Feasibility of Eco-Friendly Binary and Ternary Blended Binders Made of Fly-Ash and Oil-Refinery Spent Catalyst in Ready-Mixed Concrete Production

Large-scale recycling of new industrial wastes or by-products in concrete has become a crucial issue for construction materials sustainability, with impact in the three pillars (environmental, social and economic), while still maintaining satisfactory, or improved, concrete performance. The main goal of the paper is to evaluate the technological feasibility of the partial, or total, replacement of fly-ashes (FA), widely used in ready-mixed concrete production, with spent equilibrium catalyst (ECat) from the oil-refinery industry. Three different concrete mixtures with binary binder blends of FA (33.3% by mass, used as reference) and of ECat (16.7% and 33.3%), as well as a concrete mixture with a ternary binder blend with FA and ECat (16.7%, of each) were tested regarding their mechanical properties and durability. Generically, in comparison with commercial concrete (i) 16.7% ECat binary blended concrete revealed improved mechanical strength and durability; (ii): ternary FA-ECat blended binder concrete presented similar properties; and (iii) 33% ECat binary blended concrete has a lower performance. The engineering performance of all ECat concretes meet both the international standards and the reference durability indicators available in the scientific literature. Thus, ECat can be a constant supply for ready-mixed eco-concretes production, promoting synergetic waste recycling across industries.


Introduction
Concrete is the most widely consumed construction material, exceeding 10 billion tons/year [1] worldwide, of which 165 million tons/year is in Europe [2]. In 2012, the cement and concrete industry directly generated 20 billion Euros in value and 384,000 jobs in the European Union [3]. Since this industry has a 2.8 multiplier factor in the overall economy [3], it has a major socio-economic impact.
In addition, the vast amount of natural resources and energy required by the concrete industry, [4] as well as its remarkable CO 2 emissions [5][6][7], make this sector indispensable towards a sustainable low carbon economy [8]. Most notably, concrete's ability to incorporate industrial wastes [9,10] provides a key opportunity to implement Circular Economy [8] whilst helping the European Union to achieve its targets set for waste recycling and resource efficiency [11], and also to tackle its Societal Challenge 'Climate action, environment, resource efficiency and raw materials' [12].
It is therefore natural that several industrial and agricultural wastes incorporated in cement-based materials have been under intense scrutiny by the scientific community [1,[13][14][15]. Often, these investigations revealed that eco-concretes present improved performance including durability [15][16][17]. However, Table 1. Chemical composition, physical properties and pozzolanicity of binding materials as well chemical properties of fly ash (FA) for use in concrete defined by EN 450-1 standard. binder mortar prepared to determine pozzolans' AI. This argument is reinforced by the fact that, in a previous study [34], in which pozzolanicity of ECat, at 28 days, was evaluated by preparing the blended cement mortar with a lower cement replacement level with ECat of 20% (by mass) (as prescribed by the American standard ASTM C311), the AI value obtained was 95%. Thus, it seems that the mortar with higher cement content that forms more cement hydration products, such as Ca(OH)2, increases the extension of the ECat's pozzolanic reaction.   Four different aggregates (according to European standard EN 12620) were used: two different natural siliceous sands, one coarser designated SandC, (density 2.63, fineness modulus of 5.23, water absorption of 0.77%) and one finer designated SandF (density 2.63, fineness modulus of 2.84, water absorption of 0.48%) as well as two crushed limestone gravels, one coarser designated GravC (density 2.69, fineness modulus of 8.51, shape index of 19.30, water absorption of 1.1%) and one finer designated GravF (density 2.72, fineness modulus of 5.21, shape index of 19.50, water absorption of 1.0%). Figure 1b shows the cumulative particle size distributions of aggregates determined following the EN 933-2 standard.

CEM II/A-L Fly Ash
Two concrete admixtures were used: the third-generation high-range water reducer superplasticizer (Sp) BASF MasterEase 3530 and the plasticizer (P) MasterPozzolith 7002, both complying with European standard EN 934-2. The Sp consists of modified polycarboxylates and P consists of lignosulfonate and both are supplied in liquid form with a density of 1.07 and 1.11, respectively. Tap water was used, in accordance with European standard EN 1008.
The materials used in this study, except for the ECat, are the same of those used in the industrial ready-mixed concrete production at Betão Liz.

Concrete Mixture Design and Samples Preparation
Four concrete mixtures with binary and ternary blended binders comprising CEM II, FA and ECat, were investigated. The reference concrete has a binary blended binder containing 66.7% (by mass) of CEM II and 33.3% (by mass) of FA. This reference concrete mixture reproduces the formulation of a concrete, widely used in ready-mixed concrete plants of Betão Liz company, that complies with C25/30 XC2(P) Cl 0.2 Dmax22 S4 class of concrete in accordance with the EN 206-1 standard. Since the investigation intended to be industrial application-oriented, the innovative concretes mixtures were designed to assess if the same clinker factor reduction can be achieved. As such, the other three concretes tested incorporate ECat in their binders as a partial, or total, surrogate of the FA present in the reference concrete.
The acronyms adopted in this study for the concrete mixtures identification refer the percentage (by mass) of each addition present in the binder as follows: 33FA is the reference industrial concrete mixture, 16FA16ECat is the concrete which binder's phase contains 16.7% of FA and 16.7% of ECat and, 16ECat and 33ECat are, respectively, the concretes which binder's phases contain 16.7% and 33.3% of ECat. Table 2 presents the mixture proportions of concretes. w added /B-water-to-binder mass ratio; w eff /B-effective water-to-binder mass ratio.
The simultaneous use of cement type CEM II and a high content of additions in the concrete composition is aligned with the action plan developed by the ready-mixed concrete sector strategy to improve resource efficiency and reduce concrete embodied carbon, and thus, increasing its sustainability [46].
The small adjustments in the aggregates composition of the concretes with ECat incorporation (Table 2) were established based on preliminary mixture preparation, to ensure similar visual appearance of concretes in the fresh state and, thus, to preserve end-user acceptance. The water content was also experimentally calibrated to keep a constant slump value of 200 ± 10 mm in the range (160-210 mm) of the S4 concrete consistency class. As anticipated based on previous results [34,36,40] the water needed to achieve similar slump values tend to increase with the increase of ECat content due to its very high specific surface (150070 m 2 /kg, Table 1) with water affinity that promotes a significant water absorption of 29.7% (by mass). In fact, the water content increases 27.0% from 148 kg/m 3 in the 33FA reference concrete to 188 kg/m 3 in the 33ECat concrete which corresponds, respectively, to w added /B ratios of 0.49 and 0.63. However, the determination of the effective water-to-binder mass ratio, w eff /B, (Table 2), using Equation (1), reveals that the effective water available for binder hydration was kept mostly the same among the concretes, varying in a much narrower range within 0.49 and 0.53. Thus, the excess water added in concretes with ECat incorporation is absorbed by its particles, not remaining as free water.
where w added is the added water, and ECat and B are the ECat and binder content in concrete expressed in kg/m 3 , respectively. Concrete mixtures were prepared using a vertical axis mixer following the procedure presented in Table 3. Just after mixing, the following properties of fresh concretes were evaluated (following the standard test methods mentioned between parentheses): consistency by slump test (EN 12350-2), and bulk density (EN 12350-6). Table 3. Concrete mixing procedure.

Task Duration
wipe the inside of the mixing bowl with a damp cloth introduce the dry aggregates (in descending order of particle size) with 5% of water and mix 1.0 min resting 4.0 min add cement + additions + 70% of water and mix 1.0 min add remaining water + superplasticizer + plasticizer and mix 2.0 min resting (and scrape material adhering to the mixing bowl) 2.0 min final mix 2.0 min For each concrete mixture, various samples were prepared for posterior testing in the hardened state. As such, molds were filled and compacted following the EN 12390-2 standard. Depending on the property to be evaluated, different molds' shape and size were adopted (Section 2.3) namely, cubic molds with 150 mm side for compressive strength and ultrasonic testing, and cylindrical molds (∅100 mm × 200 mm) for chloride migration, electrical resistivity and water capillary absorption testing. The samples were unmolded after 24 h and stored in a chamber at 20 ± 2 • C and humidity greater than 95% until testing date. Three samples were tested for each test property and curing age.

Experimental Design
The use of complex concrete mixtures, namely incorporating recycled products, endorse a performance-based mixture design approach concerning the mechanical properties and the durability.
Mechanical properties evaluated were the compressive strength and the dynamic modulus of elasticity (E d ). This property was determined from ultrasonic pulse velocity (UPV) measurements because this method has proved to be useful, reliable and non-destructive both for computing E d and to estimate concrete quality.
Concrete durability is largely related to the ingress of deleterious external agents that could lead to deterioration of concrete performance over time. As such, the durability depends on the microstructure of the material (pore size distribution, connectivity and tortuosity of pore system as well as chemistry) that determines the mechanisms of substances penetration in concrete. Typically, a single parameter is not sufficient to characterize the 'potential' durability of concrete, namely regarding corrosion, due to the physicochemical complexity of processes that take place as well as due to the different driving forces involved in the transport of substances such as, concentration gradient and total pressure gradient.
The key transport properties assessed were: (i) capillary water absorption that governs the liquid moisture movement by surface tension effects. This is one of the most important features of a building material because is considered the fasted transport mechanism and may occur in a dry or semi-dry state; (ii) chloride ions migration since chloride ions can trigger reinforcement corrosion which is a major issue for the durability of concrete structures; and (iii) electrical resistivity that indicates the ability to transport the electrical charges through the material. This property besides depending on pore structure is also influenced by the composition of the pore solution. Since corrosion is an electrochemical process, resistivity also correlates with reinforcement corrosion potential.
The 'potential' durability of a given concrete mixture is specified from its classification based on transport properties values experimentally obtained. The durability classes were defined based on threshold values of the transport properties referred to as durability indicators (see Sections 3.2.4 and 3.2.5).

Test Methods
The compressive strength test was performed at 7, 28 and 91 days following the procedure described in European standard EN 12390-3.
where, L is the ultrasonic pulse path length through the sample i.e., the distance between the two transducers and t is the pulse transit time provided by the test equipment. The dynamic modulus of elasticity (E d ) of concretes was determined from the UPV values using the Equation (3) [47].
where E d . is the dynamic modulus of elasticity (MPa); UPV is the ultrasonic pulse velocity (km/s) obtained by Equation (2); ρ is the density of hardened concrete (kg/m 3 ) computed by dividing the sample mass (experimentally assessed before the UPV test) to its volume (0.15 3 m 3 ); and υ is the dynamic Poisson's ratio, assumed 0.2 in this study since it is a typical value for normal concretes [48]. The resistance to capillary water absorption of concretes, at 28 and 91 days, was evaluated following the testing procedure described in the EN 13,057 standard. In brief, the procedure consisted in preparing, for each concrete mixture, three replicates samples (∅100 × 100 mm) sliced from the cylindrical samples molded. After that, the measurements of water capillary absorption were carried out immersing 2 mm of the cut face of each dried sample into water. The mass of each sample was monitored after 12 min, 30 min, 1 h, 2 h, 4 h and 24 h from its first contact with water. The water uptake per unit area was calculated, for each time increment, from the mass of water absorbed (kg) divided by the cross-sectional area of the test face exposed to water (m 2 ). The capillary water sorption coefficient, S (kg/m 2 ·h 0.5 ), is obtained empirically from the slope of the plot of the water uptake per unit area (kg/m 2 ) against the square root of time of immersion (h 0.5 ).
The chloride migration coefficient of concretes at 28 and 91 days was evaluated from non-steady state migration experiments following the testing procedure described in NT BUILD 492 standard. In brief, the procedure consisted in preparing, for each concrete mixture, three replicates samples (∅100 × 50 mm) sliced from the cylindrical samples molded. The samples were vacuum soaked with a Ca(OH) 2 saturated solution and, thereafter, an electrical potential is applied to force chloride ions to migrate (from a 10% NaCl catholyte solution) through the samples. Each sample was subsequently axially split into two pieces, and the freshly broken surfaces were sprayed with 0.1 M silver nitrate solution. The chloride migration depth was established as the depth of visible white silver chloride precipitated. The non-steady state chloride migration coefficients, D nssm , were determined using Equation (4).
where U is the absolute value of the applied voltage (V); T is the average value of the initial and final temperatures in the 0.3 M NaOH anolyte solution (K); L is the measured values of specimen thickness (mm); x d is the average value of the penetration depths (mm); and t is the test duration (h). The electrical resistivity (ρ) of concretes was evaluated at 7, 9, 11, 14, 21, 28, 56 and 91 days using the two electrodes technique. Electrodes were composed of wet sponges in contact with stainless steel plates (∅100 mm and 2 mm thick) placed on the saturated flat faces of the samples prepared for the chloride migration experiments. The electrical current intensity was monitored by applying a force (to ensure a constant and uniform stress distribution over the entire sample face) and 60 V voltage. The electrical resistivity (ρ) was then computed using Ohm's Law (Equation (5)).
where V is the applied voltage (Volts); I, current intensity (Ampere); and, A and L are, respectively, the cross-sectional area (m 2 ) and length (m) of the test sample through which current passes.  Table 4 shows the properties of concretes in the fresh state. As previously discussed (Section 2.2), the amount of water added to the concrete mixtures was adjusted to obtain a target slump value of 200 ± 10 mm. The difference between the highest value of density exhibited by the concrete 33FA and the lowest presented by 33ECat is only 2.9% and, thus, negligible. Even though, results show that incorporation of ECat in the binders tends to decrease the concretes densities in the fresh state.  Figure 2 also reveals that the total additions content in the binder does not determine the compressive strength development over time. In fact, although the three concretes 33FA, 16FA16ECat and 33ECat have 33% of additions incorporation, the 33ECat concrete presented lower compressive strength than those of concretes with the same content of FA and the same total content of additions of half of each, FA and ECat. These might be attributed to the differences in the additions properties. Namely, FA particles are finer than those of ECat and present a polydisperse size distribution, while ECat particles are nearly monodispersive (Figure 1a). Therefore, the filling ability of the FA particles Figure 2 also reveals that the total additions content in the binder does not determine the compressive strength development over time. In fact, although the three concretes 33FA, 16FA16ECat and 33ECat have 33% of additions incorporation, the 33ECat concrete presented lower compressive strength than those of concretes with the same content of FA and the same total content of additions of half of each, FA and ECat. These might be attributed to the differences in the additions properties. Namely, FA particles are finer than those of ECat and present a polydisperse size distribution, while ECat particles are nearly monodispersive (Figure 1a). Therefore, the filling ability of the FA particles (Section 2.1) also contributes to the higher strength exhibited by concretes with this addition incorporation [44].

Fresh State
ECat exhibited a higher pozzolanic reactivity, assessed by the Chappelle test, than FA (1540 and 991 mg/g, respectively) and similarly, pozzolanicity assessed through AI (83.6% and 83.4%, respectively, Table 1). This means that the pozzolanic effect of the ECat in the cement matrix might have been limited by the absence of free calcium hydroxide (Ca(OH) 2 ), liberated by clinker hydration, since limestone Portland cement had been used in this study. In fact, the clinker content in the concretes binder's composition with CEM II/A, and 33% of additions is only 55.5%, by mass. This explanation seems to be reinforced by the faster and higher strength development of the 16ECat concrete (up to 28 days), which has the highest cement, and thus, clinker content in the binder ( Table 2). The confirmation of this hypothesis requires further investigation of Ca(OH) 2 , e.g., using thermo-gravimetric analysis or X-ray diffraction techniques. However, this deficiency in Ca(OH)2 compromising ECat's pozzolanic activity has been verified elsewhere [49].
From a technological perspective, the concretes with ECat incorporation achieved the compressive strength values specified in the EN 206-1 standard for a concrete strength class C25/30. Figure 3 presents the evolution of the UPV and dynamic modulus of elasticity (E d ) results of concretes over time. The UPV values increase over time reaching an asymptotic maximum velocity value beyond a given curing age, as expected [50]. The concretes under study approached the maximum value at 14 days of age. Moreover, the UPV values exhibited an inverse relationship with ECat content in the binders. As such, the asymptotic value of the 33FA zone is higher (ca. 3%) than those of 16FA16ECat and 16ECat, which are very similar between them, are in turn, are also ca. 3% higher than that of 33ECat concrete.  Figure 3 presents the evolution of the UPV and dynamic modulus of elasticity (Ed) results of concretes over time. The UPV values increase over time reaching an asymptotic maximum velocity value beyond a given curing age, as expected [50]. The concretes under study approached the maximum value at 14 days of age. Moreover, the UPV values exhibited an inverse relationship with ECat content in the binders. As such, the asymptotic value of the 33FA zone is higher (ca. 3%) than those of 16FA16ECat and 16ECat, which are very similar between them, are in turn, are also ca. 3% higher than that of 33ECat concrete. However, considering a classification [51][52][53] to assess concrete quality based on UPV values presented in Table 5, all concretes fell within the range of 'excellent' beyond the sixth curing day. The ECat incorporation tended to slightly delay the achievement of the 'excellent' classification to 6 and 3 curing days for 33ECat and 16ECat, respectively, whereas the 33FA and 16FA16ECat reached this class from the 1st day. However, considering a classification [51][52][53] to assess concrete quality based on UPV values presented in Table 5, all concretes fell within the range of 'excellent' beyond the sixth curing day. The ECat incorporation tended to slightly delay the achievement of the 'excellent' classification to 6 and 3 curing days for 33ECat and 16ECat, respectively, whereas the 33FA and 16FA16ECat reached this class from the 1st day. The concretes with ECat incorporation present lower E d values than that of the binary blended binder with FA, 33FA (Figure 3b). The trend of the E d plots against time is similar to those of the UPV. However, the relative difference among the values for the diverse concretes are higher, i.e, 33FA has E d values, on average, 6.5% higher than those of 16FA16ECat and 16ECat, which are very similar between them and that have E d values, after the third day, 7% on average higher than those of 33ECat.

Ultrasonic Pulse Velocity
Although there is no precise form of the relationship, there is a general agreement that the modulus of elasticity increases with an increase in the compressive strength of concrete. Thus, it would be expected that the elastic modulus of 16ECat would be the highest, at least during the first stages, since it is the concrete which exhibited the highest strength (Figure 2), but this was not verified (Figure 3b). Figure 4 plots the capillarity water uptake against the time of immersion in water after 28 and 90 days of curing age. Table 6 presents the corresponding capillary sorption coefficients, S, i.e., the slopes of the linear fit of capillary water uptake experimental results against the square root of the time as well as the fitting correlation coefficient (R 2 ).  Table 6 presents the corresponding capillary sorption coefficients, S, i.e., the slopes of the linear fit of capillary water uptake experimental results against the square root of the time as well as the fitting correlation coefficient (R 2 ).     Figure 4 and Table 6 show that for all concretes investigated both water uptake and sorption coefficients decrease from 28 to 90 days of age, as expected, in a range from 12.4% for 16ECat to 24.1% for 16FA16ECat. Moreover, concretes with ECat incorporation have higher S values of 26.9% for 16FA16ECat, 30.8% for 16ECat and 42.3% for 33ECat at 28 days of age and 25.0% for 16FA16ECat and 50.0% for both 16ECat and 33ECat at 90 days of age. Thus, an overall analysis of these results reveals that typically ECat, in these concretes composition, promotes an increase in water permeability.

Capillary Water Absorption
Typically, the pozzolanic reaction product formation promotes a decrease in the volume of capillary pores and their interconnectivity and, thus, an increase in the water absorption resistance [54]. This effect has been found in studies using ECat generated in the same [37,38], and other refineries [55] as cement surrogate up to 20%.
However, the findings of the current investigation (Table 6) show that, in general, the increase of the ECat in the binder leads to an increase in the capillary water absorption of concrete, which has also been previously seen elsewhere for high levels ECat incorporation in the binder above 20% [35,56]. Therefore, this effect emphasizes the assumption mentioned above, that the occurrence of the pozzolanic reaction of ECat, when it is present in the binder beyond a particular content, might be limited by the absence Ca(OH) 2 due to the small amount of clinker.
The criteria, available in the literature [57], to assess the concrete quality based on water absorption, refers that for S < 5 mm/h 0.5 concretes are of very good quality. Since, S values obtained for all investigated concretes (Table 6) are markedly smaller than that threshold S value all concretes possess a high potential of durability regarding this property. Figure 5 presents the non-steady state chloride migration coefficients, D nssm , of concretes at 28 and 91 days of curing. The figure also shows (by means of horizontal lines) the range of D nssm (×10 −12 m 2 /s) values that determine the concretes classification of 'potential' durability against chloride-induced corrosion [58]. The classes are the following: Very Low (VL) for D nssm > 50, Low (L) for D nssm = 10 to 50, Medium (M) for D nssm = 5 to 10, High (H) for D nssm = 1 to 5 and Very High (VH) for D nssm < 1.

Chloride Migration
The results show ( Figure 5) a marked decrease in D nssm values for the higher curing ages in a range of 94.4% for 16ECat to 98.3% for 33FA. This increasing resistance against chloride ingress over time was anticipated due to the continued binder hydration that leads to the refinement of capillary pore structure. For the investigated concretes, the chloride migration was more affected by the blended cement pastes densification over time than the water absorption by capillarity ( Table 6). Figure 5 presents the non-steady state chloride migration coefficients, Dnssm, of concretes at 28 and 91 days of curing. The figure also shows (by means of horizontal lines) the range of Dnssm (×10 −12 m 2 /s) values that determine the concretes classification of 'potential' durability against chlorideinduced corrosion [58]. The classes are the following: Very Low (VL) for Dnssm > 50, Low (L) for Dnssm = 10 to 50, Medium (M) for Dnssm = 5 to 10, High (H) for Dnssm = 1 to 5 and Very High (VH) for Dnssm < 1. The results show ( Figure 5) a marked decrease in Dnssm values for the higher curing ages in a range of 94.4% for 16ECat to 98.3% for 33FA. This increasing resistance against chloride ingress over time was anticipated due to the continued binder hydration that leads to the refinement of capillary pore structure. For the investigated concretes, the chloride migration was more affected by the blended cement pastes densification over time than the water absorption by capillarity (Table 6). At 28 days of curing age, the concretes with 33% of additions incorporation containing ECat, 16FA16ECat and 33ECat, exhibit an increase of around 11.5% in the Dnssm values in relation to the reference concrete (33FA), whereas for concrete with 16% of ECat incorporation the Dnssm value is similar with that of 33FA. At 91 days, Dnssm values of concretes with ECat incorporation increase regarding that of 33FA, being the Dnssm value of 16ECat the highest. However, considering the heterogeneity of the concretes, and that the standard deviation (SD) of the migration coefficients, Dnssm, is about 0.98 × 10 −12 m 2 /s [59], too much relevance should not be attributed to the differences in Dnssm values obtained for the different concretes at 91 days, since they are within the SD limits. As such, it can be reasoned that at this curing age, the chloride ingress resistance is very high and the difference among the tested concretes is negligible.
Previous studies have revealed that the incorporation of up to 30% of ECat in the binder promotes an increase in the chloride penetration resistance in relation to that of plain cement [37,38,60] and also that ternary blended binders with 30% of FA and ECat hinders the chloride ingress in relation to binary blended binders with only FA [41]. The increment in resistance against chloride ions migration was attributed to additional densification of the pozzolan blended cement pastes, At 28 days of curing age, the concretes with 33% of additions incorporation containing ECat, 16FA16ECat and 33ECat, exhibit an increase of around 11.5% in the D nssm values in relation to the reference concrete (33FA), whereas for concrete with 16% of ECat incorporation the D nssm value is similar with that of 33FA. At 91 days, D nssm values of concretes with ECat incorporation increase regarding that of 33FA, being the D nssm value of 16ECat the highest. However, considering the heterogeneity of the concretes, and that the standard deviation (SD) of the migration coefficients, D nssm , is about 0.98 × 10 −12 m 2 /s [59], too much relevance should not be attributed to the differences in D nssm values obtained for the different concretes at 91 days, since they are within the SD limits. As such, it can be reasoned that at this curing age, the chloride ingress resistance is very high and the difference among the tested concretes is negligible.
Previous studies have revealed that the incorporation of up to 30% of ECat in the binder promotes an increase in the chloride penetration resistance in relation to that of plain cement [37,38,60] and also that ternary blended binders with 30% of FA and ECat hinders the chloride ingress in relation to binary blended binders with only FA [41]. The increment in resistance against chloride ions migration was attributed to additional densification of the pozzolan blended cement pastes, namely with ECat, associated with its exceptional pozzolanic reactivity forming reaction products, refine the capillary pore sizes. Nevertheless, similar results were not found in the current investigation. In line with the above-mentioned arguments, one consideration is that the use of CEM II type of cement (instead of CEM I used in the studies reported in the literature) lowered the clinker content to the point that might have compromized the availability of the required Ca(OH) 2 to ensure the ECat pozzolanic reaction occurrence.
All investigated concretes, at 90 days of age, had values of D nssm ≤ 1 × 10 −12 m 2 /s ( Figure 5). As such, all concretes' durability class is VH meaning that from the technological viewpoint ECat shall not increase the risk of corrosion of the reinforcement of concretes under external chloride attack. Figure 6 presents the evolution of electrical resistivity (ρ) results of concretes over time. The figure also points out (by means of horizontal lines) the range of values proposed elsewhere [58] to classify the 'potential' durability of concretes concerning reinforcement corrosion based on ρ values. These classes are: very low (VL) potential durability for ρ < 50 Ω m, low (L) for 50 < ρ < 100 Ω m, medium (M) for 100 < ρ < 250 Ω m), high (H) for 250 < ρ < 1000 Ω m and very high (VH) for ρ > 100 Ω m. Figure 6 presents the evolution of electrical resistivity (ρ) results of concretes over time. The figure also points out (by means of horizontal lines) the range of values proposed elsewhere [58] to classify the 'potential' durability of concretes concerning reinforcement corrosion based on ρ values. These classes are: very low (VL) potential durability for ρ < 50 Ω m, low (L) for 50 < ρ < 100 Ω m, medium (M) for 100 < ρ < 250 Ω m), high (H) for 250 < ρ < 1000 Ω m and very high (VH) for ρ > 100 Ω m. Since the electrical resistivity is the material property that evaluates its resistance against the flow of the electrical current (which is mainly carried via the ions present in the liquid phase), it is mostly dependent on pore network connectivity and ions concentration [61]. Therefore, as expected, the results show ( Figure 6 The densification of concrete microstructure that corresponds to a more refined capillary pore system and thus, higher electrical resistivity typically also promotes the increasing of compressive strength, UPV, modulus of elasticity as well as of the resistance against capillary water absorption and chloride migration. Unambiguous relationships among these properties are not yet established for all concretes, because they are all affected in different ways by several factors that are also interdependent including, the raw materials properties, concretes preparation mode, cure conditions and test method adopted to evaluate properties of concretes [62]. However, the qualitative analysis Since the electrical resistivity is the material property that evaluates its resistance against the flow of the electrical current (which is mainly carried via the ions present in the liquid phase), it is mostly dependent on pore network connectivity and ions concentration [61]. Therefore, as expected, the results show ( Figure 6 The densification of concrete microstructure that corresponds to a more refined capillary pore system and thus, higher electrical resistivity typically also promotes the increasing of compressive strength, UPV, modulus of elasticity as well as of the resistance against capillary water absorption and chloride migration. Unambiguous relationships among these properties are not yet established for all concretes, because they are all affected in different ways by several factors that are also interdependent including, the raw materials properties, concretes preparation mode, cure conditions and test method adopted to evaluate properties of concretes [62]. However, the qualitative analysis of the results obtained in the current study shows that, in general, the ρ results obtained agree with those of the other properties. In fact, concretes with highest and lowest ρ values respectively, 16ECat and 33ECat also present the same relative position of compressive strength, UPV and modulus elasticity (Figures 2  and 3 respectively) and the reverse relative position in relation to capillary water uptake at 28 days ( Figure 4). Likewise, the 16FA and 16FA16ECat that show similar ρ values present similar behavior in relation to compressive strength ( Figure 2) and resistance against capillary water uptake ( Figure 4) and comparable values of the other studied properties. However, the high increase in the resistivity values over time (Figure 6), mainly for 16ECat concrete, is not comparable with the level of increment in the compressive strength, which is much smaller. A possible explanation is that the pore refinement promoted by the pozzolans, namely the ECat to the cement matrix over time led to a more significant improvement of the durability properties than on the mechanical strength.

Electrical Resistivity
The presence of pozzolanic additions promote both changes in the concretes microstructure [54] and in the ionic composition of pores solution [63]. Thus, ρ values are also affected by the specific addition used. In fact, ρ results obtained for the investigated concretes lie in the same range of those reported elsewhere [45] for self-compacting mortars also incorporating ECat in their composition although the other mixtures constituents were not the same.
In view of their technological application and adopting the above mentioned classes of 'potential' durability of concretes regarding the reinforced corrosion estimated through the ρ values (pointed out in Figure 6) both concretes, 16FA16ECat and 16ECat, represent good alternatives in relation to the ready-mix industrial concrete used as reference, 33FA. In fact, both concretes with 33% of additions incorporation in the binder (33FA and 16FA16ECat) have, from 28 days until 90 days of age, the same medium-class of 'potential' durability, whereas 16ECat, although being slightly less eco-friendly, reaches the high-class of 'potential' durability before the 90 days of age.

Conclusions
This study intended to evaluate the feasibility of producing industrial ready-mixed eco-concretes incorporating ECat, a waste generated in the oil refinery industry. To pursue this objective, a widely industrially produced binary blended concrete with 33% FA, 33FA, was used as reference. ECat partially (16FA16ECat), or entirely (16ECat and 33ECat), surrogated FA in the innovative eco-concretes studied. The main conclusions of this investigation can be summarized as follows: The compressive strength development of 16FA16ECat concrete is similar to that of the reference concrete, 33FA. The 16ECat exhibits a strength gain higher than that of the 33FA up to 28 days whereas the compressive strength of the 33ECat is ca. 20% lower than that of 33FA up to 90 days of age. Anyhow, all concretes meet the target requirements of normative strength class C25/30 Concretes with ECat incorporation present both lower UPV and dynamic elastic modulus values than those of 33FA concrete. Moreover, UPV and E d values exhibit an inverse relationship with ECat content in the binders The classification scale to assess the concretes quality based on the UPV values reveals that all concretes are of 'excellent' quality The capillary water absorption, and the sorptivity, of ECat containing concretes are higher than those of the 33FA concrete. However, this increment shall not affect their technological application since, considering the criteria of potential durability of concretes based on S values, all investigated concretes are of 'very good' quality The non-steady state chloride migration coefficients, D nssm , of all concretes markedly decrease from 28 to 91 days of age, and at this age, all have D nssm ≤ 1 × 10 −12 m 2 /s. As such, regarding the durability indicators established for this property, all concretes investigated, at 91 days, lie in the class interval of 'very high' resistance to chloride-induced corrosion Ternary 16FA16ECat concrete presents an electrical resistivity evolution similar to that of 33FA, 16ECat exhibits higher values and 33ECat lower than those of 33FA concrete. Once more these differences shall have no negative impact regarding their industrial application since, at 91 days, the 33ECat and 16FA16ECat concretes lie in the same class of durability (based on ρ values) than the 33FA, which is 'medium' and 16ECat belongs to the 'high' class.
In summary, ECat can be recycled in the production of ready-mixed concrete products, namely in the manufacture of 16FA16ECat and 16ECat concretes, with similar, or superior, technological performance, including durability when compared with that of current commercialisation. Both of these concretes contain 16.7% of ECat in binder's phase and, respectively, 33.3% and 16.7% of total waste (ECat and FA) incorporation. Since concretes were prepared with limestone Portland cement type CEM II/A-L, the clinker content is 55.5% to 69.3%, respectively, for 16FA16ECat and 16ECat. Thus, these concretes also present sustainable qualities, including low carbon footprint and resource efficiency. Moreover, the results showed that the manufacture of ready-mixed ECat-incorporating concretes with the same clinker factor reduction (of 33%) of the current industrial production is feasible.
Likewise, ECat incorporation in concretes also provides environmental benefits to oil refinery industry due to landfilling reduction as well as economic advantages by adding value to a waste.
Further investigations will be carried out to optimize the composition of concretes with ECat incorporation that lead to the best performance targeting different engineer properties and, thus, different applications. Taking in consideration the results presented in the current study, it is anticipated that the waste content in the concretes lie within the assessed range i.e., from 16.7% to 33% and that for most applications, the optimized ECat content shall be close to 16.7%.
Turning ECat into a steady supply for concrete manufacture industry fit in seamlessly with the scheme of the circular economy making a relevant contribution for a more dynamic and sustainable society. Acknowledgments: This project builds upon research developed in ECO-Zement R&D+i project which was awarded with the Jerónimo Martins/Green Project Awards 2017. Acknowledgements are also due to Sines Refinery/Galp Energia for supplying ECat and to CIMPOR TEC, namely João Pereira, for the collaboration in the binding materials characterization. The assistance of Mário Costa for participating in the concrete testing is also gratefully acknowledged.

Conflicts of Interest:
The authors declare no conflict of interest.

AI
Activity index according to with European standard EN 450-1 B Binder D nssm Non-steady state chloride migration coefficient (m 2 /s) ECat Spent equilibrium catalyst generated in oil-refinery E d Dynamic modulus of elasticity (MPa) FA Fly-ashes FCC Fluid cracking catalytic S Capillary water sorption coefficient (kg/m 2 ·h 0.5 ) UPV Ultrasonic pulse velocity (Km/s) w added /B Added water-to-binder mass ratio w eff /B Effective water-to-binder mass ratio ρ Electrical resistivity