Nanoscience and nanoengineering of cement-based materials - Nanotechnology in Eco-efficient Construction - page 22

in properties which can be signifi cantly different from their larger scale
counterparts. Several nanoscale particles are currently being considered as
nanoscale additives with the aim of improving macroscopic performance or
adding functionality to the material. Among the candidates are nanosilica,
titanium dioxide, carbon nanotube, nanoscale limestone, haematite nano-
particles, pigments, clay particles, etc. (Meng et al. , 2008; Sato and Diallo,
2010; Tregger et al. , 2010a,b; Sato and Beaudoin, 2011).

Micro- and nanosilica ( μ /n-SiO 2 )

Silica fume, also known as microsilica, is an amorphous polymorph of
silicon dioxide, silica. It is a by-product of the carbothermic reduction of
high purity quartz in electric arc furnaces in the production of silicon
and ferrosilicon alloys. The resulting product is a grey powder of surface
area on the order of 20 m 2 /g and particle sizes in the 100-200 nm range,
approximately one hundredth of the cement particles. When silica fume is
added to Portland cement concrete, it increases its compressive strength,
tensile strength and abrasion resistance. These improvements stem from
the closed packing achieved in the cement paste system that reduces the
overall porosity and improves the interfacial transition zone. Furthermore,
the material is involved in the pozzolanic reaction (Lin et al. , 2011) with
the calcium hydroxide crystals producing additional cementing material
(C-S-H) and eliminating areas of stress concentrations and prone to
failure initiation (Ca(OH) 2 crystals). The reduction of porosity produces
at the same time a material more durable and resistant to chemical degra-
dation processes like chloride ion diffusion, alkali silica reaction and
calcium leaching, which preserves the material from mechanical degrada-
tion and protects reinforcing steel from corrosion (Zhang and Li, 2011).
Several recent publications report on the signifi cant advantages of
nano-SiO 2 as compared to microsilica (Jo et al. , 2007; Ye et al. , 2007; Senff
et al. , 2010).
The already reported benefi ts persist in an amplifi ed fashion. The highly
reactive and large surface area provided by colloidal silica accelerates
cement hydration and has a signifi cant impact on the hydration process at
early ages (Bjornstrom et al. , 2004). The high surface area, however,
decreases the amount of lubricating water and interferes with the fl owing
characteristics of fresh concrete. It is therefore essential to ensure proper
workable conditions, i.e., with the use of superplasticizers, so as to avoid air
entraining in the fresh system and benefi ting from the above described
performance (Senff et al. , 2009). The use of silica reinforced cementitious
materials might fi nd applications in a variety of systems ranging from oil
well cement (Choolaei et al. , 2012) to pavement (Zhang and Li, 2011) and
high performance compacting applications (Nazari and Riahi, 2011).

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