Durability and life cycle evaluation of self-compacting concrete containing fly ash as GBFS replacement with alkali activation

Construction and Building Materials 235 (2020) 117458

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Durability and life cycle evaluation of self-compacting concrete containing fly ash as GBFS replacement with alkali activation Ghasan Fahim Huseien ⇑, Kwok Wei Shah ⇑ Department of Building, School of Design and Environment, National University of Singapore, Singapore 117566, Singapore

h i g h l i g h t s
High performance self-compacting concretes were produced by blending FA and GBFS.
FA reduces the drying shrinkage of self-compacting alkali-activated concrete.
Concretes’ resistance to acid attack improved due to inclusion of FA waste.
Use of FA in self-compacting alkali-activated concretes found beneficial for environment.

a r t i c l e

i n f o

Article history: Received 1 June 2019 Received in revised form 13 October 2019 Accepted 2 November 2019

Keywords: Self-compacting alkali-activated concrete FA GBFS Acid resistance Cost and energy saving

a b s t r a c t Nowadays, geopolymer with alkali activation binders are introduced as alternative environmentally friendly construction materials to the ordinary Portland cement for solving the carbon dioxide emission and high energy consumption problems. In the construction sectors worldwide, the durability of concrete is the major concern. Concretes produced by recycling the agricultural and industrial wastes were shown to be environmentally friendly with improved durability performance. In this view, present paper examines the effects of fly ash (FA) as replacement agent to GBFS on the durability performance of synthesized self-compact alkali-activated concrete (SCAACs). Six concrete mixes each with a different percentage of FA (30, 40, 50, 60 and 70%) in place of GBFS were designed. A control mixture with 100% GBFS content was used as base specimen to compare other five mixes. Properties such as filling and passing ability, compressive strength, drying shrinkage, carbonation depth and resistance to sulfuric acid were measured. The life cycle of proposed SCAACs were assessed in terms of CO2 emission, cost and saving energy. The resilience and the workability of the SCAAC mixtures were improved when FA was substituted with GBFS at 40%, 50% and 60%. Addition of FA could largely enhance the SCAACs durability and exhibit superior performance against sulphuric acid attack. Likewise, concrete mixtures containing FA of 50% and above showed reduction in CO2 emission over 20%, cost about 15% as well as energy consumption almost 18%. It was concluded that by substituting GBFS by FA a potential solution to the issue of trying to reduce CO2 emission and contribution to a healthier environment can be achieved. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction By definition, self-compacting concrete (SCC) is a type of concrete that can be placed and compacted under its self-weight without vibration effort, simultaneously it is cohesive enough to be handled without segregation or bleeding [1–3]. The idea of SCC was first explored in 1986 in Okamura, and two years later Ozawa at the University of Tokyo created the first sample in both fresh and set states, for specific use [4,5] In contrast to traditional concrete a ⇑ Corresponding authors. E-mail addresses: [email protected] (G.F. Huseien), [email protected] (K.W. Shah). https://doi.org/10.1016/j.conbuildmat.2019.117458 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

good SCC has three fundamental properties [6]: which are its filling ability, passing ability and segregation resistance. Filling ability is described as the concrete’s ability to completely fill the desired space under its own weight. Passing ability is the concrete’s ability to flow freely in small spaces around steel, reinforcement bars without clogging up or segregating, and segregation resistance is its capability to remain in a consistent state during transportation as well as before and after placement. As well as a high level of selfcompatibility, SCC needs to meet the standards for stability of volume, strength and resilience of the set concrete [7,8]. Studies have shown that the rheology, strength, shrinkage, durability and sustainability of SCC can be substantially affected by aspects such as the configuration of raw materials, inclusion of chemical and

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mineral admixtures, packing density, water volume, aggregate and design technique [9–12]. Over the years, the alkali-activated concrete based on GBFS incorporating by-product and agriculture waste materials such as fly ash (FA), palm oil fuel ash (POFA) and metakaolin (MK) are introduced as high performance, durable, sustainable and environmental-friendly construction materials to be ordinary Portland cement (OPC) alternate in construction sector [13–15]. In recent years, several researchers have been studied and investigated the alkali-activated properties including compressive strength (CS) performance [16], modulus of elasticity (MoE) [17], autogenous and drying shrinkage [17,18], permeability and porosity, chemical resistance included sulphuric acid and sulphate [17,19]. Generally, alkali-activated concrete presented excellent behaviour in early strength and service life in aggressive environments comparing to traditional concrete. Nowadays, GBFS is high recognized a binder based high performance alkali-activated concrete. GBFS is a derived by-product obtained from quenched iron slag and steel melt when manufactured in the blast furnace in the presence of water or steam. It appears smooth, gritty and becomes fine powder upon drying. The high levels of CaO and SiO2 in the GBFS cause it to display properties of cement and pozzolan. As a result, GBFS has been a popular choice in the construction industry to increase the mechanical properties and resilience of traditional concrete [20,21]. It was found that including GBFS in alkaliactivated materials caused a change in the resilience and microstructure of the concrete [16,22]. In alkali-activated mortars, the use of GBFS as a binder to increase its strength was concluded in a report by Yusef et al. [23]. The increase in strength was originally connected to the effects of pores filling, structuring of poorly arranged microstructure and twin creations formed during extremely polymerized units of alkali-activation and calcium silicate hydrate (C-S-H) gels [24,25]. It was proven that GBFS is related to Ca solubility, amorphous and heterogeneous nature of the substance. Additionally, the created Ca/Na aluminosilicate hydrate products so called C(A)-S-H and C(N)-A-S-H gels were shown to enhance the concretes compressive strength [22,26,27]. However, the high dioxide emission, cost, energy demand during preparation as well as the fast setting times and low durability to sulphuric acid (high CaO content) restricted to use GBFS in a wide range and construction sector specially for self-compacting applications [17,28]. FA (an offspring of burnt coal in thermal power stations) containing low level of Ca is introduced as a best waste materials to manufacture alkali-activated concrete for many reasons such as high levels of SiO2 and Al2O3 [29–31], availability in a wide range in many countries around the world and low cost and energy requirements in life cycle preparation. FA-based geopolymer concrete shows remarkable resilience to heat curing in long and short term tests [32,33]. The main obstacle to using FA is its poor strength when cured in an ambient temperature, high demand to alkaline solution and need high molarity (UP 10 M) of sodium hydroxide [34]. Therefore, no benefit can be achieved from using FA separately to manufacture alkali-activated concrete especially for self-compacting applications. Numerous experiments were carried out to find a solution to increasing the strength of FA [27], adding ingredients containing calcium. Calcium is proven to significantly affect the mechanical features of alkali-activated substances [22,35,36] where calcium (aluminium) silicate hydrate (C(A)-S-H) gels are created along with the sodium aluminium silicate hydrate (N-A-SH) products [37–39]. Many factors affect the fresh and hardened properties of SCAAC such as mixture design methods [1,9,40], chemical and physical properties of waste materials based binder [21,41], alkaline activator solution composite [42–44], molarity of sodium hydroxide

[45,46], ration of sodium silicate to sodium hydroxide [47], water and superplasticizer content [48], fine and coarse aggregate content, types and sizes [49,50]. As known the self-compacting technology is one of the important technical in construction application sector; a lacking data are available in literature evaluated the durability and sustainability performance of SCAACs under effect of binder types. For production SCAAC, the types of raw materials based binder play the main factor effect on product procedure. The high water absorbs of FA and repaid setting time of GBFS restricted use these two kinds of materials separately to SCAAC achieve the specific requirements. This study aimed to produce SCAAC incorporating FA and GBFS in various levels as the FA contributed to increase the setting time and GBFS led to reduce the water absorb of FA. The workability properties including filling ability, passing ability, segregation resistance and setting time were evaluated in wide range. The durable properties such as porosity, drying shrinkage, carbonation depth and acid resistance as well as the life cycle of SCAAC samples are investigated. The investigations were conducted on the reference sample (100% GBFS) and the samples containing FA and GBFS in various ratios. 2. Experimental programme 2.1. Materials characterization Pure GBFS, from Ipoh (Malaysia) was used as one of the resource materials to produce cement-free binder. This GBFS was further used without any handling in a laboratory and contains properties of both pozzolan and cement. It differs from other substitute cement materials and is off-white in appearance because of the water it is mixed with causing a hydraulic reaction. The composition of the GBFS is uncovered with the use of X-Ray fluorescence (XRF) spectra, showing 51.8% calcium, 30.8% silicate and 10.9% alumina [Table 1]. Tanjung Pin power station in Johor, Malaysia was a source of low calcium FA to use as an aluminasilicate in the production of SCAAC. It met the standards of the ASTM C618 for F class FA. It comprised of 57.2% silicate, 28.8% aluminium and 5.2% calcium, and was coloured grey. The estimated median of particle for FA (10 mm) and GBFS (12.8 mm) was calculated using a particle size analyser, and the Brunauer-EmmettTeller (BET) surface areas of GBFS and FA were studied in order to ascertain their physical properties. The FA showed the maximum level of exact surface area of 18.1 m2/g than that of GBFS (13.6 m2/g) as depicted in Table 1. The FA and GBFS XRD pattern analysis illustrated in Fig. 1. The XRD pattern of FA revealed pronounced diffraction peaks around 2h = 16–30°, which were allocated to the presence of crystalline

Table 1 Various properties of FA and GBFS. Material

FA

GBFS

Chemical composition (% by mass) Calcium oxide (CaO) Silica (SiO2) Aluminium oxide (Al2O3) Iron (III) oxide (Fe2O3) Magnesium oxide (MgO) Potassium oxide (K2O) Sodium oxide (Na2O) Sulphur trioxide (SO3) Loss on ignition

5.16 57.20 28.81 3.67 1.48 0.94 0.08 0.10 0.12

51.8 30.8 10.9 0.64 4.57 0.36 0.45 0.06 0.22

Physical characteristics Surface area-BET (m2/g) Average diameter (mm) Specific gravity

18.1 10 2.2

13.6 12.8 2.9

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58.2 of solution prepared for 14 M of NaOH and 2.5 of Na2SiO3 to NaOH ratio which are recommended in previous studies as the optimum ratios [23,51]. Regarding the content of Na2O, SiO2 and H2O, the prepared alkaline solution is friendlier to environment and is low in cost, energy consumption as well as the carbon dioxide emission [52]. 2.2. Mixture proportions FA replaced GBFS at rates of 30%, 40%, 50%, 60% and 70% of binder weight content. The composition of the self-consolidating alkali-activated concretes contained 484 kg/m3 of binder and an alkaline solution to binder ratio (S:B) of 0.50 across all samples [Table 2]. Na2O:SiO2 (modulus of alkaline solution) was 1.2 to achieve fresh qualities in the self-compacting concrete mixtures. All mixtures had a standard proportion of NS to NH at 0.75 and fixed levels of 844 kg/m3 fine aggregate and 756 kg/m3 course aggregate. Fig. 1. XRD patterns of FA and GBFS.

2.3. Specimen preparation and test methods silica and alumina compounds. Nonetheless, the occurrences of other sharp peaks were assigned to the presence of crystalline phases of quartz and mullite. The absence of any sharp peak in the XRD pattern of GBFS indeed verified their highly amorphous nature. GBFS contains high level of reactive amorphous SiO2 and Ca, which are extremely significant for the attainment of SCAACs. Nevertheless, inclusion of FA in GBFS was essential to surmount its low level of alumina content (Al2O3, 10.49 wt%). SEM images of FA and GBFS exhibited that the former one is comprised of regular spherical particles and the later one enclosed irregular angular particles (Fig. 2). A fine aggregate was found in local mining sand having water absorption rate of 1.13%, 2.56 of specific gravity, and 2.88 of fineness modulus. The course aggregate was made of crushed limestone with a maximum size of 10 mm, water absorption of 0.43% and specific gravity of 2.62. The experiment processes and achieved statistics were in line with ASTM C33 so that they met requirements. Analytical grade NaOH (NH) in pellet form (98% purity) was dissolved in water to make 2 M of NaOH solution, which was further cooled down (24 h) before being added to a sodium silicate (NS) mixture to get the resultant alkali solution having SiO2: Na2O proportion of 1.2. All alkali solutions contained constant NS: NH of 0.75. Analytical grade NS solution was consisted of silica of 29.5 mass%, sodium oxide 14.70 mass% and H2O of 55.80 mass%. In this study, the total Na2O, SiO2 and H2O were 10.53, 12.64 and 76.8 (by weight, %) respectively, compared to 20.75, 21.07 and

The first stage involved preparing six samples of binary blended binder (GBFS and FA) including the control batch with 100% GBFS, and storing them in plastic bags. The alkaline activator solution (NaOH + Na2SiO3) was also prepared 24 h in advance, by mixing Na2SiO3 and NaOH (2 M) with ratios of 0.75. Fine and course aggregate were combined before adding 50% binder. After adding the mixed 50% binder to fine and coarse aggregates, all materials mixed for 3 min. Then followed by adding the other 50% binder and mixed again for 3 min till homogenous. Then, the acquired mixture is activated by adding the alkaline solution. The whole matrix is mixed in the machine operating at medium speed for another 4 min. Finally, the fresh concrete is cast in moulds in three layers, where each layer was consolidated using vibration table for 30 s to allow air voids to escape. After casting, the SCAAC specimens were left for 24 h to cure at ambient temperature (27 ± 1.5) °C in relative humidity of 75%. Then, the specimens were opened and left in the same condition till testing time. As a general rule, the procedure for mixing self-compacting alkali activated concrete takes longer than the process for mixing traditional concrete. It is relevant to point out that including FA in the mixture created more difficulties in meeting the standards of tests for SCC, so the maximum FA content that was used was 70% as the ash has an increased water demand. Properties of fresh SCAACs were evaluated in line with conditions of EFNARC (2002) for passing capabilities, filling ability and resistance to segregation following mixing. Originally, the mixtures were put through T50

Fig. 2. SEM of FA and GBFS.

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Table 2 Self-compacting alkali activated concrete mix design. Mix Code

Binder (kg/m3)

Aggregate (kg/m3)

Alkali activator solution (kg/m3)

GBFS

FA

Sand

Coarse

S:B

M

NS:NH

NS

NH

SCAAC1 SCAAC2 SCAAC3 SCAAC4 SCAAC5 SCAAC6

484 338.8 290.4 242 193.6 145.2

0 145.2 193.6 242 290.4 338.8

844 844 844 844 844 844

756 756 756 756 756 756

0.50 0.50 0.50 0.50 0.50 0.50

2 2 2 2 2 2

0.75 0.75 0.75 0.75 0.75 0.75

147 147 147 147 147 147

195 195 195 195 195 195

S:B-Alkaline solution:Binder; M-Molarity of NH; NS:NH-Ratio of Na2SiO3 to NaOH

and slump flow tests (EFNARC, 2002), before experiencing Vfunnel, L-box and J-ring tests. A concrete’s filling ability is explained as it ability to modify its shape under its own weight, even in spaces with obstructions that can affect its flow. Slump flow and Orimet tests can measure the filling ability of a concrete. Slump flow tests assess the flow ability with no obstructions by measuring the mean base diameter of the concrete at the end of the test. This calculates the capability of the concrete to morph under its own weight against resistance of the surface, with no other obstructions. Due to the gelatinous quality of SCAAC the measurement of the slump flow was taken after the concrete settled and there was no more visible movement. This occurred around 1 min after the slump cone was removed. In the same test, the T50 slump flow reading was taken when the concrete was slumping, until it obtained 500 mm of slump flow. The slump flow test was conducted to assess the flow capacity of SCAACs. This procedure required untamed concrete for pouring into the mould, and a slump cone which was positioned on a non-porous, rigidlevelled mould before concrete was poured into it under nontamped conditions. It was then positioned vertically to allow the concrete to flow freely before estimating the diameter of the concrete in two equally perpendicular directions in order to work out the average. The test was performed under EFNARC standards, in which SCAAC is taken to have good capability for filling, and reliability for diameter spreading in the range of 650 mm to 800 mm. The required time was measured in seconds and considered by elevating the cone to a height that allowed the flow spread realization at 500 mm circle. It is called the T50 slump flow that measures the relative viscosity and allows evaluating the rate of free flow of the SCAAC, with better flow ability being related to a shorter time. For specimens with high viscosity, long T50 duration is not so important because it fluctuates fast. This test is normally used as a benchmark for quality control rather than for rejecting an SCAAC group. The J-ring tests were carried out along with the slump flow tests using four contrasting J-rings. Each had equal spacing between the bars, measuring 30 mm, 40 mm, 50 mm and 60 mm respectively. This allowed evaluation of the concrete to pass through different sized gaps. The J-rings which were made of Perspex rings with several round steel bars, 12 mm in diameter, placed at the specified spacing around the circumference. For each test, the J-ring was positioned outside the slump cone prior to conducting the slump flow test in the same manner as before. Following the test, readings were taken of the slump flow and compared to control sample. The capability of concrete to spread independently through confined spaces, such as those created by the presence of reinforcement bars, is defined as the passing ability. This passing ability is ordinarily improved by increasing the filling ability, but there is no guarantee of passing ability simply by having a high filling ability. The L-box test is used to assess the filling and passing abilities of SCAAC. The L-box has horizontal and vertical compartments which are sectioned off by a convenient door with vertical reinforcement bars placed before it. Before beginning the test, the

L-box was placed on a constant flat surface. The vertical part was packed with concrete before lifting the door which separates the parts. Then the concrete was poured into the narrow gaps between the bars under reinforcement at the base of the L-box. When the concrete had finished flowing, the blocking ratio (H2/H1) was computed, assuming H2 as the horizontal and H1 as the vertical section. The segregation resistance of concrete is its ability to maintain its state throughout placement until it is set, and includes both dynamic and static stability. Dynamic stability is the segregation resistance when concrete is moving; for example, while being mixed and poured, while static stability refers to the concrete’s segregation when it is still. The main reason for performing this test was to obtain the filling (flow) capability to assess the segregated resistance of SCAACs when a V-shaped funnel was used. The funnel was completely filled with approximately 0.01 m3 of concrete, without tapping or compacting, and then the trapped door was opened at the base to encourage the flowing out of the concrete. The funnel flow time is the time it took for the concrete to flow out which SCAAC required in the range of 6 to 12 s. Setting times of alkali-activated concretes are categorised into two regimes including initial and final ones. The initial setting time is the duration between mixing of binder and when there is partial loss of plasticity while the final setting time is the time taken to acquire adequate hardened form to resist a prescribed pressure. Vicat method is commonly utilized to evaluate the past setting times that follows literature specified ASTM C191 requirements. The penetration of 1.13 mm diameter needle placed on the alkali-activated past is an accurate cylindrical mould measures. The setting time corresponds to the duration between time zero and the time when the needle penetration depth is 25 mm from the bottom of the mould. The final setting time also corresponds to when the needle penetrates not more than 1 mm. The compressive strength (CS) of SCAACs were assessed using respective ASTM CC109/109 M specification. Furthermore, the recorded figures were compared with the control specimen containing GBFS of 100%. Three specimens were taken at each curing interval – 1, 3, 7, 28, 56 and 90 days - in order to gain an average value for the CS. At day 28, the proposed SCAACs were broken and their centers were ground to a fine powder for conducting microstructure tests like XRD, SEM and FTIR. XRD utilized jade software to determine the disordered phase of SCAACs. The XRD analysis was carried out in the 2h range of 5–60° (0.02° of step size and 0.5 s/step of scan speed). The specimen was positioned on a holder made of brass and tied with carbon tapes, drying was done for five minutes with IR radiation and a blazer sputtering machine was used to coat with gold layer. The surface morphologies were imaged at 20 kV with 1000 magnification. Following the ASTM C 140–07 protocol, the porosity of SCAACs was measured at 28 day of age. First, the concrete was immersed in water at 27 °C for 1 day. Afterwards, the saturated specimens (Ms) were taken outside water then followed by measure the mass. After that, all SCAACs specimens were dried in lab oven for 3 days at 105 °C. The water absorption (WA) capacity of SCAACs was

G.F. Huseien, K.W. Shah / Construction and Building Materials 235 (2020) 117458

calculated by total immersion test and the dry mass (Md) of each sample was noted. The percentage WA was calculated via:

WAð%Þ ¼

Ms  Md  100 Md

ð1Þ

Following to ASTM C157/C157M procedure, the SCAACs specimens drying shrinkage was evaluated. Three sets of SCAAC specimens each of dimension (500  100  100) mm in the form of prism are used. The specimens are prepared in accordance with ASTM C192/192M. They are cured in the ambient condition. Stainless steel studs are embedded on the specimens to facilitate measurement of length change. The specimens are de-moulded 24 h after casting and then moved to a constant environment chamber maintained at temperature of (22 ± 1) °C and relative humidity (RH) of (50 ± 5)%. Consequently, data were recorded by demec meter at 1, 3, 7, 14, 21, 28, 56, 90 and 180 days. The accelerated carbonation test was carried out using the BS 1881-210:2013 profile. A carbonated chamber was made by connecting a CO2 container to a plastic box, before introducing three SCAACs of dimension 100 mm  200 mm into it. First, all specimens were kept in vacuum chamber, where the corresponding pressure and relative humidity was maintained to 600 mmHg and 55–60% for a period of three minutes. Next, CO2 gas was inserted into the chamber for 90 days at a temperature of 26 °C and a pressure of 4%. The chamber pressure was constantly observed using a digitized pressure gauge position between the box and cylinder. After 90 days, samples were removed from the chamber and split into two pieces (parabolic cross-section) before being sprayed by phenolphthalein solution (1% concentration). The un-carbonated regions were appeared purple in color whereas the carbonated areas were unchanged. The distance between the sample boundary and the purple color edge was measured to determine the depth of carbonation. In order to determine the acid attacks in the concrete the binder paste was dissolved in the acid, which showed strength reduction of SCAACs. Effects of acid on alkali-activated compositions were evaluated using of H2SO4 acid solution supplied by QREC, Malaysia which was prepared with deionized water (concentration = 10%, pH = 1). The 28 days old specimens are weighted before immersing in the acid solution. They were immersed for duration of one year and the solution is changed every 90 days to maintain the pH all over the test period and the samples were checked. Following ASTM C267 (2012) recommendations, the performance of the proposed SCAACs specimens were qualitatively evaluated in terms of loss of weight, pulse velocity and residual strength. 2.4. Life cycle assessment (LCA) Sustainability benefits of replacing FA to GBFS in SCAACs product process was evaluated using LCA. For comparing the sustainability of FA with GBFS, three key metrics were selected such as emissions of greenhouse gases, cost and the energy saving (direct uses of fuel and electricity). These metrics formed the major basis for arguing in favor or against the usage of FA despite the significant role of other key indicators including technical performances, leaching effects, usage of water, inclusion of hazardous material, emissions of other pollutants and the waste quantity that can be overcome using FA and GBFS in SCAACs. These chosen metrics can most readily be quantified at the early stages of development for alkali-activated industrial materials. The greenhouse gases emissions, energy consumption and cost factors are obtained from life cycle approach. The evaluation was made in terms of the effects of needed feedstock and the binder manufacturing in addition to the appropriate transportation. This method is significant for validation of the two components (GBFS and FA) of concretes wherein the impact of production alone is

5

deficient to produce the full picture of the needed ‘‘embodied” energy cost and presence of CO2 in the feedstock. The mixing, laying and curing for alkali-activation and the gas emission in the entire operational life cycle are not considered because they are assumed to be comparable for every product. Thus, present method may produce an analogous life cycle effect rather than an absolute one. This method is useful for comparable products because it shortens the assessment time. Be desirable, but is the value judged to most closely approximate the Regarding every material life cycle, the emission of CO2, price and energy consumption were computed. Fig. 3 shows the life cycle of FA and GBFS, wherein the cost of assortment from the factories of these materials was considered as zero because there are treated as wastes. Furthermore, the transportation distance of every material was incorporated in the life cycle assessment. The transport distance for FA was shorter (35 km) than GBFS (500 km). In actual fact, many of the alkali-activated feedstock would be sourced from as close as possible to keep transport cost down and thus the metric values are more likely to be closer to the minimums. For all kinds of materials as depicted in Table 3 quantities such as the cost and diesel use, the type of engine in truck, the volume of fuel, the speed, price for 1 tonne/km were kept fixed. Concerning the life cycle of every concrete component, the price and amount of electricity were computed based on the capacity, operation time and electricity consumed by every machine (Table 3). In the computation of cost, the electricity price for the month of April 2019 was adopted based on the machine power (in watts). Calculation of the total carbon dioxide emission, expressed as carbon dioxide equivalent (CO2eq) per one tonne of binder produced. It included the emission of gases during the materials production, raw materials transportation and manufacturing in the lab. The approach to estimate the total carbon dioxide emission is based on the methodology reported by McLellan et al. (2011) and calculated using Eq. (2):

Total CO2 emission ¼

n X

miððdi  eiÞ þ piÞ

ð2Þ

i¼1

where left hand side indicated the net amount of greenhouse gas emitted (kgCO2eq) for every tonne of material production, mi indicates the fraction of component i, di signifies the transportation distance covered by a specific mode (km), ei denotes the emission factor for the mode of transport (kgCO2eq/(km tonne)) and pi specifies the emissions per unit mass of component i produced (kgCO2eq/tonne). Depending on the distance, the cost of transportation for the blended binder was calculated. Fig. 3 and Table 3 depicts the information related to the total consumption of fuel and electricity for every material. Similarly, the total energy requirement for every mixture was obtained based on the electricity and fuel expenditure for every type of materials wherein transportation and treatment during the preparation was included. The CO2 emission from SCAACs, required cost and energy spending were compared with the control mixture containing 100% of GBFS. 3. Results and discussion 3.1. Filling ability Table 4 enlists the slump flow readings of SCAAC with different ratios of FA to GBFS. The slump flow readings for different mixtures were recorded between 630 mm and 720 mm. Corresponding with EFNARC, the mixtures with 40%, 50% and 60% FA can be classed as slump flow class 2, which is appropriate for many normal uses such as building columns and walls. When FA content was more

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Fig. 3. Various stages of life cycle for (a) GBFS and (b) FA.

Table 3 Details of the machines and materials used. Items

Amount

Truck speed, km/hr Diesel consumption, litter/km Diesel price, $/litter Truck volume, m3 Transport charge of 1 m3, $/km GBFS density, kg/m3 FA density, kg/m3 Grinding machine power, watt Grinding machine capacity, m3 Electricity price of machine 750 W, $/kwh CO2 emission for 1 litter diesel, ton Energy consumption for 1 litter diesel, GJ CO2 emission for 1 kwh electricity, ton Energy consumption for 1 kwh electricity, GJ

80 0.09 0.55 12 0.19 1860 1350 750 0.45 0.15 0.0027 0.0384 0.00013 0.0036

than 60% the flow ability of the mixtures decreased, as shown in Table 4. For example, using a mixture with a set level of FA, the SCAAC2 slump flow was recorded as 640 mm. When the FA content was at 40%, 50% and 60%, the slump flow values were 695 mm, 720 mm and 680 mm respectively. The quick setting rate of GBFS was addressed by introducing FA. The flow diameter fell to 630 mm when FA made up 70% of the mixture. The larger surface area of FA particles compared to GBFS, which caused a high demand of water, could explain these figures. Guneyisi and Gesoglu [25] got similar results when they found that including FA in self-compacting mortar led to a gradual decrease in flow diameter. The flow ability was increased by the presence of FA at 40%, 50% and 60% of the mixture as it decreased plastic viscosity and yielding stress. Also, a higher workability (slump flow) is achieved in mixtures of these three compositions as the dispersing properties of FA reduce the level of water absorption.

The T50 flow times were recorded between 3.5 s and 5.5 s. SCAAC1 has 6 sec slump flow time because of the absence of viscosity, and the time reduced to 5.5 s, 4.0 s and 3.5 s when GBFS was replaced with FA at a rate of 30%, 40% and 50% respectively. This observation confirmed that replacing GBFS with FA by up to 50% reduced the T50 flow time of the SCAAC samples. When FA was further introduced, replacing GBFS at a rate of 60% and 70%, the flow time rose to 4.5 s and 5.5 s. Clearly, T50 was greater for samples (Table 4) with less than 40% FA which also had elevated plastic viscosity. The reduction in concrete flow with high volume content of FA was ascribed to the high specific surface area of FA which was led to increase the alkaline solution demand and loss the workability. Furthermore, it became clear that an increase in FA content (70%) decreased the workability of concrete due to high water adsorption of FA with a porous structure [53] as depicted in Fig. 2. Likewise, the rapid rate of chemical reaction of high volume GBFS content effects to decrease the plasticity of mixture which reduces the concrete’ workability [17,22].

3.2. Passing ability The findings from the L-box experiment are summarized in Table 4. The L-box blocking ratio of the concrete mix which contains FA altered from a rate of 0.80 to 0.092. Despite the control sample having a blocking ratio which was outside of the EFNARC stipulations, the majority of the samples containing FA were in line with the requirements. Results in Table 4 show that the SCAAC samples containing 40%, 50% and 60% FA displayed good blocking ratios. A blocking ratio of greater than 0.6 has been deemed appropriate for SCAAC to obtain a good filling ability [54]. Analysis of the fresh attributes of SCAAC with different FA contents shows that mixtures containing 40%–60% FA usually meet the requirements of fresh state performance in terms of deformability, passing

Table 4 Workability results of self-compacting alkali-activated concrete. Mix code

Slump flow (mm)

T50 flow (sec)

V-funnel (sec)

L-box ratio (H2:H1)

J-ring (mm)

CS at 28 days (MPa)

Acceptance criteria (EFNARC)

SCAAC1 SCAAC2 SCAAC3 SCAAC4 SCAAC5 SCAAC6

560 640 695 720 680 630

6.0 5.5 4.0 3.5 4.5 5.5

14 12.5 10 8.5 10.5 13

0.78 0.80 0.86 0.92 0.84 0.80

12 10 8 6 7.5 10.5

70.1 67.8 65.5 62.7 54.6 47.3

Not O.K Not O.K O.K O.K O.K Not O.K

6 12

0.8 1.0

0 10

According to (EFNARC) the acceptance criteria Min 650 2 Max 800 5

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ability, filling ability and good segregation resistance. This was attributed to the reduction in water adsorption and lower rate of chemical reaction which led to the enhancement in concrete workability [31]. 3.3. Segregation resistance Table 4 summarizes the results from the V-funnel tests. It shows that the V-funnel time of the control sample was 14 s and the readings from the other samples were lower. These readings for SCAAC3 was 10 s, for SCAAC4 was 8.5 s and for SCAAC5 it was 10.5 s. EFNARC [55], did not recommend a V-funnel flow time of greater than 12 s, thereby the results in Table 4 shows that samples SCAAC3, SCAAC4 and SCAAC5 all met the criteria. SCAAC2 and SCAAC6 did not. The results also indicate that V-funnel time appears to increase in line with FA content increasing above 60%. As mentioned, SCAAC5 had a V-funnel time of 10.5 s, and when FA was increased from 60% to 70%, that V-funnel time reading rose to 13 s. This finding is in line with previous studies [14,56], which found that adding FA increases the viscosity of the concrete, leading to the conclusion that up to 60% FA content has a detrimental effect on the viscosity of the composition. This also serves to prove that slump flow is not sufficient to characterize the performance of fresh self-compacting concrete. The disparity in the T50 slump flow against the V-funnel times mean that a good balance can be found. 3.4. Setting time All of the findings in the tests featuring SCAAC samples with varying degrees of FA replacing GBFS show the effect of FA on alkali-activated mixtures. The findings were compared against SCAAC1 – the control sample – which contained 100% GBFS binder. The influence of the additives on the first and last setting times of the SCAAC samples can be seen in Fig. 4. The results indicate that, due to FA’s slow setting speed in ambient temperatures, its presence in alkali-activated binders usually makes the concrete take a lot longer to set [57,58] than the control sample (SCAAC1) which set in less than 11 min. When FA was added, both first and last setting times increased. Those samples with 30% FA had an initial setting time of 18 min and a final setting time of 29 min, and the mixture with 70% FA took 66 min for the initial setting. The kind of additives included had an impact on the setting time and decreased when the level of calcium was reduced. This was also found to be the case in previous studies [22], because of the limited

Fig. 4. Influence of FA replacement for GBFS on initial and final setting time of SCAACs.

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reaction of calcium compounds in the binder. Therefore, it can be concluded that these additives in a binary blended binder decrease the setting time of GBFS based self-compacting concrete when it is cured in an ambient temperature. Besides, the addition of steel slag could accelerate the setting time and significantly improved the compressive strength, which was attributed to its latent hydraulic cementitious character of SCAACs [59]. 3.5. Compressive strength Impact of FA replaced of GBFS on CS development of SCAACs specimens presented in Fig. 5. The control sample with 100% GBFS had a compressive strength between 30 and 75 MPa. Those samples with 70% FA had compressive strength readings of between 20 and 55 MPa and, as expected, with increased age all specimens CS increase. The results show that adding FA to the samples caused a reduction of less than 26% in compressive strength, varying with the levels of FA. The filling effect of FA particles, low ratio of CaO to SiO2 and Al2O3 and deceleration of the binder hydration can cause the lower compressive strength of FA mixtures [60–62]. As can also be seen in Fig. 6, the addition of FA reduced the strength in concrete aged 1–7 days as well reducing the long term strength, measured at 28–90 days. The degree of strength loss was most noticeable in the first 7 days. Previous studies [62] have also come to the same conclusion. It has been indicated that the low ratio of CaO to SiO2 is the biggest cause of loss of strength in early age concrete [17,53,63]. Samples with FA replacement at 30%, 40%, 50% 60% and 70% were tested and the greatest strength was found in the samples with 30%-50% FA replacement. When taking into account the economic value, 50% FA is considered an appropriate alternative. Fig. 6 shows the XRD pattern of SCAACs specimens prepared with various ratio of FA to GBFS. At age of 28 days, the XRD analysis of SCAACs and the reference mix was performed. A strong overlap of the main diffraction peaks for the major phases related to the binder elements was encountered during the analysis of binder. XRD pattern of SCAAC containing FA, displayed the diffraction peaks corresponding the phases of quartz (Q), C-S-H, and mullite (M). Peaks matching to crystalline quartz (SiO2) and mullite (3Al2O32SiO2 or 2Al2O3 SiO2) phases were evidenced, which derived from the parent FA. The peak intensity was increased with increasing amount of FA from 50 to 70%, where more quartz appeared to be non- reactive with 70% of FA content. The products formed due to reaction between glassy fraction of FA and GBFS consisted of very poor crystalline phases. Kumar et al. [26] have

Fig. 5. Effect of GBFS replace by FA on developed CS of SCAACs.

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also reduced as shown by the band frequency of samples with greater FA content, compared to those with low levels of FA. The band frequency of Si-O-Al went up in conjunction with the FA level and the compressive strength of the SCAAC fell from 70.1 MPa– 47.3 MPa as FA content increased from 0% to 70%. The incensement in the bands frequency was linked to the reduction in CS of the SCAAC. Additionally, low band frequency allowed for higher levels of Si to be dissolved and AlO4, C-(A)-S-H and C(N)-A-S-H gels were created. 3.6. Water absorption

Fig. 6. XRD pattern of SCAACs with various FA content.

reported that the FA consists of reactive and refractory glass, wherein the reactive glass only participates in the hydrations. The broad and diffused background peak with maxima of around 10° was emerged from the short-range order of the hydrotalcite (Mg6Al2CO3OH164H2O) [64]. The C-S-H, Calcite (CaCO3) and Mullite peaks were observed around 28–50°. With increase of FA content, the peak intensity of the crystalline phases was increased. The quartz peak was replaced by C-S-H peak when the content of FA was raised from 30 to 50 and then to 70%. The Mullite peak at 16° for 50% FA was also replaced by quartz peak. The peaks around 24° and 33.8° were allocated to Nepheline (Na3KAl4Si4O16) where the peak intensity was enhanced with the increase in FA content. In short, the XRD analysis explained the effect of silicate, Al and Ca on the derived C-S-H gel and compressive strength of SCAACs. The C-(A)-S-H gel formation was less, that led to the lowering of CS than that of other samples. The low productivity of C-S-H enhancement in the quartz peak intensity and reduction in the compressive strength of SCAACs with the increase of FA contents from 0 to 70%, was clearly evident [65]. However, the results of XRD influenced directly by chemical composition of raw materials (Table 1). Fig. 7 shows images of unmixed SCAAC2 with 30% of FA, SCAAC4 with 50% of FA and SCAAC6 with 70% of FA which clearly indicated the spread of C-S-H gel. The sample with 30% of FA replacement is shown in Fig. 7a and it plainly shows less micro cracks and nonreactive particles than in those samples with 50% and 70% FA, indicating that the increase in FA from 30% to 50% to 70% correlates with the increase in cracking and irregular particles with micro pores in the unblended SCAAC. It could be this that causes the low level of strength compared to other, blended mixtures with 70% of GBFS. Fig. 7b is the sample with 50% of FA replacement and it shows that there are more micro pores due to the hydration process. There is less spread of C-S-H gel in this sample, creating a more even, less compact structure than that of SCAAC2. This observation can be ascribed to a little weakening in the strength of mixes than the control sample. Fig. 7c is the sample with 70% of FA replacement in which it was noticed that the pores increased on a larger scale. More cracking and more pores were seen in this sample. Therefore, the C-(A)-S-H gel will spread completely over the micrograph and create an even, solid structure when there is a low percentage of FA present. Fig. 8 is the FTIR spectra of the SCAAC samples at 28 days of curing. FTIR analysis explained the low strength of FA in the SCAAC. When the FA content was increased, the level of Al and Si also went up and Ca decreased. Thus, C-(A)-S-H and C(N)-A-S-H gels were

The effect of FA replacement of GBFS on water absorption (WA) of SCAAC specimens at 28 days of age is shown in Fig. 9. The WA capacity of studied mixes was increased with the increase in FA content. An increasing FA content from 30 to 70% as replaced of GBFS has increased the water absorption by 6.8 and 9.8%, respectively compared to 6.6% of control sample (100% GBFS). In each mixture containing FA, the results of water absorption has influenced significantly by ratio of FA replaced of GBFS. At various FA levels, water absorption was increased from 6.8% to 7.1, 7.9, 8.5 and 9.8% with increasing level of FA replaced GBFS from 30 to 40, 50, 60 and 70%. As discussed in Section 3.5, the increasing in FA level led to increase the amount of non-reacted silica and made the structure more porous (Fig. 7). Specimens with higher contents of FA revealed higher water absorption, which was attributed to the formation of gel in the binder. The low calcium level affected the re-organization of silicate, thereby reduced the C-A-S-H gel products, increased the pores (Fig. 7) and showed less homogenize structure [66]. 3.7. Drying shrinkage Drying shrinkage (DS) results of SCAACs prepared with various ratio of GBFS replaced by FA presented in Fig. 10. Inverse relationship was observed between DS values and FA content. It was observed from the results that the rate of DS was high during the early ages (up to 28) days and the rate were decreased after this age. The 90 days DS of the SCAACs was varied between 334 and 490 macro-strain. The 90 days DS values of the mixtures with 0, 30, 40, 50, 60 and 70% FA replaced GBFS were discerned to be 490, 451, 431, 376, 354 and 334 macro-strain, respectively. The low DS values were observed with increased content of FA which was ascribed to the existence of highly interconnected capillarylike networks inside the alkali-activated concrete matrix. However, GBFS replacement by the FA reduces the calcium content from the mix as the class F fly ash has a significantly low calcium content. Due to the reduction of calcium content, the rate of hydration of alkali-activated concrete reduces. As a result, FA concrete exhibits lower drying shrinkage compared to the control sample concrete (100% GBFS) [67]. 3.8. Carbonation depth (CD) On various SCAAC specimens containing different percentages of FA replaced GBFS, the carbonation test was conducted. The results of mean CD influenced directly by FA content and the depth increased as the FA replaced of GBFS increase as illustrated in Fig. 11. Thus, the prepared SCAACs revealed better performance throughout (from 0 to 70%) the substitution of GBFS by FA. Compared to control sample (100% GBFS), the CD was increased from 7.5 mm to 7.9, 8.3, 8.8, 8.9 and 9.4 mm with increasing FA level from 0 to 30, 40, 50, 60 and 70% respectively. The CD was influenced by increase FA level from 0 to 70% and led to increase the depth by 25%. It’s known the chemical composition of binder effect

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Fig. 7. SEM images of SCAACs with various FA content.

Fig. 9. Influence of FA replacement for GBFS on the WA of SCAACs. Fig. 8. FTIR of SCAACs with various FA content.

3.9. Resistance to sulphuric acid attack directly on CD, the reduction level of CaO content with increase FA replacement percentage effect negatively on increase the depth to 9.4 mm. In 70% of FA, the pore structure and high porosity of specimen’s impact negatively on the depth and increased to 9.4 mm. It can be explained respecting the existence of non-interconnected porous structures that allowed the access of CO2, leading to diminished emission. The rate of CD of concrete can be primarily influenced by the tortuosity of the porous network together with the chemistry of the binding phases, porosity and carbon dioxide transport as reported by (Basheer et al.) [68].

The effect of FA content (30, 40, 50, 60 and 70%) on the sulphuric acid resistance of SCAACs was examined. Fig. 12 shows the residual CS of alkali-activated specimen’s immersion in 10% of H2SO4 solution for 12 months. The measured CS of all SCAACs was gradually decreased with increasing exposure period. However, the increased level of FA in alkali-activated matrix led to increase SCAAC specimen’s resistance to sulphuric acid attack. The percentage of loss compressive strength was decreased from 74% to 16% with increasing FA content from 0 to 70% respectively. In each level of FA replaced of GBFS, the loss strength evaluated

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Fig. 10. Drying shrinkage of SCAACs with various FA content.

Fig. 13. UPV losses because of 10% of H2SO4 solution exposure.

Fig. 11. Carbonation depth of SCAACs prepared with various GBFS:FA. Fig. 14. Weight losses because of 10% of H2SO4 solution exposure.

Fig. 12. Compressive strength losses because of 10% of H2SO4 solution exposure. Fig. 15. XRD pattern of SCAACs immersion in 10% sulfuric acid solution.

and was observed the percentage dropped to 67, 60, 47 and 26% with 30, 40, 50 and 60% of FA level. When the specimens were exposed to H2SO4 solution, the Ca(OH)2 present in the concrete could react with SO2 4 to form gypsum (CaSO42H2O).

The formation of gypsum led to the spreading out of alkaliactivated network as well as development of extra cracks inside the concretes as observed in the UVP results (Fig. 13). The high

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Fig. 16. SEM of SCAACs immersion in 10% sulphuric acid solution of (a) 30% FA (b) 50% FA (c) 70% FA.

Table 5 Carbon dioxide emission, energy utilization and outlay of GBFS and FA. Materials

CO2 emission (t/t)

Energy (GJ/t)

Cost ($/t)

GBFS FA

0.152 0.012

2.379 0.173

112.45 8.60

calcium in control sample (100% GBFS) specimens compared to other matrixes resulted in the higher gypsum content. Therefore, the higher resistance of high FA content specimen to sulphuric acid attack and loss less than 16% of compressive strength was observed. Similar trend was observed with weight loss percentages (Fig. 14), where the increase in FA content in the alkali-activated matrix led to reduction of the weight loss. A reduction in weight loss from 2.2 to 0.4% was evidenced with increase of FA content from 0 to 70% respectively. Fig. 15 shows the XRD patterns of SCAAC samples which were submerged in 10% of H2SO4 solution for the period of 360 days. The samples which had 30% FA replacement still had clear main phases as well as albite, gypsum, gmelinite and portlandite. It is very evident that the prominent XRD peaks were matched to quartz (SiO2), particularly at 2h values of 26.8°, 40° and 46.2° than

Fig. 17. Carbon dioxide emission of prepared SCAACs.

those obtained before acid exposure. Gypsum (CaSO42H2O) was also found as a new peak at 2h = 12.8° and 31.2° in the SCAACs with 30% FA replacement. The occurrence of a new peak at 21° was

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Fig. 18. Influence SCAACs cost by FA replacing GBFS.

dioxide production and amplified outlay of GBFS. Use of higher GBFS caused higher carbon dioxide emission (0.152 tonne/tonne) than the use of more FA (0.012 tonne/tonne). Alike CO2 emission, the expenditure involved for GBFS was the maximum amongst all the concrete constituents. It was primarily due to the requirement of grinding before the utilization and transportation over long distance (Fig. 3) where an outlay of GBFS was 112.45 $/tonne and FA was 8.6 $/tonne. The less amount of GBFS inclusion in SCAAC matrix was essential to attain the sustainability in terms of low carbon dioxide emission, energy consumption and overall cost. Fig. 17 presents the total carbon dioxide emission of different SCAAC mixtures. The amounts of CO2 were calculated using the values come for individual LCA of each material (GBFS and FA). The results showed that all alkali-activated SCAAC mixtures, prepared with different FA replaced of GBFS (30 to 70) ratios revealed lower CO2 compared to control sample (100% GBFS). All the SCAAC mixtures showed a reduction in gas emission by more than half (28 to 64%). The results illustrated that the carbon emission decreased slightly when FA content in SCAAC mixtures was increased. The carbon dioxide emission was decreased from 73.6 kg CO2-e/m3 to 26.1 kg CO2-e/m3 as the FA level increased from 0 to 70%. These results confirm the promising performance of SCAAC mixtures developed in this study. The use of low molarity of NH and NS content and replacing FA with GBFS leads to an increased carbon sequestration and a reduction of carbon footprint. 3.11. Cost effectiveness

Fig. 19. Energy consumption of prepared AAMs.

assigned to the gmelinite phase in the mixes. There was less noticeable change in the peaks concentration of samples with 70% FA, before and after they were put in the acid. The XRD findings imply that FA replacement increases the concrete’s resilience to sulphuric acid attack. There could be a connection between that and the rate at which SCAAC deteriorates in sulphuric acid. Fig. 16 shows the results of the studies carried out using Scanning Electron Microscopy (SEM). After 360 days in the 10% of H2SO4 solution the microstructures of the matrices close to the outside surface of the SCAAC were analysed. When SEMs of samples containing 30%, 50% and 70% of FA were compared, the SCAAC with 30% of FA showed more cracks (Fig. 16a) and also showed gypsum and gmelinite in the samples (Fig. 16b and c).

3.10. Estimation of CO2 emission Regarding to information presented in Fig. 3 and Table 3, the LCA of each material, CO2 emission, energy consumption and the total cost, of GBFS and FA were estimated and summarized in Table 5. It was found that the preparation stages GBFS needed higher amount of electricity and diesel compared to FA, leading to enhanced cost, CO2 emission and energy expenditure. GBFS required 2.37 GJ/tonne and FA needed 0.17 GJ/tonne of energy, which is directly proportional to energy expenditure, carbon

The influence of FA in place of GBFS was studied in term of binder cost of the blended SCAACs (Fig. 18). Utilization of FA at elevated level (70%) as substituent of GBFS led to reasonable cost reduction. The materials price by weight depended on the LCA (Table 5) which had direct influence on the ultimate outlay of blended SCAACs mixes. The binder cost was reduced from 54.4 $/ m3 to 39.4, 34.2, 29.3, 24.3 and 19.2 $/m3 with rise of FA level from 0% to 30, 40, 50, 60 and 70% as replacement agent of GBFS, respectively. Comparing to control sample, all SCAAC mixes produced FA of 30% and more, revealed lesser price tag than control sample. In short, implementation of FA in place of GBFS contributed as true binders’ development with sustainability. 3.12. Energy consumption Energy consumption require of each SCAAC mixture prepared with various levels of FA replacing GBFS presented in Fig. 19. Depended on LCA and energy expenditure of every constituent, the total energy expenditure of every SCAAC mix was estimated. The energy use of studied binder was reduced with the raise in FA level as substituent of GBFS in SCAACs. The energy expenditure was correspondingly reduced to 0.83, 0.72, 0.61, 0.50 and 0.39 GJ/m3 with the increase in FA content to 30, 40, 50, 60 and 70% compared to 1.15 GJ/m3 required for the control sample. Likewise, all SCAAC mixtures revealed much lower energy utilization than the control sample. The low cost involvement of diesel and electricity usage during the life cycle of FA could directly affect the ultimate energy expenditure of SCAACs. The low carbon dioxide emission, energy spending as well as the cost of FA were the primary influential factors towards the achieved sustainability of studied binder.

4. Conclusions This study inspected the effects of GBFS replaced by FA on the sustainability performance of SCAACs. The following conclusions were drawn based on the comprehensive experimental analyses:

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i. The workability efficiency factor of SCAAC was assessed. Increasing percentage substitution of GBFS by FA in range of 40–60 wt% increase the efficiency factor of SCAAC. ii. Partially substitution of GBFS by FA increases the both of initial and final setting time of the SCAAC mixtures. The results revealed that final setting time normally up 60 min for SCC containing up to 70% FA. iii. All SCAAC mixtures with different ratio of GBFS replaced by FA show acceptable compressive strength (47–70 MPa) for construction application. However, the strength trend to decrease with increasing content of FA from 0 to 70%. iv. FA inclusion increased the water absorption and carbonation depth of SCAAC specimens. v. FA reduces the drying shrinkage of concrete specimens and led to enhance the durability. vi. Enhanced the sulphuric acid resistance of SCAAC specimens with increase FA content. vii. Incorporation of FA could significantly reduce carbon footprint by up 60% compared to binder prepared with free FA. viii. The low emission of carbon dioxide, cost and energy expenditure of FA was the primary factors responsible for the achieved sustainability of the proposed binders.

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