Cement Kiln Dust (CKD): Potential Beneficial Applications and Eco-Sustainable Solutions

Abstract

Over many decades, cement has been the primary component in construction projects and is considered one of the essential industries worldwide. At the same time, it overconsumes natural resources and can negatively impact the environment through a few byproducts, such as carbon dioxide (CO₂) and cement kiln dust (CKD). The generated quantity of CKD is estimated to be 15–20% of the produced cement, which means CKD can be induced in hundreds of millions of metric tons synchronously with annual global cement production. Unfortunately, not all materials of CKD are suitable for recycling in cement manufacturing since it contains high levels of alkalis, sulfate, and chloride, leading to excessive concentrations in the final product. Therefore, CKD industrial utilization has become highly recommended in recent research as a potential beneficial application from economic, environmental, and sustainability perspectives. This review paper highlights and discusses the recently conducted research articles that investigate the industrial applications of CKD. The obtained outcomes showed that CKD has physical and chemical properties that make it practical in many fields, such as soil stabilization, concrete mix, chemical treatment, ceramic and brick manufacturing, and mine backfill. They also indicate a lack of studies investigating CKD in mine backfill applications as a partial replacement material for cement due to the high cost of binders, optimization, and sustainability purposes.

1. Introduction

Despite the tremendous industrial and technical progress that the world is witnessing, cement manufacturing remains one of the most significant industries in various countries. Cement plays a vital role in many applications, mainly utilized as a binder in concrete used for construction projects, such as dams, buildings, roads, etc., and the decoration works. The global market statistics indicate that the world’s cement production volume has increased from 3.7 to more than 4 billion metric tons in the last six years (Figure 1) [1,2]. This vast number is approximately three times the production volume of twenty-five years ago and reflects the growing need for cement. According to government reports, in Saudi Arabia, which is experiencing a comprehensive economic renaissance, seventeen licensed cement companies have produced an average of 50 million metric tons of cement annually in the last five years [3]. This number is subject to increase due to the growing demand for cement during the past few months.

On the other hand, the cement manufacturing process can negatively impact the environment through byproduct materials, such as carbon dioxide and cement kiln dust (CKD). The generated CKD is estimated to be 15–20% of the produced cement [4,5,6]. That means that hundreds of millions of metric tons of CKD can be generated annually worldwide synchronously with cement production. If they are not recycled in the cement industry or even used in other industrial applications, these large quantities are dumped into the landfills and negatively impact the air and surface- and groundwater. CKD deposited in landfills causes environmental concerns and is not consistent with the sustainability vision [7]. CKD or bypass material is fine-grained particulate dust collected from electrostatic precipitators during clinker production under high temperatures [8]. The chemical composition of CKD is similar to the raw materials used in feeding kilns to make ordinary Portland cement in terms of calcium carbonate and silica oxides. However, it differs because it contains more significant levels of alkalis, sulfate, and chloride, which require more effort to manage and reduce dependence on landfill disposal (

Table 1. Typical chemical composition of cement kiln dust (CKD) and ordinary Portland cement (OPC) [6,11,24].

Chemical Composition		CKD (%)	OPC (%)
Calcium oxide	CaO	38–50	62–64
Silicon dioxide	SiO2	11–16	20–22
Aluminum oxide	Al2O3	3–6	4–5
Magnesium oxide	MgO	0–2	1–2.6
Sodium oxide	Na2O	0–2	<1
Potassium oxide	K2O	3–13	<1
Iron oxide	Fe2O3	1–4	3–3.6
Sulfur trioxide	SO3	4–18	2.7–3
Chloride	Cl	0-5	<0.1
Loss on ignition	LOI	5–25	1
Free lime	F-CaO	1–10	2

). The physical and chemical characteristics of CKD may be affected by several factors; the most important are raw materials and the operation process type, the fuel used, and the dust collection system [9,10]. CKD’s typical chemical composition and physical properties are shown in Table 1 and

Table 2. CKD’s typical physical properties [10,12].

Physical Property	Value
Gradation (75% passing)	30 μm (No. 450 sieve)
Particle size (Maximum)	300 μm (No. 50 sieve)
Specific surface	4600–14,000 (cm2/g)
Specific gravity	2.6–2.8

[10,11,12]. At the same time, the mechanical properties of the CKD bulk material, such as angle of repose, flowability, and bulk density, vary based on the designed facilities of handling and storage, while the average mass diameter ranges between 3 to 50 μm [13]. Cement plant kilns’ exhaust generates CKD, which has cementitious properties that make it practical for many applications [14]. Unfortunately, not all CKD materials are suitable for recycling in cement manufacturing since it contains high chloride, leading to an excessive chloride concentration in the final product [15,16,17]. Therefore, many studies have been conducted to manage CKD waste and reduce its environmental impact as well as disposal costs through utilizing it in various industrial applications [18,19,20,21,22,23].

2. Background

Cement kiln dust (CKD) can be defined as a particulate material consisting of raw material, clinker entrained particles, and some calcined raw materials collected from the exhaust gases of Portland cement kiln [25,26,27]. The production of cement kiln dust is a significant concern for cement plants, which has increased in recent years due to the direct costs associated with producing this waste material or even due to the environmental restrictions placed on the cement industry by governments.

Cement kiln dust contains fine raw materials used in kiln feed, partially some materials calcined by heat, clinker, and volatile compounds, such as alkalis, chlorides, and sulfur. These materials are entrained in the gas stream of combustion that flows countercurrent to the kiln feed and then collected in a dust collecting system (Figure 2).

3. Beneficial Applications of CKD

A large quantity of cement kiln dust is produced daily, associated with ordinary Portland cement production worldwide. Most of these quantities are disposed of in landfills to meet the environmental requirement. Meanwhile, only a small proportion can be recycled in cement manufacturing. Recently, utilizing CKD in many commercial applications has significantly increased due to the potential benefits of adding CKD, in different percentages, to the engineering properties of the prepared product. Moreover, utilizing CKD as a partial replacement for cement in industrial applications reduces the overconsumption of natural resources used for cement manufacturing and promotes sustainability [28,29]. These benefits motivated the researchers to conduct many attempts to reduce the negative impacts on natural resources and the environment, and to enhance the mechanical properties of the industrial products [11]. In the literature context, studies that investigated the practical applications of CKD are highlighted in this paper, and a discussion of their findings follows.

3.1. Utilizations of CKD in Saudi Arabia

In Saudi Arabia, the demand for cement has recently grown significantly. That comes in return for the giga-projects implemented by the government as a part of Vision 2030, such as NEOM, Red Sea Development, Amaala, Jeddah Central Project, Qiddiya, Aseer Development, and the Diriyah Gate Development project. Seventeen cement companies seek to keep up with this massive demand by producing tens of thousands of metric tons daily, which constitutes an excessive consumption of natural resources and impacts the environment. On the other hand, some cement plants suffer from the enormous quantities of cement kiln dust (CKD) generated while manufacturing the cement, since the economic and environmental problems are associated [30,31]. Furthermore, only limited amounts are recycled in cement production, while the primary proportion of produced CKD is disposed of in landfills due to alkali, sulfate, and chloride concentrations.

In recent decades, many researchers have attempted to utilize locally produced cement kiln dust (CKD) for several applications as a partial replacement for cement. Al-Refeai and Al-Karni (1999) collected various samples of CKD from cement plants located in central, southern, and eastern regions of Saudi Arabia [32]. The study aimed to overview CKD’s chemical, physical and mechanical properties, which relate to soil modification utilization (

Table 3. Physical properties of the collected CKD samples [32].

Properties	Unit	Area of Collected Samples
		S1 (Central)	S2 (Southern)	S3 (Eastern)
Specific gravity		3.01	–	3.16
Sand (%)	(%)	7.5	–	10.2
Silt	(%)	91	–	89.8
Passing Sieve # 40	(%)	100	–	100

). Three types of soil have been modified in this study: dune sand, collapsing soil, and bentonite. Conducted laboratory tests included plasticity, unconfined compression, compaction, collapse behavior, and permeability tests. Outcomes revealed improved engineering properties of the three soils and the study concluded that CKD could provide a tremendous economy as a material that comes as an alternative stabilizer to cement.

Daous (2004) utilized CKD and fly ash generated by heavy fuel oil combustion in power plants as waste materials to be blended at various ratios with Portland cement [33]. Some tests have been conducted on the blends, including normal consistency, compression and tensile strengths, and initial setting time tests. The investigation illustrated that adequate strength could be achieved using only CKD as waste material in a blend with Portland cement. Maslehuddin et al. (2008) investigated CKD usage in cement products through an intensive research review and preliminary investigation [24]. The obtained results of normal consistency, compressive strength, average drying shrinkage, autoclave expansion, and setting time tests showed that CKD could be used in cement mortar without any adverse impact on the engineering properties of the investigated cement mortar.

Mahyoup (2009) used oil fuel fly ash (FFA) and cement kiln dust (CKD) as chemical admixtures for soil treatment [17]. The compaction, California bearing ratio (CBR), durability, and unconfined compression tests were conducted for stabilized mixtures. The obtained results showed an optimization in the engineering properties of the soil compared to Portland cement when used as a traditional stabilizing agent. As the study concluded, from the strength and durability perspective, CKD is a suitable agent for stabilization compared to FFA. Ghazaly et al. (2012) reused CKD in cement concrete production [34]. Through various ratios, CKD was mixed with ordinary Portland cement. The modified cement concrete was subjected to experimental water absorption and compressive strength tests. As well, density and efflorescence tests were used for the cement bricks. The experimental results revealed that, according to standard specifications of KSA, cement bricks could be produced by mixing 30% of cement kiln dust (CKD) with ordinary Portland cement. For safety considerations, the percentage of CKD could be reduced to 20%, as the authors recommended.

Alawi (2016) conducted a laboratory investigation to study the effectiveness of CKD while using cohesive soil as a sacrificial stabilizer for roads [30]. The tested soil was soft clay collected from a depth of 3 m. The conducted testing program included unconfined compressive and direct shear strength, compaction, consistency limit, and falling head permeability tests. The results indicated that induced cementitious compounds could be gained due to the chemical reaction between high free lime in CKD and clay minerals. Calcium hydroxide Ca(OH)₂ produced from free lime in CKD is mainly responsible for the strength upon hydration. Therefore, improvement was induced on consistency limits, strain, compressive and shear strengths, compaction, and permeability of the stabilized soil. As the study concluded, adding CKD to clay soil could be used to accelerate the process of consolidation.

Al-Homidy et al. (2017) studied the feasibility of using CKD to improve the engineering properties of sabkha as weak soil [35]. CKD at 10, 20, and 30% with cement at 2% were utilized in the prepared soil and then tested using California bearing ratio, compressive strength, and durability tests. Outcomes showed that 30% of CKD with 2% of cement was adequate for effective stabilized soil. Additionally, adding CKD alone would not be sufficient for the effectiveness of sabkha stabilized soil. Alharthi et al. (2021) investigated the benefits that could be potentially achieved by replacing a proportion of cement with cement kiln dust (CKD) for cement blocks and plain concrete as construction applications [11]. The untreated raw material of electrostatic precipitators CKD was used in this study. Five tests were conducted in the experimental program: compressive strength, tensile strength, air content for plain concrete, compressive strength, and an absorption test conducted on cement blocks. The research demonstrated that CKD could be utilized as a primary component in cement blocks and concrete products, such as cement tiles, curbs, and plain concrete.

Table 4. Summary of the studies that investigated CKD materials in Saudi Arabia.

Author/s	Ref.	Year	Objective	Testing Program
Al-Refeai and Al-Karni	[32]	1999	Use CKD in problematic soil treatment.	Plasticity, unconfined compression, compaction, collapse behavior, and permeability tests
Daous	[33]	2004	Use CKD and fly ash blends modification.	Normal consistency, compression/tensile strengths, and initial setting time
Maslehuddin et al.	[24]	2008	Conduct preliminary study.	Normal consistency, compressive strength, average drying shrinkage, autoclave expansion, and setting time tests
Mahyoup	[31]	2009	Use CKD and oil fuel fly ash for indigenous soil stabilization.	California bearing ratio, durability, compaction, and unconfined compression tests
Ghazaly et al.	[34]	2012	Use CKD for modified cement concrete.	water absorption and compressive strength tests
Alawi	[30]	2016	Use CKD for clay soil stabilization.	Unconfined compressive and direct shear strengths, compaction, consistency limit, and falling head permeability tests
Al-Homidy et al.	[35]	2017	Use CKD for sabkha soil improvement.	California bearing ratio, compressive strength, and durability tests
Alharthi et al.	[11]	2021	Use CKD for modified cement blocks and plain concrete.	Compressive strength, tensile strength, and air content for plain concrete, compressive strength, and absorption tests for cement blocks

summarizes the recent studies that have successfully used cement kiln dust (CKD) for various Saudi Arabia applications. At the same time,

Table 5. Summary of chemical composition of CKD materials investigated in KSA.

Chemical Composition (%)		[33]	[24]	[11]	[35]	[30]	[32]
							S #1	S #2	S #3
Calcium oxide	CaO	42.02	49.3	49.4	49.3	40	63.59	54.79	63.18
Silica	SiO2	14.42	17.1	18.2	17.1	17	15.73	17.74	15.69
Aluminum oxide	Al2O3	4.14	4.24	4.52	4.24	–	5.01	5.26	4.22
Magnesium oxide	MgO	1.55	1.14	1.21	1.14	1.45	–	1.33	1.14
Sodium oxide	Na2O	2.6	3.84	3.84	3.84	–	0.26	0.2	0.26
Potassium oxide	K2O	3.26	2.18	2.38	2.18	10	1.33	1.38	2.14
Iron oxide	Fe2O3	3.97	2.89	2.92	2.89	2	2.41	3.01	4.66
Sulfur trioxide	SO3	1.47	3.56	5.66	3.56	5	1.97	4.06	3.24
Chloride	Cl-1	4.12	6.9	5.9	6.9	6.5	–	–	–
Loss on ignition	LOI		15.8	17.1	15.8	–	8.69	11.66	4.4

illustrates the chemical compositions of the locally investigated CKD.

3.2. Global Utilizations of CKD

Over the past few years, in support of sustainability and due to the technical developments used in the cement industry, global dramatic progress has been made in generating, managing, and reusing CKD (

Table 6. Historical statistics of managed and reused cement kiln dust (CKD) * [9].

Date	Surveyed Plants	CKD	CKD	CKD	Clinker	CKD
		Reused Quantity (KMT)	Transport to Landfill (KMT)	Reused from Landfilled (KMT)	Annual Production (KMT)	To a Landfill Per Clinker Produced, (kg/MT)
1990	84	752	2655	No data	44,360	60
1995	94	651	3146	No data	61,729	51
1998	95	768	2499	13	67,104	37
2000	92	574	2223	79	68,263	33
2001	102	924	2329	231	75,683	31
2002	101	664	1989	103	77,636	26
2003	102	718	1995	116	79,356	25
2004	102	917	1993	69	83,945	24
2005	102	987	1429	104	85,568	17
2006	101	1160	1403	261	86,686	16

Note *—Survey covered the Portland Cement Association (PCA) member companies.

and Figure 3). Along with environmental impacts, these efforts reduce costs and reduce dependency on landfills. For example, in 2006, as reported by the Portland Cement Association (PCA) in the US, 1,160,011 metric tons of CKD generated by member companies were utilized in various applications [9]. This amount was utilized in soil and waste stabilization, cement additives (blending), mine reclamation, agricultural soil amendments, sanitary landfill liners, concrete products, pavement manufacturing, wastewater neutralization, etc. The soil stabilization had the most considerable utilization, with 533,356 metric tons successfully reused. Cement kiln dust (CKD), a byproduct of the cement industry, has been introduced as a potential material that can provide economic and environmental benefits, as illustrated in many research investigations supported by positive experimental test results.

Although tremendous technical and industrial progress has recently occurred in the cement industry, which contributed to a significant decrease in the quantities produced as cement kiln dust (CKD), there are still amounts generating that constitute an essential concern regarding transportation costs and negative impacts on the environment [30]. Therefore, the focus is now on commercial applications that can use CKD.

Through a research review, Elbaz et al. (2019) discussed the beneficial use of CKD as well as fly ash (FA) from an environmental and economic point of view [36]. As revealed by the literature review, the examined CKD and FA waste materials negatively impact the environment if not disposed in an environmentally safe manner. The adverse effects of both investigated materials may include ozone depletion, biodiversity loss, acid rain, reduced crop productivity, and global warming. On the other hand, the study concluded that numerous applications, such as in the agricultural field, waste treatment, soil stabilization, and pollution control, could utilize these byproduct materials for optimization purposes. At the same time, the authors recommended that by reusing these materials, many advantages could be achieved, such environmental and human health protection and raw materials conservation.

Another study introduced by Saleh et al. (2021) presented the improvement and green sustainability that could be achieved by using CKD [37]. As stated, alkaline compounds such as CKD could activate many inert waste materials. Due to containing Portland cement as well as Ca(OH)₂, additional products of cement could be developed. Cement production depends totally on the extraction of resources, such as limestone, fossil fuels, and other natural minerals; one ton of produced clinker is required as well as 1.5–1.7 of raw materials. Moreover, cement manufacturing requires more energy with a temperature of 2000 °C. Therefore, the study concluded that CKD could partially replace cement to produce sustainable products.

3.2.1. Soil Stabilization

Ghorab et al. (2007) discussed soil chemical stabilization for producing some building units like tiles, bricks, wall plaster, and paving roads [38]. The investigation used many industrial wastes, such as blast furnace and steel slag, powder of red bricks, and cement kiln dust (CKD), to design the mixes later subjected to a compressive test to evaluate their durability. The study’s findings showed that industrial waste, including CKD, could contribute to low-cost housing and other practical advantages. Another attempt was made by Moon et al. (2008) to assess the stabilization of arsenic-contaminated soil using CKD [39]. Amounts of 10 to 25% of CKD was utilized for a prepared slurry treatment. Treatment effectiveness was evaluated in a 1–7-day curing period. As the obtained results revealed, arsenic could be immobilized due to CKD treatment. Moreover, the study recommended more investigation for the commercial stabilization and solidification of soil induced by CKD in terms of achieving cost-effectiveness as well as expanding the range of beneficial uses of CKD material.

Sreekrish et al. (2007) evaluated some samples of CKD (fresh and landfilled) for the utilization in soil treatment [40]. CKD with 8–20% replacement used for combined soil was tested using unconfined compression, compaction, swell, pH, and Atterberg limits tests. The laboratory study indicated that the fresh sample had sufficient reactivity and potentially could be utilized for soil stabilization due to induced strength improvement and swelling strain reduction, such as the results obtained with 4% Portland cement. As well, the engineering properties of CKD in the stabilization of two modified soil sample was presented by Carlson et al. (2011) [41]. Typically, the collected soils were wet and had a geotechnical pose during construction. CKD at 5, 10, 15, and 20% was added and then subjected to drying rate, unconfined compression, standard proctor, and Atterberg limits tests. The investigation results illustrated a significant improvement in the unconfined compressive and drying rate for CKD-treated soil related to increasing CKD proportions. Additionally, a higher percentage of CKD could be utilized for geotechnical construction stabilization purposes and contribute to landfill cost-saving, as the authors recommended. Ebrahimi et al. (2012) studied the effectiveness of CKD for stiffness improvement of recycled base materials, such as pavement and road surface gravel [42]. Testing programs were conducted using resilient and seismic modulus. The percentages of added CKD were 0, 5, 10, and 15% and were evaluated by a reference binder, Portland cement. The outcomes showed improvement in the modulus from 5 to 30 times based on the content of CKD and the base materials type. The study recommended that CKD is suitable for geotechnical applications, considering the change of expansion and modulus during the curing process. Albusoda and Salem (2012) conducted an extensive experimental testing program to determine the geotechnical properties of CKD-stabilized dune sand [43]. The course of tests conducted on modified soils included Atterberg limits, compaction, direct shear, triaxial compression, collapse, loading, and time of curing difficulties. As the study concluded, an irregular decrease was induced in the liquid limit by CKD when mixed with sand, in addition to a high value in cohesion with increasing the percentage of CKD. After fourteen curing days, shear strength parameters were constant, inducing no variation. The researchers recommended that soil collapse with CKD could achieve enormous economic advantages; the ultimate bearing capacity could be increased to 250% by adding 8% of cement kiln dust (CKD) materials.

In the same context, Okafor and Egbe (2013) studied the potentiality of CKD in subgrade improvement [44]. The study aimed to reduce the construction cost by converting byproducts, such as CKD, into restorative materials used in construction and soil improvement. CKD at 2–24% was utilized to stabilize the collected soil samples. Investigated properties were subjected to unconfined compression strength, compaction, California bearing ratio, and consistency limits tests. Results obtained indicated that increasing CKD content led to optimum moisture content while reducing the plasticity. Likewise, the unconfined compression strength and California bearing ratio improved while CKD content increased. The study also developed a high correlation coefficient model to be used successfully to predict the properties of soil-CKD in the case that experimental soil data are absent. As the investigation illustrated, they yielded the maximum improvement with 24% of CKD. Gupta et al. (2015) used CKD to treat cadmium-contaminated soil to improve the engineering properties and inactivate the present contaminants [45]. CKD was added at various percentages of 1, 2, 4, 6, 8, and 10%. Atterberg limits, toxicity characteristic leaching procedure (TCLP), and unconfined compressive strength tests were conducted on the modified soil. The outcomes indicated benefits induced on the engineering properties of the contaminated soil; the maximum stabilization was obtained at 8% of CKD. At the same time, the TCLP test showed that it inactivated 80.70% of the cadmium in the soil. Yoobanpot et al. (2017) used cement kiln dust (CKD) and fly ash (FA) to improve the unconfined compressive strength (UCS) of soft clay material [46]. Curing for 3, 7, 28, and 90 days was performed to prepare soft clay stabilized for UCS testing. The researchers also conducted X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques to investigate the reaction product and microstructure changes in the modified clay. The outcomes revealed that 13% CKD and 20% mixtures illustrated the optimum strength at 90 days of curing, such as the stability provided by the 10% content of ordinary Portland cement (OPC).

Arulrajah et al. (2017) investigated cement kiln dust (CKD) and fly ash (FA) as pozzolanic materials to be an alternative binder for construction and demolition aggregates [47]. The modified material was tested by undertaking the repeated load triaxial test to evaluate the durability and the unconfined compressive strength test for strength evaluation. A mixture design of 20% CKD and 10% FA achieved the optimum stabilizing performance. As the study concluded, CKD could be used in low-carbon activities of civil construction. Likewise, Mohammadinia et al. (2018) combined CKD and FA to increase the stiffness and strength of the demolition aggregates, containing an aggregate of recycled concrete, reclaimed asphalt pavement, and crushed brick [48]. The modified mixture was subjected to the repeated load triaxial test for durability and the unconfined compressive strength test for strength evaluation. The study concluded that the composite sample achieved the optimum engineering properties at 15% CKD and 15% FA. An investigation of the effect of using CKD and periwinkle shell ash (PSA) blends on the plasticity of lateritic soil was carried out by Ekpo et al. (2021) [49]. CKD at 0, 5, 10, 15, and 20% was used for soil treatment. The testing program included Atterberg limits, plasticity, UCS, X-ray diffraction (XRD), and scanning electron microscope (SEM) tests. Results concluded that CKD and PSD are viable stabilizers for tropical soils.

The results of studies that utilized CKD for soil stabilization showed that CKD could be added as an activator for pozzolanic materials. It has been successfully mixed with many industrial wastes, such as blast furnaces, periwinkle shell ash, fly ash, and steel slag, with various percentages (from 2–30%) to improve the mechanical properties of soil. Moreover, CKD exhibited strength in inactivating the contaminated soil’s toxic elements, such as arsenic and cadmium, introducing CKD as one of the low-cost solutions in the soil stabilization field. The summary of recent studies that used cement kiln dust (CKD) for soil stabilization in various applications is illustrated in

Table 7. Summary of studies that investigated the utilization of CKD for soil stabilization.

Author/s	Ref.	Year	Objective	Testing Program
Ghorab et al.	[38]	2007	Use blast furnace and steel slag, powder of red bricks, and CKD to produce building units.	Unconfined compressive strength
Moon et al.	[39]	2008	Use CKD for stabilization of arsenic-contaminated soil.	Toxicity characteristic leaching procedure, X-ray diffraction, and scanning electron microscope.
Sreekrish et al.	[40]	2007	Use CKD as soil treatment (fresh and landfilled materials).	Unconfined compressive strength, compaction, swell, pH, and Atterberg limits
Carlson et al.	[41]	2011	Use CKD for soil samples stabilization.	Drying rate, unconfined compressive strength, standard proctor, and Atterberg limits
Ebrahimi et al.	[42]	2012	Use CKD for stiffness improvement of recycled base materials.	Resilient and seismic modulus
Albusoda and Salem	[43]	2012	Use CKD for dune sand stabilization.	Atterberg limits, compaction, direct shear, triaxial compression, collapse, loading, and time of curing difficulties
Okafor and Egbe	[44]	2013	Use CKD as restorative material used in construction and soil improvement.	Unconfined compressive strength, compaction, California bearing ratio, and consistency limits
Gupta et al.	[45]	2015	Use CKD for cadmium contaminated soil treatment.	Atterberg limits, toxicity characteristic leaching procedure (TCLP), and unconfined compressive strength
Yoobanpot et al.	[46]	2017	Use CKD and fly ash (FA) to improve the soft clay material.	Unconfined compressive strength, X-ray diffraction (XRD), and scanning electron microscope (SEM) techniques
Arulrajah et al.	[47]	2017	Use CKD and fly ash (FA) as alternative binder for construction and demolition aggregates.	Repeated load triaxial and unconfined compressive strength
Mohammadi. et al.	[48]	2018	Use CKD and FA to improve demolition aggregates.	Repeated load triaxial and unconfined compressive strength
Ekpo et al.	[49]	2021	Use CKD and periwinkle shell ash (PSA) blends to improve the lateritic soil.	Atterberg limit, plasticity, UCS, X-ray diffraction, and scanning electron microscope

below.

3.2.2. Replacement for Cement

Al-Harthy et al. (2003) investigated the use of CKD in concrete and mortar as a cementitious material [50]. The study aimed to add CKD to the mixture of concrete and mortar to evaluate the effect on strength and water absorption. The conducted experimental work included compressive strength, toughness, flexural strength, initial surface absorption, and sorptivity tests. The replacement of CKD included amounts at 0 (control), 5, 10, 15, 20, 25, and 30%. The results showed that the concrete mixtures’ compressive strengths, toughness, and flexural values with 5% CKD were close to the control mix. The study revealed that no strength was gained by substituting cement with CKD for all samples investigated; meanwhile, no negative impact on strength properties occurred when the proper addition of CKD was added. Additionally, better absorption characteristics were achieved by adding a suitable proportion of CKD. Likewise, the effect of adding CKD admixture as a partial replacement for cement was investigated by Mohammad and Hilal (2010) [51]. The content of CKD was 10, 30, and 50% of the weight of cement. The study selected three mixes of CKD and one without admixture as a reference mix. A set of tests were conducted on the modified mixes such as compressive strength, splitting tensile strength, ultra-sound velocity (UPV), flexural strength, slump, and static elasticity modulus. Obtained results indicated a significant decrease in the strength of the modified concrete; at 28 days of the curing period, the compressive strength was 28, 25, and 22 MPa for 10, 30, and 50% CKD content, respectively. In comparison, the reference mix gained a 35 MPa compressive strength. Marku et al. (2012) attempted to utilize CKD as a partial replacement for cement to produce mortar and concrete [52]. The study prepared various blended materials with 0–45% of CKD. In some blends, fly ash and blast furnace slag were added to CKD. The test program included compressive strength, flexure, and durability tests. The investigation demonstrated a gain in the low strength of CKD–OPC blends due to calcium silicate absence and low fineness of CKD. However, it is possible to combine CKD with pozzolanic materials such as blast furnaces, fly ash, etc.

Through a review paper, Kunal et al. (2012) discussed the utilization of CKD in cement concrete and its leachate characteristics [5]. The review concluded that 5–10% of CKD in the mortar and concrete mixtures could achieve similar engineering properties to the control mixes. CKD could be used as an activator for copper slag, blast furnace slag, and other industrial wastes. In the future, the blend of CKD, slag, Portland cement (CKD–slag–PC) could be potentially considered as an alternative to Portland cement with good engineering properties. CKD–slag–PC blends have low alkali, making them more effective for strength improvement than CKD–PC blends. El-Mohsen et al. (2015) studied the use of CKD as a partial replacement of cement in self-consolidating concrete (SCC) to reduce production costs as well as for environmental reasons [53]. The study used a partial replacement of cement with 10, 20, 30, and 40% of CKD to produce four mixes. The engineering properties of modified mixtures were investigated using consistency, compressive and indirect tensile strengths, flexural, and shrinkage tests. The findings indicated the possibility of producing a modified SCC with engineering properties nearly like the control mix, which could introduce a cheaper product and eliminate the negative impact on the environment. SCC with 20% of CKD content was found to be the optimum mix content in terms of flexural, compression, and splitting strengths at 28 days of the curing period compared to the control mix. The values of shrinkage strain also showed that SCC with 10% of CKD is close to the control mix value.

Hussain and Rao (2014) conducted a detailed experimental work investigating CKD and fly ash as a cement replacement in the concrete [54]. They obtained fly ash residue from coal combustion and CKD, which reduces the hydration heat, and prepared cubes of 100 cubic mm with various compositions. For 28 curing days, the modified mixture was to compressive strength by destructive and non-destructive tests, such as ultra-sonic pulse velocity (UPV). The outcomes illustrated that concrete integrity is suitable for all mixtures. However, the attained strength of concrete with 10% CKD complied with the target; if CKD increases more than 10%, that reduces the concrete strength. Sadek et al. (2017) evaluated the durability and physico-mechanical properties of concrete containing CKD as a partial replacement and addition to cement in paving blocks [55]. Water absorption, compressive and tensile strengths, abrasion resistance, and slip or skid resistance tests were conducted on the designed mixtures. As the study revealed, there is a possibility to utilize a large volume of CKD in paving blocks for environmental considerations and sustainability. At the same content, using CKD as an addition is better than a replacement; adding 20% of CKD could achieve comparable engineering properties to control blocks. Up to 10% of CKD could be used as a partial replacement for cement without any adverse effect on paving block properties.

Saleh et al. (2020) utilized two problematic waste materials, CKD and poly(styrene), to produce lightweight bricks for construction applications [56]. The study aimed to improve the mechanical properties of the designed product, and some additives were used, such as iron slag, Portland cement, and crushed waste glass. The prepared mixtures were measured using compressive strength and water absorption tests. Experimental results showed that mixing the waste materials improved the combination with little cement. The mix, which contained 3% CKD, 5% iron slag, 10% crushed waste glass, and 10% cement, achieved 2.86 MPa in compressive strength; this value was higher than the minimum strength recommended by the standard (2.00 and 1.6 MPa) for non-loading-bearing bricks that. Additionally, Bagheri et al. (2020) recycled CKD and fly ash to improve some properties of concrete and reduce the carbon dioxide effect [57]. CKD at percentages of 0–40% was utilized, while fly ash content ranged from 0–30%. A compressive strength test evaluated the prepared mix at 7, 14, 28, and 90 days of the curing period. The gained outcomes indicated that the designed mixture achieved the maximum strength with 5% CKD and 15% fly ash. Moreover, if the CKD replacement exceeded 20%, the strength decreased significantly.

The outcomes of studies that utilized cement kiln dust as a replacement for cement in concrete and mortar mixtures, paving blocks, and lightweight bricks, revealed that CKD could substitute cement with various proportions (5–50%). Meanwhile, the results indicated no significant impact on prepared mixtures’ engineering properties when adding the proper percentage of CKD material. The summary of recent studies that used cement kiln dust (CKD) as cement replacement for various applications is illustrated in

Table 8. Studies that investigated the utilization of CKD in the concrete applications.

Author/s	Ref.	Year	Objective	Testing Program
Al-Harthy et al.	[50]	2003	Utilize CKD in concrete and mortar mixtures.	Compressive strengths, toughness, flexural strength, initial surface absorption, and sorptivity
Mohammad and Hilal	[51]	2010	Add CKD admixture as partial replacement for cement.	Compressive strength, splitting tensile strength, ultra-sound velocity (UPV), flexural strength, slump, and static elasticity modulus
Marku et al.	[52]	2012	Utilize CKD as a partial replacement for cement in mortar and concrete producing.	Compressive strength, flexure, and durability
El-Mohsen et al.	[53]	2015	Use CKD as a partial replacement for cement in self-consolidating concrete (SCC).	Consistency, compressive and indirect tensile strengths, flexural, and shrinkage
Hussain and Rao	[54	2014	Utilize CKD and fly ash as a cement replacement in the concrete.	Compressive strength by destructive and non-destructive tests such as ultra-sonic pulse velocity (UPV)
Sadek et al.	[55]	2017	Use CKD as a partial replacement for and addition to cement in paving blocks.	Water absorption, compressive and tensile strengths, abrasion resistance, slip/skid resistance
Saleh et al.	[56]	2020	Utilize two problematic waste materials, CKD and poly(styrene), to produce lightweight bricks.	Compressive strength and water absorption
Bagheri et al.	[57]	2020	Recycle CKD and fly ash to improve some properties of concrete.	Compressive strength

below.

3.2.3. Treatment Agent

Peetham et al. (2009) utilized CKD to treat kaolinite clay and investigate the induced physicochemical behavior [58]. CKD with high free lime was used in the treatment process to evaluate its effectiveness. The study’s testing program included tests of unconfined compressive strength, Atterberg limits, and stiffness of compacted soil. At the same time, they compared the obtained results to the results of untreated clay and clay treated with calcium oxide (quicklime). The physicochemical behavior was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray analyses. This step determined the induced morphologic and mineralogical changes on the 90-day curing period specimens. The study concluded that high free lime CKD effectively caused an improvement in the compressive strength, Atterberg limits, and stiffness of the compacted-soil-treated kaolinite clay. Another study conducted by Mackie and Walsh (2012) used CKD as a neutralization agent for acidic mine water treatment in place of quicklime [59]. In addition, the evaluated the performance of generated slurries of calcium hydroxide (Ca(OH)₂) from CKD to reduce mine water’s metal concentrations and acidity. The study used techniques to analyze the settled water quality, mine water pH, and micro-flow imaging. The obtained results showed that CKD could achieve viable active treatment by removing 98% and 97% of zinc and iron, respectively, and acidity reduction.

CKD was also successfully utilized as a treatment agent in many fields, such as removing copper and zinc contamination from acidic groundwater [60], adding an improvement in aluminum metal matrix composites [61], benefiting municipal wastewater as an effective treatment agent [62], supporting pozzolanic material production as a sustainable solution [63], improving benzene-contaminated groundwater as a capable treatment material [64], inducing benefits to magnesium phosphate cement [65], and contributing to eliminate the hazard of groundwater contaminated by cadmium ions [66]. The utilization of CKD waste material as a treatment agent could provide a wide range of advantages in this field. It creates a potential new market, reducing the cost of chemical treatments and eliminating the negative impact on the environment due to the landfill disposal.

The finding of investigations that used cement kiln dust as a treatment agent showed that CKD has chemical and physical properties that effectively treat a wide range of materials such as kaolinite clay, acidic mine water, and copper- and zinc-contaminated acidic groundwater, as well as aluminum metal matrix composites, municipal wastewater, pozzolanic materials, benzene-contaminated groundwater, magnesium phosphate cement, and cadmium ions in groundwater. The utilization of CKD as byproduct waste from cement manufacturing in the treatment field could provide many advantages related to this field, such as reducing the chemical treatment cost and eliminating the negative impact on the environment.

Table 9. Summary of studies that investigated the utilization of CKD as treatment agent.

Author/s	Ref.	Year	Objective
Peethamparan et al.	[58]	2008	Utilize CKD to treat kaolinite clay and investigate the induced physicochemical behavior.
Mackie and Walsh	[59]	2011	Use CKD as a neutralization agent for acidic mine water treatment in place of quicklime.
Sulaymon et al.	[60]	2015	Use CKD to remove copper and zinc contamination from acidic groundwater.
Hammood et al.	[61]	2017	Add CKD to improve aluminum metal matrix composites.
Mahmoued	[62]	2014	Utilize CKD as an effective treatment agent for municipal wastewater.
Abdel-Gawwad et al.	[63]	2019	Use CKD as a sustainable solution for pozzolanic materials production.
Faisal et al.	[64]	2021	Use CKD as a capable treatment material for benzene contaminated groundwater.
Baghriche et al.	[65]	2020	Use CKD to induce benefits of magnesium phosphate cement performance.
Faisal et al.	[66]	2021	Utilize CKD to eliminate the hazard of groundwater contaminated by cadmium ions.

summarizes the recent studies that used cement kiln dust (CKD) as treatment agent.

3.2.4. Ceramic and Brick

Aydin et al. (2019) used CKD as a CaO alternative source to produce ceramic wall tiles [67]. The researchers investigated the effect of CKD on the composition with various proportions (15% max) and then sintered the shaped samples at 1150 °C. Experimental work conducted to test the physical properties included water absorption, bulk density, linear firing shrinkage, and flexural strength. Meanwhile, the sintering behavior was evaluated using an optical dilatometer. The obtained results revealed CKD as an effective source of CaO and could be sufficiently used in the production of ceramic wall tile. Ewais et al. (2015) exploited CKD and quartz sand with various baths to produce wollastonite ceramics [68]. The mechanical properties of the fired batches were investigated using compressive strength and hardness tests. The outcomes indicated that wollastonite ceramics could be successfully produced by adding quartz to CKD.

Mahrous and Yang (2011) conducted a study to investigate the cement and CKD combination properties while producing cement bricks [69]. CKD with 0–40% proportions was increased gradually as a replacement for cement in the cement brick mixtures. The modified bricks were subjected to testing programs including density, compressive strength, and water absorption tests. The findings indicated that the properties of the produced cement and CKD brick with 30% of CKD content could meet the Korean standards while 40% of CKD could meet the Egyptian code. The study concluded that the modified bricks with CKD components have beneficial values from a properties and cost perspective. Ogila (2014) studied the effect of utilizing CKD on the red clay brick’s physical and mechanical properties [70]. Brick samples were prepared with dimensions of 20, 35, and 70 mm by mixing clay with different proportions of CKD (2, 4, 8, 10, and 12%). The prepared mixture batches were mixed with water to achieve plastic masses and then sintered for three hours at 950–1100 °C using an electric furnace. At the same time, the raw clay control brick was designed with 0% CKD. Atterberg limits, mineralogical composition, color and surface appearance, water absorption, firing linear shrinkage, firing weight loss, and efflorescence were investigated. The obtained results demonstrated that the properties of the produced brick strongly depend on the CKD and firing temperature. As the study concluded, CKD as a clay material substitute could provide a feasible way to make quality bricks. El-Attar et al. (2017) also recycled CKD in the brick industry through an investigation conducted for sustainability by limiting environmental concerns [71]. The study also examined the carbon footprint emitted from solid cement brick manufacturing (

Table 10. Summary of carbon footprint emitted from cement brick manufacturing [72].

CKD%	Materials (Cement, Aggregate, and Water)	Transport	Brick Production	Total Emission
	(Kg CO2e/m3 of Bricks)
0%	150.88	11.39	17.87	180.11
30%	109.03	11.39	17.87	138.26
50%	81.13	11.39	17.87	110.36

). Mixtures were prepared with 0, 30, and 50% of CKD and evaluated by compressive strength, water absorption, and unit weight. The obtained results showed acceptable properties of produced brick with CKD content up to 50% instead of cement, which provides an economical and environmentally friendly solution for concrete brick manufacturing.

Abdel-Gawwad et al. (2021) utilized CKD with the waste of red clay bricks and silica fume as the main ingredients to produce unfired building bricks [72]. The prepared composite of CKD–RCBW–SF was adjusted at various proportions of content. The ready-mix was subjected to the blending, water mixing, and casting stages. Some conditions, such as air, water, and CO₂ gas, were considered in the study for designed mixture curing. The grade of hardened bricks was evaluated based on compressive strength, saturation coefficient, water absorption, and bulk density. The study outcomes indicated that curing in water medium achieved the highest performance, with 47 MPa at 28 days and a proportion of 50 to 20 to 30 for the composite of CKD–RCBW–SF, respectively. CKD was utilized as an alkali material that activated the aluminosilicate in the silica fume and brick waste to yield a hardened product containing calcium silicate hydrate upon hydration. Moreover, the researchers used thermogravimetric and X-ray diffraction analyses to identify the calcium carbonate and calcium silicate hydrate and confirm that the occurrence of both carbonation and hydration reactions were induced with every curing medium.

Udoeyo et al. (2002) used CKD as an additive and replacement material for cement while producing hollow building blocks [73]. The properties of the prepared product were evaluated by workability (compaction factor), compressive strength, density, and water absorption. The laboratory investigation results showed that produced blocks with 5, 10, and 20% of CKD as a replacement material achieved an excellent compressive strength value that was higher than the control block strength. The study recommended that CKD as a block manufacturing replacement material has more benefits than concrete blocks, such as low production costs, and advised investigating more properties in the future, such as shrinkage and short- and long-term durability.

Abdulkareem and Eyada (2018) used two types of CKD with sand and cement to produce pressed building brick [74]. The investigation was carried out in three phases: choosing a type of CKD and sand, mixing the selected materials with cement and water, and aiming to increase the compressive strength and absorption of brick to 11 MPa without adding more cement to reduce the cost. The obtained results showed that all properties of produced bricks were satisfactory according to ASTM standards, which characterize international applications for the product made by waste. The study concluded that many benefits could be achieved by using CKD in the production of bricks, such as using economic bricks for building, reducing the dependency on natural resources, reducing pollution, and reducing negative impacts on the environment.

In the ceramic and brick industry, cement kiln dust (CKD) was introduced as an alternative partial cementitious source at various proportions (2–50%). CKD was successfully utilized with improved engineering properties in the products of ceramic wall tile, wollastonite ceramics, cement bricks, unfired building bricks, and hollow building blocks. As the gained results revealed, CKD could provide a set of advantages to the ceramic and brick industry, such as improving the product’s properties, reducing production costs, promoting sustainability through protecting the natural resources and environment, and eliminating the pollution associated with these industries. Studies conducted to utilize CKD in ceramic and brick manufacturing are summarized in

Table 11. Summary of studies that investigated the utilization of CKD in ceramic and bricks manufacturing.

Author/s	Ref.	Year	Objective	Testing Program
Aydin et al.	[67]	2019	Use CKD as a CaO alternative source to produce ceramic wall tile.	water absorption, bulk density, linear firing shrinkage, and flexural strength
Ewais et al.	[68]	2015	Exploit CKD and quartz sand with various baths to produce wollastonite ceramics.	Compressive strength and hardness
Mahrous and Yang	[69]	2011	Investigate the cement-CKD combination properties while producing cement bricks.	Density, compressive strength, and water absorption
Ogila	[70]	2014	Study the effect of utilizing CKD on the red clay brick’s physical and mechanical properties.	Atterberg limits, mineralogical composition, color and surface appearance, water absorption, firing linear shrinkage, firing weight loss, and efflorescence
El-Attar et al.	[71]	2017	Recycle CKD in the bricks industry.	Compressive strength, water absorption, and unit weight
A.-Gawwad et al.	[72]	2021	Utilize CKD with the waste of red clay bricks and silica fume as the main ingredients to produce unfired building bricks.	Compressive strength, saturation coefficient, water absorption, and bulk density
Udoeyo et al.	[73]	2002	Use CKD as an additive and replacement material for cement while producing hollow building blocks.	Workability (compaction factor), compressive strength, density, and water absorption
Abdulkareem and Eyada	[74]	2018	Use two types of CKD with sand and cement to produce pressed building brick.	Compressive strength and water absorption

.

3.2.5. Miscellaneous Applications

Due to the high produced quantities of CKD and its associated disposal cost and environmental concerns, researchers have made great efforts to find various fields that can reuse CKD [75]. Moreover, CKD has a chemical composition containing potassium, sodium, and calcium oxides, making it a suitable and inexpensive activator for pozzolanic materials, such as slag and fly ash, compared to conventional activators utilized in the activation of alkali [76]. Therefore, CKD could be used in a wide range of practical applications. Colangelo and Cioffi (2013) used CKD with two other solids, marble sludge waste and granulated blast furnace slag, in the process of cold bonding pelletization for the sustainable production of artificial aggregates [77]. The study aimed to explore the activation action of CKD components on slag hydraulic behavior by evaluating the phases that are neo-formed and presented in hydrated pastes and, especially, the effect of free lime and sulfate of CKD on slag reactivity. Finally, the produced product proved suitability in cold bonding pelletization to produce artificial aggregates.

Kalina et al. (2018) effectively used CKD as an alkaline activator for blast furnace slag pozzolanic materials considered to be latent hydraulic substances [78]. The obtained results indicated that sodium hydroxide could be created from the combination of CKD and sodium carbonate. The produced sodium hydroxide contributed to the dissolution of the particles of slag, and then formed a binder phase with better compressive and flexural strength and reduced the material cracking. Taha (2003) combined two waste products for road construction, CKD and an aggregate of the reclaimed asphalt pavement [79]. The outcomes showed that 15% CKD achieved the optimum strength. Accordingly, the study concluded that CKD could be successfully used for material stabilization of pavement in the base or subbase. In the same context, Button (2003) prepared a literature review document for the Texas Department of Transportation regarding the utilization of CKD in the stabilization of highway pavement subgrade soil and base material [80]. The study concluded that higher compressive strength is attainable by combining CKD and pozzolanic materials. They recommended that CKD should be kept dry to maintain its potential cementitious properties. Stockpiled CKD exposed to a long-period environment (aged CKD) has only a little free lime since the hydration and reactivity are consequently lost.

3.2.6. Mine Backfill

In the mining field, particularly for backfilling applications (cemented paste backfill), Lutyński and Pierzyna (2017) reused CKD as a backfill material and mineral adsorbent for CO₂ due to the high content of free lime (unreacted CaO) [81]. The study investigated the slurry properties of CKD and bottom slag through the compressive strength and water content. At 28 days of the curing period, the compressive strength of the slurry of CKD with bottom slag achieved a value of 4.7 MPa, which is higher than the threshold of the standard required strength (0.5 MPa). Excess water, observed at 0.6%, was much lower than the standard value (7%). The investigation findings indicated that CKD with CaO-high content could be considered an activator while mixing with silicate materials, significantly increasing compressive strength. Finally, the study recommended that if the leaching test results showed positive values, the slurry of CKD with bottom slag could be utilized in underground backfilling.

Likewise, Beltagui et al. (2018) utilized the blends of high free lime CKD, fly ash, and cement for mine backfill [82]. CKD with 29% free lime content was used as a high alkaline material to activate the pulverized fuel ash (PFA). The mixed and compacted blends in cylinders of 100 mm and 50 mm in length and diameter, respectively, were subjected to a compressive strength test in curing periods of 28 and 56 days to evaluate the achieved properties. Moreover, thermogravimetric and X-ray diffraction were conducted to assess the hydration products. Despite the many applications that can use CKD, reinforced concrete utilization will be impossible since the high chloride level may induce corrosion risk to the reinforcement. The results showed that the compressive strength of CKD–PFA blends ranged between 4.7 to 5.6 MPa and surpassed the required strength (3 MPa) at 56 days and a water–blend ratio of 0.2. At the same time, the maximum content of CKD that could be used and gain the required strength was 90% with 10% cement.

As concluded, the sustainability of using CKD for underground mine backfilling is confirmed through the investigation outcomes. However, some points should be considered, such as the finding that free lime in CKD rabidly absorbs the water; therefore, much water is in demand. Furthermore, mixing a large quantity of CKD with water produces large amounts of heat. Eventually, the fineness of CKD, which makes it challenging to deal with at underground mines, requires preparation on the surface and the pumping of slurry below. As recommended, further work will investigate hydrated paste’s microstructures.

In the mining industry, the daily operations extract vast amounts of materials for further processing and, simultaneously, voids are created. The induced holes may cause concerns from a safety, production, and environmental perspective. Cemented paste backfill (CPB) is a modern technology of backfill used to fill the underground voids in the presence of cement and mine tailings. The challenge related to the CPB method is the cement (binder) cost, which represents a high percentage of backfilling operation costs. The outcomes of studies that utilized cement kiln dust as a partial replacement for cement in this area indicated that CKD could effectively be used with an induced improvement on the mechanical properties of the prepared mixtures. That could provide a possible way to optimize the backfilling process. Studies that investigated the utilization of CKD in miscellaneous applications are summarized in

Table 12. Summary of studies that investigated the utilization of CKD in miscellaneous and mining applications.

Author/s	Ref.	Year	Objective
Colangelo and Cioffi	[77]	2013	Explore the activation action of CKD components on slag hydraulic behavior.
Kalina et al.	[78]	2018	Use CKD as an alkaline activator for blast furnace slag pozzolanic materials considered latent hydraulic substances.
Taha	[79]	2003	Combine two waste products for road construction, CKD, and aggregate of the reclaimed asphalt pavement.
Button	[80]	2003	Utilize CKD in the stabilization of highway pavement subgrade soil and base material.
Lutyński and Pierzyna	[81]	2017	Reuse CKD as a backfill material and mineral adsorbent for CO2 due to the high content of free lime (unreacted CaO).
Beltagui et al.	[82]	2018	Utilize the blends of high free lime CKD, fly ash, and cement for mine’s backfill.

.

4. Safety

Many health hazards are associated with dust induced by the cement industry at the various steps in the process of cement manufacturing, starting from quarrying to packing and shipping [83]. In the cement kiln, the processing of raw materials, calcination, and burning may generate heavy metals, particulate matter, sulfur dioxide, silicon, and other pollutants. The generated dust can implicate diseases in the aspiratory tract, eyes, skin, etc. [84]. The preventive actions while dealing with dust generated from cement manufacturing include using a respirator to minimize inhaled dust, washing with water and soap to avoid skin damage, and limiting the dust exposure time to prevent silicosis and bronchitis [85]. According to OSHA hazard communication standard, cement kiln dust (CKD) is considered a hazardous material due to its negative impact on the eyes, skin, and respiratory system [86]. Exposure to CKD may cause eye irritation; moreover, the calcium oxide compound may create burns while reacting with the moisture and eye protein. In addition, CKD material may irritate the skin while in contact or cause skin burns. The upper respiratory system can be irritated, resulting in coughing or difficulty breathing or inhaling CKD particles for a long time [87]. Therefore, before dealing with CKD, special instructions should be followed, including reading the safety precautions regarding a proper handling method, avoiding dust breathing, storing CKD in a ventilated area, wearing protective clothing and gloves, and maintaining eye and face protection. Moreover, exposed body parts as well as contaminated clothes should be washed.

5. Conclusions

As the literature review reveals, cement kiln dust (CKD) is a byproduct of cement manufacturing. It constitutes a concern for the cement manufacturers due to its adverse environmental effects and cost of transporting to landfills. Given the ecological risks associated with cement kiln dust landfilling, international organizations concerned with the environment must legalize the transportation of these materials to the dumped area. CKD’s physical and chemical characteristics may be affected by several factors, such as raw materials and the operation process type, the fuel used, and the dust collection system. CKD contains high levels of alkalis, sulfate, and chloride, making it inappropriate for recycling in the cement industry. The published research indicates that CKD has a cementitious property that makes it practical for many applications, such as soil stabilization, concrete mix, chemical treatment, ceramic and brick manufacturing, bituminous mixtures, and mine backfill. Previous studies’ findings illustrate the following:

• The induced cementitious compounds is gained from the chemical reaction between high free lime in CKD and clay minerals.
• The calcium hydroxide Ca(OH)₂ produced from free lime in CKD is responsible for the strength upon hydration.
• The high free lime in CKD effectively causes an improvement in the mechanical properties, such as compressive strength and elastic modulus.
• CKD has a chemical composition containing potassium, sodium, and calcium oxides, making it a suitable and inexpensive activator for pozzolanic materials, such as slag and fly ash, compared to conventional activators utilized in the activation of alkali.
• A combination of CKD and sodium carbonate creates sodium hydroxide. The produced sodium hydroxide contributes to dissolute the particles of slag and then forms a binder phase with better mechanical properties and reduces the material cracking.

The obtained sustainable benefits associated with CKD utilizations in industrial applications include saving energy, protecting natural resources, reducing carbon dioxide emissions, and minimizing CKD landfill costs and related adverse impacts on the environment. Among the literature, few conducted a limited investigation to utilize CKD in mining applications. However, CKD could be used effectively in underground mine backfill mixtures considering the hydrated paste’s microstructures, as scholars recommended for further studies in this field.