# Toward Zero-Carbon Concrete: Alkali Activation of Ladle Furnace Slag Using Cement Kiln Dust

Aleksandar Nikolov, Nicolai B Jordanov, Iliyan Djobov, Alexandar Karamanov

Karamanov

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Aleksandar Nikolov, Nicolai B Jordanov, Iliyan Djobov, Alexandar Karamanov. Toward Zero-Carbon Concrete: Alkali Activation of Ladle Furnace Slag Using Cement Kiln Dust. Buildings, 2025, 15, pp.4402. 10.3390/buildings15244402. hal-05443314

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Article

# Toward Zero-Carbon Concrete: Alkali Activation of Ladle Furnace Slag Using Cement Kiln Dust

Aleksandar Nikolov <fnref n="1" />,* , Nicolai B. Jordanov <fnref n="2" />, Iliyan Djobov <fnref n="2" /> and Alexandar Karamanov <fnref n="2" />,*

## Abstract

This study investigates the potential of producing zero-clinker, alkali-activated binders and concrete entirely from industrial by-products—ladle furnace slag (LFS), coal ash (CA), and cement kiln dust (CKD). The incorporation of CKD enhanced the workability and compressive strength properties of the alkali-activated mixtures, with the highest mechanical properties at 20% CKD addition. XRD, FTIR, and SEM analyses confirmed the formation of hydrocalumite, indicating improved hydration and microstructural densification. Mortar and concrete produced using the eco-cement reached 28-day strengths of 34.5 MPa and 32.6 MPa, corresponding to concrete class C20/25. These findings demonstrate the feasibility of manufacturing 100% waste-based construction materials suitable for sustainable, non-reinforced applications.

**Keywords:** zero-carbon; ladle furnace slag; cement kiln dust; coal ash; eco-cement; alkali-activation; one-part

<figure><img src="image_3.png" /></figure>

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Academic Editor: Salvatore Verre

Received: 6 November 2025

Revised: 26 November 2025

Accepted: 3 December 2025

Published: 5 December 2025

**Citation:** Nikolov, A.; Jordanov, N.B.; Djobov, I.; Karamanov, A. Toward Zero-Carbon Concrete: Alkali Activation of Ladle Furnace Slag Using Cement Kiln Dust. *Buildings* **2025**, *15*, 4402. https://doi.org/10.3390/buildings15244402

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

The construction industry is facing increasing pressure to reduce its environmental footprint due to intensive resource extraction, high energy consumption, and the generation of vast amounts of industrial waste [1]. Concrete is the most widely used construction material, and the production of Portland cement alone is responsible for about 7–8% of global CO₂ emissions, while also consuming significant amounts of non-renewable resources such as limestone, marl, and clay [2]. In this context, the valorization of industrial by-products and secondary raw materials into alternative binders presents a promising strategy for minimizing environmental impact and promoting circular economy principles [3]. Recycling industrial residues mitigates landfill burden and offers opportunities to minimize the carbon emissions, lower energy consumption, and save natural resources.

Alkali activation has emerged as a viable and scalable solution for transforming various types of waste and by-products into cementitious binders [4]. Alkali-activated materials (AAMs) rely on the chemical activation of aluminosilicate materials through alkaline solutions, leading to the formation of calcium-alumino-silicate-hydrates (C-A-S-H) or sodium(potassium)-aluminosilicate-hydrate (N(K)-A-S-H) gel phases [3]. This low-carbon pathway allows the production of binders with mechanical and durability performance comparable or superior to that of traditional cement, while simultaneously utilizing large volumes of waste [5]. The alkali activation enables the use of regionally available waste or by-products, tailored according to specific chemical compositions and reactivity [6].

<footnote n="1">¹ Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 107, 1113 Sofia, Bulgaria</footnote>

<footnote n="2">² Institute for Physical Chemistry, Bulgarian Academy of Sciences, IPC-BAS, Bl. 11, Acad. G. Bonchev Str., 1113 Sofia, Bulgaria; njordanov@ipc.bas.bg (N.B.J.); i.djobov@ipc.bas.bg (I.D.)</footnote>

<footnote>* Correspondence: drsashko@imc.bas.bg (A.N.); karama@ipc.bas.bg (A.K.)</footnote>
Among the various waste and by-products suitable for alkali activation, ladle furnace slag (LFS)—a secondary metallurgical residue from steel refining—is gaining attention due to its high calcium and alumina content [7]. Although traditionally considered less reactive than ground granulated blast furnace slag, recent studies have demonstrated that LFS can be effectively alkali-activated under optimized conditions [8]. Most commonly, alkali activation involves the use of commercial sodium silicate solutions in combination with sodium or potassium hydroxide [8–13]. For instance, Adenasanya et al. reported compressive strengths of up to 65 MPa in alkali-activated LFS pastes prepared using conventional alkaline solutions [14].

Despite these promising results, the widespread application of traditional silicate-
based activators is limited by their high cost, energy-intensive production, and footprint,
which challenges their sustainability in large-scale applications [15]. The use of alternative
activators is key for zero-carbon alkali-activated and geopolymer cements. Alternative
activator solutions based on industrial waste streams—such as waste-derived sodium
silicate solutions [16,17], anodizing etching solution [18], Bayer liquor [19], textile waste
water [20]—offer a promising route to reduce both environmental impact and production
costs. However, in the specific case of LFS, the vast majority of research still relies on
conventional activators (alkali silicate and hydroxide solutions), with very limited pioneer
studies exploring alternatives such as sunflower shell biomass fly ash [21]. This highlights
a significant research gap: the exploration of abundant industrial waste as an activator for
ladle furnace slag binder systems.

Furthermore, the development of dry-state activators enables the formulation of so-called “one-part” alkali-activated binders, which require only the addition of water on site, thereby simplifying handling and improving safety. Solid precursors such as desulfurization dust [22], biomass ash [21,23], and cement kiln dust [24,25] have shown potential not only as alkali sources but also as reactive mineral additions that enhance microstructure densification and performance. Cement kiln dust (CKD) and cement by-pass dust are fine particulate by-products of cement manufacturing and contain notable amounts of free lime, sulfates, and alkalis—components that can actively contribute to the activation process [26]. CKD has been employed as a partial or even complete substitute for commercial alkali activators in several systems, including pozzolanic material [24], fly ash [27], metakaolin [28], and blast furnace slag [29]. CKD is promising as an activator in applications where a moderate rather than highly alkaline environment is preferred.

In addition to the use of LFS as a primary precursor, numerous studies have high-
lighted the beneficial role of incorporating additional reactive aluminosilicate materials to
improve both the chemical reactivity and mechanical performance of alkali-activated sys-
tems [30]. These supplementary materials enhance the development of reaction products
and contribute to the improved workability, setting behavior, and long-term durability of
the hardened matrix [31]. The aluminosilicate reactive phase facilitates the formation of
a more complex and stable binder gel—often referred to as a hybrid C-A-S-H/N-A-S-H
matrix [3]. Among the most widely studied reactive aluminosilicate sources are fly ash and
bottom ash, both of which are by-products of coal combustion in thermal power plants.
These ash residues are typically composed of amorphous or semi-crystalline aluminosil-
icate phases that are reactive under alkaline conditions [32]. The co-utilization of LFS
with fly ash or bottom ash thus represents an effective valorisation strategy that not only
addresses multiple waste streams but also yields higher performance and a lower carbon
footprint. This multi-waste approach supports the transition toward circular economy
practices in construction materials engineering and aligns with global goals for reducing
the carbon footprint of the cement and concrete industry. However, often, multi-waste
systems continue to rely on commercial, energy-intensive alkali activators (e.g., sodium silicate and hydroxide), which undermines their environmental and economic benefits.

The present study introduces a novel fully waste-based cementitious system that eliminates the need for conventional activators. We formulate one-part eco-cement, mortar, and concrete, composed entirely of industrial residues—ladle furnace slag (LFS) and coal ash (CA)—which are alkali-activated exclusively using different amounts of cement kiln dust (CKD). To the best of the authors’ knowledge, this is the first report demonstrating the use of CKD as the sole alkaline activator for LFS-based systems. Unlike conventional alkali activation approaches that rely on commercial sodium silicate or hydroxide solutions, this research explores the feasibility of utilizing CKD, a highly alkaline and calcium-rich cement industry residue, as a cost-effective and sustainable alternative activator. The findings offer new insights into resource-efficient binder design and provide a pathway toward the development of circular-economy-compatible alternatives to conventional Portland cement for use in sustainable infrastructure.

## 2. Materials and Methods

The main raw material for the preparation of alkali-activated material was ladle furnace slag (LFS). LFS is a by-product of secondary refinement steel, provided by slag processor Aeiforos Bulgaria S.A., Pernik, Bulgaria. The representative sample of LFS was collected from an outside stockpile at a steel plant yard. The LFS was oven-dried to constant mass at 80 °C, then milled in steel ball mill for 1 h.

Coal ash (CA) is composite waste generated at Bulgaria's largest thermal power plant Maritsa Iztok-2 (1602 MW). The power generation technology involves the combustion of pulverized lignite coal using boilers and steam turbines. The units are equipped with flue gas desulfurization (FGD) systems to reduce sulfur dioxide emissions. The power plant generates fly ash (estimated at 1.2–3 million tons/year), bottom ash (approximately 0.12–0.6 million tons/year), and gypsum from FGD (around 0.2–0.5 million tons/year) [33]. The three waste streams are mixed with water and transported through pipelines into an ash pond. Consequently, the term “coal ash” refers to this combined mixture of fly ash, bottom ash, and FGD gypsum. The CA was oven-dried and milled in a steel ball mill for 1 h.

Cement kiln dust (CKD), supplied by Heidelberg Materials Devnya JSC, Devnya, Bulgaria, was used as the dry alkaline activator in this study. Due to its fine particle size and pronounced hygroscopic nature, precautions were taken to prevent premature reactions with atmospheric moisture and carbon dioxide. To preserve its reactivity, the CKD was stored in sealed containers under dry conditions and was used in the mixture as is. A proportion of 90% of the CKD particles were below 102 µm [26].

The powdered raw materials are presented at Figure 1.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** The used raw materials: ladle furnace slag (LFS) (**left**), coal ash (CA) (**center**), and cement kiln dust (CKD) (**right**).</figcaption></figure>

The aggregates for mortar and concrete preparation were fractioned electric arc furnace (EAF) slag provided by Aeiforos Bulgaria S.A. EAF slag is a by-product of the steel industry,
produced during the melting of iron scrap in electric arc furnaces in steelworks. The slag is poured from the furnace, cooled, and stockpiled. The resulting eco-aggregates were obtained by sieving in accordance with EN 13242:2002 + A1:2007 [34], ensuring compliance with the standard requirements for aggregates used in civil engineering works and construction.

The chemical composition of the precursors was determined by XRF using pressed pellets and analyzed on a Rigaku Supermini 200 WD apparatus, Rigaku Corporation, Osaka, Japan. The powder X-ray diffractograms were obtained on an Empyrean (Malvern-Panalytical, Malvern, UK) diffractometer using CuKα radiation at 40 kV and 30 mA. The Infrared spectra were recorded on FT-IR JASCO 4× (Tokyo, Japan) apparatus on 13 mm KBr pellets in the mid-infrared range 4000–400 cm⁻¹ with the following working parameters: absorption mode, resolution of 2 cm⁻¹, gain 4×, aperture of 2.5 mm, interferometer scanning speed of 2 mm s⁻¹, and accumulation of 64 scans for each spectrum. All spectra were manually baseline-corrected. The SEM images were obtained on a JEOL 6390 apparatus (Jeol USA, Inc., Peabody, MA, USA) at 20 kV under a secondary electron regime. The observed specimen was a fracture piece prepared with gold cover under vacuum. The particle size distribution was evaluated by a Mastersizer 3000 (Malvern-Panalytical, Malvern, UK) in dry dispersion mode. The compressive strength was measured as follows: pastes—on 3 cubic specimens with one side area of 10 cm²; mortar—prisms with dimensions 160 × 40 × 40 mm; concrete—cubes with sides of 70 mm. The absolute density was measured using a gas pycnometer (AccyPy1330, Micromeritic, Norcross, GA, USA) after milling the samples. The Vicat apparatus was used to measure the consistency of the fresh mixtures and setting time, according to EN 196-3:2016 [35]. The carbonation depth experiments were performed using a standard solution of phenolphthalein indicator on fresh split specimens, according to EN 14630:2006 [36].

# 3. Results

## 3.1. Characterization of the Precursors

### 3.1.1. Chemical and Mineral Composition

The chemical composition of the raw materials is presented at Table 1. The LFS exhibited a similar composition to Portland cement with a high content of calcium oxide (CaO, 55.50 wt.%), followed by alumina (Al₂O₃, 15.50 wt.%) and silica (SiO₂, 15.60 wt.%). CA presented a distinctly different chemical profile, with a high proportion of silica (SiO₂, 29.75 wt.%) and alumina (Al₂O₃, 15.02 wt.%), alongside a substantial amount of sulfur trioxide (SO₃, 17.61 wt.%) and iron oxide (Fe₂O₃, 14.84 wt.%). CKD displayed a dominant CaO concentration (55.51 wt.%), closely comparable to that of the LFS. In addition to its calcium content, CKD was characterized by a significant alkali content—K₂O (16.65 wt.%) and Na₂O (1.51 wt.%)—which indicates its strong potential as a dry alkaline activator, capable of contributing both to pH elevation and early-stage reaction kinetics in alkali-activated systems. The low SiO₂ and Al₂O₃ content (5.39 wt.% and 1.79 wt.%, respectively) suggests that CKD would act mainly as an activator rather than as reactive precursor. The CKD comprised significant quantities of chlorides (Cl, 11.81 wt.%) and sulfates (SO₃, 4.39 wt.%).

<table><caption>**Table 1.** Chemical composition of the used raw materials, normalized to wt.%, determined by XRF.</caption><thead><tr><th></th><th>Na<sub>2</sub>O</th><th>MgO</th><th>Al<sub>2</sub>O<sub>3</sub></th><th>SiO<sub>2</sub></th><th>SO<sub>3</sub></th><th>Cl</th><th>K<sub>2</sub>O</th><th>CaO</th><th>TiO<sub>2</sub></th><th>MnO</th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>Others</th></tr></thead><tbody><tr><td>LFS</td><td>-</td><td>6.50</td><td>15.50</td><td>15.60</td><td>2.45</td><td>-</td><td>0.10</td><td>55.50</td><td>0.70</td><td>0.80</td><td>2.69</td><td>0.16</td></tr><tr><td>CA</td><td>0.74</td><td>1.87</td><td>15.02</td><td>29.75</td><td>17.61</td><td>-</td><td>1.11</td><td>18.20</td><td>0.52</td><td>0.08</td><td>14.84</td><td>0.26</td></tr><tr><td>CKD</td><td>1.51</td><td>0.47</td><td>1.79</td><td>5.39</td><td>4.39</td><td>11.81</td><td>16.65</td><td>55.51</td><td>0.17</td><td>0.03</td><td>1.59</td><td>0.69</td></tr></tbody></table>
The powder XRD diffractograms reveal the mineral composition of the precursors (Figure 2). The LFS exhibited a predominantly hydraulic character, with the following main crystalline phases: dicalcium silicate (known as γ-belite, γ-C₂S), mayenite (C₁₂A₇), and periclase (MgO). These phases are typical for secondary steelmaking slags and are known to contribute both to early and long-term reactivity in alkaline environments, especially when finely ground and combined with suitable activators [8]. The CA was dominated by gypsum (CaSO₄·2H₂O) originating from the hydration of flue gas desulfurization products. Additional crystalline phases detected included hematite (Fe₂O₃) and anorthite (CaAl₂Si₂O₈), indicating the heterogeneous nature of the ash material and its partially crystalline aluminosilicate content. On the other hand, the main phases in cement kiln dust (CKD) were lime (CaO), larnite (β-C₂S), and sylvite (KCl). The presence of free lime and alkali salts suggests strong alkaline potential, making CKD an effective solid activator for alkali-activated systems, particularly in mixtures with calcium- and aluminosilicate-rich precursors. The diversity of phases among the three materials reflects their distinct industrial origins and contributes to the complex chemical and mineral interactions that occur during alkali activation.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Powder XRD patterns of the raw materials—ladle furnace slag (LFS), coal ash (CA), cement kiln dust (CKD). Legend: A—anorthite (CaAl₂Si₂O₈), An—anhydrite (CaSO₄), G—gehlenite (Ca₂Al₂SiO₇), Gy—gypsum (CaSO₄·2H₂O), H—hematite (Fe₂O₃), L—larnite (Ca₂SiO₄), Lm—lime (CaO), M—mayenite (Ca₁₂Al₁₄O₃₃), P—periclase (MgO), Q—quartz (SiO₂), S—sylvite (KCl), γ-γ-belite (γ-C₂S or γ-C₂SiO₄).</figcaption></figure>
### 3.1.2. Particle Size and Density

The particle size distribution and physical characteristics of the studied materials reveal notable differences in fineness and surface properties that influence their reactivity and potential role in the cementitious system (Figure 3 and Table 2). The CKD sample exhibits the finest particle size, accompanied by the highest specific surface area of 1435 m²/kg. This fine texture and large surface area suggest high potential for chemical reactivity. In contrast, the CA sample shows a significantly coarser particle distribution and the lowest surface area (503 m²/kg), indicating slower reaction kinetics and filler-type behavior rather than active participation in hydration. The LFS particles occupy an intermediate position, with surface area of 861 m²/kg. The absolute density varies from 2.50 g/cm³ for CKD to 2.96 g/cm³ for LFS, reflecting compositional differences.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Particle size distribution of the raw materials—coal ash (CA), cement kiln dust (CKD), ladle furnace slag (LFS).</figcaption></figure>

<table><caption>**Table 2.** Particle size distribution parameters and absolute density of the raw materials—coal ash (CA), cement kiln dust (CKD), ladle furnace slag (LFS).</caption><thead><tr><th>Sample</th><th>D [3;2], µm</th><th>D [4;3], µm</th><th>Dv 10, µm</th><th>Dv 50, µm</th><th>Dv 90, µm</th><th>Specific Surface Area, m²/kg</th><th>Absolute Density, g/cm³</th></tr></thead><tbody><tr><td>CA</td><td>8.3</td><td>70.5</td><td>4.7</td><td>52.4</td><td>162.0</td><td>503</td><td>2.63</td></tr><tr><td>CKD</td><td>2.9</td><td>27.2</td><td>1.1</td><td>8.7</td><td>77.9</td><td>1435</td><td>2.5</td></tr><tr><td>LFS</td><td>4.8</td><td>39.7</td><td>2.3</td><td>18.2</td><td>104.0</td><td>861</td><td>2.96</td></tr></tbody></table>

### 3.2. Alkali-Activated Paste

#### 3.2.1. Composition Design, Properties of the Fresh Mixture and Sample Preparation

To improve the reactivity and optimize the mechanical behavior of the binder, CA was introduced as an additional aluminosilicate component. The use of CA in conjunction with LFS is a well-established strategy in alkali-activated systems, contributing to improved binder gel formation, enhanced strength, and better durability characteristics [37]. Several authors have reported that blending fly ash in proportions of 20–30% by mass with steel slag precursors yields optimal mechanical properties, particularly compressive strength and microstructural refinement [38–40]. For instance, Pinheiro et al. demonstrated that 50:50 and 75:25 LFS:FA ratios resulted in the highest compressive strength at both 7 and 28 days of curing [11]. Similarly, Adesanya et al. found that increasing the silica content up to 20%, using additives such as diatomaceous earth, improved the mechanical performance of ladle slag-based binders [14].

In the present study, the proportion of CA used as a partial replacement for LFS was fixed at 25%. To this binary mixture, the effect of addition of CKD in varying amounts from 0 to 30% was evaluated on the fresh and hardened properties. The composition
design of the prepared mixtures is presented in Table 3. The dry ingredients (LFS, CA, and CKD) were dry-mixed for 30 s in a planetary mixer; then, a measured amount of tap water was added, followed by 60 s of stirring at low speed (140 rpm) and 60 s of stirring at high speed (285 rpm). The amount of water in all series was adjusted to obtain standard consistency, defined as a penetration depth of 6 ± 2 mm using the Vicat apparatus, following EN 196-3 (2016). A notable improvement in the workability of the fresh mixtures was observed with the incorporation of CKD. The improved consistency enabled a reduction in the water-to-solid (w/s) ratio from 0.36 in the reference mix (LC0) to 0.34 in the mixes containing CKD (LC-CKD10-30), while maintaining the workability required for standard consistency. The reference binary blend of LFS and CA, with no CKD addition (LC0), exhibited a rather quick setting behavior, characterized by an initial setting time of approximately 7 min. Such fast setting is often observed in ladle slags due to the high reactivity of mayenite (C₁₂A₇) [41]. A significant increase in setting time was observed with higher CKD additions (Table 3). In Portland cement systems, retardation of calcium aluminate (mainly C₃A) hydration is typically achieved through the addition of calcium sulfates, which promote the formation of ettringite crystals that suppress rapid hydration [42]. In the present binder system, the coal ash provides gypsum, which slows the hydration of mayenite and results in an initial setting time of approximately 7 min. Further significant increases in setting time were observed with progressive CKD addition. However, the exact mechanism by which CKD—rich in lime, alkalis, and chlorides/sulfates—affects the hydration of the LFS-gypsum system remains unclear.

<table><caption>**Table 3.** Composition design, consistence, and setting time of the prepared pastes.</caption><thead><tr><th rowspan="2">Series</th><th rowspan="2">LFS</th><th rowspan="2">CA</th><th rowspan="2">CKD</th><th rowspan="2">Water-to-Solid Ratio</th><th colspan="3">Vickat Apparatus Tests</th></tr><tr><th>Consistence, mm</th><th>Initial Setting Time, min</th><th>Final Setting Time, min</th></tr></thead><tbody><tr><td>LC0</td><td></td><td></td><td>-</td><td>0.36</td><td>4 *</td><td>7</td><td>10</td></tr><tr><td>LC-CKD10</td><td rowspan="3">75</td><td rowspan="3">25</td><td>10</td><td>0.34</td><td>6 *</td><td>25</td><td>40</td></tr><tr><td>LC-CKD20</td><td>20</td><td>0.34</td><td>4 *</td><td>60</td><td>140</td></tr><tr><td>LC-CKD30</td><td>30</td><td>0.34</td><td>7 *</td><td>90</td><td>180</td></tr></tbody></table>

As a result, the extended setting times provide a technological window that enhances the material's processability and application in construction scenarios, especially for cast-in-place, where transportation is required.

The fresh alkali-activated pastes were poured into steel molds (wrapped with polyethylene) to prepare cubic specimens with a side area of 10 cm². The samples were cured under laboratory conditions (25 °C, 65% relative humidity) and were demolded after one day and subsequently stored unwrapped under the same conditions.

### 3.2.2. Physical and Mechanical Properties

The addition of CKD had a positive effect on the mechanical and physical properties of the alkali-activated mixtures. As shown in Table 4, the compressive strength of the reference mixture without CKD (LC0) was 15.5 ± 0.3 MPa. The strength improved progressively with the addition of CKD content, reaching a maximum of 25.8 ± 0.4 MPa for the mixture containing 20% CKD (LC-CKD20). A slight reduction to 25.2 ± 0.6 MPa was observed at 30% CKD (LC-CKD30), indicating a possible threshold beyond which additional CKD may not contribute further to strength development probably due to the combination of factors, including the dilution of reactive LFS phases and excessive chloride/sulfate levels, where further increases do not significantly participate in the formation of stable products.

<footnote>* The consistence equal to 6 ± 2 mm is considered as standard, according to EN 196-3.</footnote>
<table><caption>**Table 4.** Mechanical and physical properties of obtained alkali-activated pastes.</caption><thead><tr><th>Series</th><th>Compressive Strength, MPa</th><th>Density, g/cm³</th><th>Water Absorption, %</th></tr></thead><tbody><tr><td>LC0</td><td>15.5 ± 0.3</td><td>1.50</td><td>28.3 ± 0.1</td></tr><tr><td>LC-CKD10</td><td>19.0 ± 0.5</td><td>1.56</td><td>25.6 ± 0.3</td></tr><tr><td>LC-CKD20</td><td>25.8 ± 0.4</td><td>1.57</td><td>25.2 ± 0.2</td></tr><tr><td>LC-CKD30</td><td>25.2 ± 0.6</td><td>1.55</td><td>25.3 ± 0.1</td></tr></tbody></table>

This enhancement is attributed to the high content of reactive lime and alkalis in CKD, which likely promoted additional hydration and binding phases within the matrix.

A similar trend was observed in the bulk density of the samples. The density increased from 1.50 g/cm³ in the reference (LC0) to a peak of 1.57 g/cm³ for LC-CKD20, with a slight decrease to 1.55 g/cm³ in LF30. The increase in density corresponds to the formation of a denser microstructure and improved particle packing due to CKD incorporation.

Water absorption also slightly decreased with CKD addition—from about 28% in LF0 to about 25% in series with CKD addition—suggesting reduced porosity and enhanced matrix consolidation. The effect of the positive change in density and water absorption on CKD addition correlates with the decrease in the water-to-solid ratio.

The compressive strength obtained in this study—up to 26 MPa—is lower than values reported for LFS activated with conventional alkali systems, such as sodium silicate and potassium hydroxide (65 MPa) [14], potassium silicate (50 MPa) [10], or sodium silicate and sodium hydroxide (44 MPa) [11]. Nevertheless, it is comparable to strengths achieved using alternative, low-alkalinity activators such as sunflower-shell fly ash, which has been reported to reach approximately 30 MPa [21], indicating that waste-derived activators can still produce mechanically competitive materials.

A cube from each mixture series was split at the 28th day and sprayed with phenolphthalein to assess carbonation depth, where the pink coloration corresponds to regions with pH > 10 (Figure 4). The CL0 series showed very light to no pink coloration, indicating a low pH (<10), likely caused by extensive CO₂ ingress into its more porous matrix or the intrinsically lower alkalinity of the system, as mayenite hydration does not produce calcium hydroxide, in contrast to calcium silicate hydration [43]. With increasing CKD content, the carbonated depth became progressively lower (up to about 2.5 mm), suggesting a denser microstructure that restricts CO₂ penetration. Additionally, the intensity and extent of the pink coloration increased with CKD incorporation, reflecting a rise in system alkalinity corresponding to the addition of lime introduced by CKD.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Image of carbonation depth using phenolphthalein solution on split specimens from each series. The appearance of a pink coloration denotes regions with pH values greater than approximately 10.</figcaption></figure>
### 3.2.3. Powder XRD

The powder X-ray diffraction (XRD) analysis provided valuable insights into the evolution of crystalline phases in the alkali-activated systems. Across all of the tested series, the highly reactive mayenite ($C_{12}A_7$) and gypsum were found to be completely reacted during the early stages of hydration (Figure 5). This indicates that these components readily participated in the formation of hydration products. In contrast, γ-belite ($\gamma-C_2S$), which is a relatively inert polymorph of belite, remained stable throughout the curing period. This observation is in agreement with previous findings that $\gamma-C_2S$ does not significantly contribute to strength development under ambient conditions due to its poor hydraulic reactivity [21].

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Powder XRD patterns of ladle furnace slag, coal ash, and cement kiln dust system. Legend: AFm—aluminate ferrite monosubstituted phases, AFt—aluminate ferrite trisubstituted phases, C—calcite, $CaCO_3$; Ett—ettringite, $Ca_6Al_2(SO_4)_3(OH)_{12} \cdot 26H_2O$; Hc—hydrocalumite, $Ca_2Al(OH)_{6.5}Cl_{0.5} \cdot 3(H_2O)$; M—mayenite, $Ca_{12}Al_{14}O_{33}$; P—periclase, MgO; Q—quartz, $SiO_2$; Str—strätlingite, $Ca_2Al_2(SiO_2)(OH)_{10} \cdot 2.5(H_2O)$; V—vaterite, $CaCO_3$; γ-γ-belite, $\gamma-Ca_2SiO_4$).</figcaption></figure>

In the control sample LC0, which did not contain CKD, the main hydration products identified were strätlingite and ettringite. Strätlingite formation is commonly associated with the interaction of aluminates—mainly derived from mayenite and fly ash—with available silica and calcium sources. Meanwhile, the complete transformation of gypsum was responsible for the extensive formation of ettringite, a typical early-age product in sulfate-rich systems. Ettringite is the main member of the AFt phases—calcium aluminate ferrite trisubstituted phases [44]. Monosubstituted calcium aluminate ferrite (AFm) was also detected in LC0 resulting from intermediate stages of ettringite decomposition or incomplete transformation of aluminate phases.

However, the addition of CKD led to a substantial modification in the hydration pathway and final mineralogical composition. The samples containing CKD showed a significant decrease in the peak intensities associated with ettringite and strätlingite, suggesting that the formation of these conventional hydration products was suppressed. Instead, new strong peaks became prominent, corresponding to an AFm phase—hydrocalumite—layered double hydroxide of calcium and aluminum. Hydrocalumite is often associated with high-alkali environments, and its formation is strongly promoted by the presence of
free lime (CaO), chlorides ($Cl^-$), and soluble alkalis ($Na^+, K^+$), all of which are typically present in high quantities in CKD. The highest amount of hydrocalumite and ettringite was observed in the series LC-CKD20, as indicated by the highest peak intensities in the XRD diffractograms. This finding correlates with the highest recorded compressive strength and density, suggesting that the formation of these phases is a key factor governing the mechanical performance of the material.

The shift from ettringite/strätlingite to hydrocalumite formation reflects a fundamental change in the thermodynamic stability of hydration products in the presence of CKD. Rather than forming expansive, needle-like phases such as ettringite, the system stabilizes into layered hydroxide structures that may influence long-term dimensional stability and durability. Chloride ions are partly incorporated into hydrocalumite through electrostatic interactions within the electric double layer of the positively charged surface, while calcium ions favor the formation of chloride-bearing hydrocalumite (Cl-AFm) and help regulate its crystal structure [45]. Chloride ions tend to inhibit the formation of calcium aluminate hydrates such as OH-AFm and $C_3AH_6$, whereas calcium ions promote the development of chloride-bearing hydrocalumite (Cl-AFm) that stabilizes chlorides [46]. This may contribute to the immobilization of chloride ions and heavy metals, offering potential advantages in terms of chemical resistance and environmental performance.

The reaction products formed during alkali activation with conventional hydroxide- or silicate-based activators are typically amorphous Ca-N(K)-A-S-H gels [14,31]. AFt and AFm phases generally do not form under these highly alkaline conditions, as their stability depends strongly on pH, with a stability domain typically between 10.5 and 13 [41,42]. Clark and Brown noted that AFm phases are inherently more stable than AFt at elevated alkalinity [47]. For instance, an AFm phase was observed in a system where LFS was activated with sodium silicate in the presence of soda-residue waste (source of gypsum), appearing as an XRD reflection near 11° 2θ, though it was incorrectly assigned as AFt in the original study [48]. Nevertheless, the formation of both AFt and AFm phases requires the presence of sulfate and/or chloride ions, which are not typically supplied by conventional alkali activators.

### 3.2.4. FTIR

The infrared spectra presented in Figure 6 reveal trends associated with hydration and carbonation processes in the prepared materials. An increase in hydration products is observed with increasing CKD content. This is evidenced by the enhanced intensity of the OH stretching bands at 3634 and 3465 cm⁻¹, corresponding to bound water, as well as the band at 1621 cm⁻¹, attributed to the H-O-H bending vibrations of interlayer water molecules. The observed OH-related bands correlate with the FTIR spectra of hydrocalumite [49], which confirms the XRD observations.

The band at 1480 cm⁻¹, attributed to the asymmetric stretching vibration of the carbonate group ($CO_3^{2-}$), also intensifies with higher CKD additions. The presence of a distinct doublet at about 1480 cm⁻¹ further confirms the formation of calcite, supported by the out-of-plane bending band at 880 cm⁻¹. The peak at 782 cm⁻¹ is characteristic of the carbonate bending mode in $CaCO_3$. The increased intensity of these carbonate-related bands suggests a higher content of carbonate species, likely formed through the natural carbonation of calcium-rich hydration products upon exposure to atmospheric $CO_2$ under the humid laboratory conditions. These findings correlate with the identification of calcite and vaterite phases in the powder XRD measurements, further supporting the occurrence of carbonation in the CKD-containing systems.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** FT-IR spectra of alkali-activated samples in the middle infrared range.</figcaption></figure>

The band observed around $1115 \text{ cm}^{-1}$ is attributed to the asymmetric stretching vibration ($v_3$) of the sulfate ion ($SO_4^{2-}$) and is typically associated with the presence of ettringite (AFt phase) [50]. This sulfate-related band partially overlaps with a neighboring band at approximately $1004 \text{ cm}^{-1}$, which corresponds to the asymmetric stretching vibrations of Si–O–T bonds (where T = Si or Al) within the aluminosilicate framework. Notably, with increasing CKD addition, the intensity of the $1004 \text{ cm}^{-1}$ band becomes more pronounced compared to the $1115 \text{ cm}^{-1}$ sulfate band. These relative intensity differences probably suggest that the CKD content promotes the formation of aluminosilicate gel at the expense of sulfate-containing phases such as ettringite.

In conclusion, the addition of CKD enhances both hydration and carbonation processes in the alkali-activated system, as evidenced by the increased intensity of OH and $CO_3^{2-}$ bands in the FTIR spectra. At the same time, the growth of the Si–O–T stretching band around $1004 \text{ cm}^{-1}$ indicates the formation of a more polymerized aluminosilicate gel.

### 3.2.5. SEM

The microstructural observations of the LC-CKD20 sample, presented in Figure 7, reveal a heterogeneous matrix with distinct morphological features. At low magnification (×100), large spherical pores can be observed, likely resulting from air entrainment during the mixing process. Several microcracks are also visible, which are probably associated with drying shrinkage occurring during sample preparation and aging. At higher magnification (×10,000), the matrix displays the presence of micron-sized plate-like crystalline formations, most likely corresponding to hydrocalumite—an AFm-type phase commonly formed in alkali-activated calcium–aluminate systems and characterized by its distinctive hexagonal plate-like morphology [51].
<figure><img src="image_10.png" /><figcaption>**Figure 7.** SEM micrographs of the LC-CKD20 sample captured using secondary electrons at magnifications of ×100 and ×5000.</figcaption></figure>

### 3.3. Waste-Based Mortar and Concrete

The feasibility of producing zero-carbon construction materials was explored by formulating mortar and concrete mixtures composed entirely of industrial by-products, without the inclusion of traditional clinker-based components and natural aggregates. The binder employed was the series showing the highest mechanical performance, LC-CKD20, composed of LFS, CA, and CKD at a ratio of 75:25:20. The used fine and coarse aggregates were derived from electric arc furnace (EAF) slag (Figure 8).

<figure><img src="image_11.png" /><figcaption>**Figure 8.** The ingredients of the prepared 100% waste mortar and concrete: eco-cement composed of LFS, CA, and CKD (**left**); eco fine aggregates (0–4 mm) (**center**); and eco coarse aggregates 4–16 mm (**right**).</figcaption></figure>
The calculations of the mix design and sieve analysis of the aggregates are provided in the supplementary materials (Appendix A). This design highlights the potential of combining multiple industrial residues to produce mortars and concretes with a reduced environmental footprint.

The results presented in Table 5 indicate that both the mortar and concrete produced entirely from industrial by-products and cured at normal laboratory conditions (20 ± 2 °C, 60 ± 5% RH) exhibit significant mechanical performance. The mortar achieved compressive strengths of 26.07 MPa at 7 days and 34.50 MPa at 28 days. The corresponding 28-day strength of the concrete reached 32.6 MPa, which is comparable to standard structural-grade materials, confirming the suitability of the 100% waste-based mixtures for sustainable construction applications. Considering the relationship between the measured average and characteristic compressive strengths, the obtained results indicate that the material achieves a strength level typical of C20/25 concrete. Nevertheless, due to the high concentrations of chlorides and sulfates, this eco-concrete is not appropriate for applications involving steel reinforcement, as these ions could initiate and accelerate corrosion processes.

<table><caption>**Table 5.** Compressive strength results of the 100% waste-based mortar and concrete.</caption><thead><tr><th colspan="3">Mortar</th><th>Concrete</th></tr><tr><th>Compressive strength, 7th day, MPa</th><th>Compressive strength, 28th day, MPa</th><th>Compressive strength, 28th day, MPa</th><th></th></tr></thead><tbody><tr><td>26.07 ± 1.20</td><td>34.50 ± 1.43</td><td>32.6 ± 1.04</td><td></td></tr></tbody></table>

## 4. Conclusions

This study demonstrated the potential of developing alkali-activated 100% waste-based construction materials by combining ladle furnace slag (LFS), coal ash (CA), and cement kiln dust (CKD). The main findings can be summarized as follows:

* The incorporation of CKD significantly improved the workability of the fresh alkali-activated mixtures, allowing for a lower water-to-solid ratio while maintaining standard consistency.

* The addition of CKD increases the setting time of the system LFS-CA and the actual mechanism of this remained unclear.

* CKD addition enhanced the mechanical performance of the hardened materials, with series LC-CKD20 achieving the highest compressive strength of 25.8 MPa after 28 days of curing under laboratory conditions.

* The observed strength gain after CKD addition is attributed to the formation of hydrocalumite (AFm) phases, leading to a denser and more cohesive microstructure. Microstructural analyses confirmed that CKD altered the hydration mechanism by promoting the formation of hydrocalumite (an AFm phase) instead of ettringite, as well as by enhancing aluminosilicate polymerization and carbonation.

* Based on these results, a binder composed of LFS-CA-CKD (75:25:20) was selected for producing 100% waste-derived mortar and concrete. The resulting mortar and concrete exhibited compressive strengths of 34.5 MPa and 32.6 MPa, respectively, after 28 days, corresponding to the strength class C20/25.

* Despite their satisfactory mechanical properties, the high chloride and sulfate content of the system limits its use in reinforced applications due to the risk of steel corrosion.

Overall, the study confirms the feasibility of producing structurally sound, zero-clinker mortars and concretes entirely from industrial by-products, offering a sustainable alternative for non-reinforced or prefabricated applications in low-carbon construction. More studies are needed to provide a more comprehensive assessment of the durability and long-term stability of the present clinker-free cement, including chloride and sulfate
leaching, late hydration of periclase, and other factors affecting performance under varying curing and environmental conditions.

**Author Contributions:** Conceptualization, A.N.; methodology, A.N., N.B.J., I.D. and A.K.; formal analysis, A.N., N.B.J. and I.D.; investigation, A.N., N.B.J., I.D. and A.K.; resources, A.N., N.B.J., I.D. and A.K.; data curation, A.N., N.B.J., I.D. and A.K.; writing—original draft preparation, A.N.; writing—review and editing, A.N. and A.K.; visualization, A.N. and N.B.J.; supervision, A.N. and A.K.; project administration, A.K.; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded as project № ПБУ-65 from 16.12.2024 г./BG-RRP-2.017-0024/.

**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

**Acknowledgments:** The authors gratefully acknowledge the company Aeiforos EAD for providing the ladle furnace slag and eco-aggregates.

**Conflicts of Interest:** The authors declare no conflicts of interest.

## Abbreviations

The following abbreviations are used in this manuscript:

CA Coal ash

LFS Ladle furnace slag

CKD Cement kiln dust

EAF Electric arc furnace

## Appendix A

Sieve analysis of the fine and coarse eco-aggregates based on fractioned electric arc furnace slag are presented at Table A1.

<table><caption>**Table A1.** Sieve analysis of the fine and coarse eco-aggregates based on fractioned electric arc furnace slag.</caption><thead><tr><th>Sieve Size, mm</th><th>Retained, %</th><th>Cumulative % Retained</th><th>Passing, %</th></tr></thead><tbody><tr><td colspan="4" style="text-align: center;">Coarse aggregates (4–16 mm)</td></tr><tr><td>32</td><td>0.0</td><td>0.0</td><td>100.0</td></tr><tr><td>16</td><td>1.9</td><td>1.9</td><td>98.1</td></tr><tr><td>8</td><td>58.0</td><td>59.8</td><td>40.2</td></tr><tr><td>4</td><td>37.1</td><td>96.9</td><td>3.1</td></tr><tr><td>&lt;4</td><td>3.1</td><td>100.0</td><td>0.0</td></tr><tr><td colspan="4" style="text-align: center;">Fine aggregates (0–4 mm)</td></tr><tr><td>8</td><td>0.0</td><td>0.0</td><td>100.0</td></tr><tr><td>4</td><td>13.8</td><td>13.8</td><td>86.2</td></tr><tr><td>2</td><td>50.8</td><td>64.5</td><td>35.5</td></tr><tr><td>1</td><td>20.3</td><td>84.8</td><td>15.2</td></tr><tr><td>0.5</td><td>8.9</td><td>93.7</td><td>6.3</td></tr><tr><td>0.25</td><td>3.5</td><td>97.2</td><td>2.8</td></tr><tr><td>0.125</td><td>1.6</td><td>98.7</td><td>1.3</td></tr><tr><td>0.063</td><td>0.9</td><td>99.6</td><td>0.4</td></tr><tr><td>&lt;0.063</td><td>0.4</td><td>99.1</td><td>0.0</td></tr></tbody></table>

The composition design of the mortar and concrete was calculated using the absolute volume method [52]. The amount of fine aggregate ($m_{FA}$) was calculated using the
presumption that the absolute volume of the cement paste fills, with an excess, the voids between fine aggregate particles in bulk state using the following equation:

$$ \frac{m_{binder}}{\rho_{c,binder}} + m_{water} = \alpha \cdot \frac{m_{FA}}{\rho_{0,FA}} \cdot \left( 1 - \frac{\rho_{0,FA}}{\rho_{c,FA}} \right) $$

$m_{binder}$—mass of the binder;

$\rho_{c,binder}$—absolute density of the binder = 2808 g/cm³ measured by gas pycnometry;

$m_{water}$—mass of the water—calculate by w/s ratio = 0.36;

$m_{FA}$—mass of the fine aggregates;

$\rho_{0,FA}$—bulk density of the fine aggregates = 1750 kg/m³ measured according EN 1097-3:2000 [53];

$\rho_{c,FA}$—specific density of the fine aggregates = 3550 kg/m³ measured by pycnometer;

$\alpha \approx (1.1 \div 1.3)$—sand void filling coefficient, related to coarseness of the sand = 1.3 for coarse fine aggregates;

Calculation per 100 kg binder:

$$ \frac{100}{2.81} + 36 = 1.3 \cdot \frac{m_{FA}}{1.75} \cdot \left( 1 - \frac{1.75}{3.55} \right) $$

$$ \Rightarrow m_{FA} = 190 \text{ kg} $$

The coarse aggregates were calculated on the presumption that the amount of sand in a bulk state fills the voids between the grains of the coarse aggregates. This condition is expressed by the following equation:

$$ \frac{m_{FA}}{\rho_{0,FA}} = \gamma \frac{m_{CA}}{\rho_{0,CA}} \left( 1 - \frac{\rho_{0,CA}}{\rho_{c,CA}} \right) $$

$m_{FA}$—mass of the fine aggregates;

$\rho_{0,FA}$—bulk density of the fine aggregates;

$m_{CA}$—mass of the coarse aggregates;

$\rho_{0,CA}$—bulk density of the coarse aggregates = 1740 kg/m³ measured according EN 1097-3;

$\rho_{c,CA}$—specific density of the coarse aggregates = 3610 kg/m³ measured by pycnometer;

$\gamma = 0.82 + 0.0024 \text{ Vt-structural empirical coefficient};$

Vt—volume of cement paste.

The mass of coarse aggregates per 100 kg binder:

$$ \frac{190}{1.75} = 0.91 \frac{m_{CA}}{1740} \left( 1 - \frac{1740}{3610} \right) $$

$$ m_{CA} = 402 \text{ kg} $$

Final recipe for 1 m³ of concrete and mortar based entirely on by-products:

<table><caption>**Table A2.** Calculated concrete and mortar recipe (kg).</caption><thead><tr><th>Ingredients</th><th>Mortar</th><th>Concrete</th></tr></thead><tbody><tr><td>Cement</td><td>799</td><td>423</td></tr><tr><td>Water</td><td>288</td><td>198</td></tr><tr><td>Fine aggregates</td><td>1519</td><td>803</td></tr><tr><td>Coarse aggregates</td><td>-</td><td>1701</td></tr><tr><td>Water-reducing admixture</td><td>5.6</td><td>3.0</td></tr></tbody></table>
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