Cheema

25th ARRB Conference Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012

ARRB Group Ltd and Authors 2012 1

LOW CALCIUM FLY ASH GEOPOLYMER CONCRETE A PROMISING SUSTAINABLE ALTERNATIVE FOR RIGID CONCRETE ROAD FURNITURE

D S Cheema, Main Roads, Western Australia

ABSTRACT Geopolymer is a material resulting from the reaction of a source material that is rich in silica and alumina with alkaline solution. This material has been studied extensively over the past few decades and shows promise as a greener alternative to ordinary Portland cement concrete. It has been found that geopolymer has good engineering properties with a reduced carbon footprint resulting from the zero-cement content.

Durability parameters depend on the pore structure of concrete matrix. Tests performed to measure compressive strength, volume of permeable void, pore structure and permeability have shown that low calcium fly ash based geopolymer concrete has the potential to be a promising sustainable alternative for rigid concrete road furniture, such as, rigid safety barrier, kerbing, traffic island infill, dual use path (DUP) and parking bay rest areas paving etc with a significant environmental benefits compared to Portland Cement concrete.

The research paper highlights potential applications of low calcium fly ash geopolymer (LCFG) concrete in non aggressive to mild environments.

INTRODUCTION Davidovits (1988) discovered that the concrete used in ancient structures is alkali-activated alumino-silicate binders and named it as geopolymer concrete because of polymerisation reaction, instead of the Calcium Silicate Hydrate (CSH) gel structure found in conventional Ordinary Portland Cement (OPC) concrete. Geopolymer binder is an inorganic material and has been reported that it may potentially be a good construction material. Curtin University, Western Australia research work has shown that LCFG concrete has significant environmental benefits over conventional concrete (Rangan 2008 GC4).

LCFG concrete rapid strength gain mechanism relies on curing at elevated temperature such as steam curing or encapsulated dry curing but for ambient curing conditions its mixture requires appropriate reagent. The use of such reagent is to offset the effect of low ambient temperature by increasing the temperature of fresh or compacted mix.

For rigid road furniture applications, LCFG concrete of non-structural concrete class (N) of strength up to 40 MPa is generally the requirement. Secondly for its desired initial strength, curing under ambient conditions is one of the essential requirements for such insitu applications.

For durable performance of reinforced concrete, the integrity of both steel and concrete components is essential. The passivating mechanism keeping the reinforcing steel intact in LCFG concrete is not fully understood yet, however the research study indicates that the reinforced LCFG concrete has the potential in non-aggressive to mild environments as a sustainable alternative of OPC concrete. The study presents the preliminary findings of slag based LCFG concrete for such applications.



EXPERIMENTAL WORK The geopolymer mix composition in the research study comprised a binding cementitious material and inert coarse and fine aggregates. The main constituents of geopolymer binder are low-calcium fly ash (Class F) and the alkaline liquid which differ from OPC concrete.

Fly ash The fly ash used in the study was sourced from Collie Power Station in Western Australia. The chemical composition of fly ash determined by X-ray Fluorescence (XRF) (mass %) is summarised in Table 1 (Hardijito & Rangan 2005).

Table1: Chemical composition of fly ash

Oxides SiO2 Al2O3 Fe2O3 CaO Na2O K2O TiO2 MgO P2O5 SO3 ZrO2 Cr MnO LOI*

Mass % 48.00 29.00 12.70 1.78 0.39 0.55 1.67 0.89 1.69 0.50 0.06 0.016 0.06 1.61 LOI* Loss on Ignition.

Alkaline liquid The alkaline liquid comprised a combination of sodium silicate solution and sodium hydroxide solution. The sodium silicate solution (Grade A53) comprised 14.7% of Na2O, 29.4% of SiO2, and 55.9% of water by mass. The sodium hydroxide solution was prepared by mixing 98% pure flakes in water. Both the solutions were mixed together at least 24 hours before use (Hardijito & Rangan 2005).

Super plasticiser A high range water reducing (naphthalene sulphonate-based) super plasticiser was used in the mixtures at the rate 1.5% of fly ash to improve the workability of the fresh geopolymer concrete (Hardijito & Rangan 2005).

Aggregates Aggregates currently used by the local concrete industry in Western Australia, supplied by BGC Concrete and Asphalt were used. Both coarse and fine aggregates used were in saturated-surface-dry (SSD) condition. The aggregates comprised 30% of 14 mm, 38% of 10 mm, and 32% of fine sand.

MIXTURE PROPORTIONS

Steam cured mix Five concrete mixtures, nominated as G40 (40 MPa) and G50(50 MPa) for LCFG concrete, S40 (40 MPa) and S50 (50 MPa) for Ordinary Portland Cement (OPC) concrete and Sx40(40 MPa) for OPC concrete with pore blocking additive were studied in this research study.The detail of these sample mixtures, curing conditions (wet curing for OPC concrete samples and steam curing for geopolymer concrete samples at 60 oC for 24 hours) and compressive strength are given in Table 2 below.



Table 2: Geopolymer concrete & OPC concrete mixture proportions

Materials Mass (kg/m3) Remarks

G40 G50 S40 Sx40 S50

Cockburn cement (GP) 400 400 420

Coarse aggregate

14 mm 647 647 920 554 554

10 mm 647 647 300 702 702

Fine sand 554 554 640 591 591

Fly ash (low calcium ASTM Class F)

409 409

Sodium silicate solution (SiO2/Na2O =2)

102 102

Sodium hydroxide solution

41 41 8M concentration

Super plasticiser (SP) 6 6

Water reducer 20 20 20 As specified by manufacturer

Target water 0 10 170 170 168

Extra water in aggregates

15.5 24.2 170 170 163.5

Admixture 2 As specified by manufacturer

Water/cement ratio 0.19 0.20 0.43 0.43 0.43

Curing temperature 60 oC 60 oC Amb. Amb. Amb.

Curing time 24 hrs

24 hrs

28 days

28 days

28 days

Conventional concrete curing included 14 days wet curing

28 days mean comp (MPA)

54.5 54 55 56 58.5

Past research has shown that LCFG concrete with 100% fly ash binder achieved initial strength in the order of 20 MPa when allowed to cure under ambient conditions with a gradual increase in strength over time (approximately 40 MPa in 4 weeks and 50 MPa in 12 weeks Rangan 2008 GC4).

This indicates that LCFG concrete without any reagent is feasible for situations where high early strength and its commissioning prior to 4 weeks is not the main requirement.

LCFG concrete ambient and steam cured mix Slag based LCFG concrete mix with composition as detailed in Table 3 below was studied for of in-situ LCFG concrete components which can achieve initial average strength of 40 MPa or more when cured under ambient conditions.

The calcium content of slag is approximately 42% while of low calcium fly ash is 1.5%. The mix proportion of binder, that is 95% fly ash and 5% slag, resulted the increase of fly ash binder calcium content to 3.5%. With this calcium content increase the low calcium fly ash binder Class F remained unaltered (Class F Low Calcium Fly Ash with calcium content less than 10).



Table 3: Geopolymer concrete mixture proportions

Materials Mass (kg/m3) Mass (kg/m3) Remarks

G40/50 G40/50

Cockburn cement (GP) - -

Coarse aggregates 14 mm 647 647

Coarse aggregates 10 mm 647 647

Fine sand 554 554

Fly ash 95%(low calcium ASTM Class F)

388.5 388.5

Slag (5%) 20.5 20.5

Sodium silicate solution (SiO2/Na2O =2)

102 102

Sodium hydroxide solution 41 41 16M concentration

Super plasticiser (SP) 6 6

Target water 0 0

Extra water in aggregates 0 0

Water/cement ratio 0.17 0.17

Curing temperature Steam(60 oC) Ambient

Curing time 24 hours Demoulding after 5 days

14 days wet curing after 7 days of casting

3 days mean comp. strength(MPa)

55.5


66.5


80.5

EXPERIMENTAL RESULTS It has been found that slag based LCFG concrete with 5% slag achieved compressive strength in excess of 50 MPa when cured under ambient conditions. High concentration of activator solution (16M) activating the slag based LCFG concrete resulted its 7 days target compressive strength of 66 MPa cured under ambient conditions. The cylinder specimens were demoulded after 5 days and were tested for compressive strength on 7th day.

This initial dry curing of slag based LCFG concrete under ambient conditions indicated the need of some protection requirement such as polyethylene sheeting to reduce quick loss of moisture for 5 to 7 days. This 7 days initial compressive strength gain of slag based LCFG concrete is three times higher than the ambient cured LCFG concrete containing 100% low calcium fly ash activated by 8M alkaline solution (Rangan GC4). This increased strength gain trend of slag based LCFG concrete is in concurrence with findings of Frantistek et al. (2006).

It was inferred that slag based LCFG concrete cured at elevated temperature of 60 oC for 24 hours resulted slightly lower strength of 55.5 MPa. This could be due to the formation of CSH products interfering with polymerisation process. An alkaline activator solution of high concentration (16M Sodium Hydroxide) tends to reduce this initial interference by allowing the



polymerisation process to proceed first, which could be due to the fast dissolution, gelation and polymerisation followed by formation of CSH products during synthesisation of mix constituents.

The higher strength gain of slag based LCFG concrete under ambient conditions is an indirect indication of its durability. The formation of CSH products in an alkali activated polymerised LCFG concrete mix will tend to increase the tortuosities of its pores structure. The research study also has shown that LCFG concrete properties compressive strength and volume of permeable voids are analogous to OPC concrete and in concurrence with the past study findings. Figure 1 and 2 below shows the trend of these properties.

Figure 1: Compressive strength and density trend of laboratory samples

Figure 2: Volume of permeable voids trend of laboratory samples

Past research has shown that 100% fly ash based LCFG concrete cured under ambient conditions has shrinkage higher than the steam cured one. This higher shrinkage occurs over the first two weeks and is approximately 13 times higher than the steam cured LCFG concrete, Rangan (2008 GC4) and Wallah and Rangan (2006 GC2). Valentin Mukhin et al. 2007 research study has shown that addition of SL cement in alkali activated fly ash geopolymer concrete in small proportion caused 50% reduction in shrinkage approximately for mix cured under ambient conditions. It is notional that small proportion of slag in slag based LCFG concrete may not be of disadvantage on account of drying shrinkage rather its slow hydration over an extended period may be useful for obtaining the desirable properties.

The quantitative study of pore structure and porosity of both LCFG concrete and OPC concrete undertaken using micro-tomography in the research study, provided better understanding of LCFG concrete long term durability properties compared to OPC concrete.



The cored samples used for the study of pore structure and micro-structural properties were from the laboratory samples. Laboratory samples were under exposure to lab-simulated severe environments mimicking the field severe environmental scenarios for two years approximately as a part of this research study. Figure 3 shows these core samples.

Figure 3: Core samples for microstructure and porosity analysis testing

650 m 650 m 650 m

Geopolymer Concrete (G40- 40 Mpa) OPC Concrete (S40- 40 Mpa) (Sx40- 40 Mpa)

OPC Concrete with pore blocking additive

Figure 4: Tomographic microstructure images



LCFG Concrete (G40) OPC Concrete (S40)

OPC Concrete with Pore Blocking Additive (Sx40)

Figure 5: Microstructure and pore size distribution

Figure 4 above shows the tomographic images of pore structures of LCFG concrete, OPC concrete and OPC concrete with pore-blocking additive while Figure 5 above shows the distribution of pores from the visual prospective, size and their shape characteristics. LCFG concrete (G40) has pore structure with pores of circular shape having isotropy index of 0.5-1. These circular shaped pores constitute 40-50% of total pore clusters, while the pore structure of OPC concrete has pores of elliptical shape with isotropy index of 0.1-0.5 and constitute 90% total pore clusters.

Micro-tomography test outputs showed that LCFG concrete (G40) binder paste had more circular shaped pores of varying size compared to OPC concrete (S40). This is notional to the fact that LCFG concrete matrix is mechanically more stable and strong compared to OPC concrete. Elongated pores were more noticeable at the interface of coarse aggregates in OPC concrete while such interface was not present in LCFG concrete. Both S40 and Sx40 have pores relatively lesser in number but of larger size than G40.

BASIS OF SUSTAINABILITY The low calcium fly ash being an industrial by-product has significant environmental benefits due to its reduced CO2 footprint. LCFG concrete has 80% lower CO2 footprint than OPC (Davividovits 2002 and CIA 2009).Subsequent illustration on this account describes the sustainable potential LCFG concrete. The research study recently undertook by Main Roads in co-operation with Curtin University and local pre-cast industry (ROCLA) manufacturing LCFG concrete box culverts of size 1200x1200x600 mm showed that LCFG concrete is a feasible construction material like OPC concrete for pre-cast concrete structure components (Cheema et al. 2008).



Sustainability potential of LCFG concrete Figure 6 below shows the feasible slag based LCFG concrete applications for rigid road furniture and rail projects in non-aggressive to mild road environments. The slag based LCFG concrete can offer significant environmental benefits for such applications.

Rigid Crash Barrier

Railway Sleeper

Curtin Wall

Dual Use Path

Island In fill

Underpass

Figure 6: LCFG concrete applications extent

Following Tables summarise the sustainable potential of LCFG concrete applications. As an illustration to demonstrate this sustainable potential of LCFG concrete applications, a project length of one kilometre of freeway (Mitchell Freeway, Perth Metropolitan) has been taken, which has six lanes divided carriageway with a fast track for metro rail through the median island barricaded by concrete safety crash barrier as shown in Figure 6 above.

The micro environments in accordance with AS3600 are summarised in Table 4 for the section under consideration of the Freeway.

Table 4: Detail of non-aggressive to mild micro environments

The production of one tonne of geopolymer binder generates 0.18 tonnes of CO2, from the combustion of carbon-fuel, compared to one tonne of CO2 from Portland cement (Davidovits 2002, Davidovits 1994 and CIA 2009). If LCFG concrete is considered as a substitute for non-structural concrete components such as concrete safety barrier, rail sleepers, kerbing and possible dual use path as an alternative construction material to OPC concrete, the significant CO2 emission cut down per kilometre (km) will be in the order of 540 tonne/km. Table 5 below summarises these CO2 emission savings.

Environment Classification Description Code classification

Environment 1 Atmospheric exposure

Not expected to contain significant concentrations of aggressive agents

Concrete exposure A2

Environment 2 Atmospheric protected

Protected areas such as under passes box culverts

Concrete exposure A1

Steel corrosion B1



Table 5: Summary of CO2 emission savings

Item Design life (AS

3600)

Non- structural concrete

class with binder

content 350 kg/m3

Approximate requirement of concrete

in m3/km

OPC CO2 emission in tonne

Geopolymer CO2 emission

in tonne @ 0.18 t/

tonne of geopolymer

cement

Net cut down on

CO2 emission in tonne/

km

Concrete safety barrier

40-60 years

N32 800 280 50

Rail sleepers

40-60 years

N32 284 100 18

Dual use path

40-60 years

N20 600 228 41

Kerbing (up to 50% extent only)

40-60 years

N20 140 50 9

Total CO2/km

658 118 540

Other concrete components such as underpasses, traffic island in-fill could be viewed as adding to this sustainable potential further. The LCFG concrete applications for road furniture components shown in Figure 6 above will raise the potential sustainability ranking of the project significantly higher than the one if OPC concrete is continued to be used.

Non-aggressive to mild environmental scenarios 1 km inland from the coast line in majority of coastal cities are the possible scenarios which can be benefitted from the sustainable potential of slag based LCFG concrete with the exceptions of some localised ones.

CONCLUDING REMARKS Preliminary research study has shown that physical properties of the LCFG concrete are on the better side and is comparable with OPC concrete on many fronts. Micro structure and pore structural properties of the LCFG concrete exhibited more mechanically stable concrete matrix than OPC concrete in addition to other properties (compressive strength, volume of permeable voids).

Slag based LCFG concrete application for rigid road furniture in non aggressive to mild environments could be of significant environmental benefits. Secondly slag based LCFG and LCFG concrete application initiatives both for in-situ and precast components of road and rail projects will provide the opportunities of improving this environmentally friendly material further and its on-going development for its extended applications.

ACKNOWLEDGEMENT The author acknowledges the support provided by RF Scanlon, Senior Engineer Structures, Main Roads Western Australia, Dr Florian Fusseis, multi-scale Earth system dynamics Western Australian Geothermal Centre of Excellence and Curtin University in undertaking this research study. The views expressed are that of the author and not necessarily of the organisation to which the author have affiliation.



REFERENCES Australian Standards (1999), Determination of water absorption and apparent volume of permeable voids in hardened concrete, AS1012.21, Standards Australia, Australia.

Australian Standards (2009), Concrete Structures, AS3600-2009, Standards Australia, Australia.

Cheema, D.S., Lloyd, N.A. and Rangan, B.V. (2008) Durability of Geopolymer Concrete Box Culverts A Green Alternative, 34th Conference on Our World in Concrete and Structure 09, Singapore.

Davidovits, J. (2002), Environmentally Driven Geopolymer Cement Application, Geopolymer Conference, Melbourne.

Davidovits, J. (1994), High alkali cements for 21st century concretes. In Concrete Technology, Past, Present and Future, Proceedings of V Mohan Malhotra Symposium, Mehta, K. (ed.). ACI SP.

Hardijito, D. and Rangan, B.V. (2005), Development and Properties of Low Calcium Fly Ash Based Geopolymer Concrete, Research Report GC1, Faculty of Engineering, Curtin University of Technology, Perth, Australia.

Rangan, B.V. (2008), Fly-ash based Geopolymer Concrete, Research Report GC4.

Recommended Practice Geopolymer Concrete (2009), Concrete Institute of Australia.

Wallah, S.E. and Rangan, B.V. (2006), Low Calcium Fly Ash-Based Geopolymer Concrete: Long-Term Properties, Research Report GC2, Faculty of Engineering. Curtin University of Technology, Perth, Australia.

AUTHOR DETAILS DS Cheema is the Geotechnical Engineer, Main Roads Western Australia, a position held since 2003, and is responsible for investigations, design and technical services in geotechnical and geomechanics, concrete technology, pavement and material engineering area.

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