Utilization of Cement Kiln Dust (CKD) with Silica Fume (SF) as a Partial Replacement of Cement in Concrete Production

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1. Introduction

CKD is an industrial by-product material during cement manufacturing that is not returned to the process but disposes of them by landfill. It was found from the experimental tests and previous research, the utilization of CKD individually as the partial replacement of cement in concrete leads to decreases in all types of strength. Also silica fume is an industrial by-product material derived during production of elemental silicon where it is defined as an alloy containing silicon; its noncrystalline silica is very fine which is produced in electric arc furnaces [1] . From experimental program, it was observed that the utilization of CKD with SF does not have any negative effects on the strength of concrete to a certain limit, but improves its performance. Therefore, it was necessary to study fresh and hardened concrete properties in the case of a partial replacing of cement with CKD and SF in the production of concrete, and determine optimum ratios for replacement. Generally increasing CKD amount leads to reduction of fresh mixes workability and also strength of hardened mortars and concretes [2] . It was observed in CKD with cement blends; the gain contribution of CKD strength is low [2] . The use of CKD as a partial replacing of cement possible in the combination with pozzolanic materials in the certain mortar mixed designs without decreasing in the main characteristics of the product [3] . The pozzolanic material such as SF is an ultra-fine powder and very reactive pozzolana; it is used in concrete because of its fine particles, large surface area and higher SiO_{2} content [4] . Silica fume as a pozzolanic material, when properly utilized as certain percent, can enhance the various properties of concrete both in the fresh state as well as in hardened state such as cohesiveness, strength, permeability and durability [5] . In the general applications, part of cement may be replaced by a much smaller quantity of silica fume. For example, one part of cement can be replaced with 0.25 to 0.33 part of silica fume (mass to mass) without losing in the strength, provided the content of water remains constant [6] .

2. Experimental Program

2.1. Materials Used

According to the Egyptian standard specifications for concrete aggregates [7] , sand with fineness modulus equal 2.59 and specific gravity of 2.57 was used, whereas the coarse aggregate used was dolomite maximum normal size (5 - 14 mm) and specific gravity of 2.74. Cement (CEM I-52.5N) used was fresh product from Sina factory and complies to E.S.S 4756/2013 [8] . CKD was obtained from the El Nahda cement factory in Qena, Egypt. SF was obtained from Egyptian Company for Ferroalloys (Alferrosilicon), Edfo factory, Aswan, Egypt. SF was confirming to ACI 234R-96 [6] . The chemical composition of Portland cement, CKD and SF were shown in Table 1. Potable water free from salts, oils, acids, sugars and other harmful substances was used for mixing and curing of specimens. Sikament-163M was used as super plasticizer (SP), for highly effective water reducing agent, density (at 20˚C) 1.200 ± 0.005 kg/liter (ASTM C494),

Table 1. Chemical composition of cement, cement kiln dust and silica fume.

*Note: These average values are supplied by the manufacturer.

Appearance/Colour is brown liquid and recommended dosage is 0.6% - 2.5% by weight of cement. The experimental dosage was 2.0% by weight of cement.

2.2. Mixtures Proportions and Specimens’ Details

In this experimental investigation, control concrete mix to achieve compressive strength of 50 N/mm^{2} at the age of 28 days was done according to ECP 203-2007 [9] . The use CKD with different percentages 5%, 10%, 15%, 20% and 25% and a fixed proportion of silica fume (SF) 15% as partial replacement by the weight of (OPC). The mixtures proportions for 1 m^{3} in all concrete mixtures were shown in Table 2. Slump test was done on the fresh concrete to find its consistency and workability. For every mix, cubes (150 mm × 150 mm × 150 mm) were cast for the compressive strength tests, cylinders (Diameter; 150 mm and height; 300 mm) were cast for the split tensile strength and modulus of elasticity tests and prisms (100 mm × 100 mm × 500 mm) were cast for the flexural strength tests. The Bond test for reinforcing steel describes by pull-out specimen as a steel reinforcing bar 12 mm diameter embedded in concrete cylinder (Diameter; 150 mm and height; 300 mm), An anchorage length equal to 300 mm. After casting, all specimens were left in the forms for 24 hours before demoulding. The curing of specimens was done by placing all specimens in water basins until the ages of testing.

3. Analysis of Test Results

3.1. Workability of Fresh Concrete

The concrete slump flow test was used according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 to determine fresh concrete workability [10] . Table 3 shows slump test results for all mixtures, in general a decrease in slump were observed with increase ratio of CKD

Table 2. Mixtures proportions.

*Mix designation is (Mix-CKD%-SF%).

Table 3. Slump test results.

with SF. The results indicate that CKD with SF leads to decrease the fresh concrete workability. The increase of (CKD) amount, reduces the fresh mixes workability and also the strength of hardened mortars and concretes in generally [2] . The workability of fresh concrete decreases with increase of silica fume amount in concrete. For getting equal workability, the concrete containing of silica fume will tend to shows less slump height than conventional concrete [11] . For each level of w/b ratio, the measured slump was the same or it decreased as the amount of CKD replacement level increased. As w/b ratio increased, the impact of CKD replacement of Portland cement on slump loss became more significant [12] . For obtained ordinary consistency with increase CKD amount, this required to increasing water, slump loss may be to high amounts of lime, sulfates, alkalis and volatile salts that require more water in concrete [13] .

3.2. Compressive Strength

The test is conducted in Compression Testing Machine with a capacity of 2000 kN and loading rate 0.6 N/mm^{2}/sec, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] . Figure 1 shows the variation of compressive strength in CKD with SF mixtures when

Figure 1. Compressive strength of mixtures contained cement kiln dust (CKD) 5%, 10%, 15%, 20% and 25% with constant 15% silica fume(SF), compared with control mixture 0% CKD with 0% SF After 7, 28 and 56 days.

compared with control mixture Mix-00-00. At all mixtures that have CKD with SF, The presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 18.66%, 10.28%, 7.22% and 4.24% respectively after 7 days, 18.34%, 11.48%, 6.17% and 3.55% respectively after 28 days and 18.02%, 9.63%, 5.42% and 2.81% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 4.93% after 7 days, 3.69% after 28 days and 6.05% after 56 days. Compressive strength of concrete mixtures were decreased with the increase in CKD percentage at the age 28 days, the reduction in strength at 10% and 40% CKD are 15% and 44% respectively [14] . The optimum replacement percentage of SF varies from 10% to 15%. Compressive strength of concrete decreases when the cement replacing is above 15% of SF [11] .

3.3. Indirect Tensile Strength

Indirect tensile strength was described by split tensile strength, the test is conducted in Compression Testing Machine with a capacity of 2000 kN and loading rate 0.04 N/mm^{2}/sec, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] . Figure 2 shows the variation of split tensile strength in CKD with SF mixtures when compared with

Figure 2. Split tensile strength of mixtures contained cement kiln dust (CKD) 5%, 10%, 15%, 20% and 25% with constant 15% silica fume(SF), compared with control mixture 0% CKD with 0% SF After 7, 28 and 56 days.

control mixture Mix-00-00. At all mixtures that have CKD with SF, the presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 18.82%, 11.53%, 6.60%, 3.71% respectively after 7 days, 18.15%, 11.14%, 6.18% and 3.96% respectively after 28 days and 17.63%, 11.71%, 7.61% and 5.59% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 4.12% after 7days, 4.91% after 28 days and 7.69% after 56 days. By the increase in CKD content, compressive strength, split tensile strength and also modulus of rupture was decreased [15] . Beyond 15% silica fume when replacement with cement in concrete, the increased in split tensile strength is almost insignificant. But get sizeable gains in the flexural strength was occurred even up to 25% silica fume replacement [16] .

Relationship between Split Tensile Strength and Compressive Strength

By analysis of the present test results, the relationship between the split tensile and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range of 10.6% to 10.76%, whereas 10.73% for control mixture.

3.4. Flexural Strength

All prism specimens were tested under four point static load according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] , by using a 1000 kN capacity (loading rate 0.06 N/mm^{2}/sec) hydraulic jack mounted on a steel frame in the R.C laboratory of Al-Azhar university. Figure 3 shows the variation of flexural strength in CKD with SF mixtures when compared with control mixture Mix-00-00. At all mixtures that have CKD with SF, the presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 17.27%, 11.36%, 7.27%, 3.64% respectively after 7 days, 17.03%, 11.15%, 7.35% and 4.41% respectively after 28 days and 18.19%, 10.92%, 5.63% and 2.82% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 5.45% after 7days, 4.41% after 28 days and 5.63% after 56 days. It was observed both of flexural strength and toughness values are decreased with the increase in CKD replacement ratios, but at 5% and 10% replacement ratios did not have an appreciable adverse effect, it is especially clear at low w/b ratios [12] . The reduction in flexural strength and toughness values attributed to a reduction in the cement amount in the blends as the content of CKD increased [11] . Content of 15% silica fume as partial replacement of cement were the optimum amount to significantly flexural strength enhancement [17] .

Figure 3. Flexural strength of mixtures contained cement kiln dust (CKD) 5%, 10%, 15%, 20% and 25% with constant 15% silica fume (SF), compared with control mixture 0% CKD with 0% SF After 7, 28 and 56 days.

Relationship between Flexural Strength and Compressive Strength

By analysis of the present test results, the relationship between flexural and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range 14.55% to 14.88%, whereas 14.71% for control mixture.

3.5. Modulus of Elasticity

The modulus of elasticity was determined by subjecting a cylinder specimen (Diameter; 150 mm and height; 300 mm) to uniaxial compression by using Universal Testing Machine of capacity 1000 Kn, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [10] . A Keithley-500 A Data Acquisition System consists of 4 channels for strain measurement and corresponding load. The deformations were measuring by dial gauges which fixed between a certain gauge length. Modulus of elasticity was found out with reference to the initial tangent drawn at the origin of the stress strain curves Figure 4. The test of modulus of elasticity was carried out for control mixture and CKD with SF mixtures. Table 4 shown the value of the modulus of elasticity after 28 days, the role of CKD with SF are observed for the increase in modulus of elasticity of mixtures; (Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15), with ratio 43.82%, 28.25%, 17.29% and 11.32% respectively, while in mixture (Mix-25-15) modulus of elasticity are decrease with 3.13% when compared with control mixture (Mix-00-00). The modulus of elasticity is decreasing with the increasing of CKD content from 10% - 50%, the reduction of modulus of elasticity within range between (5.31% to 77.87%) respectively [14] .

Figure 4. Stress-strain curves of mixtures contained cement kiln dust (CKD) 5%, 10%, 15%, 20% and 25% with constant 15% silica fume (SF), compared with control mixture 0% CKD with 0% SF after 28 days.

Table 4. Modulus of elasticity results.

Relationship between Modulus of Elasticity and Compressive Strength

By analyzing test results statistically, the relationship between modulus of elasticity and compressive strengths after 28 days for control mixture and (CKD with SF) mixtures has been obtained as the following equations in Table 5. For (CKD with SF) mixtures the direct proportional relationship between modulus of elasticity and compressive strength are observed.

3.6. Bond Strength

Bond strength was describes by pull-out test, as a steel reinforcing bar 12 mm diameter embedded in a concrete cylinder (Diameter; 150 mm and height; 300 mm), An anchorage length equal to 300 mm, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 to [10] . The test is conducted in a tensile testing machine (TTM) with a capacity of 800 kN for the direct pull out test of specimens. Specimen were inverted and positioned through the bottom platform of test machine load frame. Table 6 shows the value of bond strength and failure mode for all mixtures after 28 days. Figure 5 shows the variation of bond strength in CKD with SF mixtures when compared with control mixture. The increasing in bond strength of mixtures; (Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15), with ratio 7.20%, 6.21%, 0.69% and 0.69% respectively, while in mixture (Mix-25-15) bond strength were decreases with 7.58% when compared with control mixture (Mix-00-00). Whereas by increasing ratios over (10% CKD with 15% SF) the failure mode changed from rebar cutting to concrete splitting.

Relationship between Bond Strength and Compressive Strength

The analysis of the present test results to determined relationship between bond and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range 10.28% to 11.04%, whereas 11.34% for control mixture.

3.7. XRF Analysis

With control mixture after 28 days. The test results are shown in Table 7. It is observed that the amount of SO_{3} oxide within the range 1.89% to 2.2% for all CKD with SF mixtures, and not significant when compared with control mixture 2.13%. In general the use of CKD with SF in concrete mixtures lead to

Figure 5. Bond strength of mixtures contained cement kiln dust (CKD) 5%, 10%, 15%, 20% and 25% with constant 15% silica fume(SF), compared with control mixture 0% CKD with 0% SF after 28 days.

Table 5. Relationship between modulus of elasticity and compressive strength.

*E denote modulus of elasticity. **Fcu denote compressive strength.

Table 6. Pull out test results.

Table 7. XRF analysis results.

increase ratios of sulfate SO_{3}, chloride CL, potassium K_{2}O and silicon SiO_{2}. Where aluminum Al_{2}O_{3}, iron Fe_{2}O_{3} and calcium CaO ratios are decreased but sodium Na_{2}O ratio still constant when compared with control mixture. The total content of sulfates in concrete in the form of SO_{3}, shall not exceed 4% of the weight of cement with a hardened concrete at 28 days according to ECP 203-2007 [10] . The total content of chloride ions in the form of Cl in non-chloride-resistant of hardened concrete, shall not exceed 0.3% of the cement weight at the 28 days [10] . The amount of Cl oxide in mixture (Mix-05-15) are 0.3%, so that this mixture can be used in steel reinforced concrete, but the amount of Cl oxide in mixtures; (Mix-10-15, Mix-15-15, Mix-20-15 and Mix-25-15) within the range 0.35%, this mixtures can be used in plain concrete or in FRP reinforced concrete.

4. Conclusions

Based on the experimental results, the following conclusions could be drawn:

• We can use industrial wastes as cement kiln dust (CKD) and silica fume (SF) to produce economic concrete.

• In general, a partial replacement of cement with CKD and SF leads to decrease of fresh concrete workability. It was observed the slump decreased with increasing ratio of CKD with constant 15% of SF in concrete mixtures.

• The amount of 20% CKD in the presence of 15% SF as a partial replacement by weight of cement is the optimum ratios which can be used in concrete mixtures without lowering on compressive, split tensile and flexural strength at all the ages of concrete. Also modulus of elasticity and bond strength increased by 11.32% and 0.69% respectively after 28 days when compared with control mixture.

• At amount of 25% CKD in the presence of 15% SF as a partial replacement by weight of cement in concrete, a slight decrease was observed in compressive, split tensile and flexural strength within the range (3.69% to 7.69%) at all the ages of concrete. But bond strength and modulus of elasticity decreased by 7.58% and 3.13% respectively after 28 days when compared with control mixture.

• In concrete mixtures which contain different ratios of CKD and constant ratio 15% of SF as a partial replacing by weight of cement at the age of 28 days, it has been observed that split tensile strength equals 10.6% to 10.76% from compressive strength, flexural strength equals 14.55% to 14.88% from compressive strength and bond strength equals 10.28% to 11.04% compressive strength.

• By analyzing test results statistically of CKD with SF concrete mixtures, the directly proportional relationship between modulus of elasticity and compressive strength is observed. The relationship between modulus of elasticity and compressive strength at the age of 28 days, can be given with equations, $E=\left(\text{4}0\text{67}-\text{5448}\right)\sqrt{Fcu}$ MPa, depending on decreasing of CKD ratio (where E and Fcu denote modulus of elasticity and compressive strength respectively).

• From XRF analysis, it was observed the use of CKD with SF with gradually increased ratios in concrete mixtures led to increasing ratios of sulfate SO_{3}, chloride CL, potassium K_{2}O and silicon SiO_{2}, where aluminum Al_{2}O_{3}, iron Fe_{2}O_{3} and calcium CaO ratios are decreased but sodium Na_{2}O ratio is still constant when compared with control mixture.

Acknowledgements

Authors would like to thank AL-AZHAR UNIVERSITY—Faculty of ENGINEER (materials laboratory) for helping to carry out experimental testing of this research.

Special thanks to Assistant Professor, Dr. MOHAMMED TAHA NOOMAN, Faculty of Engineering, Al-Azhar University, Cairo, Egypt for his great assistance during the conduct of this work, valuable revisions, his directions in the presentation and analysis of test results. His effort is highly appreciated.

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