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Article

Durability Performance of Hybrid Binder Concretes Containing Non-Ferrous Slag and Recycled Aggregates

by
Pithchai P. Sivakumar
1,2,3,
Yury A. Villagrán-Zaccardi
1,4,
Thomas Lapauw
5,
Elke Gruyaert
2,
Stijn Matthys
1 and
Nele De Belie
1,*
1
Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Ghent University, Technologiepark Zwijnaarde 60, BE-9052 Ghent, Belgium
2
KU Leuven, Technology Campus Ghent, Department of Civil Engineering, Materials and Constructions, Gebroeders De Smetstraat 1, BE-9000 Ghent, Belgium
3
SIM vzw, Technologiepark Zwijnaarde 48, BE-9052 Ghent, Belgium
4
Sustainable Materials, Flemish Institute for Technological Research (VITO), Boeretang 200, BE-2400 Mol, Belgium
5
Resourcefull BV, Naamsevest 18, BE-3000 Leuven, Belgium
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(8), 6338; https://doi.org/10.3390/su15086338
Submission received: 20 February 2023 / Revised: 3 April 2023 / Accepted: 5 April 2023 / Published: 7 April 2023
(This article belongs to the Special Issue Sustainable Concrete Design)
Browse Figures
Versions Notes

Abstract

:
Novel hybrid binder concrete mixes with alkali-activated non-ferrous slag (NFS), either alone or in combination with blast furnace slag (BFS), as partial replacement of Portland cement, and containing 50% recycled aggregates, were successfully manufactured. The compressive strength, carbonation resistance, chloride resistance, frost scaling, sorptivity coefficient, and water penetration resistance were thoroughly assessed. The presence of recycled aggregates had an adverse effect on early-age strength, but after 91 days there was no difference between concrete with and without recycled aggregates. The chloride-binding capacity was enhanced in the BFS/NFS system with recycled aggregates (reduction in chloride ingress coefficients of ~28–35% compared to recycled concrete with NFS only). This is most likely caused by the binding of Cl ions in calcium alumina silicate hydrates (C-A-S-H) and ettringite phases. However, when compared to the system with virgin aggregates, BFS/NFS concrete with recycled aggregates showed increased carbonation rate (+30%) and frost scaling (+15%). Durability properties, such as sorptivity and water penetration resistance, were positively affected by the curing time for the BFS/NFS system (~35–45% further improvement from 28 to 90 days with respect to the NFS system). Specimens that were wet cured for 91 days showed improved results compared to the 28-day cured samples due to the slow pozzolanic reaction of the NFS.

1. Introduction

The eco-efficiency of concrete is essential for sustainable development in view of the large amounts of concrete used every year by mankind. In this regard, three aspects are fundamental: low carbon emissions, limited use of non-renewable resources, and extended service life.
First, concerning carbon emissions, reducing the clinker factor in concrete is essential due to the high carbon intensity of this constituent. Even though alkali-activated materials have successfully proven to be sustainable alternative binders, the versatility of Portland cement (PC) makes it a hard to fully replace it [1,2,3]. The production of Portland clinker is, however, responsible for about 6 to 8% of the global CO2 emissions, so more environmentally friendly alternative binders are still needed to take action on climate change [4,5,6]. Interesting alternatives are binders based on the alkali activation of industrial by-products, e.g., ferrous slags (FS), such as blast furnace slag (BFS) or fly ash (coal combustion by-product). Obviously, a wide range of studies have already discussed in detail the reactivity and reaction products of alkali-activated binders based on fly ash and BFS [7,8,9,10,11,12], but these industrial by-products are not available in sufficient quantities to cover the needs. Moreover, the transition in the steel industry towards production with electric-arc furnace processes will further reduce the availability of BFS in the future [13]. Alternative sources to be used in similar applications are, therefore, necessary to achieve such a reduction in the demand for BFS. An interesting alternative to investigate is the applicability of non-ferrous slags (NFS) [14,15]. NFS are industrial by-products synthesized during the production of non-ferrous metals, such as copper (Cu), lead (Pb), zinc (Zn) and others [16]. There is a large production of NFS, and usually these slags are stockpiled or used in low-value applications. NFS can find a high-value application as a sustainable binder for concrete. These NFS can be activated with alkalis similar to the BFS [17]. Such pure alkali-activated systems can show a slow reaction rate, which will affect productivity, unless they are treated with high temperatures (>40 °C) during the first hours to promote fast reactions. Hybrid systems are an intermediate solution that can provide higher strengths at early ages. In this case, the NFS is mixed with a small proportion of PC and activator, allowing the production of concrete with a similar performance to FA concrete [18].
Back in 1990, Wu et al. [19] studied the early age activation of PC blended with BFS and Na2SO4. They stated that by using Na2SO4 as an activator, early strength can be improved and the setting time can be reduced. In a recent interesting study [20], the mechanisms of early hydration by Na2SO4 in a slag–cement blend were studied with pore solution chemistry, XRD, and calorimetry. The authors claim that adding Na2SO4 in combination with PC increased the hydration of slag via a complex mechanism, and increased pH resulting from the equilibration of portlandite in the presence of reduced Ca2+ activity. Thus, reduced Ca2+ activity also appeared to promote slag dissolution by increasing the undersaturation of the slag.
Second, the depletion of non-renewable resources is also a growing environmental concern globally. Aggregates are a major ingredient of concrete. Depending on its geographic location, the construction industry mainly uses non-renewable sources, such as marine sand, dredged gravel, or crushed rocks, often granite or limestone, as aggregates. By using recycled aggregates as a primary source for the inert skeleton in concrete, a contribution can be made to the development of a circular economy. Promoting the recycling of waste concrete into high-value applications is key to developing such a circular economy. However, the mortar attached to recycled aggregate particles is a specific feature that limits structural applications [21]. The water absorption and porosity of the recycled aggregates are higher than those of most of the natural aggregates. The durability performance of recycled aggregate concrete can be affected unless its transport properties are controlled by embedding the recycled aggregate in a compact matrix that isolates its pore structure [22].
Third, durability performance is required for eco-efficient concrete to prevent the early need for intervention or replacement of structures. A considerable number of authors have investigated the durability properties of alkali-activated slag concrete [17,23,24,25,26,27,28,29,30,31]. Bondar et al. [17] studied the fresh and later-age mechanical properties of completely alkali-activated BFS slag concrete together with its chloride diffusivity. This article concluded that alkali-activated slag concrete (AASC) can be designed for different degrees of workability and grades of concrete. In addition to the good fresh properties and mechanical properties in the hardened state, the chloride-binding capacity showed excellent results in AASC concrete compared to the PC concrete. Moreover, Al-Otaibi et al. [23] studied the durability properties of BFS concrete activated by water glass. Thus, a comparison was made between a hybrid system (60% PC + BFS + water glass) and traditional PC concrete. Replacing 60% of PC with BFS resulted in increased porosity compared to PC concrete while showing similar workability. In a publication of Tennakoon et al. [24], chloride ingress and steel corrosion in AASC blended with fly ash and BFS were studied. The authors state that blending fly ash and BFS to make AASC provided improved resistance to chloride ingress over time compared to PC concrete. Furthermore, the study also concluded that blending fly ash and BFS gave higher protection with respect to corrosion of rebars (Cl-induced) compared to PC concrete.
A wide range of authors have studied the carbonation of mortar and concrete based on alkali-activated systems [32,33,34,35,36]. Bernal et al. [30] studied the natural carbonation of AASC (made with BFS). The final findings of this research stated that AASC exposed to the natural environment for 7 years showed much lower carbonation penetration than predicted from accelerated carbonation results, proposing that accelerated testing cannot be reliably used to predict natural service conditions in the same manner as for conventional concrete. In another study of Bernal et al. [37], the influence of the pore solution chemistry on the accelerated carbonation performance of alkali-activated binders was investigated. An important outcome of this study was that accelerated testing favors the formation of sodium bicarbonates instead of hydrated sodium carbonates. This mechanism favors a more rapid carbonation progress for AASC (BFS) concrete when exposed to accelerated conditions. In hybrid systems, the main advantage from a chemical point of view is that cement provides additional carbon capture capacity with the production of portlandite during hydration. This can contribute with greater resistance to carbonation in comparison with alkali-activated materials [38]. Another important durability property of concrete is its frost-scaling resistance. Usually, the frost-scaling resistance of AASC in the presence of salt is in most cases reported to be poor compared to PC concrete due to its different pore structure and possible coupling with other deterioration mechanisms, such as carbonation [39]. Carbonation is considered to be a crucial parameter in the frost resistance of AASC. The authors claim that this is due to the fact that carbonation increases the porosity and coarsens the pore system in slag-based binders, whereas the opposite behavior can be seen in PC-based concrete [40,41]. However, only limited studies can be found with respect to the frost scaling resistance of AASC [42,43,44], and no studies on the frost scaling resistance of hybrid binder concrete can be found. Generally, the pore solution of the 100% AASC is dominated by Na ions, and the CO2 concentration strongly influences the carbonate/bicarbonate equilibrium in the pore solution, causing the formation of bicarbonates at higher CO2 concentrations [45,46], further decreasing the pH of the pore solution at a higher CO2 concentration. Accelerated carbonation at CO2 concentration greater than 1% favors the formation of bicarbonates with less efficient clogging of the pore structure [47]. However, this effect is not seen in the PC-containing systems since their carbonation is dominated by CaCO3 formation without the influence of soluble alkali carbonates. It can be stated that hybrid concrete shows a mixed effect of bicarbonates and CaCO3 formation at elevated CO2 concentrations, making the structure more stable under carbonation in comparison with AASC systems. Nevertheless, accelerated carbonation testing may change the nature of the precipitated carbonation products in the hybrid systems.
Water transport of hybrid binder concrete is a topic scarcely addressed in the literature [1]. Water transport indexes are commonly used to estimate several deterioration mechanisms (e.g., chloride penetration, carbonation, and freeze–thaw) on the basis of shared physical correlations with the pore structure. Such a correlation between these transport indexes and the durability performance of concrete is likely to differ from that of conventional concrete, especially due to chemical differences that change the permeability to specific substances, such as chloride and carbon dioxide. Nawaz et al. [18] determined that the activation of Class F fly ash with sodium sulfate and Portland cement (hybrid system) is able to compensate for most of the dilution effect that fly ash produces regarding the capillary absorption rate at the age of 28 days. They obtained similar values to those of control concrete with 100% PC for the sorptivity index for concrete with 40% activated fly ash. When the fly ash content was increased to 60%, a small increase in the sorptivity index was still noted in comparison with the reference concrete, but this was only 50% of the net increase obtained without the activator. The advantages of the hybrid binder were much more noticeable for the chloride penetration rate at the age of 28 days. They obtained higher resistance to chloride penetration via migration than the control mix even with 60% activated fly ash. This added value for the particular case of chloride penetration was mainly attributed to the additional content of aluminate phases provided by the fly ash, which, after reaction, were able to bind additional chloride into the matrix and delay the further penetration. No studies on the sorptivity of slag-based hybrid binders can, at present, be found in the literature. As an intermediate system between cement-based materials and alkali-activated materials, hybrid systems benefit from the contribution of cement hydration at early ages, which also provides hydration products. Among these hydration products, the portlandite content in hybrid systems is key as it has a major impact on chemically controlled transport processes. Regarding physically controlled transport processes, there are two main aspects to address with regard to the correlation between water transport and durability performance: the pore structure of the hybrid matrix and the characteristics of the interfacial transition zone formed against the aggregates.
The interfacial transition zone (ITZ) is a key aspect when considering the mass transport in concrete. This phase is more porous than the matrix or the aggregate phase. Therefore, controlling the development of ITZ can be very effective in reducing transport rates in concrete. For the particular case of hybrid binder concrete, a reduced presence of portlandite can contribute to densifying the ITZ [38]. Conversely, the activation with sodium sulfate (Na2SO4) may contribute to increasing the amount of ettringite with an opposite effect [48]. The amount of ettringite can be reduced with the use of other activators in order to maintain a compact ITZ.
In the present study, one type of NFS was used as a precursor to make hybrid binders. The NFS used was modified ferrosilicate (MFS) slag, a by-product synthesized during the production of Cu metal from metal scraps. Thus, further on in this paper, the abbreviation “NFS” relates to this specific type of (MFS) slag. To the best of our knowledge, previous studies on hybrid binders dealt with the alkali activation of alumino-silicate sources, such as fly ash and BFS slag, and a limited number or no other studies have been conducted on the concrete scale, with iron-silicate sources such as NFS. Recently, Arnout et al. studied the mix design and performance of NFS used in a hybrid cementitious binder at the paste/mortar level [49]. The present study advances into the concrete scale with an expanded sustainability approach by also incorporating recycled aggregates in the concrete. It not only explores the feasibility of manufacturing hybrid binder concrete with NFS as a precursor and recycled aggregate but it also evaluates its durability properties.

2. Materials

2.1. Binder

The hybrid binders referred to in the text consist of Portland cement type CEM I 52.5 R (CBR) complying with EN197-1, patented NFS slag (WO 2016156394), and an alkali-activator PC95 blend of alkali–salt (ResourceFull BV) and BFS (Ecocem Moerdijk).
Wavelength-dispersive X-ray fluorescence (XRF) spectrometry was used to measure the chemical composition of the raw materials, such as CEM I 52.5 R, NFS, and BFS. Moreover, the mineralogy of the raw materials was examined by quantitative X-ray diffraction (XRD) with Rietveld analysis. For this purpose, 10% standard ZnO was added to the raw materials, and later measured on a D2 phaser diffractometer from Bruker in a range from 10° to 70° 2θ, using an acceleration voltage and current of 30 kV and 10 mA, respectively. The particle size distributions were determined using a laser diffraction particle size analyzer.
Table 1 provides the chemical composition of the binders. The NFS mainly contained oxides of Fe and Si, whereas the BFS mainly possessed oxides of Ca and Si. Table 2 also provides the mineralogy of the NFS and BFS, which was mainly dominated by an amorphous phase. In addition, NFS and BFS contain small amounts of spinel crystalline phases. Table 3 presents the particle size distribution, showing that NFS and BFS are finer than PC.

2.2. Sand and Aggregates

A combination of four different grades of aggregate was used: 0/1 mm, 0/8 mm, 4/14 mm, and 4/32 mm. In addition to these natural aggregates, coarse recycled aggregates 4/32 were used in concrete production. This commercial recycled concrete aggregate was composed of >70 wt% of waste concrete and >90 wt% of waste concrete + natural aggregate + other cementitious materials.
The specific density and water absorption of the aggregates were determined as per norm NBN EN 1097-6 (Table 4). The particle size distributions of the aggregates were determined by sieving as per the norm NBN EN 933-1 (Figure 1).

2.3. Superplasticizer

In order to improve the workability of the concrete mixture, a polycarboxylic ether-based superplasticizer (SP), Sika ViscoCrete-1020X from Sika Belgium, was added.

2.4. Concrete Composition

Three concrete mixes (designated Mix 1, Mix 2, and Ref) were prepared with novel hybrid binders. All three mixes were prepared with the same water-to-binder ratio of 0.4. This water-to-binder ratio was chosen as it is a value often specified in the standards for conventional concrete exposed to aggressive environments. The reference mix (Ref) only contained natural aggregates and a binder composition consisting of 50 m% NFS, 20 m% BFS, and 30 m% PC. Mix 1 contained coarse recycled aggregates as partial replacement (50 vol%) of the coarse natural aggregate (>4 mm). This replacement ratio is higher than the volume currently accepted in the legislation for structural concrete (30% [50]), but still a value for which acceptable durability performance can be expected [22]. Mix 2 also contains recycled aggregates but it differs from Mix 1 since only NFS was included as an alternative binder, implying that the binder for Mix 2 was 70 m% NFS and 30 m% PC. Table 5 shows the composition and fresh properties of the synthesized hybrid binder concrete mixes. The difference in density between BFS (2.9 g/cm3) and NFS (3.4 g/cm3) causes minimal differences in aggregate content for Mix 2; thus, for practical purposes, the only significant difference between Mix 2 and Mix 1 is in the binder type, and the only significant difference between Mix 2 and Ref is in the recycled aggregate content. To avoid the excessive influence of the water uptake by the aggregates, and especially the recycled aggregate, these were preconditioned by mixing them with 70% of their 24 h water absorption capacity. The reported amounts in Table 5 for the aggregates include this moisture content (i.e., wet weights are given). The lab mixing procedure was as follows. First, the binder (including all fine powder and solid activator), the fine and coarse aggregates were dry-mixed in a pan mixer for 1 min; then, the water was added and the mixing continued for 2 more minutes; finally, the superplasticizer was added and mixed for 1 extra minute. The slump and air content were determined 15 min after the end of the mixing process in accordance with standards NBN EN 12350-2 (2009) and NBN EN 12350-7 (2009). Once adequate workability was confirmed, concrete was poured in the corresponding molds and compacted using needle vibrators.

3. Methods

3.1. Mercury Intrusion Porosimetry of Paste Samples

Cement pastes were prepared in which the binders were composed according to the proportions of CEM I 52.5 R, NFS, BFS, and activator in Mix 1 and Mix 2, and having the same w/b ratio (0.40). Additionally, a cement paste with pure CEM I 52.5 R was prepared for reference. After 7 days of curing, they were manually crushed to pieces of about 5 mm in size and dried by solvent exchange with isopropanol in accordance with the procedure in [51]. MIP was performed on a Pascal 140/440 (Thermo Fisher Scientific Inc., Milan, Italy) instrument. The hardened paste samples (approximately 2 g) were pre-intruded in the low-pressure device to 200 kPa to fill the interparticle space, and then moved to the high-pressure device, where intrusion was progressively increased to a maximum of 200 MPa. The assumed surface tension of mercury was 0.48 N/m and the contact angle was 142°.

3.2. Compressive Strength

Three cubes with a side length of 150 mm were cast per mixture to investigate the compressive strength as per the standard NBN EN 12390-3. After casting, the samples were stored at a temperature of 20 °C and relative humidity (RH) > 95%. Demolding was carried out after 24 h, whereupon the samples were stored again in the same condition. Compressive strength was measured after 2, 7, 28, 56, and 91 days.

3.3. Accelerated Carbonation at 1% CO2

Three prisms with dimensions of 100 × 100 × 400 mm3 were made per mixture to investigate the carbonation resistance as per the standard NBN EN 13295. After demolding, the samples were stored in a curing room at RH > 95% and a temperature of 20 °C until the age of 28 days. Cured samples were further preconditioned by placing them in a controlled environment at 20 °C and 60% RH for 14 days. At the initial and subsequent carbonation depth measurements, the prisms were split at about 50 mm from one edge. To evaluate the carbonation depth, freshly broken surfaces were sprayed with phenolphthalein. The remaining parts of the specimens were further stored in the controlled carbonation chamber with a concentration of 1% CO2 (temperature of 20 °C and RH 60%), as specified by NBN EN 13295. For each mix, multiple time points (after approximately 2, 3, 4, and 6 weeks for Mixes 1 and 2, and after approximately 2, 4, 8, and 13 weeks for the Ref mix) were monitored to provide deeper insight into the carbonation process.

3.4. Frost–Salt Scaling

Three cubes of 150 mm side length were produced to examine the frost–salt scaling resistance of the concrete types. After curing the samples (temperature of 20 °C and RH > 95%) for 28 days, cylinders were drilled from the concrete cubes and sawn (h 50 mm; ø 100 mm). The cylinders were glued in PVC tubes (h 70 mm; ø 106 mm) in order to expose the molding surface to the de-icing salts and sealed well to prevent water penetration along the sides of the specimens. Sealing efficiency was checked by exposing the specimens to a water layer (with a thickness of ~5 mm) for 3 days. After the observation of no leakage through the bottom and side surfaces, specimens were covered on the bottom and side surfaces with a 20 mm thick Armaflex insulation. Finally, the 5 mL of the Cl solution with a conc of 3% were poured on the top layer of the test samples before placing them in the chamber. The test samples were subjected to 28 days of freeze–thaw cycles as per the standard NBN EN 1339 (2003). After every cycle of 7 days, the scaled material was collected on a filter paper by rinsing and brushing. The filter paper was subsequently dried in an oven at 105 °C and weighed after 24 h. Taking into account the dry weight of the filter paper, the amount of dry scaled material could be calculated as prescribed in NBN EN 1339 (2003).

3.5. Chloride Ingress (Non-Steady Migration Test)

The rapid chloride migration (RCM) test was performed on the molding surface of the cylinders (3 repetitions), drilled out of cubes of 150 mm side length, and sawn afterwards to the right dimensions (h 50 mm; ø 100 mm). The specimens were cured for 28 days at 20 °C and RH > 95% before being tested. Before placing the concrete into the test setup, the test specimens were first put under vacuum for 3 h, the Ca (OH)2 solution was then added, while maintaining the vacuum for another hour and the vacuum was finally released. After vacuum saturation, the test specimens were kept in the solution for another (18 ± 2) hours. Then, the test specimens, surrounded with rubber sleeves, were placed in a container with 12 L of the catholyte solution of 10% NaCl. In the rubber sleeves and on top of the cores, 300 mL of the anolyte solution of 0.3 M NaOH was added. The cathodes and the anodes were connected with the power supply, and the current was adjusted as described in the NT Build 492 standard. After the prescribed test duration, the specimens were disassembled, rinsed, and split axially. A 0.1 M silver nitrate solution was sprayed on the freshly split section to visualize the chloride penetration depth.

3.6. Chloride Diffusion

Chloride diffusion tests were performed on concrete cured for 28 days at 20 °C and RH > 95% according to the norm NBN EN 12390-11. After sawing the cores drilled out of concrete cubes of 150 mm side length to a height of 100 mm, the three specimens per concrete type were vacuum saturated with demineralized water for 24 h. After saturation, all sides except the test surface (the molding surface) were coated with epoxy. The remaining 50 mm core was stored in standard laboratory climate with a temperature of (21 ± 2) °C and a RH of (60 ± 10) %, and later used to determine the initial chloride content. The test specimens were then placed into a Ca (OH)2 solution for approximately 18 h. The following day, the specimens were placed into a 3 wt% NaCl solution. After exposing the cores to the solution for 120 days, ten layers of 2 mm were ground into powder samples. These powders were dried in an oven at 80 °C, and were later used to determine the acid-soluble chloride content as per the standard NBN EN 12390-11. The average chloride content at the exposed surface (Cs) and the effective chloride transport diffusion coefficient (Dnss) were calculated by fitting the error function derived from the solution of Fick’s second law to the measured chloride contents using non-linear regression analysis without considering the outermost layer 0–2 mm, as prescribed by standard NBN EN 12390-11.

3.7. Capillary Water Uptake and Sorptivity Coefficient

Four samples per concrete type and testing age (28 and 90 days) were tested. The procedure was based on NBN B 15–217 and IRAM 1871:2021. Each sample was obtained from the zone between 30 and 80 mm from the base after sawing a cylinder of 100 mm diameter and 200 mm height. These samples were laterally waterproofed with adhesive aluminum tape. The preconditioning, which started immediately after curing the specimens, consisted of saturating the samples under water for 72 h and drying them afterwards in an oven at 40 °C until the variation in weight was below 0.1 wt% in a 24 h period (approximately 10 days). For the capillary water uptake test, the samples were put in contact with water at their bottom face, which was positioned (3 ± 1) mm below the water level. Capillary water uptake was registered by weighing the samples at regular intervals (0, 0.5, 1, 2, 3, 4, 5, 6, 24, and 48 h). The experiment was performed in a closed container to prevent evaporation, in an environment maintained at 20 °C. The weight gain per unit area of the samples was plotted against the fourth root of time, as recommended by Villagrán-Zaccardi et al. [52]. The sorptivity coefficient was computed as the slope of the linear regression of the data between the start of the exposure and the last point before the wet front reached the top face of each sample.

3.8. Water Penetration

The method was based on NBN EN 12390-8. For the water penetration under pressure, three cylinders of 150 mm in diameter and 300 mm in height per mix and testing age (28 and 90 days) were cast and tested. By cutting each of the cylinders in half, two measurements per cylinder could be executed: one with the exposure of the molding surface (bottom surface of the cylinder) to the water and one with the exposure of the interior surface. Before testing, all specimens were laterally sealed with aluminum tape to prevent any leakage in the case of very high permeability. The testing area corresponded with the area of a circle of 100 mm diameter centered at the testing face, so the outermost 25 mm edge was sealed with tape. The preconditioning consisted of drying the samples in the laboratory environment at (21 ± 2) °C and a RH of (60 ± 10) % for 14 days. During the test, the samples were placed in the permeameter with the pressure chamber sealed by pressure against the testing face; then, a first step of water pressure of 1 bar was applied for 24 h, and a second step of water pressure of 5 bar was applied for 72 h. After this second stage, the samples were split, and the profile of water penetration under pressure was measured on both sides of the split sample.

3.9. Statistical Analysis

Statistical analysis was used to evaluate the significance of differences using SPSS Statistics 25. One-way analysis of variance (ANOVA) was carried out to assess the statistical significance of the difference between the means together with the multiple comparison of means by a Tukey test. Finally, the means with probability values (P-values) less than 0.05 were considered significantly different.

4. Results and Discussion

4.1. Mercury Intrusion Porosimetry of Paste Samples

Figure 2 presents the MIP results. After 7 days of hydration, the CEM I paste had the lowest pore volume, followed by Mix 1 and Mix 2. There was a reduction in the critical pore entry size (the pore diameter corresponding to the modal pore frequency, determined by a peak in the derivative curve) with the inclusion of the activated NFS and BFS (from 130 nm to 60 nm), showing a pore refinement due to the action of the slags. However, given the larger overall porosity in Mix 1 and Mix 2, it is anticipated that this pore refinement is slow and insufficient in reducing transport properties in view of the larger pore volume in comparison with a pure Portland cement paste. Transport processes are governed by overall porosity and pore connectivity, which normally decrease with an increasing reaction degree. A possible explanation for the difference between the critical pore entry size between the CEM I and blended pastes is a potential improvement in the particle packing achieved in blends with the inclusion of NFS and BFS. Moreover, as presented in Table 3, NFS and BFS are finer than PC, and they can, therefore, better fill the voids between PC particles, and refine the pore structure, even though they have a low degree of reaction.
A significant reduction in pore volume is observed in Mix 2 compared to Mix 1, demonstrating the significant added value of incorporating a small proportion of BFS in the binder system. The pore entry threshold is the first interconnected pore pathway in the pore network, determined by the largest pore entry size corresponding to the intersection of the two tangent lines in the sector where intrusion accelerates in the derivative curve. This threshold pore entry size is reduced from 160 nm for Mix 2 to 100 nm for Mix 1. The higher porosity of Mix 2 is present not only in the mesopore’s range (<50 nm, IUPAC terminology), but more importantly, in the macrospore’s range (>50 nm, IUPAC terminology). This contrast is indicative of a relatively more incomplete reaction of the binder in Mix 2 than in Mix 1.

4.2. Compressive Strength

Figure 3 shows the strength development up to the age of 91 days for the different concrete mixes (markers show average values, and error bars show standard deviations). This assessment depicts the evolution of mechanical properties over an extended time period, allowing to better infer the potential performance in service of the analyzed concrete mixes.
Generally, the compressive strength measured for Mix 2 is lower compared to the values recorded for Mix 1 and Ref. However, the compressive strength after 2 days for all mixes is comparable, whereas a more pronounced difference can be seen after 7 days. The compressive strength of the Ref increased by about 16 MPa between 2 days and 7 days, whereas Mix 1 and Mix 2 showed increases of 12.5 MPa and 13.8 MPa, respectively. This shows that the presence of recycled aggregates has a slightly negative effect on the early (2 and 7 days) strength development. After 28 days, the presence of BFS and its latent hydraulic behavior can be seen clearly in Figure 3. Mix 2, which contained no BFS, reached a strength of only 33.5 MPa after 28 days, whereas 39 MPa and 40.1 MPa were measured for Mix 1 and Ref, respectively. Finally, after 91 days, Mix 1 and Ref showed similar compressive strength values, suggesting that the use of recycled aggregates did not lead to a lower strength at later ages. However, binder composition played a vital role, as suggested by the fact that Mix 1 (with BFS and recycled aggregates) showed a greater strength increase than Mix 2 (without BFS and with recycled aggregates). Based on the statistical analysis, the differences in the compressive strength between the mixes at the ages of 2, 7, and 56 days were insignificant. However, at 28 days, Mix 2 showed a significantly different strength from the Ref, while this was not the case for Mix 1. At the age of 91 days, a significant difference was observed between Ref and Mix 1 on the one hand, and Mix 2 on the other hand.
The present results contrast with the results in the literature for conventional concrete, indicating that a simultaneous high replacement ratio of PC by BFS and virgin aggregate by recycled aggregate would reduce the compressive strength of concrete [53]. The addition of an alkaline activator to the binder system containing BFS and NFS would be the main difference that could explain the more favorable results in the present study. This would be because the alkaline activator enhances the reactivity of the slags and improves the formation of hydration products. Such positive effect would not only occur at the matrix level, but also on the quality of the interfacial transition zone. The interface between the aggregate and the matrix can be improved in terms of bonding, leading to a better distribution of stress and improved resistance to cracking.

4.3. Accelerated Carbonation Testing

The resistance against carbonation is dominated by physical and chemical aspects. The physical aspect refers to the connectivity of the pore structure, which allows the penetration of CO2 into the concrete. This aspect also involves the moisture content of the concrete, as when concrete has a higher saturation degree, the transport of gases is limited due to their limited solubility in the pore solution [54]. The chemical aspect refers to the CO2 binding capacity of the material, i.e., its alkaline reserve [55], which leads to the precipitation of carbonates. Moreover, a certain amount of water in the pore structure is also necessary to allow the chemical reaction between the carbon dioxide and the alkaline hydroxides to take place. The conditions of the accelerated test secure the optimal situation for carbonation. A very high linear correlation with the square root of time is demonstrated by the three mixes (Figure 4, where markers show average values, and error bars show standard deviations), as an indication of diffusion (physical) control of the process. The higher carbonation rates of Mix 2 (19.7 mm) and Mix 1 (13.9 mm) in relation to the reference mix (11.3 mm) can be mainly attributed to their larger and a more connected pore structure in relation to the recycled aggregate content. This result is consistent with the results presented later for the water transport. Based on statistical analysis, the difference in penetration depth due to carbonation for different mixes was significant.

4.4. Frost–Salt Scaling

Figure 5 presents the frost–salt scaling (expressed as mass loss in kg/m2, markers show average values, and error bars show standard deviations) for concrete mixtures Mix 1 and Mix 2 with respect to its reference. For each mix, the mass loss after each series of seven daily freeze–thaw cycles is presented. After 21 cycles, Mix 1, Mix 2, and Ref showed comparable scaling losses, although Mix 1 had a higher mass loss rate during the first 14 cycles. Mixes 1 and 2 with recycled aggregates showed a faster increase in mass loss, especially between 21 and 35 cycles. In the comparison between the mixes with recycled aggregates, the mix with the addition of BFS showed better frost-scaling properties compared to the mix with only NFS.
The poor frost scaling of the hybrid binders can be hypothetically related to the carbonation resistance. As seen in Figure 4, Mix 1 and Mix 2 already showed a carbonation depth of 2 and 4 mm before their accelerated carbonation testing. Hybrid-binder concrete acts differently than traditional binders when subjected to carbonation. As in the case of traditional binders with SCMs, for hybrid binders, carbonation leads to a coarsening of the pore structure and tends to increase the amount and size of capillary pores [56].
It should be noted that here a molding surface was exposed to the de-icing salts, indicating that a “surface layer” phenomenon should be considered while interpreting these results. After 35 cycles, a large part of the surface layer was removed for all the mixes. Gruyaert [57] states that the surface layer is probably the weakest layer of the concrete, considering that the molding surface has been exposed to carbonation. Statistical analysis indicates a significant difference between the mixes. However, when comparing the scaling results after different cycles, no significant difference could be observed for up to 21 cycles. A significant difference only starts to appear after 28 cycles.

4.5. Chloride Migration

Table 6 shows the results of the accelerated chloride testing by means of the non-steady state chloride migration coefficient (Dnssm markers show average values, and error bars show standard deviations) for all mixtures. The chemistry of BFS and NFS contains higher amounts of Al, Si, and Mg compared with PC. Mg2+ in the presence of Al3+ and CO32- tends to form hydrotalcite-like phases and AFm phases [15]. Due to the presence of this phase, the concrete could have additional binding capacity for Cl--ions, particularly via adsorption and incorporation into these phases. Mix 1 and Mix 2 showed chloride migration coefficients Dnssm of around 8.1 and 10.5 (X 10−12 m2/s), respectively. Mix 1 also possessed BFS (extra Al and less Fe), and had a higher compressive strength compared to Mix 2. The reference mix with virgin aggregates showed a slightly lower chloride migration coefficient compared to Mix 1 with recycled aggregates. However, it is clearly seen that the addition of BFS to the binder is beneficial with regard to chloride migration. Statistical analysis indicated no significant difference between the mixes.

4.6. Chloride Diffusion

Referring to Figure 6, the first trend that can be observed is that Mix 2 has the lowest percentage of chlorides in the top layers (up to 6 mm), and that the chloride content decreases more slowly when moving away from the exposed surface compared to the other mixtures. Hence, at a depth of >10 mm, Mix 2 contains the highest chloride contents. The profile of Mix 2 points out faster chloride diffusion. This mixture contains the highest percentage of NFS (70 wt%) and no BFS. Mix 1 and Ref do contain BFS and have a better chloride-binding capacity than Mix 2; they also contain a higher percentage of alumina, which promotes the formation of the calcium aluminate silicate hydrate (C-A-S-H) and ettringite or AFm phases, leading to an increase in chloride binding [58,59].
Table 7 shows the non-steady-state diffusion coefficient of all three mixes (markers show average values, and error bars show standard deviations). Mix 1 showed a lower diffusion coefficient compared to Mix 2. This can also be deduced from Figure 6, by observing the steepness with which the regression lines decrease. The steeper the decrease, the lower the Dnss value. The relatively steep decline of Mix 1 can be explained with the help of the research conducted by Glass and Buenfeld [60]. This study found that with the increasing chloride-binding capacity, the total chloride content increases at the concrete surface and decreases deeper in the concrete. This is also one of the reasons why the Mix 1 showed a higher surface concentration (Cs) compared to Mix 2 and Reference. An increase in Cs is caused by the binding effect of a greater total chloride content near the concrete surface. It is quite obvious that the presence of BFS showed a positive effect on chloride binding. When comparing Mix 1 with Ref, the influence of the recycled aggregates seems limited. Based on the statistical analysis, the difference in the chloride diffusion coefficient and average chloride content at the exposed surface for Mix 1 and Mix 2 compared to the Ref is insignificant. However, when comparing Mix 1 and Mix 2, a significant difference is observed.
The limited influence of recycled aggregates on chloride diffusion can be explained by counteracting effects. The larger concrete porosity produced with the inclusion of recycled aggregates may be partially compensated by the additional chloride-binding capacity provided by cement hydration products contained in the attached paste in these recycled aggregates [61].

4.7. Capillary Water Uptake and Sorptivity Coefficient

Figure 7 presents the results of the capillary water uptake after 28 and 90 days of wet curing (markers show average values, and error bars show standard deviations). Mix 1 demonstrated a certain reduction in capillary water transport with extended curing time after 28 days, whereas no evolution is observed for Mix 2. The difference can be attributed to the further reaction of the BFS, which at such ages should be addressed as latent hydraulic activity rather than a result of the initial activation. These results are consistent with the MIP results, showing a more limited contribution to the microstructure development of NFS in the presence of the activator in comparison with the BFS.
Statistical analysis indicated significant differences between the mixes. The values for the sorptivity index at 90 days obtained for Mix 1 and Mix 2 (w/b = 0.40) are comparable to values previously reported by [52] for conventional concrete (i.e., with natural aggregates, and Portland cement as the only binder) with w/b = 0.50 and 0.60, respectively. Despite the relatively higher sorptivity coefficient for Mix 1 and Mix 2 in comparison with the expected value for conventional concretes with the same w/b ratio, the analysis also requires considering the binder efficiency. The small amount of BFS incorporated in Mix 1 implies a significant reduction in the water transport by capillarity, allowing to keep the cement content in the concrete low.

4.8. Water Penetration

Figure 8 presents the results of the water penetration under pressure (error bars show standard deviations). The difference between the variability of results is statistically significant, and it does not allow to establish a clear differentiation between the middle and bottom faces. Yet, a trend toward deeper water penetrations for the middle face than for the bottom face is suggested, except for Mix 2 at 28 days. In agreement with previous results, increased transport values were obtained for Mix 2 in comparison with Mix 1. Moreover, a reduction in the average water penetration after prolonging the curing period from 28 to 90 days occurred for both Mix 1 (21% and 30% for bottom and middle faces, respectively) and Mix 2 (37% and 20% for bottom and middle faces, respectively).
Water penetration under pressure depends on both pore volume and connectivity. In contrast to the results from the sorptivity index, a clear evolution is noticed for Mix 2 between 28 and 90 days. The further reduction in the water penetration under pressure for Mix 2 is lower than for Mix 1, but still noticeable. Therefore, the hybrid binder based on NFS and PC is more effective in reducing the permeability of concrete than in reducing sorptivity after 28 days of age. Statistical analysis indicated an insignificant difference between the mixes at different ages.

5. Conclusions

This study compared the durability behavior of three concrete mixes containing hybrid binders with only 30% PC. An alkali activator was combined with two types of slag: 50% NFS with 20% BFS (Mix 1 and Ref) in the binder, and 70% NFS (Mix 2) in the binder. The comparison between Mix 1 and Ref provided insight on the durability effects of recycled aggregates in combination with a hybrid binder, whereas the comparison of Mix 1 and Mix 2 allowed the evaluation of the effect of BFS in a hybrid mix with NFS.
An influence of the presence of recycled aggregate can be seen in the early-age compressive strength of the mixes. However, no negative effects were observed at 91 days. On the other hand, the presence of BFS slag in the binder in addition to NFS seemed to have a positive effect (15% improvement) on increasing the compressive strength after 28 days.
Moreover, the positive effects of the presence of BFS in the hybrid binder system with NFS were observed. Improvements for Mix 1 with respect to Mix 2 are noted for the chloride-binding capacity (28% and 35% reductions in chloride migration and diffusion coefficients, respectively), carbonation resistance (25% reduction in the carbonation coefficient), and the capillary absorption rate (35% reduction at 28 days, and 45% reduction at 90 days). Additionally, despite the fact that water penetration under pressure was reduced from 28 to 90 days for both Mix 1 and 2, the performance was better for the mix containing BFS.
This study successfully explored the feasibility of manufacturing different types of hybrid binder concrete with recycled aggregates and also evaluated their durability properties. However, key aspects of the synergetic effect of alkali activation in the presence of ternary (BFS/NFS/PC) blends must still be explored. Further studies with sophisticated techniques, such as NMR, IR, and synchrotron NEXAFS spectroscopy, should be conducted to study the hydrates of ternary (BFS/NFS/PC) blended systems.
The obtained results support the use of NFS + BFS binders in conjunction with recycled aggregates in concrete to achieve adequate durability performance. However, more research on the performance of this type of concrete is required before it can be used in the construction industry. Future research involving demonstration elements appears to be a natural next step.

Author Contributions

Conceptualization, P.P.S., Y.A.V.-Z., E.G., S.M. and N.D.B.; methodology, P.P.S. and Y.A.V.-Z.; formal analysis, P.P.S. and Y.A.V.-Z.; investigation, P.P.S. and Y.A.V.-Z.; resources, P.P.S., T.L. and S.M.; writing—original draft preparation, P.P.S. and Y.A.V.-Z.; writing—review and editing, T.L., E.G., S.M. and N.D.B.; visualization, P.P.S.; supervision, E.G., S.M. and N.D.B.; project administration, E.G., S.M. and N.D.B.; funding acquisition, T.L., E.G., S.M. and N.D.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the SIM MARES program (DUSC project), grant number HBC.2017.0607; and the Research Foundation-Flanders (FWO-Vlaanderen), grants MSCA-SoE 12ZZD21N LV (Y. Villagrán-Zaccardi) and G062720N (N. De Belie).

Data Availability Statement

Data will be made available upon request.

Acknowledgments

The authors would like to thank the industrial partners Resourcefull, Metallo Belgium, Van Pelt, and Sika for providing the activator, Cu slag, aggregates, and superplasticizer.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Particle size distribution of aggregates (%).
Figure 1. Particle size distribution of aggregates (%).
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Figure 2. MIP of 7 d pastes. Left: experimental results, different mixes are indicated with different colors; continuous lines indicate the derivative curves (vertical axis on the right) and dashed lines indicate the cumulative curves (vertical axis on the left). Right: device used for the experiments.
Figure 2. MIP of 7 d pastes. Left: experimental results, different mixes are indicated with different colors; continuous lines indicate the derivative curves (vertical axis on the right) and dashed lines indicate the cumulative curves (vertical axis on the left). Right: device used for the experiments.
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Figure 3. Compressive strength development over time.
Figure 3. Compressive strength development over time.
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Figure 4. Results of accelerated carbonation testing according to NBN EN 13295 (1% CO2). Left: experimental results; right: examples after ~45–56 days of exposure.
Figure 4. Results of accelerated carbonation testing according to NBN EN 13295 (1% CO2). Left: experimental results; right: examples after ~45–56 days of exposure.
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Figure 5. Cumulative mass loss due to scaling according to CEN/TR 15177 (2006). Left: experimental results; right: example of setup prior to exposure and sample after exposure.
Figure 5. Cumulative mass loss due to scaling according to CEN/TR 15177 (2006). Left: experimental results; right: example of setup prior to exposure and sample after exposure.
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Figure 6. Chloride profile of hybrid binder concrete. Left: experimental results; right: examples of exposure in immersion and profiling of sample.
Figure 6. Chloride profile of hybrid binder concrete. Left: experimental results; right: examples of exposure in immersion and profiling of sample.
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Figure 7. Capillary water uptake. Left: 28 days; right: 90 days.
Figure 7. Capillary water uptake. Left: 28 days; right: 90 days.
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Figure 8. Water penetration under pressure. Left: experimental values; right: examples of samples, 28 d, bottom face.
Figure 8. Water penetration under pressure. Left: experimental values; right: examples of samples, 28 d, bottom face.
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Table
Table 1. Chemical composition of the starting materials in % (determined by XRF spectrometry).
Table 1. Chemical composition of the starting materials in % (determined by XRF spectrometry).
ComponentNFSBFSPC
SiO232.333.020.8
Fe2O340.90.83.5
Al2O311.011.04.7
CaO3.938.263.0
MgO0.07.51.3
Others11.99.56.7
SiO232.333.020.8
Fe2O340.90.83.5
Al2O311.011.04.7
CaO3.938.263.0
MgO0.07.51.3
Table
Table 2. Mineralogical composition of the starting materials in % (determined by XRD + Rietveld).
Table 2. Mineralogical composition of the starting materials in % (determined by XRD + Rietveld).
ComponentNFSBFS
Amorphous92.797.5
Spinel6.72.5
Iron0.60.0
Table
Table 3. Particle size distribution of the starting materials (determined by laser diffraction).
Table 3. Particle size distribution of the starting materials (determined by laser diffraction).
Starting MaterialsParticle Size Distribution (µm)
d10d50 d90
NFS 2.91245.5
BFS2.817.242.5
PC2.318.671.2
Table
Table 4. Density, water absorption, and fineness modulus of aggregates.
Table 4. Density, water absorption, and fineness modulus of aggregates.
Sand 0/1Sand 0/8Gravel 4/14Gravel 4/32Recycled Gravel 4/32
Specific density 2.602.652.672.662.37
Water absorption (%)0.20.30.90.86.6
Fineness modulus1.563.196.727.317.30
Table
Table 5. Concrete composition in kg/m3 and fresh concrete properties.
Table 5. Concrete composition in kg/m3 and fresh concrete properties.
Materialskg/m3
Mix 1Mix 2Ref
Sand 0/1126127126
River sand 0/8707712707
Gravel 4/14142143142
Gravel 4/32364366748
Recycled aggregate 4/324734860
CEM I 52.5 R 121.5121.5121.5
NFS202.5283.5202.5
BFS 81081
Superplasticizer0.50.50.5
Activator 18.718.718.7
Water 162162162
Fresh properties
Slump (mm)120125130
Air content (%)2.32.22.2
Table
Table 6. Results of rapid chloride migration (RCM) testing according to NT BUILD 492.
Table 6. Results of rapid chloride migration (RCM) testing according to NT BUILD 492.
Average Value [Standard Deviation]
Mix 1Mix 2Ref
Migration coefficient, Dnssm (10−12 m2/s)8.1 [1.6]10.3 [1.6]7.3 [1.2]
Table
Table 7. Results of chloride diffusion testing.
Table 7. Results of chloride diffusion testing.
Average Value [Standard Deviation]
Mix 1Mix 2Ref
Chloride diffusion coefficient, Dnss (10−12 m2/s)3.7 [0.5]5.8 [0.8]4.6 [0.5]
Chloride content at exposed surface, Cs (m% binder)2.6 [0.3]1.5 [0.4]2.3 [0.2]
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Sivakumar, P.P.; Villagrán-Zaccardi, Y.A.; Lapauw, T.; Gruyaert, E.; Matthys, S.; De Belie, N. Durability Performance of Hybrid Binder Concretes Containing Non-Ferrous Slag and Recycled Aggregates. Sustainability 2023, 15, 6338. https://doi.org/10.3390/su15086338

AMA Style

Sivakumar PP, Villagrán-Zaccardi YA, Lapauw T, Gruyaert E, Matthys S, De Belie N. Durability Performance of Hybrid Binder Concretes Containing Non-Ferrous Slag and Recycled Aggregates. Sustainability. 2023; 15(8):6338. https://doi.org/10.3390/su15086338

Chicago/Turabian Style

Sivakumar, Pithchai P., Yury A. Villagrán-Zaccardi, Thomas Lapauw, Elke Gruyaert, Stijn Matthys, and Nele De Belie. 2023. "Durability Performance of Hybrid Binder Concretes Containing Non-Ferrous Slag and Recycled Aggregates" Sustainability 15, no. 8: 6338. https://doi.org/10.3390/su15086338

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