Effect of cement replacement by Silica fume and fly ash
Earlier notion of using high amounts of cement for concrete has now
changed on favour of increased use of high amounts of mineral admixtures
and super plasticizers with reduced amounts of cement and water
in the concrete mixtures. Energy plays a crucial role in growth of
developing countries, like India. In context of low availability of nonrecoverable
energy sources coupled with requirements of large quantities
of energy to materials like cement, steel etc., the importance of industrial
wastes as building materials cannot be underestimated. In India about 110
million tones of fly ash has been produced by 68 major thermal power
stations and are likely to be doubled within next 10 years. It has been a
published fact from research that waste materials like fly ash; silica fume
etc, through their use as construction materials can be converted into
Also, a partial replacement of cement with fly ash is
desirable, and indeed essential due to a variety of technical, economical
and ecological reasons. Researchers have reported that silica fume smaller
in size and round shape fills the voids between the coarser cement particles
which may be otherwise occupied with water. A properly proportional fly
ash and silica fume in concrete mix improves properties of the concrete
that may not be achievable through the use of Portland cement alone. The
resulting concrete mix becomes strong, durable and economical and also
eco-friendly as it utilizes an ecological hazardous material.
MATERIALS USED – SPECIFICATIONS
1.Ordinary Portland Cement (OPC), 53 Grade.
2. Silica fume (silicon, sio2 63.1%) as mineral admixture (MA) in
dry densified form obtained from ELKEM INDIA Pvt.Ltd.,
3. Super plasticizer (chemical admixture) based on Sulphonated
Napthalene Formaldehyde-CONPLAST SP 430.
4. Locally available graded aggregate of nominal size 12.5 mm with
specific gravity 2.68 and fineness modulus, 5.08 for coarse
5. Locally available river sand confirming to Grading Zone II with
specific gravity 2.58 and fineness modulus 2.54 for fine aggregate
6. Water: potable water was used for mixing concrete.
7. Fly ash from Vijayawada thermal power station, Vijayawada, AP,
CHEMISTRY OF FLYASH AND SILICA FUME
The fly ash in concrete makes efficient use of product of hydration of
cement such as calcium hydroxide (C-H) which is otherwise a source of
weakness in normal cement concrete converts it into denser and stronger
C-S-H ompounds by pozzolanic reaction. The heat generated during
hydration initiates the pozzolanic reaction of fly ash.
Silica fume is a by-product of silicon or Ferro-Silica industry and is
100 times finer than cement. It consists of amorphous silica nd has high
reactivity towards lime. The replacement level of silica fume is generally
low at about 10%. When SF is used in concrete mix, its introduction
affects the physical arrangement of the system, particularly near the
aggregate surface where porosity exists. Silica fume starts reacting at the
early stage of hydration process. The pozzolanic action of silica fume
reduces substantially the quantity and size of “CH” crystals in hydrated
cement paste. This phenomenon along with low W/C ratio reduces the
thickness of transition zones and thus the preferential orientation of CH
crystals is considerably reduced. All these result in more uniform, stronger
transition zone potential of micro cracking.
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