Fly Ash Based Geopolymer Mortar

International Seminar on Climate ChangeEnvironmental Insight for Climate Change Mitigation

Solo, 4-5 March 2011ISBN No 979-978-3456-85-2

.

TOWARD ENVIRONMENTALLY FRIENDLY CONSTRUCTION MATERIAL WITH CLASS C FLY ASH-BASED GEOPOLYMER Investigation on Fresh and Mechanical Properties of Class C Fly Ash -

Based Geopolymer Mortar

SA Kristiawan

Civil Engineering Department, Sebelas Maret University, [email protected]

Abstract The use of cement in construction industries poses a threat to environment based on assessment on two environmental parameters i.e. climate change and fossil fuel depletion. Alternatives environmentally friendly materials that could substitute cement both partially or totally have been developed using waste material such as fly ash. This paper presents the results of investigation on fresh and mechanical properties of Class C fly ash-based geopolymer mortar. The fresh properties investigated include setting time, slump flow, slump time and rate of flow while mechanical property observed is compressive strength. Based on the observed properties, potential utilizations of Class C fly ash-based geopolymer mortar are suggested which include as flowable backfill, structural fill and overlay materials.

Keywords: Class C fly ash, compressive strength, geopolymer, mortar, rate of flow, setting time, slump flow, slump time

1. INTRODUCTION Climate change has an impact on various aspects of human life. It is also recognized that climate change poses significant risk to building and other infrastructures. Engineers are now challenged to design, build and maintain all sort of infrastructures that can adapt to the changing environment. The risks associated with climate change are the potential for increased frequency of extreme daily rainfall events, affecting the capacity and maintenance of drainage infrastructures (CSIRO Report on Infrastructure and Climate Change Risk Assessment for Victoria, 2007). As a result a housing development area, for example, when years ago expected to be free from flooding is now facing this risk and calamity. Other threat associated with climate change is the effect of temperature

rise on building: a home becomes hotter so people cannot live in comfortably anymore or has to spend more on a technology that could assist in recovering thermal comfort.As the environment affects building and infrastructures, it is also true that construction activities have a destroying effect on environment which to some extent contribute to the climate change. The effect of construction activities on environment could be traced starting from the use of materials in the construction project to the activities related to the use of the building. In term of the use of materials, it includes exploitation of construction materials without taking into consideration its impact on environmental degradation, the production process of the many construction materials in particular cement which consumes much energy and gives off CO2

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International Seminar on Climate ChangeEnvironmental Insight for Climate Change MitigationSolo, 4-5 March 2011ISBN No 979-978-3456-85-2

.emission, etc. In the meantime, operation of building also consumes energy to provide light, thermal comfort, etc for the occupants activities which, in turn, increasing pressure on environment. As awareness from the potential environmental impacts of construction activities has grown, many designers, property owners and other construction professionals have sought to take a more environmentally responsible approach to designing, constructing and maintaining building and other infrastructures. In term of selection of construction materials, environmental parameters have been proposed to evaluate performance of materials related to their environmental effects (Anderson et al, .2002). These parameters are summarised in Table 1. Using these parameters, a variety of construction materials utilized in building and other infrastructures could be compared.

2. ENVIROMENTAL IMPACT OF CEMENT AS CONSTRUCTION MATERIAL Cement is one of major construction materials used in modern building and infrastructure around the world. Indonesia, as a developing country, consumed cement at about 41 million tonne in 2009 and this number represents an increase of around 1.5% in comparison to that of 2008. It is projected that the consumption of cement will be 46.5 million tonne in 2011 (Sutiyono, 2009). The increase in cement consumption will raise environmental concern. Evaluation of

manufacturing cement impact on environment could be associated with two environmental parameters i.e. climate change and fossil fuel depletion for the following reasons: in the first, the production of 1 tonne of cement directly generates 0.55 tonne of chemical CO2 and requires the combustion of carbon-fuel to yield another 0.45 tonne of CO2. As the need of cement is increased, so the contribution of CO2 emission from cement manufactures is also increased. In the 1980, the rate of the world-cement production already exceeded the rate of atmospheric CO2 concentration. This could be interpreted as the rate of CO2

emission produced from cement industries surpassed that from other human activities that produce major CO2 emission i.e. energy consumption and transportation (Davidovits, 1994). In the second, the production of cement requires a lot of energy to heat the clinker up to 1500oC. The amount of coal required to manufacture one tonne of cement is between 100 kg and about 350 kg, depending on the process used (Neville and Brooks, 1987). With projected cement consumption in Indonesia for 2011 is about 46.5 million tonne; it means it requires at least 4.65 million tonne of coal for cement manufacturing process. Clearly, this number of coal consumption will directly be a factor in fossil fuel depletion. Being concern with those environmental impacts, attempt had been done to reduce the use of cement by replacing part of the cement with other cementitious materials to form blended cement. The cementitious

Table 1. Environmental parameters for evaluating construction materialsEnvironmental Parameter Description

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.1. Climate change2. Fossil fuel depletion3. Ozone depletion4. Human toxicity5. Waste disposal6. Water extraction7. Acid deposition8. Ecotoxicity9. Eutrophication10. Summer smog11. Mineral extraction

Global warming or greenhouse gasesCoal, oil or gas consumptionGases that destroy the ozone layerPollutant that are toxic to humanMaterial sent to landfill or incineratorMains, surface and groundwater extractionGases that cause acid rain, etcPollutant that are toxic to ecosystemWater pollutants that promote algal bloom, etcAir pollutants that cause respiratory problemsMetal ores, minerals and aggregates

materials used are pozzolanic types either from natural resources or industrial by product (waste) such as blast furnace slag and fly ash. Cement manufacturer in Indonesia now produces this type of blended cement known as pozzolanic portland cement (PPC).

3. CEMENT REPLACEMENT 3.1. Fly Ash as Partial Cement ReplacementFly ash is a residue of combustion of the finely ground coal in the generation of electric power. Fly ashes may be sub-divided into two categories, according to their origin (Wesche, 1991):—Class F: Fly ash normally produced by burning anthracite or bituminous coal which meets the requirements applicable to this class. Class F consists of low calcium (CaO<10%) and the total components of SiO2 + Al2O3 + Fe2O3 are more than70%.—Class C: Fly ash normally produced by burning lignite or sub-bituminous coal which meets the requirements applicable to this class. Class C fly ash may have lime contents (CaO) in excess of 10 % and the total components of SiO2 + Al2O3 + Fe2O3

are more than 50%.The Indonesian Suralaya Electric Power Plant (known as PLTU Suralaya) is one of the main electric powers that supply 3400 MW electricity and creates 24.000 tonne of fly ash per month as by product. With increasing electricity demand in the country by 10% per annum, more electric power has to be built with a target of generating 10.000 MW. Consequently, more waste is

produced and has to be handled. The utilization of fly ash as cement replacement in concrete production is based on its capability to react with CaOH at room temperature. This pozzolanic reaction will improve concrete properties in term of higher strength and more durable than ordinary concrete. The amount of cement replaced by fly ash in blended cement is in the range of 20-30%. A further important achievement with regard to the use of fly ash as cement replacement is the development of high-volume fly ash (HVFA) concrete that uses only approximately 40% of cement, and yet possesses excellent mechanical properties with enhanced durability performance. The test results show that HVFA concrete is more durable than ordinary concrete (Malhorta, 2002).

3.2. Fly Ash-Based GeopolymerIt is estimated that in 2010 fly ash production from two major countries i.e. China and India is about 780 million tonnes. Unfortunately, there is only a tiny fraction of the available fly ash that has been used world wide. One of the effort to increase the use of fly ash in the concrete industries is the development of fly ash as binder (cementitious material) replacing cement totally. Fly ash can be activated by alkaline so it can have cementitious propertiy as that of cement. This cementitious property does not rely on calcium silicate hydration, but it is obtained from an inorganic polycondensation, and so-called geopolymerisation. The term

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.geopolymer is familiarly used to name this material (Davidovits, 2007).Investigations on fly ash-based geopolymer has been carried in many countries. The performance of fly ash-based geopolymer is very promising in term of its strength. It is common to have concrete with strength of 40-80 MPa that are produced from fly ash-based geopolymer (Hardjito et al, 2004; Dolezal et al, 2007). It is also reported that high strength concrete (80-100 Mpa) is possible to be made from fly ash-based geopolymer (Nugteren et al, 2009). There are other advantages of using fly ash-based geopolymer such as good bond strength, low drying shringkage and creep (Fernandez-Jimenez et al, 2006), resistance to sulfate attack (Hardjito et al, 2005).

4. CALSS C FLY ASH - BASED GEOPOLYMERMost of published papers related to geopolymer concrete are those of using Class F fly ash. The present paper deals with investigation on Class C fly ash and study on the fresh and mechanical characteristics. The fresh properties investigated include setting time and slump flow while the mechanical property investigated is compressive strength.

4.1. Materials UsedFly ash used in this study is supplied from commercial ready mix concrete industry, which obtained the material from Paiton Electric Power Plant (PLTU Paiton). The chemical composition of fly ash used in this research is given in Table 2. As shown in the table, the CaO component of the fly ash is more than 10% and the total components of SiO2 + Al2O3 + Fe2O3 are more than 50%. Based on these components percentage, the fly ash could be categorized as Class C.

Table 2. Chemical composition of fly ash Oxide %

SiO2 45.27

Al2O3 20.07

Fe2O3 10.59

TiO2 0.82

CaO 13.32

MgO 2.83

K2O 1.59

Na2O 0.98

P2O5 0.41

SO3 1.00

MnO2 0.07

Alkaline used to activate the fly ash type C are sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The sodium hydroxide is available in the form of pellet, which has to be dissolved in water to provide a concentration of 8M. Meanwhile, the sodium silicate is supplied in liquid form. These two liquid alkaline are blended at proportions that will give alkaline modulus i.e. ratio of SiO2/Na2O of 1.25. The total amount of water to binder ratio w/b (water/fly ash) is varied from 0.2 to 0.3 and the ratio of binder to sand (fly ash/sand) is 1:2. The total amount of water includes that for dissolving sodium hydroxide. Detail proportions of Class C fly ash-based geopolymers mortar used in this research are presented in Table 3. For measurement of setting time, the proportions are similar to those given in Table 3 but with no sand.

4.2 Specimens PreparationSpecimens were prepared according to the following procedures: first, sodium hydroxide was mixed with water at specified proportion resulting in concentration of 8M. The dissolved sodium hydroxide was blended with sodium silicate for three minutes until homogenous solution of alkaline activator was obtained. The solution was then stored in laboratory room for 24 hour before it was used to mix up geopolymer.For measurement of setting time, the specimens were prepared by mixing the solution of alkaline activator with fly ash and water (i.e. water that had not been used to dissolve the sodium hydroxide) according to the specified proportions. The

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.mixtures were then put into Ebonite Ring for setting time measurement.Meanwhile the specimens for slump flow and compressive strength tests were produced as follows: firstly, fly ash and sand were mixed at specified proportions and then water was poured in to the dry

mixtures. The mixtures were blended with water and then the alkaline activator solution was added up to compose final mixtures of geopolymer mortars. The fresh geopolymer mortars were then separated into two groups. The first group was used

Table 3. Proportion of geopolymer mortar

for slump flow tests and the second group was poured into 50x50x50 mm moulds to produce specimens for compressive strength measurement. The compressive strength test specimens were cured in a laboratory room temperature and humidity (30oC and 70% RH) before testing them at a various ages.

4.3 Measurements of Fresh and Mechanical Properties4.3.1 Setting Time TestThere is currently no standard method to determine the setting time of geopolymer. The present research adopts that used for measuring setting time of hydraulic cement as specified in ASTM C-191. Setting time is an indication that cementitious properties is developing and achieving certain degree of resistance against penetration. Both geopolymer and hydraulic cement is cementitious materials even though the process of setting between the two is different. Setting times were measured based on the depth of the needle penetration in the geopolymer paste. The depth of penetration and the corresponding time were recorded until the paste reaching its final setting state. The setting times correspond to penetration depth of 25 and 23 mm for initial and final setting time, respectively.

4.3.2 Slump Flow Test

Slump flow test was carried out in accordance with ASTM C1611. The test was intended to specify the consistency of geopolymer mortar. Slump flow is simply the measurement of the diameter of concrete after subsidence in conventional slump test (Figure 1). In addition, it is also measurement of slump time i.e. the time required for geopolymer mortar to subside at 500 mm in diameter. Another parameter determined in slump flow test is the rate of flow. This parameter is an indication of how fast is the geopolymer will flow. The rate of flow are calculated from 500 mm, which representing the diameter of geopolymer mortar after removal of cone, divided by the slump time.

Figure 1. Slump flow test

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Mix Designation

Fly Ash (gr)

Sand (gr)

NaOH (gr)

Na2SiO3 (gr)

Water (gr)

FC-G 0.2 4396 8792 281.34 562.69 879.2FC-G 0.225 4397 8793 316.51 633.02 989.1FC-G 0.25 4398 8794 351.68 703.36 1099.0FC-G 0.275 4399 8795 386.85 773.69 1208.9

FC-G 0.3 4400 8796 422.02 844.03 1328.8


.

4.3.3 Compressive Strength TestCompressive strength of geopolymer mortars were determined by crushing 50x50x50 mm specimens following ASTM C579-01. The tests were carried out at the age of 1, 7, 14, 28 and 56 days.

4.4. Results and Discussion4.4.1 Setting TimeThe initial and final setting time of Class C fly ash-based geopolymer paste is presented in Figure 2. It indicates that w/b ratio affects both initial and final setting time in a similar manner i.e. increasing w/b ratio will result in prolonging both initial and final setting time. All the mixtures give lengthy setting times i.e. a minimum of 14 and 21 hours in initial setting time and final setting time, respectively. Even some of the mixtures have not reached their initial setting after 24 hours.

Figure 2. Setting time of Class C fly ash-based geopolymer paste

The setting time of Class C fly ash-based geopolymer paste is longer than the setting time of ordinary cement paste which normally achieved within 2-6 hours. This implies that hydration mechanism of the cement paste occurs at faster rate than geopolymerisation of geopolymer paste.The geopolymerisation process, which results in developing cementitious properties, is indicated by the penetration depth versus time. As the cementitious properties are developed, the geopolymer paste will solidify causing more resistance against penetration. The penetration depth (P) versus time (t) may be represented in mathematical formula as follows (Polivka and Klein, 1960):

P=A ( t−t0 )B

(1)where t0 denotes the time when depth of penetration equals to 50 mm and both A and B are constanta. The reason why the formulation has to be presented as a function of (t- t0) instead of t as originally suggested by Polivka and Klein (1960) is that the apparatus used for measuring penetration depth has a gauge limit up to 50 mm. When the geopolymer paste is still in fluid state and so the penetration depth is higher than 50 mm, the apparatus could not measure the value of penetration depth. It

(a) w/b 0.2 (b) w/b 0.225

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.

(c) w/b 0.25

(e) w/b 0.3

(d) w/b 0.275

Note: Mathematical formulation of penetration depth versus time for all mixtures with the value of t0 respectively as follows:

(a) t0 = 1 hour(b) t0 = 2 hour(c) t0 = 4 hour(d) t0 = 5 hour(e) t0 = 6 hour

Figure 3. Penetration depth versus time and its corresponding mathematical formula

Figure 4. Effect of w/b ratio on constanta A

is why measurement was only started after t0 when the penetration depth attained 50 mm. Figure 3 shows the penetration depth versus time for all the mixtures and its corresponding mathematical formula. Each mixture has its respective t0. As can be seen from the figure, the mathematical formula could be expressed with single value of power B i.e. -0.22. Meanwhile, the effect of w/b ratio is to alter the value of constanta A. It is found that increasing w/b ratio will cause an increase of constanta A (Figure 4).

4.4.2 Slump FlowThe results of slump flow test are presented in the form of slump flow, slump time and rate of flow versus water to binder ratio (w/b) as given in Figure 5-7, respectively. It clearly suggests that increasing w/b ratio will increase slump flow and reduce slump time almost linearly. It should also be noted that the rate of an increase value in slump flow and a decrease in slump time is dissimilar. The dissimilarity could be indentified straightly by comparing the slope of curves in Figure 5 and 6. This means that by increasing w/b ratio at the same rate will cause an increase in slump flow and a decrease in slump time at different increment. The effect of w/b on slump flow and slump time could be directly due to the fluid content in the geopolymer mortar. Having higher fluid content in the mixture with higher w/b ratio, there will be lesser friction between the solid ingredients in the mixture. Hence, there is lower yield stress which will be required to drive the geopolymer mortar freely flowing.

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.

Figure 5. Effect of w/b ratio on slump flow

Figure 6. Effect of w/b ratio on slump time

Figure 7. Effect of w/b ratio on rate of flow

Based on the value of slump flow, Class C fly ash-based geopolymer mortar could be categorized as flowable material which is similar to self consolidating concrete.In term of rate of flow, there seems that an increase of w/b ratio from 0.2 to 0.225 does not significantly enhance the value of rate of flow. However, the effect is significant when w/b ratio is increased beyond 0.25. This trend is obviously different to that of the effect of w/b on slump flow and slump time, where both effects show almost linear relationship. The explanation lays on the fact that the effect of w/b ratio on slump flow and slump time are dissimilar in rate as stated before. Since rate of flow is calculated from the 500 mm slump flow

divided by slump time, it is why that the effect of w/b ratio on rate of flow shows non linear.

4.4.3. Compressive StrengthThe rate of compressive strength development with time (Figure 8) is similar for all mixtures i.e. it is almost linear up to 14 days, after which the rate of the strength development diminishes. Each mixture gains strength in different rate, with FC-G 0.3 (mixture with w/b ratio 0.3) reaches the highest strength development. It is also interesting to recognize that after 28 days, all mixtures still develop in strength. Based on its compressive strength gain at later time (more than 28 days), it could be concluded that the effect of w/b ratio is to increase strength. This effect could be traced from the fact that as mixing water is increased; it follows by increasing the alkaline activator content since the concentration of sodium hydroxide and alkaline modulus are kept constant (see Table 3). With more alkaline activator available in the mixture, it causes more effective geopolymerisation. In turn, the strength development is higher in mixture with higher w/b ratio.In term of strength level, the Class C fly ash-based geopolymer mortar observed in this study has a moderate strength and so potentially applicable to be utilised as structural construction material.

Figure 8. Strength development of Class C fly ash-based geopolymer mortar

4.4.4. Conclusions

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. Class C fly ash-based geopolymer

exhibits lengthy setting time. The consistency of Class C fly-ash

based geopolymer mortar as measured from slump flow is affected by w/b ratio and the value indicates that this material is flowable.

Class C fly ash-based geopolymer mortar shows moderate strength and could be utilized as structural construction material.

5. POTENTIAL APPLICATION OF CLASS C FLY ASH - BASED GEOPOLYMER MORTAR.Given the stated properties, Class C fly ash-based geopolymer could be exploited in the constructions as:

flowable backfill material structural fill material structural overlay material etc

Flowable backfill material is self-compacting material with flowable consistency that is used to fill or backfill in various constructions such as sewer trenches, utility trenches, conduit encasement, other buried structure, bridge abutment, retaining wall, and road cuts. The beneficial of using material with flowable properties is that it does not require compaction effort. Most current backfill material is low in strength (less than 8.3 MPa) while Class C fly ash-based geopolymer mortar could develop moderate strength. Hence, the use of Class C fly ash-based geopolymer mortar as flowable backfill material will have advantage over the conventional backfill material such that, for example, bridge abutment and retaining wall will be more robust against horizontal forces.Utilization of Class C fly ash-based geopolymer mortar as structural fill material could be executed in the following structures: foundation sub-base, subfooting, floor slab base, pavement bases and conduit bedding. Due to moderate strength that can be achieved, the use of this material will

also provide additional load bearing when applied as structural fill material.Structural damage occurs in road, bridge deck and industrial ground floor could be remedy by overlay technique. Class C fly ash-based geopolymer mortar could be used as material for this remedial application. With its flowable properties, this material has advantage especially it could penetrate to fill the void of damage road or floor.There could be another potential utilization of Class C fly ash-based geopolymer mortar as construction material. However, further investigation has to be carried out to support the more widely applications. The investigation includes how to accelerate its setting time so rapid hardening Class C fly ash-based geopolymer mortar could be obtained and applied as patch repair material.

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Fly Ash Based Geopolymer Mortar