# Products of carbonation of cement kiln dust

Aleksandar Nikolov, Vladislav Kostov-Kytin, Mihail Tarassov, Liliya Tsvetanova, Hristina Lazarova, Tina Tasheva

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Aleksandar Nikolov, Vladislav Kostov-Kytin, Mihail Tarassov, Liliya Tsvetanova, Hristina Lazarova, et al.. Products of carbonation of cement kiln dust. Review of the Bulgarian Geological Society, In press, 85 (3), pp.163-166. 10.52215/rev.bgs.2024.85.3.163. hal-04865752

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# Products of carbonation of cement kiln dust

Aleksandar Nikolov<fnref n="1" />*, Vladislav Kostov-Kytin<fnref n="1" />, Mihail Tarassov<fnref n="1" />, Liliya Tsvetanova<fnref n="1" />,
Hristina Lazarova<fnref n="1" />, Tina Tasheva<fnref n="2" />

## Продукти на карбонизация на прах от циментови пещи

Александър Николов<fnref n="1" />, Владислав Костов-Китин<fnref n="1" />, Михаил Тарасов<fnref n="1" />, Лилия Цветанова<fnref n="1" />,
Христина Лазарова<fnref n="1" />, Тина Ташева<fnref n="2" />

**Abstract.** Cement kiln dust (CKD), a by-product of cement clinker manufacturing, embodies considerable amounts of alkali and alkaline-earth compounds, presenting a potential route for CO₂ sequestration. The study aims to clarify the potential of CKD for CO₂ capture and storage. Two experimental schemes were used: (1) direct carbonation of dry CKD and (2) carbonation of hydrated CKD. Analytical techniques such as XRF, XRD, FT-IR, SEM and DSC/TG were employed to characterize the mineralogical and morphological transformations of hydration and carbonation of CKD. The findings reveal that even in the dry state, the CKD has the ability to react with carbon dioxide at normal pressure to form stable mineral phases such as calcite and vaterite. The study confirms that CKD, both in raw and hydrated forms, holds promise for CO₂ capture and storage applications.

**Keywords:** cement kiln dust (CKD), CKD carbonation, CO₂ storage, CO₂ sequestration.

## Introduction

The building and construction sector reached new highs in 2022, making up to 37% of total global CO₂ emissions to just about 10 Gt (United Nations Environment Programme, 2024). The sector is a significant consumer of natural resources, raising concerns about resource depletion in the near future. The European policies and strategies imply circular economy principles, in which various industrial waste could become valuable by-products. Cement production is among the largest industrial contributors to pollution, responsible for approximately 8% of global CO₂ emissions. Around 55% of the emissions are produced from decarbonation of limestone and 40% as a result of heating the kiln to about 1450 °C (Müller, Harnisch, 2008). During calcination of cement clinker at the rotary

kiln, a fine particulate matter is formed, called cement kiln dust (CKD). The dust is transported in a gaseous stream flowing from the burner throughout the rotary kiln and cyclone preheaters. CKD is comprised of complex mixture of fuel residues and intermediate products between raw feed and clinker. The CKD is collected by bag filters, or other bypass system, and estimates up to 20% of the total clinker production (Al-Bakri et al, 2022). Although a substantial portion of CKD is reused within the cement plants as raw feed material, the high levels of alkalis, sulfates, and chlorides impose technological limitations, resulting in a considerable amount of the CKD being stockpiled. As a result, research efforts are increasingly focused on finding alternative uses for CKD. This also applies to a large extent to Bulgaria, where Portland cement clinker production reached 1.95 million

<footnote n="1">¹ Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Sofia; E-mail: y8sashko@yahoo.com</footnote>

<footnote n="2">² University of Chemical Technologies and Metallurgy, 8 Kliment Ohridski Str., Sofia 1797</footnote>
tons in 2022 (Executive environment agency of
Bulgaria, 2022).

Global attention has increasingly focused on mitigation of CO₂ emissions. The CO₂ capture and storage approaches are among the most relevant strategies for reducing CO₂ in the atmosphere (Vasilev et al, 2020). Mineral carbonation is a method which involves the sequestration of CO₂ into stable mineral carbonates (Sanna et al., 2014). The high content of alkali and alkaline-earth elements render CKD a suitable candidate for CO₂ capture. The carbonation of CKD is similar to natural processes of rock weathering and diagenesis observed in carbonate-rich environments. Understanding these transformations is essential to optimizing CKD reuse in sustainable construction practices and developing effective CO₂ sequestration strategies to mitigate the carbon emission released from cement production.

The focus of the present study is on the evalua-
tion of the carbonization potential of CKD for the
purpose of its reuse in CO₂ capture and storage.
Two experimental schemes were used: (1) direct
carbonization of raw CKD and (2) carbonation of
hydrated CKD.

**Materials and methods**

The chemical composition of CKD was determined using a Rigaku Supermini 200 X-ray fluorescence apparatus on pressed pellets with a diameter of 30 mm. The powder XRD patterns were recorded on an Empyrean (Malvern-Panalytical) diffractometer using CuKα radiation at 40 kV and 30 mA. The FT-IR spectra were collected by FT-IR spectrometer (Varian 600-IR) with a 3 cm⁻¹ resolution using KBr pellets. The micromorphology of the powder materials was studied on a ZEISS EVO 25LS scanning electron microscope (SEM) at 15 kV. The powder materials were scattered over carbon adhesive tape attached on SEM stubs and then coated with carbon in an Edwards 306 thermal evaporation vacuum system. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were done on BXT-STA-200 analyzer (Shanghai Glomro) on 10–20 mg sample, in the temperature range of 35–900 °C, heating rate of 10 °C/min and with an airflow atmosphere.

The CKD was provided by a local cement pro-
ducing plant in Bulgaria. The sample was stored
in closed containers to eliminate contact with the
atmosphere since CKD is characterized by a high
hygroscopicity. The chemical composition of the
initial CKD normalized to 100% was following
(wt%): CaO – 55.51; K₂O – 16.65; Cl – 11.81;
SiO₂ – 5.39; SO₃ – 4.39; Al₂O₃ – 1.79; Fe₂O₃ – 1.59;
Na₂O – 1.51; MgO – 0.47; others – 0.89.

The carbonation of the samples was carried out
in a carbonation chamber at atmospheric pressure,
optimal relative humidity (65±5%) and normal tem-
perature (22±2 °C). The samples were grinded in
agate mortar and placed in petri dishes in the cham-
ber for 28 days where periodically CO₂ gas (≥99%)
was introduced. The hydration of CKD was evalu-
ated by mixing it with water at a solid to water ra-
tio of 0.5 (sample H-CKD). The fresh mixture was
poured in a plastic mould and stored wrapped under
laboratory conditions until 28th day.

Results

The main mineral phases presented at the raw CKD are lime (CaO), sylvite (KCl) and larnite (Ca₂SiO₄) (Fig. 1a). Free lime in cement kiln dust (CKD) refers to the unreacted calcium oxide (CaO) that remains after the clinker manufacturing process. Larnite (also known as β-dicalcium silicate, or β-C₂S, belite) is one of the phases of Portland cement clinker responsible for contributing to the long-term strength development of concrete through its slow hydration process (Grutzeck, Roy, 1981). The free lime and larnite can significantly affect the properties of CKD as binder material, influencing its reactivity, pH, strength gain, and respectively potential applications. Trace amount of portlandite Ca(OH)₂ were also detected as product of hydration of lime due to contact with atmospheric moisture and high hygroscopicity nature of the CKD. A significant content of sylvite was detected and its presence in CKD is typically due to the type of fuel used in the cement kiln. Small amount of quartz and anhydrite (CaSO₄) have also been detected. The TG curve of sample CKD shows minimal weight loss (less than 1%) up to 900 °C (Fig. 1c). The small stepwise mass losses at around to 220 °C indicate the release of small amount of absorbed water.

Portlandite is found to be the main newly formed
phase as a result of the hydration process and is a
product of lime hydration (Fig. 1a). The portlandite
is associated with pozzolanic reaction in cementi-
tious binders. The presence of portlandite is marked
at DSC/TG curve with endothermic peak at about
450 °C coupled with significant mass loss of about
3.5% (Fig. 1c). The larnite phase is transformed
to hydrated amorphous C-S-H gel, not detected at
XRD pattern. Sylvite phase was still presented in the
H-CKD. However, sylvite phase is characterized by
sharper peaks and narrower full width at half maxi-
mum as compared to the raw CKD. This suggests
that sylvite has firstly dissolved when CKD has
been mixed with water and then recrystalized with
a higher degree of crystallinity. However, a part of
the chloride ions from the dissolved sylvite react
with lime and aluminium to form the highly chlo-
rine hydrocalumite phase (Ca₄Al₂(OH)₁₂Cl₂·4H₂O) detected in the H-CKD sample (Fig. 1a). Hydrocalumite is a part of calcium aluminate ferrite hydrates in cement chemistry, which refers to a group of compounds known as Afm which contributes to the mechanical properties and durability of cement-

titious materials (Abd-El-Raoof et al., 2023). The lamellar structure of hydrocalumite is formed by portlandite-like layers [Ca(OH)₂] with part of the calcium being replaced by aluminum and interlayer anion such as Cl⁻. Syngenite (K₂Ca(SO₄)₂·H₂O) was also formed in the H-CKD. Syngenite is a hydrated

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** a) and b) powder XRD diffraction patterns. Legend: A, anhydrite; C, calcite; G, gypsum; Hc, hydrocalumite; Lm, lime; Lr, larnite; P, portlandite; S, sylvite; Sn, syngenite; V, vaterite; c) and d) DSC and TGA; e) FTIR; f) SEM images of raw and hydrated CKD and their carbonation forms</figcaption></figure>
calcium potassium double sulfate and probably was
formed through a reaction involving gypsum in the
presence of potassium in high contents.

The experiments showed that very similar prod-
ucts were formed after carbonation of raw CKD and
hydrated CKD (H-CKD). Two calcium carbonate
phases were detected – calcite and vaterite (Fig. 1b).
Both lime (CaO) and portlandite (Ca(OH)₂) undergo
full carbonation to CaCO₃. Larnite remains stable in
sample CKD-C, indicating that this phase must first
be hydrated to be able to transform to CaCO₃ and
amorphous SiO₂. The sample H-CKD-C contains
higher amount of vaterite, compared to CKD-C ac-
cording XRD intensities.

The high degree of carbonation is visible at FT-
IR spectra of the H-CKD-C and CKD-C (Fig. 1e)
where we observe strong main band at 1421 cm⁻¹
and well-defined peaks at 876 cm⁻¹ and 714 cm⁻¹
assigned to the vibrations of CO₃ structural unit.
The band at 1421 cm⁻¹ is due to the degenerative
asymmetrical stretching vibration v<sub>d</sub><sup>as</sup>(E), the band
at 876 cm⁻¹ is due to the bending vibrations δ(A₂),
and at 714 cm⁻¹ is due to the asymmetric bending
vibrations δ<sub>d</sub>(E). The small band at 746 cm⁻¹ is as-
signed to vaterite. Asymmetric stretching vibrations
v<sub>d</sub><sup>as</sup>(F) and symmetric stretching vibrations v<sub>d</sub><sup>s</sup>(A₁)
of the SiO₄ structural unit are presented in the 1150-
850 cm⁻¹ region (Mihailova et al., 2023).

The XRD and FTIR data are well supported by
the SEM study (Fig. 1f): the samples CKD-C and
H-CKD-C are hardly distinguished both consist-
ing of porous aggregates of shapeless or rounded
particles with variable size 1–3 µm. Besides these
aggregates, there are also relatively large (~5 µm)
randomly shaped platy grains corresponding to the
neo-formed calcite. The original CKD sample dif-
fers from its modified forms (H-CKD-C and CKD-
C) by relatively large sylvite crystals >30 µm. All
other crystalline phases have micro- to sub-mi-
crometric sizes. SEM image of hydrated H-CKD
sample shows subhedral plate-like crystals of port-
landite (labelled as “P” in Figure 1e) and euhedral
elongated crystals of presumably hydrocalumite.
The sylvite crystals in the sample bring signs of in-
tense dissolution. It is noteworthy that, according to
XRD and SEM data, the transformations of CKD–
(H-CKD)–(H-CKD-C) and CKD–(CKD-C) lead to
very similar results, indicating that CKD materials
have good potential in CO₂ capture and storage even
without the hydration step typical for all Portland
cement and/or lime-based mortars.

Thermochemical studies show (Fig. 1d) that the
efficiency of CO₂ capture by the hydrated form of
CKD is higher than that of their dry (untreated)
forms. The weight loss corresponding to endother-

mic effect 780 °C are equal to about 10 and 2.5 wt% for at H-CKD-C and CKD-C, respectively.

Conclusion

Cement kiln dust (CKD) is valuable by-product of
cement production. The study carried out shows a
promising potential of CKD for in CO₂ capture and
storage as far as even non-hydrated the CKD mate-
rial possess the ability to react with carbon dioxide
at normal pressure to form stable mineral phases
such as calcite and vaterite. Detailed kinetic studies
are needed to reveal the full potential of CKD for
effective CO₂ sequestration strategies

**Acknowledgements:** The results in this work have been achieved in fulfilment of a project financed by the National Science Fund of Bulgaria under contract No. КП-06-H77/9 from 4.12.2023. The authors acknowledge the technical support (XRD) from the project PERIMED BG05M2OP001–1.002-0005/29.03.2018 (2018–2023).

**References**

Abd-El-Raoof, F., H. Youssef, T.M. El-Sokkary, Z. Abd El-Shakour, A. Tawfik. 2023. Fabrication and characterization of calcium aluminates cement via microwave-hydrothermal route: Mayenite, katoite, and hydrocalumite. – *Constr. Build. Mater.*, **401**, 132988; https://doi.org/10.1016/j.conbuildmat.2023.132988.

Al-Bakri, A. Y., H. M. Ahmed, M. A. Hefni. 2022. Cement kiln dust (CKD): Potential beneficial applications and eco-sustainable solutions. – *Sustainability*, **14**, 12, 7022; https://doi.org/10.3390/su14127022.

Executive environment agency of Bulgaria. *Public Annual Reports in Bulgarian*. https://eea.government.bg/bg/r-r/r-kp-kz/godishni-dokladi-14/index. Date of access: 1.10.2024.

Grutzeck, M., D. Roy. 1981. Portland cement mineralogy. – In: Frye, K. (Ed.), *Mineralogy. Encyclopaedia of Earth Science*. Boston, Springer; https://doi.org/10.1007/0-387-30720-6_113.

Mihailova, I., R. Harizanova, T. Tasheva, N. Dimova, C. Rüs-
sel. 2023. Physicochemical and structural characteriza-
tion of silicate glasses containing iron oxides. – *J. Chem.
Technol. Metall.*, **58**, 1, 3–7; https://doi.org/10.59957/jctm.
v58i1.1.

Müller, N.; J. Harnisch. 2008. *A Blueprint for a Climate Friendly Cement Industry*. Nürnberg, Germany, Gland WWF La-farge Conservation Partnership, 7.

Sanna, A., M. Uibu, G. Caramanna, R. Kuusik, M. Maroto-
Valer. 2014. A review of mineral carbonation technologies
to sequester CO₂. – *Chem. Soc. Rev.*, **43**, 23, 8049–8080;
https://doi.org/10.1039/C4CS00035H.

United Nations Environment Programme. 2024. Global Status Report for Buildings and Construction: Beyond Foundations: Mainstreaming Sustainable Solutions to Cut Emissions from the Buildings Sector. Nairobi; https://doi.org/10.5911/20.500.11822/45095.

Vassilev, S., C. Vassileva, N. Petrova. 2020. CO₂ capture and storage by carbonation of plum pits ash and walnut shells ash. – *Rev. Bulg. Geol. Soc.*, **81**, 3, 204–206.