An attempt has been made to develop Fly ash (FA) and Ground granulated blast furnace slag (GGBFS) blended self-compacting geopolymer concrete mixes with varying volume of pastes using conventionally available river sand as fine aggregate and crushed granite chips as coarse aggregate. These mixes were developed using Fly ash as a major source material along with GGBFS in the proportion 70:30 (FA:GGBFS) in the production of SCC mixes. Different amounts of Sodium silicate solutions, with specified amounts of Sodium Hydroxide flakes dissolved in them, are used as alkaline solutions. The total of four mixes were developed with varying volume of pastes in the range of 0.40 – 0.52 (within an interval of 0.04). These mixes were evaluated for their flow ability characteristics as per the relevant EFNARC guidelines. Further the mixes were evaluated for their mechanical properties in terms of compressive strength, splitting tensile strength and water absorption characteristics. Durability tests by means of exposing to acidic and sulphate environments, along with their resistances to sustained elevated temperatures upto 800ºC were carried out for all these mixes. The test results indicate better flow ability characteristics, along with their higher mechanical and durability properties.
Geopolymer concrete based on industrial by-product materials such as fly ash and slag can play a vital role in the context of sustainability and environmental issues . Approximately 5% of global CO2 emissions originate from the manufacturing of Portland cement . On the other hand, industrial by-product materials such as slag has been shown to release up to 80% less greenhouse gas emissions  and there are 80–90% less greenhouse gas emissions in the production of fly ash . Therefore, a full replacement of OPC with GGBFS or fly ash would significantly reduce the CO2 emission of concrete production. Geopolymer concrete is an alternative concrete in which an alkali activated aluminosilicate material is used as the binder instead of the traditional cement binder. One challenge for wide application of fly ash based geopolymer is the requirement of heat curing . Inclusion of blast furnace slag along with fly ash to produce geopolymer binder has been tested with some favourable results, however the fly ash/slag ratio, type, concentration and composition of activator varied widely in the mixtures [7-9].The present paper reports on the efforts being made for developing fly ash and GGBFS blended self-compacting geopolymer concrete mixes. In order to develop these mixes, different varying volume of pastes were considered namely 0.40, 0.44, 0.48 and 0.52 as the major variable, along with conventionally available natural river sand as fine aggregate and 12.5mm downsize granite chips as coarse aggregate.
In the present investigation, Class F - Fly ash, obtained from Raichur Thermal Power Corporation, Raichur, India, conforming to IS: 3812-2003 and Ground granulated blast furnace slag - GGBFS, obtained from M/S Jindal Steel Works, Bellary, India, conforming to IS: 12089-1987 were used. The Fly ash and GGBFS had a specific gravity of 2.12 and 2.9 along with their Blaine’s fineness of about 436 and 370 m2/kg respectively.
In the present study, mixtures of sodium hydroxide and sodium silicate solutions with the ratio of Sodium Silicate/Sodium Hydroxide (SS/SH) being kept as 1.0, maintaining a constant molarity of 8M are used as alkaline solutions. The distilled water along with sodium silicate and sodium hydroxide flakes were used in the preparation of alkaline solution. The alkaline solution prepared is allowed to cool and mature for 24 hours prior to mixing, in order to reduce the heat liberated during the time of mixing.
In the present study, natural river sand was used as fine aggregate and 12.5mm downsize granite chips, was used as the coarse aggregate fraction in all these geopolymer concrete mixes. The river sand and jelly was procured from a local dealer. The fine aggregate and coarse aggregate used were having a specific gravity of 2.60and 2.62. The results of the sieve analyses carried out, for both fine aggregates and coarse aggregates, indicate that they conform to the specifications of IS: 383-1970.
Mixture Proportioning, Preparation and Casting of mixtures
An initial set of only four trial mixes were formulated using a constant ratio of SS/SH as 1, maintaining a molarity of 8M and the performance of these mixes were tested in the laboratory. The details of proportions of concrete mixtures are shown in Table 1. The trial-mixes were proportioned on the basis of absolute volume method. The ratio of proportions of fine to coarse aggregates in all the mixes was maintained constant at 50:50. The cube specimens were then cast using 100mmx100mmx100mm moulds; for evaluating both the mechanical and durability characteristics. Cylindrical specimens of size 100mm x 200mm dia was cast for evaluating the split tensile strength for all these mixes. All the test specimens could be de-moulded immediately after the day of casting and were then subjected to ambient curing under the lab-environment. Sufficient numbers of specimens were cast for facilitating these tests at the age of 1, 3, 7,28-days. In each case, the averages of test results for three test specimens were considered.
|Table 1. Details of Trial Self Compacting GPC Mixes Tested with their quantities in kg/m3|
|Sl No.||Volume of PasteVp||Fly Ash
|NaOH Sol||Sodium Silicate Sol||Sand||CA
Results and discussions
Tests on Fresh Fly ash and GGBFS blended Self-compacting GPC mixes
Flow ability tests such as Slump flow, V- funnel and L-box, generally prescribed for flow able mixes were performed on all the four geopolymer concrete mixes as per relevant EFNARC guidelines. The slump flow tests were carried out using the Abram’s cone, to evaluate the filling ability of the different GP concrete mixes as shown in Figure 1. The volume of paste varying in the range 0.40-0.52 has shown a slump flow value ranging between 600mm – 700mm as shown in Table 2. Increase in the volume of paste has shown a decrease in the slump flow values for all the mixes. This behaviour may be due to increase in the volume of paste simultaneously causes an increase in the GGBFS content. Introduction of GGBFS initiates the alkali activation process at faster rate causing a decrease in the flow properties at the higher volume of pastes. Further the flow increases with the increase in the powder content up to a certain extent havinga fly ash and GGBFS content of about 371kgs and 151kg as shown in Table 1, and above which it is not possible to obtain self-compacting geopolymer concrete satisfying the EFNARC guidelines for SCC. Hence combination of fly ash and GGBFS for developing SCC is having an optimum mix with the volume of paste ranging between 0.44 – 0.48.
V – Funnel tests were also carried out on all these GPC mixes. It is observed from Table 2, the times taken for emptying the V – Funnel are in the range of 12sec – 20sec, slightly greater than the specified guidelines for V- Funnel test as per the EFNARC guidelines. Based on V- funnel test results, it can be concluded again that the mixes did not show any greater resistance to its passing ability towards filling the areas with larger congestion of rebar’s leading to better structural conditions. Hence optimum volume of paste should be selected for obtaining a better resistance in terms of filling ability.
|Table 2. Flow ability Properties of Fly ash and GGBFS blended GPC mixes|
|Sl No.||MIX ID||Volume of Paste Vp||Flow ability Tests|
|Slump Flow mm||V-Funnel in sec||L -Box Test|
Legend: FAS0.40VP - FA indicates Fly Ash, S indicates Ground granulated blast furnace slag 0.40 VP indicates volume of paste of 0.40
The Fly ash and GGBFS based GPC mixes were also tested for their passing ability by conducting the L–Box tests. It was observed that increase in the volume of paste showed an increase in their passing ability in term of their blocking ratio being in the range of 0.60 – 0.91. This behaviour can be attributed due to the increase in the paste volume, which in turn decreases the volume of aggregates leading to a better blocking ratio (H2/H1) for the GPC mixes tested herein. However all the fly ash and GGBFS based self-compacting GPC mixes tested herein are quite stable with no signs of segregation or bleeding, due to the viscous nature of the alkaline solution.
Mechanical Properties of Fly ash and GGBFS blended Self compacting GPC mixes
All the GPC mixes tested herein have shown compressive strength and splitting tensile strength values on testing as per IS 516:1959 and IS 5816:1999 as shown in Table 3. It can be observed that better compressive strengths and splitting tensile strength values in the range of 17MPa – 50MPa and 1.9 MPa – 3.4 MPa were obtained at the age of 28 days.It can observed that increase in the volume of paste has shown an increase in the strength characteristics for all the ages. These mixes have shown a higher one day compressive strength in the range of 14MPa – 34MPa and only a slight increase in the strength at the later ages. Increase in the volume of paste has simultaneously accounted for the increase in the GGBFS, which further leads to higher process of geopoly merisation. This behaviour is due to the early activation of slag grains in presence of the alkaline solution which causes the process of dissolution leading to the formation of geopolymeric reaction products.
|Table 3. Mechanical Properties of Fly ash and GGBFS blended GPC mixes|
|Sl No.||MIX ID||Vp||Compressive Strength MPa||Splitting Tensile strength MPa||Water Absorption %|
Water absorption test were also performed on all the four GPC mixes in order to have a general idea on the amount of pores present in the concrete microstructure. The water absorption values were in the range of 3.5% - 5.5%. Increase in the volume of paste has slightly shown a decrease in the water absorption values. This may be due to the filling of some amount of pores by the geopolymerisation products and also the majority of the unreacted fly ash grains have provided the pore filling effect for all the mixes tested herein.
Durability studies on Fly ash and GGBFS blended Self compacting GPC mixes
Resistance to Sulphuric acid attack
All the Fly ash and GGBFS blended Self compacting GPC mixes were exposed to concentrated sulphuric acid solution for 4 weeks after 28 days of ambient curing with their concentration being maintained at 5%.It was observed that all the specimens after the exposure to the acidic environment maintained their structural integrity without causing a major distress on the surface of the specimens. Further no significant change in colour of the specimens was observed for any of the specimens tested herein. The residual compressive strength of all the mixes was in the range of 22 %- 47 % as shown in Table 4. Increase in the volume of paste showed an increase in the residual strength values. Further this behaviour may be due to the deterioration of the geopolymeric products in highly acidic medium.
|Table 4. Sulphuric Acid and Magnesium Sulphate resistance of trial GPC mixes|
|Sl No||MIX ID||Volume of Paste Vp||Sulphuric Acid Resistance||Magnesium Sulphate Resistance|
|Actual Comp. Strength
|Residual Comp. Strength
|Actual Comp. Strength
|Residual Comp. Strength
Resistance to Magnesium Sulphate attack
The test results of magnesium sulfate attack on the compressive strength of various GPC mixes, subjected to 5 % magnesium sulfate solutions up to an duration of 4 weeks is shown in Table 4. All the fly ash and GGBFS blended self-compacting GPC mixes maintained their structural stability without showing any major cracks on the surface of the specimens. The colour on the surface of the specimens were slightly changed to whitish, due to the formation of reaction product mainly gypsum. The residual compressive strength of all the specimens subjected to sulphate environment was in the range of 8% – 29%. As similarly observed as in case of the specimens subjected to acidic environments. Increase in the volume of paste showed a decrease in the strength reduction values. Further the specimens having paste volume of 0.52 showed a strength reduction as low as 8%. This behaviour may be due to these magnesium ions which attacks the geopolymeric products leading to the formation of the expansive reaction products namely M-S-H and gypsum, causing the decrease in strength performances.
Elevated Temperature performance of Fly ash and GGBFS blended Self compacting GPC mixes
In order to evaluate the elevated temperature performance of all the GPC mixes, the specimens after 28 days of curing were placed in muffle furnace. These mixes were subjected to different increase in temperatures namely 200ºC, 400ºC, 600ºC and 800ºC for a sustained duration of 2 hours, after reaching the target temperature. The visual observation of the specimens did not show any change in the colour with the structural integrity of the specimens being maintained even after subjecting 800ºC for a longer soaking period of 2 hours.
|Table 5. Test results of percentage decrease in strength subjected to sustained elevated temperatures.|
|SL NO||SERIES||Percentage Decrease in Comp. Strength subjected to different Sustained Elevated temperatures|
|1||FAS 0.40 VP||47.6||53.3||57.5||58.6|
|2||FAS 0.44 VP||27.7||52.9||57.0||58.4|
|4||FAS 0.52 VP||19.1||33.0||48.9||57.0|
Further the specimens also did not show any signs of spalling when subjected to higher temperatures. However, minor surface cracks may be observed on the surface of specimens exposed to temperature of about 800ºC. At the lowest sustained elevated temperature 200°C, percentage loss of reduction in strength ranging between 19% - 48% was observed for all the volume of pastes as shown in Table 5. Further at 400°C increase in the percentage reduction in strength was observed ranging between 33% - 53% for all the volume of pastes. The increase in the sustained elevated temperature to 600°C and 800°C also showed higher decrease in reduction of compressive strength ranging between 49% - 57% for all the volume of pastes. These loses with the increase in the sustained elevated temperature may be attributed due to the deterioration of the reaction products formed due to the process of geopolymerisation. However fly ash combined with GGBFS based self compacting geopolymer concrete mixes are having better resistances to sustained elevated temperature when compared to normal OPC based concretes which mostly crumble at 800°C.
- Ambient cured Fly ash and GGBFS blended Self-compacting geopolymer concrete mixes can be developed with 8M of Sodium Hydroxide solution, keeping the ratio of Sodium Silicate to Sodium Hydroxide as 1.
- Based on the workability tests such as Slump flow, V- Funnel and L- Box carried out for assessing the SCC properties for Fly ash and GGBFS blended geopolymer concrete mixes shows an acceptable values for volume of pastes ranging between 0.44 -0.48. However the volume of paste 0.48 is having more optimum value.
- Fly ash and GGBFS blended Self-compacting geopolymer concrete mixes recorded a one day compressive strength greater than 30MPa and tensile strength of about 2.7 MPa for the volume of paste 0.52. Further at the later ages increase in strength values of upto50MPa in compression and 3.4 MPa in tension were obtained. Decrease in water absorption values were obtained with the increase in the volume of paste at the age of 28days.
- The limited durability studies carried out on Fly ash and GGBFS based self compacting geopolymer concrete mixes showed a greater resistances to acidic, sulphate and sustained elevated temperatures for the volume of pastes 0.48 and 0.52.
- Duxson, P., Fernández-Jiménez, A., Provis, J.L., Lukey, G.C., Palomo, A., and van Deventer,J.S.J. “Geopolymer technology: the current state of the art”.Journal of Material Science,42(9):2917–2933. (2007).
- Lawrence, C.D. “The production of low-energy cements”. In:Heweit PC, editor.Lea’s Chemistry of cement and concrete. Oxford: Butterworth-Heinemann: 421–470.(1998).
- Roy, D.M., Idorn, G.M. “Hydration, structure, and properties of blast-furnace slag cements mortars, and concrete”.Journal of American Concrete Institute,79(6):444–457. (1982).
- Duxson, P., Lukey, G., van Deventer, J.S.J. “Physical evolution of geopolymer derivedfrom metakaolin up to 1000 º C”. Journal of Material Science, 42(9):3044–3054.(2007).
- Davidovits, J. “Geopolymer chemistry and application”.2nd ed. France: Institute Geopolymer Saint-Quentin. (2008).
- Somna, K., Jaturapitakkul, C., Kajitvichyanukul, P., Chindaprasirt, P. “NaOHactivated ground fly ash geopolymer cured at ambient temperature”. Fuel,90(6):2118–2124.(2011).
- Laskar, A.I., Bhattacharjee, R.,“Rheology of fly-ash-based geopolymer concrete”.Journal ofAmerican Concrete Institute,108(5):536–542. (2011).
- Palomo, A., Fernandez-Jimenez, A., Kovalchuk, G., Ordonez, L.M., and Naranjo, M.C.“OPC fly ash cementitious systems: study of gel binders produced during alkaline hydration”.Journal of Material Science,42:2958–2966.(2007).
- Dombrowski, K., Buchwald, A., and Weil, M. “The influence of calcium content on the structure and thermal performance of fly ash based geopolymers”.Journal of Material Science,42:3033–3043. (2007).
Manjunath R is currently working as Assistant Professor in the Department of Civil Engineering, BMS College of Engineering, Bangalore. His research interests include strength and durability of concrete mixes, alternative binders for concrete, alkali activated slag (AAS) concretes and geo-polymers.