Modern Quarrying

MODERN QUARRYING

October - November 2015

when the OPC content increased. Roy et

al., (2007) used gold mill tailings mixed

with OPC, black cotton soils, and red soils

in different proportions to make bricks.

The cement-tailings bricks were cured

by immersing them in water for different

periods of time and their compressive

strengths were determined. Bricks with

20% cement and 14 days of curing were

found to be suitable.

Gold mine tailings have also been

used to produce autoclaved calcium sil-

icate bricks (Jain et al., 1983). The bricks

are cured under saturated steam and in

the process, lime reacts with silica grains

to form a cementing material consisting

of calcium silicate hydrate. Some mining

companies such as Bharat Gold Mines

in India have explored the idea of brick-

making using gold ore tailings (Be Sharp,

2012).

Table I

shows the chemical composi-

tion of some of the waste materials used

in bricks as well as the composition of

quarry clay material that comprise the

conventional feed material (Bennet et al.,

2013). The gold mine tailings are from a

Chinese mine (Yang et al., 2011).

From the chemical compositions

shown in

Table I

, it can be seen that the

waste materials have similar major oxides

in their compositions. The compositions

are also relatively similar to the typical

clay material used in brickmaking. The

waste materials all have a predominantly

high content of silica, alumina, and hae-

matite (with the exception of the gran-

ulated blast furnace slag, GGBS, which

contains no haematite), which are import-

ant in brickmaking materials.

Considering the source of the gold

deposits in the Witwatersrand Basin (river

sediments in the form of sand and gravel),

it is therefore likely that the tailings from

this area will also contain a high level of

silica.

The purpose of this work is therefore

to ascertain the technical and economic

viability of using the Witwatersrand

gold tailings for brickmaking using the

cementing method. The tailings-based

bricks will be compared with the com-

mercial bricks available on the market.

The evaluation will be based on parame-

ters such as compressive strength, water

absorption, and weight loss tests. This

work has the potential to unlock large

resources of material needed in the con-

struction industry that would help con-

serve the natural resources commonly

used. In addition it would eliminate the

land requirements for waste disposal,

thus realising savings on disposal and

landfill costs and also lessening environ-

mental damage. But above all, this work

has the potential to provide an addi-

tional revenue stream for the gold min-

ing sector.

Materials and methods

The materials used in this test work were

gold mine tailings, water, and cement as a

binding material. Gold mine tailings were

provided by a local gold mining company,

AngloGold Ashanti.

The Lafarge 42,5 kN cement was

provided by the Planning, Infrastructure

and Maintenance Depar tment at

the University of the Witwatersrand,

Johannesburg. The cement was used on

the day of delivery and tap water was

used in the mixing process.

Characterisation of gold mine tailings:

Representative samples used in all exper-

iments were prepared using a riffle split-

ter (model 15A, Eriez Magnetics, South

Africa). The gold tailings were charac-

terised by investigating the phase min-

eralogy, particle size, and quantitative

chemical analysis. The particle size anal-

ysis was done by physically screening

the samples using test sieves (Fritsch,

Germany) of various screen sizes up to

212 μm. The phase mineralogy analysis

was carried out using an X-ray diffrac-

tometer (X’Pert, PANalytical, Netherlands)

operated with Co-K radiation generated

at 40 kV and 50 mA. The chemical anal-

ysis was carried out using wavelength

dispersive X-ray fluorescence (XRF) spec-

trometry (Axios, PANalytical, Netherlands)

operated with a rhodium tube excitation

source.

The brickmaking process:

Different mixing ratios of tailings, cement,

and water were used in the brickmaking

process (

Table II

). From each mixture, a

number of bricks were cast and dried.

The three feed material (tailings,

cement, and water) were mixed in the

appropriate ratios in a commercial mix-

ing machine. Dry mixing was done first

and then a controlled amount of water

was added while continuing to mix thor-

oughly. The total mixing time was 15 min-

utes. The mixture was then cast into the

brick moulds. The brick moulds were then

placed on a vibrating machine for five

minutes in order to fill the voids in mix-

ture comprehensively and thus prevent

the formation of air pockets. The bricks

were then labelled and allowed to cure

for 24 hours.

Three curing methods were used.

These included atmospheric drying under

the sun, curing in water, and drying in an

oven at 360°C. After curing, the bricks

were de-moulded using an air compres-

sor, weighed, and tested for compressive

strength.

Unconfined compressive strength

testing:

The cast and cured bricks were tested for

compressive strength using a Tinus Olsen

compressive strength testing machine. In

the compressive strength testing process,

a force was applied on the brick until the

Composition of material used in brickmaking

Oxide

component

Fly ash

Mass %

GGBS

Mass %

Bottom ash

Mass %

Clay material

Mass %

Gold mine

tailings Mass %

SiO

53,3

35,47

56,76

61,8

38,60

29,5

19,36

21,34

7,06

10,7

5,98

12,76

CaO

7,6

33,25

2,88

29,24

1,8

0,72

3,21

FeO

0,8

MgO

8,69

1,2

7,85

0,1

2,76

Table I: Composition of material used in brickmaking (Bennet et al., 2013; Yang et al., 2011).

SPOTLIGHT ON

BRICKMAKING