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International Conference on Sustainable Infrastructure and Built Environment in Developing Countries November, 2-3, 2009, Bandung, West Java, Indonesia ISBN 978-979-98278-2-1

TheEffectofDosageandModulusofActivatorontheStrengthofAlkaliActivatedSlagandFlyAshBasedGeopolymerMortar

Andi Arham Adam1,*, Tom Molyneaux2, Indubhushan Patnaikuni2, David Law3 1Universitas Tadulako, Palu , Indonesia

2RMIT University, Melbourne, Australia 3Herriot Watt University, Edinburgh, Scotland, UK *Corresponding author: adam.arham@gmail.com

Abstract The use of industrial by-products to partly substitute ordinary Portland cement (OPC) has become common practice. However, the total replacement of OPC has only recently become a focus of research due to the environmental impact, as the production of OPC greatly contributes to the production of CO2 to the atmosphere. Research has shown that alkali activated binders can achieve similar strengths to both ordinary Portland cement and blended cements. The aim of the work reported in this paper is to compare the strength of two different alkali activated cementitious materials; an alkali activated slag (AAS), and a fly ash based geopolymer mortar. To achieve this purpose, one set of mortars has been prepared with slag activated by a low dosage of alkaline solution and another set with fly ash activated by high alkaline solution. Variable alkali moduli were employed for both types of alkaline solution. The study shows that dosage of activator has a significant influence on the strength of both AAS and fly ash based geopolymer mortars. However, there is a limit for increasing the strength by increasing the alkali modulus (Ms), beyond this limit, a reduction of strength is likely to occur. Given the alkali modulus and activator dosage, it was found that Ms =1 in 5% Na2O for AAS and Ms =1.25 in 15% Na2O for fly ash based geopolymer were the optimum mix composition providing the highest compressive strength for the mortars.

Keywords: alkali, activator, geopolymer, modulus.

1. Introduction It is widely known that the production of Portland cement has a high energy requirement

and contributes large quantities of CO2 to the atmosphere. However, at present Portland cement is still the main binder in concrete construction and the search for more environmentally friendly materials is essential.

One possible alternative is the use of alkali-activated binder using industrial by-products containing silicate materials (Gjorv, 1989, Philleo, 1989). The most common industrial by-products used as binder materials are fly ash and ground granulated blast-furnace slag (GGBS). The GGBS has been widely used as a cement replacement material due to its latent hydraulic properties, while fly ash has been used as a pozzolanic material to enhance physical, chemical and mechanical properties of cements and concretes (Swamy, 1986).

The GGBS reacts directly with water, but requires an alkali activator. In concrete, this activator is the Ca(OH)2 released from the hydration of Portland cement. By contrast fly ash is a pozzolanic material which reacts with Ca(OH)2 from Portland cement hydration forming calcium silicate hydrate (C-S-H) as the hydration product of Portland cement. Thus, when used with Portland cement, GGBS or fly ash will not start to react until some Portland cement hydration has taken place.

Recent research has shown that it is possible to use fly ash or slag as a sole binder in

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mortar by activating them with an alkali component, such as; caustic alkalis, silicate salts, and non silicate salts of weak acids (Talling and Brandstetr, 1989). There are two models of alkali activation. Activation by low to mild alkali of a material containing primarily silicate and calcium will produce calcium silicate hydrate gel (C-S-H), similar to that formed in Portland cements, but with a lower Ca to Si ratio (Bakharev and Patnaikuni, 1997, Brough and Atkinson, 2002). The second mechanism involves the activation of material containing primarily silicate and aluminates using a highly alkaline solution. This reaction will form an inorganic binder through a polymerization process (Xu, 2002, Sindhunata, 2006). The term Geopolymeric is used to characterize this reaction, and accordingly, the name geopolymer has been adopted for this type of binder (Davidovits, 1991). The geopolymeric reaction differentiates geopolymer from other types of alkali activated materials (eg. alkali activated slag) since the product is polymer rather than a C-S-H gel.

In order to investigate the effectofdosageandmodulusofactivatoronthestrengthofalkaliactivatedslagandflyashbasedgeopolymer,mortars have been prepared with GGBS activated by a low dosage of alkaline solution while others prepared with fly ash activated by high alkaline solution. Variable alkali moduli were employed for both type of alkaline solution.

2. Materials and Methods The ground granulated blast-furnace slag (GGBS) supplied conformed to Australian

Standard AS 3582.2-2001. The fly ash was a low calcium fly ash from Gladstone power station conforming to AS 3582.1-1998. The chemical analysis of these materials is given in Table 1.

Sodium silicate solution (Na2SiO3) of 1.53g/cc density with an alkali modulus (Ms) of 2 (Na2O = 14.7% and SiO2 = 29.4%) was used. Sodium hydroxide solution (NaOH) was prepared by dissolving sodium hydroxide pellets in deionised water.

Table 1 Composition of cementitious materials (%) Component Slag Fly ash SiO2 33.45 49.45Al2O3 13.46 29.61Fe2O3 0.31 10.72CaO 41.74 3.47MgO 5.99 1.3K2O 0.29 0.54Na2O 0.16 0.31TiO2 0.84 1.76P2O5 0.53Mn2O3 0.40 0.17SO3 2.74 0.27S2- 0.58 0.21

A water to binder ratio of 0.5 was used to prepare all AAS mortars. In the case of the fly ash based geopolymer mortars, the water to solid ratio of 0.37 was used instead of the water to binder ratio. The amount of water in the mix was the sum of the water contained in the sodium silicate, sodium hydroxide and the added water, while the amount of solid was the sum of the weight of fly ash, and the solid contained in the activator solution. The sand to binder ratio was 2.75

Table 2 summarizes the composition of the alkaline activators. Liquid sodium silicate and sodium hydroxide were blended in different proportions, providing the alkali modulus in solution (mass ratio of SiO2 to Na2O) ranging from 0.75 to 1.25 for AAS mix and 1 to 1.5 for fly ash based geopolymer mix. Two dosages of Na2O by slag mass in the solution, 3% and 5%

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were investigated for the AAS mix. For the Geopolymer mix, activator dosages of 10% and 15% by fly ash weight were used.

Table 2 Details of the proportions of the alkaline solution

Mortar

binder Alkaline solution

Na2O (% by binder weight)

Ms (SiO2/Na2O)

AAS3-0.75 Slag 3 0.75 AAS3-1 Slag 3 1 AAS3-1.25 Slag 3 1.25 AAS5-0.75 Slag 5 0.75 AAS5-1 Slag 5 1 AAS5-1.25 Slag 5 1.25 G10-1 Fly ash 10 1 G10-1.25 Fly ash 10 1.25 G10-1.5 Fly ash 10 1.5 G15-1 Fly ash 15 1 G15-1.25 Fly ash 15 1.25 G15-1.5 Fly ash 15 1.5

The mixing was performed using a 5-liters Hobart mixer, the mix was then poured into 5 cm cubic moulds and vibrated for 1 minute. One set of the AAS specimens was left for 24 hours at room temperature and demoulded before being subjected to 80C steam curing for a further 24 hours and then allowed to cool in a humidity cabinet at 20 C and 90% relative humidity (RH) for a further 24 hours and subsequently stored at room temperature before testing. Another set of the specimens was cured at 20C water for 6 days after demoulding and then left at room temperature prior to testing

The structural integrity of the geopolymer specimens left at room temperature was not good enough to allow the specimens to be demoulded before three days, therefore a different curing regime was applied. After leaving for 24 hours at room temperature, the specimens were wrapped with cling film and left in the oven for 24 hours at 90C and cooled before they were demoulded. The specimens were then left at room temperature until testing.

Compressive strength measurements of mortars were performed on an MTS machine under a load control regime with a loading rate of 20 MPa/min. Three to five cubes were tested for each data point. The specimens were tested at 3, 7, and 28 days after casting.

3. Results and Discussions 3.1.EffectofmodulusanddosageofalkalineactivatoronthestrengthofAASmortar

Alkali modulus which is defined as the ratio of Na2O to SiO2 in the activator has a significant influence on the strength of AAS mortars as seen in Figure 1 and 2. However the strength improvement due to the increase in alkali modulus was only observed up to Ms=1 for both 3% and 5%Na2O AAS mortars.

Increasing the activator dosage (%Na2O by slag weight) also increased the strength significantly. However it is recommended that the maximum activator dosage of 5% by slag weight is used. Higher values can result in efflorescence and brittleness problems depending on slag type, activator nature, and curing temperature. Additionally, a very high activator dosage would be uneconomical.

AAS mortar was found to be very sensitive to heat curing. As can be seen from Figure 1and 2, heat cured AAS mortars developed rapidly at an early age compared to normal cured specimens.

The 3-days strength of heat cured 3%Na2O specimens were more than 85% of its 28-days strength, while the normal cured specimens had the 3-days strength approximately 60% of the 28-days strength.

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Similar trends were also found with 5%Na2O AAS mortars, the heat curing specimens gained approximately 90% of the 28-days strength within 3 days. The 3-days strength of heat cured specimens even exceeded the 28-days strength of normal cured specimens. The 28-days strength of heat cured specimens was approximately 8-20% higher than that of normal cured specimens.

Given the alkali modulus and activator dosage it was found that Ms=1 in 5% Na2O is the optimum mix composition providing the highest compressive strength for AAS mortar.

0

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Figure 1 Strength of alkali activated slag mortars with different alkali dosage and modulus subjected to heat curing

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Figure 2 Strength of alkali activated slag mortars with different alkali dosage and modulus subjected to normal curing

3.2. Effectmodulus and dosage of alkaline activator on strength of fly ash basedgeopolymermortar

The alkali modulus of the activator has a marginal influence on the strength of the fly ash based geopolymer mortar, and at higher dosage, the increase in strength was only observed up to Ms=1.25 (as seen from Figure 3). In this study, increasing the alkali modulus means an

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increase in soluble silicates and consequently an increase in the reaction rate (a higher dosage of reactants induces a higher reaction rate). However increasing alkali modulus (for the same alkali dosage) also means reducing the amount of hydroxide which is needed to dissolve silicate and aluminate monomer from the fly ash grain. Strength development does not seem to show a clear correlation with the age, as can be seen from Figure 3. A small reduction was observed in certain mixtures as well as some increase in others at 7 days age. Although all mixtures showed a small increase in strength at 28 days, the differences appear to be caused by the variability in the batch of the mix and the effect of micro cracking due to drying at high temperature in the oven.

It can also be seen from Figure 3 that the optimum mix composition is with Ms=1.25 in 15% Na2O which gives the highest compressive strength for a fly ash based geopolymer mortar.

0

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G10-1.00 G10-1.25 G10-1.5 G15-1.00 G15-1.25 G15-1.5

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)

3 days 7 days 28 days

Figure 3 Strength of fly ash based geopolymer mortars with different alkali dosage and modulus subjected to heat curing

4. Conclusions Slag and fly ash are two industrial by products which can be used to produce alkali activated binders for concrete. The alkali activated binders produced using slag and fly ash are not only environmentally friendly but also have additional benefits such as high early and ultimate strength depending on the mix composition. Activator dosage has significant influence on the strength of both AAS and fly ash based geopolymer mortars. There is a limit to the increase of the alkali modulus, i.e 1 for AAS and 1.25 for geopolymer mortar, beyond this limit, a reduction of strength is likely to occur due to a reduction of hydroxide while the excess silicate will not contribute to the strength Given the alkali modulus and activator dosage, it was found that Ms=1 in 5% Na2O for AAS and Ms=1.25 in 15% Na2O for fly ash based geopolymer is the optimum mix composition providing the highest compressive strength on the mortars.

5. References Bakharev, T. & Patnaikuni, I. (1997) Microstructure and durability of alkali activated

cementitious pastes. In: Ong, K. C. G. (Ed.) the Fifth International Conference on Structural Failure, Durability and Retrofitting. Singapore, Singapore Concrete Institute.

Brough, A. R. & Atkinson, A. (2002) Sodium silicate-based, alkali-activated slag mortars: Part I. Strength, hydration and microstructure. Cement and Concrete Research, 32, 865-879.

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Davidovits, J. (1991) Geopolymers: Inorganic Polymeric New Materials. Journal of Thermal Analysis, 37, 1633-1656.

Gjorv, O. E. (1989) Alkali Activation of a Norwegian Granulated Blast Furnace Slag. In: Malhotra, V. M. (Ed.) third international conference on fly ash, silica fume, slag, and natural pozzolans in concrete. Trondheim, Norway, American Concrete Institute.

Philleo, R. E. (1989) Slag or other supplementary materials? In: Malhotra, V. M. (Ed.) third international conference on the use of fly ash, silica fume, slag and natural pozzolan in concrete. Trondheim, Norway, American Concrete Institute.

Sindhunata (2006) A Conceptual Model of Geopolymerisation. Department of Chemical and Biomolecular Engineering. Melbourne, The University of Melbourne.

Swamy, R. N. (1986) Cement replacement materials, Glasgow, Surrey University Press. Talling, B. & Brandstetr, J. (1989) Present State and Future of Alkali-Activated Slag

Concretes. In: Malhotra, V. M. (Ed.) third international conference on fly ash, silica fume, slag, and natural pozzolans in concrete. Trondheim, Norway, American Concrete Institute.

Xu, H. (2002) Geopolymerisation of Aluminosilicate Minerals. Department of Chemical Engineering. Melbourne, The University of Melbourne.