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Article

Performance Evaluation of Low-Grade Clay Minerals in LC3-Based Cementitious Composites

by
Nosheen Blouch
1,
Syed Noman Hussain Kazmi
2,
Nijah Akram
3,
Muhammad Junaid Saleem
4,5,
Imran Ahmad Khan
5,*,
Kashif Javed
5,
Sajjad Ahmad
2 and
Asfandyar Khan
5,6
1
Department of Building and Architectural Engineering, Faculty of Engineering and Technology, Bahauddin Zakariya University, Multan 60800, Pakistan
2
Department of Civil Engineering, Faculty of Engineering and Technology, Bahauddin Zakariya University, Multan 60800, Pakistan
3
Department of Architectural Engineering Technology, Punjab Tianjin University of Technology, Lahore 53720, Pakistan
4
Department of Textile and Fashion, Punjab Tianjin University of Technology, Lahore 54000, Pakistan
5
Department of Textile and Apparel Science, University of Management and Technology, Lahore 54770, Pakistan
6
Department of Textile Engineering, Daffodil International University (DIU), Daffodil Smart City, Birulia, Dhaka 1216, Bangladesh
*
Author to whom correspondence should be addressed.
Solids 2025, 6(3), 35; https://doi.org/10.3390/solids6030035
Submission received: 30 May 2025 / Revised: 4 July 2025 / Accepted: 7 July 2025 / Published: 10 July 2025
(This article belongs to the Topic Novel Cementitious Materials)
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Abstract

The cements industry is increasingly under pressure to reduce carbon emissions while maintaining performance standards. Limestone calcined clay cement (LC3) presents a promising low-carbon alternative; however, its performance depends significantly on the type and reactivity of clay used. This study investigates the effect of three common low-grade clay minerals—kaolinite, montmorillonite, and illite—on the behavior of LC3 blends. The clays were thermally activated and characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray fluorescence spectroscopy (XRF), and Blaine air permeability testing to evaluate their mineralogical composition, thermal behavior, chemical content, and fineness. Pozzolanic reactivity was assessed using the modified Chapelle test. Microstructural development was examined through scanning electron microscopy (SEM) of the hydrated specimens at 28 days. The results confirmed a strong correlation between clay reactivity and hydration performance. Kaolinite showed the highest reactivity and fineness, contributing to a dense microstructure with reduced portlandite and enhanced formation of calcium silicate hydrate. Montmorillonite demonstrated comparable strength and favorable hydration characteristics, while illite, though less reactive initially, showed acceptable long-term behavior. Although kaolinite delivered the best overall performance, its limited availability and higher cost suggest that montmorillonite and illite represent viable and cost-effective alternatives, particularly in regions where kaolinite is scarce. This study highlights the suitability of regionally available, low-grade clays for use in LC3 systems, supporting sustainable and economically viable cement production.

1. Introduction

The cement industry is confronted with sustainability issues, which have led to the development of various technologies aimed at reducing CO2 emissions. LC3 has emerged as a promising ternary blended composite and has gained significant attention in recent years. Its comparable mechanical properties and favorable environmental impact make it a sustainable alternative to OPC. LC3 has gained widespread importance as it uses easily sourced materials that require little processing and have a comparatively high rate of substitution for cement clinker. This results in a composite cement with lesser embodied energy and reduced carbon dioxide emissions, analogized to Portland cement.
The usual composition of LC3 is 30%, 15%, 50%, and 5% of calcined clay, limestone, cement, and gypsum, respectively. The calcined kaolinite has been extensively studied in LC3 systems and has demonstrated excellent pozzolanic reactivity [1,2,3]. Its beneficial use in LC3 is well documented; for instance, replacing 50% of cement mortar with metakaolin and limestone significantly enhances the mechanical performance and durability properties of the cement mortar [4,5,6]. Recent focus has shifted to evaluating the viability of alternative clay minerals—specifically illite and montmorillonite—as prospective pozzolanic materials in LC3 systems. Despite their natural abundance and potential for local sourcing, these clays have been less investigated. However, the pozzolanic characteristics of calcined kaolinites, compared to other clay minerals, for example, illite and montmorillonite, in LC3 systems have not been frequently studied and remain inadequately understood. Due to their use in producing paint, paper, and ceramic goods, kaolinite clays are highly competitive with other industries. The development of LC3 might be less expensive and more accessible if other commonly available clay minerals, like montmorillonite and illite, were used.
Clays are hydrous aluminum silicate-based minerals with layered sheet structures and particles that are usually smaller than 2 μm [7]. These minerals are essential to fine-grained metamorphic rocks like slate and phyllite and sedimentary rocks like shale, mudstone, and siltstone. Typically, they form through weathering and hydrothermal reactions at low temperatures. The three main clay minerals are kaolinite [Al2Si2O5(OH)4], montmorillonite [(Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O], and illite [(Al2)(Si4−xAlx)O10(OH)2·K1−x] [8] (Figure 1). These minerals have a layered structure consisting of two distinct types. The layered structure of these minerals is made up of two different kinds of layers. In the first layer, four oxygen atoms form bonds with the Si4+ cation in silicon tetrahedra. On the other hand, the second layer consists of an octahedral-shaped aluminum hydroxide layer with an Al3+ cation at its center [9].
In some clay minerals, magnesium (Mg2+) can substitute for aluminum (Al3+) in the octahedral layer [10]. Kaolinite is categorized as a 1:1 clay mineral, with H-bonds linking the OH− groups of one layer to the O2− groups of another, resulting in a stable structure that restricts cation intercalation [9]. Montmorillonite and illite, on the other hand, are 2:1-type clay minerals where the oxygen anions (O2−) from one layer are positioned next to those from an adjacent layer [11].
Heating raw clays produces metaclays, which are the source of calcined clays’ reactivity. As pozzolanic substances, these metaclays react with portlandite to produce (1) ettringite, (2) calcium silicate hydrate (C-S-H), and (3) AFm phases. The addition of limestone to Portland cement causes the calcite to react with the clinker’s C3A, forming both hemi and monocarboaluminate phases. In LC3, the aluminate from metaclays further reacts with calcite, facilitating the development of carboaluminate phases.
Most previous studies on metakaolin’s impact on hydration have focused on high- to medium-grade kaolinitic clays [12,13,14,15]. However, these materials are not economically viable, particularly in regions like the Southern Hemisphere, where they can be up to three times more expensive than Portland cement [16]. These cost disparities emphasize the economic advantage of using illite and montmorillonite in LC3 formulations, especially in regions with constrained construction budgets. Although they have lower alumina content, their structural and chemical properties indicate they could still offer satisfactory levels of pozzolanic reactivity if they are calcined correctly. There is limited information on calcined common clays, such as low-grade illite and montmorillonite, which are abundant and have significant potential as clinker substitutes [17,18,19]. Alongside chemical and strength performance, the influence of supplementary cementitious materials on microstructure and crack propagation is increasingly being explored. The behavior of fractures in cementitious composites is intricately connected to their hydration products, the refinement of pores, and the characteristics of interfacial transition zones. Supplementary cementitious materials like calcined clays have the capacity to modify these microstructural characteristics, which may enhance durability and resistance to cracking. Proposed advanced numerical models simulate this behavior, demonstrating the impact of reactive mineral additions on stress redistribution and crack development under load [20,21]. Integrating these elements offers a more comprehensive understanding of the structural effects associated with the use of alternative clays in LC3 formulations.
This research investigates the influence of calcined kaolinite, illite, and montmorillonite on the hydration mechanisms and strength development of LC3 blends. The activated clays were tested for reactivity using the modified Chapelle test. The hydration mechanisms were analyzed using XRD and TGA, while strength development was evaluated through the heat of hydration, setting time, consistency, and compressive strength evaluation.

2. Materials and Methods

2.1. Materials and Activation

The current work explored the potential use of three low-grade clays, kaolinite (Ka), illite (IL), and montmorillonite (Mn) for LC3 production from the different sources of Pakistan. Ka clays were explored from Pai Khel, district Mianwali (Punjab). Mn clay was collected from the Hub, District Lasbela, (Baluchistan), and the IL clay was collected in Jhampir (Sindh), Pakistan. Ordinary Portland Cement (OPC) and limestone used were collected from the Maple Leaf Cement Factory Pvt. Limited, Pakistan.
Kaolinite (Al2Si2O5(OH)4) consists of layers formed by two sheets linked by hydrogen bonds, resulting in a stable non-swelling structure [8]. Illite and montmorillonite are 2:1 clay minerals, having an octahedral sheet of alumina packed between two silicon tetrahedral sheets [8]. However, the montmorillonite has weaker bonds that are easy to break, while the illite layers are firmly bonded by potassium and require a high activation temperature [22]. Therefore, the raw clays were heat treated at two different temperatures. Ka and Mn were calcined at 750 °C, while IL required 850 °C due to its thermal stability. Each 1 kg batch was heated in an alumina crucible in a laboratory furnace for one hour and then rapidly cooled. The calcination temperatures were selected based on the dehydroxylation behavior observed from thermal analysis.

2.2. Characterization Techniques

The mineralogical and chemical compositions of the raw and calcined clays were evaluated using several XRD, XRF, PSD, and TGA. International standard protocols were followed during the testing procedures. X-ray fluorescence spectroscopy was conducted on powdered samples employing a PANalytical Smart Zetium X-ray fluorescence spectrometer. X-ray diffraction (XRD) analysis was carried out using a BTX III Benchtop XRD Analyzer. The samples were ground into a powder and passed through a 200-mesh screen after curing for seven and twenty-eight days. The analysis used Cu Kα radiation at 30 mA and 30 kV. The scanning angle lies between 5° and 50° 2θ, with a step size of 0.025° and a scanning time of 1.58 s per step. TG/DTA analysis was performed using a LINSEIS STA PT 1600 (TG-DSC), heating the samples at a 10 °C/min rate from 50 °C to 950 °C using N2 as a purge gas. Additionally, a correlation matrix was developed to link the clay’s reactivity with the efficiency of LC3.
Blaine fineness assessments were performed in accordance with ASTM C204 standards via a Blaine air permeability device. Samples were subjected to oven drying, grinding, and sieving through a 75 µm (No. 200) mesh prior to testing to ensure homogeneous packing and eliminate agglomerates. The fineness values were documented in cm2/g, indicating the particular surface area associated with particle arrangement and possible reactivity in cementitious systems.
Blaine’s fineness assessments revealed substantial disparities between raw and calcined clays. Upon calcination, all clays demonstrated a significant increase in specific surface area resulting from structural degradation and dehydroxylation [23]. Calcined kaolinite exhibited the highest fineness among the clays, signifying enhanced grindability and a morphology favorable for achieving a large surface area [24]. Montmorillonite demonstrated a significant enhancement in fineness following calcination, nearly matching that of kaolinite. Illite exhibited the least fineness among the calcined clays, indicative of its denser structure and reduced grinding efficiency [25]. Compared to OPC, all calcined clays exhibited significantly higher surface areas, indicating an improved potential for pozzolanic reaction. Limestone demonstrated the greatest fineness due to its soft and easily grindable characteristics. The variations in fineness are anticipated to impact the early dissolution behavior and, in turn, influence the hydration kinetics of LC3 systems [26]. The Blaine fineness analysis of clay samples, cement, and limestone are tabulated in Table 1. Figure 2 shows the particle size distribution (PSD) determined using a Malvern Mastersizer 2000 laser diffraction analysis.
The XRD analysis of Ka indicated the presence in kaolinite minerals, with a prominent peak at 11.3° 2θ (Figure 3) [27]. The IL sample exhibited diffraction peaks corresponding to illite, along with minor quantities of quartz, kaolinite, and calcite [28]. The Mn sample showed sharp peaks of montmorillonite near 6° 2θ [29], accompanied by detectable peaks of kaolinite, quartz, and traces of illite and calcite. After thermal activation, the XRD patterns of the calcined clays were analyzed to assess the effectiveness of the calcination process. Kaolinite demonstrated a significant reduction in crystallinity after being calcined at 750 °C, as indicated by the absence of kaolinite peaks, which signifies its transformation into an amorphous phase. Calcined Mn exhibited the removal of montmorillonite peaks and the appearance of weak peaks for albite and calcite [30]. In the calcined IL sample, minor illite peaks remained, indicating partial transformation. Quartz peaks remained stable in all samples, suggesting that quartz was unaffected by calcination.
The thermogram and derivative thermal analysis are revealed in Figure 4a,b. The thermograms revealed variations in mass loss patterns among the clays. Kaolinite (Ka) exhibited minimal mass loss below 300 °C, while illite (IL) showed slight mass loss, and montmorillonite (Mn) displayed the most significant mass reduction in this range [31]. This region corresponds to the removal of adsorbed water. In the 300–600 °C range, all samples showed a second mass loss peak attributed to the dehydroxylation of structural hydroxyl groups. The dehydroxylation of Mn and IL extended up to approximately 750 °C and 800 °C, respectively [32,33]. The DTG curves indicated distinct differences in thermal decomposition behavior, suggesting variations in mineral composition and structural stability among the clays. These thermal trends were consistent with XRD observations regarding phase transformation upon calcination [32]. The XRD and DTG results were consistent with each other.
The activated clays must demonstrate sufficient pozzolanic reactivity to ensure the rigorous integrity of the binder. The modified Chapelle test was employed to determine the reactivity of the calcined clays. This test measures the reactivity of clay by heating a mixture of calcium oxide and calcined clay under reflux conditions. The process involves mixing one gram of calcined clay with two grams of CaO and 250 mL of deionized water. Following this, the blend is heated for sixteen hours at 90 °C. After cooling, 250 mL of deionized water is combined with 60 g of sucrose. The resulting mixture is titrated against HCl (0.1 M). The amount of bound portlandite is calculated based on the volumes of HCl solution added for the blank and calcined clay samples. The pozzolanic activity is calculated using Formula (1). The pictorial procedure of the tests is shown in Figure 5.
P A = 2 V 1 − V 2 V 1 . 74 56 . 1000

2.3. Development of LC3

To develop LC3, a mixture of 2 parts clay and 1 part limestone was used to replace 50% of OPC based on previous research [34]. The cement paste was mixed using a mechanical mixer at 1600 rpm for 3 min. The LC3 mortar was fabricated with a 1:2.75 binder-to-sand ratio and a 0.485 water-to-binder ratio following ASTM specifications. The sand used in the mortar formulation was well-graded Lawrence pure sand, characterized by a fineness modulus of 2.7, a specific gravity of 2.65, water absorption of 1.9%, and a loose density of 1340 kg/m3. The mortar mixture design is displayed in Table 2, and Figure 6 illustrates it. A Hobart mixer was used to blend the mortar for three minutes. Three layers of mortar were poured into the 50 mm mold, and each layer was tamped 25 times. Finally, a table vibrator was used for a minute to compact it. The molds were wrapped in a low-density polyethylene sheet to prevent heat loss, keep the temperature steady at 25 °C, and maintain relative humidity at 75%. Following 24 h, the cubes were removed from the molds and cured in a humid room until they reached the necessary age.

2.4. Experimental Setup

Several tests were performed on the fresh and hardened cement mortar blend. Fresh properties, including normal consistency and setting times, were assessed according to ASTM standards, according to ASTM C187 (2016) and ASTM C230 (2020) [35,36], respectively. The compressive strength of the hardened material was evaluated at 7 and 28 days, with results averaged from three specimens using a universal testing equipment that applies a load of 2000 kN and operates at a loading rate of 1.2 mm/min, in compliance with ASTM C109 standards [35]. An isothermal heat conduction calorimeter was used to measure the heat of hydration, monitoring early-stage hydration for up to seven days following ASTM C1702 [36]. TGA and XRD were performed on powdered, cured samples to evaluate hydration mechanisms and phase development.
Figure 7 depicts the detailed methodology, workflow, and strength development mechanism of LC3 mortar incorporating kaolinite, montmorillonite, and illite. It includes raw material processing, calcination to form metakaolin, meta-smectite, and meta-illite, followed by reactivity assessment using the modified Chapelle test. LC3 mortar is developed by mixing calcined clay, limestone, gypsum, and water. The following stages involve hydration product analysis (XRD, TGA), performance testing (strength, flowability, setting time), and heat of hydration monitoring (1–7 days) to evaluate the influence of each clay type.

3. Results and Discussion

3.1. Reactivity of the Clays

3.1.1. The Modified Chapelle Test

The pozzolanic reactivity of calcined clays was assessed using the modified Chapelle test. Figure 8 illustrates the portlandite consumption by different types of calcined clay minerals, categorized by their crystalline structure. The amount of portlandite reacted varies according to the type of clay, with Ka consuming the most portlandite (1072 mg of Ca (OH)2 per gram of Ka), followed by Mn (881 mg of Ca(OH)2 per gram of Mn), and IL (783 mg of Ca(OH)2 per gram of IL). The Ka clay exhibited the highest reactivity, indicating higher reactivity due to its 1:1 kaolinitic structure [32]. The structural mineralogy of IL clay is the reason for its decreased reactivity [37]. The high testing temperature contributed to significant portlandite consumption across all clays.

3.1.2. Mineralogical Evaluation of Calcined Clays

To comprehend the reactivity tendencies identified in the Chapelle test, clays were examined for their mineralogical, chemical, and physical properties. Table 3 summarizes the reactivity-related characteristics of the three clays. Ka, a 1:1 layered clay, demonstrates the most advantageous reactivity characteristics among the examined clays. Ka possesses a highly crystalline structure, the smallest particle size (D50 = 8.5 µm), and the highest Al2O3 content (39.50%). Its unique dehydroxylation temperature range (700–750 °C) and substantial bound portlandite formation, as indicated by the Chapelle test, affirm its superior pozzolanic reactivity. Mn, a 2:1 clay mineral with medium crystallinity, shows good reactivity. It has a slightly smaller particle size (6.1 µm) and dehydroxylates within a similar range (700–750 °C) but contains lower Al2O3 (27.13%) and higher K2O (2.55%), which may limit its reactivity compared to Ka. IL, also a 2:1 clay, has the largest particle size (16.8 µm), lower crystallinity, and a less favorable oxide composition for reactivity, particularly its lower Al2O3 (18.64%) and higher Fe2O3 (7.13%). It dehydroxylates at a higher temperature (800–850 °C), and the reduced bound portlandite observed post-calcination reflects its relatively average pozzolanic reactivity.
The main focus of this research is the impact of these clays on the performance of LC3. However, the characterization conducted after calcination, such as reduced crystallinity (XRD), dehydroxylation behavior (TGA), and oxide content, offers valuable insights into the inherent reactivity of each clay. These findings correspond with the results of the Chapelle test and help clarify the differences observed in hydration and strength development. These variations highlight how structural order [38], particle fineness [39], oxide composition [40], and thermal behavior together affect the reactivity potential of clays in LC3 formulations. The mineralogical and compositional characteristics are anticipated to substantially affect the hydration mechanisms and strength development in LC3 systems, as examined in the subsequent sections.

3.2. Heat of Hydration

Figure 9 displays the cumulative heat of hydration (kJ/kg) for OPC and the various LC3 blends over the hydration period. Heat of hydration was measured on cement paste using an isothermal conduction calorimeter by ASTM C1702 (2017). The values are reported per kilogram of total binder (including clinker, calcined clay, limestone, and gypsum), ensuring a fair comparison between OPC and LC3 systems despite the difference in clinker content. OPC shows the highest total heat release, mainly due to its higher clinker content, which is only partially replaced in LC3 systems [41]. Among the various LC3 blends, LC3-Ka showcases the highest initial heat release during the first three days of hydration, attributed to the rapid pozzolanic reactions of metakaolin. This early activity underscores the high reactivity of calcined kaolinite [42]. However, after approximately three days, the heat release from LC3-Ka experiences a notable slowdown. By day five, its cumulative heat release drops below that of the other LC3 blends. Interestingly, the heat evolution of LC3-Ka begins to rise again around the seventh day, indicating a secondary phase of reactivity. The initial spike in heat release, followed by a decline and subsequent increase for LC3-Ka, reflects a typical two-phase hydration behavior in LC3 systems. The early peak is driven by the swift reactivity of metakaolin and clinker hydration, while the temporary reduction in heat release suggests a decreased availability of portlandite. The increase in heat evolution after five days is linked to the delayed formation of carboaluminate phases and ongoing pozzolanic activity, which is aligned with previous studies [34,43]. LC3-IL, despite its lower pozzolanic reactivity, demonstrates a slightly higher cumulative heat release than LC3-Mn over the same time frame [44]. This discrepancy may be attributed to variations in particle morphology or packing density, which can influence the accessibility of water and clinker particles during the hydration process. These findings emphasize the significance of clay mineralogy in shaping hydration kinetics, as both early and later-stage reactions differ based on the type of calcined clay utilized in the LC3 system.

3.3. Hydration Products

3.3.1. X-Ray Diffraction (XRD)

The hydration products were examined using XRD techniques. The type of calcined clay in LC3 significantly impacts the hydration products formed. Figure 10 presents the XRD spectra for the OPC and LC3 systems after 7 and 28 days of hydration. The hydration products formed in LC3 blends depend on the types of calcined clay present [45].
After seven days, the primary phases detected in OPC via XRD are portlandite and ettringite [46]. The peaks become more intense with increasing age. In contrast, the ternary binders LC3-Ka, LC3-Mn, and LC3-IL also show the presence of hemicarboaluminate (Hc) and monocarboaluminate (Mc), due to the interaction of calcined clay and limestone, which promotes carbonate formation. These peaks become more pronounced with age, as seen in the 28-day results [47]. Although all LC3 systems generated comparable hydration products, their kinetics and intensities exhibited significant variation depending on the mineralogy of the calcined clay. The hemi and monocarboaluminate peaks are prominent in LC3-Ka, followed by LC3-Mn and LC3-IL. LC3-IL exhibits the lowest peak intensity due to its slower pozzolanic reaction, as indicated by its lower compressive strength and reactivity results. However, the OPC spectra exhibit higher-intensity ettringite and portlandite peaks than the LC3 blends [48]. The intensity of the portlandite peaks decreases with age thanks to the limestone and calcined clay pozzolanic reaction. This is also due to LC3’s secondary hydration reactions and clinker dilution effect [45,49]. However, the decrease is slower in LC3-IL, though all binders eventually reach similar levels. The limited development of the Mc and Hc phases in LC3-IL corresponds with its lower 28-day compressive strength, as a deficiency in reactive alumina restricts the potential for advantageous secondary reactions. This relationship underscores that the mineralogical properties of clay minerals directly influence both phase development and mechanical performance. Distinct peaks of quartz (Q) are observed at approximately 26.6° 2θ, and those of calcite (Cc) at around 29.4° 2θ are more prominent in LC3 systems due to the presence of unreacted phases from calcined clay and limestone filler. These peaks serve as indirect evidence of the degree of reaction. Notably, LC3-IL exhibits more pronounced Q peaks at 28 days, indicating lower pozzolanic reactivity, which aligns with the mechanical performance data [49].
The hydration products from different ternary binders were similar, but the hydration behaviors varied based on the mineralogy of calcined clay employed. These findings emphasize that not all calcined clays exhibit uniform reactivity, and that mineralogy is key in optimizing LC3 performance, particularly in formulations where early strength and durability are essential. The pozzolanic and secondary hydration reactions in non-kaolinitic LC3 blends are determined to exhibit comparable efficiency to those involving kaolinite.

3.3.2. Thermogravimetric Analysis (TGA)

The TGA results were differentiated to obtain the derivative thermogravimeter (DTG) curve, which is shown in Figure 11. The curve mainly has three weight-loss regions. The weight loss between 150 °C and 300 °C is attributed to the decomposition of ettringite, carbonates (Mc and Hc), and C-(A)-S-H [50]. The decomposition of portlandite (Ca(OH)2 (CH) results in a peak between 400 °C and 500 °C [51]. Weight loss above 600 °C results from the disintegration of calcium carbonate (CaCO3) [52].
As hydration progresses, ettringite, Mc, Hc, and C-S-(A)-H increase significantly in all the blends. However, due to the presence of Hc and Mc phases, the loss in LC3 blends is significant. In the OPC sample, there is also a substantial increase in CH. However, in the LC3 mixtures, except for LC3-IL, the weight loss due to CH significantly decreases after 28 days. This decrease is due to the pozzolanic reaction between calcined clay and portlandite, which forms additional C-A-S-H gel and consumes CH in this process. This trend is specifically observable in LC3-Ka and LC3-Mn blends, where the reactivity of kaolinite and montmorillonite causes a significant decrease in the CH peak. Conversely, the lower reactivity of illite in LC3-IL results in a more gradual pozzolanic reaction, elucidating the relatively minor fluctuation in CH content over time. These variations underscore the mineralogical differences among the clays used and their respective reactivities, with kaolinite showing the highest reactivity and illite the lowest. This interpretation aligns with the XRD and compressive strength results, reinforcing the notion that kaolinite- and montmorillonite-rich clays facilitate more rapid portlandite consumption, while illite-rich clays exhibit a slower reaction. Moreover, the diminished availability of portlandite for carbonation results in a gradual decline in the quantity of CaCO3 produced in LC3 blends, unlike the OPC system, where CH persists and thus enhances CaCO3 generation. In LC3, the blend initially has a higher calcite content due to the limestone in the binder [53]. However, as hydration progressed, the observed calcite content reduced because there was less portlandite available for carbonation as a result of its gradual consumption in the pozzolanic reaction [14].

3.3.3. Scanning Electron Microscopy (SEM)

The SEM micrographs in Figure 12 illustrate the morphological differences between the hydration products of OPC and those generated in LC3 systems with various clay types at the age of 28 days. The OPC matrix exhibits a mostly crystalline microstructure, distinguished by many plate-like CH crystals and needle-shaped ettringite/AFm phases, interspersed within a matrix of C-S-H and secondary aluminous hydrates [54]. This illustrates the traditional hydration process characterized by the development of portlandite and ettringite [55].
The LC3-Ka system demonstrates a denser and more homogeneous microstructure, characterized by a notable reduction in visible portlandite and a fine distribution of C-S-H/C-(A)-S-H phases [56]. The reduced presence of CH indicates the increased pozzolanic activity of kaolinite, which actively consumes portlandite during secondary hydration reactions. The microstructural refinement in LC3-Ka correlates with its enhanced mechanical performance and reactivity, as demonstrated by TGA and compressive strength findings.
The LC3-Mn sample exhibits a compact structure characterized by well-developed C-S-H/C-(A)-S-H gels and moderate CH content [57]. Montmorillonite, despite having a lower alumina content than kaolinite, likely continues to dissolve over time due to its layered 2:1 structure, which aids in the formation of hydration products that are morphologically similar to those found in kaolinite-based blends [57]. The mechanical data indicate that LC3-Mn attained strength values comparable to those of LC3-Ka at 28 days.
The LC3-IL blend exhibits a more porous and less cohesive microstructure characterized by discrete CH crystals and a limited distribution of C-S-H gels. The existence of unreacted or partially reacted phases indicates that illite, although a low-cost and readily available material, demonstrates slower reaction kinetics and reduced pozzolanic reactivity at early ages [58]. The gradual development of hydration products over time, as evidenced by the progression of compressive strength, suggests that illite is a viable supplementary cementitious material for long-term or non-structural applications. Nevertheless, the incremental formation of hydration products. However, the progressive formation of hydration products over time, as evidenced by the increase in compressive strength, suggests that illite continues to be an effective supplementary cementitious material in long-term or non-structural applications.
The SEM analysis supports the TGA and XRD analysis and chemical and mechanical properties of the subsequent clays, demonstrating that the pozzolanic potential of the clays is evident in both bulk hydration behavior and the resulting microstructural development. The reduction in portlandite and the densification of the binder matrix in LC3 systems, especially with kaolinite and montmorillonite, indicate improved durability and mechanical integrity in blended cement formulations.

3.4. Setting Time and Consistency of Blends

Figure 13 displays the findings from the investigations on normal consistency and setting time. All LC3 binders demonstrated slightly superior consistency compared to OPC, with LC3-IL achieving the highest value. This improvement in consistency can be attributed to the finer particle size and increased surface area of the calcined clay particles, resulting in greater water demand [59]. Among the various blends, LC3-IL required a larger volume of water to achieve standard consistency, likely due to the platy morphology and relatively low pozzolanic activity of illite, which impedes early hydration reactions.
In terms of setting time, all LC3 binders exhibited longer initial and final setting times compared to OPC. This trend is consistent with previous findings that the incorporation of calcined clay and limestone tends to dilute the clinker content and retard the formation of C-S-H gel in the early age [57,60]. Among the various blends, LC3-IL required a larger volume of water to achieve standard consistency, likely due to the platy morphology and relatively low pozzolanic activity of illite, which impedes early hydration reactions. In contrast, LC3-Ka and LC3-Mn showed relatively moderate increases in setting time, owing to the higher reactivity of kaolinite and montmorillonite, which promote earlier nucleation and growth of hydrates. The mineralogical composition, degree of reactivity, and physical properties of the calcined clays utilized in LC3 systems significantly affect the setting time and consistency [61]. However, despite these variations, all LC3 mixtures meet the minimum setting time criteria outlined in ASTM C191.

3.5. Compressive Strength

The development of compressive strength in LC3 mortars at both 7 and 28 days, as illustrated in Figure 14, reveals notable performance trends influenced by the type of calcined clay utilized. To aid comparison, the 7-day and 28-day compressive strength values of the OPC mix are also indicated as horizontal reference lines in Figure 13. Among the ternary blends, LC3-Ka exhibited the highest compressive strength, reaching 20.4 MPa at 7 days and 30.8 MPa at 28 days, closely competing with OPC, which recorded values of 23.0 MPa and 31.9 MPa for the respective timeframes. This strong early-age performance can be attributed to the high reactivity of kaolinite, a 1:1 layered silicate mineral, which accelerates the pozzolanic reaction and promotes the rapid consumption of portlandite [62]. However, it was slightly lower than OPC due to its lower clinker content and dilution effect. The acceleration caused by the clay was insufficient to offset the dilution effect [63]. LC3-Mn, composed of montmorillonite clay, exhibited compressive strengths of 17.9 MPa at 7 days and 27.9 MPa at 28 days, slightly inferior to LC3-Ka but demonstrating significant temporal enhancement. The 2:1 layered structure of montmorillonite promotes gradual secondary hydration [30], consistent with the delayed yet consistent formation of hydration products indicated by XRD and DTG analyses (see Figure 10 and Figure 11). The moderate decrease in CH content from 7 to 28 days, as evidenced by TGA results, corroborates this postponed pozzolanic effect.
In contrast, LC3-IL, which includes illite clay, demonstrated the lowest strength at an early age, measuring 16.3 MPa at 7 days, but experienced a significant increase to 23.9 MPa by 28 days. This approximately 70% enhancement suggests that, despite illite’s relatively inert nature due to its partially crystalline and impure structure, it still contributes to long-term strength development through gradual pozzolanic reactions. The XRD spectra confirmed a reduction in carboaluminate peaks in LC3-IL, while the TGA results showed minimal consumption of calcium hydroxide at an early age, both of which support the notion of delayed pozzolanic activity. Nonetheless, the illite-based LC3 achieved approximately 75% of the strength of OPC by 28 days, indicating that even low-reactivity clays can significantly enhance long-term performance [63]. The observed trends underscore the significant influence of clay mineralogy on hydration kinetics and strength development in LC3 systems. The high reactivity of kaolinite contributes to enhanced early strength, while montmorillonite offers moderate but consistent strength gains. In contrast, illite primarily aids strength development at later ages. The interaction between limestone and various clay types, evidenced by the formation of carboaluminates and the consumption of calcium hydroxide, underscores the need for customized LC3 formulations to optimize performance. Pearson’s correlation coefficient (R = 0.998) was determined between the portlandite fixation values (modified Chapelle test) and the 28-day compressive strengths of LC3 mortars presented in Figure 15, indicating a nearly linear positive association between pozzolanic reactivity and mechanical performance.

3.6. Overall Performance Summary

Figure 16 displays a radar chart that compares the normalized performance of different LC3 blends—specifically, LC3-Ka, LC3-Mn, and LC3-IL—against OPC across seven key parameters. All parameters have been normalized based on the highest value recorded among all mixes, allowing for direct visual comparison. This approach emphasizes performance distribution instead of absolute values, complementing the detailed trends reported earlier. LC3-Ka demonstrated consistently high performance, closely aligning with OPC in compressive strength, setting time, and pozzolanic reactivity, underscoring kaolinite’s suitability for LC3 production. LC3-Mn exhibited moderate results across most indicators, while LC3-IL, despite lower early strength and reactivity, performed well in terms of water demand and setting characteristics. These performance trends reflect the underlying mineralogical differences among the clays. While all three clay types are viable for LC3 applications, kaolinite offers the most balanced and structurally robust performance. Illite, though less reactive, remains a promising low-cost alternative for non-structural or blended cement uses.

4. Conclusions

This study examined the classification and applicability of diverse low-grade clays in LC3 systems, emphasizing their thermal properties, reactivity, and contribution to mechanical performance. The results indicated that optimal calcination temperatures are specific to each clay type, with kaolinite and montmorillonite undergoing effective transformation at 750 °C, whereas impure illite necessitated higher thermal energy for complete dehydroxylation.
The findings from XRD, TGA, and reactivity assessments revealed a significant correlation between the degree of dehydroxylation and the mechanical behavior of the LC3 blends. Kaolinite-based LC3 consistently demonstrated superior performance during both early and later curing stages, highlighting its potential as a dependable supplementary SCM, even in its low-grade form. Montmorillonite also exhibited encouraging results, whereas illite, despite a slight deficiency in early-age strength, showed satisfactory strength development over time.
The modified Chapelle test has proven to be a reliable indicator of clay reactivity, providing practical support for the early screening of clay sources. This study emphasizes that, rather than focusing solely on low-grade clays, widely available low-cost non-premium clays can be significant components in sustainable cement formulations when processed accurately. Optimizing blend composition and tailoring calcination parameters are essential for utilizing these resources in low-carbon construction materials.
Despite demonstrating the highest strength and reactivity, kaolinite is frequently costly and unavailable in many regions of the Southern Hemisphere. Montmorillonite and illite, due to their cost-effectiveness and local abundance, with illite being the least expensive, present viable options. Montmorillonite, specifically, attained equal strength at 28 days and can serve as an efficient substitute for kaolinite when early strength is not critical.

Author Contributions

Conceptualization, N.B.; methodology, S.N.H.K. and N.B.; validation, A.K.; formal analysis, N.A.; data curation, K.J.; writing—original draft preparation, S.N.H.K., I.A.K. and M.J.S.; writing—review and editing, I.A.K., K.J., S.A. and A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be provided by the corresponding authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
LC3Limestone calcined clay cement (LC3)
OPCOrdinary Portland Cement (OPC)
ILIllite

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Solids 06 00035 g001
Figure 1. Clay minerals (color and their expected structure based on Li et al. [9].
Figure 1. Clay minerals (color and their expected structure based on Li et al. [9].
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Figure 2. Particle size distribution (PSD) analysis of the raw material used.
Figure 2. Particle size distribution (PSD) analysis of the raw material used.
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Figure 3. X-ray diffraction analysis of the raw clays. where K is kaolinite, M is montmorillonite, Q is quartz, H is hematite, and I is illite.
Figure 3. X-ray diffraction analysis of the raw clays. where K is kaolinite, M is montmorillonite, Q is quartz, H is hematite, and I is illite.
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Figure 4. Weight loss (a) and derivative weight loss (b) of raw and calcined clay minerals.
Figure 4. Weight loss (a) and derivative weight loss (b) of raw and calcined clay minerals.
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Figure 5. Pictorial presentation of the modified Chapelle test.
Figure 5. Pictorial presentation of the modified Chapelle test.
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Figure 6. Pictorial representation for the development of LC3 and OPC blends.
Figure 6. Pictorial representation for the development of LC3 and OPC blends.
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Figure 7. Comprehensive methodology of the performed research work.
Figure 7. Comprehensive methodology of the performed research work.
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Figure 8. The consumption mg of Ca (OH)2 per gram of clay minerals.
Figure 8. The consumption mg of Ca (OH)2 per gram of clay minerals.
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Figure 9. Heat of hydration of OPC and LC3 clay blends.
Figure 9. Heat of hydration of OPC and LC3 clay blends.
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Figure 10. XRD assessment of LC3 and OPC clay blends at the age of 7 and 28 days.
Figure 10. XRD assessment of LC3 and OPC clay blends at the age of 7 and 28 days.
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Figure 11. DTG of OPC and LC3 clay blends at 7 and 28 days.
Figure 11. DTG of OPC and LC3 clay blends at 7 and 28 days.
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Figure 12. SEM analysis of OPC and all LC3-blends (LC3-Ka, LC3-Mn, LC3-IL).
Figure 12. SEM analysis of OPC and all LC3-blends (LC3-Ka, LC3-Mn, LC3-IL).
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Figure 13. Consistency and setting time of the OPC and LC3 clay blends.
Figure 13. Consistency and setting time of the OPC and LC3 clay blends.
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Figure 14. Compressive strength of LC3 mortars containing kaolinite (LC3-Ka), montmorillonite (LC3-Mn), and illite (LC3-IL) at 7 and 28 days. The black horizontal lines indicate the corresponding compressive strength values of the OPC control (at 7 and 28 days).
Figure 14. Compressive strength of LC3 mortars containing kaolinite (LC3-Ka), montmorillonite (LC3-Mn), and illite (LC3-IL) at 7 and 28 days. The black horizontal lines indicate the corresponding compressive strength values of the OPC control (at 7 and 28 days).
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Figure 15. Correlation between 28-day compressive strength of LC3 blends and pozzolanic reactivity of calcined clays as determined by the modified Chapelle test.
Figure 15. Correlation between 28-day compressive strength of LC3 blends and pozzolanic reactivity of calcined clays as determined by the modified Chapelle test.
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Figure 16. Radar graph showing the overall performance.
Figure 16. Radar graph showing the overall performance.
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Table 1. XRF analysis of clay minerals, limestone, and mortar blends (OPC and LC3) in raw (R) and calcined (C) forms.
Table 1. XRF analysis of clay minerals, limestone, and mortar blends (OPC and LC3) in raw (R) and calcined (C) forms.
Clay TypeSiO2Al2O3Fe2O3CaOMgOK2ONa2OSO3ClBlain Finess
(cm2/g)		LO1
Ka (R)58.728.83.52.30.20.30.10.20.0655212.15
Ka (C)53.1634.283.362.670.040.010.020.060.0010,9302.97
Mn (R)41.4529.232.329.600.682.550.650.03-454510.34
Mn (C)41.4527.132.028.760.602.550.650.03-10,0000.69
IL (R)56.4019.576.984.731.711.491.390.10.72465010.29
IL (C)54.0618.647.134.641.801.571.590.080.1787650.48
Limestone19.285.442.0942.50.960.780.370.150.0011,53543.93
OPC19.44.813.5460.92.582.650.250.77032002.5
Table 2. Mix the proportions of OPC LC3-mortar pastes.
Table 2. Mix the proportions of OPC LC3-mortar pastes.
OPCClay (kg/m3)LimestoneSandWater
kg/m3KaMnILkg/m3kg/m3kg/m3
LC3-Ka170212-------981039115
LC3-Mn170----212---981039115
LC3-IL170------212981039115
Control Mix569------------1039115
Table 3. Comparative analysis of the raw (R) and activated calcined (C) clays.
Table 3. Comparative analysis of the raw (R) and activated calcined (C) clays.
Clay TypeClay StructurePSDXRDTGA Blain FinenessWeight Loss (%)Oxide Content
(%)		MCTReactivity of the Clay
D50
µm		CrystallinityDehydroxylationcm2/gLOIAl2O3
Content		SiO2
Content		K2O
Content		CaO
Content		Fe2O3
Content		Bound Portlandite Content
Ka (R)1:14.5Highly crystalline700–750 °C655212.1537.9148.080.043.151.32--Excellent
Ka (C)Disorder8.5Very low crystalline with a sharp hump at 20–40° 2θ---10,9302.9739.5050.220.061.891.551164
Mn (R)2:14.9Medium crystalline700–750 °C4545 29.2341.452.559.602.32---Good
Mn (C)Disorder6.1Low crystalline 10,000 27.1341.452.558.762.02881
IL (R)2:111Low crystals800–850 °C465010.2919.5766.401.494.736.98----Average
IL (C)Partially disorder16.8Resembles raw clay structure 87650.8918.6464.061.574.647.13783
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MDPI and ACS Style

Blouch, N.; Kazmi, S.N.H.; Akram, N.; Saleem, M.J.; Khan, I.A.; Javed, K.; Ahmad, S.; Khan, A. Performance Evaluation of Low-Grade Clay Minerals in LC3-Based Cementitious Composites. Solids 2025, 6, 35. https://doi.org/10.3390/solids6030035

AMA Style

Blouch N, Kazmi SNH, Akram N, Saleem MJ, Khan IA, Javed K, Ahmad S, Khan A. Performance Evaluation of Low-Grade Clay Minerals in LC3-Based Cementitious Composites. Solids. 2025; 6(3):35. https://doi.org/10.3390/solids6030035

Chicago/Turabian Style

Blouch, Nosheen, Syed Noman Hussain Kazmi, Nijah Akram, Muhammad Junaid Saleem, Imran Ahmad Khan, Kashif Javed, Sajjad Ahmad, and Asfandyar Khan. 2025. "Performance Evaluation of Low-Grade Clay Minerals in LC3-Based Cementitious Composites" Solids 6, no. 3: 35. https://doi.org/10.3390/solids6030035

APA Style

Blouch, N., Kazmi, S. N. H., Akram, N., Saleem, M. J., Khan, I. A., Javed, K., Ahmad, S., & Khan, A. (2025). Performance Evaluation of Low-Grade Clay Minerals in LC3-Based Cementitious Composites. Solids, 6(3), 35. https://doi.org/10.3390/solids6030035

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