Development of high-density geopolymer concrete with steel furnace slag aggregate for coastal protection structures

Construction and Building Materials 248 (2020) 118681

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Development of high-density geopolymer concrete with steel furnace slag aggregate for coastal protection structures Aziz Hasan Mahmood a,⇑, Stephen J. Foster a, Arnaud Castel b a b

Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia School of Civil and Environmental Engineering, University of Technology Sydney, NSW, 2007, Australia

h i g h l i g h t s
SFS aggregate can be accommodated in low-calcium content geopolymer binders.
The high-density geopolymer concrete results in better breakwater stability and reduced material consumption.
The geopolymer concrete developed in this research can be utilised in breakwater armour unit fabrication.

a r t i c l e

i n f o

Article history: Received 3 October 2019 Received in revised form 11 February 2020 Accepted 5 March 2020

Keywords: Coastal protection Breakwater Armour Geopolymer Concrete Steel furnace slag Aggregate High-density

a b s t r a c t Anticipated changes in coastal wave conditions due to various climate change impact scenarios along coastlines may expose coastal protection structures to greater wave energies and higher damage rates than designed for, especially during episodic storm events. Some existing coastal breakwaters need upgrading to withstand the projected conditions. Breakwater armour unit design equations and physical model tests predict a large gain in stability with a modest increase in the armour material density and indicate reduced armour unit size requirements when utilising high-density concrete. In this study, a high-density geopolymer concrete mix with steel furnace slag (SFS) aggregate was developed based on several trials; the material properties were evaluated for on-site applications under ambient curing conditions. The use of SFS aggregate offers higher bulk density to concrete and mixes were proportioned to achieve good workability and setting time. Most importantly, the fly ash-blast furnace slag blended binder used in this study leads to adequate strength gain in ambient curing and allows the diffusion of the free lime associated with the SFS aggregate into the geopolymer matrix to eliminate the delayed hydration and expansion of the aggregate. This research provides a pathway to both upgradings of existing breakwaters and construction of new structures with a reduction to the carbon footprint in breakwater construction. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Coastal structures including breakwaters are under continuing environmental attack. Moreover, owing to various climate change scenarios, they may be exposed to higher wave energy during episodic storms, noting that wave energy is a square function of the wave height [1,2]. Under current wave climate conditions, breakwaters and seawalls armoured by rock or concrete units require regular monitoring and maintenance and with anticipated changes to the coastal wave climate, these structures would be exposed to higher rates of damage. To adapt to changing conditions, some breakwaters will need to be upgraded. One possible upgrade pathway based on the ⇑ Corresponding author. https://doi.org/10.1016/j.conbuildmat.2020.118681 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

breakwater design equations and physical modelling is to place high-density armour units in layers on top of existing breakwater rubble foundations to ensure overall stability to the structure against stronger wave actions [1,3,4]. It is recognised that the individual armour unit mass required (W) can be predicted using Hudson’s Eq. [3], which takes into account several parameters including the relative density of the armour unit material (Sr = qr/ qw), design wave height (H), breakwater slope (a) etc., and is given as:

W¼

qr H3 K d ðqr =qw  1Þx cot a

ð1Þ

where qr is the armour unit density and K d is Hudson’s damage coefficient. Hudson estimated x = 3, which was later analysed and verified by van der Meer [5] through an extensive physical modelling investi-

A.H. Mahmood et al. / Construction and Building Materials 248 (2020) 118681

gation of breakwater stability in relation to several parameters including number of waves, wave period, relative mass density and stability number. A recent study by Howe and Cox [1] validates x = 3 for Hanbar armour units, a popular unit developed by NSW Public Works and extensively employed on Australian East coast. The Hudson’s armour unit design equation relates the armour mass required as an inverse cubic relation to the relative armour density and, thus, theoretical estimates dictate a significant reduction in armour size, and subsequent increase in armour stability, with a small increase in the material’s density. The theoretical estimates have been verified in several physical model tests. Ito et al. [6] investigated the effects of density of concrete armour units called Tetrapods (ranging from 1820–4270 kg/m3) on the stability of breakwaters based on physical model tests and concluded that high-density armour units have higher stability when compared with the same size units made with normal-density concrete. Moreover, Ito et al. [6] established that a structure could resist higher wave heights with high-density armour units. Zwamborn [7] studied breakwater stability for Dolos armour units of densities 2300 kg/m3, 2410 kg/m3 and 2570 kg/m3 and reported improvements in Dolos stability when using high-density armours as predicted by Hudson’s equation. Ito et al. [8] studied the effects of armour unit density on the armour layer thickness and concluded that the relative layer thickness is an inverse function of the armour unit density. Thus, with high-density concrete, it is possible to reduce the armour unit size and layer thickness, still maintaining the stability of the breakwater. Triemstra [9] studied the use of high-density concrete in armour units and determined that high-density concrete units offer similar or more stability to breakwaters. Based on physical modelling, Van Gent et al. [10] suggested that an increase in density from 2500 kg/m3 to 4000 kg/m3 can reduce the armour unit size by a factor of five, which potentially saves concrete raw materials. The theoretical estimates for Hanbar armour units were validated by Howe and Cox [1] when they compared high-density units with conventional units as illustrated in Fig. 1. The highdensity units had similar stability (in terms of damage percentage) to conventional units as shown in Fig. 2, although the high-density units had almost half the mass of conventional units. The literature reveals that high-density armour units can also result in reduced cement requirements, material and placement costs, and overall carbon footprint [1,6]. Moreover, the required individual armour unit size and mass are greatly affected by the wave height as it is a cubic function of the wave height,H. The predicted rise of mean sea levels during storm events (due to surge) leads to higher wave heights, higher storm intensities and, consequently, greater wave energy [11]. Thus, replacement armour units will need to be either of greater weight (and size), at the same normal density as currently employed, or of higher density if port shorelines are to be adequately protected. To this extent, designing of high-density armour units is a necessity and modest increase in material density results in a gain in the stability [4].

14 High density concrete 12 10 Damage (%)

2

8 6 4 2 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Hs/ΔDn50 Fig. 2. Results of physical model testing by Howe and Cox [1].

Though quarried rocks have always been used in armouring breakwaters, often, the required mass of armour units exceed the upper limit of locally available rock sizes. Also, difficulties in finding, quarrying and transporting naturally available heavy rocks has led to the use of manufactured concrete armour units as an alternative to natural stone. Concrete units can be fabricated in different shapes to ensure better interlocking between adjacent units. Since concrete armour unit size is dependent on the concrete’s density, as suggested by Eq. (1), the use of high-density concrete is a potential alternative to increasing armour weight leading to a better structural stability of the breakwater. The current armour unit construction practice utilises Portland cement concrete with naturally available aggregates resulting in a density of about 2200–2350 kg/m3. The increasing demand for high-density concrete in making armour units leads to designing an innovative concrete material with higher than conventional density and satisfactory material properties for possible on-site applications. Since aggregates contribute to more than two-thirds of concrete volume, a reasonable pathway in fabricating highdensity concrete is utilising high-density aggregates such as steel furnace slag (SFS) aggregate in armour unit concrete. SFS is a dense, non-ferrous by-product of steel making industries; it is a complex combination of silicates and oxides that solidifies on cooling and has a density of approximately 20–25 per cent greater than that of natural stones. Although the use of SFS as a filler material in riverbank stabilisation, road bases and sub-bases has been reported [12–15], the majority of the produced SFS is still disposed of and stored in open disposal sites. It is still treated as a waste material. Research on sustainable alternatives for natural aggregates and the use of SFS aggregate in construction has been conducted over a few decades [16–20]. The comparable or even superior mechanical characteristics of SFS aggregate concrete [16–21] compared to con-

Fig. 1. Comparison of Hanbars used in physical model testing [1].

A.H. Mahmood et al. / Construction and Building Materials 248 (2020) 118681

ventional concrete may make it a suitable aggregate for unreinforced breakwater armour unit construction. Although SFS offers better concrete performances, its use in construction is still limited due to the free-lime (CaO) and free-magnesia (MgO) associated with it. The free lime in SFS results from the addition of dolomite or calcite fluxes in steel making. This free-lime is not chemically bound to the SFS and, when hydrated, can volumetrically expand up to 90–100%, resulting in about 10% volume expansion in the SFS aggregates [17,22–24]. The delayed hydration of the SFS freelime in hardened concrete can lead to micro-cracks when used in bound structures [17,25,26] and severely affect durability. As a result, SFS cannot be used as an aggregate in Portland cement concrete and an alternative binder system is needed where the free lime can be accomodated. It should be noted that SFS aggregates are porous with a high water absorption capacity [24,27–29] and a certain portion of the hydrated compounds fills up the voids in SFS aggregates before expanding, leading to no apparent expansion in some cases. Also, expansion due to free magnesium is reported to be insignificant even under favourable conditions [28]. Khan et al. [30] showed that it is possible for the free lime in SFS aggregate to diffuse into a binder matrix that has a low calcium content, such as, a low calcium fly ash-based geopolymer concrete system, which is attributed mostly to their chemical interactions. Geopolymer is the result of the reaction of source materials containing aluminosilicates (i.e. fly ash, ground granulated blast furnace slag, etc.) with alkaline solutions (NaOH, Na2SiO3, KOH, K2SiO3, etc.) to produce an inorganic polymer binder. The prominent use of low calcium fly ash in the blend ensures a binder with low calcium content which has the potential to force the diffusion of the free lime into the geopolymer matrix due to the drastic calcium concentration gradient created. However, mixes studied in [30] are limited to applications where heat curing can be applied, such as precast concrete, as heat is needed to reach satisfactory levels of strength and durability for a low calcium-based system. Moreover, the high activator concentration (high alkalinity) of the mixes leads to vigorous deionisation of the aluminosilicates (binders) leading to the early setting of the concrete, limiting the workable time. The low calcium fly ash-based geopolymer concrete may not be applicable to construction projects where members are cured in an ambient environment, as the strength gain in concrete is too slow due to the low calcium content of the binder [31–33]. Thus, a high-density geopolymer concrete mix with a satisfactory setting time, workability, strength in ambient curing condition, and durability needs to be developed. Such a high-density end product can potentially be used in manufacturing Hanbar armour units to be employed on the Australian coast [1,34]. In this study, several geopolymer concrete trials with SFS aggregate were done and a high-density concrete mix was finalised for probable armour unit constructions. In the previous work, the use of only coarse SFS aggregate was successfully explored [30]. In this research, the use of both fine and coarse SFS aggregates in a geopolymer binder is undertaken. The purpose is to develop a workable concrete mix without natural aggregate and with the highest possible density. This investigation led to a mix where SFS aggregate is used in a fly ash-ground granulated blast furnace slag blended binder with a fly ash/slag ratio of 65/35 (by mass). The slag content was sufficient to meet strength gain needs in ambient curing yet proportioned to allow for adequate free lime diffusion from the aggregate into the geopolymer matrix. The fresh, hardened and transport properties of the high-density concrete were measured. Moreover, the microstructure was investigated to examine the accommodation of the SFS aggregate free lime in the geopolymer matrix. The armour unit size requirements using this high-density concrete were also analysed using Hudson’s equation [3].

3

2. Experimental program 2.1. Materials for developing high-density geopolymer concrete Several mixes were trialled to finalise a high-density geopolymer concrete mix design for the construction of high-density armour units. The aluminosilicate precursors used in this investigation are three different sources of low-calcium fly ash: Eraring fly ash sourced from Eraring Power Station, New South Wales, Australia; Gladstone fly ash obtained from Gladstone Power Station, Queensland, Australia; and Vales Point fly ash from Vales Point Power Station, New South Wales, Australia. The ground granulated blast furnace slag (GGBFS) used in this study was sourced from Australian Steel Mill Services, Port Kembla, New South Wales, Australia. The chemical compositions of the raw precursors (binders) are shown in Table 1. All fly ash used in this study are low calcium, Class F fly ash as per ASTM C618 [35] specifications. A mixture of an aqueous solution of sodium hydroxide (NaOH) solution and grade D sodium silicate (Na2SiO3) solution was used as the alkaline activator. The 98% pure NaOH pellets supplied by Ajax FineChem were dissolved in tap water to prepare the NaOH solution of different concentrations. The Na2SiO3 solution obtained from PQ Australia was added to the freshly made NaOH solution for specific modulus ratio (Ms) and Na2O% as per mix proportions. The sodium silicate solution has a chemical composition of Na2O = 14.7%, SiO2 = 29.4% and H2O = 55.9% (by mass) with a modulus ratio (Ms) of 2 (Ms = SiO2/Na2O). Both alkaline activator solutions were prepared, blended together, and cooled to room temperature prior to batching. The coarse and fine aggregates used in this study are SFS aggregates obtained from basic oxygen furnace (BOF) steelmaking and supplied by BlueScope Steel, Port Kembla, New South Wales, Australia. The SFS was crushed to obtain coarse aggregate and SFS fine was used as the fine aggregate. The aggregates were processed by the manufacturer through several wet and dry cycles to reduce the free lime associated with it. The free lime content in the aggregate was measured following ASTM C114 [36] specifications, using an ammonium acetate titration of the alcohol–glycerin solution of uncombined lime with Sr(NO3)2 as an accelerator. Based on 12 samples, the free lime measured averaged 1.6% for coarse and 4.4% for the fine aggregates. The higher free lime content in fine aggregate is attributed to its larger surface area. It should be noted that there is a possibility of the presence of calcium hydroxide in the aggregates and the test method employed in measuring freelime doesn’t distinguish between free-CaO and free-Ca(OH)2. It is assumed that the presence of free Ca(OH)2, if any, can be due to the hydration of the free lime in contact with moisture. The physical properties of the aggregates were evaluated and are presented in Table 2. The SFS coarse aggregate density is 14% higher than that of basalt (~2.80 t/m3) and SFS fine aggregate density is 25% higher than that of Sydney sand (~2.65 t/m3). The high density of the aggregates is due to the presence of iron influxes from steelmaking process [37,38]. Moreover, SFS coarse aggregate abrasion is 15.8% compared to 22% for basalt, which indicates a higher toughness and better resistance to wear. However, the water absorption of SFS fine aggregate is higher than most conventional fine aggregates due to its porous surface texture. The gradation curves for both coarse and fine SFS aggregates are illustrated in Fig. 3. Both aggregate gradations are within the limit set by ASTM C 33 [39] and can be termed as ‘well-graded’. As shown in Fig. 3(a), the SFS coarse aggregate has a nominal maximum aggregate size of about 10 mm. The chemical composition of SFS aggregate obtained by X-ray fluorescence (XRF) test is given in Table 3. There seems to have no significant differences between

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Table 1 Chemical composition of cementitious materials. Chemical composition and physical properties

Eraring fly ash (wt. %)

Gladstone fly ash (wt. %)

Vales Point fly ash (wt. %)

GGBFS (wt. %)

66.56 22.47 3.54 1.64 0.65 0.58 1.75 0.88 0.10 – –

47.9 25.7 14.7 4.11 1.36 0.81 0.67 1.39 0.19 – –

63.48 24.18 2.99 2.42 0.79 0.49 0.99 1.02 0.12 2.35 2.12

31.52 12.22 1.14 44.53 4.62 0.21 0.33 1.03 3.24 0.79 2.80

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 SO3 Loss on ignition (LOI) Specific Gravity

Table 2 Physical properties of SFS aggregates. Aggregate type

SSD density (t/m3)

Oven dry density (t/m3)

Water absorption (%)

Loss in Los Angeles abrasion test (%)

3.19 3.31

3.02 2.98

3.84 11.23

15.8 –

SFS coarse SFS fine

100

100

(b)

90

80

80

70

70

60

60

% finer

% finer

90

50 40 30 20 10

(a)

50

SFS CA gradation

40

ASTM upper limit ASTM lower limit

30 SFS FA gradation

20

ASTM upper limit

10

ASTM lower limit

0 100

10

1

0.1

0.01

Sieve opening (mm)

0 100

10

1

0.1

0.01

Sieve opening (mm)

Fig. 3. Gradation of (a) SFS coarse aggregate, (b) SFS fine aggregate.

Table 3 Chemical composition of SFS aggregates. Chemical composition

SFS coarse aggregate (wt. %)

SFS fine aggregate (wt. %)

SiO2 CaO Al2O3 MgO Fe2O3 TiO2 Na2O K2O P2O5 Mn3O4 SO3 Loss on ignition (LOI)

13.21 40.69 2.57 9.84 24.47 1.16 0.16 0.01 1.90 3.93 0.11 1.3

11.98 38.14 2.76 9.73 24.94 0.98 0.16 0.01 1.81 3.54 0.12 5.32

the coarse and fine aggregates in terms of chemical composition as they are obtained from the same source. 2.2. High-density geopolymer concrete trials and mix design Ten high-density geopolymer concrete mixes with varying fly ash/GGBFS ratio, activator concentrations, and water content were studied and the workability, flowability, setting time, strength gain

in an ambient environment, density and transport properties were monitored to finalise a mix design for probable bulk productions and practical applications. Details of the trial mixes are given in Table 4. The trials varied in terms of fly ash/GGBFS ratios, activator concentration, fly ash source and the use of admixtures. Two parameters, modulus ratio (Ms) and n were used in defining the concentration of the alkaline solutions. Modulus ratio (Ms) is the molar ratio of SiO2 to Na2O and n is the percentage mass ratio of Na2O to the total binder (precursor) in the system. Trials 1–3 used a highly alkaline solution (Ms = 1.17 and n  9.25) to activate the precursors and are predicted to have short workable times. Thus, admixtures were used to improve the setting time: 3% superplasticizer in Trial 1; 3% granular boric acid in Trial 2; and 5% Sika retarder-N in Trial 3. The admixtures were chosen based on the findings in the literature. The literature reports the use of superplasticizer [40], high-range water-reducing admixture [41], retarder [42], phosphoric acid [43] and chemical admixtures such as, calcium chloride, calcium sulphate, sodium sulphate, sucrose, etc. [44] to control geopolymer setting. The admixture dosage is significantly higher when compared to the dosages used in OPC concrete as conventional admixtures in regular dosage do not affect the geopolymer setting [40]. The fly ash/GGBFS ratio for Trials 1–3 was kept fixed at 80/20. Trials 4–10 had variations in modulus ratio (Ms) and n with minor adjustments in water content with a fly ash/GGBFS ratio of 65/35 (by mass) except for trial 5

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A.H. Mahmood et al. / Construction and Building Materials 248 (2020) 118681 Table 4 Mix proportions of high-density geopolymer concrete trials. Unit content (kg/m3)

Materials

Coarse aggregate Fine aggregate Fly ash GGBFS NaOH pellet Na2SiO3 solution Water Admixture Coarse agg./fine agg. Total binder Fly ash / GGBFS ratio Water/bindera Ms = [SiO2]/[Na2O] n = Na2O/total binder

Trial 1

Trial 2

Trial 3

Trial 4

Trial 5

Trial 6

Trial 7

Trial 8

Trial 9

Trial 10

1365 775 310 78 19.96 138.7 55.0 3% super-plasticizer 1.76 388 80/20 0.35 1.17 9.24

1365 775 310 78 19.96 138.7 75.34 3% boric acid solids 1.76 388 80/20 0.40 1.17 9.24

1394 750 340 85 21.91 151.8 99.52 5% Sika N retarder 1.86 425 80/20 0.44 1.17 9.25

1280 690 276 149 5.99 84.2 134.3 – 1.86 425 65/35 0.43 1.5 4.0

1360 725 361.25 63.75 5.99 84.22 127.3 – 1.88 425 85/15 0.41 1.5 4.0

1314 656 276 149 5.99 84.22 131.5 – 2.0 425 65/35 0.42 1.5 4.0

1314 656 276 149 6.73 94.62 120 – 2.0 425 65/35 0.41 1.5 4.5

1314 656 276 149 8.23 115.5 118.5 – 2.0 425 65/35 0.43 1.5 5.5

1314 656 276 149 7.48 105.0 123 – 2.0 425 65/35 0.43 1.5 5.0

1314 656 276 149 9.00 126.0 114.5 – 2.0 425 65/35 0.44 1.5 6.0

Source of fly ash a

Eraring power station

Gladstone power station

Eraring Power Station

Calculated considering the total water including water in the activator solutions.

having the ratio set at 85/15. All trials used Eraring fly ash except for Trial 6 and Trial 7, where Gladstone fly ash was utilised. The Trial 8 mix presented in Table 4 satisfied all the requirements and has further been studied to monitor its fresh and hardened properties and the accommodation of the free lime into the geopolymer matrix to prevent the delayed expansion of the aggregate was investigated in microstructural analyses. This will be termed as the ‘high-density concrete’ from hereon in the text. The concrete mix consists of blended Vales Point fly ash (65%) and GGBFS (35%) binder as an aluminosilicate source. The fly ash used in the characterisation of the high-density concrete is from a different source than the trial mixes to gain confidence in using locally available fly ash sources. The binder is activated by an alkaline solution of NaOH and Na2SiO3 mixed together. The modulus ratio (Ms) and n of the alkaline solution were kept at 1.5 and 5.5% respectively. The water/binder ratio of the concrete mix was calculated to be 0.43 considering all water in the system including the alkaline activator solutions. 2.3. Batching, specimen preparation and testing Similar mixing procedure was followed to batch all the trial mixes which involved dry mixing of the fly ash and GGBFS in a pan mixer to allow proper dispersion of the binder particles. The activator solution cooled to room temperature was then slowly poured into the pan and the paste was mixed for five minutes to allow for adequate dissolution of species in the raw precursors. Aggregates brought to saturated surface dry (SSD) condition were then added to the paste, sequentially adding fine aggregate followed by coarse aggregate. The mix was allowed to blend for further 5–7 min, or until a proper mix was confirmed by visual observations. The overall mixing duration was 10–15 min. After mixing, the fresh concrete was tested for slump, flow spread and fresh density according to AS 1012.3.1 [45], ASTM C 1437 [46], and AS 1012.5 [47], respectively. The spread of the fresh concrete was measured on a circular flow table of 770 mm diameter able to be raised and dropped to initiate concrete flow on the table. A flow mould with a top diameter of 170 mm and bottom diameter of 250 mm was uniformly filled with fresh concrete in two layers, each layer being tamped 20 times with a steel tamper. The top of the mould was made flush with a trowel in a sawing motion and the mould was lifted followed by immediately dropping the table 25 times in 15 s. The diameter of the concrete along the four lines scribed on the tabletop was measured and the

average of the readings was taken as the concrete flow spread. For all trial mixes, a workable time was determined as the time for which, the concrete is workable and easy to compact in preparing specimens. Meanwhile, specimens were cast in cylindrical moulds of 100 mm diameter and 200 mm height according to AS 1012.8.1 [48] specifications. In addition to the above, the paste portion (no aggregate) of the high-density concrete mix (Trial 8 in Table 4) was tested for thermal properties (heat release) and rheological properties (viscosity and yield stress). The heat evolution was measured on a TAM Air Isothermal Conduction Calorimeter at 23 0C. A parallel plate rheometer equipped with serrated test plates (35 mm diameter) was used to measure the rheological properties of the geopolymer paste. Reference cement pastes were also tested to compare the heat evolution and the rheological properties. Concrete cylinder specimens of 200 mm height by 100 mm diameter were cast and sealed and then cured in an environmentally controlled room at a temperature of 23 ± 2 0C and relative humidity of 50% until the day of testing. No heat curing of the specimens was employed to monitor the strength gain in ambient conditions so the results could be of a great benefit when manufacturing armour units on site. The specimens were tested for compressive strength according to AS 1012.9 [49], splitting tensile strength, and elastic modulus according to ASTM C 496 [50] and AS 1012.17 [51], respectively. The high-density concrete was also tested for bulk density, water absorption, and surface resistivity in accordance with ASTM C 642 [52] and AASHTO TP 95 [53], respectively. Microstructural analysis and surface morphology studies of the high-density concrete were undertaken using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) technique was employed to analyse the aggregate-matrix interface and monitor the calcium content around the aggregates. The EDS linescans were carried out to observe the Ca diffusion from the aggregate into the geopolymer matrix. Prior to scanning, SEM samples were cut from cured specimens with a slow-rotating diamond saw. Samples were then mounted in epoxy resin, degassed, and left undisturbed until set. Specimens embedded in resin were then ground and polished for optimum imaging. For EDS analysis, surfaces were coated with carbon in a desk carbon coater. A scanning electron microscope, Hitachi S3400, with an electron energy dispersive spectrometer was used for elemental analysis. The analysis was carried out at 20 kV beam strength with a 10 mm working distance.

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3. Results and discussions 3.1. Observations and results of trial mixes The properties of the trial mixes are compiled in Table 5. Trials 1, 2, and 3 respectively investigated the effects of superplasticizer, boric acid, and retarder in extending the setting time of geopolymer concrete with an apparent higher alkaline activator dosage in contrast to the rest of the trial mixes. These three mixes had a 20% GGBFS blend in the binder. The greatest drawback of the mixes was the workable time, which was not enough to be used at construction sites. Moreover, it was evident that cement chemical admixtures are not appropriate in geopolymer concrete due to the differences in cement and geopolymer physiochemistry, even at a higher dosage. Further research needs to be carried out to develop geopolymer concrete admixtures. The compressive strengths for Trials 1, 2, and 3 measured were 30.1 MPa, 24.8 MPa, and 23.8 MPa, respectively. In the later phase of the trials, a different approach was taken to ensure a longer workable time but with a fair gain in compressive strength at an early age in ambient curing. This was done by proportioning fly ash/GGBFS ratio and lowering the alkaline activator concentration to control the accelerated activation of the binders resulting in reasonably long workable time. As this study was specifically aimed at developing a highdensity concrete mix for breakwater armour units, critical parameters investigated were a workable mix with a higher than conventional concrete density. With this view, Trials 4–10 were designed with a modulus ratio (Ms) of 1.5 and n ranging from 4.0–6.0%. The fresh densities measured for these trials ranged from 2560–2630 kg/m3 while the flow diameter of the fresh concrete ranged from 440–640 mm. Trials 6 and 7 used Gladstone fly ash and resulted in the highest fresh densities. It is to be noted that Gladstone fly ash particles are about 5–6 lm in diameter, as opposed to most Eraring fly ash particles being 30 lm in diameter and, thus, Gladstone fly ash contributes to a higher density by filling up inter-particular voids in the concrete. The workable time for each trial mix was also determined as the time for which a particular mix remained workable enough for placement and compaction. Although setting time gives an idea of concrete setting, geopolymer concrete can drastically lose workability as it approaches setting. Thus, setting time measurements do not necessarily specify a workable mix for easy handling and pouring in construction. The workable time for Trials 4–9 was more than two hours while trial 10 had a significantly short workable time of only 25 min. An n = 6.0 and Ms = 1.49 was used for trial 10, which indicates a boundary for alkaline solution concentration for fairly long setting time of geopolymer concrete mix. Thus, at a fly ash/GGBFS ratio of 65/35, Ms = 1.5, W/B  0.42, and keeping the

percentage of sodium oxide (Na2O%) less than 6.0 can ensure a geopolymer concrete with long workable time. Specimens from Trials 4–10 were tested for compressive strength, splitting tensile strength, and elastic modulus and results reveal that at a constant Ms, there is an increase in strength with an increase in Na2O%. Based on the fresh and hardened properties of the trial mixes, Trial 8 was further tested for probable on-site application as it satisfied the requirements of a higher density, workable time, and strength. 3.2. Fresh properties of the high-density geopolymer concrete The greatest drawback limiting the application of geopolymer concrete in construction is the workable time, as slag blended geopolymer concrete can exhibit flash setting if the components are not proportioned well. Thus, the slump, fresh density, concrete flow, and setting time of the concrete were measured right after mixing. Although an initial slump of more than 200 mm was measured, it reduced considerably as the fresh mix approached setting in about 90 min. Fresh concrete was sieved through a 4.75 mm sieve and the setting time of the mortar obtained was 90 min, measured following the specifications of ASTM C 807 [54]. Meanwhile, the flow of the concrete was measured to be 470 mm right after mixing. The flow spread was measured every 15 min after mixing to have a better idea of the flowability of the geopolymer concrete over time and data obtained are presented in Table 6. Based on the flow spread data and visual inspection, the concrete was reasonably workable for about 90 min. Moreover, the fresh density was found to be 2610 kg/m3 indicating a gain in density compared to conventional concrete used in fabricating armour units. While discussing fresh properties, it should be noted here that when the concrete initiates setting, as soon as vibration is applied, the mix liquefies and becomes highly fluid and workable which is not usually observed in OPC concrete. Further rheological investigation was undertaken to justify the causes of this phenomenon. 3.3. Plastic viscosity and yield stress Fig. 4 illustrates the shear stress (s) vs the shear rate (c_ ) curve (also known as the hysteresis curve) for the paste portion of the concrete mix in explaining the rheological properties. The hysteresis curve fits the Bingham model (R2 = 0.99) used to describe the rheological behaviour of cement paste as follows:

s ¼ so þ lc_

ð2Þ

where the slope of the curve is the plastic viscosity (l) and the y-intercept is the yield stress (s0 ). The yield stress and plastic viscosity of the paste portion of the high-density concrete were determined using Eq. (2) to be 11.93 Pa

Table 5 Properties of trial geopolymer concrete mixes. Mix ID

Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial

1 2 3 4 5 6 7 8 9 10

FA/GGBFS

80/20 80/20 80/20 65/35 85/15 65/35 65/35 65/35 65/35 65/35

Ms

1.17 1.17 1.17 1.5 1.5 1.5 1.5 1.5 1.5 1.49

n

9.24 9.24 9.25 4.0 4.0 4.0 4.5 5.5 5.0 6.0

W/B

0.35 0.40 0.44 0.43 0.41 0.42 0.41 0.43 0.43 0.44

Fresh properties

Hardened properties

Fresh density (kg/m3)

Flow spread (mm)

Workable time (min)

Compressive strength (MPa)

Splitting tensile strength (MPa)

Elastic modulus (GPa)

– – – 2570 2560 2620 2630 2570 2570 2580

– – – 450 440 640 550 510 530 520

25 20 45 180 180 180 180 150 180 25

30.1 24.8 23.8 27.2 16.7 24.3 32.8 37.4 33.6 36.9

– – – 2.03 1.74 2.15 2.56 2.50 2.42 2.52

– – – 27.8 21.6 28.9 31.2 30.3 25.4 26.4

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Flow spread (mm)

After mixing 15 min 30 min 45 min 60 min 1 h 15 min 1 h 30 min

470 440 405 390 360 350 320

and 5.17 Pa.s, respectively. Plastic viscosity gives an idea of the ‘‘stickiness” of the paste, while the yield stress represents the minimum shear stress required to initiate flow as a result of interconnected flocs being broken down when shear is applied to the sample [55]. The rheological measurements obtained were compared to a reference General Purpose cement paste specified in AS 3972 [56], with a water-to-binder ratio of 0.43, similar to the geopolymer paste. The cement paste plastic viscosity was 0.82 Pa.s and the yield stress measured was 30 Pa. The geopolymer paste studied had a significantly higher plastic viscosity than cement paste which resulted in a viscous concrete mix observed when batching. However, the yield stress of the geopolymer paste was considerably lower compared to cement paste which theoretically indicates a lower net solid-interlocking force [57] and, thus, the paste flows when sheared at the measured stress. A low yield stress could justify the mechanism behind the regain of workability of the mix upon application of vibration as discussed in the fresh properties. The rheological findings of this research could benefit construction crew when batching in bulk. 3.4. Heat evolution Alkali-activation (and geopolymerisation) is exothermic in nature and thus, heat is released from hydration and activation, leading to product formation. To monitor the heat evolution of the high-density concrete developed, the thermal response of the geopolymer paste portion of the mix was measured at 23 0C using an isothermal calorimeter. The calorimeter allows the heat released due to the exothermic reaction of the precursors (binders) with the alkaline solutions to flow out of the calorimeter through a conduction cell, where it is measured. The sample, therefore, remains at a constant temperature. The heat flow and the heat of reaction (cumulative heat) of the geopolymer paste for the first 72 h after mixing are presented in Fig. 5 along with the thermal response of a General Purpose cement paste [56] with the same water-to-binder ratio. The heat signatures of the geopolymer paste are different from that of the cement paste owing to the differences in the overall

3.5. Hardened properties of the high-density geopolymer concrete The compressive strength gain of the high-density geopolymer concrete under ambient curing condition is presented in Fig. 6. The compressive strength reaches 37 MPa in 28 days. The splitting tensile strength measured at 14 days and 28 days were 2.4 MPa and 2.5 MPa respectively, while the elastic modulus of the highdensity geopolymer concrete was 26.7 GPa and 30.3 GPa respectively. There is an increase in both tensile strength and elastic modulus over time, consistent with the gain in compressive strength. 70 Geopolymer paste

60

Heat flow (mW/g solids)

Time elapsed

reaction mechanism of the two binders. Cement paste gives two distinct heat flow peaks – one in the very early age representing the dissolution of the cement particles in water and the initial reactions; the other around 8–12 h of mixing, representing hydration peak due to the formation of hydration products (mostly C-S-H). However, for the geopolymer paste, a single exothermic peak prominently appears in the very early age due to the wetting and dissolution of the solid materials [58] followed by no other easily noticeable peak, unlike cement paste. Although this initial peak is very similar for both geopolymer and cement pastes [59], the physiochemical reactions involved are significantly different resulting in differences in significant features. This can be better understood in the context of the cumulative heat of reaction of the two pastes as shown in Fig. 5(b). The heat evolved in the first 72 h of geopolymerisation is significantly lower than cement hydration, which explains the slower rate of strength gain of most fly-ash based geopolymer concrete in ambient curing (in this case, 23 °C). It should be emphasised that the heat response of the geopolymer paste can significantly change based on changes in the precursors and the alkalinity of the activator solutions used [59,60], especially the temperature conditioning of the calorimeter environment [60]. Therefore, monitoring the heat evolution of mixes can help predict relative strength gains between different trials.

Heat flow (mW/g solids)

Table 6 Geopolymer concrete flow spread.

Cement paste

50 40 30 20

(a)

70 60 50 40

30 20

10 0

10

0

0.2

0.4 0.6 Age (hour)

0.8

1

0 0

12

24

36

48

60

72

Age (hour) Cumulave heat (J/g solids)

300

Shear stress (Pa)

250 200 150

100 50 0

400

(b)

350 300 250 200 150

Geopolymer paste

100

Cement paste

50 0

0

10

20

30

40

Shear rate (1/s) Fig. 4. Hysteresis cycle.

50

60

0

12

24

36

48

60

72

Age (hour) Fig. 5. Heat evolution of the geopolymer paste: (a) Heat flow, (b) Cumulative heat.

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A.H. Mahmood et al. / Construction and Building Materials 248 (2020) 118681

Table 7 presents the SSD bulk density, volume of permeable voids, water absorption and resistivity of the high-density concrete. About 10% increase in the bulk density was achieved (considering the density of conventional concrete to be 2300 kg/m3). Although marginal, this increase in material density can have a significant impact on reducing armour unit size and increasing breakwater stability as the physical modelling experiment results in literature suggests and relates the Hanbar armour unit size requirement as a cubic function of the submerged relative density [1]. This is analysed further in Section 4 using armour unit design equations. The resistivity of the concrete measured indicates a ‘‘high” chloride penetrability level (low resistivity) according to AASHTO TP 95 [53]. A low resistivity of any alkali-activated binder (including geopolymer) concrete can be attributed to the presence of free metallic ions from the alkaline solutions (e.g. Na+ ion) in the pore solution. The free metallic ions are reported to conduct more current in geopolymer concrete compared to cement concrete [33]. It should be mentioned that breakwater armour units usually do not have any structural reinforcement and, thus, a lower surface resistivity is not critical to its structural integrity or its overall durability. In addition, ultrasonic pulse velocity (UPV) through the high-density concrete was measured to range in between 3.8–4.2 km/s indicating ‘‘good” quality of concrete proposed by [61].

Table 7 SSD density, permeable voids and resistivity of high-density geopolymer concrete. SSD bulk density (kg/m3) Volume of permeable voids (%) Water absorption (%) Surface resistivity (kX-cm) Bulk resistivity (kX-cm)

surface texture of the aggregate [27,64,65] and also to the chemical interaction between the aggregate and the geopolymer paste resulting from the diffusion of the free lime [30]. 4. Application of high-density geopolymer concrete in breakwater armour units Howe and Cox [1] reports that the value of x ¼ 3:0 can be used for determining Hanbar armour unit size using high-density concrete as estimated in Hudson Eq. [3]. For a specific wave height H, Hudson stability coefficient K d , and breakwater slope a, Eq. (1) can be generalised to study the effect of armour unit density on armour unit size in terms of characteristic height (l) and armour unit weight (W) as follows:

W ¼ kl qr 3

3.6. Microstructural characterisation

2520 20.5 9.0 4.7 14.7

ð3Þ

3

where, kl is a constant given by:

Compressive strength (MPa)

The intensity of calcium around the interfacial transition zone (ITZ) of the SFS aggregates was monitored using energy dispersive X-ray spectroscopy (EDS) technique. Several EDS line scans from the aggregate into the matrix were carried out on the concrete samples as shown in Fig. 7(a) to monitor the possible diffusion of free lime from the aggregate into the matrix and a representative calcium profile is presented with a solid line in Fig. 7(b). For a better understanding of the diffusion, similar EDS scans were also carried out on paste samples having the same binder and activator contents as used in the concrete mix and the calcium profile for the paste only sample is presented with a dotted line in Fig. 7(b). The line scans reveal that the calcium intensity drops steadily at the ITZ and subsequently reaches stability away from the ITZ. Most importantly, when compared to the calcium intensity in the paste samples, a significant difference in the intensities is observed. The higher intensity of calcium away from the ITZ in the concrete samples can strongly be attributed to the free lime of the SFS aggregate being diffused into the geopolymer matrix. This indicates that the free lime has been absorbed in the matrix and thus, the delayed aggregate expansion due to free lime hydration can eventually be avoided [30,62,63]. Moreover, Fig. 7(a) shows microcracks in the geopolymer paste rather than in the ITZ around the aggregates depicting a strong interface between the SFS aggregate and the geopolymer matrix. This strong ITZ can be attributed to the rough

50

46.0 37.0

40 31.0 30

25.6 21.1

20 10 0 1

10

100

1000

Age (days) Fig. 6. Compressive strength gain of high-density geopolymer concrete.

H3

3

kl ¼

kd ðSr  1Þ3 cota

ð4Þ

In Eq. (4), l denotes the characteristic height of the armour unit. If the characteristic heights of normal density (taken as 2300 kg/ m3) and high-density concrete are denoted by lN and lH, respectively, and the relative densities of normal-density and highdensity concretes are given by SN and SH, respectively. The size of armour unit for a high-density concrete relative to normal density concrete is obtained from Eq (4) as:

lH SN  1 ¼ l N SH  1

ð5Þ

In Eq. (5) lH/lN is termed the armour unit size ratio [8] and it is inversely proportional to the relative density of the concrete. Moreover, if the densities of normal-density and high-density concrete are represented by qN and qH , Eq. (1) can also be generalised to account for the effects of armour density on the overall armour mass requirement for a particular wave height H, Hudson stability coefficient, Kd, and breakwater slope a as follows:

WH q ¼ H WN qN

 3 SN  1 SH  1

ð6Þ

By Eqs. (5) and (6), the size ratio lH/ln and weight ratio WH/WN equal 0.85 and 0.68, respectively for the geopolymer concrete of the density produced in this research; meaning this concrete allows the characteristic height of armour units to be reduced by 15 per cent and the mass by 32 per cent. For an increase in density to 2600 kg/m3 these become 20 per cent and 40 per cent, respectively. Thus, there is a great potential of this concrete to be used in upgrading existing breakwaters with denser armour units with lower material demand because of the lower mass requirements. Based on the satisfactory material properties and physical modelling data, the high-density concrete developed in this research has later been upscaled for field applications. With the cooperation of industry partners, 13 high-density geopolymer concrete Hanbar units were cast and employed on the Northern breakwater at NSW Ports’ Port Kembla Harbour [62] to restrengthen it against stronger

A.H. Mahmood et al. / Construction and Building Materials 248 (2020) 118681

9

Fig. 7. Diffusion of calcium from SFS aggregate into the geopolymer matrix.

wave scenarios due to climate changes. Specimens were collected and are being tested for material properties and are monitored for durability in saline water. This research introduces a pathway to the sustainable use of industrial by-products including steel furnace slag, fly ash, ground granulated blast furnace slag etc. in construction of high-density Hanbar units. 5. Conclusions The results of the laboratory tests conducted on high-density concrete and subsequent microstructural analyses confirm that SFS aggregate geopolymer concrete can be a promising material in manufacturing high-density armour units for breakwaters. While the trials reveal that cement admixtures are not appropriate in geopolymers, they highlight the necessity of proportioning the geopolymer constituents properly in achieving desired performance of geopolymer concrete. The high-density concrete developed in this study satisfies the requirements of workability, density and strength in fabricating armour units. Using this material, there is a great potential to significantly reduce armour unit size keeping the structural performance constant, with a size reduction of as much as 30 per cent for a density of 2500 kg/m3, and 40 per cent for a density of 2600 kg/m3. This could allow the

construction of new coastal structures with reduced material requirements, placement cost, and overall footprint, or the retrofitting of concrete armour structures to provide increased stability while retaining good interlocking with existing armours. Use of no cement in the concrete coupled with reduced material requirements has the potential to reduce the carbon footprint associated with manufactured breakwater armour units. Moreover, great compatibility between SFS aggregate and low calcium fly ash based geopolymer binder was determined due to their chemical interactions. Diffusion of the free lime associated with SFS aggregate into the geopolymer binder minimises the risk of delayed expansion. Besides breakwater armour units, highdensity geopolymer concrete can also be incorporated in gravity structures that require high-density concrete. Most importantly, the use of industrial by-products as both binder and aggregates provide the lowest possible embodied carbon construction material which leads to a sustainable adaptation pathway to climate change conditions. CRediT authorship contribution statement Aziz Hasan Mahmood: Writing - original draft, Conceptualization, Methodology, Formal analysis, Investigation. Stephen J.

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Foster: Supervision, Resources, Conceptualization, Funding acquisition, Writing - review & editing, Project administration. Arnaud Castel: Supervision, Resources, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was funded by the CRC for Low Carbon Living Ltd. (grant number RP1020) supported by the Cooperative Research Centres program, an Australian Government initiative. The support of the CRC is acknowledged with thanks. References [1] D. Howe, R.J. Cox, Using high-density concrete to enhance the stability of armour units, Coasts and Ports 2017 Conference, Cairns, Australia, (2017), pp. 629–632. [2] F. Sabatier, US Army Corps of Engineers, Coastal Engineering Manual (CEM), Engineer Manual 1110-2-1100. US Army Corps of Engineers, Washington DC (6 volumes), Méditerranée. Revue géographique des pays méditerranéens/J. Mediterr. Geogr. (2007), pp. 146. [3] R.Y. Hudson, Laboratory investigation of rubble-mound breakwaters, J. Waterways Harbors Div. 85 (1959) 91–121. [4] C. Li, R.J. Cox, Stability of Hanbars for upgrading of breakwaters with sea level rise, Coasts and Ports 2013: 21st Australasian Coastal and Ocean Engineering Conference and the 14th Australasian Port and Harbour Conference, Engineers Australia, Barton, ACT (2013), pp. 477–482. [5] J.W. Van der Meer, Stability of breakwater armour layers—design formulae, Coast. Eng. 11 (1987) 219–239. [6] M. Ito, Y. Iwagaki, H. Murakami, K. Nemoto, M. Yamamoto, M. Hanzawa, Stability of high-specific gravity armor blocks, 24th International Conference on Coastal Engineering, Kobe, Japan, (1994) pp. 1143-1156. [7] J.A. Zwamborn, Dolos Packing Density and Effect of Relative Block Density, 16th International Conference on Coastal Engineering, Hamburg, Germany, (1978), pp. 2285-2304. [8] M. Ito, Y. Iwagaki, H. Murakami, K. Nemoto, M. Yamamoto, M. Hanzawa, On the effect of 2-layer thickness by high-specific gravity armor blocks on wave reflection, Coast. Eng. (1998) 1625–1637. [9] R. Triemstra, The Use of High Density Concrete in the Armourlayer of Breakwaters, Delft University of Technology, Netherlands, 2000. [10] M. Van Gent, K. d’Angremond, R. Triemstra, Rubble mound breakwaters: Single armour layers and high-density concrete units, Breakwaters, Coastal Structures and Coastlines: Proceedings of the International Conference Organized by the Institution of Civil Engineers, Thomas Telford Publishing, London, UK, (2002), pp. 307–318. [11] Climate change 2014: synthesis report. Contribution of working groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzerland (2014). [12] B. Fronek, P. Bosela, N. Delatte, Steel slag aggregate used in portland cement concrete: U.S. and international perspectives, Transp. Res. Rec. 2267 (2012) 37–42. [13] National Slag Association. Steel Slag: A Premier Construction Aggregate. http:// www.nationalslag.org, Coatesville , PA 19320. [14] W. Xuequan, Z. Hong, H. Xinkai, L. Husen, Study on steel slag and fly ash composite Portland cement, Cem. Concr. Res. 29 (1999) 1103–1106. [15] A. Zhang, Study on Step-recycling Use of Metallurgy Slag, Xian University of Architecture and Technology, 2005. [16] X. Huang, Z. Wang, Y. Liu, W. Hu, W. Ni, On the use of blast furnace slag and steel slag in the preparation of green artificial reef concrete, Constr. Build. Mater. 112 (2016) 241–246. [17] A.S. Brand, J.R. Roesler, Steel furnace slag aggregate expansion and hardened concrete properties, Cem. Concr. Compos. 60 (2015) 1–9. [18] X. Yu, Z. Tao, T.-Y. Song, Z. Pan, Performance of concrete made with steel slag and waste glass, Constr. Build. Mater. 114 (2016) 737–746. [19] N. Palankar, A.U. Ravi Shankar, B.M. Mithun, Durability studies on eco-friendly concrete mixes incorporating steel slag as coarse aggregates, International Journal of Sustainable, Built Environ. 4 (2015) 378–390. [20] T.U. Mohammed, M.N. Rahman, A.H. Mahmood, T. Hasan, S.M. Apurbo, Utilization of steel slag in concrete as coarse aggregate, 4th International Conference on Sustainability of Construction Materials and Technologies (SCMT4), Las Vegas, Nevada, (2016), Paper No. 184. [21] J. Saravanan, N. Suganya, Mechanical properties of concrete using steel slag aggregate, Int. J. Eng. Inventions 4 (2015) 7–16. [22] B. Erlin, D. Jana, Forces of hydration that can cause havoc in concrete, Concr. Int. 25 (2003) 51–57.

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