EFFECT OF CEMENT KILN DUST SUBSTITUTION ON CHEMICAL AND PHYSICAL PROPERTIES AND COMPRESSIVE STRENGTH OF PORTLAND AND SLAG CEMENTS

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1 EFFECT OF CEMENT KILN DUST SUBSTITUTION ON CHEMICAL AND PHYSICAL PROPERTIES AND COMPRESSIVE STRENGTH OF PORTLAND AND SLAG CEMENTS S. Abd El-Aleem Faculty of Science, Cairo University, Fayoum Branch,Fayoum, Egypt M.A. Abd-El-Aziz Faculty of Engineering, Cairo University, Fayoum Branch,Fayoum, Egypt M. Heikal* Chemistry Department, Faculty of Science, Zagazig University, Benha Branch,Benha, Egypt and H. El Didamony Faculty of Science, Zagazig University,Zagazig, Egypt Key words: Cement Kiln Dust (CKD), Ordinary Portland Cement, Slag Cement, Physico-Chemical Properties, and Compressive Strength. 1. INTRODUCTION Large quantities of cement kiln dust (CKD) are produced during the manufacture of cement clinker by the dry process. The technico economical problems that arise in the transportation of dust from the plant to outside as well as the severe pollution to the surrounding atmosphere show the utmost necessity of utilizing CKD as one of the main objectives during the last decade. CKD is produced as a solid waste from the pre-heater by-pass systems during the manufacture of Portland cement clinker, by using the dry process, as a result of the presence of some minor volatile constituents in the kiln feed and fuel. Actually, the properties of CKD depend on the kind of raw materials and the fuel used, as well as the production method or kiln type. Therefore, due to the high alkali and salt contents of the raw materials for cement production, the dry processes are normally equipped with pre-heater by-pass systems. In these systems, the circulating alkalis are removed through withdrawal of the hot gas stream along with the dust it contains. * Address for correspondence. Porf. Mohamad Heikal, Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. ayaheikal@hotmail.com 263

2 In Egypt, production of the different types of cement reached nearly 30 million tons, with 3 millions tons CKD/year in dry lines. Up to twenty-five years ago, cement was produced by the wet process in Egypt. Nevertheless, the on-going shift in the cement industry to the dry method is expected to increase the accumulated dust. The dry process of cement production produces three times more dust than the wet process. CKD represents a mixture of raw feed, partly calcined cement clinker, and condensed volatile salts [1]. The chemical composition of CKD is influenced by the size of particles carried away by the kiln gases. The exit dust shows considerable amount of alkalis volatilized in the burning zone and condensed on the particles of the dust. Approximately 12% of the kiln feed exits from the kiln with the gas and about 73% of CKD is recycled to the cement making process [2]. El-Didamony et al. [3] studied the feasibility of washed and calcined CKD with anhydrite as an activator for granulated slag. It was found that high alkaline medium from calcined cement dust may be used to provide this alkalinity. The activation of slag by anhydrite leads to the formation of ettringite at the early ages of hydration. At later ages of hydration CSH (tobermorite-like phase) is formed. The rate of reaction was studied utilizing chemical, DTA, and XRD techniques. The activation of slag increases with calcination temperature as well as with anhydrite content. This also indicated the possibility of utilization of washed and calcined CKD in supersulfated cement. Aldiyarov et al. [4] prepared clinker from CKD directly in a separate furnace. The clinker obtained had a lower hydraulic activity and higher water demand than the conventional clinker due to its much higher alkali content. The most prevalent method for discarding CKD is dumping it on the plant site. This may be done in piles on unused areas or may be transported to abandoned sections from which raw materials have been extracted. Davis and Hooks [5] reported that leachate and run-off from CKD piles have ph values in the range of This is highly alkaline leachate. Abd El-Fattah and El-Didamony studied the phase composition of the CKD by using X-ray diffraction analysis technique [6]. They found that CKD consists mainly of limestone as a main component, and minor amounts of quartz, together with CaSO 4, KCl, NaCl, K 2 SO 4, KNaCl 2, 2(2CaO.SiO 2 ).CaSO 4, and sulfospurite {2(2CaO.SiO 2 ).CaCO 3 }[6]. The effect of CKD on the properties of fly ash-slag Portland cement was examined [7]. According to the author, CKD could be categorized into three types: (a) low alkali; (b) low chloride high sulfate and (c) high chloride low sulfate. The blends were studied for strength, shrinkage, autoclave expansion, sulfate expansion and alkali aggregate expansion. The addition of CKD to slag cement or slag silica fume cements activates the hydration process. A superplacticizer must be used to reduce the mixing water [8]. Amin et al., [9] and El-Didamony et. al, [] studied the effect of calcination temperature on the chemical and mineralogical composition of the CKD from 800 up to l000 o C. The calcined CKD activates the BFS. The activation of slag increases with calcination temperature of up to 1300 o C. El-Didamony et. al. [11] and Heikal [12] studied the role of cement dust in some blended cements. The calcined CKD was used with Portland cement clinker, silica fume, granulated blast furnace slag, and gypsum. The kinetics of hydration, as combined water and free lime contents, and compressive strength of the hardened cement pastes were determined. It was concluded that 5% of calcined CKD fired at 00 o C for 2 hours could be used with % silica fume and 80% Portland cement clinker and 5% gypsum. Also, blended cement was prepared from 35% cement clinker, 55% BFS, 5% CKD, and 5% gypsum. The aim of the present work is to evaluate the influence of different amounts of CKD on physico-chemical properties and compressive strength of OPC and slag cements. 2. EXPERIMENTAL TECHNIQUES The materials used in this work were OPC, slag cement (40% slag), and CKD, obtained from Beni Suef Cement Company, Egypt. The chemical analyses and Blaine surface area of investigated materials are shown in Table 1. The mix composition of the investigated mixes is shown in Table 2. The X-ray diffraction pattern of the CKD was shown in a previous work [13]. CKD contained CaCO 3 as a major phase with NaKCl 2, α-c 2 S, quartz, and anhydrite. Each dry mix was homogenized for one hour in a porcelain ball mill using four balls to assure complete homogeneity. The mixing of specimens was done as described in a previous work [12]. After the predetermined curing time, the hydration of the pastes was stopped using an acetone methanol mixture [14]. The water for normal consistency and the setting times (initial and final) were determined according to ASTM C187 and C191, respectively [15,16]. 264 The Arabian Journal for Science and Engineering, Volume 30, Number 2B October 2005

3 Table 1. Chemical Composition of OPC, Slag Cement, and CKD. Oxides OPC Slag Cement* CKD SiO Al 2 O Fe 2 O CaO MgO SO Na 2 O K 2 O Cl FL** L.O.I Surface area, g/cm * Slag cement contains 60% OPC and 40% granulated blast-furnace slag. ** Free lime. Mix No. Table 2. Mix Composition of the Investigated Specimens. Compositions, % OPC Slag Cement Cement Kiln Dust I I I I I I II II II II II Hydration behavior of each mix was followed by the determination of free lime as well as evaporable and chemically combined water contents at each curing time. The free lime content was determined by the alcoholic ammonium acetate method [17]. The combined water content (W n %) was estimated by the ignition of the dried paste at 00 o C for 2 hours. The total water was determined by the ignition of saturated sample (W t %). Combined water content = ignition loss of the hydrated sample (ignition loss of the raw mix + water of free lime). The evaporable water content of the hardened cement paste was obtained as: W e % = (W t W n )%. The compressive strength was determined according to ASTM C9 [18] using 50 mm cube specimens. The mortar was prepared by mixing one part of cement and 2.75 parts of sand by weight with water that was sufficient to obtain a flow of 1 ± 5% with 25 drops of the flow table. The W/C ratio for each mix is given in Table

4 Table 3. Water / Cement Ratio (W/C) of Portland and Slag Cement Mortars Containing CKD. Mix No. I.1 I.2 I.3 I.4 I.5 I.6 II.1 II.2 II.3 II.4 II.5 W/C RESULTS AND DISCUSSION Substitution of OPC with CKD Water of Normal Consistency and Initial and Final Setting Time The water for normal consistency and setting time are plotted as a function of CKD content in Figure 1. The water for normal consistency of blended cement pastes is higher than that for OPC paste. Addition of CKD increases the water for normal consistency. This may be attributed to high amounts of alkalis, sulfates, and volatile salts as well as the higher surface area of CKD that requires more water. The setting times (initial and final) of blended cement pastes are less than that in the OPC paste. As the CKD replacement increases, the setting time decreases. This is also due to the high amounts of lime and alkalis in CKD, which accelerate the hydration process, leading to fast setting. Generally, the addition of CKD accelerates the hydration process and reduces the setting times of the blended cement [9]. INITIAL AND FINAL SETTING TIMES, MIN Final Set Initial Set Consistency WATER OF CONSISTENCY, % BY-PASS CEMENT, Mass% Figure 1. Water of consistency and setting times of OPC pastes with different percentages of cement kiln dust Free Lime Content The free lime in OPC and blended cement pastes is plotted in Figure 2. The quantity of free lime increases with curing time in all the cement pastes. This is due to the continuous hydration of the main cement phases, such as -C 2 S and C 3 S liberating free lime in addition to that leached from CKD. At a given time, the quantity of free lime increases with CKD content in the blends. The free lime in CKD-blended cement pastes increases sharply up to 7 days, and then at a slower rate. This may be attributed to the leaching of Ca 2+ ions from CKD, in addition to the lime liberated due to the hydration of OPC. 266 The Arabian Journal for Science and Engineering, Volume 30, Number 2B October 2005

5 FREE LIME CONTENT, % I.1 I.2 I.3 I.4 I.5 I.6 5 Figure 2. Free Lime Content of OPC Pastes Containing Different Quantities of Cement Kiln Dust Evaporable (Free) Water Content The evaporable (free) water content is graphically represented as a function of curing time in Figure 3. The free water content decreased gradually with curing time in all the hardened cement pastes. This is due to the continuous hydration of cement phases, which leads to a decrease in the evaporable water. It was found that the evaporable water content in CKD blended cement pastes increased with the dust content. This is attributed to the increase in mixing water and low or no hydraulic properties of CKD, in comparison to the high hydraulic properties of cement phases. EVAPORABLE WATER CONTENT,% I.1 I.2 I.3 I.4 I.5 I.6 Figure 3. Evaporable Water Content in OPC Pastes With Varying Cement Kiln Dust as a Function of Curing Time Chemically Combined Water Content The chemically combined water content in OPC and blended cement pastes is illustrated in Figure 4. Generally, the chemically combined water content increased with curing time in all hardened cement pastes due to the continuous hydration and accumulation of hydration products mainly as calcium silicate and sulfoaluminate hydrates. The blended cement pastes exhibited lower values of combined water content than the OPC paste. This indicates that the C-S-H formed in the blended cements is less than that in the OPC. 267

6 CHEMICALLY COMBINED WATER CONTENT, % 15 I.1 I.2 14 I.3 I.4 I.5 13 I Figure 4. Combined Water Content in OPC Pastes with Varying Cement Kiln Dust as a Function of Curing Time Compressive Strength The compressive strength of the hardened cement mortars containing OPC with different amounts of CKD is graphically represented as a function of curing time in Figure 5. The compressive strength increased with curing time for all hardened cement mortars. However, the compressive strength decreased slightly with CKD content of up to 6 %. Above this percentage the compressive strength decreased sharply. The reduction of compressive strength is attributed to the reduction in the cement content and an increase in the water to cement ratio as well as the increase in free lime content in cement dust; the higher amount of Ca(OH) 2 weakened the hardened matrix. Also, the formation of chloro- and sulfoaluminate phases leads to the softening and expansion of the hydration products. The porosity also increases, due to the high chloride (7.5%) and sulfate (5.%) content of the CKD. The formation of these products enhances the crystallization of hydration products leading to an opening of the pore system. The crystallization of hydration products may be accompanied by an increase in the pore size due to a change in the packing between the crystals, which leads to a decline in the strength. It can be said that the replacement of CKD dust up to 6 % in OPC can be utilized in the manufacture of the blended cement Substitution of Portland Slag Cement with CKD Water for Normal Consistency and Initial and Final Setting Time The water for normal consistency and setting times are graphically represented as a function of cement dust replacement in Figure 6. It can be observed that, the water for normal consistency increases with the quantity of CKD. This is due to the presence of high amount of alkalis, sulfates, volatile salts, and lime in cement dust, as well as the high surface area leading to high water demand. The setting times are shortened with an increase in the quantity of CKD. This may be attributed to the high alkalinity of the CKD that accelerates the hydration of cement Free Lime Content The free lime content in the hardened slag cement paste is plotted in relation to the curing time in Figure 7. The results show that, the free lime content increases with curing time for all cement pastes up to 7 days. This is mainly attributed to the hydration of slag portion that begins with slow rate and increases with time. Therefore, the consumption of the liberated lime increases with time. The hardened slag cement pastes with the CKD give higher values of free lime than only slag cement paste. This is due to the fact that there are two main sources of lime, the hydration of cement clinker phases and the residual lime containing CKD. Therefore, the free lime is increased with CKD content. At a given age, the free lime content increases with the quantity of CKD. 268 The Arabian Journal for Science and Engineering, Volume 30, Number 2B October 2005

7 COMPRESSIVE STRENGTH, MPa I.1 I.2 I.3 I.4 I.5 I.6 Figure 5. Compressive Strength of OPC Mortars with Varying Cement Kiln Dust as a Function of Curing time. INITIAL AND FINAL SETTING TIMES, MIN Initial Set Final Set Consistency WATER OF CONSISTENCY, % BY-PASS CEMENT, Mass% Figure 6. Water of Consistency and Setting Times of Slag Cement Pastes with Varying Quantity of Cement Dust Evaporable Water The evaporable water content in the cement paste is plotted as a function of curing time in Figure 8. It is clear that, the evaporable (free) water content decreases gradually with curing time for all cement pastes This is due to the continuous hydration of the pastes leading to decrease in the free water. At a given time, the free water content increases with the quantity of CKD. This is due to an increase in the water for normal consistency in the CKD cement paste. 269

8 FREE LIME CONTENT, % II.1 II.2 II.3 II.4 II.5 5 Figure 7. Free Lime Content in Slag Cement Pastes with Different Percentages of Cement Kiln Dust as a Function of Curing Time. EVAPORABLE WATER CONTENT,% II.1 II.2 II.3 II.4 II.5 Figure 8. Evaporable water in Slag Cement Pastes with Different Percentages of Cement Kiln Dust as a Function of Curing Time Chemically Combined Water Content The chemically combined water content in the hardened slag cement pastes is plotted in Figure 9. The combined water increases with curing time for all hardened cement pastes. This is due to the continuous hydration of cement compounds. CKD has high alkali and sulfate content, which makes it an excellent activator for pozzolanic materials. The dissolution rate of materials with latent pozzolanic properties, such as BFS generally depends on the alkali concentration of the reacting system [19]. Latent hydraulic materials develop pozzolanic activity and act as hydraulic cements once their glass network disintegrates when attacked by OH ions. BFS is an artificial pozzolana and it can hydrated in the presence of Ca(OH) 2. The Ca(OH) 2 liberated in OPC acts as an activator, leading to more accumulation of hydration products, mainly as calcium silicate and sulfoaluminate hydrates. This is attributed to the formation of C-S-H (II), which has lower water content than C-S-H (I) in the OPC. 270 The Arabian Journal for Science and Engineering, Volume 30, Number 2B October 2005

9 Compressive Strength The compressive strength of the hardened slag cement mortar prepared with CKD is graphically represented in Figure. The compressive strength increases with curing time for all hardened cement mortars. This is due to the continuous hydration and accumulation of hydration products in water filled pores to form a more compact body. At a given time, the hardened slag cement mortar with 5% CKD gives higher values of compressive strength than slag cement mortar. 5% CKD is the optimum amount to activate the hydration of slag [11]. Also, it can be seen that compressive strength decreases slightly with CKD content of up to mass%. It can be concluded that, the substitution of slag cement with 5 to % CKD has a slight effect on the compressive strength. As the CKD content increases up to 20 mass%, the compressive strength decreases significantly. COMBINED WATER CONTENT, % II.1 II.2 II.3 II.4 II.5 Figure 9. Combined water Content in Slag Cement Pastes Containing Different Percentages of Cement Kiln Dust as a Function of Curing Time. COMPRESSIVE STRENGTH, MPa II.1 II.2 II.3 II.4 II.5 Figure. Compressive Strength of Slag Cement Mortars Containing Different Percentages of Cement Kiln Dust as a Function of Curing Time. 271

10 4. CONCLUSIONS From the above results, it can be concluded that: 1. Addition of CKD increases the water for normal consistency of OPC and BFS cement pastes, whereas the setting time decreases. 2. Free lime content of CKD OPC cement pastes increases sharply up to 7 days, and then increases more slowly, whereas, the free lime content of CKD BFSC pastes increases with curing time for all cement pastes up to 7 days, and then increases with time. 3. The free water increases with the quantity of CKD. 4. The combined water content in the blended cements is less than that in OPC and BFS cement pastes. 5. The substitution of OPC with CKD up to 6 mass% has no significant effect on the compressive strength of hardened cement mortar. However, a CKD content of more than 6% adversely affects the compressive strength. A similar effect was noted in the slag cement. This indicates that CKD acts as an activator for slag cement hydration. The optimum value of CKD ranges between 5 to mass%. At 15 and 20% CKD, the compressive strength of the slag cement decreases. REFERENCES [1] K.E. Daugherty and A.O. Wist, Review of Cement Industry Pollution Control, Bull. Am. Ceram. Soc., 54 (1975), p [2] R.F. Smith, J.E. Levin, and A.T. Kearney, EPA: , Cincinnati, OH, 1979, p.44. [3] H. El-Didamony, A.H. Ali, A.M. Sharara, and A.M. Amin, Assessment of Cement Dust with Anhydrite as an Activator for Granulated Slag, Silic. Ind., 62 (1-2) (1997), p.31. [4] D.A. Aldiyarov, M. Stepanov, V.V. Timashev, A.T. Suleimenov and C. Sichkareva, Use of Dust by Calcining in a Separate Furnace, Tsem. Zavod, Chimkent, USSR, 7 (1978), p.11. [5] T.A. Davis and D.B Hooks, EPA: , Cincinnati, OH, [6] W.I. Abd El-Fattah and H. El-Didamony, Thermal Investigation of Electrostatic Precipitator Kiln Dust, Thermochim. Acta., 51 (1981), p.297. [7] M.S.Y. Bhatty, Kiln Dust Cement Blends Evaluated, Rock Prod, 88 () (1985), p [8] H. El-Didamony, S.A. Abo-El-Enein, A.H. Ali, and T.M. El-Sokkary, Effect of Silica Fume on the Slag Cement Containing Wet Cement Dust, Indian J. Eng. & Mater. Sci., 6 (1999), pp [9] A.M. Amin, E. Ebied, and H. El-Didamony, Activation of Granulated Slag with Calcined Cement Kiln Dust, Silic Ind., LX (3-4) (1995), pp. 9. [] H. El-Didamony, A.H. Ali, A.M. Sharara, and A.M. Amin, Assessment of Cement Dust with Anhydrite as an Activator for Granulated Slag, Silic. Ind., 62 (1-2) (1997), p.31 [11] H. El Didmony, A.A. Amer, E. Ebied, and M. Heikal, The Role of Cement Dust in Some Blended Cements, il Cemento, 90 (4) (1993), pp [12] M. Heikal, Silica Fume as an Ingredient in Blended Cements, M. Sc. Thesis, Faculty of Science, Zagazig University, [13] M. Heikal, I. Aiad, and I.M. Helmy, Portland Cement Clinker, Granulated Slag and By-Pass Cement Dust Composites, Cem. Concr. Res., 32 (2002), pp [14] H. El-Didamony, Application of Differential Thermogravimetry to the Hydration of Expansive Cement Pastes, Thermochim Acta, 35 (1980) pp [15] ASTM Standards, Standard Test Method for Normal Consistency of Hydraulic Cement. ASTM Designation, C187-83, 1983 p [16] ASTM Standards, Standard Test Method for Time of Setting of Hydraulic Cement, by Vicat Needle. ASTM Designation, C191-83, 1983, p The Arabian Journal for Science and Engineering, Volume 30, Number 2B October 2005

11 [17] R. Kondo, S.A. Abo-El-Enein, and M. Daimon, Kinetics and Mechanisms of Hydrothermal Reaction of Granulated Blastfurnace Slag, Bull. Chem. Soc., Jpn, 48 (1975), pp [18] ASTM Standards, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, ASTM Designation, C9-92, 1992, p. 62. [19] S. Song and H.M. Jennings, Pore Solution Chemistry of Alkali-Activated Ground Granulated Blast-Furnace Slag, Cem. Concr. Res., 29 (2) (1999), pp Paper Received 5 February 2003; Revised 6 April 2004; Accepted 1 June 2004 (as a Technical Note). 273