Page 5 - Open-Access-Sep-2019

TECHNICAL PAPER

aggregates in concrete is accepted in many countries and is observed including at early age. In order to overcome the
significant amount of research have been conducted with good reduction in the mechanical properties observed in concrete
understanding on the properties of concrete containing these containing recycled aggregates and those containing high
recycled aggregates and SCM materials. volume fly ash and slag in the above studies, one promising
approach is to use ultrafine pozzolanic materials e.g. silica fume,
The construction industry in Australia is a fast-growing industry nano silica and other ultrafine materials. This paper presents
due to the increasing demand for infrastructure as a result of the effect of nano silica and ultrafine fly ash on compressive
increase in population growth in recent and coming years, and indirect tensile strengths of recycled aggregate concretes
predicted to be 72% between 2007 and 2030 (Hyder consulting, measured from early (7 days) to later ages (91 days) and the
2009), as well as the anticipated growth and demand for effect of silica fume on the compressive and indirect tensile
infrastructure in the future. Construction and demolition strengths of recycled aggregate concretes containing high
(C&D) waste accounts for about 40% of Australia’s solid waste volume fly ash and slag as partial replacement of cement. In
materials, with 160 million tonnes of virgin aggregates being addition, the effect of inclusion of SCMs and nano silica on
mined every year (Australia’s sustainable aggregates industry, the carbon footprint of recycled aggregate concrete is also
2012). In 2008 a total of 19 million tonnes of C&D waste was analysed.
disposed of in Australia. Only 10.5 million tonnes, or 55%,
of this waste was recycled or recovered, leaving 45%, or 8.5 2. ExpEriMEntal prograM
million tonnes, of waste sent to landfills around Australia. With
increasing target recovery of C&D wastes in future and the Materials
shortage of natural aggregates in some parts of the world, there Ordinary Portland Cement (OPC) was used in all mixes. The fly
is a need to develop recycled aggregates from C&D waste as an ash used in this study was class F fly ash, while the blast furnace
alternative source of aggregates.
slag used in this study was obtained from a local supplier.
In order to further improve the environmental friendliness of Commercial Silica Fume (SF) supplied by Ecotec was used in this
concrete and to maximise the use of SCM and C&D wastes in
concrete the use of high volume fractions of these materials
in concrete need to be incorporated. Significant amount of
research is also conducted on the individual use of high volume
fractions of SCM in concrete. Numerous researches on both
mechanical and durability properties of recycled aggregate
concretes have also been conducted (Chen et al. 2003; Yong
and Teo 2009; Zhang and Ingham 2010; Xiao et al. 2005; Yang
et al. 2008; Tam et al. 2006; Padmini et al. 2009; Ahmed 2013).
It is shown that the old adhered mortars in recycled aggregates
are more porous than that of natural coarse aggregates and
micro crack exists in recycled aggregates due to crushing and
grinding during their manufacturing (Etxeberria et al. 2006; Katz
2004). Moreover, the interfacial transition zone (ITZ) between
the old mortar and the old aggregate are also porous and weak
(Otsuki et al. 2003; Poon et al. 2004). Those factors are claimed
to be responsible for inferior properties of recycled aggregate
concretes. The SCM can reduce these pores and micro cracks
through pore filling due to their finer particle sizes than OPC as
well as the formation of additional hydration product (calcium
silica hydrate (CSH)) through pozzolanic reaction in concrete.
A number of studies have evaluated the effect of fly ash on the
properties of concrete containing recycled aggregates (Somna
et al., 2012, Ahmed, 2013 and 2014, Corinaldesh and Moriconi,
2009, Kou et al., 2007 and 2013) and the effect of slag on the
properties of concrete containing recycled aggregates (Kou et
al., 2011, Ann et al., 2008, Cakir, 2014, Majhi et al., 2018). In all
the reported studies on the use of fly ash and slag in concrete
containing recycled aggregates the reduction in mechanical Figure 1: Dispersion of nano silica in water and superplasticiser solution
properties of concrete with increase in fly ash and slag contents in ultrasonication machine.

56 The IndIan ConCreTe Journal | SepTember 2019