USE OF CEMENT BYPASS DUST AS A SUSTAINABLE CEMENT REPLACEMENT

Cement bypass dust (CBPD) is a waste dust extracted from rotary cement kilns at around 2%
of cement clinker by weight. CBPD is often landfilled due to its excessive contents of chlorides,
sulphates, and high alkalinity, at high monetary and environmental cost. Due to its high pH and
lime content, it is hypothesized that CBPD could be used as a cement replacement with
activating potential to make a sustainable binder. The aim of this research is to create a
sustainable binder composed of CBPD and pozzolanic materials that can perform similarly to
OPC. This was achieved through extensive lab work, including a thorough microstructural
analysis to properly characterise the behaviour of binders.
Preliminary investigations showed that ambient temperature cured binders composed of CBPD
and granulated ground blast furnace slag (GBFS) at ratios of 1:1 are possible. Heat curing
proved to worsen the long-term strength benefits of CBPD binders, leading to lesser hydration
products and a more porous microstructure. This is primarily due to accelerated hydration
leading to uneven reaction products and consequently a less dense microstructure. Results also
indicated that the inclusion of 20% cement content enhanced properties such as strength and
workability.
Further research investigated the differences between CBPDs sourced from different locations.
It was found that CBPDs with high free lime contents lead to deleterious expansion causing
poor mechanical properties. It was also identified that the strength of CBPDs largely comes
from its ettringite content. Following this, research involving carbonation curing revealed that
the free lime content of CBPDs can be exploited due to the reaction of lime and CO2. Thus, the
properties of the lime rich CBPD binders were significantly enhanced relative to the sulphate
rich CBPD. This showed that different forms of curing may be more suitable for different types
of CBPD.
A milled CBPD mix composed of 10% metakaolin, 45% CBPD and 45% GBFS showed
optimum mechanical and durability properties outperforming the control cement mix. CBPD
mixes generally outperformed OPC when tested for alkali-silica reaction. Chlorides from
CBPD binders did not cause any significant corrosion to steel fibres. Life cycle analysis
indicates that all the studied binders had significantly less global warming potential than OPC.
This work shows that a cementless, environmentally friendly binder that can perform similarly
to OPC is possible with CBPD. This can lead to reductions in landfilling and to make cement
manufacture overall more efficient and less polluting.