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Development of Alkali-activated Cementitious Material using Copper Slag
Construction and Building Materials 211 (2019) 73–79
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Development of Alkali-activated Cementitious Material using Copper Slag Jagmeet Singh, SP Singh 1,⇑ Dr B R Ambedkar National Institute of Technology, Jalandhar, India
h i g h l i g h t s CS was use as source of aluminosilicate for development of ACM. The mixes were activated using alkali content of 5% and 7% by weight of binder. Addition of mineral admixtures in alkali-activated CS improves its performance. MK was found to be more effective than FA.
a r t i c l e
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Article history: Received 18 September 2018 Received in revised form 15 March 2019 Accepted 18 March 2019
Keywords: Alkali-activated Cementitious Material Compressive strength Copper Slag Microstructure Porosity
a b s t r a c t The paper presents results of an investigation conducted to study the feasibility of use of Copper Slag (CS) as aluminosilicate material for developing Alkali-activated Cementitious Material (ACM). The effect of addition of mineral admixtures such as Fly Ash (FA) and Metakaolin (MK) on the performance of alkali-activated CS was also investigated. In addition to a control mix which was prepared using 100% CS; two other mixes were also prepared by replacing CS with 30% FA and 30% MK respectively. These mixes were activated using alkali content of 5% and 7% by weight of binder. Sodium hydroxide and sodium silicate were used as alkali-activators. Compressive strength tests were performed on alkaliactivated cement mixes at different curing ages. The pore-size distribution, mineralogy and microstructure of selected alkali-activated cement mixes were determined using Mercury Intrusion Porosimetry (MIP), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS). Test results indicate that CS has great potential as aluminosilicate material for developing ACM. Further, the addition of mineral admixtures in alkali-activated CS cement improves its performance; however, the addition of MK was found to be more effective than FA in terms of compressive strength and microstructure development. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Alkali-activated Cementitious Material (ACM) is a product of a chemical reaction of aluminosilicate material and an alkali component at high temperature. Purdon in 1940 developed the first ACM using slag as aluminosilicate material and caustic soda as an alkali component [1]. In 1978, Davidovits introduced a new term for this reaction, which was known as geopolymerization [2]. The sources of aluminosilicate materials may be industrial wastes such as Ground-Granulated Blast Furnace Slag (GGBFS), Fly Ash (FA), Rice Husk Ash (RHA) and natural pozzolans i.e. calcined clay, volcanic ⇑ Corresponding author. E-mail address: [email protected] (SP Singh). Department of Civil Engineering, Dr B R Ambedkar National Institute of Technology, Jalandhar 144011, India. 1
https://doi.org/10.1016/j.conbuildmat.2019.03.233 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ashes and zeolites. The reaction products of alkali-activation depend upon the chemical composition of aluminosilicate materials. High calcium binders react with alkali activators, produce C(A)-S-H–type gel and low calcium binders produce N-A-S-(H) type gel, where N represents Na+ or K+ cations [3,4]. Alkali-activated cementitious materials provide comparable mechanical properties to Portland Cement (PC) and have greater resistance to corrosion, acid attack and chemical attack [5–7]. Alkali-activation of industrial waste takes many economic and environmental benefits as the production process of ACM releases less CO2 and requires less energy as compared to PC [8,9]. In earlier studies, ACMs were mainly derived from precursors such as GGBFS and FA and its chemical composition, physical properties and optimum synthesis parameters (i.e. activator type, activator content, curing type and curing temperature) are briefly described in the literature [5,6]. However, the growing demands
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of ACMs and supplementary cementitious materials lead to the consumption of significant amount of GGBFS and FA. According to a rough estimate, the use of GGBFS and FA in cement production was of the order of 43.7 million tons and 39.8 million tons respectively in the year 2015 [10]. As cement consumption increases rapidly, these materials are expected to fulfill approximately 20% requirement of total cement production by 2050 [11]. So, there is a need to identify the new aluminosilicate materials such as Copper Slag (CS), which may be used as a precursor for the development of ACM. Copper Slag is metallurgical waste generated during the extraction of copper from copper ores. During smelting phase reducing agent (Silica) separates the sulphides and oxides, which are present in the Copper ores. The sulphides do not make any anions and develop matte phase (Copper rich) and reducing agent combines with oxide to form slag phase (Impurities rich). The water quenching of slag produces glassy, amorphous and granulated slag. Copper Slag has a specific gravity in the range of 3.4–3.7 and consists of Fe (30–40%), SiO2 (35–40%), Al2O3 (<10%), CaO (<10%) and Cu (0.5–2.1%) [12]. It was estimated that in the smelting process, 2–3 times more CS is generated than copper and approximately 24.6 million tones of CS is generated annually worldwide [12]. Copper slag usually dumped at sites, which creates environmental issues and utilizes the valuable land. As per National Green Tribunal, around 3.52 lakh tonnes of CS has dumped at Thoothukudi (Tamil Nadu, India), which affecting the surrounding agricultural land and water ways [13]. Copper Slag can be used as an abrasive material, manufacturing of floor tiles and glass-ceramic [14–16]. Copper slag has been used in construction industry as aggregates [17,18] and also cementitious materials [19,20]. The presence of alumina and silica in the CS makes it suitable as precursor in the alkali-activation [21–23]. Ahmari et al. [21] developed the ACM using CS and mine-tailing (MT) as source of aluminosilicate. It was found that ACM synthesis using 100% CS and cured at 90 °C for 7 days exhibits the maximum compressive strength up to 50 MPa. Nazer et al. [22] studied the effect of curing time and curing temperature on the performance of alkali activated CS cement matrices. The performance of heat cured specimens at 65 °C for 3 days was similar to ambient cured specimens at 20 °C for 90 days. Qianmin et al. [23] studied the effect of elevated temperature on the mechanical properties of alkaliactivated CS. The optimum elevated temperature for alkaliactivated CS was 200 °C, above this mechanical properties of alkali-activated CS was reduced due to the decompositions of hydration products. From above literature, it can be observed that heat curing of alkali-activated CS gives better results than ambient curing. However, the effect of addition of mineral admixtures on the performance of alkali-activated CS is scanty in literature. Therefore, the aim of the present study is to develop the alkali-activated CS with addition of mineral admixtures. The addition of mineral admixtures in alkali-activated CS could improve its performance and will make it more suitable as cementitious material. The alkaliactivation process of CS was evaluated through compressive strength tests conducted on mortar specimens with different percentages of CS and mineral admixtures. The results of microstructure analysis, porosity and reaction products of different mixes were also evaluated and compared. It is mentioned here that the work reported in this paper is part of a larger investigation on the strength and durability properties of alkali-activated copper slag concrete. First, the optimum synthesis parameters for alkali-activation of copper slag such as alkali content, type of alkali, curing regime and mineral admixtures will be selected. The effects of these synthesis parameters on the mechanical and durability properties of concrete will be evaluated.
After that the final concrete mix will be evaluated in terms of cost, energy suitability and environmental sustainability. 2. Experimental programme 2.1. Materials used Copper Slag obtained from Rajasthan, India was used in the investigation. To enhance the performance of alkali-activated CS, FA and MK were added as additional source of alumina and silica. The chemical composition of CS, FA and MK was determined using X-ray fluorescence and is shown in Table 1. Commercially available MK was used, which is supplied as MetaCem 85C and has calcining temperature of 600–800 °C. It can be seen from Table 1 that there is significant amount of Fe2O3 in CS. The role of Fe2O3 in the geopolymerization process is vague. Other than CS, Fe rich slags have been thoroughly studied as potential precursors, although it remains yet unclear exactly how it participates at the atomic level in geopolymerization. The Standard sand conforming to IS 650 [24] was used to prepare alkali-activated cement mortars. Locally available Sodium Hydroxide (SH) and Sodium Silicate (SS) was used as alkali-activators. The SH was supplied in pellet form with 98% purity, whereas SS was supplied in solution form with Ms = 3.3 (Si2O = 26.5%, Na2O = 8%, H2O = 65.5% by weight). Only CS was ground in Laboratory Ball Mill using 20 mm and 25 mm stainless steel balls for 2 h to attain its fineness equal to FA. As such there in no standard available regarding specification of CS for use as cement. However, the specification of FA for use with cement is given in Indian Standard 3812. According to this standard, FA particles retained on 45 mm IS sieve should be less than 34%. Therefore, to attain equal and more fineness than FA, CS was sieved through 45-mm sieve. The ratio of stainless steel balls (kg) to CS (kg) was 2. The parentage passing through 45-mm sieve for CS, FA and MK was 95%, 82% and 100% respectively. The specific gravity and BET surface area of used materials is given in Table 1. The X-ray Diffraction patterns of CS, FA and MK are shown in Fig. 1. The XRD patterns show that CS is a fully amorphous material with minor peaks of crystalline mineral i.e. Fayalite (F) and Clinoferrosilite (C) and slags with a high degree of amorphous content reflect higher reactivity towards alkali-activation [5]. Strong deviations of crystalline minerals; Quartz (Q) and Mulite (M) were detected on 2h range of 15° to 40° in MK and FA and some peaks of Magnetite (MT) and Sillimanite (SM) were also detected. Except these crystalline peaks, MK and FA were observed to be partially amorphous materials. 2.2. Mix proportions of alkali-activated cement mortars Alkali-activated cement mortars were prepared using CS, MK and FA as a binder with standard sand in the ratio of 1:3. In addition to a control mix which was prepared using 100% CS; two other mixes were also prepared by replacing CS with 30% FA and 30% MK respectively. These mixes were activated using alkali content of 5% and 7% by weight of the binder. Sodium hydroxide and sodium silicate were mixed to prepare the alkali activator solution of modulus Ms = 1.25 (Ms = Si2O/ Na2O = 1.25), which was constant for all the mixes. A water-to-solid ratio of 0.3 was fixed for all the mixes. Mix proportions of different mixes for 1000 g of the binder are given in Table 2. 2.3. Mixing, casting and curing To prepare alkali activator solution, sodium hydroxide flakes were dissolved in water and then sodium silicate was added in this solution and stirred constantly.
Table 1 Chemical composition and physical properties of CS, FA and MK. Compound (%)
Si2O Al2O3 Fe2O3 CaO Na2O K2O TiO2 SO3 MgO MgO P2O5 MnO Loss on ignition (%) Specific gravity BET surface area (m2/kg)
Composition CS
FA
MK
37.6 11.5 42.4 3.80 0.74 0.76 0.37 0.01 0.57 0.62 0.13 0.01 1.50 3.21 503
56.6 31.5 4.72 1.10 0.10 0.35 1.62 0.32 0.36 0.02 – – 3.3 2.34 465
58.1 39.1 1.21 0.23 0.08 0.09 0.83 0.13 – – – – 0.23 2.62 1055
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Fig. 1. XRD patterns of CS, FA and MK.
Table 2 Mix proportions for 1000 g of binder (By weight). Mix
CS (g)
FA (g)
MK (g)
Sand (g)
SH (g)
SS (g)
Water (g)
Alkali content
W/S ratio
1 2 3 4 5 6
1000 700 700 1000 700 700
– 300 – – 300 –
– – 300 – – 300
3000 3000 3000 3000 3000 3000
40 40 40 56 56 56
236 236 236 330 330 330
181 181 181 135 135 135
5% 5% 5% 7% 7% 7%
0.3 0.3 0.3 0.3 0.3 0.3
W/S ratio = Water/Solid ratio, where water is water in activator and extra water; Solid is solid part of activators and binders. The alkali activator solution was prepared 24 h prior to mixing to cool down the temperature of the solution. First, dry materials such as CS, FA, MK and standard sand were weighed in the required quantity and put into the mortar mixture and mixed for 5 min. Subsequently, alkali activator solution was added in the mixer and mixed for another 2 min. After the mixing, the mortar was poured into cube specimens of size 50 50 50 mm and sealed to prevent any moisture loss. Though expected, the authors did not observe any significant loss in workability on account of addition of MK. It can be seen from literature that the performance of heat cured alkali-activated CS specimens was better than ambient cured specimens [21–23]. Keeping this in view, the authors selected heat curing for the alkali-activation of CS. After the casting, the cube specimens in sealed condition was placed into oven and heat cured for 24 h at 80 °C along with moulds. The specimens de-moulded and placed in laboratory conditions at a temperature of 25 ± 3 °C and relative humidity of 50 ± 5% until testing. The specimens were cured in laboratory conditions for 3, 7 and 28 days until testing.
2.6 Microstructure and mineralogy The microstructure of alkali-activated samples was observed using highresolution field emission Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS). Samples for SEM were put in ethanol to stop the reaction and then dried in an oven up to constant weight and for better observations; all samples were gold-coated. Mineralogical study of alkali-activated samples was carried out using X-Ray Diffraction (XRD). The XRD patterns were obtained using a Cu-Ka radiation under conditions of 45 kV and 40 mA. For SEM and XRD tests, samples were collected from broken cubes after conducting compressive strength tests at 28 days of curing of some of the mixes made with an alkali content of 5%. The XRD test was also performed on raw aluminosilicate materials such as CS, FA and MK.
3. Results and discussion 2.4. Compressive strength test
3.1 Compressive strength The compressive strength tests on alkali-activated CS cement mortars were conducted as per IS 516 [25]. The compressive strength was determined using compression testing machine of capacity of 2000 kN at a loading rate of 0.6 kN/s. 2.5. Pore-size distribution and porosity Pore-size distribution and porosity of alkali-activated CS cement mortars were determined using Mercury Intrusion Porosimeter (MIP) at a pressure range of 0.2– 50,000 psia. For MIP, samples were collected from broken cubes after conducting compressive strength tests at 28 days of curing of some of the mixes made with an alkali content of 5%. Prior to testing, the samples were dried in an oven at a temperature of 100 °C until constant weight was achieved.
The compressive strength results of different mixes of alkaliactivated CS cement mortars with different alkali contents and curing ages are shown in Fig. 2. The 28 days compressive strength of alkali-activated CS mortars for 5% and 7% alkali content was 27.56 MPa and 34.12 MPa respectively, which seems to be sufficient for structural applications in various construction works. The amorphous nature of CS helps in the dissolution of silicates, which promotes the alkali-activation process and improves compressive strength [22]. The amorphous materials are structurally
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3.1.1. Effect of curing ages on compressive strength From Fig. 2, it was observed that the compressive strength of all mixes was increased with increase in curing age. The increment in compressive strength with curing age was symmetric in the all mixes; however major portion of compressive strength was gained in just 3 days of curing. In all mixes, up to 80% of 28 days compressive strength was achieved in 3 days of curing. This can be attributed to the heat curing of mixes, which accelerate the alkaliactivation process of binders and develop the compressive strength in short curing periods. It was also observed in the literature that during the heat curing of alkali-activated CS mortars, maximum gain in compressive strength was achieved in just 3 days of curing period [22]. 3.1.2. Effect of alkali content on compressive strength The effect of alkali content on the compressive strength of different mixes is presented in Fig. 2. It was found that the compressive strength of all mixes was increased with an increase in alkali content. With the increase in alkali content, the compressive strength of mix CS was increased from 23.75 MPa to 31.46 MPa at 3 days, 25.63 MPa to 32.41 MPa at 7 days and 27.56 MPa to 34.12 MPa at 28 days of curing as shown in Fig. 2(a). A similar trend was also observed for mix CS + FA shown in Fig. 2(b), wherein compressive strength was increased from 25.27 MPa to 33.55 MPa at 3 days, 26.74 MPa to 36.27 MPa at 7 days and 30.72 MPa to 39.01 MPa at 28 days of curing. It can be seen that with the increase in alkali content from 5% to 7%, the compressive strength of mix CS + MK as shown in Fig. 2(c) was increased by 12.95%, 11.47% and 11.11% at 3, 7 and 28 days of curing respectively. Higher content of alkali provides extra species of Si and Na, where Na dissolves the silicate and aluminate from the binders and extra Si species from activator helps in formation aluminosilicate gel [26,27]. The solidification of aluminosilicate gel results in the formation of the alkali-activated binder, which improves the compressive strength of mixes. 3.1.3. Effect of mineral admixture on compressive strength The effect of different mineral admixtures on the compressive strength of alkali-activated CS mortars is shown in Fig. 2. It was observed that the addition of mineral admixtures improved the compressive strength of alkali-activated CS mortars. However, the addition of MK was more effective in improving the compressive strength than the addition of FA in alkali-activated CS mortars. For 5% and 7% alkali content, CS + MK mix exhibits maximum compressive strength of 40.78 MPa and 45.31 MPa at a curing age of 28 days as presented in Fig. 2(c). The performance of MK additive mixes was better than other mixes due to its chemical composition and fine particles. Metakaolin contains some large amount of silicate and alumina than FA and CS (Table 1), which is released in the presence of alkali media and combines with alkali cations to form cross-link gel, which acts as a binder and strengthen the alkali-activated CS mortars [28]. Also, the fine particles of MK than FA and CS make it more reactive and provide the large surface area than FA and CS for alkali-activation. 3.2. Pore-size distribution and porosity Fig. 2. Effect of alkali content on compressive strength of alkali-activated mixes (a) CS (b) CS + FA and (c) CS + MK.
disordered and these materials can easily breaks down in to its constituents. Due to amorphous nature of CS, when it meets highly alkaline environment, dissolution or break down of different constituents such as alumina and silica takes place. The effect of curing ages, alkali contents and mineral admixtures on compressive strength of alkali-activated CS mortars is briefly described in the following sections.
The porosity and pore-size distribution of selected samples were determined by MIP. Samples were collected from broken cubes of 28 days compressive strength tests of mixes made with CS, CS + FA and CS + MK activated with 5% alkali content. The porosity of different mixes measured is shown in Fig. 3. The porosity of alkali-activated CS mix was highest amongst all the mixes i.e. 15.12%, followed by mix made with CS + FA = 11.97% and mix containing CS + MK = 10.11%. It can be seen that with the addition of FA and MK, the porosity of alkali-activated CS mixes was
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Fig. 3. Porosity of alkali-activated CS, CS + FA and CS + MK mixes, 5% alkali content.
decreased. However, the addition of MK was found to be more effective in reducing the porosity than FA. The reduced porosity of mix containing MK may be attributed to the finer particles of MK compared to FA, which is responsible for better particle packing and results in the dense microstructure. The pore-size distribution of different mixes is shown in Fig. 4, where the pores are separated into mesopores (<50 nm diameter) and macropores (>50 nm diameter). It has been found that MK-based mixture consists of macropores, however the macropores compared to FA and CS mix were negligible. The alkali-activation of MK produces NASH type gel, which fills most of the macropores and gives more refined structure than mixes containing FA and CS. The porosities have a direct relationship with the compressive strength of ACM. Lower porosity gives the refined structure and a more refined structure of ACM can acquire uniform stress distribution, which leads to a higher compressive strength [29–31]. The mix containing MK having least porosity shows better compressive strength than mixes containing FA and CS.
3.3. Mineralogy X-ray diffraction patterns of mixes activated with 5% alkali content are shown in Fig. 5. The XRD patterns of various mixes tested in this investigation indicate crystalline peaks of quartz and mul-
Fig. 4. Pore size distribution of alkali-activated CS, CS + FA and CS + MK mixes indicating Mesopores and Macropores, 5% alkali content.
Fig. 5. XRD patterns of alkali-activated CS, CS + FA and CS + MK mixes, 5% alkali content.
lite. The crystalline peaks of quartz and mulite were initially present in MK and FA (Fig. 1), which were also reflected after alkaliactivation in Fig. 5. The alkali-activated CS mix also shows the crystalline peak of quartz and mullite, which may be due to nonreactive silicate species. It was observed that new hydration products formed during alkali-activation process were not detected in XRD patterns may be due to their amorphous nature. However, the MK mix contains very small peaks of zeolite X, which is aluminosilicate mineral and represents the existence of aluminosilicate gel. 3.4. Microstructure Micrographs of alkali-activated CS, CS + FA and CS + MK mixes with 5% alkali content were detected using SEM and are shown in Fig. 6. It can be observed that alkali-activated CS mix shows maximum micro-cracks as seen in Fig. 6(a). Some micro-cracks were also observed in alkali-activated CS + FA mix as can be seen in Fig. 6(b), but alkali-activated CS + MK mix shows most compact and dense microstructure in Fig. 6(c). In addition to this, some geopolymer gel phase was also detected as a brightened portion in the alkali-activated CS + MK mix. Microstructure analysis reflects that alkali-activated CS mix has most porous structures, but the addition of FA and MK makes its structure dense due to the formation of geopolymer gel and smaller particle size of FA and MK also helps in solidification of the microstructure of alkali-activated CS mix. Due to the densest microstructure of alkali-activated CS + MK mix, it demonstrates the highest compressive strength amongst all mixes. Test results of EDX analysis of alkali-activated mixes are also shown in Fig. 6. The Al/Si ratio is most important factor in alkaliactivated materials for determination the behavior of geopolymer gel. The increase of Al/Si ratio indicates more dissolved aluminum in alkaline medium and which enhances its contribution in formation of geopolymer gel [32]. In Fig. 6(f), alkali-activated mix CS + MK shows maximum Al/Si ratio, which indicates its wellformed geopolymeric matrix. Some atoms of Fe are shown in EDX of alkali-activated mixes of CS and CS + FA in Fig. 6(d) and (e) respectively. The lower compressive strength of alkaliactivated CS and CS + FA mixes is due to presence of Fe in these mixes. The existence of Fe in alkali-activated mixes provides resistance to discharge of Si and Al in an alkaline environment, which
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Fig. 6. SEM micrographs of alkali-activated mixes (a) CS (b) CS + FA (c) CS + MK and EDX analysis results of alkali-activated mixes (d) CS (e) CS + FA (f) CS + MK, 5% alkali content.
opposes the alkali-activation process and thus results in reduced compressive strength [21]. 4. Conclusions Following conclusions can be drawn from the present study: (a) All alkali-activated mixes achieved 80% of 28 days compressive strength within 3 days due to heat curing. (b) The compressive strength of all alkali-activated mixes was increased by more than 11%, when alkali content was increased from 5% to 7%.
(c) Alkali-activated CS mix activated using 7% alkali content exhibits compressive strength of the order of 34.12 MPa, which seems to be sufficient for structural applications in various construction works. (d) The compressive strength of alkali-activated CS was increased by more than 11% and 32% with addition of FA and MK respectively. The microstructure of alkali-activated CS was also improved with addition of FA and MK. (e) In general it can concluded that CS can be used as aluminosilicate in the development of ACM. The addition of FA and MK in alkali-activated CS makes it more suitable as cementitious material in construction industry.
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Acknowledgments The authors acknowledge financial assistance in the form of a fellowship to the first author from the Ministry of Human Resource Development (MHRD), Government of India. The authors also acknowledge the help from the staff of Concrete Technology and Structures Testing Laboratories at Dr B R Ambedkar National Institute of Technology, Jalandhar during the experimentation work.
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Conflict of interest
[18]
The authors declare that there is no conflict of interests regarding the publication of this article.
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Development of Alkali-activated Cementitious Material using Copper Slag Jagmeet Singh, SP Singh 1,⇑ Dr B R Ambedkar National Institute of Technology, Jalandhar, India
h i g h l i g h t s CS was use as source of aluminosilicate for development of ACM. The mixes were activated using alkali content of 5% and 7% by weight of binder. Addition of mineral admixtures in alkali-activated CS improves its performance. MK was found to be more effective than FA.
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Article history: Received 18 September 2018 Received in revised form 15 March 2019 Accepted 18 March 2019
Keywords: Alkali-activated Cementitious Material Compressive strength Copper Slag Microstructure Porosity
a b s t r a c t The paper presents results of an investigation conducted to study the feasibility of use of Copper Slag (CS) as aluminosilicate material for developing Alkali-activated Cementitious Material (ACM). The effect of addition of mineral admixtures such as Fly Ash (FA) and Metakaolin (MK) on the performance of alkali-activated CS was also investigated. In addition to a control mix which was prepared using 100% CS; two other mixes were also prepared by replacing CS with 30% FA and 30% MK respectively. These mixes were activated using alkali content of 5% and 7% by weight of binder. Sodium hydroxide and sodium silicate were used as alkali-activators. Compressive strength tests were performed on alkaliactivated cement mixes at different curing ages. The pore-size distribution, mineralogy and microstructure of selected alkali-activated cement mixes were determined using Mercury Intrusion Porosimetry (MIP), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS). Test results indicate that CS has great potential as aluminosilicate material for developing ACM. Further, the addition of mineral admixtures in alkali-activated CS cement improves its performance; however, the addition of MK was found to be more effective than FA in terms of compressive strength and microstructure development. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Alkali-activated Cementitious Material (ACM) is a product of a chemical reaction of aluminosilicate material and an alkali component at high temperature. Purdon in 1940 developed the first ACM using slag as aluminosilicate material and caustic soda as an alkali component [1]. In 1978, Davidovits introduced a new term for this reaction, which was known as geopolymerization [2]. The sources of aluminosilicate materials may be industrial wastes such as Ground-Granulated Blast Furnace Slag (GGBFS), Fly Ash (FA), Rice Husk Ash (RHA) and natural pozzolans i.e. calcined clay, volcanic ⇑ Corresponding author. E-mail address: [email protected] (SP Singh). Department of Civil Engineering, Dr B R Ambedkar National Institute of Technology, Jalandhar 144011, India. 1
https://doi.org/10.1016/j.conbuildmat.2019.03.233 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ashes and zeolites. The reaction products of alkali-activation depend upon the chemical composition of aluminosilicate materials. High calcium binders react with alkali activators, produce C(A)-S-H–type gel and low calcium binders produce N-A-S-(H) type gel, where N represents Na+ or K+ cations [3,4]. Alkali-activated cementitious materials provide comparable mechanical properties to Portland Cement (PC) and have greater resistance to corrosion, acid attack and chemical attack [5–7]. Alkali-activation of industrial waste takes many economic and environmental benefits as the production process of ACM releases less CO2 and requires less energy as compared to PC [8,9]. In earlier studies, ACMs were mainly derived from precursors such as GGBFS and FA and its chemical composition, physical properties and optimum synthesis parameters (i.e. activator type, activator content, curing type and curing temperature) are briefly described in the literature [5,6]. However, the growing demands
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of ACMs and supplementary cementitious materials lead to the consumption of significant amount of GGBFS and FA. According to a rough estimate, the use of GGBFS and FA in cement production was of the order of 43.7 million tons and 39.8 million tons respectively in the year 2015 [10]. As cement consumption increases rapidly, these materials are expected to fulfill approximately 20% requirement of total cement production by 2050 [11]. So, there is a need to identify the new aluminosilicate materials such as Copper Slag (CS), which may be used as a precursor for the development of ACM. Copper Slag is metallurgical waste generated during the extraction of copper from copper ores. During smelting phase reducing agent (Silica) separates the sulphides and oxides, which are present in the Copper ores. The sulphides do not make any anions and develop matte phase (Copper rich) and reducing agent combines with oxide to form slag phase (Impurities rich). The water quenching of slag produces glassy, amorphous and granulated slag. Copper Slag has a specific gravity in the range of 3.4–3.7 and consists of Fe (30–40%), SiO2 (35–40%), Al2O3 (<10%), CaO (<10%) and Cu (0.5–2.1%) [12]. It was estimated that in the smelting process, 2–3 times more CS is generated than copper and approximately 24.6 million tones of CS is generated annually worldwide [12]. Copper slag usually dumped at sites, which creates environmental issues and utilizes the valuable land. As per National Green Tribunal, around 3.52 lakh tonnes of CS has dumped at Thoothukudi (Tamil Nadu, India), which affecting the surrounding agricultural land and water ways [13]. Copper Slag can be used as an abrasive material, manufacturing of floor tiles and glass-ceramic [14–16]. Copper slag has been used in construction industry as aggregates [17,18] and also cementitious materials [19,20]. The presence of alumina and silica in the CS makes it suitable as precursor in the alkali-activation [21–23]. Ahmari et al. [21] developed the ACM using CS and mine-tailing (MT) as source of aluminosilicate. It was found that ACM synthesis using 100% CS and cured at 90 °C for 7 days exhibits the maximum compressive strength up to 50 MPa. Nazer et al. [22] studied the effect of curing time and curing temperature on the performance of alkali activated CS cement matrices. The performance of heat cured specimens at 65 °C for 3 days was similar to ambient cured specimens at 20 °C for 90 days. Qianmin et al. [23] studied the effect of elevated temperature on the mechanical properties of alkaliactivated CS. The optimum elevated temperature for alkaliactivated CS was 200 °C, above this mechanical properties of alkali-activated CS was reduced due to the decompositions of hydration products. From above literature, it can be observed that heat curing of alkali-activated CS gives better results than ambient curing. However, the effect of addition of mineral admixtures on the performance of alkali-activated CS is scanty in literature. Therefore, the aim of the present study is to develop the alkali-activated CS with addition of mineral admixtures. The addition of mineral admixtures in alkali-activated CS could improve its performance and will make it more suitable as cementitious material. The alkaliactivation process of CS was evaluated through compressive strength tests conducted on mortar specimens with different percentages of CS and mineral admixtures. The results of microstructure analysis, porosity and reaction products of different mixes were also evaluated and compared. It is mentioned here that the work reported in this paper is part of a larger investigation on the strength and durability properties of alkali-activated copper slag concrete. First, the optimum synthesis parameters for alkali-activation of copper slag such as alkali content, type of alkali, curing regime and mineral admixtures will be selected. The effects of these synthesis parameters on the mechanical and durability properties of concrete will be evaluated.
After that the final concrete mix will be evaluated in terms of cost, energy suitability and environmental sustainability. 2. Experimental programme 2.1. Materials used Copper Slag obtained from Rajasthan, India was used in the investigation. To enhance the performance of alkali-activated CS, FA and MK were added as additional source of alumina and silica. The chemical composition of CS, FA and MK was determined using X-ray fluorescence and is shown in Table 1. Commercially available MK was used, which is supplied as MetaCem 85C and has calcining temperature of 600–800 °C. It can be seen from Table 1 that there is significant amount of Fe2O3 in CS. The role of Fe2O3 in the geopolymerization process is vague. Other than CS, Fe rich slags have been thoroughly studied as potential precursors, although it remains yet unclear exactly how it participates at the atomic level in geopolymerization. The Standard sand conforming to IS 650 [24] was used to prepare alkali-activated cement mortars. Locally available Sodium Hydroxide (SH) and Sodium Silicate (SS) was used as alkali-activators. The SH was supplied in pellet form with 98% purity, whereas SS was supplied in solution form with Ms = 3.3 (Si2O = 26.5%, Na2O = 8%, H2O = 65.5% by weight). Only CS was ground in Laboratory Ball Mill using 20 mm and 25 mm stainless steel balls for 2 h to attain its fineness equal to FA. As such there in no standard available regarding specification of CS for use as cement. However, the specification of FA for use with cement is given in Indian Standard 3812. According to this standard, FA particles retained on 45 mm IS sieve should be less than 34%. Therefore, to attain equal and more fineness than FA, CS was sieved through 45-mm sieve. The ratio of stainless steel balls (kg) to CS (kg) was 2. The parentage passing through 45-mm sieve for CS, FA and MK was 95%, 82% and 100% respectively. The specific gravity and BET surface area of used materials is given in Table 1. The X-ray Diffraction patterns of CS, FA and MK are shown in Fig. 1. The XRD patterns show that CS is a fully amorphous material with minor peaks of crystalline mineral i.e. Fayalite (F) and Clinoferrosilite (C) and slags with a high degree of amorphous content reflect higher reactivity towards alkali-activation [5]. Strong deviations of crystalline minerals; Quartz (Q) and Mulite (M) were detected on 2h range of 15° to 40° in MK and FA and some peaks of Magnetite (MT) and Sillimanite (SM) were also detected. Except these crystalline peaks, MK and FA were observed to be partially amorphous materials. 2.2. Mix proportions of alkali-activated cement mortars Alkali-activated cement mortars were prepared using CS, MK and FA as a binder with standard sand in the ratio of 1:3. In addition to a control mix which was prepared using 100% CS; two other mixes were also prepared by replacing CS with 30% FA and 30% MK respectively. These mixes were activated using alkali content of 5% and 7% by weight of the binder. Sodium hydroxide and sodium silicate were mixed to prepare the alkali activator solution of modulus Ms = 1.25 (Ms = Si2O/ Na2O = 1.25), which was constant for all the mixes. A water-to-solid ratio of 0.3 was fixed for all the mixes. Mix proportions of different mixes for 1000 g of the binder are given in Table 2. 2.3. Mixing, casting and curing To prepare alkali activator solution, sodium hydroxide flakes were dissolved in water and then sodium silicate was added in this solution and stirred constantly.
Table 1 Chemical composition and physical properties of CS, FA and MK. Compound (%)
Si2O Al2O3 Fe2O3 CaO Na2O K2O TiO2 SO3 MgO MgO P2O5 MnO Loss on ignition (%) Specific gravity BET surface area (m2/kg)
Composition CS
FA
MK
37.6 11.5 42.4 3.80 0.74 0.76 0.37 0.01 0.57 0.62 0.13 0.01 1.50 3.21 503
56.6 31.5 4.72 1.10 0.10 0.35 1.62 0.32 0.36 0.02 – – 3.3 2.34 465
58.1 39.1 1.21 0.23 0.08 0.09 0.83 0.13 – – – – 0.23 2.62 1055
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Fig. 1. XRD patterns of CS, FA and MK.
Table 2 Mix proportions for 1000 g of binder (By weight). Mix
CS (g)
FA (g)
MK (g)
Sand (g)
SH (g)
SS (g)
Water (g)
Alkali content
W/S ratio
1 2 3 4 5 6
1000 700 700 1000 700 700
– 300 – – 300 –
– – 300 – – 300
3000 3000 3000 3000 3000 3000
40 40 40 56 56 56
236 236 236 330 330 330
181 181 181 135 135 135
5% 5% 5% 7% 7% 7%
0.3 0.3 0.3 0.3 0.3 0.3
W/S ratio = Water/Solid ratio, where water is water in activator and extra water; Solid is solid part of activators and binders. The alkali activator solution was prepared 24 h prior to mixing to cool down the temperature of the solution. First, dry materials such as CS, FA, MK and standard sand were weighed in the required quantity and put into the mortar mixture and mixed for 5 min. Subsequently, alkali activator solution was added in the mixer and mixed for another 2 min. After the mixing, the mortar was poured into cube specimens of size 50 50 50 mm and sealed to prevent any moisture loss. Though expected, the authors did not observe any significant loss in workability on account of addition of MK. It can be seen from literature that the performance of heat cured alkali-activated CS specimens was better than ambient cured specimens [21–23]. Keeping this in view, the authors selected heat curing for the alkali-activation of CS. After the casting, the cube specimens in sealed condition was placed into oven and heat cured for 24 h at 80 °C along with moulds. The specimens de-moulded and placed in laboratory conditions at a temperature of 25 ± 3 °C and relative humidity of 50 ± 5% until testing. The specimens were cured in laboratory conditions for 3, 7 and 28 days until testing.
2.6 Microstructure and mineralogy The microstructure of alkali-activated samples was observed using highresolution field emission Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS). Samples for SEM were put in ethanol to stop the reaction and then dried in an oven up to constant weight and for better observations; all samples were gold-coated. Mineralogical study of alkali-activated samples was carried out using X-Ray Diffraction (XRD). The XRD patterns were obtained using a Cu-Ka radiation under conditions of 45 kV and 40 mA. For SEM and XRD tests, samples were collected from broken cubes after conducting compressive strength tests at 28 days of curing of some of the mixes made with an alkali content of 5%. The XRD test was also performed on raw aluminosilicate materials such as CS, FA and MK.
3. Results and discussion 2.4. Compressive strength test
3.1 Compressive strength The compressive strength tests on alkali-activated CS cement mortars were conducted as per IS 516 [25]. The compressive strength was determined using compression testing machine of capacity of 2000 kN at a loading rate of 0.6 kN/s. 2.5. Pore-size distribution and porosity Pore-size distribution and porosity of alkali-activated CS cement mortars were determined using Mercury Intrusion Porosimeter (MIP) at a pressure range of 0.2– 50,000 psia. For MIP, samples were collected from broken cubes after conducting compressive strength tests at 28 days of curing of some of the mixes made with an alkali content of 5%. Prior to testing, the samples were dried in an oven at a temperature of 100 °C until constant weight was achieved.
The compressive strength results of different mixes of alkaliactivated CS cement mortars with different alkali contents and curing ages are shown in Fig. 2. The 28 days compressive strength of alkali-activated CS mortars for 5% and 7% alkali content was 27.56 MPa and 34.12 MPa respectively, which seems to be sufficient for structural applications in various construction works. The amorphous nature of CS helps in the dissolution of silicates, which promotes the alkali-activation process and improves compressive strength [22]. The amorphous materials are structurally
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3.1.1. Effect of curing ages on compressive strength From Fig. 2, it was observed that the compressive strength of all mixes was increased with increase in curing age. The increment in compressive strength with curing age was symmetric in the all mixes; however major portion of compressive strength was gained in just 3 days of curing. In all mixes, up to 80% of 28 days compressive strength was achieved in 3 days of curing. This can be attributed to the heat curing of mixes, which accelerate the alkaliactivation process of binders and develop the compressive strength in short curing periods. It was also observed in the literature that during the heat curing of alkali-activated CS mortars, maximum gain in compressive strength was achieved in just 3 days of curing period [22]. 3.1.2. Effect of alkali content on compressive strength The effect of alkali content on the compressive strength of different mixes is presented in Fig. 2. It was found that the compressive strength of all mixes was increased with an increase in alkali content. With the increase in alkali content, the compressive strength of mix CS was increased from 23.75 MPa to 31.46 MPa at 3 days, 25.63 MPa to 32.41 MPa at 7 days and 27.56 MPa to 34.12 MPa at 28 days of curing as shown in Fig. 2(a). A similar trend was also observed for mix CS + FA shown in Fig. 2(b), wherein compressive strength was increased from 25.27 MPa to 33.55 MPa at 3 days, 26.74 MPa to 36.27 MPa at 7 days and 30.72 MPa to 39.01 MPa at 28 days of curing. It can be seen that with the increase in alkali content from 5% to 7%, the compressive strength of mix CS + MK as shown in Fig. 2(c) was increased by 12.95%, 11.47% and 11.11% at 3, 7 and 28 days of curing respectively. Higher content of alkali provides extra species of Si and Na, where Na dissolves the silicate and aluminate from the binders and extra Si species from activator helps in formation aluminosilicate gel [26,27]. The solidification of aluminosilicate gel results in the formation of the alkali-activated binder, which improves the compressive strength of mixes. 3.1.3. Effect of mineral admixture on compressive strength The effect of different mineral admixtures on the compressive strength of alkali-activated CS mortars is shown in Fig. 2. It was observed that the addition of mineral admixtures improved the compressive strength of alkali-activated CS mortars. However, the addition of MK was more effective in improving the compressive strength than the addition of FA in alkali-activated CS mortars. For 5% and 7% alkali content, CS + MK mix exhibits maximum compressive strength of 40.78 MPa and 45.31 MPa at a curing age of 28 days as presented in Fig. 2(c). The performance of MK additive mixes was better than other mixes due to its chemical composition and fine particles. Metakaolin contains some large amount of silicate and alumina than FA and CS (Table 1), which is released in the presence of alkali media and combines with alkali cations to form cross-link gel, which acts as a binder and strengthen the alkali-activated CS mortars [28]. Also, the fine particles of MK than FA and CS make it more reactive and provide the large surface area than FA and CS for alkali-activation. 3.2. Pore-size distribution and porosity Fig. 2. Effect of alkali content on compressive strength of alkali-activated mixes (a) CS (b) CS + FA and (c) CS + MK.
disordered and these materials can easily breaks down in to its constituents. Due to amorphous nature of CS, when it meets highly alkaline environment, dissolution or break down of different constituents such as alumina and silica takes place. The effect of curing ages, alkali contents and mineral admixtures on compressive strength of alkali-activated CS mortars is briefly described in the following sections.
The porosity and pore-size distribution of selected samples were determined by MIP. Samples were collected from broken cubes of 28 days compressive strength tests of mixes made with CS, CS + FA and CS + MK activated with 5% alkali content. The porosity of different mixes measured is shown in Fig. 3. The porosity of alkali-activated CS mix was highest amongst all the mixes i.e. 15.12%, followed by mix made with CS + FA = 11.97% and mix containing CS + MK = 10.11%. It can be seen that with the addition of FA and MK, the porosity of alkali-activated CS mixes was
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Fig. 3. Porosity of alkali-activated CS, CS + FA and CS + MK mixes, 5% alkali content.
decreased. However, the addition of MK was found to be more effective in reducing the porosity than FA. The reduced porosity of mix containing MK may be attributed to the finer particles of MK compared to FA, which is responsible for better particle packing and results in the dense microstructure. The pore-size distribution of different mixes is shown in Fig. 4, where the pores are separated into mesopores (<50 nm diameter) and macropores (>50 nm diameter). It has been found that MK-based mixture consists of macropores, however the macropores compared to FA and CS mix were negligible. The alkali-activation of MK produces NASH type gel, which fills most of the macropores and gives more refined structure than mixes containing FA and CS. The porosities have a direct relationship with the compressive strength of ACM. Lower porosity gives the refined structure and a more refined structure of ACM can acquire uniform stress distribution, which leads to a higher compressive strength [29–31]. The mix containing MK having least porosity shows better compressive strength than mixes containing FA and CS.
3.3. Mineralogy X-ray diffraction patterns of mixes activated with 5% alkali content are shown in Fig. 5. The XRD patterns of various mixes tested in this investigation indicate crystalline peaks of quartz and mul-
Fig. 4. Pore size distribution of alkali-activated CS, CS + FA and CS + MK mixes indicating Mesopores and Macropores, 5% alkali content.
Fig. 5. XRD patterns of alkali-activated CS, CS + FA and CS + MK mixes, 5% alkali content.
lite. The crystalline peaks of quartz and mulite were initially present in MK and FA (Fig. 1), which were also reflected after alkaliactivation in Fig. 5. The alkali-activated CS mix also shows the crystalline peak of quartz and mullite, which may be due to nonreactive silicate species. It was observed that new hydration products formed during alkali-activation process were not detected in XRD patterns may be due to their amorphous nature. However, the MK mix contains very small peaks of zeolite X, which is aluminosilicate mineral and represents the existence of aluminosilicate gel. 3.4. Microstructure Micrographs of alkali-activated CS, CS + FA and CS + MK mixes with 5% alkali content were detected using SEM and are shown in Fig. 6. It can be observed that alkali-activated CS mix shows maximum micro-cracks as seen in Fig. 6(a). Some micro-cracks were also observed in alkali-activated CS + FA mix as can be seen in Fig. 6(b), but alkali-activated CS + MK mix shows most compact and dense microstructure in Fig. 6(c). In addition to this, some geopolymer gel phase was also detected as a brightened portion in the alkali-activated CS + MK mix. Microstructure analysis reflects that alkali-activated CS mix has most porous structures, but the addition of FA and MK makes its structure dense due to the formation of geopolymer gel and smaller particle size of FA and MK also helps in solidification of the microstructure of alkali-activated CS mix. Due to the densest microstructure of alkali-activated CS + MK mix, it demonstrates the highest compressive strength amongst all mixes. Test results of EDX analysis of alkali-activated mixes are also shown in Fig. 6. The Al/Si ratio is most important factor in alkaliactivated materials for determination the behavior of geopolymer gel. The increase of Al/Si ratio indicates more dissolved aluminum in alkaline medium and which enhances its contribution in formation of geopolymer gel [32]. In Fig. 6(f), alkali-activated mix CS + MK shows maximum Al/Si ratio, which indicates its wellformed geopolymeric matrix. Some atoms of Fe are shown in EDX of alkali-activated mixes of CS and CS + FA in Fig. 6(d) and (e) respectively. The lower compressive strength of alkaliactivated CS and CS + FA mixes is due to presence of Fe in these mixes. The existence of Fe in alkali-activated mixes provides resistance to discharge of Si and Al in an alkaline environment, which
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Fig. 6. SEM micrographs of alkali-activated mixes (a) CS (b) CS + FA (c) CS + MK and EDX analysis results of alkali-activated mixes (d) CS (e) CS + FA (f) CS + MK, 5% alkali content.
opposes the alkali-activation process and thus results in reduced compressive strength [21]. 4. Conclusions Following conclusions can be drawn from the present study: (a) All alkali-activated mixes achieved 80% of 28 days compressive strength within 3 days due to heat curing. (b) The compressive strength of all alkali-activated mixes was increased by more than 11%, when alkali content was increased from 5% to 7%.
(c) Alkali-activated CS mix activated using 7% alkali content exhibits compressive strength of the order of 34.12 MPa, which seems to be sufficient for structural applications in various construction works. (d) The compressive strength of alkali-activated CS was increased by more than 11% and 32% with addition of FA and MK respectively. The microstructure of alkali-activated CS was also improved with addition of FA and MK. (e) In general it can concluded that CS can be used as aluminosilicate in the development of ACM. The addition of FA and MK in alkali-activated CS makes it more suitable as cementitious material in construction industry.
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Acknowledgments The authors acknowledge financial assistance in the form of a fellowship to the first author from the Ministry of Human Resource Development (MHRD), Government of India. The authors also acknowledge the help from the staff of Concrete Technology and Structures Testing Laboratories at Dr B R Ambedkar National Institute of Technology, Jalandhar during the experimentation work.
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Conflict of interest
[18]
The authors declare that there is no conflict of interests regarding the publication of this article.
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