Engineering Properties and Durability of Super-Classified Fly Ash Concrete

Research Student: Bruce K.T. KandieSupervisor: Dr. Ewan A. Byars

Figure 3. Comparison between SPFA and various types of PFAs

Figure 2. Burning coal to produce electricity and fly ash

Aim of this Research

The aim of this research is to develop specialist concrete mixes for high performance applications using optimised mix proportions with SPFA and compare this on a performance and economic basis with MS and BS3892: Part 1 fly ashes.

Objectives

To achieve the project aim, the following objectives have been identified:

i) To determine how the various replacement levels of SPFA for cement affects the fresh, hardened and the durability properties of concrete. Water demand, early and long term strength development, self-compacting concrete, creep, shrinkage, carbonation resistance, chloride resistance and sulfate resistance will be studied.

ii) To measure the effect of SPFA on the microstructure of concrete.

iii) On the basis of (i) and (ii), perform economic analysis to determine the most appropriate uses of SPFA in concrete.

Blended cement2.8%

Disposal43.6% Bricks and ceramics

0.2%

Other uses0.5%

Cement raw material3.2%

Stockpiled6.9%

Lwt. Aggregate2.3%

Non-aerated blocks0.9%

Aerated concrete blocks10.2%

Concrete addition8.0%

Landfill, land reclamation and

restoration9.8%

In-fill1.1%

Structural fill3.2%

General fill2.3%

Grouting5.1%

Figure 4. Fly ash disposal and utilisation in the UK

PHASE ONEFly Ash Characterization

•Fineness•Loss on Ignition

•Pozzolanicity

PHASE TWOFresh Concrete Properties

•Water Requirement•Setting Time

•Admixture Compatibility

Research Programme

PHASE THREEMechanical & Physical Properties

•Compressive Strength•Tensile Strength•Dry Shrinkage

PHASE FOURPermeability Properties

•Oxygen Diffusion•Porosity

PHASE FIVEDurability

•Chloride Ingress•Sulfate Resistance

•Carbonation•Freezing/Thawing

Extra refinement of fly ash produces Super-Classified Fly Ash-(SPFA), an ultra fine powder with average 8µm particle size. It is a very reactive pozzolan compared to BS3892: Part 1[1] fly ashes and has high potential for use as an alternative to micro-silica (MS) for high strength concrete and other applications [2].

Figure 1. Coal

(a)

Conclusion

The use of SPFA in concrete has achieved the following:

• A water deduction of 18% for 30% replacement of cement (Fig 5a).

• Concrete with a strength of 125 N/mm2 at 28 days (fig 5c) in water curing-for normally compacted concrete, 150N/mm2 at 90-days.

• Self compacting concrete made with only 380 kg/m3 cementitious cement and minimalplasticizer[3].

Sponsored by: Kenya Government

Introduction

Combustion of coal (Fig 1) in modern power plants (Fig 2) produces some bottom ash, but most of the burnt minerals escape with the flue gases and is called fly ash (PFA), Fig 3. This ash is subsequently removed from the gas by electrostatic precipitation.

SPFA

BS3892: Part 1 PFAs

1

23 5

6

7

4

8 µm

% W

ater

req

uire

men

t of

OP

C

Pozzolan replacement (%)

Water Demand

PHASE SIXEconomic Analysis

Results and Discussions

Fig 5a shows that SPFA has a high water reducing effect compared to other cementitious material, due to it’s hydrophilic nature and spherical shape.

Fig 5b shows the strength development, for a range of concrete mixes designed for 28-day in strength, it can be seen that 5% and 10% SPFA concrete has higher strength than control up to 10 days. This is in contrast to conventional BS3892 PFA, all of which lags behind the OPC control up to design age, as indeed do the micro-silica concrete mixes. Fig 5c shows the strength development curve for 125 N/mm2 strength at 28 days. This was made with a blend of SPFA and MS at replacement of 15% each.

References

1. BRITISH STANDARDS INSTITUTION. Specification for pulverized-fuel ash for use as a cementitious component in structural concrete. BS 3892 : Part 1. BSI, London, 1993.

2. Fossey, S. D., Byars, E.A. and Zhu, H.Y. 2003. Super-Classified PFA For Self-Compacting Concrete. ICCC-2003. Durban, South Africa, (in press).

3. Tsartsari, A. and Byars, E. A. 2000. Ultra-High Strength Concrete Using Conventional Casting.  Concrete, Vol. 36 , No 1, pp 16-17.

Fly ash poses a major waste disposal problem in the world (Fig 4). However, when used in blended cements it can reduce greenhouse gas emissions, the cost of concrete and improve the strength, durability and other properties of concrete .

22

28

34

40

0 7 14 21 28

OP C40

SP 40/ 05

SP 40/ 10

BS3892: P ar t 1 P FA

P FA5/ 30

MS40/ 05

MS40/ 10

Com

pres

sive

Str

engt

h N

/mm

2

(b)

Figure 5. Shows (a) Water demand and (b and c) the compressive strength curves for the SPFA and BS3892: Part 1 fly ashes

70

80

90

100

110

120

130

0 7 14 21 28

SP / MS120/ 15/ 15

Com

pres

sive

Str

engt

h N

/mm

2

(c)

Grade 120 MixGrade 40 Mixes

Age-days Age-days