Greenhouse gas emissions of different fly ash based geopolymer concretes in building construction

Accepted Manuscript Greenhouse gas emissions of different fly ash based Geopolymer concretes in building construction

Malindu Sandanayake, Chamila Gunasekara, David Law, Guomin Zhang, Sujeeva Setunge PII:

S0959-6526(18)32672-6

DOI:

10.1016/j.jclepro.2018.08.311

Reference:

JCLP 14095

To appear in:

Journal of Cleaner Production

Received Date:

11 December 2017

Accepted Date:

29 August 2018

Please cite this article as: Malindu Sandanayake, Chamila Gunasekara, David Law, Guomin Zhang, Sujeeva Setunge, Greenhouse gas emissions of different fly ash based Geopolymer concretes in building construction, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro. 2018.08.311

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ACCEPTED MANUSCRIPT Greenhouse gas emissions of different fly ash based Geopolymer concretes in building construction Malindu Sandanayakea*, Chamila Gunasekarab, David Lawb, Guomin Zhangb, Sujeeva Setungeb a

School of Engineering and Science, Victoria University, Melbourne, VIC 3011, Australia of Engineering, RMIT University, Melbourne VIC 3001, Australia

b School

*Corresponding author, email: [email protected], Tele: +613 99195746

ACCEPTED MANUSCRIPT 1 2

Greenhouse gas emissions of different fly ash based Geopolymer Concretes in Building Construction

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ABSTRACT

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Replacing virgin materials with recycled or sustainable materials to reduce energy consumptions and emissions is the focus of

6

contemporary research to reduce building related emissions. Geopolymer concrete produced using 100% fly ash is a similar

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sustainable construction material capable of replacing Portland Cement (PC) concrete. As a replacement for PC, fly ash seems

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to be a sustainable solution, however the benefits from the production process of fly ash geopolymer (FAGP) concrete is the

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subject of considerable debate. In addition, factors such as local availability and transportation issues could potentially increase

10

the emission profile of FAGP concrete. Thus, this study aims to evaluate the emission profiles for different types of fly ash in

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Australia considering availability and transportation. A case study and a scenario analysis are also presented to demonstrate

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the factors that affect the Green House Gas (GHG) emission profile of FAGP manufacture. The results indicate that to the

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GHG emission profile for FAGP concrete changes considerably based on the material availability, transportation

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and mix design. Alkali activators and elevated heat curing processes also significantly contribute the total GHG

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emissions of FAGP concrete production. The results further signify that the case study location could influence

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the employment of FAGP concrete in terms of GHG emissions. Further studies are encouraged on optimizing the

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cost, GHG emissions, availability and strength characteristics to strike a balance between in sustainability for

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selecting the best FAGP type for construction. The results also provide the initial factors to be considered in

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developing a guideline for employing sustainable materials in the building industry.

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Keywords: Greenhouse gas emissions, Fly ash Geopolymer, Alkali activated slag, Construction, Building

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1. Introduction

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The global sustainability focus is converging towards carbon-neutral communities within the next

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decade and the race to achieve more environmental friendly products and process continues [1-3]. With

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this intention, new standards and policies have been initiated in many countries to achieve carbon-

25

neutral cities. However this is a distant task without achieving a sustainable building industry as it is

26

responsible for 40% of the greenhouse gas (GHG) emission and 60% of the wood harvest and 50% of

27

the water consumption [4]. 1

ACCEPTED MANUSCRIPT 1

Previous research studies have emphasized that construction materials contribute a significant

2

portion of the life cycle emissions of a building [5-7]. Amongst materials, the majority of the

3

environmental emissions are split among concrete and steel due to high energy requirements related

4

upstream production processes [8, 9]. However, concrete has been the principal focus of this research

5

due to the possibility of achieving environmental savings by replacing ingredients such as cement,

6

coarse aggregates and fine aggregates in concrete [10].

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Geopolymer concrete is deemed a sustainable material that replaces cement from concrete and thus

8

reducing considerable environmental emissions [11-16]. Geopolymer concrete can be produced using

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waste materials, such as fly ash, activated directly with an alkaline activator, without the presence of

10

PC [12, 13, 17-19]. Fly ash production had increased to 900 million tonnes per year by 2008 and it is

11

anticipated to increase up to about 2000 million tonnes in 2020 [20]. While about 45% of this is

12

being utilized for various purposes including cement and concrete production the balance is disposed

13

in landfills and storage lagoons at significant cost and posing a potential risk to local aquifers due to the

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possible leaching of heavy metals. Thus, usage of fly ash as a source material for geopolymer production

15

is an added benefit in converting a waste product into a useful by-product, conserving landfills and

16

storage lagoons [21].

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In the geopolymerization process, alumina and silica species in fly ash rapidly react with highly

18

alkaline activator solution and produce a three-dimensional polymeric chain and ring structure

19

consisting of Si–O–Al–O bonds. It has been noted that production of 100% fly ash based geopolymer

20

requires heat curing in order to accelerate the fly ash-alkaline reaction [22-24]. The principal

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geopolymer product is sodium-aluminosilicate (N–A–S–H) gel, which governs the properties of low

22

calcium fly ash geopolymer concrete [25, 26]. Geopolymer concretes can achieve comparable strengths

23

to PC and blended cement concretes [19, 27, 28]. Moreover, the durability resistance of geopolymer

24

concretes against elevated temperature [10, 29], sulphate and acids [30-32], carbonation [33, 34] and

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chloride penetration [35-37] has been widely studied, and showed comparable or superior properties to

26

that of equivalent PC concrete. Hence, 100% fly ah Geopolymer concretes can be used to produce

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precast elements in large scale. 2

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Several research studies have argued that downstream transportation and operation effects of a

2

product or process can considerably affect its sustainability [38, 39]. The real environmental savings of

3

using sustainable materials therefore, depends on the life cycle savings of the product used. Thus, the

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objective of the study is to compare the life cycle GHG emissions of different 100% fly ash based

5

geopolymer concretes and the implementation of them in a building project in Australia. A case study,

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consisting of three different low calcium fly ash based geopolymer concretes are compared with normal

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PC concrete, in order to determine the potential environmental savings. The presented results will be

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beneficial in providing insights of the context of utilisation of each geopolymer concrete type in building

9

construction projects. It will also provide guidance in developing environmental policies and guidelines

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to use geopolymer concrete in the construction industry.

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2. Experimental Procedure

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Three types of low calcium, Class F fly ash conforming to AS 3582.1 standard [40], obtained from

13

Gladstone (Type A), Pt. Augusta (Type B) and Collie (Types C) power plants in Australia were used to

14

manufacture the geopolymer concrete. The chemical composition of the three fly ashes, determined by

15

X-ray fluorescence analysis, is shown in Table 1. The particle size distribution and mineralogical

16

composition of the three fly ash, determined by Malvern particle size analyser and X-ray diffraction

17

techniques, are shown in Table 2. Brunauer Emmett Teller (BET) method by N2 absorption was used

18

to determine the fly ash surface area. The alkaline liquid used in the geopolymer production consisted

19

of a mixture of commercially available sodium silicate solution (specific gravity=1.53, Na2O=14.7%

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and SiO2=29.4% by mass), and sodium hydroxide solution (15M). Selection of alkaline activator was

21

based on cost and availability in the market.

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Table 1 Chemical composition of fly ash

Component (wt. %)

Fly ash SiO2

24

Al2O3 Fe2O3 CaO P2O5 TiO2 MgO K2O SO3 MnO

Na2O

LOIa

Type A

47.87

28.0 14.09 3.81 1.81 1.99

0.93 0.62 0.27

0.21

0.41

0.43

Type B

49.37

31.25 4.47 4.80 1.65 2.94

1.28 2.21 0.24

0.04

1.30

0.51

Type C

53.82

29.95 9.24 1.03 1.28 2.19

0.58 0.79 0.34

0.04

0.75

0.63

PC

22.5

0.67 0.51 0.17

2.8

0.10

0.74

a Loss

4.5

0.4 66.3 0.15 0.20

on ignition (unburnt carbon content)

3

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Table 2 Physical and Mineralogical properties

Properties investigated

Gladstone

BET Surface Area, (kg/m2)

Fineness (%)

Al coordination

Collie

2363

1228

1095

at 10 microns

43.1

46.7

40.9

at 20 microns

61.9

62.1

54.6

at 45 microns

82.7

80.2

70.0

71.8

59.5

72.5

Quartz

6.8

29.2

18.2

Mullite

17.9

7.5

8.7

Others

3.5

3.8

0.6

Tetrahedral (AlIV)

54.3

40.2

Octahedral (AlIV)

45.7

59.8

0.43

0.51

Amorphous (%)

Crystalline (%)

Pt. Augusta

Unburnt carbon content (%)

0.63

3 4

Previously reported [37] optimum mix designs for each fly ash geopolymer concrete are

5

used and shown in Table 3. The AM is characterized by the blended sodium silicate and sodium

6

hydroxide solutions as the mass ratio of SiO2 to Na2O in alkaline activator. In all cases, the

7

Na2O dosage (i.e. mass ratio of Na2O content in alkaline activator to fly ash) is fixed at 15%.

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A control PC concrete with a similar binder content (410 Kg/ m3) was used as a comparison. Moreover,

9

the total aggregate in the concrete is kept to 64% of the entire mixture by volume for all mixes. The

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ratio of ingredients (fly ash, chemical activator, aggregate, and water) was calculated based on the

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absolute volume method [41], as a result, the total weight of binder and water was varied to keep the

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volume of material and water/solid ratio (0.37) constant. All geopolymer concrete specimens were heat

13

cured in a dry oven for 24 hours at 80°C temperature. Table 2 summarizes the optimised mix details.

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Table 3 Mix design details (kg/m3)

Concrete

Fly ash Cement (kg) (kg)

Aggregates (kg) Sand

7mm

10mm

Activator (kg) Na2SiO3

NaOH

(Liquid)

(15 M)

Added Water (kg)

*water/solid

ratio

Type A

416

–

699

309

618

292

65

8

0.37

Type B

416

–

699

309

618

292

65

8

0.37

Type C

420

–

706

312

624

241

92

15

0.37

4

ACCEPTED MANUSCRIPT PC

–

410

615

550

550

–

–

220

–

1 2 3

*Water/Cement

4

3. Assessment methodology and Data

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3.1 Emission scope and functional unit

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3.1.1 Greenhouse gas emissions According to the Kyoto Protocol, six groups of greenhouse gases are regulated namely: carbon

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dioxide (CO2), Methane (CH4), Nitrous Oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons

9

(PFCs) and hexafluoride (SF6) [42]. However, emissions related to the construction industry are often

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consequences of fossil fuel combustions [38, 43, 44]. Therefore, the current study considers only CO2,

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CH4 and N2O emissions as GHG emissions. The GHG emission factors for different fossil fuel and

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electricity consumptions are adopted from Australian Greenhouse gas accounts [45]. All the emissions

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are converted in terms of CO2 equivalent (CO2-eq) by conversion through global warming potentials

14

(GWP).

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3.1.2 Functional unit The GHG emission study is divided into two sections. In section 1, the emission factors for different

17

concrete types are compared. Thus, the functional unit is set to kgCO2-eq/ m3 of concrete. Whereas in

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section 2 the functional unit is chosen as kgCO2-eq/ m2 of the building floor area to maintain uniform

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comparison of emissions across the building.

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3.1.3 Quantitative approach According to ISO 14040 and 14044, Life Cycle Assessment (LCA) is a comprehensive technique

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that can assess a range of environmental impacts in a product or process along its life cycle. However,

23

based on the objective and the scope of the study, the focus can be limited to several emissions and

24

specific life cycle stages [6, 46-48].

ratio of 0.54 is used for PC concrete Note: Water=Mass of water contained in Na2SiO3, NaOH and added water. Solid= Mass of fly ash and solids contained in Na2SiO3 and NaOH solution.

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Input-output, process and hybrid are the three major quantitative approaches used in LCA studies

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[49-52]. Selection of the appropriate quantitative approach depends on data availability, objectives and

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limitations of the study. Input-Output and process are top-down and bottom-up approaches respectively

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that can be used to evaluate emissions of a product life cycle. Input-output approach is more suitable 5

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when upstream information is not available and process based approach is suitable when comprehensive

2

data is available for the specific process considered. Hybrid approach can be both input-output hybrid

3

and process hybrid based on the quality and the amount of data available for the process. Hybrid

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analyses are more comprehensive and are a combination of both input-output and process based

5

approaches. A process based quantitative approach was selected since the current study considers two

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different process stages (manufacture and construction) separately and sufficient data is available for

7

process modelling.

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3.1.4 Emission calculation models Total GHG emissions (kgCO2-eq) can be estimated from the following equation.

Etot 

n

E

m ,GHG

(1)

m ,1

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Etot is the total GHG emissions and Em,GHG is the GHG emissions from the mth emission source. m represents either materials or transportation in the current study.

12

Emissions from materials (E1) can be determined from equation (2). Qi is the amount of ith concrete

13

material used in kg and eim is the energy factor or the emission factor for ith concrete material in kgCO2-

14

eq/kg. µi is the waste factor for the ith concrete type when used in the building construction case study.

E1 

 (1   ) * Q i

i

 eim

(2)

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Equation (3) is used to determine GHG emissions from transportation. E2 is the GHG emissions from

17

transportation, EFj is the GHG emission factor for fuel type (j) in kgCO2-eq/MJ and fT is the energy

18

content factor for the transport mode T in MJ/tonne-km, dm is the distance travelled by the vehicle in

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km, wi is the weight of the material i in kg. Different emission factors corresponding to different

20

transportation modes are tabulated in Table 5.

E2  

EF j * f T * d m  wi 1000

(3)

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3.2 System Boundary

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3.2.1 System boundary for concrete manufacture The system boundary for concrete manufacture considered in the current study involves FAGP

4

concrete and OPC concrete manufacture. The system boundary includes emission processes associated

5

with manufacture of ingredients for the three types of concrete. Energy consumption during raw

6

material extraction, transportation to production plant and production process are considered during the

7

emission factor estimation. The system boundaries for concrete manufacture are shown in Figure 1.

8 Alkali Activator Production Production & & Transport Transport

Fly ash Transport Transport

FAGP Concrete

Heat Curing

Production Production & & Transport Transport

Portland Cement (PC)

9 10

Production Production & & Transport Transport

PC Concrete

Production Production & & Transport Transport

Fine and coarse Aggregate

Figure 1 System boundary for concrete manufacture

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3.2.2 System boundary for the building case study The second section of the study compares the use of different concrete types in a building

13

construction case study. A comprehensive system boundary for construction stage emission assessment

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should include emissions from materials, emissions from machine and equipment usage and emissions

15

due to transportation. Assuming the equipment operation remain the same, only the emissions from

16

materials and transportation are considered for the case study of construction. Figure 2 highlights the

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system boundary for the building construction case study.

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Figure 2 System boundary for the building construction case study

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ACCEPTED MANUSCRIPT 3.3 Inventory analysis 3.3.1 Emission factor for Na2SiO3 manufacture Sodium Silicate (Na2SiO3) can be manufactured using the furnace route or using the hydrothermal route. However in Australia, the furnace route of production is often used where silicate lumps are dissolved at elevated temperature and pressure levels [53]. Figure 3 illustrates the generic furnace based manufacturing process of Na2SiO3. During the manufacturing process the solution is filtered continuously to maintain particular specifications and to remove residue turbidity. The practical difficulty of obtaining sensitivity information from the local manufacturers persuaded the research team to collect information from overseas manufacturers. Emission factor inventories are adopted from Australian National Greenhouse Gas accounts (NGA) [45]. The energy consumptions are obtained from a large-scale manufacturer who uses sophisticated energy consumption techniques to maintain a sustainable production process. It is assumed that the production process of Australian local manufacturers is the same. The resulting value was found to be 0.78 kgCO2-eq/kg of Na2SiO3. The manufacturer and product details are not made available due to commercial sensitivity of information. The value determined is lower than a previous study conducted on a similar production process [11]. This could be due to the new emission control standards and the novel energy consumption reduction techniques adopted by the manufacturer. However the result falls within the tolerance level suggested by McLellan et al. in their study on costs and carbon emissions of Geopolymer pastes [54].

Figure 3 Flow diagram for Na2SiO3 Manufacture process

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ACCEPTED MANUSCRIPT 3.3.2 Emission factor for NaOH manufacture Sodium Hydroxide (NaOH) is often a by-product of chlorine production by electrolysis. The major mechanism of electrolysis is that electric current is passed through a brine solution. The process of brine purification can be quite poor in energy efficiency if not managed effectively [55]. Use of diaphragm cells is one technique to counteract this issue [55]. Based on the data and information availability, a system boundary for NaOH production is considered to include material extraction, production and transportation to the manufacturing plant. The resulting emission factor was estimated as 1.425 kgCO2-eq/kg of NaOH production. 3.3.3 Emission factors for aggregate manufacture The coarse and fine aggregate required for concrete production is assumed to be acquired and processed from local quarries. Previous studies have emphasized that the impact of fine and coarse aggregates carry less significance over emissions in concrete production [11]. Hence, emission factors of 0.0408 kgCO2-eq/kg and 0.0139 kgCO2-eq/kg are adopted for coarse and fine aggregates respectively [11]. The value can be assumed comprehensive as it considers quarrying and crushing of materials and transportation of raw materials. 3.3.4 Emission factor for superplasticizer Based on the previous literature review studies, the emission factor for superplasticizer was found to be significantly less than the other components used in OPC concrete production. Moreover the quantities of these admixtures in unit volume of concrete production is relatively small. Thus the emission factor for superplasticizer is assumed to be 5.2 x 10-6 kgCO2-eq/kg of material [56]. 3.3.5 Emission factor for cement manufacture Emissions from PC manufacture can vary based on the composition of the limestone used for production and the energy consumption during the production process. Thus, according to various research studies published across the world, the emission factor for PC production varies from 0.7 to 1.0 kgCO2-eq/kg [10, 56]. In Australia, the emission factor value for cement production is adopted as 0.82 kgCO2-eq/kg and thus the same is adopted the current study [10].

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ACCEPTED MANUSCRIPT 3.3.6 Emission factors for fly ash Even though fly ash is a waste product it needs to be cleansed and processed prior using as a cement replacement material in concrete. The emission factor for this process has been reported as a low value but was considered in the assessment process. 3.3.7 Case study specific emission factors for various concrete types Three low calcium fly ashes considered in this study (Type A, B and C) are predominantly available in Queensland, South Australia and Western Australia respectively. Assuming air freight as the transport mode of fly ash to Melbourne the following emission factors are obtained for FA-GP concrete manufacture as shown in Table 4. Air transportation mode was selected because the fly ash for this case study was transported using the air freight mode. The emission factors derived in Table 4 are case study specific as they are modelled for a building construction case study in Melbourne. Table 4 Case study specific GHG emission factors for different concrete types

Type

Concrete type

Available State

A B C -

FAGP FAGP FAGP PC

Queensland South Australia Western Australia Victoria

Transportation distance to Melbourne (km) 1653 655 2730 -

GHG emission factor (kgCO2-eq/m3) 58.01 36.21 107.75 393.60

4. Results and Discussions The resulting cradle-to-gate GHG emissions of different concrete types observed in the case study are illustrated in Figure 4. According to obtained results, type A and type B FAGP concrete records a 4.94% and 9.70% reduction of GHG emissions as compared to PC concrete. Conversely type C FAGP concrete demonstrates a comparative 6.17% increase of GHG emissions to PC concrete. This is due to the increased transportation distances for type C as compared to Types A and B. The high transportation distance of fly ash from Perth to Melbourne significantly increases the GHG emissions from fly ash transportation which results in increase of total GHG emissions. It can also be observed that transportation for all the three types of FAGP have a considerable emission component for fly ash transportation as compared to PC concrete. The major reason for the increase is due to the lack of local availability of fly ash as compared to cement.

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ACCEPTED MANUSCRIPT Moreover, it is also noted that the alkali activation process significantly contributes to the total GHG emissions of FAGP concrete. In addition to the alkali activated process, heat curing temperature also further influences the GHG emission increase. The results from Turner et al. studies highlights that these implications reduce GHG emissions savings of FAGP concrete to 9% as compared to PC concrete [11]. 600.00

GHG Emissions (kgCO2-eq/m3)

500.00 Heat Curing Water NaOH Na2SiO3 10mm coarse aggregate 7mm Coarse Aggregate Sand Fly ash or OPC

400.00

300.00

200.00

100.00

Type A

Type B

Type C

OPC

Figure 4 GHG emission profiles for different concrete types

5. Scenario Analysis The GHG emission assessment revealed that alkali activation and heat curing processes contribute significantly to the GHG emission profiles of GP concrete. Thus it is important to further investigate the sensitivity of each parameter that contributes to the total GHG emissions of each FAGP concrete as compared to PC concrete. In view of these circumstances, the following scenarios are considered to inform the reader on the most sensitive parameters for GHG emissions on concrete production.

5.1 Scenario 1 (SC1) - Different transportation modes for materials The GHG emission results revealed that transportation was a significant factor in the total GHG emissions for the various concretes. In the initial analysis freight is considered as the mode of transport of fly ash. In order to investigate the effect transport mode on total emission three alternative forms of 12

ACCEPTED MANUSCRIPT transports are considered for scenario analysis. Energy content factors for different modes of transports are shown in Table 5. Table 5 Energy content factors for different transportation modes for fly ash

Mode of transport

Energy content factor (MJ/ton-km)

Sea

0.216

Rail

0.325

Road – Transport truck

2.275

The resulting emission factors for the three types of FAGP are displayed in Figure 5. The maximum reduction is observed for sea transport mode. The observation is that type C FAGP emission factor is reduced significantly as compared to road and freight transport modes. Based on sea transport mode all the three FAGP types can be considered as sustainable in terms of GHG emissions. Use of road transport mode for fly ash transportation is found not-sustainable for FAGP concrete production in Melbourne using the three FAGP types. Sea or train transport modes are the preferred modes of transport in consideration of reducing GHG emissions of FAGP as compared to PC concrete production.

Emission factor kgCO2-eq/m3

700.00 600.00 500.00 400.00

OPC - No transprrt Freight

300.00

Road Rail

200.00

Sea 100.00 Type A

Type B

Type C

Type of FAGP Figure 5 Emission factor variations for different transport modes

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ACCEPTED MANUSCRIPT 5.2 Scenario 2 (SC2) – Change of Mix design Previous literature has reported a range of different mix designs for FAGP concrete production. The following scenario aims to investigate the effect of different mix designs on total GHG emissions. Five different mix designs are obtained from previously published research studies and are listed in Table 6. Table 6 Different mix designs considered for SC2

Material

Mix 1 (M1)

Mix 2 (M2)

Mix 3 (M3)

Mix 4 (M4)

Mix 5 (M5)

Fly ash (kg)

408

494

444

309.9

350

Coarse Aggregate (kg)

1202

858

1190

1204

1200

Sand (kg)

647

691

793

648.4

645

Na2SiO3

41

99

106

59

103

NaOH

103

99

42

27.7

41

Added water (kg)

26

-

-

83.7

20

[11]

[57]

[57]

[58]

[10]

Reference

During the scenario analyses the variation of compressive strength and other characteristics are not considered. Thus it is assumed that the desired characteristic strength can be achieved through fly ash types A, B and C for all the mix designs. Energy consumption for heat curing is assumed to be a linear function of the fly ash quantity. This is because there is lack of information in the previous studies on heat curing. The results indicate that optimisation of the mix design can minimize GHG emissions considerably. Further studies are encouraged on investigating the opportunities to optimise the mix design by reducing the alkali activated materials. All the mix designs considered in SC2 demonstrated a reduction of GHG emissions as compared to the original mix design. Mix 4 and Mix 2 recorded the lowest and the highest emissions with range of 190.74 to 323.91 kgCO2-eq/m3 respectively. The deviation is because the alkali activator quantities are relatively smaller as compared to the original mix design. This confirms the careful optimisation of the alkali activator quantity could reduce the GHG emission without compromising the strength characteristics.

14

ACCEPTED MANUSCRIPT 450 400

GHG emissions (kgCO2-eq/m3)

350 300 250 200 150 100 50 0 Mix 1

Mix 2

Mix 3

Mix 4

Mix 5

Original

Figure 6 GHG emissions for different mix designs in kgCO2-eq/m3

5.3 Scenario 3 (SC3) - Variation of inventory information Different studies have used different inventories in comparing FAGP concrete with emission factors with PC concrete. These inventories could change due to factors such as approximations, material availability and compositions, manufacturing technique, energy consumption, type of energy/ fuel used, mix design proportions and transportation. This scenario aims to further investigate the deviation of GHG emissions as a result of these inventory changes. Based on the significant GHG emission contribution of each material/process observed for FAGP concrete, alkali activators and heat curing were chosen for this scenario. Table 7 highlights the inventory range considered for each material/element used in different concrete types. Heat curing inventory range was determined to include emission factors of different states in Australia and energy consumption of different sources. The most likely distribution for each inventory is decided based on curve fitting and goodness of fit. The output was revaluated with 100,000 iterations to obtain the sensitiveness of the inputs in the material GHG emission model. The modelling was conducted for the original mix design by keeping the other variables as constant.

15

ACCEPTED MANUSCRIPT Table 7 Inventory (emission factor) for different materials used in concrete

Material

Inventory Range

Most likely distribution

References

Na2SiO3 (kgCO2-eq/kg)

0.75 – 1.75

Weibull

[46, 53]

NaOH (kgCO2-eq/kg)

1.0 – 1.915

Normal

[36]

Heat Curing (kgCO2-eq/m3)

14.8 – 45.3

Linear

-

Heat Curing

NaOH

Na2SiO3

GHG emission variation (kgCO2-eq/m3) Figure 7 GHG emission variations of FAGP concrete for different inventory information

The results indicate that Na2SiO3 emission factor is the most sensitive parameter among the three while heat curing was observed the least sensitive. The difference between the minimum and the maximum as a result of Na2SiO3 emission factor variation is almost 100%. This signifies the importance of accurate determination of the energy consumption process of the alkali activators manufacturing process. Moreover, further research should focus on minimising energy consumption and finding sustainable production techniques of alkali activators. 6. Case study – Practical Implementation of different concrete types in a building The preceding sections conducted in-depth analyses on factors that contribute to GHG emissions of FAGP concrete production. The following case study aims to further investigate the GHG emissions of 16

ACCEPTED MANUSCRIPT materials when implemented in a building case study. The case study is a commercial building construction located in Melbourne Central Building District (CBD). The material and transportation details of the case study is adopted from a previous research study [44]. Since heat curing is essential for FAGP production with 100% FA, it can only be used as prefabricated concrete. Table 8 highlights the material and transportation details of the case study considered. Table 8 Case study details [44]

Construction stage

Material

Amount (tons)

Distance from production to construction (km)

Percentage of materials to the total

Foundation

PC concrete

22266.00

22

18.33

Structure

PC concrete

97300.34

12

80.12

Structure

Pre-fabricated concrete

1892.98

25

1.55

A total GHG emission reduction of 0.03% was observed by adopting FAGP prefabricated concrete as 1.74% of the total concrete usage. The results were re-calculated based on varying quantities of prefabricated compositions to investigate if an optimum percentage of FAGP material composition could be obtained or not. The effect of transportation distance of pre-fabricated materials and use of different GP (with varying emission factors) are also considered in the investigation. As expected a linear reduction in total GHG emissions is observed for varying quantities of prefabrication for a given transportation distance as shown in Figure 8. The first point in the graph (1.74% pre-fabrication) corresponds to the total emissions in the current case study. It is quite interesting to note that even with waste reduction, use of pre-fabrication does not demonstrate a significant GHG emission reduction. This is mainly because the transportation distance is almost double as compared to PC concrete. Figure 9 illustrates the combined effect of transportation and pre-fabrication variation on total GHG emissions. In this sensitivity assessment both the pre-fabrication quantity and the prefabrication transportation distance is varied keeping the total concrete amount and the in-situ transportation distance a constant. Based on the results the total GHG emissions do not significantly increase until the transportation distance is 29 km and a pre-fabricated percentage of 14%. However

17

ACCEPTED MANUSCRIPT beyond that the emissions start to increase rapidly. Thus, these observations further justify the influence transportation distance of materials has on total GHG emissions.

Total Emissions (thousand tonnesCO2-eq)

54.20 54.15 54.10 54.05 54.00 53.95 53.90 53.85 53.80 53.75 53.70 53.65 53.60 1

8

18 28 Prefabrication quantity in precentage

38

48

Figure 8 Total GHG emission variation with pre-fab percentage

Total Emissions (thosand tonnesCO2-eq)

64.00 62.00 60.00 58.00 56.00 54.00 52.00 50.00 5

20

35

50

65

80

Transportation distance for pre-fabricated materials (km) Figure 9 Total GHG emission with variation of pre-fab quantities and transport distance

Figure 10 highlights the total GHG emission variation considering the combined change of emission factor, transportation distance and pre-fabrication amount in the building. The results exemplify that 18

ACCEPTED MANUSCRIPT the total GHG emissions are minimum for a FAGP concrete emission factor between 408 – 412 kgCO2eq/m3 of concrete. The corresponding transportation distance and pre-fabrication quantity range is observed as 20-25 km and 26-30% respectively. This optimum range obtained in the analysis is case study specific and will vary for different case studies based factors such as transport vehicle characteristics, local environment and case study location. The case study specific results indicate that when using Geopolymer concrete in building construction factors such as transportation distance, local availability of materials and pre-fabricated percentage are critical factors to be considered in design stages.

54,200.00 Total Emissions (tonnesCO2-eq)

54,100.00 54,000.00 53,900.00 53,800.00 53,700.00 53,600.00 53,500.00 53,400.00 53,300.00 53,200.00 53,100.00 380 382 384 386 388 390 392 394 396 398 400 402 404 406 408 410 412 414 416 418 420 422 424 426 428 430 432

53,000.00

Emission factor for GP (kgCO2-eq/m3) Figure 10 Total emission variation as a result of combined effects

The results consider only GHG emissions and does not involve the design alterations as a result of pre-fabrication usage in the building. The optimum composition and selection of FAGP and PC concrete should be further subjected to design considerations, availability and cost other site constraints before practical implementation. These calculations are performed for a building construction site located at Melbourne CBD. However, buildings at different locations across Australia, will exhibit different results based on transportation distance and availability of materials. This study highlights the importance of conducting an in-depth analysis prior selecting the composition of FAGP concrete in a building construction. 19

ACCEPTED MANUSCRIPT 7. Assumptions and Limitations Any LCA study will incur assumptions and limitations based on the objectives and the scope of the study. The following analysis was established for a Melbourne based case study. Therefore, emissions related to transportation, manufacturing and erection of concrete members are based on this assumption and is case study specific. The results and conclusions are also relevant to a building construction based in Melbourne. However, the same methodology can be implemented to investigate the emission variations of using different concrete types in various locations. The study is also subjected to the following major assumptions and limitations: 

Emission factor for NaOH and Na2SiO3 production are based on the energy consumption

information collected from local manufacturers are assumed to be accurate 

Emission factors for different concretes were modelled based on locally collected data. These

factors may vary depending on the energy consumptions and the type of energy used during the production of ingredients. Thus, the study states case study specific emission factors for the different concrete considered 

Other effects in concrete such as carbonisation and carbon sequestration are not considered in the

current study due to lack of information and hence the effects are deemed negligible 

The emission factors for each element is modelled using the fuel and energy consumption

information provided by local manufacturers. These emission factors may differ based on the energy source and energy consumption patterns. 

The availability of the three types of flyash employed are limited as the recent closure of power

stations. However, the study is intended illustrate the effect of using different fly ash types. 

Wherever sufficient energy consumption information was not available, emission factors were

adopted from previously published literature relevant to Australian conditions. 

The possibility of practical implementation of different pre-fabrication combinations are not

investigated and provided only for comparative basis 

The comparative studies were executed only based on GHG emissions perspective without due

consideration on long term strength and durability characteristics and cost implications 

The practical aspects such as workability and handling issues of using FAGP are not considered in

the study 8.

Conclusions and Further Research Sustainable concrete is identified as having the potential to significantly reduce GHG emissions

from a buildings’ life cycle. The current study compares GHG emissions associated with four such 20

ACCEPTED MANUSCRIPT sustainable concrete production cases, i.e., three fly ash geopolymers. A process based assessment model was considered to evaluate and compare the significant GHG emission characteristics of various concrete type and their behaviour when implemented in a building construction. The results exemplified that the GHG emissions savings from vary from 4% to 9%. This contrasts with the 80% GHG savings suggested by the majority of the emission studies on GP concrete. However the results are within the range proposed by Turner et al. in their emission study on GP concrete [11]. The major reason is that the majority of these previous studies do not consider the alkali activation process and the energy consumptions during the heat curing process in FAGP production. Another important finding is that the transportation of fly ash significantly contributes to total GHG emissions as they are not available in all the regions across Australia. Therefore, local availability of fly ash could be key factor in implementing FAGP in building construction. The scenario analyses provided critical insights of the factors to be considered in minimising GHG emissions of FAGP concrete manufacture. Availability of fly ash is a critical factor to be contemplated in using FAGP concrete in building construction. This is because transportation emissions significantly contribute to the increase of total GHG emissions in FAGP concrete. In addition, mix design optimisation and accurate inventory analysis are also critical factors that should be given due consideration in order to minimise emission. However in the case of mix design optimisation, structural characteristics should be given more emphasis in the optimisation algorithm. The case study results further justified the factors to be considered in using FAGP in building construction. In an ideal case, a combination of on-site and pre-fabricated concrete types could provide minimum GHG emissions. In addition to material related factors, transportation distance from prefabrication plant to construction site, amount of pre-fabrication can also influence the emissions from materials. For the case study, optimum range of transportation distance, emission factor and prefabrication composition were determined to minimise the GHG emissions from materials. The results did not follow structural or other site specific considerations during the optimisation. All the outcomes unanimously confirmed importance of optimising the different factors prior choosing FAGP as a sustainable material. This is because sustainability of FAGP concrete depends on various factors such 21

ACCEPTED MANUSCRIPT as availability of materials, transportation, optimum mix design and pre-fabrication composition in the building. Moreover, strategies to reduce transporting distance during material transportation and the use of other waste materials should be compared with FAGP concrete to investigate GHG emission variations. The results obtained are case study specific and are relevant to buildings constructed in Melbourne. Further research can also be focused on optimising the cost, strength and emission characteristics to investigate the sustainability of different types of concrete. The conclusions and suggestions are based on the GHG emissions of different concrete types. However, based on other environmental factors such as reuse of waste material, addressing material scarcity and land usage the potential results may differ significantly. Therefore, further studies are also encouraged on optimising these factors to investigate the total sustainability aspects of FAGP concrete as compared to PC concrete.

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ACCEPTED MANUSCRIPT

   

FAGP concrete use in a building was observed 4 to 9% GHG emission savings Optimum use of alkali activated materials in Geopolymer concrete is vital to minimise GHG emissions Transportation is major factor to be considered in minimising GHG emissions from FAGP concrete production Sustainability of FAGP concrete depends on availability of materials, optimum mix design and pre-fabrication composition