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Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume
HBRC Journal (2014) xxx, xxx–xxx
Housing and Building National Research Center
HBRC Journal http://ees.elsevier.com/hbrcj
FULL LENGTH ARTICLE
Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume M.S. Amin *, S.M.A. El-Gamal, S.A. Abo-El-Enein, F.I. El-Hosiny, M. Ramadan Faculty of Science, Ain Shams University, Cairo, Egypt Received 11 December 2013; revised 19 June 2014; accepted 2 July 2014
KEYWORDS Electric arc furnace slag (EAFS); Silica fume (SF); Thermal analysis; Phase composition
Abstract Filled-pozzolanic cement pastes were made by different replacements of OPC by electric arc furnace slag (EAFS) with silica fume (SF) at water/cement ratio of 0.27. The pastes were hydrated up to 90 days. At each time interval, the physico-chemical characteristics of the hardened cement pastes were studied and related to the structure of the hardened pastes and the role of EAFS replacement as a filler in the hardened OPC-EAFS pastes. It was found that the optimum replacement of OPC by EAFS for the improvement in hydraulic properties of filled cement pastes is 6%. High replacement of OPC by EAFS (10% or 15%) causes a notable deterioration in the compressive strength at all ages of hydration. The replacement of EAFS in Mix (90% OPC + 10% EAFS) by 4% SF causes a marked improvement in the mechanical properties for the hardened pastes of Mix (90% OPC + 6% EAFS + 4% SF). The DSC thermograms for all pastes indicated the formation of nearly amorphous calcium silicate hydrates, calcium sulphoaluminate hydrates, calcium aluminate hydrates and portlandite. The SEM micrographs showed that the partial substitution of OPC by EAFS and/or SF leads to more dense structures as compared to the neat OPC paste. ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.
Introduction
* Corresponding author. E-mail address: [email protected] (M.S. Amin). Peer review under responsibility of Housing and Building National Research Center.
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Manufacturing of Portland cement (PC) is a resource exhausting, energy intensive process that releases large amounts of the green house gas (CO2) into the atmosphere. At present, efforts have been made to enhance the use of cementitious materials such as pozzolana and other industrial wastes to partially replace Portland cement [1,2]. The term pozzolana was defined as siliceous/aluminous materials, either natural or artificial, which react chemically with calcium hydroxide (CH) or with materials that can release calcium hydroxide (Portland cement clinker) in the presence of water to form compounds that pos-
http://dx.doi.org/10.1016/j.hbrcj.2014.07.002 1687-4048 ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
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M.S. Amin et al.
sess cementitious properties. Fly ash (FA), rice husk ash (RHA), silica fume (SF), slag and also calcined clay in the form of metakaolin (MK) are good examples of pozzolanic materials [3–6]. Condensed silica fume (SF) is a byproduct of silicon or ferrosilicon alloy industries. It consists of 95% amorphous silica with a very high surface area; these characteristics increase its pozzolanic activity [7,8]. Metakaolin (MK) is ultrafine artificial pozzolana powdered form of anhydrous alumino-silicate derived from the calcination of raw kaolin at a specific temperature range to form a phase transition which is highly disordered, amorphous and with pozzolanicity [9]. Slag represents one of the industrial wastes of various metal extractions and refining processes. In the specific case of making steels, the slag is generated at 3 different stages of processing and accordingly classified as: blast-furnace slag, electric arc furnace slag and ladle slag [10]. The electric arc furnace slag (EAFS) has a chemical composition more close to that of the cement clinker compared to the ground granulated blastfurnace slag (GGBFS). Hence, recently it was shown that it has potential application as a partial substitute for raw materials in clinker production. Addition of up to 20% EAFS in the kiln feed was found to improve burnability index of the raw material mix [11]. The cementitious and pozzolanic behaviour of electric arc furnace steel slag, both as received and treated, has been studied. The as received slag was completely crystalline with monticellite as the predominant phase. Treatment of this slag, remelting and water quenching, results in several phases with merwinite as the dominant phase with an increase in basicity index which is more hydraulic [12]. Hydration of multi-blended cements composed of (OPCFA-SF-MK), (OPC-MK-GGBFS) and (OPC-SF-GGBFS) mixes was studied. The results have shown that even 40 wt.% replacement of OPC by the pozzolanic materials does not deteriorate the compressive strength. This is attributed to that the pozzolanic material acts as a filler as well as, pozzolana which increases the formation of the C–S–H phase [13–17]. The object of this work is to study the hydration characteristics of OPC-EAFS blends with SF. The hydration characteristics were investigated by the determination of compressive strength, chemically combined water and free lime contents at different hydration ages. In addition, the phase composition was examined using DSC. The morphology and microstructure of some selected cement pastes were also examined using SEM.
was supplied from Ezz Flat Steel Company, Egypt, with the same Blain surface area of OPC. The chemical oxide compositions of OPC and EAFS are shown in Table 1. Condensed silica fume (SF) was supplied from ferro-silicon company, KomOmbo, Egypt. Silica fume particles are spherical and have an average diameter of about 0.1 lm. It consists of 95% amorphous silica with a specific surface area 2 · 105 cm2 g 1. The chemical composition of SF is shown in Table 1.
Experimental
Compressive strength
Materials
Fig. 1 shows the results of compressive strength versus age of hydration for all of the investigated hardened pastes made of neat OPC and OPC – EAFS blends. Obviously, the values of compressive strength of the neat OPC paste increase continuously with the age of hydration. This increase is mainly
Ordinary Portland cement (OPC) used in this study was supplied from South Valley Cement Company, Egypt, with a Blaine surface area of 2945 cm2 g 1. Electric arc furnace slag (EAFS)
Table 1
Preparation of the hardened blended cement pastes Different cement pastes were prepared using a W/S ratio of 0.27. Each paste was prepared by mixing the dry mix with the required amount of water for about 3 min. After complete mixing, the resulted paste was moulded in 1 inch cubic moulds, cured in 100% relative humidity for 24 h and then demoulded and cured under water at room temperature up to 90 days. Table 2 shows the mix designations and their compositions. Methods of investigation Compressive strength tests were performed after 1, 3, 7, 14, 28 and 90 days. At each curing time, three cubes of each mix were subjected to compressive strength test and the average value was recorded as kg cm 2. This was accomplished using a Ton-industrie machine (West Germany) with a maximum load of 60 tons. After the determination of compressive strength the crushed specimens of the hardened cement pastes were then ground and the hydration reaction was stopped using the method described in an earlier publication [18]. Samples were then dried at 80 C for 3 h in CO2-free atmosphere and maintained in a desiccator containing soda lime and CaCl2 until the time of testing. Hydration kinetics was studied by the determination of chemically combined water and free lime contents at different ages of hydration using the ground dried samples according to the methods reported in earlier investigation [18,19]. The phase composition of the formed hydrates was investigated for some selective samples by differential scanning calorimetric (DCS) technique. Also, the morphology and microstructure of hydrated phases were examined using a JSM-5410 scanning electron microscope (SEM). Results and discussion
Chemical oxide compositions of OPC and EAFS, mass %.
Oxide/mass %
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
Cr2O3
Mn2O3
TiO2
L.O.I
OPC EAFS SF
20.46 13.9 94.7
5.14 5.82 0.26
3.53 36.1 0.25
61.28 33.4 1.13
2.8 5.71 –
0.2 0.2 0.36
0.11 – 2.45
2.82 0.3 0.6
– 0.73 –
0.11 2.82 –
0.33 0.6 –
3.15 – –
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes Table 2 Compositions of the different mixes and their designations. Mixes
Blank E1 E2 E3 E4 ES
Mix proportion, mass % OPC
EAFS
SF
100 96 94 90 85 90
– 4 6 10 15 6
– – – – – 4
3 neat OPC paste as shown in Fig. 1. The presence of silica fume causes a notable improvement in the mechanical properties for the hardened paste of Mix ES as compared to Mix E3; Silica fume consists of very fine amorphous particles, large surface area, high SiO2 content and acts as a very reactive pozzolana when used as a blend in cement and concrete. Silica fume plays two important roles: (i) it acts as a filler, due to its fine particle size, fills the spaces between the cement grains and this effect reduces the pore size and (ii) it acts as a pozzolanic material due to its ability to react with free lime liberated from the hydration of OPC leading to the formation of additional amounts of calcium silicate hydrates with a consequent increase in compressive strength [7]. Hydration kinetics
Fig. 1 Compressive strength values (kg/cm2) with curing time for the neat OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
attributed to the formation of hydration products mainly as CSH, within the pore system of the hardened paste. The partial substitution of OPC by EAFS as supplementary cementitious material, leads to a discrepancy in the values of compressive strength for the hardened pastes made of OPC-EAFS blends; these discrepancies in the compressive strength for the different blends are mainly attributed to the role of EAFS, as a partial substituent of OPC, which has weak pozzolanic characteristic and filler effects that lead to a noticeable decrease in the total porosity of the hardened pastes. Therefore, the compressive strength increases [20]. On the other hand, the results also revealed that the paste made of Mix E2 (94% OPC + 6% EAFS) possesses the highest strength values at intermediate and later stages of hydration as compared with other neat OPC pastes and hardened blends. On contrast, the partial replacement of OPC by 10% or 15% EAFS causes a marked decrease in the compressive strength values at all ages of hydration as compared to the neat OPC paste. This decrease may be attributed to a reduction in OPC content that is responsible for the decrease in the amount of formed hydration products, mainly as CSH, which represents the main binding centres of the hardened cement pastes. The replacement of OPC by 6% EAFS and 4% SF (Mix ES) causes a considerable increase in compressive strength values at all ages of hydration as compared to those of the paste made of Mix E3 (90% OPC + 10% EAFS) and comparable results with the compressive strength values obtained for the
Hydration kinetics of the investigated pastes was followed by determining the free lime and chemically combined water contents with age of hydration. The results of chemically combined water versus age of hydration for all hardened pastes made of neat OPC and OPC-EAFS blends are shown in Fig. 2. It is clear that the values of combined water increase with the age of hydration for all hardened cement pastes up to 28 days. This increase is attributed to the formation and later accumulation of hydration products mainly as CSH. Also the results indicate that a fast hydration reaction takes place from the time of mixing up to one day of hydration. After that, the initially formed hydration products shield the cement grains leading to a slower rate of hydration reaction; this was followed by a gradual increase in the rate of hydration up 28 days. At 90 days of hydration, all the tested OPC-EAFS pastes (except mixes E4 and ES) showed a slight decrease in the values of combined water content; this decrease is attributed to the phase transformation of the hydration products (mainly as CSH) from hydrates having high water contents to other hydrates having low water contents [21]. Obviously, the data of Fig. 2 also show that the paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) possesses the highest combined water contents at intermediate and later stages of hydration as
Fig. 2 Chemically combined water content with curing time for OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
4 compared to those of the other mixes (blank, E1, E2, E3 and E4) indicating the higher pozzolanic activity of silica fume with the free lime liberated from OPC hydration leading to the formation of excessive amounts of CSH with higher water contents. Obviously, the Wn-values with age of hydration show almost the same trend observed for the changes in the values of compressive strength [7]. Fig. 3 shows the free lime contents with age of hydration for various cement pastes. Evidently, the free lime content increases continuously with the age of hydration for the neat OPC paste; this is due to the continuous liberation of free portlandite (CH) from OPC hydration. On the other hand, for all OPC-EAFS blended cement pastes (E1–E4), the free lime contents increase continuously during the early hydration ages up to 28 days. This increase is attributed to the fact that the amount of free lime (CH) released by OPC hydration exceeds the amount of CH consumed by EAFS hydration. Therefore, a net increase in the free lime content was observed up to 28 days. During the hydration ages from 28 to 90 days, however, there appeared a slight decrease in the free lime content due to the activation of EAFS hydration for the pastes made of mixes (E1–E4). Finally, the results indicate that by increasing the percentage replacement of OPC by EAFS, lower values of free lime content are obtained. This decrease in the free lime contents during all ages of hydration compared to those of neat OPC pastes is due to the pozzolanic reaction of EAFS with the free lime liberated from OPC hydration [22]. The results also indicate that the observed free lime content for the paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) is a net result of two opposite effects, first the increase in the amount of free lime liberated from OPC hydration and second the consumption of this liberated lime via the pozzolanic reaction with both SF and EAFS. Thus, the paste made of Mix ES possesses the lowest values of free lime contents at all ages of hydration as compared to those of the pastes made of OPC and mixes (E1–E4). In addition, the marked decrease in the free lime content during the hydration ages from 28 to 90 days attributed to the higher pozzolanic activity of SF in Mix ES as compared to EAFS in mixes (E1–E4).
Fig. 3 Free lime content with curing time for OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
M.S. Amin et al. Phase composition and microstructure Differential scanning calorimetry (DSC) The DSC thermograms obtained for the specimens made of the neat OPC (blank), 94% OPC + 6% EAFS (Mix E2) and 90% OPC + 6% EAFS + 4% SF (Mix ES) are shown in Figs. 4–6, respectively, after 1, 28 and 90 days of hydration. The DSC curves of the neat OPC paste (blank) as shown in Fig. 4, indicate the presence of three endothermic peaks located at 80–165, 460–486 and 730–860 C. The first endotherm located at 80–165 C is mainly due to the removal of free water, the dehydration of sulphoaluminate hydrates (ettringite) and the nearly amorphous calcium silicate hydrates (CSH). The area of this endotherm is extended towards higher temperatures up to 165 C at later ages (28 and 90 days) indicating the formation of excessive amount of microcrystalline CSH. The second endotherm located at 460–490 C is mainly related to the dehydroxylation of portlandite (CH) phase [8]. The intensity and area of this endotherm increase with the age of hydration as a result of the increase in the liberated free lime content from OPC clinker hydration. The third endotherm located at 730–860 C is due to the decomposition of calcium carbonate (CaCO3) of varying degrees of crystallinity. Evidently, the nearly amorphous CaCO3 formed after 1 day of hydration decomposes at a lower temperature (730 C) while the microcrystalline form at longer curing ages decomposes at a relatively high temperature (860 C); this result accounts for the degree of carbonation of the hardened specimens [23,24]. The differential scanning calorimetry (DSC) endotherms for the specimens made of Mix E2 (94% OPC + 6% EAFS), hydrated for 1, 28 and 90 days are shown in Fig. 5. The DSC thermograms indicate the same three endothermic peaks as those of the neat OPC paste (blank). The first endotherm located at 77–160 C is mainly due to the removal of free moisture as well as the dehydration of sulphoaluminate hydrates and both of the amorphous and microcrystalline forms of calcium silicate hydrates (CSH). The areas of this endotherm are extended towards higher temperatures (up to 160 C) with time of hydration (28 and 90 days); this is mainly associated with the increase in the degree of crystallinity of CSH from the nearly amorphous phase to the microcrystalline phase. The compressive strength values showed a continuous increase with increasing age of hydration from 1 to 90 days of hydration. This increase in the compressive strength values is attributed to the increase in the amount of formed hydrates (mainly as tobermorite like CSH) as previously mentioned in case of neat OPC paste. The second endotherm located at 460–485 C is mainly related to the decomposition of portlandite (CH). The enthalpy of this endotherm increases from 18.98 to 23.23 J/g with curing time of hydration from 1 to 28 days then decreases to 19.86 J/g after 90 days of hydration. These results are explained in terms of net increase in the amount of free lime liberated from OPC up to 28 days and the net slight decrease in free lime content as a result of the very poor pozzolanic activity of EAFS. The third endotherm located at 720– 780 C is due to the decomposition of calcium carbonate with variable degree of crystallinity; this result is accounted for the degree of carbonation of the hardened specimens. The DSC thermograms for the specimens of the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF),
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes
Fig. 4
Fig. 5
5
DSC thermograms for the hardened neat OPC paste (blank) at various ages of hydration.
DSC thermograms for the hardened paste made of Mix E2 (94% OPC + 6% EAFS) at various ages of hydration.
hydrated for 1, 28 and 90 days at room temperature under water, are shown in Fig. 6. The thermograms indicate the same three endothermic peaks as those of the neat OPC pastes. The first endotherm located at 77–160 C is mainly due to the removal of moisture as well as the dehydration of the sulphoaluminate hydrates and calcium silicate hydrates (CSH). The increase in both intensity and area of this endotherm, up to final age of hydration, is attributed to the formation of additional amounts of the sulphoaluminate hydrates and calcium silicate hydrates (CSH) which formed via pozzolanic reaction of SF with CH liberated from OPC hydration; since the compressive strength values showed a continuous increase with age of hydration from 1 to 90 days of hydration. The second endotherm located at 460–485 C is mainly related to the decomposition of portlandite (CH). The third endotherm located at 720–750 C is due to the decomposition of calcium carbonate.
Scanning electron microscopy (SEM) The microstructure of the neat OPC paste hydrated for 28 days is given in Fig. 7. The SEM micrograph indicates the formation of nearly amorphous and microcrystalline particles of tobermorite-like CSH phases as the dominant hydration products, hexagonal calcium aluminate hydrates and hexagonal calcium hydroxide; these hydrates are deposited in the originally water-filled spaces as well as unhydrated parts of cement grains. The microstructure of the hardened paste made of Mix E2 (94% OPC + 6% EAFS) after 28 days of hydration is given in Fig. 8. The SEM micrograph indicated the formation of poorly crystalline particles of tobermorite-like CSH phases engulfed as well as the remaining unhydrated parts of cement and slag grains. The pores that appeared in the neat OPC paste (without EAFS) relatively disappeared in this paste as a result of the filler effect of EAFS. The filling effect of EAFS causes
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
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Fig. 6
M.S. Amin et al.
DSC thermograms for the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) at various ages of hydration.
Fig. 7 SEM micrograph for the hardened neat OPC paste (blank) after 28 days of hydration.
Fig. 9 SEM micrograph for the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) at 28 days of hydration.
the notable increase in the values of compressive strength for these hardened pastes made of Mix E2. The microstructure of the hardened cement paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) after 28 days of hydration is shown in Fig. 9. The SEM micrograph displayed a dense structure composed of crumpled fibrous particles of tobermorite-like CSH phases as the dominant hydration products and thin platelets of CH as well as unhydrated cement and slag grains. Conclusions On the basis of the results obtained in this investigation, the following conclusions can be derived:
Fig. 8 SEM micrograph for the hardened paste made of Mix E2 (94% OPC + 6% EAFS) at 28 days of hydration.
1. The optimum replacement of OPC by EAFS which causes an improvement in compressive strength compared to the neat OPC paste is 6%.
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes 2. The presence of silica fume improves the compressive strength for the hardened paste of Mix ES (90% OPC + 6% EAFS + 4% SF) as compared to Mix E3 (90% OPC + 10% EAFS); this reflects the higher pozzolanic activity of SF as compared with EAFS. 3. The variations of Wn-values with the age of hydration show the same trend for the values of compressive strength. 4. DSC thermograms for all pastes indicate the formation of nearly amorphous calcium silicate hydrates, calcium sulphoaluminate hydrates, calcium aluminate hydrates, portlandite and CaCO3. 5. The hardened OPC-EAFS and OPC-EAFS-SF pastes possess a more dense microstructure as compared with the neat OPC paste.
Conflict of interest There is no financial or other relationship with people or organizations that may inappropriately influence the work. References [1] J. Davidovits, M. Moukwa, S.L. Sarkar, K. Luke, Geopolymer Cement to Minimize Carbon-dioxide Greenhouse Warming Ceramic Transaction Cement-based Material: Present Future Environmental Aspect, American Ceramic Society, Westerville, 1993, vol. 37, pp. 165–181. [2] N.J. Saikiaa, P. Senguptaa, P.K. Gogoib, P.C. Borthakura, Cementitious properties of metakaolin-normal Portland cement mixture in the presence of petroleum effluent treatment plant sludge, Cem. Concr. Res. 32 (2002) 1717–1724. [3] B.B. Sabir, S. Wild, J. Bai, Metakaolin and calcined clay as pozzolans for concrete, Cem. Concr. Compos. 23 (2001) 441– 454. [4] Manjit Singh, Mridul Garg, Reactive pozzolana from Indian clays––their use in cement mortars, Cem. Concr. Res. 36 (2006) 1903–1907. [5] M.S. Morsy, Y.A. Al-Salloum, H. Abbas, S.H. Alsayed, Behavior of blended cement mortars containing nanometakaolin at elevated temperatures, Constr. Build. Mater. 35 (2012) 900–905. [6] M.S. Mansour, H. Kadri, S. Kenai, M. Ghrici, R. Bennaceur, Influence of calcined kaolin on mortar properties, Constr. Build. Mater. 25 (2011) 2275–2282. [7] M.S. Amin, S.A. Abo-El-Enein, A. Abdel Rahman, K.A. Alfalous, Artificial pozzolanic cement pastes containing burnt clay with and without silica fume, J. Therm. Anal. Calorim. 107 (2012) 1105–1115. [8] S.M.A. El-Gamal, F.S. Hashem, M.S. Amin, Thermal resistance of hardened cement pastes containing vermiculite and expanded vermiculite, J. Therm. Anal. Calorim. 109 (2012) 217–226.
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Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
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FULL LENGTH ARTICLE
Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume M.S. Amin *, S.M.A. El-Gamal, S.A. Abo-El-Enein, F.I. El-Hosiny, M. Ramadan Faculty of Science, Ain Shams University, Cairo, Egypt Received 11 December 2013; revised 19 June 2014; accepted 2 July 2014
KEYWORDS Electric arc furnace slag (EAFS); Silica fume (SF); Thermal analysis; Phase composition
Abstract Filled-pozzolanic cement pastes were made by different replacements of OPC by electric arc furnace slag (EAFS) with silica fume (SF) at water/cement ratio of 0.27. The pastes were hydrated up to 90 days. At each time interval, the physico-chemical characteristics of the hardened cement pastes were studied and related to the structure of the hardened pastes and the role of EAFS replacement as a filler in the hardened OPC-EAFS pastes. It was found that the optimum replacement of OPC by EAFS for the improvement in hydraulic properties of filled cement pastes is 6%. High replacement of OPC by EAFS (10% or 15%) causes a notable deterioration in the compressive strength at all ages of hydration. The replacement of EAFS in Mix (90% OPC + 10% EAFS) by 4% SF causes a marked improvement in the mechanical properties for the hardened pastes of Mix (90% OPC + 6% EAFS + 4% SF). The DSC thermograms for all pastes indicated the formation of nearly amorphous calcium silicate hydrates, calcium sulphoaluminate hydrates, calcium aluminate hydrates and portlandite. The SEM micrographs showed that the partial substitution of OPC by EAFS and/or SF leads to more dense structures as compared to the neat OPC paste. ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center.
Introduction
* Corresponding author. E-mail address: [email protected] (M.S. Amin). Peer review under responsibility of Housing and Building National Research Center.
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Manufacturing of Portland cement (PC) is a resource exhausting, energy intensive process that releases large amounts of the green house gas (CO2) into the atmosphere. At present, efforts have been made to enhance the use of cementitious materials such as pozzolana and other industrial wastes to partially replace Portland cement [1,2]. The term pozzolana was defined as siliceous/aluminous materials, either natural or artificial, which react chemically with calcium hydroxide (CH) or with materials that can release calcium hydroxide (Portland cement clinker) in the presence of water to form compounds that pos-
http://dx.doi.org/10.1016/j.hbrcj.2014.07.002 1687-4048 ª 2014 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
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M.S. Amin et al.
sess cementitious properties. Fly ash (FA), rice husk ash (RHA), silica fume (SF), slag and also calcined clay in the form of metakaolin (MK) are good examples of pozzolanic materials [3–6]. Condensed silica fume (SF) is a byproduct of silicon or ferrosilicon alloy industries. It consists of 95% amorphous silica with a very high surface area; these characteristics increase its pozzolanic activity [7,8]. Metakaolin (MK) is ultrafine artificial pozzolana powdered form of anhydrous alumino-silicate derived from the calcination of raw kaolin at a specific temperature range to form a phase transition which is highly disordered, amorphous and with pozzolanicity [9]. Slag represents one of the industrial wastes of various metal extractions and refining processes. In the specific case of making steels, the slag is generated at 3 different stages of processing and accordingly classified as: blast-furnace slag, electric arc furnace slag and ladle slag [10]. The electric arc furnace slag (EAFS) has a chemical composition more close to that of the cement clinker compared to the ground granulated blastfurnace slag (GGBFS). Hence, recently it was shown that it has potential application as a partial substitute for raw materials in clinker production. Addition of up to 20% EAFS in the kiln feed was found to improve burnability index of the raw material mix [11]. The cementitious and pozzolanic behaviour of electric arc furnace steel slag, both as received and treated, has been studied. The as received slag was completely crystalline with monticellite as the predominant phase. Treatment of this slag, remelting and water quenching, results in several phases with merwinite as the dominant phase with an increase in basicity index which is more hydraulic [12]. Hydration of multi-blended cements composed of (OPCFA-SF-MK), (OPC-MK-GGBFS) and (OPC-SF-GGBFS) mixes was studied. The results have shown that even 40 wt.% replacement of OPC by the pozzolanic materials does not deteriorate the compressive strength. This is attributed to that the pozzolanic material acts as a filler as well as, pozzolana which increases the formation of the C–S–H phase [13–17]. The object of this work is to study the hydration characteristics of OPC-EAFS blends with SF. The hydration characteristics were investigated by the determination of compressive strength, chemically combined water and free lime contents at different hydration ages. In addition, the phase composition was examined using DSC. The morphology and microstructure of some selected cement pastes were also examined using SEM.
was supplied from Ezz Flat Steel Company, Egypt, with the same Blain surface area of OPC. The chemical oxide compositions of OPC and EAFS are shown in Table 1. Condensed silica fume (SF) was supplied from ferro-silicon company, KomOmbo, Egypt. Silica fume particles are spherical and have an average diameter of about 0.1 lm. It consists of 95% amorphous silica with a specific surface area 2 · 105 cm2 g 1. The chemical composition of SF is shown in Table 1.
Experimental
Compressive strength
Materials
Fig. 1 shows the results of compressive strength versus age of hydration for all of the investigated hardened pastes made of neat OPC and OPC – EAFS blends. Obviously, the values of compressive strength of the neat OPC paste increase continuously with the age of hydration. This increase is mainly
Ordinary Portland cement (OPC) used in this study was supplied from South Valley Cement Company, Egypt, with a Blaine surface area of 2945 cm2 g 1. Electric arc furnace slag (EAFS)
Table 1
Preparation of the hardened blended cement pastes Different cement pastes were prepared using a W/S ratio of 0.27. Each paste was prepared by mixing the dry mix with the required amount of water for about 3 min. After complete mixing, the resulted paste was moulded in 1 inch cubic moulds, cured in 100% relative humidity for 24 h and then demoulded and cured under water at room temperature up to 90 days. Table 2 shows the mix designations and their compositions. Methods of investigation Compressive strength tests were performed after 1, 3, 7, 14, 28 and 90 days. At each curing time, three cubes of each mix were subjected to compressive strength test and the average value was recorded as kg cm 2. This was accomplished using a Ton-industrie machine (West Germany) with a maximum load of 60 tons. After the determination of compressive strength the crushed specimens of the hardened cement pastes were then ground and the hydration reaction was stopped using the method described in an earlier publication [18]. Samples were then dried at 80 C for 3 h in CO2-free atmosphere and maintained in a desiccator containing soda lime and CaCl2 until the time of testing. Hydration kinetics was studied by the determination of chemically combined water and free lime contents at different ages of hydration using the ground dried samples according to the methods reported in earlier investigation [18,19]. The phase composition of the formed hydrates was investigated for some selective samples by differential scanning calorimetric (DCS) technique. Also, the morphology and microstructure of hydrated phases were examined using a JSM-5410 scanning electron microscope (SEM). Results and discussion
Chemical oxide compositions of OPC and EAFS, mass %.
Oxide/mass %
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
Cr2O3
Mn2O3
TiO2
L.O.I
OPC EAFS SF
20.46 13.9 94.7
5.14 5.82 0.26
3.53 36.1 0.25
61.28 33.4 1.13
2.8 5.71 –
0.2 0.2 0.36
0.11 – 2.45
2.82 0.3 0.6
– 0.73 –
0.11 2.82 –
0.33 0.6 –
3.15 – –
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes Table 2 Compositions of the different mixes and their designations. Mixes
Blank E1 E2 E3 E4 ES
Mix proportion, mass % OPC
EAFS
SF
100 96 94 90 85 90
– 4 6 10 15 6
– – – – – 4
3 neat OPC paste as shown in Fig. 1. The presence of silica fume causes a notable improvement in the mechanical properties for the hardened paste of Mix ES as compared to Mix E3; Silica fume consists of very fine amorphous particles, large surface area, high SiO2 content and acts as a very reactive pozzolana when used as a blend in cement and concrete. Silica fume plays two important roles: (i) it acts as a filler, due to its fine particle size, fills the spaces between the cement grains and this effect reduces the pore size and (ii) it acts as a pozzolanic material due to its ability to react with free lime liberated from the hydration of OPC leading to the formation of additional amounts of calcium silicate hydrates with a consequent increase in compressive strength [7]. Hydration kinetics
Fig. 1 Compressive strength values (kg/cm2) with curing time for the neat OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
attributed to the formation of hydration products mainly as CSH, within the pore system of the hardened paste. The partial substitution of OPC by EAFS as supplementary cementitious material, leads to a discrepancy in the values of compressive strength for the hardened pastes made of OPC-EAFS blends; these discrepancies in the compressive strength for the different blends are mainly attributed to the role of EAFS, as a partial substituent of OPC, which has weak pozzolanic characteristic and filler effects that lead to a noticeable decrease in the total porosity of the hardened pastes. Therefore, the compressive strength increases [20]. On the other hand, the results also revealed that the paste made of Mix E2 (94% OPC + 6% EAFS) possesses the highest strength values at intermediate and later stages of hydration as compared with other neat OPC pastes and hardened blends. On contrast, the partial replacement of OPC by 10% or 15% EAFS causes a marked decrease in the compressive strength values at all ages of hydration as compared to the neat OPC paste. This decrease may be attributed to a reduction in OPC content that is responsible for the decrease in the amount of formed hydration products, mainly as CSH, which represents the main binding centres of the hardened cement pastes. The replacement of OPC by 6% EAFS and 4% SF (Mix ES) causes a considerable increase in compressive strength values at all ages of hydration as compared to those of the paste made of Mix E3 (90% OPC + 10% EAFS) and comparable results with the compressive strength values obtained for the
Hydration kinetics of the investigated pastes was followed by determining the free lime and chemically combined water contents with age of hydration. The results of chemically combined water versus age of hydration for all hardened pastes made of neat OPC and OPC-EAFS blends are shown in Fig. 2. It is clear that the values of combined water increase with the age of hydration for all hardened cement pastes up to 28 days. This increase is attributed to the formation and later accumulation of hydration products mainly as CSH. Also the results indicate that a fast hydration reaction takes place from the time of mixing up to one day of hydration. After that, the initially formed hydration products shield the cement grains leading to a slower rate of hydration reaction; this was followed by a gradual increase in the rate of hydration up 28 days. At 90 days of hydration, all the tested OPC-EAFS pastes (except mixes E4 and ES) showed a slight decrease in the values of combined water content; this decrease is attributed to the phase transformation of the hydration products (mainly as CSH) from hydrates having high water contents to other hydrates having low water contents [21]. Obviously, the data of Fig. 2 also show that the paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) possesses the highest combined water contents at intermediate and later stages of hydration as
Fig. 2 Chemically combined water content with curing time for OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
4 compared to those of the other mixes (blank, E1, E2, E3 and E4) indicating the higher pozzolanic activity of silica fume with the free lime liberated from OPC hydration leading to the formation of excessive amounts of CSH with higher water contents. Obviously, the Wn-values with age of hydration show almost the same trend observed for the changes in the values of compressive strength [7]. Fig. 3 shows the free lime contents with age of hydration for various cement pastes. Evidently, the free lime content increases continuously with the age of hydration for the neat OPC paste; this is due to the continuous liberation of free portlandite (CH) from OPC hydration. On the other hand, for all OPC-EAFS blended cement pastes (E1–E4), the free lime contents increase continuously during the early hydration ages up to 28 days. This increase is attributed to the fact that the amount of free lime (CH) released by OPC hydration exceeds the amount of CH consumed by EAFS hydration. Therefore, a net increase in the free lime content was observed up to 28 days. During the hydration ages from 28 to 90 days, however, there appeared a slight decrease in the free lime content due to the activation of EAFS hydration for the pastes made of mixes (E1–E4). Finally, the results indicate that by increasing the percentage replacement of OPC by EAFS, lower values of free lime content are obtained. This decrease in the free lime contents during all ages of hydration compared to those of neat OPC pastes is due to the pozzolanic reaction of EAFS with the free lime liberated from OPC hydration [22]. The results also indicate that the observed free lime content for the paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) is a net result of two opposite effects, first the increase in the amount of free lime liberated from OPC hydration and second the consumption of this liberated lime via the pozzolanic reaction with both SF and EAFS. Thus, the paste made of Mix ES possesses the lowest values of free lime contents at all ages of hydration as compared to those of the pastes made of OPC and mixes (E1–E4). In addition, the marked decrease in the free lime content during the hydration ages from 28 to 90 days attributed to the higher pozzolanic activity of SF in Mix ES as compared to EAFS in mixes (E1–E4).
Fig. 3 Free lime content with curing time for OPC and various OPC – EAFS and OPC-EAFS-SF hardened pastes.
M.S. Amin et al. Phase composition and microstructure Differential scanning calorimetry (DSC) The DSC thermograms obtained for the specimens made of the neat OPC (blank), 94% OPC + 6% EAFS (Mix E2) and 90% OPC + 6% EAFS + 4% SF (Mix ES) are shown in Figs. 4–6, respectively, after 1, 28 and 90 days of hydration. The DSC curves of the neat OPC paste (blank) as shown in Fig. 4, indicate the presence of three endothermic peaks located at 80–165, 460–486 and 730–860 C. The first endotherm located at 80–165 C is mainly due to the removal of free water, the dehydration of sulphoaluminate hydrates (ettringite) and the nearly amorphous calcium silicate hydrates (CSH). The area of this endotherm is extended towards higher temperatures up to 165 C at later ages (28 and 90 days) indicating the formation of excessive amount of microcrystalline CSH. The second endotherm located at 460–490 C is mainly related to the dehydroxylation of portlandite (CH) phase [8]. The intensity and area of this endotherm increase with the age of hydration as a result of the increase in the liberated free lime content from OPC clinker hydration. The third endotherm located at 730–860 C is due to the decomposition of calcium carbonate (CaCO3) of varying degrees of crystallinity. Evidently, the nearly amorphous CaCO3 formed after 1 day of hydration decomposes at a lower temperature (730 C) while the microcrystalline form at longer curing ages decomposes at a relatively high temperature (860 C); this result accounts for the degree of carbonation of the hardened specimens [23,24]. The differential scanning calorimetry (DSC) endotherms for the specimens made of Mix E2 (94% OPC + 6% EAFS), hydrated for 1, 28 and 90 days are shown in Fig. 5. The DSC thermograms indicate the same three endothermic peaks as those of the neat OPC paste (blank). The first endotherm located at 77–160 C is mainly due to the removal of free moisture as well as the dehydration of sulphoaluminate hydrates and both of the amorphous and microcrystalline forms of calcium silicate hydrates (CSH). The areas of this endotherm are extended towards higher temperatures (up to 160 C) with time of hydration (28 and 90 days); this is mainly associated with the increase in the degree of crystallinity of CSH from the nearly amorphous phase to the microcrystalline phase. The compressive strength values showed a continuous increase with increasing age of hydration from 1 to 90 days of hydration. This increase in the compressive strength values is attributed to the increase in the amount of formed hydrates (mainly as tobermorite like CSH) as previously mentioned in case of neat OPC paste. The second endotherm located at 460–485 C is mainly related to the decomposition of portlandite (CH). The enthalpy of this endotherm increases from 18.98 to 23.23 J/g with curing time of hydration from 1 to 28 days then decreases to 19.86 J/g after 90 days of hydration. These results are explained in terms of net increase in the amount of free lime liberated from OPC up to 28 days and the net slight decrease in free lime content as a result of the very poor pozzolanic activity of EAFS. The third endotherm located at 720– 780 C is due to the decomposition of calcium carbonate with variable degree of crystallinity; this result is accounted for the degree of carbonation of the hardened specimens. The DSC thermograms for the specimens of the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF),
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes
Fig. 4
Fig. 5
5
DSC thermograms for the hardened neat OPC paste (blank) at various ages of hydration.
DSC thermograms for the hardened paste made of Mix E2 (94% OPC + 6% EAFS) at various ages of hydration.
hydrated for 1, 28 and 90 days at room temperature under water, are shown in Fig. 6. The thermograms indicate the same three endothermic peaks as those of the neat OPC pastes. The first endotherm located at 77–160 C is mainly due to the removal of moisture as well as the dehydration of the sulphoaluminate hydrates and calcium silicate hydrates (CSH). The increase in both intensity and area of this endotherm, up to final age of hydration, is attributed to the formation of additional amounts of the sulphoaluminate hydrates and calcium silicate hydrates (CSH) which formed via pozzolanic reaction of SF with CH liberated from OPC hydration; since the compressive strength values showed a continuous increase with age of hydration from 1 to 90 days of hydration. The second endotherm located at 460–485 C is mainly related to the decomposition of portlandite (CH). The third endotherm located at 720–750 C is due to the decomposition of calcium carbonate.
Scanning electron microscopy (SEM) The microstructure of the neat OPC paste hydrated for 28 days is given in Fig. 7. The SEM micrograph indicates the formation of nearly amorphous and microcrystalline particles of tobermorite-like CSH phases as the dominant hydration products, hexagonal calcium aluminate hydrates and hexagonal calcium hydroxide; these hydrates are deposited in the originally water-filled spaces as well as unhydrated parts of cement grains. The microstructure of the hardened paste made of Mix E2 (94% OPC + 6% EAFS) after 28 days of hydration is given in Fig. 8. The SEM micrograph indicated the formation of poorly crystalline particles of tobermorite-like CSH phases engulfed as well as the remaining unhydrated parts of cement and slag grains. The pores that appeared in the neat OPC paste (without EAFS) relatively disappeared in this paste as a result of the filler effect of EAFS. The filling effect of EAFS causes
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
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Fig. 6
M.S. Amin et al.
DSC thermograms for the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) at various ages of hydration.
Fig. 7 SEM micrograph for the hardened neat OPC paste (blank) after 28 days of hydration.
Fig. 9 SEM micrograph for the hardened paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) at 28 days of hydration.
the notable increase in the values of compressive strength for these hardened pastes made of Mix E2. The microstructure of the hardened cement paste made of Mix ES (90% OPC + 6% EAFS + 4% SF) after 28 days of hydration is shown in Fig. 9. The SEM micrograph displayed a dense structure composed of crumpled fibrous particles of tobermorite-like CSH phases as the dominant hydration products and thin platelets of CH as well as unhydrated cement and slag grains. Conclusions On the basis of the results obtained in this investigation, the following conclusions can be derived:
Fig. 8 SEM micrograph for the hardened paste made of Mix E2 (94% OPC + 6% EAFS) at 28 days of hydration.
1. The optimum replacement of OPC by EAFS which causes an improvement in compressive strength compared to the neat OPC paste is 6%.
Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002
Physico-chemical characteristics of blended cement pastes 2. The presence of silica fume improves the compressive strength for the hardened paste of Mix ES (90% OPC + 6% EAFS + 4% SF) as compared to Mix E3 (90% OPC + 10% EAFS); this reflects the higher pozzolanic activity of SF as compared with EAFS. 3. The variations of Wn-values with the age of hydration show the same trend for the values of compressive strength. 4. DSC thermograms for all pastes indicate the formation of nearly amorphous calcium silicate hydrates, calcium sulphoaluminate hydrates, calcium aluminate hydrates, portlandite and CaCO3. 5. The hardened OPC-EAFS and OPC-EAFS-SF pastes possess a more dense microstructure as compared with the neat OPC paste.
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Please cite this article in press as: M.S. Amin et al., Physico-chemical characteristics of blended cement pastes containing electric arc furnace slag with and without silica fume, HBRC Journal (2014), http://dx.doi.org/10.1016/j.hbrcj.2014.07.002