ASSESSMENT OF THE POZZOLANIC REACTIVITY OF METAKAOLIN AND GROUND AERATED CONCRETE WASTE FOR SUSTAINABLE CEMENTITIOUS MATERIALS

УДК 666.972.16:691.311

Abstract

This study investigates the pozzolanic reactivity of metakaolin and ground aerated concrete waste as potential supplementary cementitious materials for sustainable construction applications. The increasing demand for environmentally friendly building materials and the necessity to reduce cement consumption have encouraged the utilization of alternative mineral additives derived from industrial and construction waste. The primary objective of this research is to evaluate and compare the pozzolanic performance of thermally activated metakaolin and recycled aerated concrete waste. Experimental investigations included the characterization of raw materials and the determination of their pozzolanic activity using rapid assessment methods. Metakaolin samples were produced by thermal activation of kaolin under different temperature and time regimes, while aerated concrete waste was processed through grinding to obtain a fine mineral powder. The results demonstrated that metakaolin exhibited significantly higher pozzolanic reactivity due to the formation of highly reactive amorphous aluminosilicate phases. Ground aerated concrete waste also showed noticeable pozzolanic activity, attributed to the presence of amorphous silica compounds and hydration products. The findings indicate that both materials can be effectively utilized as supplementary cementitious materials, contributing to reduced cement consumption, enhanced resource efficiency, recycling of construction waste, and the production of more sustainable and environmentally responsible cement-based composites.

Аннотация

В данной статье исследуется пуццолановая активность метакаолина и измельчённых отходов газобетона в качестве перспективных минеральных добавок для устойчивых цементных композиционных материалов. Растущая потребность в экологически безопасных строительных материалах и необходимость снижения расхода цемента обусловливают актуальность использования альтернативных минеральных компонентов, получаемых из промышленных и строительных отходов. Основной целью исследования является оценка и сравнительный анализ пуццолановой активности термически активированного метакаолина и переработанных отходов газобетона. В ходе экспериментальных исследований были изучены физико-химические свойства исходных материалов и определена их пуццолановая активность с использованием экспресс-методов оценки. Образцы метакаолина были получены путём термической активации каолина при различных температурно-временных режимах, тогда как отходы газобетона подвергались измельчению до тонкодисперсного состояния. Полученные результаты показали, что метакаолин обладает значительно более высокой пуццолановой активностью благодаря образованию высокореакционноспособных аморфных алюмосиликатных фаз. Измельчённые отходы газобетона также продемонстрировали заметную пуццолановую активность, что обусловлено присутствием аморфного кремнезёма и продуктов гидратации. Результаты исследования свидетельствуют о возможности эффективного использования обоих материалов в качестве дополнительных цементирующих компонентов, способствующих снижению расхода цемента, повышению эффективности использования ресурсов, переработке строительных отходов и производству более экологичных и устойчивых цементных композитов.

Keywords: metakaolin, aerated concrete waste, pozzolanic activity, mineral additive, cementitious composites, amorphous silica, sustainable construction, secondary resources.

Ключевые слова: метакаолин, отходы газобетона, пуццолановая активность, минеральная добавка, цементные композиты, аморфный диоксид кремния, устойчивое строительство, вторичные ресурсы.

Introduction

The construction industry is one of the largest consumers of natural resources and a major contributor to global carbon dioxide (CO₂) emissions. In particular, the production of Portland cement accounts for approximately 7–8% of total anthropogenic CO₂ emissions worldwide, making it a significant environmental concern. Consequently, reducing cement consumption and partially replacing it with supplementary cementitious materials (SCMs) has become one of the key strategies in the development of sustainable concrete technologies [1, 2]. Such an approach not only decreases the environmental footprint of construction materials but also contributes to the conservation of natural resources and energy.

Pozzolanic materials react with calcium hydroxide generated during cement hydration, producing additional calcium silicate hydrate (C–S–H) and calcium aluminate hydrate compounds. These secondary hydration products enhance the microstructural development of cementitious composites by refining the pore structure, reducing porosity, and improving both mechanical and durability-related properties [4, 5]. As a result, considerable research efforts have been devoted to the utilization of natural and artificial pozzolans as sustainable alternatives in cement-based materials.

Among various pozzolanic materials, metakaolin has emerged as one of the most promising highly reactive supplementary cementitious materials. Metakaolin is an amorphous aluminosilicate obtained through the thermal activation of kaolinitic clays at temperatures ranging from 600 to 800 °C. Owing to its high chemical reactivity and large specific surface area, metakaolin effectively consumes calcium hydroxide and promotes the formation of additional C–S–H gel within cementitious systems [6, 7]. Previous studies have demonstrated that the incorporation of metakaolin significantly enhances the compressive strength of concrete, improves its microstructural characteristics, and contributes to a denser and more homogeneous cement matrix [8, 9]. Furthermore, metakaolin has been reported to reduce water permeability, increase resistance to sulfate and chloride attack, and improve the long-term durability of concrete structures exposed to aggressive environmental conditions [10].

In parallel with the development of advanced pozzolanic materials, the recycling and valorization of construction and demolition waste have become essential components of sustainable construction practices [11]. Significant quantities of aerated concrete waste are generated during the manufacturing of autoclaved aerated concrete products and the demolition or renovation of existing buildings. Due to the presence of silica-rich phases, calcium-containing compounds, and partially amorphous constituents, aerated concrete waste exhibits considerable potential for use as a supplementary cementitious material [12, 13]. The effective utilization of such waste materials not only mitigates environmental problems associated with landfill disposal but also promotes the circular economy by transforming industrial and construction waste into valuable resources for cementitious composites.

Several researchers have reported that ground aerated concrete waste exhibits pozzolanic activity in cementitious systems and can contribute to the improvement of certain properties of concrete [14, 15]. However, the pozzolanic reactivity of aerated concrete waste has not yet been sufficiently investigated in comparison with highly reactive supplementary cementitious materials such as metakaolin. In particular, the evaluation of the pozzolanic performance of these materials under identical experimental conditions and the comparative analysis of the obtained results are of considerable scientific and practical significance.

In this context, the primary objective of the present study is to experimentally assess the pozzolanic reactivity of metakaolin and ground aerated concrete waste, to comparatively evaluate their performance, and to determine their potential applicability as supplementary cementitious materials in sustainable cement-based composites. The findings of this research are expected to provide a scientific basis for the efficient utilization of industrial and construction waste materials while promoting the development of environmentally friendly and resource-efficient cementitious systems.

Materials and Methods

The materials used in this study were selected from locally available resources within the Republic of Uzbekistan. In particular, aerated concrete waste generated during the production of autoclaved aerated concrete (AAC) blocks at ARTON LLC, located in the Tashkent region, was utilized as a secondary mineral material. In addition, kaolin obtained from the Angren district of the Tashkent region was investigated as a precursor for the production of metakaolin.

Pozzolanic reactivity characterizes the ability of a mineral additive to react with calcium hydroxide (Ca(OH)₂) released during cement hydration. Reactive silica (SiO₂) and alumina (Al₂O₃) contained in pozzolanic materials interact with calcium hydroxide to form additional calcium silicate hydrate (C–S–H) and calcium aluminate hydrate phases. The formation of these hydration products contributes to the densification of the cement matrix, resulting in enhanced strength, durability, and long-term performance of cementitious composites. Various physical, chemical, and mechanical methods are commonly employed to evaluate the pozzolanic reactivity of supplementary cementitious materials.

Strength Activity Index(SAI)

The Strength Activity Index (SAI) is one of the most widely applied practical methods for assessing pozzolanic performance. In this method, a specified portion of Portland cement is replaced with the investigated mineral additive, and the resulting specimens are compared with a control mixture. After curing periods of 7, 28, and 90 days, the compressive strength of the specimens is determined [5, 6].

Generally, materials exhibiting an activity index greater than 75% are classified as effective pozzolanic materials.

Frattini Test

The Frattini test, standardized in EN 196-5, is used to evaluate the calcium hydroxide consumption capacity of mineral additives. A cement–additive blend is prepared and stored under controlled conditions for a specified period. Subsequently, the concentrations of Ca²⁺ and OH⁻ ions in the solution are measured [7].

If the measured calcium ion concentration lies below the calcium hydroxide saturation curve, the material is considered pozzolanically active. The Frattini method provides a direct assessment of the pozzolanic reaction.

Chapelle Test

The Chapelle test is based on determining the capacity of a material to fix calcium hydroxide. During the test, a known quantity of the mineral additive is reacted with an excess amount of CaO or Ca(OH)₂ at elevated temperature. The amount of unreacted calcium is then determined through titration [16].

The results are typically expressed as milligrams of Ca(OH)₂ fixed per gram of material (mg/g). Materials exhibiting a Chapelle activity greater than 700 mg Ca(OH)₂/g are generally classified as highly reactive pozzolans.

Electrical Conductivity Method

This method is based on monitoring changes in the electrical conductivity of a calcium hydroxide solution. When a pozzolanic material is introduced into the solution, Ca²⁺ ions participate in the pozzolanic reaction, resulting in a decrease in electrical conductivity. The rate of conductivity reduction provides an indication of the material’s pozzolanic reactivity [17].

Due to its simplicity and rapid execution, this method is widely used for preliminary screening of potential pozzolanic materials.

X-Ray Diffraction (XRD) Analysis

X-ray diffraction analysis enables the identification of hydration products formed during the pozzolanic reaction. The incorporation of a reactive pozzolan typically results in a reduction in the intensity of calcium hydroxide peaks, accompanied by the formation of additional C–S–H and C–A–S–H phases [11].

Thermal Analysis (TG–DTA)

Thermogravimetric analysis (TG) and differential thermal analysis (DTA) are employed to quantify the reduction in calcium hydroxide content within cementitious systems. As the pozzolanic reaction progresses, calcium hydroxide is consumed, leading to a decrease in the mass loss associated with its thermal decomposition [11, 12].

Chemical Composition Assessment

According to ASTM C618, the combined content of SiO₂, Al₂O₃, and Fe₂O₃ is an important criterion for evaluating pozzolanic materials [5].

Although compliance with this requirement indicates potential pozzolanic characteristics, chemical composition alone cannot fully describe the actual reactivity of a material.

Experimental Procedure

In the present study, the pozzolanic reactivity of the selected materials was determined using a rapid assessment method. Alongside conventional testing techniques, rapid evaluation methods are increasingly employed to estimate the ability of mineral additives to react with calcium hydroxide within a relatively short period. These methods provide an efficient approach for the preliminary screening and comparative assessment of new pozzolanic materials, thereby facilitating the identification of promising supplementary cementitious materials for sustainable construction applications [1, 2].

Results and Discussion

During the course of the study, the rapid assessment (express) method was employed to determine the pozzolanic reactivity of metakaolin produced through the thermal activation of kaolin at elevated temperatures. Initially, the collected bulk kaolin samples were crushed into smaller fragments with dimensions of approximately 20 ± 5 mm, and a total of 16 specimens were prepared for the experimental program.

The prepared samples were subsequently subjected to thermal treatment in a muffle furnace, where kaolinite underwent dehydroxylation and structural transformation into metakaolin. This thermal activation process is essential for enhancing the pozzolanic reactivity of the material, as it converts the crystalline kaolinite structure into a highly reactive amorphous aluminosilicate phase. Following calcination, the specimens were conditioned and prepared for pozzolanic activity evaluation using the express testing procedure.

Figure 1 presents the thermal treatment of the samples in the muffle furnace and the preparation stages of the specimens for the rapid pozzolanic reactivity test.

Figure 1. Sample preparation procedure: (a, b) kaolin specimens prepared for metakaolin production; (c) pozzolanic activity determination using the rapid assessment (express) method.

In the present study, metakaolin specimens were produced through the thermal activation of kaolin under various temperature and time regimes. The raw kaolin samples were calcined at temperatures of 600, 700, 800, and 900 °C for durations of 1, 2, 3, and 4 hours, respectively. During the thermal treatment process, the crystalline structure of kaolinite was progressively disrupted, resulting in the formation of an amorphous aluminosilicate phase with high pozzolanic reactivity, commonly known as metakaolin. The transformation of kaolinite into metakaolin significantly enhanced the material’s chemical activity, making it a promising supplementary cementitious material for sustainable cement-based composites.

Results and Discussion

It is well established that the dehydroxylation of kaolinite and its transformation into metakaolin generally occur within the temperature range of 550–800 °C. As the calcination temperature increases, the crystalline structure of kaolinite progressively collapses, leading to an increase in pozzolanic reactivity. However, excessively high temperatures or prolonged thermal treatment may induce recrystallization of metakaolin, thereby reducing its pozzolanic performance. Consequently, evaluating the influence of different temperature–time regimes on metakaolin reactivity constituted one of the primary objectives of this study.

The experimental results demonstrated that both the calcination temperature and duration exert a significant influence on the pozzolanic reactivity of metakaolin. This behavior is primarily associated with the structural breakdown of kaolinite, dehydroxylation processes, and the formation of highly reactive amorphous aluminosilicate phases.

At 600 °C, which represents the initial stage of kaolinite transformation into metakaolin, the specimen calcined for 1 h exhibited relatively low pozzolanic activity (32%), indicating incomplete dehydroxylation of the kaolinite structure (Figure 2a). Extending the calcination period to 2 h increased the activity to 53%, reflecting substantial disruption of the crystal lattice and an increase in the amorphous phase content. A decrease to 41% was observed after 3 h, which may be attributed to localized structural rearrangements within the material. Nevertheless, after 4 h, the activity increased again to 54%, suggesting further activation of the kaolinite. Overall, the results indicate that the transformation process remained incomplete at 600 °C.

A temperature of 700 °C is generally considered favorable for the conversion of kaolinite into reactive amorphous metakaolin. However, the measured pozzolanic activity ranged only between 35% and 45%, with no pronounced increase observed (Figure 2b). This behavior may be attributed to variations in the mineralogical composition of the raw material and differences in the degree of amorphization among the specimens. Although all samples exhibited pozzolanic activity at this temperature, none reached the highly reactive category, suggesting that higher calcination temperatures may be required for complete activation.

A substantial increase in pozzolanic activity was observed for specimens calcined at 800 °C. In particular, the sample treated for 4 h achieved a pozzolanic activity of 64%, indicating extensive destruction of the kaolinite crystal structure and the formation of highly reactive amorphous aluminosilicate phases. Samples calcined for 1–3 h exhibited activity values ranging from 39% to 48%, confirming that the activation process was still progressing (Figure 2c). The prolonged calcination period of 4 h provided conditions favorable for maximum activation.

The highest pozzolanic activity was recorded for the specimen calcined at 900 °C for 2 h, reaching 68%. This result indicates that the material attained its highest degree of amorphization and developed a substantial amount of highly reactive aluminosilicate phases. However, when the calcination duration was extended to 3 h and 4 h, the activity decreased to 46% and 44%, respectively (Figure 2d). This reduction can be attributed to the partial recrystallization of metakaolin and the formation of less reactive spinel-type phases. Similar decreases in pozzolanic reactivity at elevated temperatures and prolonged calcination times have been widely reported in the literature.

The pozzolanic activity of the cement-based aerated concrete waste was determined to be 48.6%. This value is comparable to those obtained for several metakaolin specimens and may be attributed to the presence of amorphous silica phases and hydration products within the aerated concrete matrix. The results indicate that cement-based aerated concrete waste has potential for utilization as a secondary supplementary cementitious material.

In contrast, the lime-based aerated concrete waste exhibited a pozzolanic activity of only 15.3%, placing it within the low-reactivity category. This relatively low value may be associated with the predominance of crystalline calcium silicate hydrate phases formed during autoclave curing and the comparatively low content of amorphous phases. Therefore, the direct use of lime-based aerated concrete waste as a pozzolanic material may be limited unless additional activation treatments are applied.

Based on the obtained results, calcination at 900 °C for 2 h was identified as the optimum thermal activation regime for the investigated kaolin. Under these conditions, the produced metakaolin exhibited the highest pozzolanic reactivity and can therefore be considered a promising supplementary cementitious material for use in advanced and sustainable cement-based composites.

Conclusion

In this study, the pozzolanic reactivity of metakaolin produced through the thermal activation of kaolin under various temperature and time regimes, as well as that of cement-based and lime-based aerated concrete waste, was evaluated using the rapid assessment (express) method.

The experimental results confirmed that the thermal activation regime has a significant influence on the pozzolanic reactivity of metakaolin. Calcination of kaolin at temperatures ranging from 600 to 900 °C for durations of 1–4 h resulted in pozzolanic activity values varying between 32% and 68%. The highest activity (68%) was achieved by the specimen calcined at 900 °C for 2 h, indicating that this material can be classified as a highly reactive pozzolan.

A relatively high pozzolanic activity (64%) was also observed for the metakaolin sample calcined at 800 °C for 4 h. The results demonstrate that the thermal activation process enhances the reactivity of kaolinite through its transformation into an amorphous aluminosilicate phase. However, extending the calcination duration to 3–4 h at 900 °C led to a reduction in pozzolanic activity. This decrease is attributed to the partial recrystallization of metakaolin and the formation of less reactive mineral phases.

The investigation of aerated concrete waste revealed that the cement-based aerated concrete waste exhibited a pozzolanic activity of 48.6%, indicating its potential as an effective supplementary cementitious material. This finding highlights the feasibility of utilizing aerated concrete waste as a secondary raw material in cementitious composites. In contrast, the lime-based aerated concrete waste demonstrated a considerably lower pozzolanic activity of 15.3%, placing it within the low-reactivity category.

Based on the obtained results, thermal activation of kaolin at 900 °C for 2 h is recommended as the optimum calcination regime for metakaolin production. Metakaolin produced under these conditions exhibited the highest pozzolanic reactivity and can serve as a promising mineral additive for reducing cement consumption, improving the physical and mechanical properties of concrete, and promoting the development of environmentally sustainable construction materials.

Furthermore, the moderate pozzolanic reactivity exhibited by cement-based aerated concrete waste demonstrates its potential for recycling and reuse in the construction industry. The incorporation of such waste materials into cement-based products can contribute to resource conservation, reduction of construction and demolition waste, and mitigation of the environmental impacts associated with disposal practices, thereby supporting the principles of sustainable construction and circular economy.

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