Utilization of alkali-activated copper slag as binder in concrete

Abstract

This study was focused on developing concrete using alkali-activated copper slag (AACS) as a binder. The properties of alkali-activated copper slag concrete (AACSC) were compared with portland cement concrete (PCC). Different AACSC mixes were prepared with varying Na₂O dosage (6% and 8% of the binder by weight) and curing methods. Hydration products in AACSC were retrieved using Fourier-transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD) techniques. The test results indicate that the workability of AACSC was lesser than that of PCC. The AACSC mix with 6% Na₂O dosage has exhibited similar mechanical properties as that of PCC. The mechanical properties of AACSC were higher than PCC when 8% of Na₂O dosage was used. Heat curing was effective to upgrade the strength properties of AACSC at an early age of curing, but at a later age mechanical properties of ambient cured and heat-cured AACSC were comparable. The hydration products of AACSC were not identified in XRD patterns, whereas, in FTIR spectra of AACSC some alkali-activated reaction products were reflected. The AACSC mixes were found to be more sustainable than PCC. It has been concluded that AACSC can be produced similarly to that of PCC and ambient curing is sufficient.

Graphical abstract

Keywords

binder / concrete / mechanical properties / mineralogy / workability

1 Introduction

Portland cement (PC) is used as a binding material in concrete, but its production is associated with high CO₂ emissions. The production of one tone of PC releases around 0.6 to 0.8 tons of CO₂ and with 4.1 billion tons of annual production, it contributes to 6%–8% of global CO₂ emissions [1]. These emissions can be controlled with the development of sustainable alternative binding materials. A new class of binders has been introduced called alkali-activated cementitious materials (ACMs), which can be used as a full replacement of PC in concrete [2]. The ACMs have exhibited comparable mechanical and durability properties to PC and CO₂ emissions of ACMs are also lower than PC [1]. The ACMs are derived from reactive solid aluminosilicates, that become hardened when reacting with an alkaline activator [3]. Most industrial wastes are used as solid aluminosilicates in ACMs, but the most preferred aluminosilicate sources are fly ash (FA) and blast furnace Slag (BFS). These materials are also used as alternative binding materials and it is projected that around 40 million tons of each FA and BFS are used in cement [4]. Owing to their large demand for cement, these materials (FA) cannot fulfill the future demands of solid aluminosilicates in ACMs. So, it is essential to determine the non-conventional source of aluminosilicate for ACMs, and it would be better to use industrial waste as a new aluminosilicate source.

The smelting process of copper generates a significant amount of solid waste named copper slag (CS) [5]. With the production of 21 million tons of copper, approximately 40 million tons of CS are generated globally [6]. CS has some applications in the manufacturing of abrasion tools, glass, and ceramics, but most of the CS has been dumped near smelter units, which harm soil and groundwater [5]. Recently, the construction industry has found some applications of CS in the form of sand replacement in concrete, cement manufacturing, road sidewalk, and geotechnical applications [7]. CS has the potential as an aluminosilicate source for synthesis ACM and alkali-activated copper slag (AACS) can be used as a binder in mortar/concrete [5]. The AACS mortar was synthesized using CS as a precursor and acceptable compressive strength of mortar was found even at ambient curing [8–10]. The influence of metakaolin and FA on the performance of AACS mortar was studied [11] and found that metakaolin was better than FA to improve the microstructure and strength of AACS mortar. These studies reflected that the AACS could be used as a binder in mortars, but the potential use of AACS as a binder material in concrete is scanty.

This study has been conducted to develop concrete using AACS as binder material. The mix design procedure is presented and the workability and strength properties of alkali-activated copper slag concrete (AACSC) were compared with that of portland cement concrete (PCC) using the same amount of binder content. The influence of Na₂O dosage and curing method on the performance of AACSC has also been studied.

2 Materials and methods

2.1 Materials

In this study, powdered CS was used as a solid aluminosilicate source. Coarse CS was originally obtained from the industry and was ground in the ball mill to convert to powder form. The high surface area of CS was achieved by grinding. After 1 h of grinding in a planetary ball mill using stainless steel balls (10 mm diameter), up to 50% of CS was passed through the 45-micron sieve. After 2 h of grinding, up to 85% of CS was passed through the 45-micron sieve. The PC conforming to IS 8112 [12] was used and its properties are given in Tables 1 and 2 along with CS. The coarse aggregates of a nominal maximum size of 12.5 mm and fine aggregates of grading zone of II by IS 383 [13] were used in PCC and AACSC. Different properties and grading requirements of aggregates are given in Tables 3 and 4, respectively. For alkali-activation of CS, sodium hydroxide (SH) solution (10 mol/L) and sodium silicate (SS) solution was used and their properties are given in Table 5. The 10 mol/L SH solution was prepared by dissolving the required amount of commercial-grade SH flakes into water.

2.2 Mix proportions

One reference concrete mix of grade M25 was made using PC, based on the guidelines of IS 10262 [14]. The mix proportions for PCC and AACSC mix are given in Table 6. There is no standard available for mix proportions for AACSC. So, in this study, some guidelines of IS 10262 [14] were adopted and the mixed proportions of AACSC were obtained based on several trials. This study aimed to achieve a comparable performance of AACSC to PCC. Therefore, it was decided to use an equal amount of binder content in both AACS and PCC. To achieve 25 N/mm² of characteristic strength of PCC, 400 kg/m³ of PC was used as a binder. In AACSC, the same amount of (400 kg/m³) powdered CS was used as a binder. Based on previous studies [9–11], the alkali-activator solution of Na₂O dosage of 6% or 8% by weight of CS and silicate modulus (Si₂O/Na₂O) of 1.5 was used to prepare two AACSC mixes (AACSC6 and AACSC8).

By the guidelines of IS 10262 [14], the W/B ratio of 0.475 was selected in PCC to achieve the required strength and slump in the range of 50–75 mm. The W/B ratio of 0.30 was fixed in AACSC mixes. The W/B ratio in AACSC was selected after some initial trials in the laboratory. To calculate the water content in AACSC, the solid content of alkali-activators was taken in the binder portion and the water content of alkali-activators was also considered.

The total volume of all aggregates was determined using the absolute volume method by IS 10262 [14], where the volume of binders, alkali-activator, and water was subtracted from the total volume of 1m³ concrete. To calculate the volume of binders and alkali-activators, and the specific gravity of materials is given in Tables 3 and 5. The proportions of the volume of aggregates were calculated by IS 10262 [14] with consideration of different factors such as the maximum size of coarse aggregates, grading zone of fine aggregates, and W/B ratio.

2.3 Mixing, casting, and curing

In PCC, the required amount of cement, aggregates, and water was mixed in the laboratory mixer for 5 min to achieve a homogenous mixture of concrete. In AACSC, the first CS and aggregates were mixed in a laboratory mixer for 5 min. Then, alkali-activator solution and water were put into a laboratory mixer and mixed further for 2 min. Fresh concrete was poured into cube molds of size 100 mm× 100 mm × 100 mm for mechanical testing. In PCC, cube specimens were removed from molds after 24 h of casting and placed in a curing tank for water curing until testing. The temperature of water in the curing tank was maintained at 27°C±2°C. In AACSC, two types of curing regimes were selected; ambient and heat curing. In ambient curing, specimens were de-molded after 24 to 48 h of casting and placed under laboratory conditions (t = 25°C±3°C and RH= 50%±5%) until testing. In heat curing, specimens were put into the oven along with molds for 24 h at 65°C. The specimens were placed in the oven directly after casting. The specimens were then de-molded and placed under laboratory conditions (t = 25°C±3°C and RH= 50%±5%) until testing.

2.4 Testing procedure

The workability of both fresh PCC and AACSC was determined using the slump test by the procedure laid down in IS 1199 [15]. The desired slump range for both PCC and AACSC was in the range 50–75 mm.

The compressive strength (f_c) tests on different PCC and AACSC specimens were conducted by IS 516 [16] at the curing age of 7 and 28 d. The f_c tests were performed on cube specimens of size 100 mm× 100 mm × 100 mm on a compression-testing machine (capacity of 2000 kN) at a loading rate of 14 N/(mm²·min). The average of three specimens was taken to compute the f_c for a particular mix.

The split tensile strength (f_sp) tests on different PCC and AACSC specimens were conducted by IS 5816 [17] at the curing age of 7 and 28 d. The f_sp was determined by diagonally splitting of cube specimens of size 100 mm × 100 mm× 1000 mm on the compression-testing machine (capacity of 2000 kN) at a loading rate of 2.4N/(mm²·min). The f_sp was determined using an expression, tensile stress (σ) = 0.5187load/side².

Reaction products formed in the AACSC were determined using Fourier-transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD) techniques. Samples were scanned in the range from 4000 to 400 cm⁻¹ in the FTIR test and 0 to 60° 2ϕ in the XRD test. The samples were taken from the crushed cubes of f_c tests and placed in ethanol to terminate the reaction.

3 Results and discussion

3.1 Workability

The workability of PCC and AACSC mixes was determined using the slump value, which is shown in Fig. 1. It was found that the slump value for PCC and AACSC mixes was in the range of 60–70 mm, which was required by the calculation of mix proportions. The slump values for AACSC were slightly lower than the PCC due to the presence of SS solution. The sticky characteristic of SS solution may have reduced the workability of AACSC [18]. However, during casting of the specimen lesser vibrations were required for AACSC than PCC. Although the SS solution has sticky characteristics, the concrete produced was uniform due to its binding action. Therefore, with small vibrations relatively more AACSC can be placed into specimens. The slump value of AACSC8 was marginally lower than AACSC6. The AACSC8 mix consists of more Na₂O dosage than the AACSC6 mix, which increases the viscosity of the alkali-activator solution and decreases the workability. It was also found in the literature that the high dosage of the alkali-activator solution decreases the workability [19,20]. The slump values for PCC and AACSC were similar, but the W/B ratio in AACSC was lower than that of PCC. It was due to the glassy surface of CS, which reduced the water absorption capacity of CS and provided free water content in the AACSC mix to make it more workable [21].

3.2 Compressive strength (f_c)

The average of three specimens was taken to compute f_c of each mix. All individual values of f_c of PC and AACS mix are given in Table 7. The test results of average f_c of ambient cured PCC and AACSC mix are shown in Fig. 2. It was seen that the 28 d f_c of AACSC6 mix was comparable to PCC and, the AACSC8 mix exhibited higher f_c than PCC. The same observation was also seen for the 7 d f_c of PCC and AACSC mixes as shown in Fig. 2. It implies that the AACS has good binding properties and could attain f_c comparable to that of PCC with an equal amount of binder content. Alkali-activator solution breakdowns the aluminosilicate, which are present in CS. These aluminosilicates react with Na₂O and silicates of the alkali-activator solution to form sodium aluminosilicate hydrate (NASH) type gel, which provides the binding action in the AACSC [11]. It was also seen that the AACSC mix with more Na₂O dosage achieves higher f_c values. The 28 d f_c of AACSC8 mix was 36.7 MPa compared to 32.6 MPa for the AACSC6 mix. A high Na₂O dosage increases the pH value of the alkali-activator solution, which enlarged the rate of breakdowns of aluminosilicate. It generates a high amount of NASH gel and improves the f_c of AACSC.

The effect of heat curing on the average f_c of AACSC is shown in Fig. 3. It was found that the f_c of heat-cured specimens was more than the f_c of ambient cured specimens and this observation was similar in both AACSC mixes. The 7 and 28 d f_c of the heat-cured mix were increased up to 40% and 7%, respectively. The heat curing accelerates the alkali-activation of CS and results in high f_c at an early age of curing. This is in agreement with literature for heat-cured AACS mortars, where up to 80% of f_c was attained after 3 d of curing [11]. Although the 7 d f_c of heat-cured mixes were high than ambient cured mixes, the f_c for ambient and heat cured mixes were almost similar at 28 d of curing. So, it is suggested to use ambient cured AACSC mix, because it provides almost similar f_c to heat-cured after 28 d of curing. Also, ambient curing is more practical than heat curing for in situ construction.

3.3 Split tensile strength (f_sp)

The f_sp of PCC and AACSC mixes is given in Table 8. Same as the f_c results, the f_sp of the AACSC6 mix was close to PCC, but the AACSC8 mix exhibited higher f_sp than the PCC mix. It was due to high Na₂O dosage, which increases the hydration products in the mix and provides high f_sp properties [22]. The effect of heat curing was also observed on the f_sp and it was improved with heat curing. For AACSC8 mix, 7 and 28 d f_sp was upgraded by 17% and 11% after heat curing, respectively. Similar observations were observed in the f_c test results of AACSC. The f_sp of concrete is directly proportional to its f_c and the relationship between f_c and f_sp given by ACI Committee 318 [23] is reproduced below:

(1)f_{\mathrm{sp}}=\mathrm{0.56}{f}_{\mathrm{c}}^{\mathrm{0.5}}.

A comparison of the experimental and predicted values of f_sp of PCC and AACSC mixes is shown in Fig. 4. It can be observed that the experimental values of f_sp are close to the predicted values and all experimental values are within the error bar of ±5%, which is acceptable. However, this observation is based on limited test data and the results may vary if more data points are available. It suggests that the f_sp of AACSC exhibits a similar relationship with f_c as PCC.

The relationship between f_sp and f_c of AACSC mixes was also established from the available test data in this study. The regression equation for f_sp of AACSC is given below:

(2)f_{\mathrm{sp}}=\mathrm{0.47}{f}_{\mathrm{c}}^{\mathrm{0.56}}.

The power equation was found to be more suitable to predict f_sp of AACSC from f_c and the coefficient of determination for this power equation was 0.99.

3.4 Mineralogical characterization of CS and AACS

The XRD pattern of CS and 28 d of ambient cured AACSC mixes is shown in Fig. 5. Copper slag seemed as fully amorphous materials, however, small crystalline peaks were detected, which indicates Fe content in the CS. It has been revealed in Ref. [5] that the amorphous slags are reactive to alkali-activation. Therefore, the amorphous characteristic of CS makes it a potential source of aluminosilicate for alkali-activation. In AACSC mixes, crystalline peaks were not generated. It shows that the crystalline part of CS (which is small) does not participate in the polymerization and aluminosilicate gels mainly form due to the amorphous part of CS [24]. Therefore, FTIR analysis of the AACSC mixes was conducted to detect the hydration products. It may be noted that the characteristic peaks of SS and SH were not detected in the XRD pattern. During the alkali-activation process, dissolution of alkali-activators, i.e., SS and SH, and aluminosilicate (CS) occurs. The rearrangement and exchange among the dissolved species lead to the generation of new products. This may be the possible reason that SS and SH were not detected in the XRD pattern.

The FTIR spectra of CS and 28 d ambient cured AACSC mixes are shown in Fig. 6. The NASH type gel was found as a key hydration product in a low calcium-based alkali-activated system [2]. In FTIR spectra, the vibrating band at wavenumber 1000 cm⁻¹ was spotted in CS and AACS, which signifies the stretching of T–O–T bonds (t = Al or Si) and gave details regarding the hydration product such as NASH type gel. The T-O-T bond changes to a lower wavenumber, when the amount of tetrahedra Al and Si increases in the system, which signifies the generation of N-A-S-H gel [25–27]. In CS, the T-O-T bond was at 1000 cm⁻¹, which was reduced to 950 cm⁻¹ after alkali-activation. It shows that the hydration products of AACS contain tetrahedral SiO₄ and AlO₄⁻, which establishes the NASH gel. There is a further shift in the band with a rise in Na₂O dosage in AACSC8 mix that implies the development of the high amount of NASH gel in AACSC8 mix and gives more f_c to AACSC8 mix than AACSC6 mix. Thus, FTIR results reflect similar observations as the test results of f_c. In CS and AACS mixes, Fe was present in the crystalline phase. In FTIR spectra, the vibrating band related to Fe was also not found. The participation Fe in the alkali-activation system is not fully established. Except for copper slag, Ferro-silicate slags have been used as starting materials for alkali-activation, but the role of Fe in the binding action of alkali-activation was not cleared [10,28,29]. So, it may be possible that Fe remains unreacted in alkali-activated CS.

It has been observed in this study that AACS shows good potential as a binder material in concrete. The use of Na₂O dosage of 6% and ambient curing is recommended to develop AACSC with f_c comparable to M25 grade of PCC. It is also suggested to examine the durability characteristics of AACSC before it is considered as an alternative to PCC.

3.5 Ecological and economic analysis of AACSC and PCC mix

The ecological analysis of AACSC and PCC mixes was done based on the embodied energy (EE) and Embodied carbon dioxide emission (ECO₂e) of different raw materials used. The economic analysis of AACSC and PCC mixes has been conducted using the cost of raw materials, which is given in Table 9. The EE and ECO₂e values for different raw materials were taken from the literature [30–35] and given in Table 9. The ungrounded CS has been reported to have no EE and ECO₂e [34,35]. However, to grind one tone of CS, approximately 17.6 to 40 kWh (63.36 to 144 MJ) of EE is required [36] and a corresponding range of ECO₂e becomes 8.36 to 19 kg (475 g CO₂/kWh) [37]. Thus, for 1 kg of milled CS, the average values of EE and ECO2e are 0.1037 and 0.0137, respectively.

From Table 9, it was observed that the EE of AACSC6 and AACSC8 mixes were approximately 33.91% and 15.14% lower than that of the PCC mix, respectively. Similarly, the ECO₂e of AACSC6 and AACSC8 mixes was 35.25% and 14.69% lower than that of the PCC mix, respectively. The cost of AACSC6 and AACSC8 mixes is around 33.59% and 24.89% lower than that of PCC mix, respectively. It shows that all AACSC mixes were ecological and economically better than PCC mix, but the use of AACSC6 mix is more sustainable than that of AACSC8 mix.

4 Conclusions

Alkali-activated materials have been widely accepted as alternative binders due to their low carbon footprints and equivalent performance to PC. This study was focused on developing concrete using AACS as a binder. The workability and mechanical properties of AACSC have been compared with PCC using an equal amount of binder content. Different AACSC mixes were prepared with variations in Na₂O dosage and curing methods. Based on the experimental results, conclusions are as follows.

1)‚ The workability of AACSC is slightly lower than PCC and it further reduces with the rise in Na₂O dosage due to the high viscosity of the activator solution.

2)‚ The f_c of AACSC6 mix has been found identical to PCC. However, for the AACSC8 mix, the f_c is more than PCC. The f_sp of AACSC mixes shows similar behavior as f_c of AACSC mixes.

3)‚ Heat curing has been observed to be more effective to improve the mechanical properties of AACSC at 7 d of curing, but at 28 d of curing, the mechanical properties of ambient cured and heat-cured AACSC are almost equal.

4)‚ The hydration products of alkali-activation of AACSC were not reflected in XRD patterns. However, the main alkali-activated reaction product was identified in the FTIR spectra.

5)‚ The AACSC mixes were found to be ecological and economically better than the PCC mix, but the use of AACSC6 mix is found to be more sustainable than that of AACSC8 mix.

6)‚ This study suggests that AACS shows good potential as a binder material in concrete. The use of Na₂O dosage of 6% and ambient curing is recommended to develop AACSC with f_c comparable to M25 grade of PCC. It is also suggested to examine the durability characteristics of AACSC before it is considered as an alternative to PCC.

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