CHEMISTRY OF FLY ASH 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 silica fume 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.

Effect of cement replacement by silica fume and fly ash:

– The higher the percentage of FA the higher the values of concrete compressive strength until 30% of FA. However, the highest value of concrete compressive strength is obtained from mix containing 15% SF.

– SCC mixes with 15% silica fume as a replacement of cement content give higher values of concrete compressive strength than those with 30% FA by about 12% and 10% for 550 kg/m3 and 450 kg/m3 cement content respectively.

– The highest value of compressive strength for all test cases is obtained from specimens cured in water for 28 days and the lowest value is obtained from air cured specimens.

– SCC with 10% fly ash and 10% silica fume gave the highest value of concrete compressive strength for SCC consisted of combination of fly ash and silica fume.