EFFECT OF MIX DESIGN ON CONSISTENCE AND SETTING TIME OF ALKALI ACTIVATED CONCRETE

Kofi Abora1, Keith Quillin2, Kevin A Paine3, Andrew M Dunster4

1,3BRE Centre for Innovative Construction Materials, University of Bath2,4Building Research Establishment, UK

Abstract: In order for alkali activated systems to become technically and economically viable construction materials, a number of technical and economic barriers need to be addressed. As with Portland cement concretes, issues related to the mix design, curing and use of alkali activated concrete have to be clearly understood; inappropriate mix designs will lead to products that are not fit for purpose. This paper reviews two form of sodium silicate based solution (water glass) at different concentration to assess its effects on consistence, one major barrier in the use of alkali activated system. Achieving better control of these parameters whilst retaining appropriate compressive strength development will help move these promising low carbon dioxide (CO2) binders towards mainstream use. The results obtained have demonstrated that the change in concentration of sodium oxide (Na2O) within the activator has an effect on the volume of activator required for a consistence mix. Additionally the results give indication of the optimum activator binder ratio and the compressive strength, when compositional design of the activator is varied. Although the concretes made with the lower concentrated activator had the highest level of consistence, it could only achieve less than 25% of the strength value of the concretes made with the higher concentrated activator. Thus the type of activator to be used will depend on the application of the concrete and the level of strength required.

Keywords: alkali activated concrete, strength, consistence, fly ash, ggbs

1. IntroductionIn a bid to reduce CO2 emissions associated with the use of concrete, the cement

industry is providing incremental process improvements and the use of additions such as fly ash and ground granulated blastfurnace slag (ggbs) in blended cements and combinations is having some effect. However, significant savings from decarbonation can only be made by changing the composition of the cement. This is due to the fact that, in addition to emissions arising from intensive energy use, CO2 is generated from the calcinations of the calcium carbonate; one of the main components of Portland cement manufacture. Global data from the European Cement Association (Cembureau) on CO2 emissions from cement manufacture estimates an average of 0.8 tonnes of CO2 per tonne of Portland cement produced.

Current research on alkali activated technologies indicates a more enhanced environmental impact than Portland cement. This is due to the fact that an alkali activated

1 Researcher, k.abora@bath.ac.uk 2 Associate Director, quillink@bre.co.uk 3 Senior Lecturer, k.paine@bath.ac.uk 4 Principal Consultant, dunstera@bre.co.uk

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system utilises industrial by-products, providing massive savings on energy consumption and CO2 emissions.

In order for alkali activated systems to become technically and economically viable construction materials, a number of technical and economic barriers need to be addressed. As with Portland cement concretes, the key performance requirements from any binders used in concrete are that they provide good consistence of the wet concrete and an appropriate rate of strength development.

Actual formulations, hardening characteristics, strength and durability are dependent on the composition of the materials used, the relative proportions, the formulation of the alkali activator and the curing conditions; however the exact reaction mechanism leading to the setting of the alkali activated binders is still being investigated.

1.1. Alkali activated technologyAlkali activated technology involves the use of various raw materials and industrial by-

products activated with a high pH alkali metal solution (generally sodium silicate-based) and cured at room or higher temperatures. The source materials predominantly used are fly ash, ggbs, metakaolin, and sewage treatment and paper processing residues. The high alkali environment breaks down Si-O chemical bonds in the glassy phase of the material. These phases then become available for reaction with the alkaline species in the liquid. Reaction products are zeolitic in nature and the activator is consumed in the reaction. The chemistry is dependent on the composition of the source material and the type of activator used.

Basic research on alkali activated materials has been carried out at a number of institutions, since the 1940’s (Roy 1998). Most work has been done in Eastern Europe, Australia, New Zealand, Ukraine, France, Finland and the former Soviet Union. However, comprehensive work was carried out at the Building Research Establishment, UK in the 1970’s. Smith and Osborne (1977) reported that by activating a combination of fly ash with either ggbs or specially prepared synthetic granules with sodium hydroxide, there is the potential of producing cement. However, at that time, the use of fly ash and ggbs as partial Portland cement replacements was seen as being of more immediate benefit, in terms of reducing the CO2 emissions associated with Portland cement production. However, building construction using alkali activated slag cements in Russia, Ukraine and Poland has been reported (Shi et al, 2006).

1.2. Alkali activated technology and CO2

The manufacturing of cement produces a more significant source of CO2 than any industrial process other than electrical power generation and can be associated with two or three contributions:

Č{total} → Č{raw materials} + Č{fuels} + Č{electric power} (1)

The most contributing factor of the CO2 emissions during manufacturing of Portland cement is as a result of the electricity and energy used in firing the kiln to high temperatures in the region of 1450°C, and the calcination of limestone and silica. The chemical equation representing the above scenario can be expressed as:

5CaCO3 + 2SiO2 → (3CaO.SiO2 )(2CaO.SiO2) + 5CO2 (2)

Where energy consumption equates to about 0.4 tonnes of CO2 per tonne of clinker (Quillin, 2007).

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When industrial by-products such as fly ash and ggbs are used as the binders for alkali activated technologies, the major source of the CO2 arises from the production of the sodium silicate-based chemical activator through thermal processing and decarbonation of sodium carbonate. Information from silicate manufacturers indicates that the CO2 emission should be less than 0.2 tonnes per tonne of activator.

Whilst fly ash may be activated alone, its use in combination with ggbs allows for the curing to be done at room temperature, and therefore there is no need for elevated temperatures which would increase the level of CO2 emissions further.

2. Experimental InvestigationThe consistence and compressive strength characteristics of alkali activated concrete

using two different concentrations of alkali activators were investigated in this project. The specimens for the compressive strength test were 100 mm cubes and these were seal cured in air at room temperature. A flow table test was carried out on the alkali activated concrete made from different activators at different volumes.

The same ggbs, fly ash, alkali activators, fine and coarse aggregates were used throughout the investigation.

2.1. Cementitious materialsThe chemical and physical properties of UK sourced fly ash and ggbs, conforming to

BS EN 450 and BS 6699 respectively, are given in Tables 1 and 2 below.

2.2. Alkali activatorsTwo forms of alkali activators with the same molecular ratio SiO2:Na2O (Ms) = 1.54

were used. The first was a concentrated sodium silicate solution (A1) with 20% SiO2 and 13% Na2O and a density of 1420 kg/m3. The second activator was a less concentrated sodium silicate solution (A2) with 12.3% SiO2 and 8% Na2O and a density of 1320 kg/m3.

2.3. Mixture proportionsA series of alkali activated concrete using fly ash and ggbs were made and tested for

consistence and compressive strength. The compositions given in Table 3 were studied:The mix design used in each case was as follows:

Total binder content: 440 kg/m3

Total activator volume: 170 – 205 litres per m3 of concreteTotal aggregate: binder ratio: 3.5 : 110/20 mm Thames valley aggregate: 35%4/10 mm Thames valley aggregate: 25%0/4 mm Thames valley aggregate: 40%

Each mix of about 7 litres was made in a pan mixer to produce six cubes to allow for a 7, 28 and 91 days compressive strength testing. The aggregates and the binders were mixed for a few seconds to ensure all the materials were mixed thoroughly. The activator was then added and mixing continued for about 3.5 minutes. The concrete was then subjected to a flow table test before casting into a 100 mm cube moulds that using a vibration table.

The samples were subject to a standard curing regime involving 24 hours in moulds under damped Hessian, prior to demoulding, being wrapped in polythene and stored in a controlled room maintained at room temperature until the test date.

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Figure 1: Fresh alkali activated concrete mix

Table 1: Typical chemical and physical properties of ggbsOxide % Physical propertiesSiO2 35.3 Specific surface: 450 m2/kgAl2O3 11.3 Specific gravity: 2.91Fe2O3 0.2 Colour: off whiteCaO 40.0MgO 10.6SO3 4.0

Na2O 0.3K2O 0.4

Table 2: Typical chemical properties of fly ash

Oxide %SiO2 44.0Al2O3 24.6Fe2O3 11.7CaO 5.5MgO 2.1SO3 0.2

Na2O 1.1K2O 2.4

Table 3: Concrete mix proportions

MIX ID Fly ash GgbsActivator

type

Volume of activator per m3 of concrete

(litres)

Activator : binder ratio

MIX 1 90% 10%A1

205 0.66MIX 2 90% 10% 186 0.60MIX 3 90% 10% 170 0.55MIX 4 90% 10%

A2205 0.61

MIX 5 90% 10% 186 0.55MIX 6 90% 10% 170 0.50

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2.4. Flow table test The consistence of the concrete was investigated using a standard flow table in

accordance with the method specified for determining the flow of fresh concrete in BS EN 12350-5:1999. The measurement of spread was as shown in Figure 2.

Figure 2: Spread measurement of a fresh alkali activated concrete

2.5. Compressive strength100 mm cubes specimens (Figure 3) were prepared to determine the effect of varying

the composition of the activator as well as the change in volume of activator on the compressive strength. Tests in accordance with BS EN 12390-3:2009 were carried out at 7, 28 and 91 days.

Figure 3: 100 mm cube specimen of alkali activated concrete

3. Results and Discussion

3.1. Consistence of alkali activated concreteInitial and final setting times of alkali activated concrete are difficult to determine,

because unlike Portland cement the consistence changes rapidly over a short period of time (Atiş et al., 2009), making the fresh concrete difficult to place. Initial trials using the slump test was used to measure the consistence of alkali activated concrete. However, this was quickly identified as unsuitable because most of the mixes flowed freely under their own weight and therefore there was no correlation between slump and the degree of consistence. To ensure comparison of the consistence between the various mixes, a flow table test was therefore used.

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It was observed that the concrete made with the less concentrated activator solution exhibited the highest consistence value, largest flow value, when comparing the volume of activator used per mix (Figure 4). It can also be observed that the higher the volume of activator, the higher the consistence value.

The ability to control and increase the setting time of alkali activated concrete needs a more detailed study and therefore lies outwith the scope of this paper.

Table 4 : Measurement of spread from the flow table test

400

450

500

550

600

650

165.00 175.00 185.00 195.00 205.00

Volume of activator/m 3 of concrete (litres)

Mea

sure

men

t o

f sp

read

(m

m)

activator A1 activator A2

Figure 4: Relationship between the activator type and consistence

3.2. Compressive strength developmentThe compressive strengths of the concrete are shown in Table 5. The compressive

strength of the alkali activated concrete was greater with the more concentrated activator solution, A1. This is attributed to the high concentration of the Na2O in the activator, which has been reported by previous authors using various ranges of activator concentration (Atiş et al.). The results presented in Figure 5 indicate that the analysis is valid for each of the curing ages of the mixes made with the A1 solution (MIX 1, 2 and 3).

Furthermore, the increase in compressive strength correlated to the decrease in volume of activator used as shown in Figure 6 and Figure 7. Analysis of the concretes made with solution A1 and A2 indicated that overall the highest compressive strength was obtained when the lowest volume of activator was used. For the alkali activated concretes at a constant concentration, as shown in Figure 5, the highest compressive strengths were associated with the activator with the highest sodium concentration, A1. Among the six concrete mixes, MIX 3 exhibited the highest compressive strength at all testing ages.

Volume of activator per m3 of concrete (litres)

Measurement of spread (mm)Activator A1 MIX ID Activator A2 MIX ID

205 565 1 630 4

186 485 2 510 5

170 415 3 415 6

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Table 5: Compressive strength test results at 7, 28 & 91 days

Volume of activator per m3 of concrete

(litres)

Compressive strength (N/mm2)A1 A2

7 days 28 days 91 days 7 days 28 days 91 days205 14.0 27.5 46.5 3.3 4.4 5.8186 15.5 30.5 49.5 2.8 4.5 5.8170 17.5 33.8 50.3 4.0 5.3 6.8

0

10

20

30

40

50

60

7 28 91

Test age (days)

com

pre

ssiv

e st

ren

gth

(N

/mm

2)

activator A1 activator A2

Figure 5: Relationship between strength development and type of activator used

0

5

10

15

20

25

30

35

170.00 186.00 205.00

Volume of activator/m 3 of concrete (litres)

Co

mp

ress

ive

stre

ng

th (

N/m

m2)

50%

55%

60%

65%

70%

acti

vato

r :

bin

der

rat

io

7 days 28 days activator:binder ratio

Figure 6: Relationship between strength development and volume of activator (A1)

7

0

2

4

170.00 186.00 205.00

Volume of activator/m 3 of concrete (litres)

com

pre

ssiv

e st

ren

gth

(N

/mm

2)

50%

55%

60%

65%

acti

vato

r :

bin

der

rat

io

7 days 28 days activator:binder ratio

Figure 7: Relationship between strength development and volume of activator (A2)

4. ConclusionThe overall analysis has demonstrated that when the compositional design of the

activator is varied, in this instance the change in concentration of NaOH within the activator, there is an effect on the volume of activator required for a mix of a given consistence and compressive strength. The properties of the alkali activated concrete are dependent on the type of activator used. The following observations were made during the investigations:

The results identified from the research indicates that fly ash in the presence of the ggbs, which provides calcium to the binders leads to higher strength and quicker hardening of the concrete within 24 hrs.

The more concentrated the NaOH within the activator solution, the higher compressive strength. This indicates that the strength of the concrete is influenced by the concentration of NaOH.

The analysis indicates that the lower the volume of activator for the mix, the higher the compressive strength.

The consistence value of the alkali activated concrete increased with the use of lower concentrated activator solutions when compared to more concentrated activators based on the same volume of activator used.

Observation of the concrete over time indicates that the consistence reduces much more quickly with an increase in the sodium concentration of the activator.

A slump test could not be used for determining the consistence of the concrete. This is due to the fact that alkali activated concrete fell freely under its own weight during the slump test.

In summary although the concretes made with the lower concentrated activator had the highest level of consistence, it could only achieve 25% of the strength value of the concretes made with the higher concentrated activator. Thus the type of activator to be used will depend on the application of the concrete and the level of strength required.

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5. AcknowledgementsThe authors wish to express their gratitude and sincere appreciation to the laboratory

technicians at the Building Research Establishment especially Les White and Clive Tipple, whom without their assistance this paper would not have been possible.

6. ReferencesATIŞ C. D., BILIM C., ÇELIK Ö AND KARAHAN O., 2009. Influence of activator on the

strength and dry shrinkage of alkali-activated slag mortar. Construction and building materials, 23(2009), pp.548 – 555.

EUROPEAN CEMENT ASSOCIATION (Cembureau), Energy is a sensitive factor in cement manufacture, www.cembureau.be, 2000.

QUILLIN K, 2007. Calcium sulfoaluminate cements. CO2 reduction, concrete properties and applications. Bracknell: IHS BRE Press.

ROY D. M., 1998. Alkali-activated cements, opportunities and challenges. Cement and concrete research, 29(1999), pp.249-254.

SHI C., KRIVENKO P. V AND ROY D., 2006. Alkali-activated cements and concrete. Oxon: Taylor & Francis Ltd, pp.298-319.

SMITH M.A. AND OSBORNE G. J., 1977. Slag/fly ash cements. World Cement Technology, Nov\Dec, pp.223-233.

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