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Article

Performance Enhancement of Alkali-Activated Electric Arc Furnace Slag Mortars through an Accelerated CO2 Curing Process

by
Dany Kassim
,
Ghandy Lamaa
,
Rui Vasco Silva
* and
Jorge de Brito
CERIS—Civil Engineering Research and Innovation for Sustainability, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(3), 1662; https://doi.org/10.3390/app12031662
Submission received: 10 January 2022 / Revised: 26 January 2022 / Accepted: 1 February 2022 / Published: 5 February 2022
(This article belongs to the Special Issue Alkali-Activated Materials: Advances and Novel Applications)
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Abstract

:
The use of electric arc furnace slag (EAFS) as sole precursor to produce alkali-activated mortars has been experimentally investigated. EAFS, a by-product of the steel recycling industry, is a coarse material with unevenly distributed and size-extensive particles. Milling of EAFS was required to achieve a cement-like sized powder before it could be used as precursor. Different combinations of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used, by varying the Na2O/binder concentration (4%, 6%, 8%, 10%, 12%) and SiO2/Na2O ratio (0, 0.5, 1.0, 1.5, 2.0, 2.5) to maximize the mechanical performance. The alkaline solutions were prepared 24 h prior to mixing to unify temperatures for all mixes. The results showed that the SiO2/Na2O ratio and strength development are directly proportional. The maximum 28-day compressive strength obtained, after being subjected to an initial 24 h thermal curing at 80 °C, was 9.1 MPa in mixes with 4% Na2O/binder and 2.5 SiO2/Na2O. However, after an additional 28 days of accelerated carbonation, the maximum compressive strength (i.e., 31 MPa compared to 3.9 MPa in uncarbonated mixes, corresponding to an 800% increase) was obtained in mixes with 12% and 1.0 for Na2O/binder and SiO2/Na2O, respectively, thus showing an alteration in the optimal alkaline activator contents.

1. Introduction

The world’s annual population is continuously increasing by 1.05% on average, with an increase of 81 million people in 2020 [1]. This growth seems to be proportionally linked to the rising demand for housing and other infrastructure that will simultaneously increase the need for construction materials, such as Portland cement. In 2016, the amount of cement produced worldwide reached 4.2 billion tonnes, 57% of which was produced by China alone [2,3]. It is a well-known fact that for each one tonne of cement produced, around 0.5–0.6 tonnes of CO2 (0.59 according to IEA, 2020 [4]) are being released into the atmosphere, meaning that in 2016, the cement industry itself released nearly 2.5 billion tonnes of CO2, which accounted for 8% of the global CO2 emissions of the same year [5,6]. Solving this issue cannot be done by cutting the production of cement alone; other cement-replacing materials need to be found to meet the growing demand for this vital building material. Alkali-activated materials (AAM) are a relatively recent technology as integral cement replacement in the production of concrete and with reduced environmental impact and cost [7]. However, some of the precursors generally used for their production, such as ground granulated blast furnace slag (GGBFS) and fly ash (FA), are facing shortage in availability [8]. To maintain a constant flow in the supply chain, other aluminosilicate waste precursors, prone to little variation in yearly production over time, are needed. One contender is the electric arc furnace slag (EAFS), as the steel industry is shifting from new steelmaking to a recycling system, consequently reducing the amount of GGBFS. It has an aluminosilicate-rich chemistry giving it the possibility to be the next cement-replacing AAM used in concrete. In terms of availability, about 190–290 million tonnes of steel slag are generated each year, 15–20% of which are EAFS [9]. EAFS generation is based on the recycling of scrap metal and pig iron that can be limited at some locations [10]. It is being used mostly as an aggregate substitute for road base course layers and asphalt pavements [11,12], which is a downcycling process from what they can actually offer.
According to several studies [12,13,14,15,16,17,18,19,20,21], EAFS has led to notable improvements in mechanical strength when used as coarse and/or fine aggregates replacement. However, its richness in calcium oxide (free-CaO) and Ferric oxide (FeO) has pushed researchers to further explore the possibility of using EAFS as a cement substitute [22,23]. Xu and Deventer [24] found that any silica–alumina source material that has a pozzolanic component is suitable for dissolution in alkaline solutions. Since EAFS is rich in aluminium and silica, it is desirable to examine whether its activation in alkaline solutions can occur, considering this knowledge [25,26].
The use of EAFS as partial or full cement replacement has led to improvements in mechanical and microstructural performances [25,27]. Muhmood et al. [28] conducted an experimental study to understand the mechanical performance of partially replaced clinker with EAFS. The study concluded that a compressive strength of 50 MPa can be achieved with 30% clinker–EAFS replacement, compared to 58 MPa with 20% replacement and 58.6 MPa for the reference specimen with 100% cement. It has also been found that the substitution of untreated EAFS with treated EAFS can lead to an increase in strength [28]. Another study has recorded a 28-day compressive strength of 44.38 MPa using 25% EAFS as cement replacement compared to 59.34 MPa for the reference specimen with 100% cement [29]. Moreover, Zhao et al. [30] investigated the influence of cement’s partial substitution with EAFS, focusing on the EAFS’ particle distribution. The results revealed that concrete specimens containing properly ground EAFS showed better microstructure, higher compressive strength, lesser porosity and better durability performance compared to the reference blast furnace slag blended cement specimens. Other studies have reported similar results [29,31,32,33]. In addition, raising the fineness of EAFS particles, as well as integrating early thermal curing in the curing stages, led to better hydration performance and a faster early-age hydration [22,31,34,35]. Similar studies found that the compressive strength of alkali-activated concrete cured at 70 °C was higher than that of concrete cured at 30 °C [22,32,36]. Furthermore, Roslan et al. [36] reported later-age improvements in compressive, tensile and flexural strengths with a 20% cement–EAFS replacement.
The use of EAFS as a precursor for AAM production usually shows some shortcomings in performance in comparison with FA or GGBS. Ozturk et al. [25] studied the influence of the optimization of SiO2/Na2O ratio and Na2O/binder percentage on the mechanical performance of mortars with 100% EAFS as a precursor. For a silicate modulus of 1, 1.5 and 2, the highest compressive strengths reported were 16.55 MPa, 17.11 MPa and 22.02 MPa, respectively. In addition, an increase in flexural strength was noticed with the increase in silicate modulus resulting in values of 2.85 MPa, 3.09 MPa and 4.18 MPa for SiO2/Na2O ratios of 1, 1.5 and 2, respectively. Moreover, the highest mechanical strengths were all reached under an 80 °C curing temperature and 12 h curing time. Nikolic et al. [37] investigated the influence of the NaOH and KOH concentration, solid to liquid ratio and temperature on the dissolution kinetics of Si and Al in EAFS. In the study, the increase in alkaline solution concentrations (NaOH or KOH) increased the dissolution of Si and Al of EAFS. Similarly, a decrease in the solid to liquid ratio and an increase in temperature also catalyzed this reaction. Furthermore, Turker et al. [35] studied the influence of thermal curing on the microstructural performance of alkali-activated EAFS mortars activated by a solution with 5% and 1.0 of Na2O/binder and SiO2/Na2O ratios, respectively. A compressive strength of 25.6 MPa was reported for specimens cured at ambient temperature (21 °C), compared to 40.7 MPa for a similar sample cured at 60 °C for 6 h (59% increase). A similar study also demonstrated the benefits of early-age thermal curing on the strength development and shrinkage of alkali-activated EAFS mortars, after reporting an increase in compressive strength from 14 MPa to 21 MPa after a thermal curing stage at 70 °C with durations ranging between 3 h and 24 h, respectively [38]. Cesnovar et al. [39] studied the influence of different slag mixtures containing EAFS and ladle furnace basic slag (LS) with different mixing ratios and activated using potassium silicate with a fixed ratio to slag of 1/2. A compressive strength of 56.7 MPa was reported using an EAFS/LS ratio of 1/1. This optimal mixture was used to understand the influence of thermal curing on strength development. The results indicated that the specimens cured at 70 °C for 3 days attained similar compressive strength (~56 MPa) to that of specimens cured at room temperature for 28 days.
Despite the effort made, little is known on the use of EAFS as full binder replacement, and more experimental studies should take place to further explore this interesting research idea. In addition, the optimization of the alkali activators may pave the way toward more encouraging results, but the mechanical performance values recorded so far are not market competitive. This eagerness toward achieving better performance raised the idea of carbonation as a curing method. Exploiting the possibility of curing the EAFS concrete with CO2 [40,41,42], by taking advantage of its CaO-rich chemistry, will not only lead to better mechanical performance but also an added value to CO2-capturing revolutionary technologies. Monkman and Shao [40] assessed the carbonation behavior of six cementitious materials (cement, FA, GGBFS, EAFS, and hydrated lime) to determine and compare the strength development, %CO2 uptake and carbonation degree for each one of them. The samples were exposed to 100% CO2 at a pressure of 5 bar for 2 h. Afterward, X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDS) analyses were performed to determine the products of carbonation and to observe the morphology of the carbonation products. The results showed a 12% CO2 uptake for EAFS, FA and cement, unlike lime and BFS that presented 40% and 7% CO2 uptake, respectively. Similarly, cement and EAFS were comparable in terms of carbonation degree with a value of around 25%. The compressive strength (at 2 h) of EAFS reached 16.6 MPa, while that of FA was only 3.5 MPa.
In this paper, the use of alkali-activated electric arc furnace slag (AAEAFS) as a sole precursor in the production of mortars has been investigated. Its main objective is to reach the optimization of the activator’s composition based on maximized mechanical performance. Twenty-one different mixes were produced to study the influence of different alkali activator concentrations and ratios, to find the best alkaline solution for the EAFS mortars. Different silicate modulus ratios (0, 0.5, 1.0, 1.5, 2.0, 2.5) and Na2O/binder concentrations (4%, 6%, 8%, 10%, 12%) were selected for this study. Their effect on the performance was evaluated through comprehensive testing methodologies, including slump, shrinkage, flexural strength (FS), compressive strength (CS) and carbonation degree. A 28-day curing period, including a 7-day accelerated carbonation stage, followed. Finally, the correlations between the distinctive preparation conditions and the different tested parameters were graphically illustrated, serving the upcoming research studies toward the best future use of EAFS as a more sustainable cement-replacing material.

2. Materials and Methods

2.1. Electric Arc Furnace Slag (EAFS)

The EAFS used in this study is a by-product of steel recycling, collected from Siderurgia Nacional de Portugal and provided by HARSCO (Portugal). It presents an extensive and coarse particle size distribution that requires preparation and grinding before it can be used as a binder.

2.2. Alkaline Activator

The alkaline activator was prepared in the form of a liquid solution. Reactive grade anhydrous sodium hydroxide pellets (NaOH) from Crimolara (Portugal), with 98% purity and a density of 2.13 g/mL, then dissolved in tap water, complying with Directive 98/83/CE [43]. A commercial solution of sodium metasilicate (Na2SiO3) from Crimolara (Portugal) was then added, containing 8 ± 0.6% of sodium oxide (Na2O), 26.4 ± 1.5% of silicon oxide (SiO2) and 65.6 ± 2% of water (H2O). The Na2SiO3 has a relative density of 1.355 g/mL.

2.3. Fine Aggregate

The mortars were produced using two types of fine siliceous aggregates (i.e., coarse and fine river sands) to maximize compacity through an extensive particle size distribution. The particle size distribution showed maximum nominal sizes (NS) of 1 mm and 4 mm for fine and coarse aggregates, respectively. The 24 h water absorption (WA24), including the values of the apparent (ρa), rodded-dry (ρrd), saturated surface-dry (ρSSD), and bulk (ρb) densities are shown in Table 1.

2.4. Water-Reducing Admixture

The water-reducing admixture (WRA) used in this research was SikaPlast-717, consisting of a synthetic organic water-based naphthalene-based dispersant, with a density of 1.21 ± 0.03 kg/dm3 and a pH of 10 ± 1.

2.5. Mortar Mix Design

The experimental research focused on optimizing the alkaline activator based on the mortar’s mechanical performance. To achieve the optimum activator for EAFS, different mixing combinations were used. The Na2O/precursor concentrations used were 4%, 6%, 8%, 10% and 12%, while the adopted SiO2/Na2O mass ratios were 0, 1.0, 1.5, 2.0 and 2.5 (Table 2). The amount of each constituent was calculated based on the volumetric and mass ratios of the components, in accordance with their densities. The binder/aggregate volumetric ratio (VB/VA) was 0.33, while the mass ratio of water/binder (MW/MB) was fixed at 0.3. The mass WRA/precursor ratio varied from 0% to 1% to achieve constant workability.

2.6. Production Method

The production of mortars was based on EN 196-1 [44]. The alkaline solution was prepared by gradually dissolving the NaOH pellets in water and then leaving them to cool down for 24 h. The 40 × 40 × 160 mm3 three-gang steel moulds were wrapped entirely with thin plastic film to demould the samples without the need for a release agent. The alkaline solution was poured first into the mixer followed by WRA and the precursor, before mixing for 3 min. After that, the mixer was paused to add the fine aggregates, and then mixing was resumed for another 2 min, followed by one additional minute of high-speed mixing. Next, the slump was tested using the slump table according to EN 1015-3 [45]. Afterward, the mix was moulded and covered with plastic film and immediately placed in the thermal curing chamber. Finally, the specimens were demoulded, and each specimen was sealed and left to cure in a dry chamber at a temperature of 23 ± 2 °C and relative humidity (RH) of 65%. The samples were left in the dry chamber until the testing day.

2.7. Curing Conditions and Testing Methods

As stated previously (Section 2.6), the moulded mix was thermally cured at 80 °C for 24 h. After that, the mortar specimens were demoulded and placed in their designated curing condition depending on the desired test method, as shown in Table 3. The loading rate for the flexural and compressive strength tests had a constant value of 30 N/s and 300 N/s, respectively, complying with its corresponding standard (Table 3). The compressive test was applied to both halves resulting from the flexural test. The shrinkage values of the specimens were measured using a mortar shrinkage apparatus following a certain measuring regime in accordance with EN 1015-13 [46].

3. Results

3.1. Characterization of EAFS

This material presents an apparent density of 3770 kg/m3. The oxide chemical composition of the raw material, obtained from X-ray fluorescence (XRF), is shown in Table 4. The studied EAFS contains 28.5% of Fe2O3, 28.2% of CaO, 17.7% of SiO2, and 10.1% of Al2O3. The high amount of iron in EAFS could induce magnetic properties on AAEAFS concrete [49] and it was confirmed to be a strongly magnetic precursor with the use of neodymium magnets.
Figure 1 presents the results of the particle size distribution of the EAFS after the milling process. Ordinary Portland and fly ash (FA) from a coal power plant were tested as reference materials. EAFS showed a bimodal particle distribution curve similar to FA and cement. The highest peak of both the EAFS and cement curves indicated a similar particle diameter of 25 μm. However, the peak corresponding to cement’s curve was sharper than the one of EAFS, showing a higher distribution percentage. EAFS and cement showed another peak at 0.35 μm, indicating the presence of very fine particles. In contrast, FA showed a wider peak at 20 μm and a lower distribution percentage at 0.35 μm compared to EAFS and cement.
In Figure 2a, the EAFS particles are irregular in shape, dispersed, and have angular morphology. In addition, finer particles were observed on the surface of the larger ones (Figure 2b). On a larger scale view, vitrified surfaces are seen resembling those of materials exposed to elevated temperatures (i.e., greater than 1600 °C) suggesting the possibility of pozzolanicity and reactivity to alkali activation (Figure 2c). Moreover, the EDS test was carried out on the area presented in Figure 2d and the results were graphically represented in Figure 3. The corresponding EAFS particle is composed of around 50.6% oxygen, 18.0% calcium, 7.1% silicon, 6.2% of iron, and other components. This primary composition matches the mineralogical composition of the EAFS XRD results (Section 3.1) showing high calcium content seen as white particles (Figure 2d), as well as silicon oxide and iron oxide.
Figure 4 shows the EAFS’ mineralogical composition obtained by XRD. The results showed three fundamental phases: wustite (FeO), gehlenite (Ca2Al2SiO7), and dicalcium silicate (Ca2SiO4) (Figure 4a). Other minor phases appeared as magnesioferrite. After heating the sample to 1000 °C in thermogravimetric analysis (TGA), the fundamental phases were gehlenite and magnesioferrite, as presented in Figure 4b; wustite was no longer present and hematite was formed as a result of the former’s oxidation (mainly between 400 and 620 °C).

3.2. Fresh State Mortar Properties

The mortar’s workability was evaluated by their slump according to the EN 1015-3 standard [45]. The target slump had been initially set at 140 ± 20 mm. The superplasticizer was used and adjusted accordingly between 0% and 1.0% depending on the Na2O/binder and SiO2/Na2O ratios to achieve constant workability. Increasing the Na2O/binder ratio led to a reduction in workability. This was due to the higher viscosity of the alkaline solution after the addition of a solid solute. It was also expected that the mixes would lose workability after a short time for higher SiO2/Na2O ratios due to flash setting; Ca2+ ions from the EAFS quickly react with the silicate ions from the solution, leading to the precipitation of an initial C-S-H, which is responsible for the setting [34]. Therefore, it can be stated that there is a linear correlation between the Na2O/binder and SiO2/Na2O ratios and the workability. However, mixes N8S1, N10S1 and N12S1 presented a slightly lower slump between 109 and 117 mm (Figure 5a) due to the increase in viscosity of the alkaline activator. Therefore, the superplasticizer content was increased to 1.0% for mixes with silicate modulus ratios of 1.5 and higher, to adjust the workability and the slump which ranged between 130 and 147 mm (Figure 5b).

3.3. Hardened State Mortar Properties

3.3.1. Compressive Strength

The average compressive strength of the AAEAFS uncarbonated samples is presented in Figure 6, which shows how the change in parameters affects the mechanical performance of the samples. The values presented in the figure have coefficients of variation ranging between 0.5 and 7.5%. Mixes N4S0.5, N6S0.5, N10S0.5, and N12S0.5 were eliminated due to inconsistent values and unexpected outcomes during the mixing process.
The alkaline solutions of mixes N10S0.5 and N12S0.5 showed precipitation, possibly due to the saturation of silica, during its 24 h of cooling before the mixing day. Thus, the solution must undergo different conditions to avoid any fluctuation in the results. Correspondingly, the alkali activator is the most important factor controlling compressive strength.
Different studies on alkali-activated EAFS used at least one of the alkaline activators stated in this study; Turker et al. [35], Ozturk et al. [25], and Peys et al. [50] used both sodium hydroxide and sodium silicate to achieve compressive strength values of 40.7 MPa, 22.0 MPa, and 16.0 MPa, respectively. Abdollahnejad et al. [51] used only sodium hydroxide and recorded 27.0 MPa. This study presented maximum average compressive strength for uncarbonated specimens of 9.61 MPa for mixes with 4% and 2.5 of Na2O/binder and SiO2/Na2O ratios, respectively. Mixes N10S0 and N12S0, which were specimens with insufficient stability, exhibited values close to zero and thus were not reported. This could be due to the excess amount of sodium hydroxide and the lack of sodium silicate. Increasing the amount of Na2O to a given percentage increases the strength, after which the performance starts to deteriorate. Nevertheless, excess amount of OH-, due to the Ca(OH)2 across the particles of EAFS, reduces the interaction of Ca2+ ions from the surface of EAFS [34]. Therefore, it can be stated that the low strength results from the inadequate amount of C-S-H gels produced by the reaction of Ca2+ with Si4+.
Sodium hydroxide and sodium silicate have a significant influence on the mechanical properties of AAEAFS. Si4+ and Al3+ ions within the EAFS dissolve much more with a high concentration of OH− [52]. Song et al. [53] observed that increasing the concentration of the alkali activator increases the reaction rate as a result of a high alkali medium. Wang et al. [54] stated that sodium hydroxide and sodium silicate are directly proportional to compressive strength only to a given level (from 3–5% by weight of the mix). The authors also stated that, if the amount of sodium concentration increased over a given limit, it would cause efflorescence, as shown in Figure 7. This is due to the migration of Na+ ions to the surface of the specimens, leading to the precipitation of sodium carbonate.
Another parameter affecting the compressive strength for all mixes was the curing time. The specimens in this study experienced 7-day, 28-days, and 91-days of curing in a dry chamber at 23 ± 2 °C and RH of 65%. Since all specimens were sealed, there was no exchange of humidity with the surrounding environment. The maximum compressive strength (9.61 MPa) reported for the uncarbonated specimen N4S2.5, was obtained at 91-days of curing age.

3.3.2. Flexural Strength

Figure 8 presents the results reported for the flexural strength of AAEAFS mortars. The values presented in the figure have coefficients of variation ranging between 0 and 11% except in one case that showed 17%. The maximum flexural strength for uncarbonated specimens reached 2.45 MPa for mix N12S1. The surface of the specimens presented some micro-cracks, possibly caused by the heat curing process and the expedited nature of the reactions at relatively high-temperature levels [55], thus causing a decline in strength. Although a higher SiO2/Na2O ratio is a good indicator of an improved performance [52,56,57,58], the low performance in the case of EAFS may be due to the low amount of amorphous phases present in the precursor, which did not react with the alkaline activator. Furthermore, even though one would expect to have improved performance from the interaction of Ca from EAFS with SiO2 from the activator to produce C-S-H gels, it is possible that the Ca-bearing mineralogical phases were stable at high pH levels, thereby minimizing the dissolution of Ca2+ ions to the solution. In this context, flexural strength will only be enhanced when exposed to accelerated carbonation. This improvement in strength could be caused by the reaction of OH- from the alkaline activator with the Ca2+ ions released from the decalcified phases of EAFS to generate Ca(OH)2 and subsequently CaCO3.

3.3.3. Carbonation

The average compressive and flexural strength for the carbon-cured EAFS mortars are presented in Figure 6 and Figure 8. The specimens in this study, after 28 days of curing in a dry chamber followed by 7 days of exposure to CO2 (i.e., 35 days), were considered fully carbonated, as there was no indication of a pinkish hue from the phenolphthalein solution pH indicator, thereby making the CO2 penetration impossible to read. Alkali-activated materials are known to present a fast decline in pH with ongoing polymerization reactions due to the consumption of the OH− ions present in the pore solution. This phenomenon, in combination with the carbonation of Ca-bearing phases, led to an overall decline of the specimens’ pH.
The average compressive and flexural strengths of the specimens subjected to accelerated carbonation were tested following EN 1015-11 [47]. A noticeable improvement in the mechanical performance of AAEAFS mortars was observed. The additional 28 days of accelerated carbonation following the 28 days in a dry chamber (i.e., 56 days) led to a near 800% and 500% increases in compressive (i.e., from 3.9 MPa to 31 MPa) and flexural (i.e., from 1.6 MPa to 7.85 MPa) strengths, respectively. The maximum mechanical performance was obtained in mixes with 12% and 1.0 for Na2O/binder and SiO2/Na2O ratios, respectively, thus showing a shift in the optimal alkaline activator contents. It is likely that Ca2+ ions were released from the EAFS’ Ca-bearing phases and reacted with CO2 forced into the microstructure. This resulted in the precipitation of CaCO3 polymorphs, which significantly densified the microstructure. The resulting SiO2 gels from decalcified calcium silicate phases also led to a widespread polymerization thus a more effective binding of the microstructure. These processes resulted in enhanced performance [25,53].
The silicate modulus and sodium concentration also had an obvious effect on the mechanical performance. Contrary to that expected, the flexural strength of mixes with lower performance is often not correlated with the compressive strength, as presented in Figure 9.
Carbonated specimens with enhanced performance, in which exponential relationships were observed between the flexural and compressive strength, suggested otherwise. The 28-day relationship of carbonated specimens was close to that obtained by standard 40 × 40 × 160 mm cement mortar prims from other studies [59,60,61,62,63,64,65,66]. The improvement in flexural strength between 7 days and 28 days is likely due to the greater polymerization of SiO2 gels from the greater amount of decalcified calcium silicate phases (e.g., C-S-H). The isotropic growth of these phases, but especially of those growing perpendicularly to the loading vector, led to improved flexural strength.

3.3.4. Shrinkage

In Figure 10, the change in length (i.e., shrinkage) of sealed specimens (with little to no humidity exchange with the surrounding environment) was tested for 91 days. Most specimens presented considerable shrinkage, with one of them close to 3500 μm/m, which is three times what is typically observed in standard cement mortars. All specimens presented at least 45% of their total 91-day shrinkage in the first 28 days, except for mix N8S0.5. Mix N8S0.5 slightly expanded, with minor fluctuations, for the first 28 days and started to shrink later on to settle at 198 μm/m after 91 days. This behaviour also occurred in fly ash mortars studied by Atiş et al. [67]. The authors hypothesized that the expansion of mortars containing FA could be from the MgO and the high content of SO3, which can result in long-term instability due to the formation of expansive calcium sulphate phases [67]. In Figure 10a, the shrinkage of specimens with a SiO2/Na2O ratio of 1.0 decreased with increasing Na2O/binder ratio and somewhat correlated with their mechanical behaviour. It is likely that the greater dissolution of aluminosilicate phases from the higher pH level of the alkaline solution led to the formation of more strength-enhancing and densifying phases, thus leading to effective restrained shrinkage. However, the opposite was observed for mixes with a SiO2/Na2O ratio of 1.5. Further research is required to ascertain the trend of specimens with varying silicate modulus and Na2O content.

4. Conclusions

In this study, electric arc furnace slag (EAFS) was studied as a potential full replacement for cement in the production of alkali-activated mortars. The results obtained in this study allowed a conclusion that EAFS as the sole precursor will result in mixes with relatively low performance. According to the XRD results, this is most likely due to the lower number of amorphous phases compared to other common aluminosilicate pozzolans.
The SiO2/Na2O ratio and compressive strength are generally directly proportional. The compressive strength was also affected by curing time. The continued reaction was observed after the 24 h thermal curing stage. The specimens in this study experienced 7 days, 28 days and 91 days of curing in a dry chamber at 23 ± 2 °C and RH of 65%. The maximum recorded compressive strength was obtained from specimens tested on day 91 corresponding to 4% and 2.5 of Na2O/binder and SiO2/Na2O ratios, respectively.
Despite the shortcomings of EAFS as a sole precursor, the mechanical performance increased significantly after subjecting the specimens to an accelerated carbonation stage. After subjecting the specimens to a CO2-enriched environment for 28 days, AAEAFS showed an average compressive strength increase of ~500%, with one case reaching 800% (i.e., from an initial 3.9 MPa in uncarbonated mixes to 31 MPa for carbonated ones). The maximum performance was observed in mixes with 12% and 1.0 for Na2O/binder and SiO2/Na2O ratios, respectively, thus showing a shift in the optimal alkaline activator contents.
The sealed shrinkage test showed considerable dimensional variability over time. Great autogenous shrinkage may have occurred due to the continuous alkali activation reaction. Nevertheless, this phenomenon is still widely unknown and must be further researched.
The complete replacement of cement with alkali-activated aluminosilicate waste may translate into significant reductions in cost and minimal environmental impacts, especially with incorporating a forced carbonation curing stage using industrial CO2-rich flue gases. Therefore, greater focus should be given to this curing technique in future research.

Author Contributions

Conceptualization, R.V.S.; methodology, R.V.S.; formal analysis, D.K.; investigation, D.K.; resources, R.V.S.; writing—original draft preparation, D.K. and G.L.; writing—review and editing, R.V.S. and J.d.B.; supervision, R.V.S. and J.d.B.; project administration, R.V.S.; funding acquisition, R.V.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by FCT—Foundation for Science and Technology, through research projects PTDC/ECI-CON/29196/2017 (RInoPolyCrete) and EXPL/ECI-EGC/0288/2021 (ECO2Alkrete).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The authors acknowledge the support of the CERIS Research Institute, IST, University of Lisbon. The authors would also like to acknowledge the support of HARSCO and SIKA for part of the materials provided for this experimental research.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 01662 g001 550
Figure 1. Particle size analysis.
Figure 1. Particle size analysis.
Applsci 12 01662 g001
Applsci 12 01662 g002a 550Applsci 12 01662 g002b 550
Figure 2. SEM micrographs of EAFS sample (magnification of (a) 100×; (b) 500×; (c) 5000×; (d) 1000×).
Figure 2. SEM micrographs of EAFS sample (magnification of (a) 100×; (b) 500×; (c) 5000×; (d) 1000×).
Applsci 12 01662 g002aApplsci 12 01662 g002b
Applsci 12 01662 g003 550
Figure 3. EDS coupled to SEM of EAFS sample.
Figure 3. EDS coupled to SEM of EAFS sample.
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Applsci 12 01662 g004 550
Figure 4. XRD pattern; (a) EAFS; (b) EAFS—TGA.
Figure 4. XRD pattern; (a) EAFS; (b) EAFS—TGA.
Applsci 12 01662 g004
Applsci 12 01662 g005 550
Figure 5. Slump values for mortar mixes (WRA content of (a) 0.5% and; (b) 1.0%).
Figure 5. Slump values for mortar mixes (WRA content of (a) 0.5% and; (b) 1.0%).
Applsci 12 01662 g005
Applsci 12 01662 g006 550
Figure 6. Average compressive strength: (a) 4% sodium concentration; (b) 6% sodium concentration; (c) 8% sodium concentration; (d) 10% and 12% sodium concentration (Mix code-C represents carbonated specimens).
Figure 6. Average compressive strength: (a) 4% sodium concentration; (b) 6% sodium concentration; (c) 8% sodium concentration; (d) 10% and 12% sodium concentration (Mix code-C represents carbonated specimens).
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Applsci 12 01662 g007 550
Figure 7. Efflorescence caused by Na2CO3 on the surface of a mortar specimen.
Figure 7. Efflorescence caused by Na2CO3 on the surface of a mortar specimen.
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Applsci 12 01662 g008 550
Figure 8. Average flexural strength: (a) 4% sodium concentration; (b) 6% sodium concentration; (c) 8% sodium concentration; (d) 10% and 12% sodium concentration (Mix code-C represents carbonated specimens).
Figure 8. Average flexural strength: (a) 4% sodium concentration; (b) 6% sodium concentration; (c) 8% sodium concentration; (d) 10% and 12% sodium concentration (Mix code-C represents carbonated specimens).
Applsci 12 01662 g008
Applsci 12 01662 g009 550
Figure 9. Flexural vs. compressive strengths of mortars from this study compared with other studies from the literature [59,60,61,62,63,64,65,66].
Figure 9. Flexural vs. compressive strengths of mortars from this study compared with other studies from the literature [59,60,61,62,63,64,65,66].
Applsci 12 01662 g009
Applsci 12 01662 g010 550
Figure 10. Shrinkage for AAEAFS mortars: (a) values for mixes with silicate modulus of 1.0; (b) values for mixes with silicate modulus of 0.5 and 1.5.
Figure 10. Shrinkage for AAEAFS mortars: (a) values for mixes with silicate modulus of 1.0; (b) values for mixes with silicate modulus of 0.5 and 1.5.
Applsci 12 01662 g010
Table
Table 1. Characterization of the aggregates used in the mixes.
Table 1. Characterization of the aggregates used in the mixes.
AggregatesNSρaρrdρssdρbWA24
mmkg/m3kg/m3kg/m3kg/m3%
Fine sand0/126522624263715440.4
Coarse sand0/426362601261715560.5
Table
Table 2. Mix code of EAFS mixes.
Table 2. Mix code of EAFS mixes.
Na2O (%)SiO2/Na2O
00.511.522.5
4N4S0N4S0.5N4S1N4S1.5N4S2N4S2.5
6N6S0N6S0.5N6S1N6S1.5N6S2-
8N8S0N8S0.5N8S1N8S1.5--
10N10S0N10S0.5N10S1---
12N12S0N12S0.5N12S1---
Table
Table 3. Curing conditions and testing methods assigned for the specimens.
Table 3. Curing conditions and testing methods assigned for the specimens.
TestStandardNo. of SpecimensCuring Conditions
Flexural strengthEN 1015-11 [47]6Sealed specimens in a dry chamber until testing age.
Compressive strength
Accelerated carbonationEN 13295 [48]414 days sealed + 14 days unsealed in the dry chamber; then placed in the carbonation chamber until testing age.
ShrinkageEN 1015-13 [46]2Sealed specimens in a dry chamber after demoulding until the end of the test.
Table
Table 4. Chemical composition of EAFS obtained from XRF (%).
Table 4. Chemical composition of EAFS obtained from XRF (%).
Fe2O3CaOSiO2Al2O3MgOMnO2Cr2O3TiO2P2O5SO3Na2OBaOK2OV2O5CuOZnO
28.4828.1817.6610.135.665.442.380.650.420.330.190.170.030.110.020.02
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Kassim, D.; Lamaa, G.; Silva, R.V.; de Brito, J. Performance Enhancement of Alkali-Activated Electric Arc Furnace Slag Mortars through an Accelerated CO2 Curing Process. Appl. Sci. 2022, 12, 1662. https://doi.org/10.3390/app12031662

AMA Style

Kassim D, Lamaa G, Silva RV, de Brito J. Performance Enhancement of Alkali-Activated Electric Arc Furnace Slag Mortars through an Accelerated CO2 Curing Process. Applied Sciences. 2022; 12(3):1662. https://doi.org/10.3390/app12031662

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Kassim, Dany, Ghandy Lamaa, Rui Vasco Silva, and Jorge de Brito. 2022. "Performance Enhancement of Alkali-Activated Electric Arc Furnace Slag Mortars through an Accelerated CO2 Curing Process" Applied Sciences 12, no. 3: 1662. https://doi.org/10.3390/app12031662

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