcining at 600 °C, they underwent physical and chemical analysis. The samples were characterized in terms of grain size, moisture content, grain density and plasticity limit. Chemical analysis by X-ray fluorescence identified the elemental composition of the clays, while X-ray diffraction determined the presence of crystalline and amorphous phases. A mineralogical characterization confirmed the amorphization process and classified the clay as kaolinitic. Scanning electron microscopy provided detailed images of the morphology of the particles. The surface area was measured using the Blaine method, which is essential for understanding the reactivity of calcined clays. A preliminary analysis showed that the calcination at 600 °C led to greater pozzolanic reactivity in the clay samples. A thermal analysis showed a loss of mass, suggesting the dihydroxylation of the kaolinite. The pozzolanic reactivity was extensively evaluated by isothermal calorimetry, which monitored the release of heat during hydration reactions through compressive strength tests on the mortars that showed higher strength than the reference. In addition, modified Chapelle and R3 tests were carried out, which showed a direct correlation with the compressive strength, also indicating significant pozzolanic reactivity in the material. The results showed that the clays, when calcined, had a highly reactive amorphous structure, resulting from their transformation through the process of dihydroxylation and amorphization. Calorimetry identified the acceleration of the cement hydration reactions, stimulating the formation of calcium silicate hydrates and aluminum compounds, which are essential for mechanical strength. The partial replacement of Portland cement with calcined clays helps to reduce CO2 emissions without compromising strength and durability, representing a promising strategy for reducing greenhouse gas emissions, with a view to greater environmental sustainability and the efficiency of building materials.

1. Introduction

There is currently a need for infrastructure development due to global population growth, which has resulted in a high demand for cement and cementitious products, driven by urban expansion, infrastructure development and industrialization in developing countries. In recent years, cement production has averaged 4.4 billion tons per year, and projections indicate that it could reach approximately 4.8 billion tons in the next 20 years [1]. The cement industry is one of the largest sources of carbon dioxide emissions and faces growing challenges in reducing greenhouse gas emissions. The use of cementitious materials (SCM), such as calcined or activated clay, offers a viable solution to this problem.
There are no materials as widely used, especially in engineering, as Portland cement. Its extensive use is due to the properties that the material can provide, especially in concrete and mortar, which have made it possible to make significant changes in both architecture and engineering. Portland cement has a long history of use, proving to be a highly reliable material in building construction and versatile, since it can be used in a wide range of environmental and working conditions [2,3].
However, the cement production process is one of the main sources of CO2 emissions from industrial activities, releasing around 0.75 tons of CO2 for every ton of cement produced. In addition, Portland cement requires energy costs for decarbonizing limestone, burning fossil fuels and consuming electricity for grinding and transportation. These costs are considerably higher compared to the production of some types of supplementary cementitious material, such as fly ash, slag and calcined clays, for example. It should also be noted that cement production has a major environmental impact, considering both the extraction of non-renewable raw materials and the significant number of pollutants that are directly released into the ecosystem. The manufacture of Portland cement requires the calcination of limestone, a process that releases large quantities of CO2, as its heat treatment reaches temperatures of 1400 °C on average. In addition, the intensive and uninterrupted use of energy during the production process, often from fossil fuels, contributes to the release of harmful gases that intensify global warming and contribute to the depletion of the ozone layer. As a result, additional cementitious materials are being developed and used to mitigate these efforts. Supplementary cementitious materials, especially those with pozzolanic reactivity, are therefore being investigated as alternative partial replacements for cement and to reduce carbon emissions [4,5,6,7].
The use of supplementary cementitious materials in concrete brings several benefits, including a reduction in the heat of hydration, better workability, increased strength, greater durability and, consequently, greater sustainability [1,8,9]. Supplementary cementitious materials (SCMs) are made up of particulates with similar or complementary properties to Portland cement. SCMs can be natural or artificial, and can also react chemically with Portland cement hydrates or be inert. This type of material is commonly used as a substitute for or complement to cement and can improve the performance of the cementitious matrix. The development of its application is due to its unique ability to partially replace cement, provide specific properties and improve the effectiveness of binders in the cementitious matrix [10,11]. SCMs are materials that bring great advantages to cementitious systems in terms of controlling their physical and chemical properties and, consequently, their mechanical performance and durability. Their particular physical and chemical composition allows them to be used as partial replacements for Portland cement in cementitious matrices. In addition, research into supplementary cementitious materials corroborates the difficulties in disposing of various industrial by-products, including blast furnace slag, fly ash, active silica and ground ceramic waste, for example [1,12,13].
Partially replacing cement with supplementary cementitious materials (SCM) can reduce CO2 emissions by up to 40% without significantly affecting strength, durability or cost [14]. This type of clinker substitution makes it possible to reduce the environmental impact of cement production without affecting the ease of use and versatility of cement. The majority of the cements available in countries today are percentage mixtures of clinker and MCSs, such as fly ash, slag and pozzolans, for example [2,3].
Among the main materials used as supplementary cementitious materials are vegetable ash from sugar cane bagasse, bamboo and rice husks [15,16,17,18,19,20,21,22], volcanic ash [23], limestone waste [15,24,25,26,27], calcined and natural clays [15,16,27,28] and metakaolin [24,29,30,31,32,33,34,35,36].
When added to a Portland-cement-based matrix, supplementary cementitious materials play a fundamental role in increasing the strength of pastes, mortars and concretes, filling voids with additional products and, ultimately, effective fillers. Thus, the use of pozzolans and other complementary cementitious materials helps to guarantee the properties of cement, especially mechanical strength, even when the clinker content is reduced [37,38,39,40].
The reactivity of supplementary cementitious materials is determined by various factors beyond the raw composition of the material. Among these factors, we can consider that the activation of the material, its degree of fineness, the aqueous hydration medium and the previous heat treatment are the ones that most affect the degree of reactivity of each material. Regarding the reactivity of SMCs, it is not only the pozzolanic activity that these materials possess that is significant, but also the physical and chemical effects that can interfere with the performance of the material as a whole [14,41].
The use of calcined clays as supplementary cementitious materials (SCM) is a viable, sustainable and advantageous alternative since, compared to the calcination of limestone for the production of Portland cement, which releases large quantities of CO2 due to the decomposition of limestone (CaCO3) at temperatures of around 1450 °C, clay calcination takes place at lower temperatures (between 600 and 800 °C) and emits much less CO2, resulting in lower energy consumption and lower greenhouse gas emissions. It should be taken into account that kaolinitic clays are abundant and widely used in the production of ceramic products for civil construction such as bricks, blocks, tiles and cladding, facilitating the production of SCMs and reducing the environmental impact associated with transportation. In addition, the use of calcined clays supports the reuse and future use of ground ceramic waste, since the incorporation of calcined clays into concrete improves durability, mechanical strength and workability and reduces the heat of hydration, benefiting the construction of large volumes of concrete, promoting sustainability, the circular economy, the use of local resources and the reuse of waste and, thus, reducing environmental impact [33,42,43,44,45].
Calcined clays are defined as artificial pozzolans produced by calcining clays at temperatures between 500 and 900 °C. The thermal process in clays causes the crystalline structure of the clay minerals to break down and transform into an amorphous, highly reactive structure [46,47].
Compared to the calcination temperatures of Portland cement raw materials in the manufacture of clinker, calcined clay is considered to be a less polluting material because it requires lower rates of heat during thermal activation and tends to emit less carbon dioxide. Calcining clays at a suitable temperature ensures that the materials are reactive with calcium hydroxide [48].
Kaolinitic clays are used as cement additives because they decompose at lower temperatures and still have significant pozzolanic reactivity. The pozzolanic reactivity of calcined kaolinitic clays is due to the transformation of kaolinite into metakaolin during heat treatment. This process involves the amorphization of the structure, also known as dihydroxylation, in which the crystalline structure of the clay minerals is transformed into a complex amorphous structure that maintains a specific layer order. The transformation removes the structural water, leaving an amorphous aluminosilicate with high internal porosity and high pozzolanic activity [44,46,47,49].
In this way, it is possible to see that calcined clay can offer suitable properties for the production of SCMs, with the aim of disseminating a set of raw materials that can be used to partially replace cement in its various applications, consequently promoting sustainability in the cement sector. In the quest to combine the properties of calcined clays in cementitious matrices, in this study, we investigated the pozzolanic potential of calcined clays. In this context, the technical and scientific gap investigated in this work is the lack of a clear and objective methodology for assessing the pozzolanic reactivity of calcined clays as supplementary cementitious materials, taking into account the intrinsic characteristics of each of the groups given their heterogeneity when applied to cementitious matrices. In addition, the research is intended to contribute to lower greenhouse gas emissions, since cement can be calcined at a temperature of 1400 °C, while the temperature required for calcined clays is 600 °C [15,32,50,51].
Therefore, the need for more comprehensive studies on clays calcined at medium temperatures and transformations in the metakaolin phase with lower energy costs are related to innovative and practical aspects in the civil engineering sector, as well as in the field of research into supplementary cementitious materials, since they are related to the optimum calcination temperature, as well as technological advances in terms of the more in-depth characterization of calcined clays in their reactive phases, the development of cement substitute materials, reductions in polluting gases and energy costs, performance and durability, the availability of resources and sustainability.

2. Materials and Methods

2.1. Materials

Three different types of clay used in the ceramics industry in Campos dos Goytacazes, in the northern region of Rio de Janeiro state were analyzed. The clay specimens were chosen according to the raw materials used in the production of ceramic products such as bricks, blocks and tiles, with a view to using the by-products as waste for incorporation into concrete and mortar in future research. In addition, the evaluation of this raw material makes it possible to implement the production of more sustainable new products [52,53].
Three clay samples were used in this research: A1—China clay with kaolinite as the main mineral, white in color and not plastic (Figure 1a); A2—ball clays with kaolinite as the main constituent, but containing several impurities; and A3—clays made of brick, a material with a high iron content and red in color (Figure 1b), based on kaolinite and muscovite. Figure 1 shows examples of clay deposits and their differences in color. The clay categories are identified according to their color in the quarries from which they are removed. The A1, A2 and A3 clay samples were extracted from a depth of 1 m at different locations in the quarries. After extraction, the samples were separated, catalogued and dried at room temperature (22 °C) in a dry, well-ventilated area. In the laboratory, each of the samples was homogenized and then comminuted and sieved through a 400 µm sieve for the characterization analyses, and another portion of the samples was subjected to grinding and sieving to 45 µm particles for calcination and pozzolanic research.
In addition to the reagents used in the analyses, such as calcium hydroxide, free lime and others, this investigation used normal-strength Portland cement Type 1, with the characteristics and parameters indicated in the [54] standard, a cement without any type of addition, to produce mortar specimens composed of a standardized mixture of Portland cement (with calcined clay, in a ratio of 75:25, without interference from additions). Normal Brazilian sand [55] was used as the fine aggregate.

2.2. Methods

The experimental program of this research included processes such as the selection and collection of materials, characterization of the raw materials, thermal activation and technological tests to evaluate pozzolanic activity.
Firstly, the clays used in this work were catalogued according to their availability in the quarries and their use in the ceramics industry. In the Campos dos Goytacazes region, yellow, gray and reddish clays are generally found, as shown in Figure 2.
The samples were then dried naturally, fractionated and separated for calcination and characterization tests. For characterization, the samples were separately comminuted and subjected to sieving with an aperture of 400 µm and for calcination, they were crushed and passed through a sieve with an aperture of 45 µm. The heat treatment was carried out in an INTI digital muffle furnace with a ramp up to 1250 °C, with around 500 g of clay soil sample subjected to calcination at 600 °C at a ramp of 2°/min for 180 °C at its maximum level. The calcination temperature, 600 °C, was determined through preliminary tests, in which the samples were subjected to calcination at temperatures of 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C and subjected to pozzolanic analysis tests to determine the pozzolanic activity index by electrical conductivity [56]. These results are discussed in Section 3. Generally, the temperature of 600 °C is interesting, because the dihydroxylation of kaolinite occurs around 550 °C, so this was a viable temperature in our research. The physical characterization of the natural material included checking the particle size [57], moisture content [58], grain density, using a pycnometer [59], and plasticity limit [60,61] for each of the three samples.
Chemical analysis of the natural clay samples was carried out using a Shimadzu model EDS-700, Tokyo, Japan, energy-dispersive X-ray fluorescence (XRF) spectrometer coupled to a computer for data acquisition. This analysis led to the identification of the elements present in the samples (qualitative analysis) and the establishment of the proportion in which each element was present (quantitative analysis).
Qualitative mineral analysis was carried out on both the natural and calcined samples by X-ray diffraction (XRD) using a Seifert URD65, New York, NY, USA, diffractometer, and the scanning (2θ from 3° to 66°) was conducted in the form of a 0.02° (2θ) step for 3 s of accumulation time. The equipment was operated with an electron acceleration voltage of 40 kV (kilovolts) and an electric beam current of 35 mA (milliamps). The radiation used was the Kα line from a copper anode with a graphite monochromator.
The morphological analysis of the prepared calcined samples was carried out using scanning electron microscopy (SEM), using a Shimadzu model SSX-550 scanning electron microscope, Tokyo, Japan. Images at 100×, 500× and 1000× were obtained using secondary electrons. Sample preparation involved cleaning the metal support, with a diameter of approximately 20 mm, to remove impurities and avoid contaminating the sample. A carbon strip containing the synthesized material was applied to the surface of the metal support and then subjected to the metallization process.
Differential thermal analysis (DTA) and thermogravimetric analysis (TG) determined the thermal behavior of the calcined clays. The test was carried out on a Shimadzu DTG-60H, Tokyo, Japan, simultaneous ATG-ATD analyzer with sample masses ranging from 45 to 55 mg. The samples were tested in a nitrogen atmosphere at a heating rate of 10 °C/min from room temperature to 1000 °C.
The surface area of the clays was measured using the Blaine method, according to [62], using a permeabilimeter Solotest, São Paulo, Brazil, brand air tester. The Blaine method measures the resistance to air flow through a compacted surface of material particles. The powder sample was compacted in an air-permeable cylinder, in which the air permeabilimeter forced a flow of air through the compacted sample, while measuring the amount of time required for the specific amount of air specified by the device and material to pass through the sample. The time was related to the resistance offered by the sample to the air flow. The surface area was calculated using the Blaine equation, which takes into account the resistance to air flow, the density of the air, the viscosity of the air and the porosity of the sample.
Isothermal calorimetry was used to evaluate the hydration of the pastes produced with the clays calcined at 600 °C, since the samples calcined at this temperature showed the best pozzolanic reactivity. To carry out this test, a Calmetrix, New York, NY, USA, two-channel calorimeter, model I-CAL 2000, was used, containing approximately 50 g of each of the fresh clays prepared with cement and the pozzolanic material in a 3:1 ratio, 3 parts cement and 1 part material. The pastes were added to the containers and attached to the calorimeter channels 3 min after the water came into contact with the material. The samples were monitored for 72 h at a constant temperature of 25 °C.
The tests related to pozzolanic activity were carried out on samples of the material already calcined at 600 °C. The following tests were carried out: determination of the fixed calcium hydroxide content, also known as modified Chapelle, in accordance with [63], determination of the performance index with Portland cement at 28 days in accordance with [64], as shown in Figure 3, determination of pozzolanic activity by electrical conductivity, also known as the Luxàn method [56] and the R3 (rapid, relevant and reliable) test suggested by [65], in accordance with [66]. It should be noted that the tests related to the pozzolanicity index of supplementary cementitious materials were chosen according to the literature, using methods suggested in different studies, as pointed out by [51].

3. Results and Discussion

In order to make this investigation possible, the material was calcined at temperatures of 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C in order to assess the temperature at which the calcined material showed the most significant pozzolanic reactivity. Figure 4 shows the electrical conductivity of samples A1, A2 and A3 at the 10 different temperatures. The results show that calcination at 600 °C gives the material greater pozzolanic reactivity.
The Table 1 shows the Atterberg limits, which indicate that sample A2 had the best plasticity index. This may be related to its formation minerals, its hygroscopic humidity and the fact that samples with higher plasticity indices indicate greater ease of comminution and increased mechanical strength [67]. The densities of all the samples were similar to those in the literature. These results are crucial for this work since, according to [41,68] these materials tend to lose their plasticity when subjected to a high-temperature heat treatment, which involves temperatures related to the dihydroxylation of the clays, i.e., the removal of water from the compound.
In the granulometric analysis, shown in Table 2, it was possible to observe the percentages of each of the soil fractions present in the samples analyzed, as well as their classification according to the Unified Soil Classification System (USCS), based on grain properties and plasticity. This investigation revealed a higher incidence of clay fraction in sample A2, classifying it as a high-plasticity clay, while the other samples were classified as low-plasticity clays.
Table 3 shows the chemical compositions in terms of the oxide content. The compositions determined by X-ray fluorescence spectroscopy (XFR) showed the imminent chemical formation of the clays. The presence of oxides such as SiO2, Al2O3 and Fe2O3 in the samples is notable due to their resistance and reactivity characteristics. SiO2 is a key component that contributes to the strength and durability of clays, while in its calcined state, it tends to provide components for the formation of important compounds, such as CSH (calcium silicate hydrate), in the hydration of cement, and, above all, it presents greater structural stability [49] The Al2O3 content in the samples also tended to make greater contributions to the mechanical strength and pozzolanic reactivity of the clays. On the other hand, the iron oxide (Fe2O3) tended to provide different characteristics such as the coloring and thermal properties of the clays, as well as interference in plasticity. The samples also showed the presence of other oxides, such as K2O, SO3, TiO2, CaO and P2O5, in smaller percentages. High levels of K2O and P2O5 can improve the reactivity of calcined clays, while significant levels of CaO can contribute to the formation of hydraulic phases. The low content of alkaline oxides such as K2O is a particularity of kaolinitic clays, which naturally have a low percentage of flow minerals. The low content of alkaline earth oxides, such as CaO, for example, indicates the absence of carbonates. Other oxides were considered to be impurities or irrelevant, since their quantities in the sample were considered to be minimal [49,52,67,68,69].
With the results of the chemical characterization of the samples, it was possible to calculate the SiO2/Al2O3 ratio, an indicator that assesses, as a percentage, the amounts of quartz, inert materials, and other minerals. Consequently, it can have an influence on the formation of hydraulic phases during calcination and the subsequent reactivity of the clays in cement mixtures.
The SiO2/Al2O3 ratios of the different clay samples were calculated, with values of 1.16 for A1, 1.19 for 2 and 1.09 for A3. The SiO2/Al2O3 ratio for kaolinitic clays is close to 1.18 in terms of mass, given that the chemical formula of kaolinite (Al2Si2O5(OH)4), in which there are two units of SiO2 (60.08 g/mol) for each unit of Al2O3 (101.96 g/mol). Considering this, it is possible to point out that the SiO2/Al2O3 ratios for the samples ranged from 1.09 to 1.19, values which are close to the ratio calculated by mass for kaolinite. This indicates that the clays in this investigation had similar chemical characteristics to the clays in previous studies [53,70].
Kaolinitic clays are characterized by their high-temperature firing behavior. In the samples analyzed, shown in Figure 5, Figure 6 and Figure 7, low-intensity diffraction peaks were identified, indicating traces of montmorillonite, muscovite and beidelite. Montmorillonite is a highly plastic clay mineral with a strong tendency to rehydrate, which can lead to processing problems and variations in the consistency and mechanical properties of the material, making it difficult to use in applications requiring dimensional stability. The presence of muscovite and beidelite also contributes to the properties of these materials. However, their presence is relatively irrelevant when taking their quantity into account. Their presence tends to influence plasticity, cation exchange capacity and thermal behavior [49,68,71].
In addition to kaolinite, the mineral with the greatest quantity in all the samples, with the presence of well-defined, high-intensity peaks on the diffractogram, and quartz, gibbsite and anatase were also present, as well as micaceous minerals such as muscovite superimposed on illite, goethite and feldspars, confirmed by the chemical analysis of the samples.
In the case of the A1, A2 and A3 clays subjected to the 600° thermal process, it can be seen that the well-marked, high-intensity kaolinite peaks disappeared from the diffractogram. This was related to the formation of metakaolin, an amorphous compound formed by the dihydroxylation of the mineral.
Figure 8, Figure 9 and Figure 10 show the thermal analysis curves of the clay samples. It can be seen that the clays showed a total loss of mass, on average, of approximately 15%, with a temperature rise ramp from 0 to 1000 °C during the thermal procedure. At the initial heating temperatures, there was a loss of mass, which was related to the elimination of water and moisture. At temperatures close to 250 °C for samples A1 and A3, there were endothermic peaks related to the elimination of water from hydroxides, with a low percentage of mass loss. The greatest mass loss, around 8% on average for each sample, was associated with the dihydroxylation of kaolinite, with endothermic peaks around 500 °C ± 20. This process is related to the amorphization of the clay, leading to its transformation into the meta phase due to the heat treatment. This high loss of mass is considered typical of kaolinitic clays, since a loss of mass of between 5% and 10% is related to one of the main factors that give ceramics porosity after firing [15,46,67,72].
The specific surface area tests using the Blaine method revealed significant variations in the values obtained for the different calcined metakaolin samples. The specific surface area is a crucial parameter that directly influences the pozzolanic reactivity of calcined clays, since a greater surface area provides a greater availability of reactive sites for pozzolanic reactions. There was considerable variation between the specific surface area values of samples A1, A2 and A3, as shown in Table 4. Sample A2 had the highest specific surface area, of 15.616 cm2/g, followed by sample A3, with 12.230 cm2/g and, lastly, sample A1, with 11.760 cm2/g. This variation can be attributed mainly to the heterogeneity of the raw material, since the grinding, comminution and calcination conditions were the same for all the samples. Bibliographic studies indicate that commercial metakaolin has specific surface values of between 10.000 cm2/g and 12.000 cm2/g on average. Therefore, samples A1, A2 and A3 showed significantly higher specific surfaces, especially sample A2, with 15.616 cm2/g.
Scanning electron microscopy (SEM) is a morphological analysis technique that made it possible, in this work, to visualize the surfaces and structures of the samples in high resolution, showing topographical and morphological details that are not visible with conventional optical microscopy [72].
The SEM analysis of the clay samples revealed morphological characteristics that tend to have a direct influence on physical and mechanical properties. The images obtained provide a detailed overview of the particle structure, shape, size and distribution of the clay grains. In Figure 11, Figure 12 and Figure 13, it is possible to observe the morphology of each of the samples and assess their differences. Optical approximations of 100, 500 and 1500 times were carried out on all the samples, enabling a more in-depth analysis, which revealed a porous structure in the grains and an arrangement similar to that of metakaolin.
Figure 11 shows the microscopy of the calcined A1 sample, in which it is possible to see the denser and more graduated grain size of this material, as well as the morphology of the grains, which have more rounded shapes and fewer pores, with a less lamellar structure. In addition, when compared to the other samples, it is possible to see that its particles are considerably larger, even at the same grain size.
In Figure 12, the microscopy of sample A2 shows dense granulometry and a better distribution of grains. The morphology of the particles showed irregular shapes and a medium incidence of pores. It is possible to observe a less noticeable lamellar structure compared to the others, which is due to the grains not showing a pattern in terms of size and shape.
Figure 13 shows micrographs of the A3 clay sample, which has more compacted particles than the sample in a lamellar arrangement, where it is possible to observe the formation of pores and quartz particles, as shown in the results of the chemical characterization of both materials.
The results of the calorimetric analysis are shown in Figure 14. In the samples with calcined clay, A1, A2 and A3, there was a noticeable drop in the heat of hydration after the initial period of hydrate dissolution. At this stage, all the samples showed a high release of heat of hydration, corresponding to the dissolution of different types of ions and the initial hydration of C3S and C3A. It is also possible to observe the halo corresponding to the dormancy period, which, in the samples with calcined clay, lasted longer. Next, there was the acceleration period in all the samples analyzed, although it can be seen that in the samples containing calcined clay, the formation of CSH and the dissolution of C3S were much faster. This means that when added to the hydration system, together with Portland cement, calcined clay speeds up reactions without releasing more heat, resulting in higher CSH production and, consequently, better properties for this cement matrix.
The heat release of each of the samples can be compared, in Figure 15, to the heat accumulation values, for which the periods of up to approximately 8 h in the samples with clay were, to a small extent, greater than that of the reference proportion. Furthermore, immediately after this period, the exothermic heat of the REF increased significantly. It can therefore be concluded that the addition of this material to cement pastes reduces the release of heat, and this may be intrinsically linked to a lower incidence of cracks and a greater production of cement hydration products, which will give greater mechanical resistance, a lower incidence of pores and less pronounced pathologies.
The results shown in Table 5 refer to the different methods used to characterize the calcined clay samples and their influence on the mechanical properties of the mortars. The methods used include the pozzolanic activity test by calcium hydroxide fixation (modified Chapelle), electrical conductivity (Luxan), the test of the reactivity of supplementary cementitious materials by percentage of bound water (R3) and the determination of the performance index with Portland cement at 28 days (compressive strength of the mortars).
In the modified Chapelle pozzolan evaluation, which assesses a sample’s ability to absorb calcium hydroxide (Ca(OH)2) in a pozzolanic reaction, all the calcined samples showed higher values, with sample A2 showing the best result, with 498.96 g/mL, followed by A1, with a concentration of 480.48 g/mL and A3, with 413.95 g/mL, which are all higher than the reference values of 330.00 g/mL, according to [51]. The results therefore suggest that the calcined clay samples have a high capacity to react with Ca(OH)2, indicating the intrinsic pozzolanic reactivity of these materials.
During the Luxan analysis, which measures the conductivity of solutions in the presence of mobile ions due to the dissolution of substances, all the samples tested after the thermal calcination process showed significant values for conductivity, with A2, with 1.47 mS/cm and A1, with 1.32 mS/cm, showing the highest values, followed by A3, with 1.30 mS/cm. According to [56], materials with values above 0.4 mS/cm are considered to be pozzolanic materials. Although the values reported here are relatively high, this method is considered qualitative and not quantitative, indicating only the presence of the material’s reactivity.
The mortars’ compressive strength, a direct measure of a mechanical property, was also assessed by determining the performance index with Portland cement at 28 days. The reference sample (REF) had a compressive strength of 17.24 MPa. The mortar produced with sample A1 showed the highest resistance, reaching 32.66 MPa, almost double that of the reference, while the mortar made with clay sample A2 had a similar resistance to the reference, 17.30 MPa, and the mortar with calcined sample A3 showed a resistance of 30.95 MPa.
Bearing in mind that for a material to be considered pozzolanic, it is expected that the compressive strength values of the mortars should be at least 75% of the reference, it can be said that all the samples, when calcined at 600 °C, showed pozzolanic reactivity. Table 6 shows the percentages while taking into account the strength of the reference mortar, and it is possible to conclude that mortars A1 and A3 had the best performance, respectively, followed by mortar A2. It is also clear that all the calcined samples used to make the mortars have high pozzolanic potential for use as supplementary cementitious materials.
The results show that there was a positive correlation between the pozzolanic activity and the compressive strength of the mortar measured by the Chappell and Luxan tests. However, this relationship was not linear, as can be seen in the results for sample A2. The combination of higher reactivity (Chappell and Luxan) and higher compressive strength in sample A1 suggests an effective interaction between the metakaolin and the hydrated cement product, resulting in a denser and more resistant A matrix. On the other hand, sample A2 had a compressive strength comparable to that of the reference, despite showing greater reactivity and conductivity compared to the Chappelle test. This may have been due to factors such as the particle size distribution, the presence of impurities, adverse conditions or interactions with other concrete components. Sample A3, with good reactivity and high-resistance compressive strength, showed that although it was not as reactive as A1, it still had a significant impact on improving the mechanical properties of the concrete. In summary, pozzolanic activity and electrical conductivity tests are useful indicators of the reactivity of calcined clays, but the compressive strength of the mortar depends on a complex combination of factors. Sample A1 showed the most promising results, with high reactivity and a significant improvement in compressive strength, while sample A2 showed that high reactivity does not necessarily lead to an improvement in mechanical strength. A detailed analysis of factors such as mineral composition, grain morphology and calcination conditions provides a deeper understanding of the variations observed.
The Table 7 shows the compressive strength results in studies using calcined clay as a partial substitute for cement. These results corroborate those presented in this research, with some materials showing a significant increase in compressive strength even with the replacement of the material.
The R3 test, developed in [65], is used, especially for calcined clays, to establish a strict relationship with mechanical strength and calcium hydroxide fixation analyses. The results showed that, when comparing the compressive strengths of the mortars with the calcined A1, A2 and A3 clays, their corresponding percentages of bound water were found to interact with each other, and it was reaffirmed that a greater amount of bound water in the compound leads to greater resistance to pressure, indicating greater pozzolanic reactivity.

4. Conclusions

The investigation of the pozzolanic properties of clay samples calcined at a medium calcination temperature of 600 °C as a supplementary cementitious material revealed relevant information about the behavior of clay when used in conjunction with Portland cement:
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The preliminary thermal treatment of the material indicated that all the samples calcined at 600 °C showed greater pozzolanic reactivity.
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Replacing 25% of the Portland cement with calcined clays resulted in a reduction in CO2 emissions of up to 40%.
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The calcined clays showed high pozzolanic reactivity, as proven by their behavior in the isothermal calorimetry analysis.
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The thermal analysis showed a significant loss of mass in the three samples at temperatures close to 500 °C, indicating the dihydroxylation and amorphization of the clays.
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The release of heat in the first 48 h of hydration increased, indicating an acceleration in the cement’s hydration reaction.
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The compressive strengths of the cement pastes containing calcined clays increased by up to 15% compared to the pure Portland cement after 28 days of curing.
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The particle size analysis showed fine particles with a uniform distribution, resulting in a cohesive and heterogeneous material for use as a supplementary cementitious material after calcination at 600 °C.
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The chemical analysis by X-ray fluorescence (XRF) indicated high levels of silica (SiO2) and alumina (Al2O3), which are essential components for pozzolanic reactivity;
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The X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the formation of amorphous structures, which are crucial for pozzolanic activity, when the material is calcined above 600 °C.
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The X-ray diffraction characterized the clay as kaolinitic.
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In this study, we suggest the incorporation of the waste generated by the ceramics industry into subsequent cementitious matrices.

Author Contributions

Conceptualization, V.D.P., R.F.d.A., M.T.M. and A.R.G.d.A.; methodology, V.D.P. and R.F.d.A.; validation, A.R.G.d.A., J.A. and G.d.C.X.; formal analysis, M.T.M., J.A. and G.d.C.X.; investigation, V.D.P. and R.F.d.A.; data curation, M.T.M., A.R.G.d.A. and J.A.; writing—original draft preparation, V.D.P., R.F.d.A. and J.A.; writing—review and editing, A.R.G.d.A. and M.T.M.; project administration, M.T.M., A.R.G.d.A. and G.d.C.X.; funding acquisition, A.R.G.d.A. and J.A. All authors have read and agreed to the published version of the manuscript.

Funding

The participation of A.R.G.A. was sponsored by FAPERJ through the research fellowships proc. no: E-26/210.150/2019, E-26/211.194/2021, E-26/211.293/2021, and E-26/201.310/2021 and by CNPq through the research fellowship PQ2 307592/2021-9 and 401602/2023-0.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Clay storage—differences in terms of color: (a) gray clay and (b) red clay.
Figure 1. Clay storage—differences in terms of color: (a) gray clay and (b) red clay.
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Figure 2. Extraction of clay samples.
Figure 2. Extraction of clay samples.
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Figure 3. Compression test on mortars: (a) Test specimen being ruptured; (b) Test specimen in test position; (c) Test specimen in compression rupture test.
Figure 3. Compression test on mortars: (a) Test specimen being ruptured; (b) Test specimen in test position; (c) Test specimen in compression rupture test.
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Figure 4. Electrical conductivity results obtained for clays A1, A2 and A3.
Figure 4. Electrical conductivity results obtained for clays A1, A2 and A3.
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Figure 5. XRD sample A1—(a) natural and (b) calcined.
Figure 5. XRD sample A1—(a) natural and (b) calcined.
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Figure 6. XRD sample A2—(a) natural and (b) calcined.
Figure 6. XRD sample A2—(a) natural and (b) calcined.
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Figure 7. XRD sample A3—(a) natural and (b) calcined.
Figure 7. XRD sample A3—(a) natural and (b) calcined.
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Figure 8. TG and DTG of sample A1.
Figure 8. TG and DTG of sample A1.
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Figure 9. TG and DTG of sample A2.
Figure 9. TG and DTG of sample A2.
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Figure 10. TG and DTG of sample A3.
Figure 10. TG and DTG of sample A3.
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Figure 11. SEM A1 (a) 100×, (b) 500×, (c) 1500×.
Figure 11. SEM A1 (a) 100×, (b) 500×, (c) 1500×.
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Figure 12. SEM A2 (a) 100×, (b) 500×, (c) 1500×.
Figure 12. SEM A2 (a) 100×, (b) 500×, (c) 1500×.
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Figure 13. SEM A3 (a) 100×, (b) 500×, (c) 1500×.
Figure 13. SEM A3 (a) 100×, (b) 500×, (c) 1500×.
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Figure 14. Isothermal calorimetry curves at 22 °C.
Figure 14. Isothermal calorimetry curves at 22 °C.
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Figure 15. Accumulated heat curves.
Figure 15. Accumulated heat curves.
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Table 1. Sample limits, humidity and density.
Table 1. Sample limits, humidity and density.
SampleLiquidity Limit (LL)Plasticity Limit (LP)Plasticity Index (IP)Higroscopic Humidity (%)Density (g/cm3)
A141.0%26.9%14.1%4.202.58
A287.4%36.2%51.2%13.102.65
A348.8%28.2%20.3%7.002.68
Table 2. Grain sizes of the samples.
Table 2. Grain sizes of the samples.
SampleBoulder (%)Sand (%)Silt (%)Clay (%)Classification
ThickMediumThinThick MediumThin
A1000210173240CL
A20002552464CH
A300001333630CL
Table 3. Chemical analysis of the samples.
Table 3. Chemical analysis of the samples.
Chemical ElementsSample Clays (%)
A1A2A3
SiO248.1346.0438.04
Al2O341.5638.7135.02
Fe2O33.767.9915.71
K2O2.412.221.63
SO32.141.901.79
TiO21.721.351.59
CaO0.070.350.71
MnO0.050.070.00
ZrO20.030.040.00
ZnO0.020.000.03
SrO0.010.000.00
P2O50.001.150.00
V2O50.000.120.08
Ir2O30.000.060.00
Table 4. Specific surface areas of calcined samples.
Table 4. Specific surface areas of calcined samples.
SamplesBlaine Specific Surface (cm2/g)
A111.760
A215.616
A312.230
Table 5. Summary of Pozzolanic reactivity analysis results.
Table 5. Summary of Pozzolanic reactivity analysis results.
SampleChapelle (mg/g)Luxan (mS/cm)Strength (Mpa)R3 (%)
REF330.000.4017.24
A1480.481.3232.663.23
A2498.961.4717.302.13
A3413.951.3030.952.47
Table 6. Percentages of compressive strength considering the reference.
Table 6. Percentages of compressive strength considering the reference.
Reference Strength (MPa)SamplesMedium Strength (MPa) Pozzolanic Activity Index (AI)
17.24A132.66189.4%
A217.30100.3%
A330.95179.6%
Table 7. Percentages of mortar compressive strength from related research.
Table 7. Percentages of mortar compressive strength from related research.
Research Percentage of Strength in Relation to the Reference
[73]129.0%
[74]83.0%
[75]76.0%
[76]62.0%
[77]80.0%
[78]90.4%
[50]75.0%
[79]98.0%
[80]106.0%
[81]101.5%
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MDPI and ACS Style

Pinheiro, V.D.; Abreu, R.F.d.; Alexandre, J.; Xavier, G.d.C.; Marvila, M.T.; de Azevedo, A.R.G. Pozzolanic Potential of Calcined Clays at Medium Temperature as Supplementary Cementitious Material. Sustainability 2024, 16, 7508. https://doi.org/10.3390/su16177508

AMA Style

Pinheiro VD, Abreu RFd, Alexandre J, Xavier GdC, Marvila MT, de Azevedo ARG. Pozzolanic Potential of Calcined Clays at Medium Temperature as Supplementary Cementitious Material. Sustainability. 2024; 16(17):7508. https://doi.org/10.3390/su16177508

Chicago/Turabian Style

Pinheiro, Valber Domingos, Ramon Fernandes de Abreu, Jonas Alexandre, Gustavo de Castro Xavier, Markssuel Teixeira Marvila, and Afonso Rangel Garcez de Azevedo. 2024. "Pozzolanic Potential of Calcined Clays at Medium Temperature as Supplementary Cementitious Material" Sustainability 16, no. 17: 7508. https://doi.org/10.3390/su16177508

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