Fly ash exploited in pavement layers in environmentally friendly ways

A. Athanasopoulou* and G. Kollaros

Department of Civil Engineering, Democritus University of Thrace, Xanthi, Greece

(Received 14 July 2014; accepted 23 December 2014)

Increased care about the environment is currently evidenced by governmental,industrial, and consumer concern for ozone depletion, solid and liquid waste disposal,and pollutants. This concern has led to an increase in marketing of the“environmentally friendly” aspects of products. In the past, fly ash was generallyreleased into the atmosphere, but pollution control equipment mandated in recentdecades now requires that it is captured prior to release. In order to upgrade expansivesoils as construction materials, fly ash, which is a waste material, has been selectedand successfully used for stabilizing expansive clays in the Thrace region. Thestrength characteristics of the stabilized soils were measured. Depending upon the soiltype, the effective fly ash content for improving the engineering properties of the soilvaried between 8% and 12%. Using fly ash in roadwork projects will help theenvironment reducing the deposited amounts.

Keywords: industrial by-products; environmental impacts; soil stabilization; fly ash

1. Introduction

For the construction of pavements, large volumes of materials are required. Waste and

byproduct materials augmenting in volume and disposal cost, after recycling and recover-

ing, can be used in six major highway construction applications: (1) flowable fill (a mix-

ture of coal fly ash, water, and Portland cement that flows like a liquid, sets up like a

solid, is self-leveling, and requires no compaction or vibration to achieve maximum den-

sity); (2) asphalt concrete; (3) Portland cement concrete; (4) granular base; (5) embank-

ment or fill; and (6) stabilized base. In each application category, there is at least one

potential material use, for example, as aggregate, supplementary cementitious material,

pozzolan, pozzolan activator, or self-cementing material. A great variety of waste and

byproduct materials could be used in pavement construction works, including coal fly

ash, coal bottom ash/boiler slag, steel slag, baghouse fines, blast furnace slag, kiln dusts,

foundry sand, mineral processing wastes, nonferrous slags, flue gas desulfurization, scrap

tires, scrubber material, quarry by-products, reclaimed concrete material, reclaimed

asphalt pavement, sewage sludge ash, municipal solid waste incinerator ash, sulfate

wastes, waste glass, etc. (USEPA 2010).

Soil stabilization is the permanent physical and chemical alteration of soils to enhance

their physical properties. Stabilization can increase the shear strength of a soil and/or con-

trol the shrink-swell properties of a soil, thus improving the load-bearing capacity of a

subgrade to support pavements and foundations. A wide range of subgrade materials

from expansive clays to granular materials can be treated using stabilization processes.

Stabilization can be achieved with a variety of chemical additives including lime, fly ash,

*Corresponding author. Email: gkoll@civil.duth.gr

� 2015 Taylor & Francis

Toxicological & Environmental Chemistry, 2015

http://dx.doi.org/10.1080/02772248.2015.1005090

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and Portland cement, as well as with by-products such as lime-kiln dust and cement-kiln

dust. The proper design and testing of any stabilization project allows for the establish-

ment of design criteria as well as the determination of the proper chemical additive and

admixture rate to be used to achieve the desired engineering properties. Benefits of the

stabilization process can include: higher resistance values, reduction in plasticity, lower

permeability, reduction of pavement thickness, elimination of excavation and base impor-

tation, aided compaction, “all-weather” access onto projects sites. Previous research has

shown that self-cementing fly ash can be an effective binder for stabilizing soils for

highway bases (Edil, Acosta, and Benson 2006). However, limited information exists on

the reuse of high carbon off-spec fly ash in construction of highway pavements (Cetin,

Aydilek, and Guney 2010). This is particularly important when high carbon fly ash is

non-cementitious and calcium-rich activators are required to generate pozzolanic reac-

tions (Takhelmayum et al. 2013). The addition rate depends on the nature of the soil, the

characteristics of the fly ash and the required strength and must be determined by labora-

tory mix design testing. Higher fly ash addition rates generally result in higher compres-

sive strength (Hatipoglu, Edil, and Benson 2008). Any fly ash having self-cementitious

properties can practically be used in transportation projects.

2. Fly ash

Fly ash is the finely divided mineral residue that results from the combustion of pulver-

ized coal in electric and steam generating plants and is transported from the combustion

chamber by exhaust gases. Heat is extracted from the boiler by tubes; the flue gas cools

and the molten residue hardens forming ash. Coarse ash particles, termed bottom ash or

slag, fall to the bottom of the combustion chamber. Light fine ash particles, referred to as

fly ash, suspend in the flue gas. Before exhausting the flue gas, fly ash is removed by elec-

trostatic precipitators or filter fabric baghouses.

Fly ash has a unique spherical shape and particle size distribution, typically between

10 and 100 micron, which make it good mineral filler to be used in hot mix asphalt appli-

cations and improve the fluidity of flowable fill and grout (Lav, Lav, and Goktepe 2005).

Depending on its chemical and mineral constituents, fly ash can have a tan to dark gray

color. Tan and light colors are associated with high lime content. Iron content gives fly

ash a brownish color. Dark gray to black color can be attributed to high quantities of

unburned carbon. The color is very consistent for each power plant and coal source.

The annual world fly ash production is estimated to be 480 million tones (Vom Berg

2000). This reflects an ascendant tendency in the production of electric energy from coal

combustion, since an earlier estimation (Manz 1997) referred to 459 million tones. Fly

ash is found in large quantities in Peloponnesus (Megalopolis, low calcium, 2£106 tons

per year) and Western Macedonia (Ptolemaida, Kozani, high calcium, 13£106 tons per

year) regions (Koukouzas et al. 2007; Kolias et al. 2005) where power stations are in

operation. Because the cement industry uses only 10% of these quantities as raw material,

fly ash is a good candidate for use in structural fills and other highway applications.

Although non-concrete applications, such as structural fills, are not affected by fly ash

fineness, others, such as asphalt filler, are dependent on the fly ash fineness and its particle

size distribution. For fly ash applications in concrete, fineness is defined as the percent by

weight of the material retained on the No. 325 (0.044 mm) sieve. Fineness of fly ash is

associated with the operation of the coal crushers and the grindability of the coal, as lig-

nites and anthracites are more resistant to grinding than are bituminous coals. Fineness

contributes to the pozzolanic reactivity of fly ash. Finely divided pozzolans in the

2 A. Athanasopoulou and G. Kollaros

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presence of water react with calcium hydroxide at ordinary temperatures to produce

cementitious compounds. A coarse gradation can lead to a less reactive ash with higher

carbon contents.

Fly ash consists primarily of oxides of silicon, aluminum, iron, and calcium, and, in a

lesser degree, of magnesium, potassium, sodium, titanium, and sulfur. Based on its chem-

ical composition, fly ash is classified as either Class C (self-cementing) or Class F (non-

self-cementing) defined by AASHTO M 295 (2011) (ASTM C 618 2001) specifications.

Class C or high calcium (>20% free lime, CaO) fly ash is derived from sub-bituminous

coals and consists primarily of calcium alumino-sulfate glass, quartz, tricalcium alumi-

nate, and free lime. Class F or low-calcium ashes are derived from bituminous and anthra-

cite coals and they consist of an alumino-silicate glass, quartz, mullite, and magnetite.

The hydration (“cementitious”) and the pozzolanic reactions that occur when fly ash is

blended with water form the products that bond soil grains or agglomerates together to

develop strength within the soil matrix.

Large volumes of coal from multiple sources are burnt by electric-generating stations

the environmental performance and productivity of whom may be improved by blending

coals. The technology used to control pollution can affect the chemical composition of

the fly ash, which is directly related to the parent coal chemistry along with the fuels or

additives used in the combustion process.

Most Northern Greek lignite deposits are found in the Florina�Ptolemaida�Kozani

geologic basin, a region full of open-air mines supplying nearby lignitic power stations.

In the aforementioned basin, four thermoelectric power stations are located having a total

installed power of 4108 MW. In an annual base, fly ash production in the region comes

up to 13£106 tons. Most of these fly ash quantities are laid down at solid waste disposal

sites covering an area of 135,000 acres, while only a small percentage is channeled to

cement industry (Iordanidis 2012).

Moreover, the fly ash emissions in the atmosphere from the burning of lignite are not

negligible and cause local pollution problems (Grigoratos 2012). Iordanidis (2012) men-

tioned that fly ash particles of varied morphology, size, and constitution have been recog-

nized in the atmosphere. In the area of the plants, an increase of suspended particles has

been associated with increased mortality, and the respiratory and cardiovascular diseases

(Sichletidis et al. 2005). It is thus obvious that the estimation of suspended fly ash contri-

bution in the region’s atmosphere is important.

3. Laboratory testing of fly ash stabilized soils

The highway New Egnatia (A2) crosses areas in Thrace, Northern Greece with abundant

clayey soils having poor technical properties. Treatment of physical soils with some sub-

stances could bring up new materials, which would operate better under the given traffic

and environmental conditions. This has led to the decision to investigate the possibilities

of improving the existing soil materials using fly ash as additive. Five soil samples were

collected from regions covering all three prefectures in Thrace, Xanthi, Rhodope, and

Evros.

All five soils tested were fine-grained clays differing in color, texture and swelling

potential. They are characterized as CH or CL according to the Unified Soil Classification

System. Both categories are judged as moderate to poor for the construction of pavement

subgrade.

The fly ash used for the preparation of the laboratory specimens was supplied by the

Ptolemaida Power Station (6,000,000 ton/year). It is an off-specification ash, meaning

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that it does not meet the Class C or Class F criteria in ASTM C 618. The specific gravity

of this coal residue found to be 2.5. Loss on ignition (LOI) is a measure of unburned

carbon remaining in the ash, with limits specified by AASHTO and ASTM. The use of

Class F pozzolan containing LOI higher than 6.0% may be approved if either records of

acceptable performance or laboratory test results are available (Dockter and Jagiella

2005). For the stabilization of recycled base materials, Cetin (2009) studied three fly

ashes from power plants in Maryland having LOI values 10.7%, 13.4%, and 20.5%. Fly

ash from a power station in Wisconsin used for the stabilization of soft fine-grained soils

had a LOI of 53.4% (Edil, Acosta, and Benson 2006). The fly ash grain size distribution

is shown in Figure 1.

The chemical properties of fly ash along some specification values for Class C fly ash

material are depicted in Table 1.

Figure 1. Grain size distribution of Ptolemaida fly ash.

Table 1. Fly ash chemical properties

Percent ofspecifications for Class C fly ash

Parameter composition ASTM C618 AASHTO M 295

SiO2 (silicon dioxide), % 29.95

Al2O3 (aluminum oxide), % 10.85

Fe2O3 (iron oxide), % 4.57

SiO2 C Al2O3 C Fe2O3, % 45.37 50 Min 50 Min

CaO (calcium oxide), % 20.00

MgO (magnesium oxide), % 1.90

K2O (potassium oxide), % 0.95

Na2O (sodium oxide), % 0.32

S (sulfur, 1000 �C), % 2.92 5 Max 5 Max�

C (carbon), % 3.80

Loss on ignition, % 13.90 6 Max 5 Max

�Specifications refer to SO3 (sulfur trioxide).

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The effect of fly ash on the physical soil characteristics and their engineering behavior

has been studied as a function of Atterberg limits (Table 2). It must be noted that the pul-

verized fly ash is a non-plastic material, which when blend with the soil reduces both LL

and PI. The effect of the fly ash on the PI was more pronounced for the S1 soil, particu-

larly at higher contents in the mix.

The air-dried soil materials passing the 4.75 mm sieve were mixed in different propor-

tions by weight with fly ash. Water was added until the optimum moisture content was

reached. The mixing process continued till a uniform product was achieved. Cylindrical

specimens 50 mm in diameter and 100 mm high were formed in special molds. The mate-

rial was placed in the mold in three layers of equal thickness. The quantity of the material

for each sample was determined by the optimum moisture content (OMC)�maximum

dry density (MDD) relationship. The compaction to the Proctor MDD was achieved by

compressing the required mass with an automatic hydraulic press. The quantity of the

material for each sample was determined by the OMC�MDD relationship. For each fly

ash percentage, a set of three specimens was prepared. The MDD and OMC values for

different fly ash contents in the soil-additive mixes are shown in Table 3. Generally, the

max dry density decreased with an increase in fly ash content, while OMC increased.

Table 2. Atterberg limits for soil�fly ash mixes.

Fly ash(%)

Soil samples

Soil S1 Soil S2 Soil S3 Soil S4 Soil S5

Liquid limit, LL 0 23.0 76.0 51.0 77.0 56.0

2 21.0 73.0 50.0 75.0 54.0

4 19.0 69.0 49.0 71.0 48.0

8 NP� 64.0 47.0 64.0 45.0

12 NP 59.0 45.0 58.0 42.0

Plasticity limit, PL 0 15.0 29.0 23.0 33.0 17.0

2 17.0 30.0 26.0 31.0 16.0

4 18.0 32.0 33.0 32.0 13.0

8 NP 35.0 37.0 34.0 18.0

12 NP 39.0 40.0 36.0 25.0

Plasticity index, PI 0 8.0 47.0 28.0 44.0 39.0

2 4.0 43.0 24.0 44.0 38.0

4 1.0 37.0 16.0 39.0 35.0

8 NP 29.0 10.0 30.0 27.0

12 NP 20.0 5.0 22.0 17.0

�NP D Non plastic.

Table 3. Variation of compaction parameters for soil�fly ash mixes.

Max dry density (kg/m3) Optimum moisture content (%)

Fly ash (%) S1 S2 S3 S4 S5 S1 S2 S3 S4 S5

0 1994 1588 1707 1441 1522 10.40 21.70 17.80 27.20 22.30

4 1915 1526 1611 1421 1498 12.35 25.60 22.10 28.40 24.70

8 1895 1487 1577 1398 1466 12.50 26.40 24.50 30.30 26.30

12 1870 1422 1477 1391 1372 13.05 31.20 28.10 31.40 29.20

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Care was taken to cure the specimens under stable temperature and moisture condi-

tions. The specimens for the unconfined pressure test were cured for 7, 28 and 90 days

before their testing. The fly ash contents by weight were 4%, 8%, and 12%. The OMC

was determined using the standard Proctor method according to the AASHTO T99 01

(2001) specification. For the unconfined compression tests a machine with a strain rate

1.25 mm/min was used. Unconfined compression strength (UCS) values for the five clay

soils cured for 90 days are shown in Figure 2.

The addition of fly ash resulted in a reduction of the liquid limit in comparison with

the natural soil. The admixture of fly ash rapidly initiates flocculation and cation

exchange reactions, leading to a reduction of the specific area of the soil. The reduction

of the thickness of the diffused double layer causes the reduction of the liquid limit. The

admixture of fly ash resulted in a reduction of the MDD of the soils. On the other hand,

an increase in OMC was observed for the same compaction effort. The reduction in

MDD, following the treatment with fly ash, reveals the increased resistance to the com-

paction effort offered by the flocculated soil structure. The OMC increased as a conse-

quence of the excess of water retained in the voids of the flocculated soil structure

(formation of soil aggregates), which results from the soil�fly ash interaction.

There is no specified strength for fly ash soil stabilization. It is usually thought satis-

factory to obtain strength values of about 100�200 psi (700�1400 kPa) (Rossow 2003)

for road subgrades. Considering the strength change of the soils, the UCS increased with

the percentage of the fly ash. The strength increased in an almost constant rate when the

percentage of fly ash in the mixture was increased, with the exception of sample S5, where

the rate of strength increases after the addition of 6% fly ash was very intense. The differ-

ence in the soil behavior is certainly due to the differences in the mineralogy of the soil

and the kind of the exchangeable cations present.

4. Conclusions

In Greece, there is a problem with the disposal of fly ash, while very little information is

available for the possible technical use of fly ash in highway construction projects.

Figure 2. Unconfined compressive strength of soil specimens stabilized with different fly ashcontents.

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When used to improve soil conditions, fly ash eliminates the need for expensive bor-

row materials, and reduces the need for more expensive natural aggregates in the pave-

ment cross-section, greatly helping the reservation of the environment. If engineers

consider the environmental degradation costs due to the consumption of top soil and

aggregates from borrow areas quarry sources, as well as the loss of fertile agricultural

land caused by ash deposition, then the savings will be high. In view of this reasoning, fly

ash use will be justified even for long transport distances.

In general, the addition of fly ash in the case of the five studied soils led to a decrease

of the liquid limit and to an increase of the plastic limit; only the sample S4 showed a

slight reduction of PL when 2% of fly ash had been added. Therefore, in all cases the plas-

ticity index has been reduced.

The modification of soil properties with special emphasis on their strength has been

examined in the laboratory. The fly ash-stabilization of all soils tested led to increased

soil strength. The experimental results have shown that the unconfined compressive

strength increased as a function of the percentage of fly ash in the mixture. The soils

tested, when treated with fly ash contents 8% or higher could be used as a subgrade or

even as a sub-base layer in roadway pavements (UCS ranging from 760 to 1103 kPa).

Therefore, fly ash can be considered as cost-effective and efficient material for use in

road construction, embankment, and earth fills.

In order to successfully exploitate the waste material from power stations, it is neces-

sary to have specifications for the high calcium fly ash. This exploitation is associated

with the development of a fly ash management system which will include their qualifica-

tion taking into account the existing infrastructure for the fly ash treatment in the Western

Macedonia basin power stations, especially in Ptolemaida, where most of the fly ash is

produced.

Disclosure statement

No potential conflict of interest was reported by the authors.

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