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

This research aimed to clarify the role of by-product materials, such as CKD with SF as partial replacement by weight of cement in concrete manufacturing and inclusion on different characteristics of concrete. Concrete test specimens were mixed with 0%, 5%, 10%, 15%, 20% and 25% (CKD) with 15% (SF) as partial replacement by weight of Cement (CEM I-52.5N). Fresh concrete properties have been evaluated by workability measurement slump test. While hardened concrete properties have been evaluated by compressive, split tensile and flexural strengths tests at ages 7, 28 and 56 days, but evaluated for bond strength, modulus of elasticity and chemical composition measurement with X-Ray Fluorescence at age of 28 days. The test results have revealed that the increase of CKD amount with fixed amount of SF in concrete mixtures as partial replacement by weight of cement leads to gradual decrease of fresh concrete workability. In concrete mixtures, 20% CKD in the presence of 15% SF as partial replacement by the weight of cement are the optimum ratios which can be used without any negative effect on mechanical properties compressive, indirect tensile, flexural and bond strength at all the ages of concrete. Also modulus of elasticity and bond strength increased by 8.81% and 0.69% respectively at the age 28 days compared with control mixture.


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].

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), A. E. Elhelloty et al. 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.

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

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 A. E. Elhelloty et al.  Mix-20-15 20 Mix-25- 15 16 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].

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   [15].

Indirect Tensile Strength
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.

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.  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]. A. E. Elhelloty et al.

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.

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.

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

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.

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 A. E. Elhelloty et al. 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.

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 in-A. E. Elhelloty et al.
creased 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,

( )
4067 5448 E 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.