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Article

Influence of Minor Additives on the Performance of Calcined Clay and Blast Furnace Slag Based One Part Alkali-Activated Mortars

by
Suat Çalbıyık
1,
Tarik Omur
2,
Hakan Ozkan
2,3 and
Nihat Kabay
2,*
1
CIMPOR Serviços, S.A, 1099-020 Lisbon, Portugal
2
Department of Civil Engineering, Yildiz Technical University, 34220 Istanbul, Türkiye
3
Betão Liz, S.A., 3040-326 Lisbon, Portugal
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(20), 3776; https://doi.org/10.3390/buildings15203776
Submission received: 15 September 2025 / Revised: 8 October 2025 / Accepted: 10 October 2025 / Published: 20 October 2025
(This article belongs to the Special Issue Recent Advances in Sustainable Low-Carbon Materials for Green Concrete)
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Abstract

The availability of key precursors of alkali-activated binder (AAB) systems is declining, requiring sources. Calcined clays (CCs) stand out as a promising alternative due to their widespread accessibility. Although the properties of CC and blast furnace slag (BFS)-based two-part AABs have been well reported in the literature, the effect of minor additives on the properties of a one-part AAB system composed of CC and BFS remains unexplored. In this research, calcined magnesia (CM), aerial lime (AL), hydraulic lime (NHL), quicklime (QL), borax (BR), and zeolite (ZP) have been used as minor additives and incorporated into the AAB system at between 2% and 15%. The specimens were activated with sodium–metasilicate, and the fresh, physical, mechanical, durability and microstructural properties of mortars have been investigated. Key findings indicate that all minor additives, except for BR, enhanced the early- and later-age mechanical properties. Notably, 10% QL addition significantly increased compressive strength by up to 55% at 28 days (50.9 MPa), compared to the reference. BR and ZP usage eliminated the efflorescence formation without compromising other properties. Furthermore, incorporating QL, AL, CM, and BR markedly reduced the chloride permeability of the mortars and decreased Dnssm value by as much as 81%, compared to the reference.

1. Introduction

1.1. Overview

The processes of urbanization and the growth of the construction sector have significantly contributed to global greenhouse gas emissions and rising energy demand. As urban areas expand and infrastructure requirements escalate, the built environment presently constitutes a substantial share of global energy consumption and carbon emissions. The significance of the built environment in this context is evident from the statistics; the building and functioning of structures utilize about 36% of all energy generated globally, and contribute to around 39% of worldwide CO2 emissions [1]. Besides this, cement production is responsible for approximately 7–8% of global anthropogenic (human-caused) CO2 emissions annually [2,3]. This trend creates significant environmental issues, such as exacerbated climate change, resource depletion, and air pollution, requiring urgent and innovative solutions to foster sustainable urban development and mitigate the environmental impact of construction activities, especially in terms of cement production. In light of the environmental and performance constraints associated with conventional Portland cement, the construction sector has seen the introduction of various innovative binders and alternative types of cement. As such, investigations focused on minimizing the cement content, even in specialized materials like ultra-high-performance concrete, are garnering significant interest [4,5]. Significant advancements encompass alkali-activated binders (AAB) sourced from industrial by-products like fly ash (FA) and blast furnace slag (BFS), calcium sulfoaluminate cements, limestone calcined clay cements (LC3), belite cement, and magnesium-based cements [6,7,8,9,10]. These alternatives suggest reduced CO2 emissions while delivering performance characteristics that are comparable to or even superior to those of traditional Portland cement binders. Furthermore, innovative technologies like biochar-enhanced cements, algae-based cements, and carbon-negative binders are starting to attract interest due to their sustainable properties [11,12].
AABs provide considerable advantages over the aforementioned binders owing to their remarkable adaptability in raw material utilization, facilitating the valorization of a diverse array of industrial and post-consumer wastes. AABs integrate several aluminosilicate sources, including FA, BFS, rice husk ash, glass waste, calcined clays, and demolition debris, providing environmental advantages and consistent mechanical performance across different formulations [13,14,15]. This adaptability not only diverts waste from landfills, but also expands the possibilities for sustainable binder design, exceeding the raw material constraints of conventional Portland cement and other alternative binders. Nevertheless, in recent years, there has been a significant reduction in the availability of FA, which is the most favored precursor material for the AAB systems. The depletion of reserves with the shutdown of coal-fired power plants has resulted in supply shortages [16]. It is also noteworthy that the quantity of commonly identified by-products is evidently insufficient to meet the targets for reducing the amount of Portland cement to consistently meet the demand for conventional concrete production. For example, the annual worldwide generation quantities of BFS and FA are approximately 390 and 800 million tons, respectively; however, the quality of the majority of FA is substandard, particularly regarding its low binding capacity [8,17]. This creates the need to explore alternative sources for sustainable AAB systems. Various clay deposits are capable of acquiring binding properties through calcination, and reports indicate that kaolin-type clays alone are available in excess of 5 billion tons annually, surpassing the cement production levels [8].
Clay sources are vital precursors in the synthesis of binding materials, especially for alkali-activated materials. The primary clay sources commonly used comprise kaolinite (convertible to metakaolin (MK)), illite, smectite, and other mixed-layer clays [18,19,20]. High-quality clays like MK are favored for their increased reactivity and capacity to create robust aluminosilicate networks through alkali activation, resulting in binders with enhanced mechanical characteristics. AAB mixes using MK cured at 40 °C and 60 °C for 5 h have been documented to achieve a compressive strength of 50 MPa or greater at 28 days [21]. Likewise, when combined with a calcium source like BFS, it can demonstrate even higher strength (>55 MPa) under ambient curing by forming a strength-giving gel combination, such as calcium aluminate silicate hydrate (C-A-S-H) or sodium aluminate silicate hydrate (N-A-S-H) [22]. In addition, Alcamand et al. [23] examined the sulfate resistance of several AABs including diverse calcium sources and MK, revealing that the MK-rich AABs exhibit superior resistance to magnesium sulfate attack.
The superior binding capabilities of MK, derived from the calcination of high-grade clays, provide a viable alternative for the building sector. Nonetheless, the difficulty with high-grade kaolinitic clay lies in its limited geographical availability and considerable cost, attributed to its applications in the ceramic, paper, and paint industries [24]. Therefore, other clay sources except MK, abundantly accessible globally, are regarded as a potential binding agent owing to their affordability and availability. Indeed, the sources of some clay may also be linked to waste or by-products from other economic activities, like dredging, mining operations, or excavating clays from substantial civil infrastructure projects [20]. They can originate from waste minerals or mining operations containing elevated levels of silica and alumina. Mining activities generate around 65 billion tons of clay waste annually, which is frequently linked to risks connected with its storage and environmental management [25]. Despite this significant potential and environmental concern, a limited number of studies in the literature have examined the assessment of clays other than MK as binding agents. Numerous investigations have indicated that clay-based AABs frequently demonstrate diminished compressive strength compared to commonly used binders such as FA or BFS. For instance, the study of Khan et al. [26] revealed that the restricted surface area and reactivity of kaolinite minerals impede acid/alkali treatment, leading to inferior strength development when compared to conventional AAB precursors. Another study by Karozou et al. [27] investigated the long-term durability of alkali-activated clay mortars, revealing an intrinsic low mechanical strength and significant susceptibility to water ingress. Yamchelou et al. [28] revealed that untreated clay-based AAB could attain a strength of 32 MPa at 7 days with heat curing (24 h at 120 °C), and noted that the compressive strength of corresponding mixes increased by roughly 34% following the calcination of this impure clay. In another study, AAB mixes using calcined clay have achieved a compressive strength of approximately 30 MPa when cured at ambient temperatures. Furthermore, it was found that minimal quantities of phosphogypsum could enhance the strength of clay-based AABs [29]. Recent studies have concentrated on identifying the most suitable calcination parameters, the improvement of calcined clay properties by various treatments, and examining the influence of different alkali activator sources on the characteristics of clay-based AABs. An investigation into the reactivity of various clays revealed that the mechano-chemical process enhances pozzolanic activity and the binding potential in AAB systems. However, it was noted that extended grinding durations may result in significant energy consumption when compared to binders like FA or BFS [30]. The optimal calcination temperature and duration are mostly influenced by the chemical and mineralogical composition of the clay source; however, research indicates that elevating the calcination temperature of clays to a specific threshold enhances the mechanical properties of the resulting mortars [31,32]. Furthermore, it has been reported that mortar mixes utilizing both sodium silicate and sodium hydroxide as alkaline activators exhibit superior mechanical properties in comparison to those having solely sodium hydroxide [33]. The mechanical activation of clay was also found to be another effective treatment method for improving the mechanical properties of alkali-activated clay-based mortars [34]. Furthermore, it was previously reported that the physical and durability properties of clay-based AABs, including water absorption, sulfate resistance, and acid resistance, can be ameliorated with the incorporation of calcium sources including BFS and lime [35,36,37].
Despite the numerous studies referenced earlier aimed at enhancing the performance of AAB mixtures with untreated or calcined clay, except for MK, it remains evident that a comparable performance to conventional cement-based mortars regarding mechanical, physical, and durability properties has not yet been achieved. In this context, the incorporation of minor additives can lead to substantial enhancements in properties. Dener et al. [38] showed that the performance of BFS-based AABs activated with sodium sulfate can be notably enhanced with the addition of minimal amounts of calcium sources. As a result, an 86% enhancement in ultimate compressive strength was achieved by incorporating 5% quicklime (QL) in comparison to the control mixture. Furthermore, the early-age compressive strength exhibited a threefold increase compared to the control mixture, with minimal additions of cement or hydrated lime. It was also found that the mechanical performance of ambient-cured MK-based geopolymers can be significantly improved by the incorporation of various calcium-modifiers such as calcium carbide residue, QL, and slaked-lime [39]. Thus, it was clearly noted that the carefully selected and balanced minor calcium additives can significantly enhance the performance of AABs. However, it was also mentioned that finding the optimal dosages for these calcium sources in AABs is crucial to prevent adverse effects [40,41]. Along with the beneficial effects of minor calcium-modifiers, magnesia sources also have positive impacts on boosting the AAB’s performance. Calcined magnesia (CM) was particularly found to be effective for performance and durability enhancements of AABs through shrinkage mitigation, strength gain, and microstructural refinement [42,43]. Similarly, recent studies also emphasize the roles of xylitol, tannic acid and borax (BR) as minor additives, which can modify the pore structure and durability of AABs, while also aiding in the stabilization of the matrix [44,45]. Some certain additives, including BR, sucrose, and citric acid, were observed to negatively impact the early-age compressive strength; however, they generally enhanced later-age performance, particularly at optimal dosage levels (typically 2–5% by mass of binder). It was also stated that a refined pore structure and increased formation of binding gels at later ages result in improved micro-cracking resistance and long-term durability [46]. Furthermore, the microstructural analysis revealed a reduction in micro-cracking and a denser microstructure in BR-modified AABs [47].

1.2. Research Significance

The accessibility of aluminosilicate deposits is the primary determinant of large-scale AAB production. The predominant study in this field concentrates on BFS-, MK-, and FA-based AABs. The availability of the most common aluminosilicate source, FA, may be constrained by reasons such as economic conditions, environmental concerns, and geographic location. Furthermore, their availability is diminishing annually as the conventional production methods in these industries are converting into more sustainable methods. Access to high-grade clay deposits with minimal impurities or industrial MK may provide a challenge to industrial-scale AAB synthesis in certain countries. Thus, the utilization of abundantly found clays suggests a promising alternative. However, the potential of these clays to be used in the synthesis of AABs has not garnered substantial attention thus far compared to MK-based AABs. Currently, it can be observed that the majority of the calcined clay-based or calcined clay–BFS-based AAB mixes analyzed in the literature were manufactured using the conventional two-part mixing method, requiring an aqueous alkali source, whereas the one-part alkali activation of calcined clay and BFS-based AABs cured at ambient conditions has been minimally researched. The examined mechanical properties of non-MK clay-based AABs are often inferior, and the use of the two-part methodology and oven curing to enhance the mechanical properties diminish their feasibility and increase energy consumption. The current literature indicates that incorporating calcium modifiers or various minor additives can markedly enhance the physico-mechanical and durability properties of AABs based on FA or BFS. Nonetheless, the impact of different minor additives on the properties of calcined clay and BFS-based AABs remains unexplored. Thus, this study has sought to improve the mechanical, physical, and durability properties of calcined clay and BFS-based AABs utilizing several minor additives, including calcined magnesia (CM), aerial lime (AL), natural hydraulic lime (NHL), quicklime (QL), borax (BR), and powdered zeolite (ZP). The minor additives utilized in this study were selected based on their expected contributions to the physical, mechanical, and durability properties. Calcium sources (AL, NHL, and QL) were utilized to enhance the strength under ambient conditions, whereas CM, BR, and ZP were used to enhance durability and mitigate efflorescence. The one-part alkali activation and ambient curing methodologies are adopted to ascertain the feasibility, ease of production and effectiveness of various proportions of the above-mentioned additives, depending on their utilization ratio.

2. Materials and Methods

2.1. Raw Material Characteristics

The BFS and calcined clay were utilized as main precursors in the formulation of AAB mixes. Finely ground BFS was procured from OYAK Cement Concrete Paper Company, Istanbul, Türkiye. The ground calcined clay was sourced from CIMPOR, Ivory Coast plant. The calcined clay was obtained by incinerating the raw clay at 750 °C for about 40 min, followed by grinding it into a fine powder using a laboratory-type grinder. The oxide components of binders and minor additives obtained via X-ray fluorescence (XRF) are summarized in Table 1. The X-ray diffraction (XRD) patterns of precursors are shown in Figure 1. The XRD results indicate that BFS exhibits a significant level of amorphousness, whereas calcined clay demonstrates prominent peaks of quartz and some minor kaolinite peaks. The particle size distribution and sieve analysis of the precursors and minor additives are depicted in Figure 2a. The particle size distributions of the minor additives and the binders are comparable, with mean particle size (D50) values ranging between 4.5 and 11.2 µm. On the other hand, the D50 value of BR was 135 µm, indicating a coarser particle size distribution in comparison to other materials. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of raw clay and calcined clay are depicted in Figure 2b. Two prominent peaks located at around 75 °C and 520 °C were observed in the DTG curve of the raw clay, which correspond to the removal of physically and chemically bonded water from raw clay, respectively. The former peak represents the evaporation of loosely bound water from mineral surfaces, while the latter peak is associated with the removal of structural or chemically bound water, typically from the dehydroxylation of clay minerals like kaolinite. In addition, the TG and DTG curves of calcined clay demonstrate a successful calcination process exhibiting minor weight change and no notable peaks. The micro-morphology of binders was interpreted via scanning electron microscopy (SEM) analysis and the results are illustrated in Figure 3. Both BFS and calcined clay are characterized by their angular and irregular grain shapes.
Six different minor additives were used to enhance the properties of one-part calcined clay and BFS-based AABs. CM powder was procured from Kümaş Manyezit (Kütahya, Türkiye), whereas the AL, QL, BR, ZP, and NHL were sourced from the local market (Istanbul, Türkiye).
The sodium metasilicate anhydrous (SMS) powder, exhibiting a purity exceeding 95% and a silicate ratio (SiO2/Na2O) of around 1.0, was employed as the alkali source for the preparation of one-part AAB mixes. Siliceous sand with a Dmax of 2.36 mm as per EN 196−1 was utilized as the fine aggregate in the formulation of one-part mortar mixes. A high-range water-reducing admixture based on polycarboxylate ether was employed to attain an appropriate consistency.

2.2. Mix Design and Specimen Preparation

The design parameters for one-part AAB mortars are detailed in Table 2. The experimental investigation is conducted by adjusting the proportions of minor additives, specifically CM, AL, NHL, QL, BR, and ZP, while maintaining the other mix design parameters constant. Preliminary tests were conducted to identify the ideal amounts of the precursors and the activator. In these tests, the primary objective was to maximize the proportion of calcined clay in mortar formulations; however, it was noted that the utilization of calcined clay beyond 50% considerably impaired the workability of fresh mortar due to its higher water demand. Thus, the ratio of BFS to calcined clay was maintained at 1:1 by mass in all mixtures to ensure adequate mechanical strength development and appropriate fluidity. In a similar manner, it was determined that an activator dosage of 25% relative to the total weight of the precursors was optimal for achieving satisfactory mechanical strength. In addition, the ratios of sand to binder and water to binder were determined as 2.0 and 0.40, respectively. The superplasticizer was added to each mixture at a rate of 1.5% of the total binder to ensure sufficient flowability. The minor additives examined were integrated into the mixtures at different proportions ranging from 2% to 15% by mass of binders, as detailed in Table 2.
The one-part paste AAB samples were produced without siliceous sand to examine the microstructural properties while utilizing the same design parameters as in mortars.
At first, all powder components, including precursors, activator and sand, were meticulously dry mixed with a trowel. Subsequently, the powders and tap water were combined in a mortar mixer and stirred for a period of 3 min to achieve a uniform mixture. Following this, the freshly stirred mortar was poured into designated molds and compacted with a vibration table to reduce the presence of voids. The hardened specimens were demolded 24 h after casting and then kept in a regulated environment at a temperature of 22 ± 1 °C and a relative humidity of 55 ± 2% until the specified testing age.

2.3. Experimental Schedule

The experimental schedule included the assessment of fresh-state, physico-mechanical, and durability tests for a subset of mortar AAB specimens, while the microstructural properties of the mixes were examined on the selected paste samples. The mechanical, durability, microstructural and mineralogical properties were determined at 28 days, while the drying shrinkage of mortars was observed over a period of 56 days. Figure 4 provides a schematic representation that outlines the process of sample preparation and testing, accompanied by subsections that elaborate on the experimental procedures.
The setting times of paste mixtures incorporating varying amounts of minor additives were assessed in line with the ASTM C191 [48] standard. The penetration depths were documented at 10-min intervals, utilizing an automatic Vicat apparatus. The moment the Vicat needle penetrated to a depth of 25 mm in the specimen was documented as the initial setting time. The final setting time was recorded when the Vicat needle ceased to penetrate the AAP sample. The fluidity of mortar mixes was determined in accordance with the ASTM C1437 [49] standard. The spreading diameters of the fresh mortars on the flow table were measured after the table was dropped 25 times. The results are illustrated as the percentage increase in relation to the initial diameter of 10 cm.
The semi-adiabatic calorimeter test was conducted to observe the temperature changes and key temperature-related factors of the mixes, such as the duration to attain the peak temperature and the highest exothermic temperature [50]. The semi-adiabatic tests were performed on fresh paste samples with a water-to-binder ratio of 0.40. The binders, minor additives, and SMS were mixed in a bowl for 2 min and then transferred into cylinders with a diameter of 50 mm and a height of 100 mm. The mold was placed into the insulating container, and a K-type thermocouple was promptly immersed in the fresh paste mixture. Temperature measurements were recorded for approximately one day in a climate-controlled room.
The compressive strength of hardened AAB specimens with the dimensions of 50 × 50 × 50 mm3 was tested at 3, 7, and 28 days. The loading rate was selected as 1.5 kN/s in compliance with the ASTM C109 [51] standard, and the averages for the three identical samples were reported as the compressive strength.
The water absorption characteristics of AAB specimens were measured at 28 days of curing age as per ASTM C642 [52] standard. The hardened mortar specimens were first oven-dried until constant weight (A) and then fully soaked in water for 48 h. The saturated and surface-dried (SSD) mass of mortars was recorded after the water immersion process (B). Finally, the specimens were kept in boiling water for 5 h. After the boiling-immersion process, the SSD mass (C) and submerged mass (D) of mortar specimens were recorded, and the water absorption (WA) and volume of permeable pores (VoPP) were calculated using Equations (1) and (2).
W A % = B − A A × 100  
V o P P % = C − A C − D × 100
The drying shrinkage of mortars was assessed using prisms measuring 25.4 × 25.4 × 285 mm3 in accordance with the ASTM C596 [53] standard. The AAB samples were demolded 24 h after production, and the initial length measurement was promptly conducted using a length comparator with a precision of ±0.001 mm. Subsequently, the specimens were stored in a humidity-regulated cabinet maintained at 22 ± 2 °C and 55 ± 2% relative humidity. The following measurements were performed weekly over a duration of 56 days. The length change readings were taken on two replicate prisms for each mix, and the average results were documented.
The efflorescence of AABs was evaluated both quantitatively and qualitatively on three identical cubic mortar specimens based on the referenced studies [54,55]. As shown in Figure 5, AAB samples were immersed in water at a depth of 3–5 mm under laboratory conditions of 22 ± 2 °C and 60 ± 5% relative humidity. The periodic efflorescence formation on the specimens’ surfaces was visually monitored after 1 h, 3 h, 6 h, 1 day, 3 days, 5 days, and 7 days. Subsequently, to evaluate the intensity of efflorescence, the white efflorescence deposit was scraped from the surface of the specimens, and their mass values were recorded.
The resistance of mortar specimens to chloride migration was assessed at 28 days following the NT Build 492 [56] standard. The non-steady-state migration coefficient (Dnssm) of specimens was determined using cylindrical disc specimens of 50 mm in height and 100 mm in diameter. The mortar slices were initially preserved in a container at a vacuum pressure of 50 mbar for 3 h, followed by complete immersion in lime-saturated water for 18 h. After the preconditioning process, the mortars were positioned in the testing apparatus, which consisted of the catholyte (10% NaCl) and the anolyte (0.3 N NaOH) solutions. A preliminary voltage of 30 V was supplied to the mortar samples to ascertain the test voltage and duration based on the initial current measurement. Following the exposure period, the specimens were axially bisected, and a 0.1 M silver nitrate solution was applied to the exposed surfaces of the mortars. The penetration depth from the catholyte surface was assessed using the visible white silver hue, and the mean depth was determined. The test results were expressed by calculating the Dnssm (×10−12 m2/s), as specified in Equation (3).
D n s s m = 0.0239 273 + T L U − 2 t × x d − 0.0238 273 + T L x d U   −   2  
Here, T (°C) denotes the temperature of the anolyte solution, while U (V) and t (h) signify the applied voltage and its duration of exposure, respectively. Additionally, L (mm) and xd (mm) illustrate the height of the mortar sample and the average penetration depth of chloride ions, respectively.
The mineralogical and microstructural characteristics of one-part AAB mixes were determined on paste samples, utilizing XRD and SEM analysis. The freshly prepared paste samples were placed in plastic falcon tubes and kept in an ambient condition until 28 days of curing age. The PANalytical X’Pert PRO diffractometer (Almelo, Netherlands), utilizing copper tube radiation, was employed for the XRD analysis with a step size of 0.017. The diffraction patterns were plotted within the scan range of 5–70° (2ϴ). Furthermore, the micromorphology of pastes was scanned utilizing the Zeiss EVO® LS 10 SEM instrument (Jena, Germany) coupled with energy-dispersive spectroscopy (EDS).

3. Results and Discussion

3.1. Setting Time and Flowability

The influence of minor additives on the initial and final setting times of AABs is illustrated in Figure 6a. The initial setting time of fresh paste mixtures varied between 4 min and 370 min, whereas the final setting times spanned from 6 min to 480 min. The addition of 15% AL, NHL, QL, and ZP resulted in reductions in final setting times by 75%, 61%, 98%, and 28% respectively, in comparison to the REF sample. Accordingly, ZP marginally decreased the setting time by offering nucleation sites for additional gelation due to its fine particles and high specific surface area, as presented in Figure 2 [57,58]. On the other hand, AL, NHL or QL had a more pronounced impact on the setting time by engaging directly in chemical reactions. The interaction of QL with water yields exothermic slaking reactions and produces calcium hydroxide, resulting in an alkaline environment favorable for chemical reactions. Additionally, Ca2+ ions are released into the pore solution and play a crucial role in the setting phenomena of AABs. Ca2+ ions precipitate into calcium aluminosilicate hydrate (C-(A)-S-H) gel during hydration, accelerating setting time and early strength, whereas silicon ions (Si4+) and aluminum ions (Al3+) from precursors and sodium metasilicate can enhance the reaction degree of the system [59]. These reactions significantly elevated the inner temperature of specimen and accelerated the plasticization (see Figure 7). Similarly, the fast-setting characteristic with the addition of 15% AL and NHL is due to the additional alkalinity (CaOH from AL) and calcium silicates they provided, respectively.
The incorporation of BR consistently prolonged the setting time of AABs. The REF mixture completed its setting in 250 min; however, with the addition of 5% BR, the final setting time increased to about 480 min, representing an increase of approximately 1.92 times. The effect of anhydrous borax on the setting and hardening characteristics of AABs mainly changes depending on the calcium and aluminosilicate contents of the precursors. This study reveals that the use of both calcium (i.e., BFS) and an aluminosilicate source (i.e., calcined clay) as primary binders in equal proportions resulted in the setting-retarding effect of BR manifesting in two distinct manners. Initially, the borate ions interacted with the dissolved calcium ions to create a calcium borate complex, characterized by a semi-permeable structure [46]. This semi-permeable structure envelopes the BFS particles and decelerates the reactions. Simultaneously, borate ions interact with [SiO4] tetrahedra to create Si-BR oligomers, thereby diminishing the availability of free [SiO4] tetrahedra [60]. Thus, elevating the dosage of BR leads to a reduction in the early reaction capacity of BFS, while simultaneously decreasing the concentration of free [SiO4] in the hybrid precursor system, resulting in a consistent prolongation of the setting time of AABs. It is also noteworthy that the addition of BR exceeding 10% was found to be inappropriate, as it prolonged the setting time of pastes to as much as 24 h. Consequently, the utilization of BR was restricted to 5%.
Figure 6b shows the fluidity results of mortars. The flow percentages of mixes varied in a narrow range between 32% and 40% (corresponding to an 8 mm difference in spread diameter). It is generally observed that an increase in the minor additive ratio reduced the workability of mortars. However, this reduction did not cause any handling or consolidation problems.

3.2. Semi-Adiabatic Calorimetry

The temperature profiles of the selected paste mixtures are illustrated in Figure 7. The semi-adiabatic profiles of the mixes exhibited three distinct phases. The initial portion of the curves, denoted as “1” in Figure 7, represents the quick dissolving of SMS in water, resulting in considerable heat release from the formation of soluble silica species. Consequently, the internal temperature of the pastes experienced a rapid escalation within the initial minutes of mixing, sustaining this climb for around 10 min across all mixes. Subsequently, a slower acceleration phase emerged (indicated by “2” in Figure 7), marked by a progressive increase in temperature, indicating the starting of exothermic reactions due to the binder’s dissolution. The acceleration phase concluded with a singular exothermic peak, signifying the wetting and dissolving of precursors, as well as the development of hydration products [50,61,62]. Finally, the deceleration phase (indicated by “3” in Figure 7) began, and the internal temperature of specimens progressively decreased following the exothermic peak, ultimately reaching ambient temperature within 1 day.
The characteristics of the initial phase primarily rely on the SMS dosage, regardless of the minor additives. A previous study similarly reported that the presence of SMS in one-part AAB mixes caused a rapid rise of temperature [62]. In the initial 10 min, the temperature increased to the 45–50 °C range across all mixes. Upon the shift to phase “2” on the curve, a gradual increase ensued, with peak temperatures ranging between 53 °C and 59 °C. On the other hand, in the QL-containing paste mixture, the transition from phase “1” to phase “2” was not distinctly observable, and the temperature escalated rapidly from ambient temperature to a peak of 111.6 °C within about 26 min. This results from the exothermic slaking reaction that transpires when QL interacts with water along with the dissolution of SMS. The significant temperature rise noted in paste combinations with QL clearly validates their characteristics of rapid setting, elevated early strength, and the prompt achievement of ultimate strength values. This scenario resembles the heat-cured geopolymer mixes, where the strength values at 7 days and 28 days are nearly identical [63,64].

3.3. Compressive Strength

Figure 8 plots the compressive strength development of mortar mixes. The compressive strength values varied from 9.8 to 25 MPa at 3 days and 16.5 to 39 MPa at 7 days. At early ages (3 and 7 days), it was observed that the influence of QL, AL, and BR on the compressive strength was more pronounced compared to other minor additives. Accordingly, the compressive strengths of mixtures containing 15% AL and QL were found to be 49% and 79% higher than that of the REF mixture at 3 days, respectively. The improvement ratios at 7 days were 24% for AL15 and 88% for the QL15 mixture, indicating a superior early-age reaction capacity achieved by incorporating AL and QL. The prominent contributions of AL and QL to the early-age strength of AABs were mainly due to the increased pH of the pore solution and additional calcium ion promotion. When AL (Ca(OH)2) is added into the mixture, the alkalinity of the mortar system increases, which accelerates the dissolution of BFS and clay particles into Ca2+, Si4+, and Al3+ ions to form strength-giving gel structures [65,66]. Similarly, when QL reacts with water, it forms calcium hydroxide through an exothermic reaction. This creates a highly alkaline environment and additional calcium ions that promote strength improvement. Furthermore, the simultaneous hydration process of sodium metasilicate and QL following mixing leads to increased reaction heat (Figure 7), which enhances ion diffusion and gel formation similarly to heat-cured geopolymer mixtures. Unlike QL and AL, the inclusion of BR reduced the early age compressive strength of mortars. The 3-days compressive strength of Br5 was found to be 30% less than that of the REF mortar. The situation arises from the distinct responses that BR induces in the initial phases with calcium and silicon ionic species. The presence of BR diminishes strength at an early age due to its effect on the availability of free silicon in the pore solution [60]. It also creates a semi-permeable film on the surfaces of BFS particles, which slows down hydration by obstructing water contact and binding essential cations that are crucial for strength-giving gel formation [45].
The 28-days compressive strength of mortars ranged between 28.7 MPa and 50.9 MPa. The results indicate that all minor additives, except for BR, enhanced the later age compressive strength of mortars compared to the reference. In addition, the increase in the amount of CM, AL, NHL, and ZP consistently improved the later age compressive strength, whereas the mortars with QL showed an optimal utilization dosage. For example, the compressive strengths of REF, CM10, AL15, NHL15, QL15, ZP15, and Br5 mortars at 28 days were 32.9 MPa, 37.3 MPa, 42.4 MPa, 34.8 MPa, 41.0 MPa, 38.5 MPa, and 29.4 MPa, respectively. The contribution of NHL to compressive strength was minimal, primarily due to the supplementary silicate structures it introduces and the reactions of C2S [67]. Meanwhile, CM and ZP enhance the later-age strength of mortars by offering additional nucleation sites, attributed to their fine particle size distribution [68,69].
Notably, a 10% inclusion of QL yielded the highest compressive strength at 28 days (50.9 MPa), with a 55% increment compared to REF mortar. Beyond this range (i.e., 15% QL), the high amount of heat released during the early hours, resulting from the reactions of QL and sodium metasilicate with water, generates a reaction ring on the binder particles, which constitutes a diffusion barrier on the precursor particles [70,71]. This reaction ring hinders the progress of polycondensation reactions, and prevents further strength gain in AAB samples.
Figure 9 illustrates the t-test analysis of compressive strength results of investigated mortars compared to REF mixture. A high t-value signifies a more substantial effect, whereas the sign of this value indicates whether the associated minor additive leads to an enhancement or a decline in compressive strength compared to the REF mixture, respectively. Accordingly, AL15, NHL15, QL10, and QL15 mortars exhibited rather high positive impacts on the early age compressive strength of AABs. The negative impact of 5% BR on early age strength is also shown in Figure 9a. Upon analysis of the t-values from the 28-day compressive strength tests, it was observed that mixtures with elevated levels of QL, AL, and ZP exhibited a notable impact compared to other minor additives. Figure 9b shows the p-values for compressive strength test results. A p-value below 0.05 indicates a statistically significant effect, whereas a p-value above this value suggests the absence of a significant effect. Statistically significant effects of NHL-containing mortars on early-age compressive strength were noted; however, no significant effects were found at later ages. In the case of CM addition, no statistical significance was noted in compressive strength. In addition, the high p-value (>0.05) observed in the Br5 mixture at 28 days indicates that this mixture does not cause a statistically significant decrease in strength at a later age.

3.4. Physical Properties

Figure 10 presents the water absorption and volume of permeable pore (VoPP) percentage of one-part AABs. The water absorption and VoPP values ranged between 2.1 and 3.7% and 5.5 and 10.1%, respectively. The REF mortar demonstrated a water absorption rate of 3% and a VoPP of 10.1%, while the Br5 mixture showed the lowest values with 2.1% water absorption and 6.2% VoPP. Similar to BR-containing mortars, QL5 and QL10 mortars showed superior water absorption of 2.2% and 2.4%, respectively. The superior absorption characteristic of QL-containing mortars is due to the dense microstructure and higher gel formation with an optimal Ca/Si molar ratio (see Figure 8 and Section 3.4 for further details). The Ca/Si atomic molar ratio in the range of 0.8–1.2 restricts the connection of the pore network and limits water ingress by providing optimal polymerization in AAB samples containing the calcium source [72,73]. On the other hand, the lowest water absorption observed in mortar mixes incorporating BR can be credited to the refined pore structure and increased tortuosity [45]. [BO4], produced from borax hydrolysis, exhibits analogous coordination properties to [SiO4] and [AlO4] in the alkali activation, and augments the complexity and polymerization of gel structures [74]. This phenomenon probably increases gel tortuosity and reduces the water absorption in BR-containing mortars.
Figure 11 depicts the drying shrinkage evolution of mortars with elapsed curing days. The time-dependent drying shrinkage of mortars indicated a rapid rise up to 14 days. Beyond this age, the drying shrinkage slightly increased. The shrinkage values of mortars recorded after 56 days varied between 2598 µɛ and 8192 µɛ. The REF mortar exhibited a 56-days drying shrinkage of 4916 µɛ. The incorporation of smaller quantities of CM, AL, BR, and NHL did not notably affect the final shrinkage, which varied between 4666 µɛ and 5012 µɛ. On the other hand, incorporating 2% QL (2598 µε) and 10% CM (3040 µɛ) reduced the final shrinkage by 47% and 38%, respectively, when compared to the REF mixture. The beneficial effect, realized through the incorporation of a suitable quantity of QL alongside an adequate amount of CM, arises from the substantial reaction products generated by the interactions of these two components. The formation of reaction products like Ca(OH)2 through the addition of CaO-QL, and Mg(OH)2 and hydrotalcite-like phases from the addition of CM, leads to a volume increment that counteracts the volume reduction, thereby effectively minimizing shrinkage [75,76,77,78].
In contrast, the mixes AL15, Br5, QL10, QL15, and ZP15 demonstrated higher shrinkage deformation than the REF mortar after a period of 56 days. The observed higher shrinkage values of these lime-rich mortars (AL15, QL10, and QL15) are due to the excessive calcium content, which disrupts the stable geopolymer gel formation for volume stability. The previous studies also reported that the addition of excessive CaO or Ca(OH)2 can lead to the generation of inadequately polymerized gel structures [79,80]. The highest shrinkage values among the investigated mortars were found in ZP5, ZP10, and Br5 with a shrinkage deformation of greater than 7400 µɛ. The inferior effect of ZP on shrinkage behavior is possibly due to its porous nature and increased water demand, which creates additional porosity and capillary tension in the mortar specimen. Moreover, the intricate pore structure present in zeolite particles provides supplementary capillary pathways throughout the drying process [58,81,82]. Similarly, the higher shrinkage of Br5 mortar is primarily attributed to the reduced early age mechanical strength and stiffness (Figure 8).

3.5. Efflorescence

The efflorescence potential of AAB mixes and its impact on the mechanical properties of mortars were assessed by quantifying the deposit mass on the mortar surface and the residual compressive strength values. Table 3 illustrates the evolution of efflorescence in the mixtures over time and the quantities of deposit mass accumulated. During the initial 6 h of the test, no efflorescence was observed. The first signs of efflorescence were noted at 1 day in all mixtures, with the exception of those incorporating ZP and BR. In addition, the emergence of efflorescence in ZP-containing mortars was initially noted after 3 days, whereas in Br2 and Br5 mixes it was observed after 5 and 7 days, respectively. As shown in Table 3, the deposit mass values ranged between 0.005 g and 4.143 g. Figure 12 illustrates that the minimum and maximum deposit mass were observed in Br5 and QL5 mortars, respectively; meanwhile, REF, CM5, CM10, AL5, and AL10 mixtures demonstrated moderate levels of efflorescence, with a deposit mass range of 2.626 to 3.069 g.
The impact of NHL and QL additions on the severity of efflorescence was notably affected by the quantities utilized, with efflorescence formation often comparable to or exceeding that of the REF mixture. Accordingly, the introduction of 5% NHL led to a 25% reduction in deposit mass, whereas the incorporation of 15% NHL resulted in a 17% increase in deposit mass. On the other hand, employing 5% QL resulted in a discernible increase in deposit mass by 1.6 times, whereas utilizing 15% QL led to a reduction of 14%, as shown in Figure 12. The distinct efflorescence behavior noted with the inclusion of NHL and QL can be elucidated in the following manner.
The phenomenon of efflorescence in AABs is fundamentally distinct from that observed in cement-based systems. In the context of Portland cement, efflorescence mainly results in the formation of calcium carbonate, whereas in AABs, the predominant efflorescence products are sodium carbonate hydrates (Na2CO3.xH2O). Initially, the free sodium ions from the pore solution leach the surface of AAB specimens, which is subsequently followed by atmospheric carbonation and crystallization that leads to the formation of sodium carbonate hydrate complexes [83]. Thus, it is suitable to assume that the intensity of efflorescence is significantly influenced by the presence of free Na+ ions, the characteristics of pore structure, and the mechanisms of moisture transport in the AAB system [54]. The incorporation of NHL into the mixture at suitable proportions can refine the pore structure and minimize efflorescence by facilitating the development of an additional C-(N)-A-S-H gel, potentially containing sodium, due to the hydration of calcium silicate structures. Simultaneously, the inclusion of excessive NHL, owing to its Ca(OH)2 formation, may diminish the effectiveness of alkali activation reactions, resulting in a greater availability of free alkalis for efflorescence. Consequently, the addition of 5% NHL was determined to be more effective compared to 15% NHL for enhancing the efflorescence performance of the AAB system. In a manner akin to the circumstances observed in the NHL, the incorporation of QL leads to the formation of Ca(OH)2, which markedly enhances the overall alkalinity of the system. The improved alkalinity can elevate the solubility and movement of sodium compounds, facilitating increased sodium transfer to surfaces. Conversely, with increased QL contents, the presence of calcium ions assures that the majority of carbonate species interact with calcium instead of sodium, thereby effectively obstructing pathways for sodium efflorescence. It has also been demonstrated that the formation of calcium carbonate is thermodynamically favored compared to sodium carbonate when there is an adequate supply of calcium [84]. Moreover, as illustrated in Figure 8, the mortars QL10 and QL15 exhibited increased compressive strength, attributed to their densified matrix that partially limits the movement of sodium ions. Consequently, the balance of alkalinity, calcium content and pore structure in these mixtures controls the efflorescence performance of QL-containing AABs.
The BR or ZP addition was found to be beneficial in reducing the efflorescence severity of AABs. Accordingly, the incorporation of 2% and 5% BR was observed to nearly eliminate efflorescence in the mortar, whereas the inclusion of 5% and 15% ZP demonstrated a reduction in deposit mass formation by 41% and 59%, respectively. The fundamental mechanism underlying the BR addition entails the creation of intricate borate networks capable of incorporating and immobilizing Na+ ions. Previous studies indicate that [BO4]− tetrahedra derived from borax exhibits analogous coordination properties to [SiO4] and [AlO4] units within the AAB gel framework, resulting in the development of C-S(B)-H gel structures where boron atoms integrate into the C-S-H matrix [74]. This incorporation results in enhanced cross-linked, more polymerized, and stable gel structures that are more effective in retaining sodium ions within their framework, preventing their migration to surfaces [45,47]. Furthermore, the incorporation of BR leads to decreased pore connectivity as in Figure 10, effectively limiting sodium ion transportation and enhancing the efflorescence performance of mortars. Natural zeolites are noted for their remarkable ion exchange properties, attributed to their crystalline aluminosilicate framework structure [85]. Their elevated cation exchange capacities enable them to selectively bind and immobilize Na+ ions from the pore solution, effectively diminishing the severity of efflorescence in AAB systems.
Figure 13 and Figure 14 visualize the evolution of efflorescence deposits on the alkali-activated mortar surfaces. Except for the region in contact with water, the development of white crystalline deposits on all sides of the sample evolved over time, demonstrating a notable increase. Particularly, it was observed that these crystals were almost negligible within the Br5 mixture. Consequently, it was found that BR and ZP could effectively mitigate or eradicate this problem in BFS-calcined clay-based AABs. Conversely, it has been observed that calcium sources like QL, AL, and NHL warrant careful consideration regarding their usage in terms of efflorescence formation.

3.6. Chloride Migration Resistance

The resistance to chloride migration in one-part AAB specimens was assessed through non-steady state migration (Dnssm) coefficients, with higher values indicating a reduced resistance to chloride migration. The variations in Dnssm values of mortar mixtures incorporating various minor additives are illustrated in Figure 15. The highest Dnssm value was observed in the REF mortar at 45.4 × 10−12 m2/s. In mortars with higher amounts of AL, QL, or BR, the Dnssm values were consistently low, varying between 8.4 × 10−12 m2/s and 11.5 × 10−12 m2/s. Among these mixtures, the mortar containing 15% QL demonstrated the lowest chloride migration coefficient.
The incorporation of CM, AL, QL or BR consistently improved the chloride migration resistance of mortars. Accordingly, the Dnssm values of CM10, AL15, QL15, and Br5 mortars were found to be 48%, 76%, 81%, and 75% lower compared to the REF mortar, respectively. The positive effect of CM on the resistance of mortars to chloride migration arises from the development of hydrotalcite-like phases resulting from the polymerization of CM. The development of substantial hydrotalcite-like phases effectively sequesters chloride ions and partially occupies the pores, thereby limiting the subsequent entry and mobility of chloride species [86,87]. The beneficial impacts of AL and QL can be attributed primarily to the enhanced gelation, increased strength, and minimized void contents that lead to matrix densification (refer to Figure 8). On the other hand, the enhanced resistance to chloride observed with the incorporation of 5% BR in the mortars can be explained by variations in the composition of the gel structure (Br-containing gel structure) and resulting more tortuous pore structure, rather than the increased matrix densification noted in AL and QL. The increased tortuosity, resulting from the addition of borax, as also observed in the water absorption test (refer to Figure 10), complicated the transport of water and chloride ions within the mortar, leading to a notable decrease in the Dnssm value.
The incorporation of NHL or ZP, similar to other minor additives, enhanced the mortar’s resistance to chloride ingress; however, raising the proportion of NHL or ZP from 5% to 15% did not result in a notable alteration in Dnssm values. The observed moderate positive influence of NHL or ZP at optimal ratios on chloride resistance arises from the supplementary silicate content and the nucleation site characteristics they contribute, respectively. Optimal levels of NHL, combined with a suitable increase in alkalinity and silicate reactions, effectively restrict chloride ion transport. Furthermore, the incorporation of ZP may enhance the tortuosity of the void structure with its fine crystal structure.

3.7. Microstructural Investigations

Figure 16 depicts the crystalline phases of hardened paste specimens at 28 days of curing age. The incorporation of minor additives resulted in slight alterations in the mineral composition of the mixtures, with gismondine (COD: 9016969) and quartz (COD: 9010146) consistently identified across the diffraction patterns in all series. The presence of quartz is mainly due to the partially reacted clay particles, whereas the detection of gismondine is due to the alkali activation of BFS. It was previously reported that the main crystalline reaction product of BFS-containing AAB mixes is gismondine [88], and it is stable in the presence of low Ca-tobermorite, such as in C-S-H, silica, and C-(A)-S-H phases [89]. The incorporation of CM resulted in the formation of additional hydrotalcite (COD: 9009272) structures located at approximately 11.5–12° (2Ɵ) and 60° (2Ɵ). Moreover, the AL15 mixture exhibited peaks for portlandite (COD: 7020138) and calcium silicate hydrtate (CSH) (COD: 9001689) around 18° (2Ɵ) and 34° (2Ɵ), which were not present in the REF mixture. These findings corroborate the positive effect of AL on strength development and reactions provided by the additional calcium content and alkalinity. The inclusion of NHL and QL led to the identification of additional peaks, including gismondine, calcite (COD: 1010962), and portlandite, unlike in the REF mixture. However, the mineral compositions of mixtures with ZP or BR closely resemble that of the REF mixture, and these did not yield secondary reaction products as in AL-, QL-, or NHL-containing mixtures.
Figure 17 and Figure 18 depict the microstructural images of AAB specimens. The SEM image of the REF paste exhibits a gel-rich microstructure. CM10, NHL15, ZP15, and Br5 had similar micromorphologies with a dense microstructure. On the other hand, the inclusion of AL led to the formation of additional gel clusters in the vicinity of the voids. As shown in Figure 18a, the QL10 mixture exhibited a highly dense structure. This corroborates the improved strength and durability characteristic of QL10 mortar compared to other mortars.
The atomic molar ratios of Ca/Si and Si/Al derived from AAB matrices via EDS analysis are shown in Table 4. The ratios of Ca/Si and Si/Al in the mixtures were within the ranges of 0.45–0.93 and 2.35–3.20, respectively. The REF, CM10, and ZP15 matrices exhibited comparable Ca/Si and Si/Al molar ratios, suggesting that CM and ZP influence the properties through physical effects such as nucleation and the formation of secondary reaction products, rather than altering the gel structure itself. Conversely, the incorporation of AL, QL, and NHL, which possess elevated amounts of calcium oxide, resulted in an increase in the Ca/Si ratios of the matrix, while the Si/Al ratios experienced a gradual decrease. The Ca/Si molar ratios of AL15 and QL10 mixtures varied between 0.8 and 1.0, a range commonly observed in C-A-S-H-type gel structures within AAB matrices [72,73]. These findings verify the lower water absorption, increased strength, and better resistance to chloride penetration observed in these mixes.

4. Conclusions

This paper focused on the development of one-part alkali-activated mortars utilizing calcined clays and BFS. The binders were activated with sodium metasilicate to produce AAB samples. The properties of these AABs were improved by incorporating various minor additives, including calcined magnesia (CM), aerial lime (AL), natural hydraulic lime (NHL), quicklime (QL), borax (BR), and powdered zeolite (ZP). The performances of these mixtures were assessed via mechanical, physical, and durability tests, alongside microstructural analyses and an assessment of their efflorescence potential. The key findings and major observations of this research are outlined below:
  • The incorporation of CM resulted in the development of secondary voluminous reaction products like hydrotalcite, which consistently enhanced the compressive strength and chloride resistance of mortars, while simultaneously decreasing water absorption and drying shrinkage. On the other hand, it was found that CM had a negligible influence on the setting time and efflorescence characteristics of mortars;
  • The physicomechanical properties of AABs were generally positively influenced by AL and NHL, which promote the entry of additional alkalinity and calcium ions into the microstructure. Accordingly, the addition of AL or NHL resulted in a moderate enhancement of the compressive strength (up to 29%), water absorption, and chloride penetration properties of mortars, while also shortening the setting time by 75% and 61% at a 15% AL and NHL content, respectively;
  • The test results indicate that the incorporation of QL emerged as the most effective among the minor additives regarding its influence on the characteristics of AAB mixtures. Even at 2% QL addition, the mixtures exhibited a remarkable reduction in setting time and drying shrinkage, achieving decreases of 62% and 47%, respectively. The optimal ratio identified was a 10% addition, which resulted in a 55% increase in strength development, an 82% decrease in the Dnssm coefficient, and a 20% decrease in water absorption. Incorporating over 10% QL resulted in an excessively rapid reaction of the mixture, and the elevated hydration heat subsequently impeded strength development at later ages;
  • The incorporation of BR markedly enhanced water absorption and resistance to chloride penetration. The mixtures containing BR, which are capable of limiting sodium ions within their framework, showed efflorescence diminished to almost negligible levels. Similarly, ZP addition reduced the efflorescence formation on mortar surfaces due to its elevated cation exchange capacity;
  • Overall, the results of this research indicate that the mechanical, physical, and durability characteristics of calcined clay and BFS-based AABs can be enhanced with the use of minor additives even at low ratios. Thus, employing QL, AL, NHL, and CM in suitable proportions generally enhances strength and durability characteristics, whereas ZP and BR effectively mitigate the intensity of efflorescence without compromising strength and physical properties. Nevertheless, it was noticed that the excessive usage of QL or BR led to quick and prolonged setting behaviors, respectively, which restrict their usage in specific civil engineering applications. QL, characterized by its outstanding early strength and quick setting capabilities, is particularly well-suited for applications requiring rapid repair. In contrast, the other minor additives analyzed are appropriate for civil engineering uses, including screed and plastering, owing to their well-balanced setting times and material characteristics. Subsequent investigations may explore the efficacy of calcined clay and BFS-based one-part AABs with QL as repair mortars, and preliminary trials could be carried out for various field applications involving each minor additive.

Author Contributions

S.Ç.: Conceptualization, funding acquisition, resources, project administration, formal analysis, supervision. T.O.: Methodology, software, investigation, validation, visualization, writing—original draft preparation. H.O.: Methodology, resources, conceptualization. N.K.: Methodology, formal analysis, investigation, writing—review and editing, data curation, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was funded by CIMPOR Serviços, S.A.

Data Availability Statement

The data of this research are available from the corresponding author upon reasonable request.

Acknowledgments

The authors acknowledge Deniz Sarıalioğlu from OYAK Cement group for providing the raw materials, and CIMPOR Serviços, S.A. for their financial support for the APC.

Conflicts of Interest

Author Suat Çalbıyık was employed by the company CIMPOR Serviços, S.A. Author Hakan Ozkan was employed by the company Betão Liz, S.A. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AABAlkali-activated binder
CMCalcined magnesia
ALAerial lime
NHLNatural hydraulic lime
QLQuicklime
ZPZeolite powder
BRAnhydrous borax
DnssmNon-steady state chloride migration coefficient
BFSBlast furnace slag
FAFly ash
LC3Limestone calcined clay cement
MKMetakaolin
C-A-S-HCalcium aluminate silicate hydrate
N-A-S-HSodium aluminate silicate hydrate
XRDX-ray diffraction
XRFX-ray fluorescence
SEMScanning electron microscopy
EDSEnergy dispersive spectroscopy
SMSAnhydrous sodium metasilicate
WAWater absorption
VoPPVolume of permeable pores
Na2CO3.xH2OSodium carbonate hydrates

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Figure 1. XRD patterns of (a) binders and (b) minor admixtures.
Figure 1. XRD patterns of (a) binders and (b) minor admixtures.
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Figure 2. (a) The particle size distribution of binders and minor additives, (b) TG and DTG analysis of raw and calcined clay.
Figure 2. (a) The particle size distribution of binders and minor additives, (b) TG and DTG analysis of raw and calcined clay.
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Figure 3. The microsructural morphology of (a) BFS and (b) calcined clay.
Figure 3. The microsructural morphology of (a) BFS and (b) calcined clay.
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Figure 4. Flow chart of experimental study.
Figure 4. Flow chart of experimental study.
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Figure 5. Schematic demonstration of efflorescence test setup.
Figure 5. Schematic demonstration of efflorescence test setup.
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Figure 6. (a) The setting time results of paste mixes, (b) the fluidity of mortar mixes.
Figure 6. (a) The setting time results of paste mixes, (b) the fluidity of mortar mixes.
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Figure 7. The temperature evolution of paste mixes under semi-adiabatic conditions.
Figure 7. The temperature evolution of paste mixes under semi-adiabatic conditions.
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Figure 8. The strength development of mortars.
Figure 8. The strength development of mortars.
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Figure 9. The t-test analysis of compressive strength results: (a) t-values and (b) p-values.
Figure 9. The t-test analysis of compressive strength results: (a) t-values and (b) p-values.
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Figure 10. The water absorption and VoPP of mortars.
Figure 10. The water absorption and VoPP of mortars.
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Figure 11. The drying shrinkage results of mortars.
Figure 11. The drying shrinkage results of mortars.
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Figure 12. The effect of minor additives on the efflorescence of AABs relative to the reference.
Figure 12. The effect of minor additives on the efflorescence of AABs relative to the reference.
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Figure 13. The efflorescence formation on the mortar surfaces.
Figure 13. The efflorescence formation on the mortar surfaces.
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Figure 14. Visual appearance of AAB specimens before and after efflorescence.
Figure 14. Visual appearance of AAB specimens before and after efflorescence.
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Figure 15. The chloride migration coefficient of mortar specimens.
Figure 15. The chloride migration coefficient of mortar specimens.
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Figure 16. The diffraction patterns of one-part paste AABs.
Figure 16. The diffraction patterns of one-part paste AABs.
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Figure 17. SEM images of hardened paste specimens: (a) REF, (b) CM10, (c) AL15, and (d) NHL15.
Figure 17. SEM images of hardened paste specimens: (a) REF, (b) CM10, (c) AL15, and (d) NHL15.
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Figure 18. SEM images of hardened paste specimens: (a) QL10, (b) ZP15 and (c) Br5.
Figure 18. SEM images of hardened paste specimens: (a) QL10, (b) ZP15 and (c) Br5.
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Table 1. The chemical oxide percentages of binders and additives.
Table 1. The chemical oxide percentages of binders and additives.
Oxide
Composition (%)		BFSCalcined ClayCMALNHLQLZPBR
SiO237.6568.234.401.311.750.6465.39-
Al2O313.6120.110.100.63.590.1711.31-
Fe2O30.712.740.600.31.550.091.45-
CaO27.451.795.0070.152.4285.363.47-
MgO11.240.3789.901.32.910.841.15-
SO30.510.15-0.61.431.010.03-
K2O1.011.68-0.20.900.043.46-
Na2O0.590.74-0.00.160.090.3030.41
TiO2-1.96-0.50.330.351.26-
P2O5-0.05-0.10.060.020.03-
Cr2O3--------
Mn2O34.250.03--0.04-0.05-
ZnO-0.01------
SrO0.080.01--0.070.050.33-
B2O3-------68.42
Cl-0.12-0.10.09-0.06-
LOI *2.551.100.8025.424.5511.7012.630.23
*: Loss on ignition.
Table 2. The mix design parameters of one-part mortar specimens (g/l).
Table 2. The mix design parameters of one-part mortar specimens (g/l).
Mixture CodeBFSCalcined ClaySMSWaterSandSuperplasticizerMinor Additive
REF325.7325.7162.8260.513009.8-
CM5325.7325.7162.8260.513009.832.6
CM10325.7325.7162.8260.513009.865.1
AL5325.7325.7162.8260.513009.832.6
AL15325.7325.7162.8260.513009.897.7
NHL5325.7325.7162.8260.513009.832.6
NHL15325.7325.7162.8260.513009.897.7
QL2325.7325.7162.8260.513009.813.0
QL5325.7325.7162.8260.513009.832.6
QL10325.7325.7162.8260.513009.865.1
QL15325.7325.7162.8260.513009.897.7
ZP5325.7325.7162.8260.513009.832.6
ZP15325.7325.7162.8260.513009.897.7
Br2325.7325.7162.8260.513009.813.0
Br5325.7325.7162.8260.513009.832.6
Table 3. The presence of efflorescence on mortar surfaces (“×” indicates no efflorescence formation, “✓” indicates efflorescence formation).
Table 3. The presence of efflorescence on mortar surfaces (“×” indicates no efflorescence formation, “✓” indicates efflorescence formation).
Mixture Code1 h3 h6 h1 Day3 Days5 Days7 DaysDeposit Mass (g)
REF×××✓✓✓✓2.626
CM5×××✓✓✓✓2.779
CM10×××✓✓✓✓2.977
AL5×××✓✓✓✓3.028
AL15×××✓✓✓✓3.069
NHL5×××✓✓✓✓1.957
NHL15×××✓✓✓✓3.913
QL2×××✓✓✓✓2.568
QL5×××✓✓✓✓4.143
QL10×××✓✓✓✓2.993
QL15×××✓✓✓✓2.256
ZP5××××✓✓✓1.548
ZP15××××✓✓✓1.087
Br2×××××✓✓0.478
Br5××××××✓0.005
Table 4. EDS analysis results of paste samples.
Table 4. EDS analysis results of paste samples.
Mixture CodeEDS SpotONaMgAlSiCaCa/Si *Si/Al *
REF148.729.610.986.9322.53110.52 ± 0.032.99 ± 0.18
247.0610.642.468.0819.3510.13
CM10147.529.881.227.2221.4511.220.51 ± 0.022.63± 0.26
255.182.6535.331.234.291.31
AL15158.98.071.45.2214.0111.930.93 ± 0.112.59 ± 0.13
253.080.946.436.4616.2216.41
NHL15161.578.214.65.3611.687.640.71 ± 0.022.53 ± 0.25
263.422.450.761.423.9228.02
QL10157.713.150.747.3415.2412.710.83 ± 0.012.35 ± 0.12
253.554.980.777.1217.2114.25
ZP15140.710.13--59.16-0.53 ± 0.013.20 ± 0.02
247.165.330.016.225.1913.54
Br5159.8910.64-6.2814.577.080.45 ± 0.082.61 ± 0.32
257.248.575.885.2416.316.53
*: The atomic molar ratios were calculated using the average value of a minimum of three data points from the gel phase.
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Çalbıyık, S.; Omur, T.; Ozkan, H.; Kabay, N. Influence of Minor Additives on the Performance of Calcined Clay and Blast Furnace Slag Based One Part Alkali-Activated Mortars. Buildings 2025, 15, 3776. https://doi.org/10.3390/buildings15203776

AMA Style

Çalbıyık S, Omur T, Ozkan H, Kabay N. Influence of Minor Additives on the Performance of Calcined Clay and Blast Furnace Slag Based One Part Alkali-Activated Mortars. Buildings. 2025; 15(20):3776. https://doi.org/10.3390/buildings15203776

Chicago/Turabian Style

Çalbıyık, Suat, Tarik Omur, Hakan Ozkan, and Nihat Kabay. 2025. "Influence of Minor Additives on the Performance of Calcined Clay and Blast Furnace Slag Based One Part Alkali-Activated Mortars" Buildings 15, no. 20: 3776. https://doi.org/10.3390/buildings15203776

APA Style

Çalbıyık, S., Omur, T., Ozkan, H., & Kabay, N. (2025). Influence of Minor Additives on the Performance of Calcined Clay and Blast Furnace Slag Based One Part Alkali-Activated Mortars. Buildings, 15(20), 3776. https://doi.org/10.3390/buildings15203776

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