Effect of silica fume on the characterization of the geopolymer materials
Hisham M Khater
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Housing and Building National Research Centre (HBNRC)
, 87 El-Tahreer St., Dokki, Giza, P.O. Box 1770, Cairo,
Egypt
The influence of silica fume (SF) addition on properties of geopolymer materials produced from alkaline activation of alumino-silicates metakaolin and waste concrete produced from demolition works has been studied through the measurement of compressive strength, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy (SEM) analysis. Alumino-silicate materials are coarse aggregate included waste concrete and fired kaolin (metakaolin) at 800C for 3 h, both passing a sieve of 90 m. Mix specimens containing silica fume were prepared at water/binder ratios in a range of 0.30 under water curing. The used activators are an equal mix of sodium hydroxide and silicate in the ratio of 3:3 wt.%. The control geopolymer mix is composed of metakaolin and waste concrete in an equal mix (50:50, wt.%). Waste concrete was partially replaced by silica fume by 1 to 10 wt.%. The results indicated that compressive strengths of geopolymer mixes incorporating SF increased up to 7% substitution and then decreased up to 10% but still higher than that of the control mix. Results indicated that compressive strengths of geopolymer mixes incorporating SF increases up to 7% substitution and then decreases up to 10% but still higher than the control mix, where 7% SF-digested calcium hydroxide (CH) crystals, decreased the orientation of CH crystals, reduced the crystal size of CH gathered at the interface, and improved the interface more effectively.
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Introduction
Silica fume (SF) known as micro-silica is a by-product of
the reduction of high-purity quartz with coal in electric
furnaces in the production of silicon and ferrosilicon
alloys. Because of its extreme fineness and high silica
content, silica fume is a highly effective pozzolanic material.
Silica fume is used in concrete to improve its properties
like compressive strength, bond strength, and abrasion
resistance; it reduces permeability and therefore helps in
protecting the reinforcing steel from corrosion.
Silica fume has been used as a high pozzolanic reactive
cementitious material to make high-performance concrete
in the severe conditions (Kohno 1989; Gautefall 1986).
This mineral admixture has highly been used in severe
environmental conditions despite its several mixing and
curing problems because of its acceptable early-age strength
development (Cheng-Yi and Feldman 1985; Grutzeck
et al. 1983). The hydration mechanism and properties of
secondary C-S-H made by pozzolanic reaction have been
studied by many investigators (Cheng-Yi and Feldman
1985). However, CSH formed by silica fume-calcium
hydroxide reaction might be different with respectto the
amount of molecular water, C/S ratio, and density (Cohen
and Bentur 1988). Moreover, because of its rather different
characteristics, pozzolanic gel has a high potential to
contribute in reactions with other internal or external ions
such as Al, Cl, and alkalies (Maage 1989; Sellevold and
Nilsen 1987).
On the other hand, the search for a new
environmentally friendly construction material that will match the
durability of ancient concrete has provoked interest into
the study of alkali-activated cementitious systems over
the past decades. Alkali-activated cements refer to any
system that uses an alkali activator to initiate a reaction
or a series of reactions that will produce a material that
possesses cementitious property (Yip et al. 2005).
Alkali-activated cement, alkali-activated slag and fly
ash, and geopolymers are all considered to be
alkaliactivated cementitious systems; however, it is expected
that the structures of these materials are vastly different
and result from different chemical mechanistic paths. It
is commonly acknowledged that calcium silicate hydrate
(CSH) is the major binding phase in Portland cement
(Taylor 1964) and alkali-activated slags (Richardson and
Cabrera 2000); however, the binding property of geopolymers
is generally assumed to be the result of the formation of a
three-dimensional amorphous aluminosilicate network (van
Jaarsveld and van Deventer 1999a; Davidovits 1991; Phair
and van Deventer 2002; Lee and van Deventer 2002).
The production of geopolymeric precursors is carried
out by calcinations of aluminosilicates, natural clay
materials. Their source can be also some industrial
aluminosilicate waste materials. The result of the hardening
mechanism is a three-dimensional zeolitic framework,
unlike traditional hydraulic binders in which hardening
is the result of the hydration of calcium aluminates and
silicates (Davidovits 1991; Phair and van Deventer 2002;
Lee and van Deventer 2002; Davidovits 1993); this
circumstance is a cause of significant differences in the
quality and variety of the engineering properties of the
composites based on geopolymer and current cements.
As a means of converting waste materials to useful
p (...truncated)