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EMS5ARA Advanced Research Learning Literature Review

EMS5ARA Advanced Research Learning Literature Review

Mar 13,23

Question:

Background:

Department of Engineering

EMS5ARA Advanced Research Learning Literature Review

Grades and Assessment Comments

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(Student’s) Knowledge/Understanding

Please provide a letter grade only: i.e. NS, N, D-, D, D+, C-, C, C+, B-, B, B+, A-, A, A+, AA-, AA, AA+ or AAA for each of the above categories (using Rubric in the next page). Although if you provide a fail grade then please also indicate an indicative mark e.g. N (35).

Please provide below summary assessment review comments roughly 100-200 words. These will be provided to the student along with the letter grades above.

Please give your summary here and indicate the marks out of 100.

Assessment task: Literature Review
	POOR (1-4)	AVERAGE (4-6)	GOOD (6-8)	EXEMPLARY (8-10)
OVERALL PRESENTATION & FORMAT (0.75%)	Has not used appropriate format; or has used standard but not included all required components. Poor formatting and inconsistencies in text throughout the report.	Has used appropriate format and included all required components; but formatting of headings and spacing within text has significant inconsistencies	Has used appropriate format, has all required components; formatting of headings and spacing within the text may have some minor inconsistencies	Has used appropriate format and included all required components. Formatting is consistent throughout the document
LITERATURE REVIEW	No evidence of	Evidence of relevant	Evidence of relevant	Evidence of relevant
BODY (6%)	relevant literature	literature review however	literature review with	literature review with
	review, no critical	no critical analysis and	critical analysis and	critical analysis and
	discussion and	discussion.	discussion.	discussion, and outcome
	analysis.			of proper conclusions.
LANGUAGE (1%)	Poor sentence structure, punctuation and inappropriate vocabulary. Language lacks professional conventions. Dot points used instead of paragraph structure.	Ideas are expressed in a professional style, although there may be minor sentence structure, punctuation and spelling errors.	Ideas are clearly expressed in a professional manner, using appropriate vocabulary. There are very few grammar, punctuation and spelling errors.	Ideas are eloquently expressed in a professional manner, using appropriate vocabulary. There are no grammar, punctuation and spelling errors.
TABLES AND FIGURES (0.25%)	Tables and figures are not referred to in the text. Tables and figures are not numbered sequentially. Figures and tables are unclear or unreferenced.	Tables and figures are numbered sequentially. There is some reference to them in the text. Most figures are clear and the tables appropriate.	Tables and figures are numbered sequentially and integrated into the text. Tables and figures are clear and appropriate.	Tables and figures are referred to sequentially and integrated to add value to the text. Figures and tables are clear and appropriate and correctly formatted.
CITATIONS (1%)	References are either poorly cited or not cited in the text at all. They are not all in the reference list. They not formatted correctly in APA system format in either citation or reference list.	References are mostly correctly cited in text and most are formatted correctly in reference list	References are all correctly cited, but may have a few errors in the format of the reference list.	References are all correctly formatted in citations and reference list.
QUALITY OF REFERENCES (1%)	References are of poor quality, generic in nature and show no evidence of wide research. References are not relevant.	References are of adequate quality and show some evidence of relevant research/reading.	References are of good quality and show evidence of relevant wide research/reading.	References are of excellent quality, showing strong evidence of relevant reading/research.

The performance of fly ash as an alkali-activated binder in geopolymer concrete and development of guidelines for mix design of cast-in-situ geopolymer concrete

Authored by Bradley Connolly – 17697347

Under the supervision of Dr D. Ionescu and Dr V. Patel

This document has been developed and submitted in partial fulfilment of EMS4EMP in the Bachelor of Civil Engineering (Honours) Degree at La Trobe University,

Bendigo

June 2017

Abstract

Geopolymer concrete has a number of environmental advantages when compared to Ordinary Portland Cement (OPC) concrete, yet its adoption is delayed by gaps-in-knowledge, particularly for concrete cured in ambient conditions [1]. Fly ash, a waste material, has shown promise to replace OPC as a concrete binder in some circumstances. This yields benefits including the reducing the carbon footprint of infrastructure development. Recent research has found that chemical composition of fly ash reagents is a poor indictor of the performance of geopolymer concrete, instead Zhang et al (2016) have proposed a ‘reactivity index’ to assess the suitability of fly ashes. In this discourse the mechanical properties of fly ash based geopolymer concrete has been examined from available literature. Fly ash based geopolymer concrete is seen to behave in a similar manner to OPC concrete, with differences in magnitude in some parameters and effectively identical in others. The largest gap-in-knowledge is the performance of geopolymer concrete in in-situ applications.

Abstract……………………………………………………………………………………………………………………………….. 2

Table of Contents…………………………………………………………………………………………………………………… 3

List of Figures………………………………………………………………………………………………………………………… 4

List of Equations…………………………………………………………………………………………………………………….. 5

List of Tables…………………………………………………………………………………………………………………………. 6

Literature Review…………………………………………………………………………………………………………………… 7

Environmental Impact of Ordinary Portland Cement…………………………………………………………………. 8

Chemistry of Aluminosilicate Gel…………………………………………………………………………………………… 9

Fly Ash…………………………………………………………………………………………………………………………… 10

Stress-Strain Relationship…………………………………………………………………………………………………… 13

Suitability as Flexural Members…………………………………………………………………………………………… 15

Beams…………………………………………………………………………………………………………………………. 15

Columns………………………………………………………………………………………………………………………. 16

Water content and Workability…………………………………………………………………………………………… 18

Mixing and Placement Requirements……………………………………………………………………………………. 19

Tensile Strength………………………………………………………………………………………………………………. 20

Fire Resistance………………………………………………………………………………………………………………… 21

Drying Shrinkage……………………………………………………………………………………………………………… 22

Creep…………………………………………………………………………………………………………………………….. 24

Conclusion……………………………………………………………………………………………………………………… 25

References………………………………………………………………………………………………………………………….. 26

List of Figures

Figure 1: Greenhouse Gas Emissions of Constituent Materials [1]……………………………………………………… 8

Figure 2: Structure of Sodium-polysialate as proposed by Barbosa, MacKenzie and Thaumaturgo [16]………. 9

Figure 3: Particle size distribution of 5 Australian fly ashes [2]………………………………………………………… 10

Figure 4: SEM images of 5 Australian fly ashes [2]………………………………………………………………………… 12

Figure 5a, 5b and 5c : Stress strain comparison of equation 4 and sample specimens of different composition[24]………………………………………………………………………………………………………………………………………… 14

Figure 6: Failure of Geopolymer composite beams in single curvature [27]……………………………………….. 15

Figure 7: Failure of geopolymer columns [27]…………………………………………………………………………….. 17

Figure 8: Concrete slump versus excess water [24]………………………………………………………………………. 18

Figure 9: 7-day f’c versus Water/Geopolymer ratio of specimens cured at different temperatures [24]……. 18

Figure 10: Compressive strength versus mixing time [24]………………………………………………………………. 19

Figure 11: Compressive Strength vs temperature exposure for a Fly Ash Based Geopolymer [32]…………… 21

Figure 12: Thermal Shrinkage versus exposed temperature of a Fly Ash Based Geopolymer [32]……………. 21

Figure 13: Drying Shrinkage Strain [17]……………………………………………………………………………………… 22

Figure 14: Drying Shrinkage of Heat Cured Geopolymer Concrete compared to predictions as per AS3600-2009 [17], [26]……………………………………………………………………………………………………………………………. 23

Figure 15: Drying Shrinkage of Ambient Cured Geopolymer Concrete compared to predictions as per AS3600-2009 [17], [26]…………………………………………………………………………………………………………………….. 23

Figure 16: Creep of a Heat Cured Geopolymer Concrete [17]…………………………………………………………. 24

List of Equations

Equation 1: Reactivity Index (RI) as proposed by Zhang et al [2]………………………………………………………. 11

Equation 2: SSA [2]……………………………………………………………………………………………………………….. 11

Equation 3: Sauter Mean Diameter [23]…………………………………………………………………………………….. 11

Equation 4: Stress Strain Curve of OPC concrete as proposed by Collins et. al (1993) [3] [24]…………………. 13

Equation 5: Modulus of elasticity (in Megapascals) as per AS3600-2009 [26]……………………………………… 13

Equation 6: Muo as per AS3600-2009 [26]…………………………………………………………………………………… 15

Equation 7: Characteristic Flexural Tensile Strength of Concrete [26]……………………………………………….. 20

Equation 8: Characteristic Uniaxial Tensile Strength of Concrete [26]……………………………………………….. 20

Equation 9: Uniaxial Tensile Strength of Concrete from Indirect Tensile Testing as per AS1012.10-2000 [26]20Equation 10: Characteristic Tensile Strength of Concrete as proposed by Neville (2000) [24]…………………. 20

List of Tables

Table 1: Comparison of geopolymer concrete beam performance and AS3600-2009 [26], [27]………………. 16

Table 2: Comparison of geopolymer concrete beam performance and AS3600-2009 [26], [27]………………. 17

Table 3: Tensile Strengths of Fly Ah Based Geopolymer Concrete [24], [26]……………………………………….. 20

Literature Review

Concrete is the most widely used construction material in the world, with the production of the Ordinary Portland Cement (OPC) binder accounting for an estimated 5% of global carbon dioxide pollution [4]. In 1978 the French researcher, Joseph Davidovits, proposed an alternative concrete binders, ‘geopolymers’ [5]. Such geopolymers involve the formation of an inorganic SOL-gel polymer from the alkalisation of materials rich in silicon oxides and aluminium oxides, that solidifies to form a cement comparable with OPC [6][6], [7]. Fly ash has been proposed as a source material for geopolymerisation, with the mechanical behaviours assessed and a mix design methodologies proposed by numerous researchers [6], [8], [9], [10].

Recent research has assessed the mechanical performance of various fly ashes, proposing chemical composition is a poor indicator of performance [2]. This suggests parameters in previous studies, such as the silica to alkaline ratio or the fly ash to alkaline ratio, are ill founded. It is logical therefore to draw on this new knowledge to propose an alternate mix design methodology considering an empirical ‘reactivity index’ as proposed by Zhang et al (2016). A standard methodology for fly ash based geopolymer formulation which considers the heterogeneous nature of fly ash would be beneficial in enabling full adoption by industry [2], [3].

Environmental Impact of Ordinary Portland Cement

Globally, the production of OPC accounts for between 5 and 7% of CO2 emissions [1]. Australia’s cement industry, accounts for approximately 1.3% of total national emissions [1]. McLellan et al. (2011) suggests adopting geopolymer concrete as a substitute for OPC concretes has the potential to halve the atmospheric pollution of the cement industry in some circumstances. It is important however to look at localised areas on a case-by-case basis for geopolymer concrete adoption, since silica and alumina sources can be significant distances away, resulting in vast transportation distances which may be more detrimental environmentally when compared to OPC concretes [1].

Given geopolymer concretes can utilize fly ash as a source of alumina and silicate, the adoption of geopolymer concrete has added environmental benefits. Fly ash is the residue extracted from coal fired power-plant exhaust gasses [11]. It is mostly landfilled with some being utilized in pavement stabilization or in OPC concretes as a pozzolan [4], [12]. One tonne of OPC emits nearly one tonne of CO2, the reduction in CO2 emissions noted with fly ash based geopolymer concrete is due to the minimal processing of the material required, with the main pollutants been attributed to the transportation of the fly ash and the production of the alkaline activators [1], [4] .

Portland cement is a synthetic product comprised mostly of lime [13]. The process of lime manufacture includes the quarrying of appropriate material, generally limestone, and superheating it in a process known as calcination with the resulting product being ground to a fine powder [7]. An additional benefit of geopolymer adoption is the reduced required mining of feedstock materials for OPC formulation.

Geopolymer concrete had shown promise as a means of industrial waste product removal. Waste materials such as Ground Granulated Blast Furnace Slag, Fly Ash and Bottom Ash are all possible sources of the alumino-silicate reagent [3]. The average equivalent CO2 emissions for common constituent materials in geopolymer concrete is shown in Figure 1, with Sodium Hydroxide being the most carbon intensive [1]. A proposition has been suggested by Van Riessen et al (2013) that alkaline waste materials obtained from industrial alumina synthesis can be employed in geopolymer manufacture [14].

Figure 1: Greenhouse Gas Emissions of Constituent Materials [1]

Chemistry of Aluminosilicate Gel

‘Geopolymer’ is a general term proposed by Davidovits, used to describe an inorganic polymer consisting of an alumina-silicate SOL-gel matrix [3], [7]. Under acidic or basic conditions, alkoxides such as silicon oxides and aluminium oxides are interchanged with hydroxyl groups [7]. Such groups are mobilised in the solution and interlink with oxygen, while the cation from the hydroxyl activator, which is also mobile, acts as a charge balancer [7], [15]. At a molecular level three main basic structures can be identified, these are sialate units [– Si–O–Al–O–], sialate siloxo units [–Si–O–Al–I–Si–O–], and sialate disiloxo units [–Si–O–Al–O–Si–I–Si–O–] [15], [16]. These monomer units interconnect throughout the solution to form a gel comprised of liquid and solid phases, this is shown in Figure 2 with a solid polysialate network entrapping the liquid [7], [16]. When larger portions of the network are examined however, less order can be appreciated due to a far greater complexity of the reaction.

Although complex and mostly indeterminable, this is not dissimilar to OPC and other hydraulic binders [7]. Hardened OPC is comprised of mostly interlinked calcium-silicate hydrate as well as crystalline calcium hydroxide [7]. This too is of equivalent complexity and can only be examined at the basic constituent level, thus leaving technical investigation as the most reliable means of assessment into mechanical properties.

The rate of reaction for geopolymers is analogous to OPC concrete with a characteristic compressive strength being reached at 28 days, this is a statistical determination since the strength of geopolymer concrete can been observed to increase with time [6], [17]. Temperature also is an important consideration in the chemistry of the SOL-gel matrix , with heat cured specimens of identical proportioning reaching greater characteristic strengths [17].

Figure 2: Structure of Sodium-polysialate as proposed by Barbosa, MacKenzie and Thaumaturgo [16].

Fly Ash

Fly ash is defined by AS3582.1-2016 as the “solid material extracted from the flue gasses of a boiler fired with pulverized coal” [18]. This standard also specifies grades of fly ash as one of three: Special Grade, Grade

1 and Grade 2. This can be seen as supplementary grading to the often quoted ASTM C618-2005 classification, which classes fly ash as either class F or class C [11]. By comparing these two referencing tools, it can be seen that AS3582.1-2016 is a further categorisation of ASTM class F fly ashes, allowing for greater distinction between fly ashes. AS3582.1-2016 largely disregards all fly ashes that would otherwise be categorised as ASTM class C, this is of little concern since these ashes have been found to be unsuited to geopolymer concrete due to noted durability issues as well as unpractical setting times owing to their relatively high calcium oxide content [3], [9], [17].

AS3582.1-2016 refers to a number of additional standards which specify methods for testing of various properties, one such property is ‘strength index’ which is to be undertaken as per AS3583.6-1995 (reconfirmed 2016). This stipulates a direct comparison between the compressive strength of a binder with and without the supplementary fly ash. Such a strength index, although a similar concept, should not be confused with the reactivity index as proposed by Zhang et. al. (2016) since the performance of fly ash may be vastly different under different binding mechanisms, as an alkalised inorganic polymer or a pozzolan [12].

Fly ash is of small particle size, with a particle size distribution of 5 Australian fly ashes displayed in Figure 3. AS3582.1-2016 stipulates Special Grade, Grade 1 and Grade 2 fly ashes to have a minimum of 85, 75 and 55 percent by mass passing a 45μm sieve, respectively. Based solely on the particle size and neglecting the other properties as specified in AS3582.1-2016, fly ash A in Figure 3 could be classed as Special Grade, with fly ashes B and C being Grade 1 whereas fly ashes D and E could only make Grade 2 limits.

Figure 3: Particle size distribution of 5 Australian fly ashes [2]

It has been shown that fly ash generally has a spherical shape, with a Scanning Electron Microscope (SEM) image presented in Figure 4 [2], [12], [19], [20]. Considering this as well as additional parameters discussed below, Zhang et. at (2016) propose a reactivity index for fly ashes, as the inconsistencies in the fly ash properties leads to variation in performance [2]. This novel approach proposes a formula for reactivity index, see Equation 1.

Network Formers (NF) are identified as available Si, Al, Fe and Ti and Network Modifiers (NM) are Na, Al, K, Ca, Mg and Fe [2], [21]. These ions can have complex functions in geopolymerisation, for example Al and Fe can behave as both a NF and as a NM; varying weighting coefficients may apply for each elemental parameter [2], [22]. There is little available information however that permits the enumeration of network modifiers and network formers interacting; due to the complexity of their interaction with some behaving as either, under different conditions [2]. A method of Empirical Potential Structure Refinement (EPSR) modelling should be undertaken on fly ashes to further understand the influence of such ions, Zhang et. al (2016) was able to confirm the applicability of their model by fitting results of a EPSR study of silicate network glasses [2]. This is an area needing significant research that could allow the performance of fly ashes to be assessed without the need for in depth mechanical testing.

Where;

IPV is the inter-particle volume, which can be found by using the Archimedes method of comparing the density of the fly ash with the particle density, thus to determine the volume of voids which defines the IPV [2]

SSA is the geometric specific surface area; this is given by Equation 2. This however relies on the Sauter Mean Diameter (SMD), given in Equation 3. The SMD is the diameter of a theoretical sphere that has the same surface area to volume ratio as the actual particle [23]

Figure 4: SEM images of 5 Australian fly ashes [2]

Stress-Strain Relationship

The stress-strain relationship of geopolymer concrete was investigated by Hardjito and Rangan (2005), confirming that it is comparable to that of OPC concretes [3], [6], [24]. This study noted that the gradual loading of 100x200mm compression cylinders resulted in a reduced compressive strength when compared to more brisk loading, this is a similar phenomenon as what is observed with OPC concretes [24]. Equation 4 was proposed by Collins et. al (1993) for OPC concretes, and has been found to simulate geopolymer concretes well also, graphs comparing the applied formula to test specimens are presented in

Figure 5 [24], [25]. Due to the similar stress-strain relationship of geopolymer concrete when compared to OPC concrete, the current provisions in AS3600-2009 for the modulus of elasticity given in Equation 5canbe considered applicable [24],[26].

Figure 5a, 5b and 5c : Stress strain comparison of equation 4 and sample specimens of different composition [24]

Suitability as Flexural Members

Beams

Investigations of the applicability of geopolymer concretes to current design standards has been undertaken to the degree which the Concrete Institute of Australia has published its recommended practice guidelines for in geopolymer concrete, informing the members of the institute that AS3600-2009 can be used to design geopolymer concrete members [3]. Testing of beams in single curvature has been investigated and compared AS3600-2009 by Sumajouw & Rangan (2006), Table 1 tabulates the measured properties of the test specimens of this study and their anticipated performance determined as per AS3600-2009 [26], [27]. Such testing yielded identical failure modes to under-reinforced concrete beams with tensile crack precipitation occurring at the bottom of the beams before compression failure of the concrete at the top of the beam, this can be seen in Figure 6 [27]. The bending moment induced where compression failure of the concrete occurs is indicative of the ultimate moment capacity of the beam, this is referred to as Muo in AS3600-2009 [26], [28]. Compressive failure of concrete is considered brittle as no warning is given, however since the beams tested were under-reinforced the tensile reinforcement has already yielded before compressive failure and the beam had shown sufficient sign of strain through deflection and large tensile cracks [27], [28]. It is worth noting however the specimens tested in the study were head cured and confirmation that some of the conclusions drawn in this study apply for ambient cured geopolymers is yet to be provided.

Table 1: Comparison of geopolymer concrete beam performance and AS3600-2009 [26], [27]

Beam	Tensile Reinforce -ment Ratio (%)	Nominal Ast (mm)	F’c (MPa)	α₂	fsy (MPa)	Mid span deflection (mm)	Muo from testing (kNm)	Muo as per Equatio n 6	Muo Ratio Test/calc
GBI-1	0.55	330	37	0.85	550	56.63	56.30	51.83	1.09
GBI-2	1.00	600	42	0.85	560	46.01	87.65	92.89	0.94
GBI-3	1.55	930	42	0.85	560	27.87	116.85	137.25	0.85
GBI-4	2.25	1350	37	0.85	557	29.22	160.50	180.64	0.89
GBII-1	0.55	330	46	0.85	550	54.27	58.35	52.34	1.11
GBII-2	1.00	600	53	0.84	560	47.2	90.55	94.47	0.96
GBII-3	1.55	930	53	0.84	560	30.01	119.00	141.03	0.84
GBII-4	2.25	1350	46	0.85	557	27.47	168.70	189.43	0.89
GBIII-1	0.55	330	76	0.77	550	69.75	64.90	53.05	1.22
GBIII-2	1.00	600	72	0.78	560	40.69	92.90	95.80	0.97
GBIII-3	1.55	930	72	0.78	560	34.02	126.80	144.23	0.88
GBIII-4	2.25	1350	76	0.77	557	35.85	170.59	201.49	0.85
Average	0.96
Std Dev.	0.12

Columns

As with beams, the Concrete Institute of Australia has publicised that geopolymer concrete complies to AS3600-2009 for concrete structural members, this includes columns also [3]. Columns can be subjected to flexural loading, but are subjected to axial loading also [28]. Such members were also tested extensively by Sumajouw and Rangan (2006) with induced moment and axial loading, these specimens were found to behave analogous to OPC concrete columns [27]. The failure of such members, presented in Figure 7, can be seen to fail with compression failure of the concrete and buckling of the steel reinforcement, tensile cracking can also be observed due to the induced moment in the column [27]. It can be seen in Table 2 that AS3600- 2009 can be confidently used to design slender columns using geopolymer concrete [3], [26], [27].

Figure 7: Failure of geopolymer columns [27]

Table 2: Comparison of geopolymer concrete beam performance and AS3600-2009 [26], [27]

Column	f’c (Mpa)	e (mm)	p (%)	*N (kN)**	Nᵤₒ	ratio *N/Nᵤₒ**
GCI-1	42	15	1.47	940	962	0.98
GCI-2	42	35	1.47	674	719	0.94
GCI-3	42	50	1.47	555	573	0.97
GCII-1	43	15	2.95	1237	1120	1.10
GCII-2	43	35	2.95	852	832	1.02
GCII-3	43	50	2.95	666	665	1.00
GCIII-1	66	15	1.47	1455	1352	1.08
GCIII-2	66	35	1.47	1030	1010	1.02
GCIII-3	66	50	1.47	827	760	1.09
GCIV-1	59	15	2.95	1559	1372	1.14
GCIV-2	59	35	2.95	1057	1021	1.04
GCIV-3	59	50	2.95	810	800	1.01
average	1.03
std dev	0.06

Water content and Workability

Being an important physical restraint in concrete placement, workability is important to be specified for a given application [3]. It has been found that like OPC concrete, the addition of water increases workability but too reduces the quality of the concrete, the increase in slump with water is presented in Figure 8 [3], [10], [24]. Figure 9 shows the results of a study in mixing proportions of heat cured fly ash concrete displaying that an increase in excess water, that is water surplus to the absorption capacity of the aggregates, is detrimental to the mechanical strength of the geopolymer concrete [10], [24]. Certain admixtures can be used to modify the properties of plastic of geopolymer concrete, for example a naphthalene based superplasticiser has been employed in previous studies; admixtures however have been developed specifically for OPC concrete and mostly not required or are not effective for geopolymer concrete [3], [13], [24], [29]. It is appropriate to note however that water is an integral part of the system, performing the role of the medium in which allows the polymerisation process to occur, this has been discussed previously [6], [7].

Figure 9: 7-day f’c versus Water/Geopolymer ratio of specimens cured at different temperatures [24]

Mixing and Placement Requirements

When mixing geopolymer concrete it is preferable to use high shear folding mixers, although conventional mixer are acceptable [3]. The Concrete Institute of Australia has identified that an area in need of development is measuring the viscosity of geopolymer concrete as mixing times increase and as it undergoes polymerisation, this will allow for better manufacture of geopolymer concrete as well as assurance it can be placed after a delay between batching and placement [3]. It has been observed that as mixing times increase, the slump of the concrete increases, this is obviously up to a point as when the SOL-gel develops the viscosity increases until it is rigid structure [3]. It too has been noted that as mixing times increase the mechanical performance of the concrete is improved as shown in Figure 10.

Figure 10: Compressive strength versus mixing time [24]

It has been observed that geopolymer concrete is more adhesive than regular concrete [3], [30]. This has been noted as being a significant issues since additional cleaning of handling equipment may generally be required [3], [30]. Due to the different chemical nature when compared to OPC concrete, geopolymer concrete tends to breakdown conventional formwork release agents, as such different products need to be identified overcome this [30].

The recommended practice for geopolymer concrete in Australia suggests that, like for OPC concretes, air voids can be of concern and low amplitude internal vibration is the preferred compaction technique, with conventional methods of compaction for no-slump concretes applicable to geopolymers [3].

Tensile Strength

The characteristic tensile strength of geopolymer concrete has been assessed by Hardjito and Rangan (2005) as specified in AS1012.10-2000 (reconfirmed 2014) and compared it to AS3600-2009, this is presented in Table 3 [26], [31]. AS3600-2009 deems the characteristic flexural and uniaxial tensile strengths of concrete shall be determined from their characteristic 28-day compressive strength, presenting Equation 7 and Equation 8 [26]. Equation 10 perhaps is the best method of determining the tensile strength of geopolymer concrete, more economic that the current provisions in AS3600-2009 yet meeting an important factor of safety of AS3600-2009 presented in Equation 9 [26]. These equations suggest the characteristic uniaxial tensile strength and the characteristic flexural tensile strength are considered similar, with a factor of difference of 0.04 (0.40-0.36=0.04). The existing provisions in AS3600-2009 are suitable for geopolymer concretes, with the characteristic uniaxial tensile strength determined from the characteristic 28-day compressive strength being around 1.8 times the uniaxial tensile strength as determined from the indirect tensile splitting test. As such the current provisions can be considered conservative, for full realisation of the environmental and technical benefits of geopolymer concrete, adaptations for geopolymer concrete’s superior tensile strength should be made [3], [6], [24].

Fire Resistance

The development of geopolymer concrete was originally developed as a heat resistant building material after a number of fires in France in the early 1970’s [29]. Fly ash based geopolymers have been investigated for their fire resistance and found to increase in mechanical strength when temperatures exceed about 600⁰C as shown in in Figure 11, this strength increases is due to a sintering liquid phase [32]. This sintering however also responsible for an increase in the thermal shrinkage, this is presented in Figure 12 [32]. In the circumstance that thermal shrinkage reaches a threshold value, usually in the range of 1 to 2% the material may fail despite the increase in its mechanical strength, this is due to cracking and changes in the material volume [32]. When compared to OPC concrete however it is a vast improvement with OPC concretes deteriorating in strength at around 300⁰C, characterised by explosive failure as the water molecules inside the concrete vaporise [6], [13]. The failure mode for geopolymers however is not explosive but results by melting of the inorganic polymer [32]. Certain geopolymers have been investigated and are in use in ultra- high temperature applications, however these systems perform differently to geopolymer binders used in concrete due to variation in the monomers that make up the geopolymer network [6], [33].

Drying Shrinkage

Shrinkage is a phenomenon observed with OPC concretes, as the OPC hydrates into a crystalline matrix it reduces in volume, this makes sense since crystalline structures are much denser than the overall concrete so the remaining volume is made up of millions of small air voids [7]. Additional shrinkage is due to aggregates in the concrete absorbing water that would otherwise contribute to the volume of the concrete [34]. It has been observed that drying shrinkage of geopolymer concretes is less than that of OPC concretes [3], [17], [35]. Since geopolymer concrete undergoes polymerisation not hydration into a crystalline matrix it makes sense that the shrinkage of geopolymer concretes is less than OPC concrete [7]. As drying shrinkage occurs it, creates strain within the concrete structure [34]. Since most of the studies of geopolymer concrete have been on heat cured specimens it is interesting for Wallah and Rangan (2006) to report that shrinkage of ambient cured geopolymer concrete is far greater than that of heat cured geopolymer concrete as can be seen in Figure 13. This cause for concern since the Recommended Practice for Geopolymer Concrete in Australia suggests that the shrinkage joints or saw-cuts may not be required [3]. It can be seen in Figure 14 that provisions AS3600-2009 are conservative, yet are inadequate for ambient cured geopolymer concrete as presented in Figure 15. An experimental footpath was constructed at Curtin University of Technology in Perth, Australia, with no shrinkage cracks observable in 1.5x12m slabs, yet cracking at 3m centres for an OPC concrete reference footpath [3]. This is obviously contradictory to the findings of Wallah and Rangan (2006), and with lack of research on ambient cured geopolymer concrete this is an area requiring further research.

Figure 15: Drying Shrinkage of Ambient Cured Geopolymer Concrete compared to predictions as per AS3600- 2009 [17], [26]

Creep

Creep is a phenomenon that can be defined as the increase in strain under a sustained stress [34]. It can also be seen as the relaxation of stress with time for concrete subject to constant stress [34]. For the design of concrete members, AS3600-2009 employs a creep coefficient which is defined as the “mean value of the ratio of creep strain to elastic strain under conditions of constant stress” [26]. The creep strain of heat cured geopolymer concrete was investigated by Wallah and Rangan (2006) noting that creep can reduce the strength of geopolymer concrete [17]. The current provisions in AS3600-2009 however are conservative for heat cured geopolymer concrete [6], [17].

Figure 16Figure 16 presents a predicted creep strain curve calculated as per AS3600-2009 and an actual test specimen [17], [26]. Like for drying shrinkage, little research has been undertaken for geopolymer concrete cured at ambient temperatures.

Conclusion

By reviewing the properties of fly ash based geopolymers it is clear there is a substantial lack of knowledge on the effect of curing conditions on the various mechanical parameters of geopolymer concrete. Although heat cured specimens have largely been presented in in this discourse, a mix design methodology and an assessment of the applicability of Australian design codes for in-situ geopolymer concrete is the objective of this associated research project. Since recognition of geopolymer concrete by some professional institutions and governmental authorities is based on heat cured concrete, there have been assumptions made that are yet to be confirmed for in-situ concretes [3], [36]. As research progresses it is likely that these gaps-in- knowledge will be addressed since the behaviour of geopolymer concrete is similar to that of OPC concretes, but with differences in magnitude. Future research lies in investigating the in-situ performance of geopolymer concretes and subsequent commercial realisation by the adaptations of existing standards to economically realise the mechanical performance of geopolymer concretes.

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