17-08-2012, 12:37 PM
DEVELOPMENT AND PROPERTIES OF LOW-CALCIUM FLY ASH-BASED
GEOPOLYMER CONCRETE
1DEVELOPMENT AND PROPERTIES.pdf (Size: 1.77 MB / Downloads: 185)
INTRODUCTION
GENERAL
Concrete usage around the world is second only to water. Ordinary Portland cement
(OPC) is conventionally used as the primary binder to produce concrete. The
environmental issues associated with the production of OPC are well known. The
amount of the carbon dioxide released during the manufacture of OPC due to the
calcination of limestone and combustion of fossil fuel is in the order of one ton for
every ton of OPC produced. In addition, the extent of energy required to produce
OPC is only next to steel and aluminium.
On the other hand, the abundant availability of fly ash worldwide creates opportunity
to utilise this by-product of burning coal, as a substitute for OPC to manufacture
concrete. When used as a partial replacement of OPC, in the presence of water and in
ambient temperature, fly ash reacts with the calcium hydroxide during the hydration
process of OPC to form the calcium silicate hydrate (C-S-H) gel. The development
and application of high volume fly ash concrete, which enabled the replacement of
OPC up to 60% by mass (Malhotra 2002; Malhotra and Mehta 2002), is a significant
development.
LOW-CALCIUM FLY ASH-BASED GEOPOLYMER CONCRETE
In this work, low-calcium (ASTM Class F) fly ash-based geopolymer is used as the
binder, instead of Portland or other hydraulic cement paste, to produce concrete. The
fly ash-based geopolymer paste binds the loose coarse aggregates, fine aggregates
and other un-reacted materials together to form the geopolymer concrete, with or
without the presence of admixtures. The manufacture of geopolymer concrete is
carried out using the usual concrete technology methods.
As in the case of OPC concrete, the aggregates occupy about 75-80 % by mass, in
geopolymer concrete. The silicon and the aluminium in the low-calcium (ASTM
Class F) fly ash react with an alkaline liquid that is a combination of sodium silicate
and sodium hydroxide solutions to form the geopolymer paste that binds the
aggregates and other un-reacted materials.
SCOPE OF WORK
The research utilized low-calcium (ASTM Class F) fly ash as the base material for
making geopolymer concrete. The fly ash was obtained from only one source. As far
as possible, the technology and the equipment currently used to manufacture OPC
concrete were used to make the geopolymer concrete.
The concrete properties studied included the compressive and indirect tensile
strengths, the elastic constants, the stress-strain relationship in compression, and the
workability of fresh concrete.
CONCRETE AND ENVIRONMENT
The trading of carbon dioxide (CO2) emissions is a critical factor for the industries,
including the cement industries, as the greenhouse effect created by the emissions is
considered to produce an increase in the global temperature that may result in climate
changes. The ‘tradeable emissions’ refers to the economic mechanisms that are
expected to help the countries worldwide to meet the emission reduction targets
established by the 1997 Kyoto Protocol. Speculation has arisen that one ton of
emissions can have a trading value about US$10 (Malhotra 1999; Malhotra 2004).
The climate change is attributed to not only the global warming, but also to the
paradoxical global dimming due to the pollution in the atmosphere. Global dimming
is associated with the reduction of the amount of sunlight reaching the earth due to
pollution particles in the air blocking the sunlight. With the effort to reduce the air
pollution that has been taken into implementation, the effect of global dimming may
be reduced; however it will increase the effect of global warming (Fortune 2005).
From this point of view, the global warming phenomenon should be considered more
seriously, and any action to reduce the effect should be given more attention and
effort.
USE OF FLY ASH IN CONCRETE
One of the efforts to produce more environmentally friendly concrete is to reduce the
use of OPC by partially replacing the amount of cement in concrete with by-products
materials such as fly ash. As a cement replacement, fly ash plays the role of an
artificial pozzolan, where its silicon dioxide content reacts with the calcium
hydroxide from the cement hydration process to form the calcium silicate hydrate (CS-
H) gel. The spherical shape of fly ash often helps to improve the workability of the
fresh concrete, while its small particle size also plays as filler of voids in the
concrete, hence to produce dense and durable concrete.
An important achievement in the use of fly ash in concrete is the development of
high volume fly ash (HVFA) concrete that successfully replaces the use of OPC in
concrete up to 60% and yet possesses excellent mechanical properties with enhanced
durability performance. HVFA concrete has been proved to be more durable and
resource-efficient than the OPC concrete (Malhotra 2002).