Faculty of Engineering
Department of Civil Engineering
THE MIX DESIGN DEVELOPMENT OF GEOPOLYMER
CONCRETE UNDER AMBIENT CURING CONDITIONS
by Darryl Hole 13110853 October 2009PROJECT DOCUMENTATION
SHEET
Title: The Mix Design Development of Geopolymer Concrete Under Ambient Curing Conditions Author: Darryl Hole Date: 19th October 2009. Supervisor: Dr. Natalie Lloyd ABSTRACT: Continued increase in the focus and restriction on global carbon dioxide emissions requires the research for a cleaner alternative to the use of Portland cement. The manufacture of this product is responsible for the release of millions of tons of carbon dioxide worldwide every year. Geopolymer concrete consists of 100% fly ash replacement of the ordinary Portland cement. A binder is formed by a reaction from an alkaline liquid and the aluminium and silicon present in this fly ash. The present report deals with advancing the mix design research in geopolymer concrete applications.
The laboratory work carried out for this report was based upon developing geopolymer concrete mixes that were able to be used in an industry based application, and therefore having appropriate ambient curing properties. The conditions that would be found on a large scale concrete project within industry were replicated to form a comparison. Such measures taken included no specific aggregate preparation (saturated surface dry) or steam room curing. The aim initially was to consistently produce geopolymer concrete mixes that set quickly and exhibited a 28 day compressive strength of at least 30 MPa.
Previously successful geopolymer concrete mix designs were used as a basis, with additives such as silica fume and calcium hydroxide included in anticipation of developing a faster setting concrete mix with a higher early strength. Seven concrete mixes were produced during the year with varying mix design properties. Experimental results were based on compressive strength primarily, with mixes being tested at 7, 14, 21 and 28 days of age in majority of situations. Tensile strengths were also tested for the first four mixes produced.
Indexing Terms:
ACKNOWLEDGEMENTS
I would first of like to particularly thank both my project supervisor Dr. Natalie Lloyd, and Professor Arie van Riessen from the Curtin University Centre for Materials Research, for their assistance and direction throughout the year. Further to this, the assistance of Dr. Dan Churach from the Centre for Sustainable Resource Processing and Evan Jamieson of Alcoa is acknowledged.
The majority of experimental work for this research was carried out in the Civil Engineering laboratories at Curtin University, Western Australia. For this I thank the technical staff including Mr. John Murray, Mr. Mike Ellis, Mr. Ashley Hughes and Mr. Mike Appleton. I would also like to thank Ms. Monita Olivia for the support and guidance throughout the year. The progress of this research would have been delayed significantly without the assistance of these individuals.
The assistance in this research from the Chemical Engineering laboratory at Curtin University is also acknowledged. Thank you to Ms . Karen Hayes and Ms. Ann Carroll for their support and assistance throughout the year. Further to this, I would also like to acknowledge the assistance provided from post‐graduate students from the Curtin Centre of Materials Research, in particular Ms. Emily Carter, Ms. Melissa Lee and Mr. Ross Williams.
A final thank you goes to all my friends from the Curtin University Class of Civil Engineering 2009.
CONTENTS PAGE
PROJECT DOCUMENTATION SHEET ... i ACKNOWLEDGEMENTS ... ii CONTENTS PAGE ... iii LIST OF FIGURES ... v LIST OF TABLES ... vi 1. INTRODUCTION ... 1 1.1 Background ... 1 1.2 Geopolymer Concrete ... 2 1.3 Research Aims ... 3 1.4 About this Report ... 4 2. LITERATURE REVIEW ... 5 2.1 Ordinary Portland Cement and the Environment ... 5 2.2 Alternatives to Portland Cement in Concrete ... 7 2.3 Fly Ash based Concretes ... 9 2.4 Geopolymer Concrete ... 11 2.5 Mix Proportioning of Geopolymer Concrete ... 14 2.6 Curing of Geopolymer Concrete ... 15 2.7 Aiding the Early Strength of Concrete ... 17 3. EXPERIMENTAL PROCEDURE ... 20 3.1 Introduction ... 20 3.2 Safety ... 21 3.3 Materials ... 22 3.3.1 Fly Ash ... 22 3.3.2 Sodium Hydroxide ... 22 3.3.3 Sodium Silicate ... 23 3.3.4 Calcium Hydroxide ... 23 3.3.5 Silica Fume ... 23 3.3.7 Alkaline Liquid ... 24 3.3.8 Aggregate ... 243.4 Preliminary Laboratory Work ... 26 3.4.1 Mixing Procedure ... 27 3.4.2 Mixture Proportions ... 29 3.4.3 Curing of Geopolymer Concrete ... 33 3.5 Testing of Concrete Specimens ... 34 4. EXPERIMENTAL RESULTS AND DISCUSSION ... 35 4.1 Introduction ... 35 4.2 Experimental Results Overview ... 35 4.3 Compressive Strength and Observations of Geopolymer Concrete Mixes ... 40 4.3.1 Initial Geopolymer Concrete Reference Mix ... 40 4.3.2 The Use of Silica Fume to Aid Ambient Curing ... 43 4.3.4 The Effect of Free Water Content on the Strength of Geopolymer Concrete . 48 4.3.3 The Use of Calcium Hydroxide to Aid Ambient Curing ... 53 4.4 Indirect Tensile Strength of Geopolymer Concrete ... 63 5. SUMMARY AND CONCLUSIONS ... 68 5.1 Introduction ... 68 5.2 Production of Geopolymer Concrete ... 68 5.2.1 Pre‐production Issues ... 68 5.3 Results and Observations ... 69 5.3.1 The Use of Silica Fume to Aid Ambient Curing ... 69 5.3.2 The Effect of Free Water Content on Geopolymer Concrete ... 69 5.3.3 The Use of Calcium Hydroxide to Aid Ambient Curing ... 70 5.3.4 Other Observations During Research ... 70 6. RECOMMENDATIONS ... 72 REFERENCES ... 75 APPENDIX A ... 79 APPENDIX B ... 81
LIST OF FIGURES
Figure 2.1: Compressive Strength of 30% Fly Ash Substituted Concrete ... 18 Figure 3.1: Grading Curve of Combined Aggregates ... 25 Figure 3.2: Pan Mixer Used for Production of Concrete ... 29 Figure 3.3: Setting of Wet Geopolymer Concrete... 29 Figure 3.4: Ambient Curing of Geopolymer Concrete ... 33 Figure 3.5: Rough Surface of Cured Geopolymer Cylinder ... 34 Figure 4.1: Compressive Strength of all Carried Out Mixes ... 38 Figure 4.2: Indirect Tensile Strength of Mixes One to Four ... 39 Figure 4.3: Compressive Strength of Mix One ... 41 Figure 4.4: Efflorescence Formed on the Outside of Cylinders ‐ Mix One at 14 days .... 42 Figure 4.5: Efflorescence on the Outside of Cylinders ‐ Mix One at 28 days ... 43 Figure 4.6: Expansion of Mix Two (right) Relative to Mix One (left). ... 45 Figure 4.7: Expansion of Mix Two Cylinders ... 45 Figure 4.8: Compressive Strength for Mixes One and Two ... 46 Figure 4.9: Excess Water in Geopolymer Concrete ... 51 Figure 4.10: Compressive Strength of Mixes One and Four ... 52 Figure 4.11: Compressive Strength of Mixes Three and Four ... 55 Figure 4.12 : Rapid Setting Effects and Efflorescence on Mix Three Cylinders ... 56 Figure 4.13: Cross Section of Small Cylinder ‐ Mix Three ... 56 Figure 4.14: Mix Six at Two Hours after Pouring ... 59 Figure 4.15: Mixes Seven (Left) and Six (Right) at One Hour after Pouring ... 60 Figure 4.16: Compressive Strength of Mixes Five, Six and Seven (MPa) ... 61 Figure 4.17: Efflorescence Beginning to Form after De‐moulding ‐ Mix Five ... 63 Figure 4.18: Indirect Tensile Strength of Mixes One to Four ... 65LIST OF TABLES
Table 3.1: Grading of Combined Aggregates ... 26 Table 3.2: Free Water Content of Mixes One and Two ... 31 Table 3.3: Mix Design Summary of Carried Out Research ... 32 Table 4.1: Mix Design One ... 40 Table 4.2: Compressive Strength of Mix One (MPa) ... 41 Table 4.3: Mix Designs One and Two ... 43 Table 4.4: Compressive Strength of Mix Two (MPa) ... 46 Table 4.5: Mix Design Four ... 48 Table 4.6: Free Water Content of Mix One ... 49 Table 4.7: Free Water Content of Mix Four ... 50 Table 4.8: Compressive Strength of Mixes One and Four (MPa) ... 52 Table 4.9: Mix Design Three ... 53 Table 4.10: Compressive Strength of Mix Three ... 55 Table 4.11: Mix Designs Five, Six and Seven ... 58 Table 4.12: Compressive Strength of Mixes Five, Six and Seven (MPa) ... 62 Table 4.13: Indirect Tensile Strength of Mixes One to Four ... 65 Table 4.14: Relationship Between Compressive and Tensile Strength ... 661. INTRODUCTION
1.1 Background
Concrete is the most widely used structural material in the world, and therefore the production of it and its constituents are greatly relied upon in industry. The manufacture of ordinary Portland cement (OPC), the primary binder in a conventional concrete mix however, is well known for its environmental impacts. Approximately 1.35 million tons of greenhouse gases are emitted through the manufacture of OPC each year and therefore raises the concern for a cleaner alternative to be developed (Malhotra 2002).
Continued increase in the focus and restriction on global carbon dioxide emissions requires the research for a cleaner alternative to the use of Portland cement. Concrete made using a binder that does not present such environmental issues has been investigated in the past using fly ash and an alkaline solution. The method of substituting fly ash for portions of cement in a concrete mix has been established and is well documented (Huntzinger and Eatmon 2009). However, the use of 100% fly ash made concrete is limited in industrial applications, partly due to the cost of fly ash and, in contrast, the availability and convenience of cement. Research fields though are well interested in the production of concretes with 100% fly ash because of the sustainability of using this industrial waste product for a construction material.
This report investigates the effects of altering the mix design and properties of geopolymer concrete. Additives such as silica fume and calcium hydroxide have been used in anticipation of aiding the ambient temperature curing properties of the concrete. Further to this, properties of the concrete such as the effect the free water content has on the final strength have also been investigated. This research deals exclusively with the ambient curing of geopolymer concrete. This is to simulate site conditions that a concrete structure may be exposed to, and therefore investigate the feasibility of in‐situ cast geopolymer concrete.
1.2 Geopolymer
Concrete
The investigation into the use of fly ash‐based geopolymer concretes has increased since 2000 due to the environmentally sustainable option of using an industrial waste to form a useful material. Research and industry groups are excited about the prospect of a concrete made from industrial by‐products that would therefore negate the need for waste disposal of these materials. The development of geopolymer concrete mix design has been carried out previously at Curtin University, Western Australia. Hardjito and Rangan (2005) investigated the effects of aspects such as alkaline parameters, water content and curing conditions in “Development and Properties of Low‐Calcium Fly Ash‐Based Geopolymer Concrete”. Further to this, the production and testing of low scale beams has also been carried out (Hardjito and Rangan, 2005). The physical properties of geopolymer concrete such as creep, drying shrinkage and sulfate and acid resistance were also researched at Curtin (Wallah and Rangan, 2006).
The Centre of Materials Research at Curtin has investigated the use of including chemical additives to geopolymer pastes in order to increase the early strength under ambient curing conditions. This paste is essentially an aggregate‐less concrete that is made in much smaller quantities than the research for this current report. The mix design properties of geopolymer concrete were investigated by scaling up the production of geopolymer paste in the form of quantity and by adding aggregate to the product.
The concrete produced consisted of 77% by mass of aggregate, which is bound by a geopolymer paste formed by the reaction of the silicon and aluminium within the fly ash and the alkaline liquid made up of sodium hydroxide and sodium silicate solutions. Specimens produced were cured only under ambient conditions within the Civil Engineering laboratory at Curtin University.
1.3 Research
Aims
The present study aims to carry out a scaling up exercise of past work with geopolymer pastes and mortars that was undertaken by the Materials Research Centre at Curtin University, and therefore furthering the mix design knowledge of geopolymer concrete applications. This exercise focuses on progressing towards the production of a geopolymer concrete with additives included in the mix designs to develop a quicker setting concrete mix with a higher early strength. It must be noted however, that the mixing procedure differs greatly between the paste and concrete, as the handling time and the quantity of material produced is much greater in the production of geopolymer concrete.
The practical research for this report differs to many previous fly ash based concrete reports, as the fly ash based concrete is mixed using zero ordinary Portland cement. Although the production of fly ash does produce large amounts of carbon dioxide through the burning of coal, the use of it in concrete is seen as a sustainable option that negates the need for disposal of this waste.
The aim initially was on achieving appropriate mix designs and a mixing procedure that would consistently provide a 28 day compressive strength of at least 30 MPa.
A conventionally made geopolymer mix utilizing just sodium silicate and sodium hydroxide with no mix additives was made initially to act as a reference mix. All subsequent mixes produced were based primarily on this reference mix with materials either substituted in for fly ash or just as an additive. The mix designs are judged upon their compressive and tensile strengths accordingly. The main aims of the laboratory research for this thesis included: - To familiarize with the making of fly ash based geopolymer concrete.
- To develop an understanding of an appropriate mix procedure in the production of fly ash based geopolymer concrete.
- To develop an understanding of appropriate mix proportioning in the production of fly ash based geopolymer concrete.
- To observe the strength development of fly ash based geopolymer concrete under ambient curing conditions.
1.4 About this Report
This report is structured as follows; Chapter 2 presents a brief review of selected literature related to the environmental tribulations of ordinary Portland cement, the alternatives to mixing concrete utilizing OPC, and the previous research conducted in the use of fly ash‐based geopolymer concrete. The general background of geopolymer concrete production is investigated, along with mix proportioning, mixing procedures and curing properties.
Chapter 3 describes the experimental process in conducting the research for this report. Attention is paid to the materials used, mix designs, mixing procedures, curing conditions and the method of testing the geopolymer concrete specimens produced.
Chapter 4 presents and discusses the results of the research, drawing a comparison between the final strength and strength development of geopolymer concrete with varying mix designs cured under site conditions. Any observations noted during the experimental research being carried out are also stated, with explanations and justifications to clarify any unknowns. The present report’s summary and conclusions are included in Chapter 5. This section is based upon all results and observations discovered in the research throughout the year.
Further to this, a list of recommendations is given in Chapter 6, detailing suggested steps in furthering the research in the mix design development of geopolymer concrete. Concluding this report is a list of references and all relevant appendices.
2. LITERATURE REVIEW
Chapter 2 presents a background into the environmental impact of the manufacture of ordinary Portland cement (OPC) and other suggested alternatives to the use of cement. Research has been undertaken into the previous use of geopolymer concrete, and the mechanical properties resulting from mix design properties and different methods of curing.
Research was also conducted into the sustainable qualities of the use of production by‐ products in the manufacture of geopolymer concrete.
2.1 Ordinary Portland Cement and the Environment
Disregarding water, concrete is the most widely used material in the world. Unfortunately, the manufacture of the integral constituent, ordinary Portland cement (OPC), proves to be unsustainable with regards to it’s the environmental impact due to the emissions of carbon dioxide (CO2) and large requirement of energy in the production procedure. However, due to the high demand for structural materials, the requirement for cement and concrete will be substantial until an equally effective and economic alternative is available, and therefore deeming it necessary to either overlook the environmental impact of standard concrete production, which is highly unlikely, or develop alternatives that will decrease these effects.
Concrete International recognizes the situation at hand, and the article titled “Sustainable Development and Concrete Technology” quotes the current issues. ‘The
contribution of ordinary Portland cement production worldwide to greenhouse gas emissions is estimated to be approximately 1.35 billion tons annually or approximately 7% of the total greenhouse gas emissions to the earth’s atmosphere (Malhotra 2002)’.
The reason large amounts of CO2 are released during the manufacturing of cement is due in part to the immense heat that is required. The kiln used is heated to
temperatures of up to 1400°C, and therefore energy requirements to yield this temperature account for approximately half of the released CO2 in the production of cement, the second half is released during the calcination process in which calcium carbonate is reduced to calcium oxide (Hendriks et al. 2003).
The production of cement alone accounts for approximately 5% of the worlds carbon dioxide emissions. According to the International Energy Agency, approximately 0.81 kilograms of CO2 is generated per kilogram of cement produced annually throughout the world. The production of cement also produces millions of tons of Cement Kiln Dust (CKD) which is harmful to the respiratory system (Hendriks et al. 2003).
Due to the increasingly popular requirements for sustainable development within industry, research into methods of reducing greenhouse gas (GHG) emissions while maintaining the structural convenience of concrete has been carried out.
The US Concrete Industry has addressed the current GHG emissions incorporated with the production of concrete in “Vision 2030: A Vision for the US Concrete Industry.” In this, focus is put on making concrete an environmentally friendly construction material whilst maintaining its status as the mostly widely used material in industry (Mehta 2001). In recent times, researchers have attempted to produce concrete as an environmentally friendly product by replacing amounts of ordinary Portland cement from the mix with industrial by‐products such as fly ash and blast furnace slag.
Global warming continues to be a current concern within the public awareness, and what effects it will have on the human population in day to day life in the future. The continuing release of GHG through the burning of fossil fuels and land use change further increases the risk on earth of a rise in average surface temperatures and the flow on effects that it will have on sea levels.
Huntzinger and Eatmon (2009) uses life‐cycle analysis (LCA) to evaluate the environmental impacts and therefore the ‘global warming factor’ of the manufacture of Portland cement and three other technologies. The three alternatives discussed
include, “blended cement (natural pozzolans), cement where 100% of the CKD is
recycled into the kiln process, and Portland cement produced where CKD is used to sequester a portion of the process related to CO2 emissions.”
It was discovered that the most environmentally solution of the three was the blended cement. Substituting natural pozzolans for OPC will effectively reduce the ‘global warming factor’ of the product proportional to the amount replaced. In reality though, it will be seen that in industry, because of the consistent high demand of cement, most kilns are operating at above their effective capacity. This therefore means that using ‘blended cement’ in industrial applications would not be likely to reduce the net emissions of carbon dioxide (Huntzinger and Eatmon 2009).
As can be gathered through this review, most of the previous research available looks at the current situation of cement production and the damage it is causing to the atmosphere. The next logical step into this investigation is to either prevent this damage or offer alternatives to concrete using ordinary Portland cement.
It is in the opinion of many, that the use of ordinary Portland cement in concrete is not going to slow down, despite the ongoing research into alternative binders. It would therefore be a sustainable decision to investigate further into the mix design of concrete whilst minimizing the volumes of OPC being used. It would be seen that if the use of OPC is going to remain strong over the coming decade, keeping its use to a minimum whilst retaining both durable and workable concrete would provide great benefit to the GHG emissions.
2.2 Alternatives to Portland Cement in Concrete
As the growth in the world of infrastructure continues, the demand for concrete that is usable in an industrial application will be high for the foreseeable future. Concrete using binders other than ordinary Portland cement that leave a smaller carbon footprint, are therefore heavily investigated within the cement and concrete industry. The use of these concretes within an industrial application is limited however, and it
would be fair to say that concrete made without cement has not made a significant impact into the construction industry yet.
The research into environmentally sustainable concretes is however not limited to replacing the OPC. Suggestions have been put forward into forming ‘blended cement’ where products such as pozzolans are added to OPC in order to reduce the environmental effect of the concrete. Concrete that has had OPC replacement commonly consists of industrial by‐products such as coal fly ash or ground blast furnace slag (GBFS).
It has been suggested (Damtoft et al. 2008) that the cement and concrete industry is working positively in the hope of achieving sustainable solutions in environmentally friendly concrete. He suggests that using hydraulic binders, those which are based on Portland cement, have an incredible impact on the environment and sustainable development due to being easily the most widely used construction material worldwide.
Damtoft et al. also discussed in which ways the industry is acting in order to provide sustainable development within the field of reducing the environmental impact of concrete production.
The techniques discussed are as listed below:
- The addition of extra materials to the list of approved supplementary cementious materials (SCM’s) within current standards.
- Allowing more complex composite cements within current cement standards. Greater attention to be paid to blending properties.
- Development of methodology for the design of optimal performance for the use of blended cements.
Damtoft et al. (2008) clearly supports the use and further development of blended cements in industry, which directly reduces the CO2 emissions to the environment through replacing volumes of OPC.
The current amount of research into the use of fly ash as a hydraulic binder is far from limited. The use of coal fly ash in concrete has been investigated for years with very
positive results for its use in industry. Unfortunately, apart from this research and a small amount of testing using blast furnace slag, there is little information regarding the use of other industrial wastes to substitute of ordinary Portland cement in concretes. The development of a concrete mixture using a new OPC substitute would greatly progress the process of producing an environmentally friendly concrete. This coincides with Damtoft’s discussion in which additional materials should be investigated into their effectiveness of working as a supplementary cementious material.
2.3 Fly Ash based Concretes
The production of concrete that incorporates the complete replacement of OPC with industrial by‐products such as coal fly ash has been developed, yet it is far from fully established. The use of waste products promises to be a sustainable option in any case, as it negates the need for disposal of these materials, which can become both costly and of an environmental concern. Fly ash is a residue that is formed during the combustion of coal. In the past fly ash was released to the atmosphere during production, but in recent times as research presents that this previously useless by‐product can be used for other applications, the capture of it has been instigated. Fly ash’s main constituents are amounts of silicon dioxide (SiO2), aluminium oxide (Al2O3) and iron oxide (Fe2O3). Fly ash that is destined for experimental use can be examined in more depth in order to determine its chemical composition. X‐ray Fluorescence (XRF) analysis is used to determine the proportions of materials present within the fly ash.
The use of fly ash for concrete production is a popular option in theory, as it is available abundantly worldwide. The use of 100% fly ash based concrete however, is limited to date in structural uses. The Ash Development Association of Australia (ADAA) states that in 2007, Australia alone produced 14.5 million tons of fly ash, and that only 1.50 million tons (11%) was used in high valued applications such as
Though the practical use of 100% fly ash based in concrete is limited in structural applications, frequent laboratory work has been conducted into the investigation of mechanical properties of fly ash based concrete. A recent study at Montana State University, USA, discussed the method of preparation of fly ash based concrete. It should be noted that in their experiments, class F fly ash is utilized, in which the chemical reaction occurring is the hydration of water with the calcium in the fly ash (Cross, Stephens and Vollmer 2005). The major hurdle into the use of fly ash based concrete that was noted was the rapid rate of chemical reaction that occurs once water is added to the mix. It was discovered that the use of a retarder in these trials were essential, and that in the case that there was no retardation, hydration would occur immediately and that the concrete would ‘flash set’ in a matter of minutes. It was recommended to the researchers to attempt the use of borax to delay any setting of the mix, as has been discovered effective with OPC concretes. It then became an aim of the trials to gain an understanding of under which conditions the borax needs to be present to extend the placement time before setting (Cross, Stephens and Vollmer 2005). It was discovered that the effectiveness of borax was not determined equivalently to that in OPC based concretes. Rather than a simple relationship connecting the amount of borax required to the amount of cement used, it was determined that the effectiveness of the admixture was dependant on its physical properties and the rate at which it is added to the mix. When the relationships used for OPC based concretes were attempted to correspond to fly ash based concrete, it was found that the predicted setting times were largely inaccurate (Cross, Stephens and Vollmer 2005).
The development of fly ash based concrete has a promising future. Laboratory research carried out worldwide are consistently yielding compressive strengths equal to or greater than equivalent mix designs utilizing Portland cement. In the above mentioned report, Cross and Stephens also discovered that fly ash based concrete
gained strength at a rate equal to or faster than OPC concretes of similar mixes (Cross, Stephens and Vollmer 2005).
One a physical level, it is thought that the rounded shape of the fly ash particles maintain the workability of the concrete prior to setting. Fly ash particles are also smaller in size than that of OPC and therefore produce and more compacted a denser concrete set.
It should be recognised that the next step in the development of fly ash based concrete would be research into its durability as a structural material. To date, concretes with 100% fly ash have been limited to use on low strength applications. Before this material is to be introduced as a structurally safe and durable material though, development of mixes, prediction and control of strength, workability and set times must be obtained. The use of 100% fly ash concrete in these environments would require the knowledge that it develops soundly under site conditions, such as curing under ambient temperatures.
2.4 Geopolymer
Concrete
Davidovits first proposed that concrete could be made with a hydraulic binder, where in which the silicon and aluminium from the inclusive fly ash would react with an alkaline liquid. The reaction that occurs, polymerization, is significantly faster due to the alkaline conditions. The resultant three dimensional structures consisting of Si‐O‐ Al‐O bonds is a polymeric chain (Davidovits 1999).
The most conventional method of producing geopolymer concrete is the incorporation of a reaction between the fly ash and an alkaline solution formed from a metal hydroxide and silicate that forms an alkaline liquid. It is not uncommon for the constituents of the geopolymer alkaline solution to be sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). It is common practice in the mix design of geopolymer concrete, that aggregates occupy anywhere from 70 – 80% in volume by mass.
Johnson (2008) states that, “Geopolymer consists of silicon and aluminium atoms
bonded via oxygen into a polymer network. Geopolymers are prepared by dissolution and poly‐condensation reactions between alumino‐silicate binder and an alkaline silicate solution such as a mixture of an alkali metal silicate and metal hydroxide.”
Again, in the research carried out by Johnson it was noted how quickly the chemical reactions take place with the addition of the alkaline solution to the fly ash in the mixing process, therefore limiting the able handling time before setting begins to occur. Therefore, an aim of the research became that of determining a mixing procedure that would enable sufficient handling time whilst maintaining a concrete of workable consistency and could be used in industrial applications. It was discovered that if a preliminary mixture of the total aggregate volume and the metal hydroxide solution were formed first, and then the fly ash added, no reaction would take place until the metal silicate solution was added to the mix. This process of mixing generally was found to extend handling time up to 45 minutes consistently, and therefore provide a more suitable application for use on site (Johnson 2007).
Hardjito and Rangan (2005) concluded that it was favourable to mix the sodium hydroxide and sodium silicate solution at least one day prior to adding it to the dry materials. This was carried out under recommendation from Davidovits, who observed that when this was carried out, bleeding and segregation of the concrete no longer occurred. This combination was then added to the dry mixture.
This is in contrast to advice given to the author of this report by Curtin post graduate student M. Olivia (personal communication, 25 May, 2009). She advised that the mixing of sodium hydroxide and sodium silicate solution should occur on the day of mixing the concrete, otherwise the solution may solidify and the production of concrete will be extremely difficult. She stressed that research has shown experiences of the alkaline solution crystallizing before it is to be added to the dry materials, therefore deeming the pour to be a failure. Situations had also occurred in which the concrete mix had hardened to a point that it is unable to be poured whilst still in the mixer.
Sofi et al. (2007) had similar findings to Johnson (2007) in the research paper entitled “Engineering Properties of Inorganic Polymer Concretes (IPCs).” In it, he praised the use of concrete utilizing materials other than OPC in terms of their mechanical properties. He suggests that inorganic polymer concretes comprising of materials such as fly ash, can exhibit superior mechanical properties to ordinary Portland cement concretes. This is heavily dependent though on the chemical composition of the fly ash used.
The hurdle though, still stands at managing the quick setting nature of geopolymer concrete, and maintaining the concrete’s ultimate characteristics such as strength and durability is a prime concern when introducing mixture additions in order to retard the rapid setting. The fast setting characteristic of IPC, Sofi writes, can be taken as an advantage or disadvantage. Though the setting of IPC’s can occur rapidly, and the polymerization reaction occurs straight away, it continues over a length of time which extends beyond seven days. This contributes to the strength gaining characteristic of geopolymer which has a distinct behaviour in comparison to OPC based concretes. It was found within the IPC mixes that between 7 and 28 days, a development of compressive strength occurred of up to 15 MPa (Sofi et al. 2007).
The use of Geopolymer, to date has only been limited to low strength applications. This seems to remain the case, when in fact a lot of researchers praise the characteristics of the product. Johnson (2007) writes in the aforementioned report that the heat, fire and acid resistance of geopolymer concrete will be greater than that of Portland cement based concrete. Johnson used the geopolymer’s fast setting characteristic as an advantage, as he proposed that it be used in the production of concrete pipes and poles. Such manufacturing requires the use of concrete with zero slump, and processes that involve centrifugal stages, roller suspension and vertical casting. It was discovered that by manipulating the mix design, and therefore producing ‘no slump’ concrete, it was possible to utilize geopolymer concrete in preparing pipes and other consolidated moulded products.
setting, and therefore losing any plasticity, must be long enough to incorporate any required transport of the product. This becomes important because if the concrete is at its hardened state during transport, cracking is likely to occur and therefore a reduction in the final strength will be experienced. To overcome the associated problems of rapid strength gain will require control of the setting times of the concrete, or reverting back to casting products on site or in‐situ, therefore making it unlikely to have the availability of steam rooms or kilns available for the particular application.
2.5 Mix Proportioning of Geopolymer Concrete
The aim of the research conducted for this report was to further the mix design of geopolymer concrete by improving its ambient curing properties. Therefore, the mix proportioning carried out for this research was in the form of using additives to the geopolymer concrete mix, rather than re‐establishing standard mixes again. This meant that initial mix designs were based largely upon previously successful geopolymer concrete mixes that had already yielded substantial results.
It was found by Hardjito and Rangan (2005) that consistent results were gained upon keeping the alkaline solution at a sodium silicate‐to‐sodium hydroxide ratio of 2.5. This ratio was favoured over a lesser one because of the reliable results that it yielded, and because the sodium silicate solution is considerably cheaper than the sodium hydroxide pellets. A general proportion of alkaline solution‐to‐fly ash was settled upon at approximately 0.35. Upon investigation of the affects of the concentration of the sodium hydroxide solution, it was found that in mix designs of exact proportions, the mix with the higher concentration in molarity of the sodium hydroxide solution would yield a higher compressive strength. This was examined through the use of varying the molarity of the solution between 8 molars and 14 molars in mix designs of exact proportions.
Liu reports how geopolymer concrete can be produced by using other industrial wastes such as bauxite residues. It is noted how past research into the re‐use of these
products such as bayer liquor has been used to produce materials such as ceramics, cements, clay bricks and glazes. In the production of unsintered construction and building products, Liu suggested that the optimal proportions of raw materials show following: Bauxite Residue : 25 – 40% Fly Ash : 18 – 28% Sand : 30 ‐35% Lime : 8 – 10% Gypsum : 1 – 3% Portland Cement : 1% This composition has been used to produce building materials that has reached the 1st grade of Chinese standards for a brick (Liu et al. 2009).
2.6 Curing of Geopolymer Concrete
The present report deals with the ambient curing of geopolymer concrete, yet changing the method of curing has previously researched in geopolymer concrete. The ability of concrete to cure at ambient temperatures becomes useful in industrial applications when concrete is cast in‐situ or on site, as the availability of a kiln, especially on larger scale projects, is unlikely.
The conditions under which geopolymer concrete is cured directly relates to the durability and strength of the mix, as displayed by Hardjito et al. (2004). His results found that the curing of concrete at higher temperatures, up to 60°C, yielded a higher compressive strength than at a lower temperature, yet any increase in curing temperature over this threshold made no substantial difference to its strength. A proportional relationship was discovered between the length of curing time and compressive strength. The rate of setting of geopolymer concrete is well documented, yet it is likely that these cases were resultant upon short curing times. Hardjito et al. discovered the fast rate of polymerization only stalled the strength gain when the concrete was cured for short times, such as 24 hours. This contrasts with the strength
over a length of time when being steam cured, therefore increasing in strength with age. This strength development over time can be achieved with geopolymer concrete when curing time is extended. It was discovered that as the curing time increases in the range of 6 hours to 96 hours (4 days), the polymerization process is improved and therefore yields a higher compressive strength. It is noted though, that the strength increase after 48 hours of steam curing is not significant.
It is recommended that during curing of geopolymer concretes at elevated temperatures, samples should be wrapped and then sealed, this should be present for the duration that the samples are being cured at temperatures up to 100°C. This precaution has been suggested in order to prevent excessive evaporation of the samples during curing. This would cause a less dense concrete with a weaker compressive strength. It was also discovered that in wrapping the geopolymer concrete specimens, the mix did not harden immediately under ambient conditions. At room temperatures of below 30°C, hardening of the concrete did not occur for at least 24 hours (Hardjito and Rangan 2005).
Whilst interesting to know that it is possible to achieve a time‐dependant, strength‐ development behaviour with geopolymer concrete, in industry, it would not be very applicable. Rarely would you see concrete cast and then kept under controlled curing conditions for any more than 24 hours, and if it was cast in situ, all curing would be under ambient conditions. In a rare situation where formwork turnover is not as critical in a precast concrete environment, it would be possible to achieve an extended curing time under controlled conditions.
Wallah and Rangan (2006) reported how the strength development of geopolymer concrete varied with the conditions under which they were cured. Three batches of the same mix were produced at varying times in the year; May, July and September 2005, and cured under ambient conditions within the laboratory. The cylinders were released from their moulds one day after casting. It was discovered that specimens cured under ambient conditions exhibited significantly lower 7 day compressive strengths than those cured under elevated temperatures for the first 24 hours.
It was reported that under ambient curing conditions of geopolymer concrete, the 7th day compressive strength and subsequent strength gain with respect to age lies dependent upon the average ambient temperature at the time of curing. As the ambient temperature at casting increased, as did the 7th day and subsequent compressive strength’s tested at later dates. The compressive strength of the geopolymer concrete during July exhibited a 28 day strength of 31 MPa in comparison to 47 MPa for the mix poured in May. The average temperature experienced within July 2005 ranged from 8°C to 18°C, and 18°C to 25°C in May (Wallah and Rangan 2006).
2.7 Aiding the Early Strength of Concrete
The reaction between elements in fly ash based concretes is a slow process, and therefore only contributes to the strength development at later dates of age. This causes a problem in the utilization of fly ash concrete in ambient cured precast concrete applications, due to the low early strength and formwork turnover routines. Previous research in OPC based concretes has indicated that the inclusion of silica fume and hydrated lime (calcium hydroxide) yields positive results in increasing the early strength of concrete, as well as having the concrete mix set quicker.
Barbhuiya et al (2009) investigated the use of including silica fume and calcium hydroxide to concretes with a fly ash substitution of 30% of the ordinary Portland cement based content. Silica fume was added to the mix at 5% by mass of the cement content as a final addition when mixing the concrete. Hydrated lime on the other hand was substituted at a rate of 5% by mass of the total cementious materials. In order to investigate the early strength development of this concrete specimens were tested at 3, 7 and 28 days after casting. Specimens were cured in curing rooms at constant temperatures. The first 24 hours were spent at 20°C and then transferred to a moist curing room at 23°C and kept in water until testing.
Workability is seen to decrease upon the addition of hydrated lime, however to improve this, a super plasticiser was added. The addition of silica fume to the mix had no effect on the workability of a concrete mix. It was discovered that the addition of both silica fume and calcium hydroxide increased the early compressive strength of the concrete mixes. Testing at 3 days of age showed that the strength of both silica fume and hydrated lime mixes were equally higher, (30 MPa) than the standard concrete mix at 24 MPa. The major differences in compressive strengths were apparent at 28 days with a constant progression from the standard mix (49 MPa), fly ash inclusive of hydrated lime (53 MPa) and then the concrete mix incorporating silica fume with a 58 MPa 28 day compressive strength (Figure 2.1).
Figure 2.1: Compressive Strength of 30% Fly Ash Substituted Concrete
The use of calcium based additives into geopolymer pastes was researched by Temuujin, van Riessen and Williams (2009). Both calcium hydroxide and calcium oxide were substituted into geopolymer pastes for fly ash in order to accelerate the ambient curing (on average at 20°C) of the paste, and increase the compressive strength under these curing conditions. To form a proper comparison between the effects from curing conditions, specimens were oven cured being subjected to heats of 70°C.
It was found that the addition of calcium compounds improved the mechanical properties of geopolymer pastes cured at ambient temperatures, yet reduced the strength of those cured under elevated temperatures.
The results also showed that the addition of calcium hydroxide (Ca(OH)2) aided the ambient curing strength more so than calcium oxide (CaO). It is suggested that this is apparent because the calcium hydroxide is a reactive constituent of the geopolymer mixes. The use of calcium hydroxide would appear to present incomplete hydration of the product as it reacts with the alkaline solution in the formation of calcium hydroxide. Specimens with CaO added presented compressive strengths approximately 20% lower than those with calcium hydroxide.
It is suggested that the lower compressive strength in the pastes that is cured under elevated temperatures is due to the water evaporation within the mix, exhibited by lower density and higher porosity. At elevated temperatures, it is also suggested that the presence of calcium doesn’t allow the formation of three dimensional geopolymer network due to the fast dissolution of the paste. This therefore results in reduced mechanical properties of the final product. Under ambient conditions, it was found that by increasing the percentage of added calcium compound, the compressive strength increased with it. With a 3% addition of calcium hydroxide the compressive strength of 29 MPa compared to a geopolymer paste with no calcium additive which exhibited a strength of 12 MPa. In comparison, geopolymer with a calcium hydroxide inclusion of 1% and 2% showed strength of 24 MPa and 28 MPa respectively (Temuujin, van Riessen and Williams 2009).
3. EXPERIMENTAL PROCEDURE
3.1 Introduction
Chapter 3 presents the details of the research that was carried out in order to investigate the inclusion of additives in the development of geopolymer concrete mix design.
Due to the limited research conducted using fly ash‐based geopolymer concrete with zero OPC, a large part of the experimental work for this report focused on the mix proportioning and procedure for developing this concrete. The project’s aim included mix design development that would constantly yield concrete mixes with a consistent compressive strength of at least 30 MPa.
Due to the lack of previous mix design information using geopolymer concrete, initial mix design and procedures closely followed regular conditions for the production of geopolymer concrete using sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) to form the alkaline solution. A trial and error process was then used for fine tuning the strength of the mixes, including materials such as silica fume and calcium hydroxide in anticipation of developing a concrete mix that would cure faster and develop a higher compressive strength.
Experimental results were based upon compressive and tensile strengths, this is not unusual because compressive strength has a fundamental importance in the design of concrete structures. Tests for these parameters were for the majority of the mixes conducted at 7, 14, 21 and 28 days after casting. This was conducted to observe the short term strength development in concrete with the primary binder not being cement.
The current methods of producing and testing of ordinary Portland cement concrete were followed as closely as possible in the production of this geopolymer concrete.
This was a key point in the research, as it was important that a relevant comparison between the two products was formed and therefore investigating whether geopolymer concrete would be suitable to be produced on site. This included a general aggregate quantity of 77% by mass within the mix. The aggregate used within the different mixes originate from the same source throughout the year, in order to minimize the effect of varying aggregate properties.
3.2 Safety
Prior to any research beginning, an aim for this report was to develop a geopolymer concrete mix utilizing bayer liquor. This product is an industrial waste that is formed in the stage of removing bauxite in the refining of alumina. It has previously been used in the production of geopolymer pastes by the Centre of Materials Research department at Curtin University. The aim was to carry out a scaling up exercise of this paste by adding aggregates to the mix and increasing the quantity produced.
In order to gain access to the bayer liquor, numerous precautions needed to be carried out, due to the caustic nature of the product. The most important of these was the Edusafe risk analysis and compliance. This took into account what measures needed to be put in place so that safe handling procedures of this material could be carried out. A major influence that this programme had on the preparation was the requirement for a safety shower to be installed in the laboratory. Strict methods of storage and disposal also had to be planned that comply with the appropriate measures as outlined on the product’s Material Safety Data Sheet.
Due to the unsafe nature of the product and the relatively tight schedule to put all safety measures in place, the bayer liquor was not able to be brought to Curtin for use in concrete. These measures were also carried out for production of the conventional geopolymer concrete, however the procedure for the bayer was quite a bit more stringent due to it never having been used in concrete at Curtin University.
3.3 Materials
3.3.1 Fly Ash
The carried out experimental work utilized low calcium Class F fly ash obtained from Collie Power Station located south of Perth, Western Australia. Throughout the research, the fly ash used was from the same delivered batch. The fly ash was obtained in bulk bags and measured from here into the respective amounts required.
3.3.2 Sodium Hydroxide
A sodium hydroxide solution was utilized in all mixes as a constituent in the alkaline reactor. The product was obtained from a local supplier in the form of pellets with a purity of 98%. The solution was prepared by dissolving the pellets into distilled water at specified concentration in molars, M, for the concrete. In the laboratory research carried out, the solution was prepared with a concentration of approximately 10 M by dissolving the sodium hydroxide solids into distilled water.
To produce 1 kg of sodium hydroxide solution, 416.8 grams of pellets was dissolved into 583.2 grams of distilled water. The solid was added to the water gradually and stirred for approximately 20 minutes until all solid had dissolved. It was noticed that upon addition of the solid to water, the solution became hot as the exothermic reaction of dissolution carried out.
Upon preparation of the first mix produced, the sodium hydroxide solution was prepared 4 days prior to its addition to sodium silicate, and then production of concrete. It was discovered that after 4 days of standing, some sodium hydroxide solids had appeared in the solution after being dissolved completely when initially combined, this required stirring of approximately half an hour to reduce the solid content. Subsequent sodium hydroxide solutions made throughout the year were not prepared to a schedule prior to mixing the concrete. Generally though, dilution of
sodium hydroxide occurred a few days before concrete production in order to limit the time spent preparing the chemicals on the concrete mixing day.
3.3.3 Sodium Silicate
The sodium silicate was obtained in 30.5 kilogram pallets from a local chemical supplier, PQ Australia. The grade of material used is known as PQ‐D with a SiO2/Na2O ratio of 2.0. The pH of this liquid was 11.9 and was in the form of a heavy syrup.
The weight analysis of this material was as given by the supplier:
Na2O : 14.7%
SiO2 : 29.4%
Water : 55.9%
No dilution was required, after being weighed out it was used in the concrete as delivered.
3.3.4 Calcium Hydroxide
The calcium hydroxide used in Mix Three is known as HYLIME by Cockburn Cement. It was an industrial grade powder obtainable from the local hardware store, typically used in masonry mortars or plastering applications. This product was used in anticipation of developing a faster curing concrete with a higher early strength.
XRF analysis carried out on the product shows the majority of the composition of HYLIME to consist of 84% by mass of calcium oxide, 7.2% silicon dioxide and 5.3% magnesium oxide. 3.3.5 Silica Fume Silica fume was used in Mix Two as a fly ash replacement in hope that it would aid the ambient curing properties of the concrete.
The silica fume used was obtained from local supplier Simcoa, Western Australia. The product was delivered in bags of 10 kilograms, and was known just as Microsilica or densified silica fume. This same product is also used in concrete batching plants in Western Australia, in particular for marine applications. The silica fume is in the form of extremely fine particles and therefore makes the concrete less impermeable upon addition. 3.3.7 Alkaline Liquid
The alkaline solutions for all mixes produced during the research were prepared by combining the sodium hydroxide solution to sodium silicate gradually. This mixture was then stirred moderately for a few minutes and then sealed in the buckets with lids until addition to the concrete mix. This process took place immediately prior to beginning production of the concrete, the ratio of sodium silicate to sodium hydroxide was kept consistent at 2.5 upon recommendation from Hardjito and Rangan (2005).
3.3.8 Aggregate
The aggregate used was supplied by Cemex to Curtin University, stored outside uncovered in storage divisions. The aggregate supplied consisted of two components; coarse aggregate obtained from the Cemex Gosnells Quarry and a fine aggregate that originated from Baldivis Sand. For the purpose of this research, coarse aggregates were used with nominal sizes of 7mm, 10mm and 20mm, and fine aggregates in the form of sand.
The aggregate was measured approximately a week prior to pouring and sealed in bins. The moisture content of the aggregate was measured at the time of being used in the concrete, and subsequently used to determine the free water content of the concrete mix.
The aggregate proportions were found in accordance with utilizing British Standards BS 882.92 (Neville 2000, 172) grading requirement limits for all‐in aggregate. The grading curve was constructed in order to satisfy the grading limits with an application sourced from the University of Patras.
As can be seen the sieve analysis of the utilized aggregate displayed a grading‐gap, which is displayed on the grading curve below (Figure 3.1). This made proportioning the aggregate components a more stringent process. Neville (2000) suggests that a grading curve closer to the bottom limit is comparatively workable, and can therefore be used in mixes with a low liquid/binder ratio. The results of sieve analysis and grading combinations of the utilized aggregates can be seen below in Table 3.1. 30.5 30.1 27.5 5.5 0.7 1 0 10 30 95 6 35 50 100 0.063 0.15 0. 6 4.75 19 0 10 20 30 40 50 60 70 80 90 100 0 . 0 1 mm 0 . 1 mm 1 mm 1 0 mm 1 0 0 mm P e rcent a ge pas s ing Particle size (mm) BS Sieve ISO Sieve Figure 3.1: Grading Curve of Combined Aggregates (Grading Curve 2009)
Table 3.1: Grading of Combined Aggregates 20 mm 10 mm 7 mm Fine 19.00 mm 98.10 100.00 100.00 100.00 99.43 95‐100 9.50 mm 0.19 90.68 100.00 100.00 68.66 4.75 mm 0.14 1.16 44.67 99.94 41.37 35‐55 2.36 mm 0.14 0.71 1.49 99.78 30.46 1.18 mm 0.14 0.69 0.52 99.47 30.12 600 μm 0.14 0.69 0.37 70.98 21.53 10‐35 300 μm 0.13 0.68 0.25 17.81 5.55 150 μm 0.11 0.66 0.11 1.96 0.75 0‐8 Ratio 30 15 25 30 BS 882.92 Sieve Aggregates Combination
3.4 Preliminary
Laboratory
Work
The aim of this research was to gain a knowledge and understanding of the effect of altering mix designs in a geopolymer concrete mix. Due to the lack of experience in any geopolymer concrete production by the author, it was suggested that to begin with, a standard geopolymer concrete mix using the established sodium hydroxide and sodium silicate alkaline solution would be made first to familiarize with the process and use a reference to other mixes.
The first two mixes were undertaken at the beginning of June, 2009, with the use of the 70 litre capacity pan mixer (Figure 3.2) to produce approximately 65 litres (156 kilograms) of geopolymer concrete. Samples were placed in test specimens, 100mm x 200mm compression cylinders and 150mm x 300mm tensile cylinders, and cured under the ambient conditions after pouring.
The preliminary laboratory works focused on the following main objectives:
- To familiarize with the making of fly ash based geopolymer concrete.
- To develop an understanding of an appropriate mix procedure in the production of fly ash based geopolymer concrete.
- To develop an understanding of appropriate mix proportioning in the production of fly ash based geopolymer concrete.
- To observe the strength development of fly ash based geopolymer concrete under ambient curing conditions. 3.4.1 Mixing Procedure The mixing procedure plays a vital role in the production of geopolymer concrete due to the unstable nature of some mixes. If constituents are added in the wrong order, it is possible that the concrete may flash set in the mixer, causing both a failed mix and a tough clean up. For this reason, a particular order was followed in the concrete mixing during this research. Cylinder moulds were first prepared for concrete pouring by coating them with mould release. For the use of geopolymer concrete, a product by the name of Valsof PE‐40 was used as the mould release, as the usual grease would not work the same as with cement based concretes.
The alkaline solution consisting of sodium hydroxide and sodium silicate was combined at the beginning of the day when producing concrete. This came under recommendation in order to avoid the solution crystallizing over a long stationary period, an outcome that would deem the concrete mix design to differ if water was used to dissolve the solid again. The sodium hydroxide solution was added carefully to the second constituent and mixed thoroughly, before being sealed with lids prior to mixing time.
The mixing procedure for geopolymer concrete was similar to that of conventional OPC concrete. All dry aggregates and fly ash were first added to the pan mixer and mixed for a few minutes to properly combine all sizes. After this dry mixing, the alkaline solution and any extra water was then added gradually and then mixed for a further three minutes, or, until an adequately combined mixture was formed.
